Author Archives: Kimberly Ennico

About Kimberly Ennico

Dr Kimberly Ennico Smith is a research scientist at NASA Ames Research Center. Trained with a PhD in astrophysics, with an emphasis on astronomical instrumentation, Kimberly serves several NASA missions in a mission or instrument scientist role. At present she is Deputy Project Scientist New Horizons Pluto Fly-by Mission; Instrument Scientist Regolith & Environment Science and Oxygen & Lunar Volatile Extraction (RESOLVE) Near-Infrared Volatile Spectrometer System; and Instrument Scientist Mid-Infrared Spectroscopy Mode for the Stratospheric Observatory for Infrared Astronomy (SOFIA) FORCAST Instrument. She is also Principal Investigator for a small-sat collapsible telescope design and is actively working to mature low-cost, quick turn-around suborbital and balloon payloads that deliver focused science measurements and promote broader hands-on experience.

Focus is the name of the game! Flight Day (Nov 17, 2013): Part 2 Experiencing Microgravity for the First Time!

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My final blog summarizes my experiences of the flight and my evolving perspective on this type of platform for doing engineering, science and technology experiments. (Earlier posts, part 1 here, part 2 here).

First Impressions. So for a first timer, the first question asked is, “So what was it like?” I am so glad I had an audio recorder since my first experience on the onset of micro-gravity for the first time (and hopefully not the last time) in my life was said in deadpan fashion (totally not typical for me) “Alright. That’s interesting. Oh, wow. Okay. Yeah. We’re good.”

(The second question is “Did you get sick?” Well, it was challenging to keep disciplined to keep my head straight, especially during the 1.8-2G periods. I did not get sick, but got close to being sick on Parabola #25. But it was totally my fault since I looked out the window between Parabola #24 & #25 and saw the horizon almost vertical and that messed with my head. Lesson learned: don’t look out the window.)

In the interest of full disclosure, one payload had been having some intermittent issues that, like all intermittent issues, reared its head during a pre-flight end-to-end test a day before the flight. Luckily I had a contingency operations sketched out which performed perfectly. So when were on the plane and were doing the set-up and startup, I was really “uber-focussed” on the payload and not on myself for the first few cycles. When things started to get into a rhythm around Parabola #5 I had no idea we were 1/5th of the way done. Wow.

In between parabolas

“That was short. That was very short.” My comments after the very first parabola, which was a Martian (0.33 G) scenario. This image shows our team’s positions in between parabola 1 & 2. We did not have space enough to fully lay down so we reclined against the side of the aircraft. Left to right is Con Tsang, myself (monitoring a payload via a table), Cathy Olkin, and Alan Stern (face not visible). The photo is taken via Go-Pro camera on the head of Dan Durda who was across the way. Eric Schindhelm, who rounded out our team, was next to Dan and not in this view.

The rapid change between the onset of low-gravity for about 10-15 seconds followed by 2-3 sec transition to what appeared to be about 30s of 1.8-2G forces was very unexpected. With each parabola I did start to realize that the set-up time for the manual operation of one payload took way too long. (Lesson learned)

Sometimes we had unexpected escapes (I escaped my foot-holds on Parabola #7) and Eric Schindhelm (shown below) escaped the next one.

Parabola 8

Con was monitoring BORE and deftly diverted Eric’s collision path. For BORE, the key thing was to keep the box free from any jostling by others or the cables.

The payloads. We had two payloads, each with different goals for the flight. The fact that a decision to tether them together (made a few weeks before the flight) complicated the conops (concept of operations). One was a true science experiment: BORE, the Box of Rocks Experiment. The other was primarily an operations test for the SWUIS, the Southwest Universal Imaging System. Both experiments are pathfinder experiments for the emerging class of reusable commercial suborbital vehicles. Providers like Virgin Galactic, X-COR, Masten Space Systems, Up Aerospace, Whittinghill Aerospace, etc. You can read more about this fleet of exciting platform at NASA’s Flight Opportunity page https://flightopportunities.nasa.gov/, where they have links to all the providers.

Setting up SWUIS & BORE early parabolas

From left to right: Dan & Con monitoring BORE (aluminum box with foamed edges) while Cathy holds onto the SWUIS camera doing a “human factors” test using a glove (yellow). Image from Go-Pro camera affixed to the SWUIS control box.

View of SWUIS Target and Control Box

View of the SWUIS control box and Go-pro camera (used for situation awareness) while Dan’s holding it. You can see the SWUIS target that we used for the operations testing.  Image from a Go-Pro camera affixed to Dan’s head. Multiple cameras for context recording were definitely a must! (Lesson Learned)

Dan doing SWUIS targeting during Parabola

Dan Durda taking a test run with SWUIS on Parabola #23 (19th zero-G).

With BORE, we ask the question: how do macro-sized particles interact in zero gravity? When you remove “gravity” from the equation, other forces (like electro-static, Van der Waals, capillary, etc.) dominate. In a nut shell, BORE is a simple experiment to examine the settling effects of regolith, the layer of loose, heterogeneous material covering rock, on small asteroids.

Our goal is to measure the effective coefficient of restitution (http://en.wikipedia.org/wiki/Coefficient_of_restitution) in inter-particle collisions while in zero-g conditions. The experiment consists of a box of rocks. There are two boxes, one filled with rocks of known size and density, one filled with random rocks. Video imagery (30fps) is taken of the contents of each box during the flight. After the flight, the plan is to use different software (ImageJ, Photoshop, and SynthEyes) to analyze the rocks and track their movements from frame to frame. The cost of BORE is less than $1K in total, making it in reach of a the proceeds of a High School bake sale!

BORE does need more than 20 s of microgravity to enable a better assessment of rock movement, and this is exactly why this experiment is planned for a suborbital flight where 4-5 minutes of microgravity conditions can be achieved. Here, we used the parabolic flight campaign to test the instrumentation and get a glimpse of the first few seconds of the rock behavior. With this series of 15-20s of microgravity, we made leaps forward from previous tests using drop towers which provide only 1-2s of microgravity.

Today’s Microgravity platforms and durations of zero-G from https://www.nasa.gov/audience/foreducators/microgravity/home/.

  • Drop Towers (1-5 s)
  • Reduced-Gravity Aircraft (10-20s)
  • Sounding Rockets (several minutes)
  • Orbiting Laboratories such as the International Space Station (days)

Box of Rocks Sample Zero G Data

Some BORE images from one of the zero-G parabolas. Top Row: (left) Rest position of and (right) free-floating bricks of known size (they are actually bathroom tiles from Home Depot) but have the ratio L:W:H of 1.0:0.7:0.5. Surprisingly this is near the size and ratio of fragments created from laboratory impact experiments (e.g. Capaccioni, F. et al. 1984 & 1986, Fujikawa, A. et al. 1978) and similar to the ratio of shapes of boulders discovered on the rubble-pile asteroid Itokawa (see below).

Why is this important? Well, if you want to visit an asteroid someday and are designing tools to latch onto it, drill/dig into it, collect samples, etc. the behavior of collisional particles in this micro/zero-gravity environment is important. Scientifically, if you want to understand more about the formation, history and evolution of an asteroid where collisional events are significant, knowing more about how bombardment and repeated fragmentation events work is a key aspect.

Itokawa and ISS for Scale

Source: NASA & JAXA. The first unambiguously identified rubble pile. Asteroid 25143 Itokawa observed by JAXA’S Hayabusa spacecraft. (Fujiwara, A. et al. 2006). The BORE experiment explored some of the settling processes that would have played a role in this object’s formation.

SWUIS was more of a “operations experiment.” This camera system has been flown on aircraft  before to hunt down elusive observations that require observing from a specific location on earth. For example, to observe an occultation event, when a object (asteroid, planet, moon) in our solar system crosses in front of a distant star, the projected “path” of the occultation on our planet is derived from the geometry and time of the observation, similar to how the more familiar solar and lunar eclipses only are visible from certain parts of the Earth at certain times. Having a high-performance astronomical camera system on a flying platform that can go to where you need to observe is powerful. So, SWUIS got its start in the 1990s when it was used on a series of aircraft. You can read more about those earlier campaigns at http://www.boulder.swri.edu/swuis/swuis.instr.html.

Over the past few years I have been helping a team at the Southwest Research Institute update this instrument for use on suborbital vehicles that get higher above the earth’s atmosphere compared to conventional aircraft. Suborbital vehicles can get to 100 km (328,000 ft.; 62 miles) altitude, whereas aircraft fly mainly at 9-12 km (30,000-40,000 ft.; 5.6-7.5 miles). Flying higher provides a unique observational space, both spectrally (great for infrared and UV as you are above all of the water and ozone, respectively), temporarily (you can look along the earth’s limb longer before an object “sets” below the horizon) and from a new vantage point (you can look down on particle debris streams created by meteors or observe sprites & elves phenomena in the mesosphere). 100km altitude is still pretty low compared to where orbiting spacecraft live, which is 160-2000 km (99-1200 miles) up (LEO/Low Earth Orbit). For example, our orbiting laboratory, the International Space Station is 400 km (250 miles) in altitude.

Suborbital Flight Trajectory and access highlights

The SWUIS system today consists of a camera and lens, connected by one cable to a interface box. The interface box, which is from the 1990s version, allows one to manually control gain and black-level adjustments via knobs. It also provides a viewfinder in the form of a compact LCD screen. Data is analog but then digitized to a frame-grabber housed in a laptop. The 1990s version had a VCR to record the data, but since we are in the digital age, the battery-operated laptop augmentation was a natural and easy upgrade. The camera electronics are powered by a battery which makes it portable and compact. For this microgravity flight I introduced the notion of a tablet to control the laptop, to allow for the laptop to be stowed away. In practice this worked better than expected and my main take away is that the tablet is best fixed to something rather than hand-held to prevent unwanted “app-closure.” However, having a remote terminal for the laptop also would work.

Here’s a series of three short videos (no sound) of three legs when I got to hold the Xybion camera on Parabolas #13, 14 & 15. This captures how terribly short all the parabolas are and if you are doing an operations experiment, how utterly important it is to be positioned correctly at the start. One test was to position myself and get control of the camera and focus on a test target. A second test was to practice aiming at one target and then reposition for another target within the same parabola.

SWUIS_ZeroG_Parabola13_Web2

SWUIS_ZeroG_Parabola14_Web2

SWUIS_ZeroG_Parabola15_Web2

Above, the links are for lo-res (to fit within the upload file size restrictions on this site), no sound Videos of Parabola #13,14,15  (7,8 &9th in microgravity). By the third time I was getting faster at set-up and on-target time.

In 1-G this camera and lens weigh 6.5 lbs. (3 kg) . Held at arms length, when I was composing the test in my lab, as I scripted the steps, I had trouble controlling the camera. In fact, I was shaking to keep camera on target after some seconds. I was amazed at how easy it was to hold this in zero-G, and complete the task. The Zero-G flight told us many things we need to redesign. One issue we learned was the tethering cabling was not a good idea and in some cases the camera, held by one person, was jerked from the control box, held by another person. In the next iteration, one of those items will need to be affixed to a structure to remove this weakness.

My lessons learned from the whole experience: Everything went by very quickly. Being tethered was difficult to maintain. Design the conops differently (what we did seemed awkward). Laptop and tablet worked better than expected. Hard to concentrate on something other than the task at hand. Don’t plan too much. Have multiple cameras viewing the experiment. Need to inspect the cable motion via video, as it was hard to view it in-situ. Very loud, hard to heard, hard to know what other people were working on. The video playback caught a lot more whoops during transitions to zero-G than I remembered. Heard the feet-down call clearly but not the onset of zero-G. The timing between parabolas is very short. The level breaks were good to reassemble the cabling then. Next time, don’t hang onto the steady-wire which is attached to the plane (I got that idea from Cathy & Alan next to me) as it caused more motion than needed (the plane kept moving into me): instead remain fixed with the footholds and do crouch positions like Con & Dan did and let the body relax (Con & Dan were most elegant).

And, my biggest take-away of all: If you want to do a microgravity experiment, I strongly recommend doing a “reconnaissance” flight first. Request to tag along a research flight to observe, perhaps lend a hand as some research teams might need another person. Observe the timing and cadence and space limitations. Use that to best perform your experiment. It is an amazing platform for research and engineering development and can truly explore unique physics and provide a place to explore your gizmo’s behavior in zero-G and find ways to make it robust before taking it to the launch pad.

I am very much hoping to experience microgravity again! With these same two payloads or with others. One of the key points of these reduced-gravity flights, they fly multiple times a year, so in theory, experiment turn-around is short. Ideally I wished we flew the next day. I could have implemented many changes in the payload-operations and also in Kimberly-operations.

Our team is now working to assess what worked and what did not work on this flight. We achieved our baseline goals, so that is great! Personally, I wished I had not been that focused on certain aspects of the payload performance and made more time to look around. However, that said, my focus keyed me on the task at hand, the payload performed better than expected, and when you have 10-15s, focus is the name of the game!

It’s time to fly and go weightless! Microgravity Flight Day (Nov 17, 2013): Part 1, TSA Check, Board, Ascent, and Flight Profile.

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This is a second entry (part one here, part three here) of a three part blog series about my recent experience in microgravity.

Team Photo Before Flight

The team is outfitted in their flight suits ready to go! left –to-right Kimberly Ennico (me), Con Tsang, Eric Schindhelm, Dan Durda, and Cathy Olkin. The photo was taken by Alan Stern, another member of the team, rounding us to six flyers. Con & Cathy had flown once before. Dan & Alan had multiple flight histories. It was Eric & I to savor the first-time-flyer award. All my colleagues work at the Southwest Research Institute in Boulder, Colorado.

In writing this blog entry, I still giggle at recalling the moments before the flight. We actually had a TSA check before boarding the plane. Now, pretty much every person has a go-pro or a recorder strapped to some limb, all carefully secured in the pockets of his/her flight suit. So as each person went through security, all the pockets had to be emptied before the TSA wand-scan, and then all the devices got re-pocketed ready for the adventure.

So what were in my pockets? I had some spare duct-tape affixed to plastic (for easy removal) to do patch-taping (came in super handy), a Nexus tablet for one of the experiments (with velcro on its backside ass it needed to be velcroed to the floor), 6 AA fresh batteries (for me to putt in one of the payloads during the setup leg), two checklists (both velcroed to me), and an audio recorder (affixed to my arm with a iPod armband). Any items that did not have some sort of way to be strapped or velcroed down had to be lanyard to you (such as a camera).

Author boarding plane

That’s me outfitted with documentation. I’m “walking documentation.” My right shoulder holds an audio recording device, my right thigh our checklists, and my left wrist (not shown) the list of tests vs. parabola. If you look closely my name badge is upside down, indicating I am a first-flyer.  (Photo by Dan Durda).

I was assigned seat 3C for takeoff (and yes, they actually gave us boarding passes!). There are a few rows of seats in the back which all fliers have to be buckled in for take up.  We boarded from the rear of the 727-200. There was an in-flight safety briefing (oxygen, life jacket, seatbelts). There is an emergency card, tailored for Zero-G, similar to what was provided for SOFIA. The plane is operated by Zero-G corporation, but registered under Amerijet. Its call sign was AJT213. The main body is empty with padded floors, walls and ceilings. There are specific areas to bolt down footstraps and equipment. For those items that cannot be bolted down, there are a series of Velcro strips we placed the day before. This turned out to be important as during the in between microgravity parabolas, you experience 1.2-2 G and holding free-floating equipment will immediate come crashing down. So this experiment which involve 5 separate free-floating equipment, having a “safe place to store.”

At approximately 9:16 am EST (local time), we taxied and the takeoff felt just like a normal plane. At about 10 minutes after takeoff, we were instructed we could begin our set-up. This set-up leg is about 15-30 minutes in length. From our practice sessions last week we knew that setting up SWUIS took about 15 minutes (with no glitches). BORE took a similar amount and they are dovetailed in such a way that we need to go in parallel but also stage certain setup first. So the checklist came in handy to remind us our “dance” for setup. We put in fresh batteries for our equipment and got it up and running in a we bit more than 15 minutes, after experiencing a momentary pause when a known interference issue might have reared its head, but it played nice that morning.  We had a pretty complex set-up, which I realized we should simplify on future flights and I made some oral notes into the audio-recorder.

We knew from the review the day before we would be experiencing 25 parabolas in total, performed in bunches of five with a flat 1-2 minutes of 1 G of “level” in between. The first “set” would be four Martian (1/3 G) and one zero-G. The second set would be one lunar (1/6 G) and 4 zero-G. And all the remaining parabolas would be zero-G. There was only one experiment on board who had requested the Martian gravity, all others needed zero-G. I gathered that the tourist flights get 15 parabolas also similarly put in 5-sets, and depending on the experiments on the flight, the number of Martian & lunar parabolas are tailored appropriately.

Besides the research teams, Zero-G assigns at least one “coach” per experiment group. He or she can help with the experiment logistics, and also provide assistance if one of the team comes down with motion sickness. To avoid motion sickness, I was strongly advised not to turn my head, or if I had to turn my head, to ensure I turned my entire upper torso and slowly, and this especially important during the high-G parts of the parabolas.

Let me divert from the experience to summarize what the plane is supposed to do to provide these “periods” of reduced gravity. This “reduced-gravity environment” is created as the plane flies on a parabolic path: the plane climbs rapidly at a 45 degree angle (“pull up”), traces a parabola (“pushover”), and then descends at a 45 degree angle (“pull out”). During the pull up and pull out segments, everything on board, then crew and experiments, experience accelerations of about 2 g (and boy did I feel this! This was actually more striking than the <1 g). During the parabola (pushover), net accelerations are supposed to drop as low as 1.5×10-2 g for about 15-20 seconds. For me, this was the largest take-away of the entire experience: those periods of zero-G went by very, very, very quickly. Also the period of 2G felt like they went by much slower, but essentially they were the of similar duration. I was very surprised, but when I decoded my voice recorder results and looked at the camera data taken by our two experiments (which were time stamps) those “pushover” events were indeed in “20 s duration time chunks.”

After  5 parabolas, the aircraft was leveled off to get us back to “old familiar” 1 G. This was a key time I learned to help re-position cables (and in many cases, people!) to get ready for the next series of five. We erred in our conops design to rotate things in threes, which did not work very well with the break after 5 parabolas. Having known now the importance of using those breaks, I would have designed the operations-experiment differently. The other science experiment was not affected by that issue.

After speaking with other folks, apparently, the “20s duration of zero-G” is driven by safety limits on the aircraft’s flight profile, to drop only a few thousand feet during the parabolas. Here’s where the suborbital rockets (one-use) and the emerging new reusable commercial suborbital platforms come in, as they promise 4-5 minutes of microgravity in a single flight. This longer duration of zero-G is highly attractive for some experiments. However, others may still want multiple zero-G test times in a short time and those are nicely provided by these aircraft doing parabolic flight profiles.

Our entire flight from nose-up to nose-down was only 2 hrs. The time between the start of parabola 1 and the end of parabola 25 was about 1 hr. It was quick.

After the flight I looked up the flight path on flightaware.com and we were doing some pretty neat aerobatics over the Gulf of Mexico. Our altitude ranged from 25,000 ft. to 20,000 ft. during the parabolic maneuvers.

Flight Path Actuals

Flight Altitude Profile

My final blog summarizes my experiences of the flight and my evolving perspective on this type of platform for doing engineering, science and technology experiments.

This little scientist’s first taste of microgravity research aboard a reduced gravity aircraft. Flight Day Minus 1 (Nov 16, 2013): Briefing, Test Readiness Review (TRR), and Load Up the Plane.

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This is the first of a three-blog series (part 2 here, part 3 here) of this little scientist’s first foray into microgravity research. I participated in a research flight provided by the Zero-G corporation. To read more about their company go to https://www.gozerog.com/.

Zero-G operates a Boeing 727-200F aircraft, “G-Force One,” specially modified for reduced gravity operations. They provide opportunities for research flights (people and equipment) and also opportunities for you to experience zero-G (people). For experiment/research flights, you can apply directly to Zero-G where they organize a flight once they have enough researchers to fill a flight, or apply to NASA through their Flight Opportunities program https://flightopportunities.nasa.gov/, when NASA organizes the flight-manifest and Zero-G provides the flight platform. University students have additional opportunities to get flights through NASA’s Microgravity University, http://microgravityuniversity.jsc.nasa.gov/, with annual proposal calls. Had I known this when I was at school, I totally would have been a veteran flyer by now! Aircraft doing parabolic flight profiles are not restricted to the USA or to NASA. One list is provided here http://en.wikipedia.org/wiki/Reduced_gravity_aircraft.

The day before the flight, the flight director and series of “coaches” provided by the Zero G Corporation, came around to each of the research groups to look at the payloads and ascertain safety items. Prior to our arriving at our departure airport (in our case, Titusville, FL, but the “G-Force One” does fly from many airports, see their website), each team had to complete a Research Package, which contains the usual information such as mass, volume, power (including specifying “kill switch” items) and particular requests for gravity (the pilots can fly the airplane to simulate Martian and Lunar gravity in addition to near zero-G).  A series of weekly telecons were held in the weeks leading up to the flight to discuss interface needs and potential interference issues with others sharing the flight.

We meet the other teams for the first time. There were 6 experiments aboard this flight along with a BBC crew for the show Stargazing Live. One of the BBC presenters, Dara Ó Briain, joined us on this flight. So Kimberly gets to be an (unnamed) extra on TV show!

The other research experiments included (1) CubeSat solar-sail deployment mechanism, (2) testing a new IMU (inertia measurement unit), (3) evaluating sedimentations under Martian gravity, (4) Australian company developing ways to brew, and pour beer in zero-gravity, and (5) a mystery payload as it was under a NDA (non-disclosure agreement) with Zero G. Our Box of Rock science experiment (BORE) plus the SWUIS (Southwest Universal Imaging System) operations experiment rounded us to a total of 7 unique experiments. It was very fun getting to know the other experimenters, many who were also first timers!

payloads reduced gravity flight fit into suitcases

(left) Our two payloads in the hotel before driving out to the airport. (right) Easy to transport our suitcase-sized payloads to the airfield.

Setting Up Payloads for TRR

(left) Heading to the Test Readiness Review (TRR). Payloads coming in cardboard boxes, pelican cases and roller bags. (right) Julia Laystrom-Woodard, a senior engineer from CU Aerospace (another first time flyer like me) describing her solar sail deployment experiment at the TRR.

The Test Readiness Review (TRR) was held in a hangar near the plane. Each group had to show the Zero-G staff the exact payload and configuration and describe the experiment in more detail and call attention to unique configurations and requests. In our case, we were going to use both blue tooth and wireless communication to monitor our payloads during the flight, so this meant an interference test with the airplane would need to be scheduled later in the day. The reviewers were mainly concerned about safety, safety to ourselves, safety to fellow passengers and equipment nearby, and safety to the airplane. For example, we had filed down edges on our payload, but due to the free-floating nature of the experiment, they requested we “foam our edges.” As we had experienced flyers with us, we had brought foam pipe-insulation with us and, of course, the ever-essential duct tape.  However, we had to send a few members of our team over lunch to Home Depot to pick up some more. Duct tape and foam were the order of the day!

Assembling the Box of Rocks Experiment

Assembling BORE for the TRR. No shortage of duct tape and foam.

Assembling the Southwest Universal Imaging System

Assembling SWUIS and showing the layout of the tethering cables.

After the TRR, we waited for our time to set up in the aircraft. Along the floor of the aircraft are designated hook points. All payloads need to be secured to the floor with straps. For those that are “free-floating” they need to be secured within a storage box, whose dimensions were given to use prior. In our case, we configured our two free floating payloads to the size of two suitcase volumes. With our team of six, we identified where wanted our “footholds” to help keep us in place. These footholds were manually installed by the Zero-G folks and torqued down.

Loading payloads onto the plane

Loading up the plane via the back door to this Boeing 727. Not shown is that is another way to enter the aircraft via a large cargo bay door that can be opened on the side of the fuselage for larger payloads. For this flight, all the researcher’s experiments were all hand carried and broken down into smaller suitcase sized parts.

Setting up on the plane

(left) Securing one of the other experiments to the floor of the aircraft. You can see the large cargo door opened to the left. It was a hot day in Titusville, FL so it made setting up a bit cooler to have air circulating. (middle) Using loads of Velcro to provide “temporary” binding for our free-floating experiments during the high-G times. (right) Setting up and installing the foot straps (red cords) to specific locations on the  floor.

During this setup we learned where each group would be physically situated on board and we could re-assess interference items not previously considered. Each experimenter group was assigned a 10 foot x10 foot area on the plane and were designated by the color of their socks. We were the “grey team” and had a spot about half-way down the aircraft near the exit windows.

After the configuration of all the mechanical hold-down areas, we did our powered tests and also checked for interference. All looked good. Anything we would bring the next day to board the flight had to fit in our flight suit. We next stowed our two suitcase payloads for takeoff and headed back to the hotel for a team briefing and light dinner.

The next blog entries follow the flight day.

 

 

The Pluto science community is rich and diverse, just like its target of study: the ever-fascinating Pluto and its satellite moons.

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This blog entry concludes my series of talk summaries for the July 22-26, 2013 Pluto Science Conference, “The Pluto System on the Eve of Exploration by New Horizons: Perspectives and Predictions.” You can read more about the conference and browse through the abstracts at the conference website https://dnnpro.outer.jhuapl.edu/plutoscience/Home.aspx

In his closing comments, Alan Stern (SwRI), the lead scientist (Principal Investigator) for NASA’s New Horizons fly-by mission to Pluto, told us about the last time a scientific discussion gathering specifically about Pluto occurred. It was twenty years ago, a 3-day meeting in July 1993, in Flagstaff, Arizona. The talks and presentations from that workshop led to ten contributed papers in a special issue in 1994 in Icarus (Vol 108, Issue 2) and, in 1997, the publication of a book entitled “Pluto and Charon” by The University of Arizona Press.

When the group gathered in 1993, the 1989 Voyager 2 fly-by of Neptune’s moon Triton’s was still “fresh data”, the prime Pluto-Charon “Mutual Events Period” of the 1985-1990 had just ended, and the Hubble Space Telescope (HST) would be soon coming back on-line with its fixed optics (the 1st HST servicing mission would occur in December 1993). It was a busy time for the Pluto science community.

Pluto Scientists July 1993

Some attendees at the open workshop meeting on Pluto & Charon in July 1993, Flagstaff, Arizona.

This five-day July 2013 meeting has demonstrated that the quest to better understand Pluto and its environment is a very rich and diverse field of study. With each new data set about Pluto and its companions, surprises are uncovered and new questions are posed. When the New Horizons spacecraft reaches the Pluto system in July 2015, a true “first encounter experience,” its on-board suite of modern instruments will transform our current-best resolution ~800 km/pixel (from Hubble observations) to a resolution of 0.46 km/pix (hemisphere) with 0.09 km/pix (regional) resolution with the LORRI instrument. You can be certain there will be a lot more surprises in store. Combining this with new and unique data sets from New Horizons’ particle & dust instruments and the UV and IR spectrometers, our understanding of the Outer Solar System will find a new grounding.

With 103 oral talks + 30 posters + 13 “topical sessions” this was a jammed pack week of sharing old information, sharing new data from the past few years, sharing “hot off the press data” (it’s Pluto observing season right now and during the conference attendees were doing observations of Pluto & Charon with IRTF, Keck and other telescopes, remotely or with their colleagues at the telescopes), identifying what computations or experiments are needed before the 2015 encounter, and in some cases, providing predictions of what might be detected at Pluto and Charon. Several papers presented at this conference will be submitted to the Icarus journal.

Pluto Scientists July 2013

Attendees at the “The Pluto System on the Eve of Exploration by New Horizons: Perspectives and Predictions,” held July 22-26, 2013, in Laurel, MD. The topical sessions covered Atmospheres, Charon, Dust & Rings, Interiors, Kuiper Belt Context, Laboratory Studies, Magnetosphere, New Horizons Mission, Origins, Satellites, Surface Composition, Surface Geology, and Surface-Atmosphere Interactions.

The stage is set for a summer 2017 Pluto Science Conference. New Horizon’s flyby of the Pluto System is on July 14, 2015, but it will take a bit over a year for all the data to come down losslessly (i.e. without compression). Deliveries to the NASA’s Planetary Data System are planned in 2016 and early 2017.

I hope you enjoyed this blog series reporting on these intriguing topics. You can follow the New Horizons mission status at any time by visiting the New Horizons Mission Website at http://pluto.jhuapl.edu/ and https://www.nasa.gov/mission_pages/newhorizons/main/index.html.

To Pluto and Beyond!!!!

Pluto Exotica. Atoms. Pick Up Ions. Bow Shocks. Suprathermal Tails. X-Rays. UV airglow.

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The morning of the last day of this week’s July 22-26, 2013, Pluto Science Conference opened up the discussion with outer atmosphere (far out) and magnetosphere (really far out) talks.

Fran Bagenal (University of Colorado) started the session with a talk on “The Solar Wind Interaction with Pluto’s Escaping Atmosphere.” Pluto’s interaction with the solar wind was first suggested in 1981 by Larry Trafton. There are two generally predicted regimes of what this interaction might look like: (1) Venus-like (small escape rate) and (2) Comet-Like (high escape rate). A key parameter distinguishing the two is what the atmospheric escape rate might be, that is, how many atmospheric molecules (assumed to be nitrogen) are escaping from Pluto, no longer being bound by gravity. Current estimates for the escape rate, based on a number of approaches, notably a recent one by Darrell Strobel (2012), have this number at 2-5×1027 molecules/sec.  This is large enough to suggest Pluto will appear to be “comet-like” in its interaction with the solar wind. However, we need to wait until 2015 for the New Horizons fly-by with their in-situ particle instruments SWAP & PEPSSI to make the interaction measurements.

When describing the Pluto System in terms of solar wind interaction, Fran Bagenal showed this image, which superimposed one of Darrell Strobel’s atmospheres (characterized with an exobase at 12 Pluto radii). Pluto becomes a “large object” for interaction with the solar wind.

Solar Wind Perspective Pluto

When solar wind particles (protons) interact with the Pluto atmosphere, their path through space is bent along the magnetic field lines, and to convert momentum, pickup ions (neutral hydrogen atoms from the heliosphere that undergo a collisional charge-exchange interaction with solar wind protons, get ionized, are “picked up” by the solar magnetic field) get tossed onto new trajectories. Those ions are charged and will begin to rotate and follow electrical field lines. Where do the ionized particles go? A weak magnetic field will create large gyro-radii of pick-up ions which can extend millions of kilometers upstream of Pluto.  This is best modeled with a kinetic interaction.

Peter Delamere (University of Alaska, Fairbanks) spoke in greater detail about “The Atmosphere-Plasma Interaction: Hybrid Simulations.” Plasma interaction is an atmospheric diagnostic tool. Neutral gases are not easily picked up, but ions and how they interact with the solar wind can be detected with in-situ instruments such Hew Horizons’ SWAP and PEPSSI. He discussed his model plasma interaction mode, which was validated using Comet 19P/Borrelly that had been visited by Deep Space 1 on Sept 22, 2001.

Comet Borrell Solar Wind Interaction

Example of Comet 19B/Borelly environment time vs. energy reveals the structure of the interaction between a comet and the solar wind. The X-axis is time from closes approach, with the Y-axis energy. The color code is the number of particles counted by the PEPE instrument aboard Deep Space 1. This is similar to what the data is expected to look at for Pluto when New Horizons reaches it in 2015, however, the solar wind at 33 AU may be more extended and more diffuse and therefore the signal strength (in terms of counts) will be much less.

If we can understand where the bow shock forms, this becomes a diagnostic of the atmosphere, and if indeed the exosphere extends out to 10 Pluto radii as suggested by recent work by Darrell Strobel (2012) and other models, then this is a sizable ‘obstacle.’ But is it inflated enough to form a bow shock? Peter Delamere thinks so. He stepped us through a variety of simulations. One of the simulations predicts a partial bow shock. If you increase Qo (the escape rate parameter, predicted to be in the 2-5×1027 N2 molecules) or increase magnetic field strength you can create a full bow shock. Future work includes adding the pickup part of the solar wind model as input.  If there is a very slow momentum transfer, perturbed flow could extend out to an AU.

Simulations predict all sorts of shock structures (Mach cones, bow shocks), but these structures depend on the escape rate parameter.

Solar Wind at Pluto Model

Example of a plasma interaction mode for three escape rates, decreasing from right to left. This is a slide in space of plane vs. distance from.  The white lines are sample solar wind proton trajectories. The color scale indicates ion density. The solar wind (and hence, the direction from the sun) is incident from the left. Pluto is at (0,0).

Predictions at Pluto. He anticipates significant asymmetry. The predicted bow show could be as far as 500 Pluto radii.

Heather Elliot (SwRI, San Antonio) in her talk “Analysis Techniques and Tools for the New Horizons Solar Wind around Pluto” described the New Horizons SWAP instrument and the different rate modes (sampling rate and scan types) it will be using during the 2015 encounter.

NH SWAP Instrument Cruise Data

Measurement of the solar wind taken with the SWAP instrument aboard New Horizons during the last 6 years of cruise. This data set covers AU=10 (Saturn distance) out to AU=23 in 2012. The solar wind is mostly protons (H+). The second most abundant species are alphas (He++). The colors are the intensity of species. The vertical axes are energy per charge units and the horizontal axis is time.

Fitting the SWAP data to a solar wind model requires making adjustments for view angle and during the hibernation period, when they do not have attitude information, they have modeled the Sun-probe-Earth angle to estimate the attitude and this works well to fit their data.

John Cooper (NASA Goddard) spoke about the  “Heliospheric Irradiation in Domains of Pluto System and Kuiper Belt.”  He is interested in computing the “radiolytic” dosage onto bodies in the outer solar system (that is, the effect of how molecules break down or change molecular band structure due to the influence of radiation, such as by cosmic rays, particles, UV, etc.). For this he needs measurements of the particle flux at large AU.  New Horizons joins its cousins Voyager 1 & 2, Pioneer 10 & 11 and Ulysses in exploring the outer solar system.

S/C Outer Solar System

Location of the NH spacecraft (orange on the left, purple on the right) for two different views of the solar system. Also plotted are deep space missions Voyager and Pioneer, among many. The left view is s top down view of the solar system with the Sun at (0,0), the axes are in AU, where 1 AU (Astronomical Unit) is the distance between the Earth and Sun. The right is a view of time vs. latitude for the crafts. Comparative data sets to New Horizons, which travels along the solar ecliptic, are Pioneer 10 and early Voyager 2 data.

He showed computations of irradiation dosage when applying those particle rates measured by New Horizon’s PEPSSI instrument and instruments aboard Voyager 2 and Pioneer 10.

He maintains a database of all particle instrument flux measurements at the Virtual Energetic Particle Observatory http://vepo.gsfc.nasa.gov.

Thomas Cravens (JHU/APL) with ”The Plasma Environment of Pluto and X-Ray Emission: Predictions for New Horizons,” asked “What happens when you get to within 1000 km of Pluto?“  Pluto is anticipated to be “Comet-Like” in its interaction with the solar wind, however when you get closer to Pluto (around 1000 km), it may more closely resemble “Venus-like” interaction. He is trying to compute where the charge-exchange boundary could be, probably around r~5000km. This is boundary between the kinetic (r>5000km) and fluid (r<5000 km) regimes, essentially probing the ionosphere regime of Pluto.

Switching to slightly lower energies, Casey Lisse (JHU/APL) gave a talk on “Chandra Observations of Pluto’s Escaping Atmosphere in Support of New Horizons.” X ray interactions (charge exchange, scattering and auroral precipitation) require an extensive neutral atmosphere, which is what is expected at Pluto. Interaction of solar wind with comets has consistently shown X-ray emission. He expects to see X-ray emission from Pluto. If detected it would tell us about the size and mass of Pluto’s unbound atmosphere. The best time to look for x-rays at Pluto is about 100 days after a large CME (corona mass ejection) event, which is about the time it takes for CME to get to Pluto at 33 AU.

He and his colleagues applied for, and got, time on NASA’s Chandra X-ray telescope. On Chandra, Pluto & Charon will appear to fill one Chandra pixel using the Chandra HRC instrument.  He ended his talk suggesting that looking at background counts with the LORRI and RALPH CCDs might serve as a poorman’s x-ray detector. It is also possible that PEPSSI background counts could be used to infer presence of lower X-rays.

Kandi Jessup (SwRI) gave a talk addressing the “14N15N Detectability at Pluto.” We care about 14N15N because it can be used to determine the 15N to 14N isotropic fractionation. This can help tell us about the evolution of Pluto’s atmosphere. Learning about Pluto’s atmospheric evolution history also provides vital suggestions for the evolution of equivalent TNOs (Trans-Neptunian Objects) and other objects in the Kuiper Belt, and hence, the outermost parts of our Solar System

The measurement will be the UV spectral observations during the solar occultation of Pluto by the Alice instrument during the New Horizons fly-by. N2 is the dominant absorber between 80-100nm. To identify the molecule 14N15N they use an atmosphere model from Krasnopolsky & Cruikshank (1999). That model does not have a troposphere. Next they need absorption cross-sections (a parameter that quantifies the ability of a molecule to absorb a photon of a particular wavelength) for 14N2 and 14N15N. 14N2 is the more dominant species and they are trying to find a very small percentage for 14N15N. Using these simulations they anticipate the Alice instrument will be sensitive enough to detect at least a 14N15N to 14N2 ratio of 0.3%. They will be look at the UV spectrum between 88 and 90 nm where the 15N lines spectrally shifted from 14N line. 14N15N to 14N2 ratio has been measured on Mars (0.58%), Titan (0.55%), and Earth (0.37%). What ratio will Pluto have? New Horizons data will hopefully tell us.

Randy Gladstone (SwRI, San Antonio) spoke about “Ly-alpha at Pluto.” Pluto ultraviolet (UV) airglow line emissions will be very weak, except at HI Lyman-alpha (Ly-a). Ly-a at Pluto could have both a solar (Sun) and an interplanetary (IPM/interplanetary medium) source. Ly-a should be scattered by Hydrogen atoms in Pluto’s atmosphere.  He uses the Krasnopolsky & Cruikshank (1999) Pluto atmosphere model that predicts the number of Hydrogen atoms at altitude. There are several observations near Pluto closest approach planned with the New Horizons Alice instrument to measure Lyman-alpha emissions.  This data will provide information about the vertical distribution of H and CH4 in Pluto’s atmosphere. Observation of the IPM Lyman-alpha source will be unique and provide important information to model Pluto’s photochemistry, especially for the nightside and winter pole region.

Randy Gladstone (SwRI, San Antonio) ended the session with a talk about “Pluto’s Ultraviolet Airglow.” He presented a model by Michael Stevens (Naval Research Lab), which has been used to explain the Cassini UVIS (Ultraviolet Imaging Spectrograph) observation of UV airglow at Titan over the 80-190 nm wavelength, emissions arising from processes on N2 (Stevens et al 2011). The model is called AURIC, the Atmospheric Ultraviolet Radiance Integrated Code. This model will be used for interpreting Pluto atmosphere data taken at UV wavelength with the New Horizons Alice instrument.

If Pluto was not already an exotic place to visit with all the predictions about its formation, its interior, its surface, it surface-atmosphere interaction, its composition, etc., it certainly will prove to be an amazing place if any or all of these predicted upper atmosphere and mesosphere molecular species, ions, and high energy particles are measured with the New Horizons spacecraft!

Winds. Fog. Frost. Global weather predictions on Pluto.

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Talk summaries from the Pluto Science Conference held July 22-26, 2013 in Laurel, MD continues. This blog entry is about atmosphere presentations on July 26th.

Angela Zalucha (SETI) began the discussion with her talk entitled “Predictions of Pluto’s vertical temperature and wind structure from the MIT Pluto general circulation model.”

A general circulation model (GCM) solves conservation of momentum in 3D, conservation of mass, conservation of energy and equation of state (P=rRT). It can tell us some fundamental atmospheric properties such as composition (what is it made of), pressure (how much is there?), temperature (how hot is it?), and wind (how does it move?). In particular, understanding wind is one of the most important things a general circulation model gives you, because it is so hard to observe remotely.

She presented her model, based on the MIT (Massachusetts Institute of Technology) GCM that was originally designed as an ocean model. She turned it upside down to make it an atmosphere model. It has multiple layers, CH4 mixing ratio at 1%, CO mixing ratio at 0.05%, includes atmosphere models (Strobel et al 1996) and runs for a 15 year Earth integration rate (she notes that is probably not enough time to have the atmosphere equilibrate). She sets frost layers on the surface as a parameter, and explored different surface pressures (8 16, 24 microbars). She uses the Ecliptic North convention. One output from this model are curves of temperature vs. altitude, called a temperature profile. She reported the presence of a frost predicts a much colder atmosphere. Future work will be to investigate other ice distributions, put in a CH4 transport model, and improve surface model.

Temperature Profile

Example of a suite of temperature profile curves from the Pluto MIT GCM. Temperature in Kelvin is shown for a range of altitudes in kilometers. The MIT GCM has assumed a particular Pluto radius to set zero altitude.

Melanie Vangvichith (LMD, Paris) in her talk “A Complete 3D Global Climate Model (GCM) of the Atmosphere of Pluto” presented another general circulation model for Pluto, the LMD (Dynamic Meteorology Lab) GCM. For a thin atmosphere that is expected on Pluto, their model uses careful parametizations of the nitrogen condensation and sublimation surface-atmosphere processes, which they claim is key (Forget et al 1998). They also adopt a particular initial frost distribution, the distribution from Lellouch et al 2000.  Their model is run for 140 Earth years, starting with 1988 adopting initial conditions based on observations. Conclusions. When adopting a 20 MKS thermal inertia, the model is in agreement with occultation data to date, but this model does not predict a troposphere, just a “big stratosphere.”

Winds from the LMD GCM

Example of a wind prediction from the Pluto LMD GCM. The temperatures (in K) are represented by the color and the arrows represent the wind direction and speed at particular height. This is mapped onto a lat/long grid using the right-hand-rule (i.e. matches the Marc Buie convention).

In the previous entry, I had commented on thermal inertia and its role in atmosphere dynamics. To recap here, thermal inertia is a measure of the ability of a material to conduct and store heat. In the context of planetary science, it is a measure of the subsurface’s ability to store heat during the day and reradiate it during the night. This has natural consequences for deriving what happens to processes that require an exchange of heat. A GCM uses thermal inertia of the surface as a key parameter. There is a currently big disconnect in the community over what Pluto’s thermal inertia is. In E. Lellouch’s talk on Jul 23 he reported that Spitzer & Herschel have measured Pluto’s thermal inertia as 20-30 MKS (Lellouch et al 2011). However, Pluto atmosphere pressure models needed to match occultation data by C. Olkin & L. Young require Pluto have a much higher thermal inertia >1000 MKS to explain their occultation measurements (this meeting). Thermal inertia is usually quoted in MKS units, where MKS is an abbreviation for “J K-1 m-2 s-1/2.”

Anthony Toigo  (JHU/APL) with his talk “The Atmosphere and Nitrogen Cycle on Pluto as Simulated by the PlutoWRF General Circulation Model” presented a third general circular model. Their GCM is based on the terrestrial model used for Weather Research and Forecasting (WRF). It has been adopted for Mars, Titan and Jupiter, and they have adopted it for Pluto.  They ran their model for two extremes of thermal inertia, as this is a current open question in the community. They are just attempting to see what effect this has on the predictions.  They also looked at the effect of the nitrogen cycle adjusting amount of nitrogen ice. Conclusions. The model is in agreement with the increase in pressure derived from observations, supports large volatile abundances, and shows a pole-to-pole transport. Future work for Pluto includes constraining the volatile cycle and looking at surface wind relations.

The three modelers sparked a lively debate at the Pluto Science Conference. Sometimes they agree and in many cases they diverge greatly. It was neat to see how different groups tackle the same physics problem. It came down to the details and initial assumptions. GCMs have become such powerful tools to describe dynamics (changes) in atmospheres, but because there are still so many assumptions about Pluto’s surface and atmosphere, it will only be until New Horizons provides measurements to start anchoring down these models.

Coordinate systems do matter. Brush up on that Right Hand Rule, y’all.

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This is a blog series covering the talks presented at the Pluto Science Conference, held July 22-26, 2013 in Laurel, MD.

In an engaging talk by Amanda Zangari (SwRI) entitled “Plutography: A Meta-Analysis of Coordinates on Pluto From Charon’s Discovery to the Present Day,” she compared and contrasted two coordinate systems used by Pluto researchers. Her motivation is that data sets, past, present and future will be compared to the New Horizons dataset, and so it will be very important that all use the same coordinate system.

Coordinates

It comes down to two coordinate systems, although she mentioned that some researchers sometimes use a hybrid-definition.  In her visual summary, the Red (left) is the Ecliptic North configuration where Pluto’s North Pole is “North of the invariable plane.” The Green (right) is the where “Pluto’s North Follows the Angular Momentum Vector” aka Right Hand Rule (RHR).  When the planet’s north pole is aligned closely with the Ecliptic North, the former is normally okay (like for Earth). However, for Pluto, Uranus and Venus the two are definitely very different. She suggests that the Right Hand Rule is the more appropriate definition for Pluto. Note that JPL Horizons (their official ephemeris generating software), GEOVIZ (New Horizons Planning software) and SPICE uses the other convention (Ecliptic North Pole).

How they differ are summarized below (i.e. both axes are flipped). The pole that is visible from Earth is what is seen in the lower-right quadrant of each schematic. Since the 1980s we have been is observing Pluto’s Northern Pole per the Right-Hand-Rule (RHR) convention.

Coordinates 2

Alan Stern, New Horizon’s Principal Investigator (lead scientist) mentioned that the New Horizons Spacecraft will not change its system prior to the Jul 2015 encounter. After the encounter, the plan will be to adopt the new SPICE files, etc. He stressed that Pluto Data that gets released in the Planetary Data System will be in the Right-Hand-Rule convention (RHR). Leslie Young, New Horizons’ deputy Project Scientist, said that there are new SPICE files available using a Pluto coordinate system using the RHR Convention, although the JPL/SPICE official release  is still Ecliptic North.

So, huh, which way is up? I’m sure this topic is far from over. In fact, during discussions at the meeting, agreeing on a coordinate system for planetary bodies is no stranger to this community.

Did you know it’s northern springtime on Pluto right now? Pluto is far from a cold lump of rock we were told about in school. It’s a dynamic world and has seasons.

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The afternoon session of Jul 25th of the Pluto Science Conference started with John Stansberry’s (STScI) talk entitled “Interactions between Pluto’s Surface and Atmosphere.” He stated, “The similarities between Pluto and Triton are remarkable.”

Pluto & Triton

The main properties of Pluto and Neptune’s moon Triton are summarized above.

Pluto has a volatile-rich atmosphere (N2, CO, CH4) and interacts with the surface to bring about mass and energy exchange. N2 dominates the surface ices and the atmosphere. N2 is also globally much at the same temperature mainly due to N2’s large latent heat of sublimation that balances out changes in temperature. To probe deeper at surface-atmosphere interactions, he looked specifically at the methane to nitrogen mixing ratio (abundance of one component of a mixture relative to that of all other components). But there are many open mysteries about surface-atmosphere interactions.

Overabundance of Methane Mystery.  Pluto’s upper atmosphere has X_CH4 ~0.5% (X_CH4 is the methane mixing ratio) based on occultation measurements. Infrared spectral measurements (Jason Cook et al 2007) from the lower atmosphere derive a much higher X_CH4 ~ 4%. Surfaces models for the N2+CH4 ice predict  X_CH4 ~0.5%. So in order to explain the overabundance of methane in Pluto’s lower atmosphere, two models were introduced to help provide additional sources of methane. This is an active area of study.

Changing Atmospheric Structure. Stepping through the light curve changes shown by Mike Pearson (see previous blog entry) there is something changing the structure of the atmosphere at ~1200 km. Comparing 1988 (equinox) and 2006 (N mid-Spring; Northern ) could be explained partially by geometry changes. Not all the changes are understood.

Pluto’s lower atmosphere is a mystery. We can probe down to the stratosphere with occultation measurements (scale height 50 km). So below 50km they need to resort to models.

Other influences on surface-atmosphere interaction included effects due to topography and winds. Winds have been observed on Triton (Hansen et al 1990).

Predictions for New Horizons. X_CH4 will be ~0.5% in stratosphere, a few % in troposphere. For the atmosphere structure he predicts r_tropopause = 1185 km, h_tropopause = 10km, r_atmosphere_base = 1200km, weak inversion, and bottom of atmosphere at r=1175 km, pressure at surface > 15 microbar. Winds will be Triton-like. A bright north polar cap. Potentially morning frosts.

Bonnie Buratti (JPL) presented a talk on “Pluto’s Light Curve over Time as an Indication of Seasonal Volatile Transport.” They are looking at historical light curves, plus new ones and fit with a fixed frost model. Changes in light curves tell you about the albedo (reflectance) of the surface. You do need to do a correction to phase angle because of Pluto’s high obliquity. They do find the data over 2012-2013 consistent with a constant frost model. She showed the data of long-term monitoring of Triton and it indicated a volatile change and they got HST imaging data which when compared with the Voyager fly-by they did see that areas of high albedo got dark and others got brighter, supporting their interpretation of the light curve approach. They really need to get a good light curve prior to New Horizons’ 2015 fly-by encounter.

Erin George (University of Colorado), working with Marc Buie (SwRI) in “Pluto Light Curve in 2010,” described her work in analyzing data from Lowell Observatory over 2007-2013. The challenge had been to find stars to use as relative flux calibrators that were well separated. They also used a technique to remove the template of background stars (eliminate field confusion).

Marc Buie (SwRI) next took us on a tour of “Seasonal Variations on the Surface of Pluto.” He reports on visible (B & V filters)  photometry from data taken with photographic plate, photoelectric detectors, and CCDs from 1954-2010. All the measurements are of “Pluto+Charon” as the two bodies are not resolved from the ground for the majority of this data (large pixels). He showed the trend of the light curve which indicates that “something happened in 1992” (he hypothesizes it occurred very fast) to change the “color” of Pluto because the light curves in B & V passbands deviate. He’s working towards removing Charon from the data using a model for its brightness from his HST data.

Leslie Young (SwRI) presented a talk on “Modeling Pluto’s Diurnal and Seasonal 3-Dimensional Volatile Transport with VT3D. ” She asks, “Why should we care about volatile transport?” Three key reasons: (1) Mobile volatiles control the surface appearance (albedo, composition), (2) Volatile transport depends on the hidden subsurface (thermal properties, depth of volatile deposits), and (3) Volatile transport models can predict atmospheric behavior at other times (escape rates, atmospheric chemistry, winds).

Her new code, VT3D, uses physics from standard volatile transport models (Hansen &  Paige, Spencer & Moore). She has been validating it against other codes. Types of parameters that she investigated in her code are emissivities, thermal inertia, albedo, and nitrogen abundances. She found her results clumped into three categories, atmospheres characterized by: (1) Permanent Northern Volatiles, (2) Exchange with Pressure Plateau, and (3) Exchange with Early Collapse.

A description of her model can be found in L. Young (2013) http://arxiv.org/abs/1210.7778.

Her model fails to predict a bright south pole seen in the 1990-1994. But then she counters, could that be possible because it’s covered with bright CH4 frost?

Pluto Volatile Transport Model

Three classes of atmosphere models from Leslie Young (2013). The graphs are the surface albedo and the pressure at  1175km in u=microbars vs. time. The time runs from 1866 to 2116, a full Pluto year about the Sun. The seasons on Pluto are shown with the vertical lines and the equinox in 1990 is highlight with the circle.

To learn more about Pluto’s seasons check out this blog from the Planetary Society http://www.planetary.org/blogs/emily-lakdawalla/2013/05021212-plutos-seasons-new-horizons.html.

Candy Hansen (PSI) described her model in her talk entitled “Pluto’s Climate Modeled with  New Observational Constraints.” She described her model, HP96, named after Candy Hansen and her collaborator Dave Paige, which was coded in 1996. She showed an output of the model for Pluto from 1000 to 2100 AD, over a good four Pluto-orbits about the sun. The model now has been updated to address new knowledge learned about the Pluto system.  To derive solutions that do not have a zonal band (an observable characterized by sharing a range of latitude, appearing as a ‘band’), eliminates high thermal inertia cases, cold frosts and large abundances of N2. The model does meet constraints from the observed albedo. She does not include an atmosphere in her model, so she is excluding wind and other atmosphere layer issues. After seeing the data from the May 2013 occultation presented at this meeting on Tuesday made her change her models and she presented a true “hot off the press” new result today. Predictions for New Horizons. 2.4 Pascals at the surface at the time of the New Horizons fly-by.

As a closing comment during the discussion session, Rick Binzel is hopeful that Charon-illuminated image of Pluto’s south pole during the New Horizons fly by will be a key to helping understand what may be going on at Pluto’s south pole!

Pluto's Climate

A nice summary of seasons of Pluto. Since the 1950s we are seeing more of Pluto’s northern hemisphere. We are in Pluto’s northern spring time right now.

Laurence Trafton (University of Texas) gave a talk on  “Driving Seasonal Sublimation and Deposition on Pluto-Uncertainties in Evaluating the Vapor Pressure.”  Pluto’s atmosphere is supported by the vapor pressure of its surface ice.  For most models, N2, CH4, and CO are assumed to exist solely in solid solution on Pluto’s surface, and are well mixed in atmosphere. However, this did not explain the mystery of Pluto’s elevated atmosphere CH4 amounts (see above talk by Stansberry). Two models were suggested: “Detailed balancing model” (DBM) (Trafton 1990) and the “Hot CH4 Patch Model” (Stansberry et al 1996).  The latter only needs 1-3% of Pluto’s surface to have this extra source of CH4. Neither model can explain widespread pure CH4 ice hypothesized to be on Pluto’s surface. He is in need of lab experiments to establish vapor pressures for the saturated areas of the phase diagram.

Tim Michaels (SETI) spoke about “Global Surface Atmosphere Interactions on Pluto. ” He is using the OLAM (Ocean Land Atmosphere Model, Wakko and Avissar, 2008) model. He is using the Northern summer in 21st century convention (which is the IAU/Ecliptic North coordinate system). Their approach stats with a simple surface nitrogen ice model (no methane). When run for 1990 and 2015, they show distinctly different trends. Next steps are to add albedo distribution, methane cycle, and gravity changes. This is a rich atmosphere-surface system.

Kevin Baines (University of Wisconsin-Madison) spoke about “Chemistry in Pluto’s Atmosphere and Surface: Predictions of Trace Aerosol and Surface Composition, and a Potential Geologic Chronometer.” There are many sources that drive atmosphere surface chemistry and albedo. For example, volatile transport dominates on days, months, and year timescales. There is UV photochemistry (decade timescale) in this rich atmosphere. Hydrocarbons could be raining out 1mm every 50,000 years. Solar wind and accretion activities (impacts, dust from satellites or KBOs) occur in 1Myr timescales. So, he asked, “What would Galactic Cosmic Rays (GCR) do?“ The types of products by irradiation from CGRs include CH4, CnH2n+2, C6H10, NH3, HCN, etc. over 5-20 Myr. And he proposes they may be viewable by darkening on airless KBOs.

Vladimir Krasnopolsky spoke on “Pluto’s Photochemistry:  Comparison with Titan and Triton.” There are three bodies with N2/CH4 atmospheres in the solar system: Titan (moon of Saturn), Triton (moon of Neptune), and Pluto. He proposes that Titan is a better analog to Pluto rather than Triton. He presented the main results of his Pluto Model (Krasnopolsky & Cruikshank 1999). He observed Pluto with HST in the UV 180-256nm and was able to fit molecular species predicted by his model and saw no albedo changes from his predictions (Krasnopolsky 2001). New Horizons data will provide useful information to update their photochemical model.

Francois Forget (CNRS, Paris, France) closed the day with “3D Modeling of the Methane Cycle on Pluto). He presented the LMD Pluto Global climate model. Their model does include the methane cycle, but they are neglecting microphysics of N2-CH4. They assume the terrain description of Pluto from Lellouch et al 2000 (areas of N2 Ice, CH4-rich areas and dark albedo areas due to tholins). They ran the model starting from 1988 and the model has a resolution of 170km. The model predicts an observable X_CH4 of 0.35% and this in the agreement of the ~0.5% from observational data by Lellouch et al (2011). The models predict methane cloud formation at the pole and that could be matching observed data from J. Cook who understanding his methane detection comes form cold parts of the atmosphere. He presented a model for cloud formation for 2015 and clouds do appear in the low, colder parts of the atmosphere. Their model will be made available to the community to use. They do not have a cold troposphere in their model.

Prediction for New Horizons. More CH4 ice deposits at the summer pole in 2015 than in 2010.

Who’d have known that that little cold world out there in deep, dark space, would have such a fascinating trip around the Sun? Data from New Horizons from its July 2015 fly-by of the Pluto System will literally confirm or refute all of these predictions. Then the models can be updated to reflect what summer will be like on Pluto in 2240.

Today: Geology of unmapped worlds. 2015: Pluto will never be the same as New Horizons brings you a Pluto, Charon, Nix, Hydra, Kerberos, and Styx, in ways never seen before.

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This a blog entry for a series about the Pluto Science Conference being held at JHU’s APL in Laurel, MD, July 22-26, 2013. This entry summaries surface geology talks presented on July 25th.

Paul Schenk (LPI) began the session with his talk entitled “The Improbable Art of Predicting Pluto-Charon Geology.” Thanks to Voyager, Galileo and Cassini we have a wealth of knowledge about icy bodies in the solar system. However, in comparison to the icy satellites about Saturn, the Pluto-Charon bodies are expected to have key differences: volatile ice content is probably higher, their geological histories are not influenced by the existence of large giant planet near by (tidal forces), etc.

He asked, Is Pluto just another Triton? No. It may be similar in size and composition, but geology will be different. The main sources for heating for Pluto are assumed to be heating from radiogenic rock component (heating by radioactive decay) and energy from the giant impact that formed it. Is Charon one of many? Charon is of similar size to Uranian satellites Dione, Tethys, Ariel, Umbriel, but it may be that Charon-forming impact did not much impact much heating. In any event, all these icy satellites are diverse, from dead cold worlds to those with active erupting volcanoes, so in Paul Schenk’s words, “Who knows [what Charon will resemble]?”

He next stepped up through the key geological processes that would alter and/or create surface features. For example, volcanism creates smooth plains, calderas, vents, ridges, and active venting. Volcanic processes have been seen on many icy moons. There is diapirisim, a type of solid-state resurfacing due vertical ice movement. To have this process, you need an ice shell, preferably a thin shell with a source of heat. This is seen only two icy bodies in the solar system: Triton (“cantaloupe terrain”) and Europa (domes, spots). There is also tectonism, revealed through fractures.

Solid State Resurfacing

Vertical movement is one of the processes creating unique features on Triton & Europa. The Earth shows this behavior (described above, called diapirism) in local regions of salt deposits.

He next pointed out the possibility that satellites could have rings too. For example, there is a giant ridge on Iapetus (Ip 2007) and a similar feature on Rhea (Schenk et al 2011). It is hypothesized that these surface features were the result of a ring that had collapsed onto the surface. His advice to the New Horizons team is to pay special attention to the Pluto-Charon equatorial regions to look for a ring-remnant.

Impact Cratering tells us about the impactor population that are “fluxing into the system,” reveals surface stratigraphy, reveals interior stratigraphy (what the underlying layers look like), and reveal thermal history. Counting crater impacts on Pluto & Charon will be used to evaluate the Kuiper Belt population.

Predictions for New Horizons. He expects craters to look like those found on Ganymede (simple craters) for Pluto and Dione & Thetys for Charon (craters with dominant peaks). Charon may be “bland geologically.”

What about viscous relaxed craters? They have been found on Ganymede & Enceladus. Crater shapes can be used to reveal the properties about the object. However, he added this caveat that all previous work on modeling crater shapes was on water ice dominated surfaces, not methane (which is expected on Pluto), so more lab work is needed. Basin morphologies are important too. They tell us about evolution. Anomalous morphologies are also a key thing to look for on Pluto & Charon.

Tyre crater on Europa

The Tyre crater on Europa (one of Jupiter’s moons) from http://www.lpi.usra.edu/science/kiefer/Education/SSRG2-Craters/europa_tyre.jpg.

Europa has these intriguing multi-ring systems, such as shown above with the image of Tyre crater. The hypothesis is that on Europa we are seeing an impact penetrating into the ice shell of 10-20 km. Crater falls in on its self, creating this ringed structure.

“There will be impact craters. We are going to be captivated. We are going to be befuddled.” – Paul Schenk

Geoffrey Collins (Wheaton College) “Predictions about Tectonics on Pluto and Charon.” His premise: “Tides raised by giant planets appear to be an important factor in icy satellite tectonics, but what about the Kuiper Belt?” He is interested in the time period after the initial Charon-forming impact. He and his colleagues have created models to calculate the interior viscosity for a range of Charon’s orbital evolution scenarios. They looked at the three main possible interior models for Pluto: (1) ice shell, ocean, rock core; (2) thick ice shell, rock core; and (3) uniform density. Another parameter they looked at was the formation distance of Charon from Pluto.

Interior Models Pluto

Possible interior models for Pluto are shown above: (left) ice-shell, ocean, core, (middle) thick ice-shell, core, (right) uniform density (undifferentiated).

Conclusions. Pluto will need to have an interior > 200 K. It  is likely that due to tidal heating (when orbital and rotational energy are dissipated as heat in the crust, which would be happening for Pluto despinning after formation), Pluto would melt and differentiate. The most self-consistent models include an ocean.

Beau Bierhaus (SwRI) talked about “Crater and Ejecta Populations on Pluto and its Entourage.” Craters are the most abundant landform in the solar system. They tell us about the target on/in which they reside. By studying the numbers and sizes of craters, and assuming an estimated impact rate, one can use crater density to estimate surface age. They are also indirect indicators of the impactor and in the case of Pluto, this could tell us information about the Kuiper Belt makeup.

Mass ejected from a crater follows an inverse relationship with velocity. Less mass is ejected at higher velocities, but to have ejecta (the material thrown out after an impact), the speed has to be above a particular Vmin, minimum velocity.  And if that velocity is lower than the escape velocity for an object (the speed above which would allow the object to escape the gravity well and go into orbit), you can create secondary craters. For ice, Vmin ~150-250 m/s. Pluto has Vescape ~1180 m/s; Charon has Vescape ~550 m/s. So we expect to see secondary craters on Pluto & Charon. Sesquinary craters might form when Vmin>> Vescape and the ejecta does escape the surface and then falls back onto its surface or onto another body. These are expected to be rare, but it’s possible you can have a crater formed on Pluto from a secondary ejected from Charon.

Predictions for New Horizons. Expect to see secondary craters on Pluto & Charon. Most of the craters will be primary impacts and they may have unusual morphology due to low impact speeds.

Veronica Bray (LPL, Arizona) continued the conversation with her talk on  “Impact Crater Morphology on Pluto.” The crater morphology (shape) depends on properties of impactor and also the gravity and surface/subsurface of the receiving body. For Pluto, we expect the crater morphologies to match those expected for impacts to icy bodies: shallow wall slopes, smaller rim heights, and central pit instead of peak-rings.

Crater Morphology

Comparative crater morphology. Top row: Impact Craters on a rocky body (Earth’s Moon). Bottom Row: Impact craters on icy bodies. Left to right indicate increasing crater diameter. The multi-ring basin, shown at the bottom right, is the Tyre crater on Europa, hypothesized to be the remnant of an object that penetrated into the subsurface.

Lower impact velocity will provide less impact melt. New Horizons will resolve large and small-scale features. However, with respect to things like Isis-style floor pits, New Horizon’s best resolution is not high enough.

What will New Horizons data tell us? New Horizons will address answers to how velocity affects peak development and primary crater depth, central pit formation in relation to melt drainage, and the d/D (depth to Diameter) trend to address heat flow models over time.

Olivier Barnoiun (JHU/APL) talk was entitled  “Surfaces Processes on the Moons of Pluto: Investigating the Effects of Gravity.” He is interested in processes on the smaller Pluto moons: Nix, Hydra, Kerberos, and Styx. He identified analogies in the solar system like 25413 Itokawa, a 100m asteroid that had been imaged by JAXA’s Hayabusa spacecraft in 2005. Hayabusa’s images revealed boulders that clustered together and were aligned; the leading hypothesis is that they were placed there by motion due to gravity. Other processes due to the presence of gravity, such as slope motions, are seen in the images. He notes that New Horizons will not have the resolution to duplicate the resolution Hayabusa had on Itokawa.

He and his colleagues have developed a computation “plate model” approach to tackle the motion of surface objects caused by “acceleration due to gravity” and this can be applicable to non-spherical bodies, for which Pluto’s smaller moons are highly suspected to be.

Marc Neveu (Arizona State University) asked “Exotic Sodas: Can Gas Exsolution Drive Explosive Cryovolcanism on Pluto and Charon?” Charon’s surface looks geologically young and could have an environment suitable for cryovolcanism. The term cryovolcanism was coined to explain the condition where the volcano erupts volatiles such as water, ammonia or methane, instead of molten rock . He presented a geochemical model where he has gas exiting a liquid as the mechanism for the cryovolcanism. The model involves the host liquid with different gas materials added and requires a crack in the surface ice layer. They also applied their model for a object that has a top crust; the crust acts like a “pressure seal” and prevents the gas from exsolving (separating from the liquid).

Lynnae Quick (JHU) presented some additional unique physics at Pluto with her talk on “Predictions for Cryovolcanic Flows on the Surface of Pluto.” She started with the statement that candidate bodies where cryovolcanism may be taking place are Enceladus, Europa, Titan and Triton. Imaging data from Voyager 2’s flyby of Neptune’s moon Triton provides strong evidence of cryovolcanism through interpretations of the terrain characterized by a lack of craters, geyser-like plume, walled plains, ring paterae (smooth circular), pit paterae, guttae (drop features).  She and her colleagues are modeling the cooling of (surface) lava flows and used a variety of “candidate lava compositions,” mixtures of H2O, NH3, CH3OH, CO, CH4, N2 ices. They compute cooling time for variety of lava thicknesses and compositions.

Predictions for Plutonian Lavas. Their work suggests N2-CO and/or N2-CH4 lavas could have 62-68 K melting temperatures, so essentially they could stay “molten” on Pluto for long duration. They will need high-resolution topography of Pluto, Triton, and Io, lab data for 50-273 K and New Horizons imagery of Pluto data to advance their models.

Alan Howard (University of Virginia) spoke about “Landforms and Surface Processes on Pluto and Charon. ” He walked us through multiple ways to add and subtract material from a surface: Accrescenence is the addition (e.g., condensation) of material normal to the surfaces, resulting in outward facing surfaces getting rounded and inward faces surfaces being sharpened. Decrescence is the removal (e.g., sublimation) of material normal from the surface, resulting in the outward facing surfaces being sharpened, and inward faces getting smoothed. Mass wasting is the bulk movement of material downslope aided by gravity (e.g. avalanches, debris flow, landslides). Gas Geysers are caused when solar energy hits ice, heating the underlying surface that then expands and erupts through the surface. Aeolian (wind) processes form things like sand dunes. To address this complex series of multiple surface activities, landscape evolution models have been developed to model such these processes with time.

Landform Models

Result of a landscape evolution mode. Shown here is one result where large fluvial (flow) networks tend to get disrupted when you have a lot of impacting events.

Predictions for New Horizons. Expect to see scarps (steep banks) on the surface of Pluto. Expect to see the unexpected on Pluto and Charon.

Timothy Titus (USGS) gave a thought-provoking talk about using the “Mars Seasonal Caps as an Analog for Pluto: Jets, Fans, and Cold Trapping.”  His premise is that the Mars’ polar processes are suitable analogs to explain what may be happening on Pluto. He stepped us through the Mars polar volatile transport model.

Mars and Pluto

A key item is thermal inertia. On Mars the soil absorbs heat in summer, but high enough thermal inertia (200 MKS) to delay ice formation until late in the year, and this, as you would expect, affect the whole ‘ice cycle.’ There is a big disconnect in the community over what Pluto’s thermal inertia is. In E. Lellouch’s talk on Jul 23 (see earlier blog entry) he reported that Spitzer & Herschel have measured Pluto’s thermal inertia as 20-30 MKS (Lellouch et al 2011). However, Pluto atmosphere pressure models needed to match occultation by C. Olkin & L. Young require Pluto have a much higher thermal inertia >1000 MKS to explain their occultation measurements (presented at this meeting).

Thermal inertia is a measure of the ability of a material to conduct and store heat. In the context of planetary science, it is a measure of the subsurface’s ability to store heat during the day and reradiate it during the night. This has natural consequences for deriving what happens to processes that require an exchange of heat such as transport models. Thermal inertia measurements can also be used to infer types of surfaces, e.g. distinguish between fine dust/few rocks and coarse sand/many rocks. MKS is a short form for the units “J K-1 m-2 s-1/2.”

He makes a tantalizing comparison between the north/south asymmetry in the Mars polar ice caps due to topography and suggests whether this could possibly be an analog to why the methane and N2 have longitudinal distributions (shown yesterday in Will Grundy’s talk). Jets, plumes, fans, and spiders on Mars are results of active gas geyser activities. He postulates, could the same be occurring on Pluto? If we miss the gas/plume season, we could see the leftover signatures in fans and spiders.

Predictions for New Horizons. Asymmetric seasonal caps. Methane lags surrounding the N2 seasonal cap. Optically thick layers of methane on top of N2 ice. Solid green house gas jets or at least spiders and fans.

David Williams (Arizona State University) talked about “Using Geologic Mapping as a Tool to Investigate the Geologic Histories of Pluto and its Satellites.” Geologic mapping documents the main geologic units and features and their relative ages and other characteristics. This is an iterative process using greyscale images, topographic data, and compositional and spectral data. They also identify structural features (crater rims, ridges, toughs, graben, lineaments, scarps, pits, etc.) They can use crater model ages to define a model-derived stratigraphy. The Geologic Information Software (GIS) is used to make the maps. Geologic maps are being made of the Moon, Jupiter system, Saturn system, Mercury, etc. from orbiter data. Data from fly-by missions have been used to make maps, such as Mariner 10 (Mercury), Voyager-Galileo (Galilean satellites), Cassini RADR (Titan). There will be a challenge of the vastly changing resolution data sets from the New Horizons flyby, but they would like to make these maps.

John Spencer (SwRI) ended the session with “What will Pluto Look Like?”  He began, “Will Pluto Look like Triton?” And his answer: Geologically, Yes.  He does not expect to see lots of craters. He expects that Pluto will have a surface that is as young and geologically active as Triton’s. One of the surprising thing about Triton’s surface is that it is lightly cratered. Are we seeing a situation where Triton had been completely resurfaced a lot since its capture by Neptune? And it should also have sufficient internal heating from radiogenic heating (radioactive decay from rock) rather than rely on Neptune to provide resurfacing mechanisms.

When looking at albedo (reflectance) contrasts between Triton (Voyager 1989) vs. Pluto (HST data 2004), you see factors of 10 across short distances on both bodies. Non-volatile surfaces on Triton are “bright” explained as H2O and CO2 exposed.  However non-volatile surfaces on Pluto are “dark” explained as H20 and CO2 buried by dark material. So Pluto will not look compositionally like Triton. John Spencer then drew our attention to Iapetus, a moon of Saturn, that also has a range of albedos, dark to light, with the hypothesis being an “exogenic trigger” and was suggestive of an analog there.

In answering a question about a prediction for topography, the discussion led to Paul Schenk (who spoke earlier) suggesting that Charon would look pretty flat (like Triton), with +/- 1km range. Bill McKinnon reminded us that Iapetus is an example of a body with extreme topography, ~ +/- 15km.

After such a visualizing intriguing morning, one thing can be certain: Pluto and Charon surfaces will have an impact (pun intended!) on our understanding the nature of icy bodies in our solar system.

It’s more than skin deep. Interiors of Pluto and Charon, a Discussion.

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This entry is a summary of talks presented at the Interiors session July 24th, 2013, during the Pluto Science Conference in Laurel, MD being held this week July 22-26, 2013.

Christophe Sotin (JPL) began the session with a talk entitled “Processes involved in the evolution of Pluto’s interior Structure.” He started his talk with a comparison of model of the interiors of Ceres, Callisto, Enceladus, Pluto (McCord & Sotin, 2005; McKinnon & Mueller, 1986; and Simonelli & Reynolds 1989). More recent models propose the existence of a liquid layer between an icy surface and a rocky core (Hussman et al 2006). This layer of liquid changes the way heat is transferred to the ice crust. If liquid methane could form at the base of the ice layer, forming a “sub surface ocean”, it would react with water and form stable methane clathrates. The presence and thickness of the “clathrate layer” affects the thickness of the ice crust above it.

Presence Liquid Layer

Conclusions. In their interior models, minor components (e.g. NH3) play important roles in both the characteristics (e.g. thickness) and dynamics of the ice crust. They need laboratory experiments to study the relative stability of the clathrate hydrates.  Hydrated silicates (e.g. antigorite) are likely to be the make-up of the Pluto core.  It will undergo dehydration some 100 Myrs to Gyrs after accretion. Convection within the core. The presence of a subsurface ocean depends on the presence of minor components.

The term clathrate is used to describe a structure that consisting of a lattice that traps or contains molecules.

Francis Nimmo (UC Santa Cruz) followed to provide some suggestions of surface observational evidence to probe the “Interiors of Pluto and Charon.”

Shape is Important. Shape tells us whether a body responds like a fluid. If the body behaves like a fluid (i.e., behaves hydrostatically), you can compute the moment of inertia. This, in turn can tells us something about the interior (i.e., is it differentiated or not). There is a caveat that differentiation can also occur due to radioactive decay. Differentiation happens when ice melts, so it tells us about thermal evolution.

Icy Body Relatives

Comparative study of other bodies in the outer solar system and what we know about their interiors.

Evolution of Shapes. Early on, Pluto & Charon are rotating quickly, and are distorted. Pluto and Charon change shapes in the first few to 100 Myr if fluid or elastic, respectively, as their spin rate slows down and Charon moves outward. The spin rates influence their shapes.

What leads to Oceans? A conductive (no convection) ice shell is required to make an ocean (Desch et al 2009). This shell basically lets the heat out from the core. This heating then melts the bottom of the ice shell creating an ocean. The presence of an ocean changes the stress history. In the creation of an ocean, you are replacing low-density ice with higher density water and this introduces compression stresses. If you see things like the Tyre crater on Jupiter’s moon Europa, a multi-ring impact that implies there was an ocean. Whether or not an ocean is present has important astrobiology & geophysics consequences. If you introduce an ocean, you never have a fossil bulge. If you do not have an ocean, you could get a fossil bulge.

A fossil bulge is a bulge that froze into shape before the satellite synchronized its rotation.

Martin Paetzold (Universitat zu Koln, Germany) spoke about “Mass Determination of Pluto and Charon from NH’s REX Radio Science Observation.” During the fly-by, Pluto will perturb the New Horizons spacecraft velocity just slightly and this will be recorded as a tiny Doppler shift of the X-band (8.4 GHz) radio carrier frequency. This information will be used to measure the mass, or more specifically, the product GM (universal gravity constant times the mass), of Pluto.  There are two different ways to obtain this data during the New Horizons mission: (1) Using two-way ranging a week before the encounter and week after the encounter, and (2) During the encounter, the REX uplink instrument (operating at 7.1 GHz) will have a series of measurements during the days around closest approach. They hope to obtain 0.15% accuracy for the first method and 0.04% for the second method. The best results utilize both methods potentially deriving an estimate of Pluto & Charon masses with an accuracy of 0.01%. They are currently looking at the small forces file, which is the measure of the attitude performance during thruster firing.

James Roberts (JHU/APL) spoke about “Tidal Constraints on the Interiors of Pluto and Charon.” Thermal evolution of Pluto & Charon is a key question for scientists to answer. But thermal models are dependent on interior structure. At present we do not know whether Pluto or Charon are homogeneous (i.e. same material throughout) or differentiated (split into a core and crust, or maybe core, subsurface ocean and crust). Typical methods used to probe interiors are Kepler’s 3rd Law, Bulk density, Moment of Inertia, Gravity, Magnetism, Radar, Seismology, Tides. For New Horizons, as the fly-by is not that close, Gravity is not a viable method; the lack of a magnetometer aboard rules out Magnetism, and Seismology requires the spacecraft to land, also not possible. He described their approach  that will use Shape Modeling to measure a Tidal Bulge. Both Pluto and Charon may each raise a tidal bulge on the other. He cautioned that we may not be able to determined the existence of an ocean using a shape model.

Steve Desch (Arizona State University) spoke on “Using Charon’s Density to Constrain Models of the Formation of the Pluto System.” The unique Pluto-Charon system has been modeled as arising from the impact of two large Kuiper Belt Objects (KBOs). This had been presented on July 23rd by Robin Canup. Steve Desch’s model takes two differentiated KBO bodies (but they must have a thick crust) in a disk and collides them. Parts from the bodies’ inner core, plus some ice, forms Pluto and the outer icy mantles form Charon & the other moons. He addressed that the initial differentiated KBOs with r=600-1200 km could exist (Desch et al 2009, Rubin et al 2013). The outcomes of this model create a dense Charon (density= 1.63 g/cm3) because Charon would have been formed from the outer regions of the initial KBOs, and those objects are characterized with thick crusts.  This is the alternative model that was not preferred by Robin Canup in her talk yesterday. This remains an active area of study.

Wrapping up this 3rd day of a dynamic conference we learned that we still have a lot more questions about the formation and the interior or Pluto, Charon or any of these icy bodies in the Outer Solar System. New Horizons will indeed bring an revolutionary dataset to allow to direct investigation of surface features, overall shapes, masses, and orbital dynamics, all which will constrain models of what these bodies are made of and how they formed.

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