New Horizons – Mission: Ames

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

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 1^st HST servicing mission would occur in December 1993). It was a busy time for the Pluto science community.

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.

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.

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 2-5×10²⁷molecules/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.

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.

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 Q_o (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.

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.

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.

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.

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 26^th.

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.

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.”

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.

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.

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.

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.

The afternoon session of Jul 25^th 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.”

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?

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!

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 21^st 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, C_nH_2n+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.

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.

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.

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.

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 V_min, 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, V_min ~150-250 m/s. Pluto has V_escape ~1180 m/s; Charon has V_escape ~550 m/s. So we expect to see secondary craters on Pluto & Charon. Sesquinary craters might form when V_min>> V_escape 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.

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.

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.

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.

This entry is a summary of talks presented at the Interiors session July 24^th, 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.

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.

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 3^rd 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 23^rd 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 3^rd 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.

Some insights into Charon and what roles laboratory work can play in New Horizons science.

These are talk summaries from the afternoon of July 24th at the Pluto Science Conference being held this week, July 22-26, 2013 at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

Marc Buie (SwRI) walked us through “The Surface of Charon.” Charon was detected by Jim Christy in April 1978, in what were originally dubbed “bad images” from the Naval Observatory, but not confirmed as a satellite by the IAU until February 1985. Charon is about 1 arcsecond from and ~1.5x mag fainter than Pluto. An occultation measurement in April 1980 confirmed the detection.

“Mutual Event Season” is when every half orbit of Charon passes in front or behind Pluto. This occurred over 1985-1990 time frame. For the specific orientation where “Charon went behind Pluto,” as observed from Earth, you can directly measure’s Charon’s albedo, the size ratio between Pluto and Charon and start deriving its composition. So, work in earnest to determine Charon’s surface started in the mid-1980s.

In 1987, Marc Buie and his colleagues got IR spectra using a single-channel detector and a circular variable filter, the best in spectrographs at the time, and this revealed Pluto’s atmosphere is methane dominated and Charon’s atmosphere is water dominated, and they do not look like each other.

Hubble Space Telescope (HST) entered the scene and a series of observations of Pluto and Charon with HST started in 1992. The first rotational light curve of Charon was obtained in 1992-1993, indicating a 8% variation in the brightness, much smaller than that for Pluto and the data also confirmed that Charon was tidally locked with Pluto (just like our Moon is tidally locked with Earth, showing the same face). Marc Buie and his colleagues obtained HST NICMOS near-infrared spectrum in 1998 of both Pluto & Charon.

Comparison of Pluto and Charon infrared spectra, taken in 1998 at the same epoch (near in time with each other), with HST NICMOS (near infrared camera and spectrometer aboard Hubble).

A mystery. Spectra from Tethys, one of Saturn’s moons, has a remarkable agreement with Charon’s spectra, despite the bodies are of different temperatures and albedos? Will they have similar compositions when the New Horizons spacecraft flies by? The spectra is also not fit precisely with just water, so there is another unidentified species there.

Marc Buie was observing Pluto & Charon just last night (July23^rd, 2013) with the Adaptive Optics mode of the OSIRIS instrument on Keck. This instrument achieves comparable spatial resolution as Hubble. At the conference, he showed off the latest image, “hot off the press.”

Predictions for New Horizons: Charon to have a heavily cratered surface with modest (subtle) albedo and color features. Expect to see differences between the Pluto and anti-Pluto hemispheres.

Francesca DeMeo (MIT) talk was entitled “Near-Infrared Spectroscopic Measurements of Charon with the VLT.” She began her talk stating that TNOs (Trans-Neptunian Objects) can be characterized as (1) volatile-rich (lots of N₂, CO, CH₄), (2) volatile-transition, (3) water+ammonia rich (H₂O, NH₃), and (4) volatile-poor (neutral to very red colors, maybe some water ice). No TNOs, to date, show evidence for CO₂. Her analog is to Charon is Orcus, a TNO with its own moon Vanth. Both are water and ammonia-rich bodies.

Comparison of two water and ammonia-rich bodies: the TNO Orcus and Pluto’s moon Charon.

She observed Charon in 2005 using the VLT (8m telescope) with AO (adaptive optics), which separates Pluto. Her Pluto data is published in DeMeo et al 2010. Charon data was presented here in her talk and showed a comparison with Jason Cook’s data from 2007 and F. Merlin’s data from 2010, as they were looking at the same surface location. She is using the JPL Horizons longitude system.

For a review of Trans Neptunian Objects, she recommends Mike Brown’s 2012 Review Paper http://adsabs.harvard.edu/abs/2012AREPS..40..467B.

Gal Sarid (Harvard) followed with “Masking Surface Water Ice Features on Small Distant Bodies.” Minor (icy) bodies (TNOS, Centaurs, comets) are a diverse population with varied size, composition and structure. Their surface compositions show evidence for water ice and other volatile species. They are understood to be remnants of a larger population of planetesimals. He stepped us though his thermal and physical model of a radius=1200km object to reveal the possible insides of these minor icy bodies. Observationally this could be tested by inspecting impact crater that could eject subsurface material. From his computations he varies the ratio of carbon (dust) to water ice to give predictions for water band depth. When he compares the colors of the computed spectra they match very ice-rich TNO bodies, but his work reveals questions to explain the B-R colors. The models may need more other ices (methane, methanol).

Reggie Hudson (NASA GSFC), a laboratory spectroscopist, presented “Three New Studies of the Spectra and Chemistry of Pluto Ices.” At NASA Goddard, they have equipment to test ices with their vacuum-UV (vacuum-ultraviolet). He showed 120-200 nm results of N₂ + CH₄ at 10 K. A second study was to measure CH₄ ice in the infrared. CH₄ has three phases: high temp crystalline T > 20.4 K, low temp crystalline T < 20.4 K, and amorphous CH₄ forms around 10 K. He showed results for solid CH₄ from 14-30 K over 2.17 to 2.56 microns and 7.58 to 7.81 microns. Their lab also has the ability to irradiate the samples, and when they have done so, certain phases recrystallize, but that is a function of temperature. Future work involves completing lab data of C₂H₂, CH₄ and C₂H₆. Their lab website is http://science.gsfc.nasa.gov/691/cosmicice/.

Brant Jones (University of Hawaii) discussed “Formation of High Mass Hydrocarbons of Kuiper Belt Objects.” They irradiate their ices with a laser and their measurement technique is a “Reflectron time-of-flight mass spectrometer.” They have identified 56 different hydrocarbons wit their highest mass C₂₂H_m where 36 < m < 46. Future work is to investigate PAHs, look at “processed ices” and study different compositions, and study exact structures.

Christopher Materese (NASA Ames) spoke on “Radiation Chemistry on Pluto: A Laboratory Approach.” Reporting on their laboratory work at NASA Ames, in their setup, they radiate their ices with ultraviolet (UV). Now for Pluto, the atmosphere will be opaque (not-transparent) to UV radiation. Secondary electrons generated by ion processes, however, drive the chemistry and their energy (keV-MeV) is similar to that provided by UV radiation. He presented NIR (near infrared) and MIR (mid-infrared) spectra of his irradiated ices. They have completed over 20 molecular components. They also have a GC-MS (gas chromatograph–mass spectrometer) to measure the masses of the molecules they create.

The importance of laboratory work cannot be underestimated. It can help with predictions and equally important help with identification of molecules. Then once molecules and their abundances are determined, that can fold into more complicated models to look at volatile transport.

Pluto, the Orange Frosty, served with a dash of Nitrogen, a pinch of Methane, and smidgen of Carbon Monoxide.

Summary talk entries for the Pluto Science Conference continues. This is from the morning of July 24, 2013^th on the topic of “Composition.”

Dale Cruikshank (NASA Ames) set the stage with a spectra-rich presentation and gave an overview talk about the “Surface Compositions of Pluto and Charon.” Putting it in context, even 45 years after Pluto was discovered, we did not know much about Pluto only where it was in the sky and its rotation period. That rapidly changed when Dale and colleagues saw strong evidence for solid methane on Pluto in 1976 (Cruikshank, Pilcher, Morrison, 1976 Science 194, 835), Jim Christy discovered the companion moon Charon in 1978, and repeated observations were made of Pluto and Charon in the 1980s.

Spectroscopy, the technique which spreads light into different wavelengths, has been a powerful diagnostic tool for the identification of molecular species, and therefore tells us the composition of the object. Low-resolution (R~100-500) spectra is sufficient to identify ice-solid features which are characterized by wide features, but higher resolution (R~1,000-10,000s) helps constrain models that determine temperature and also . New Horizons’ LEISA spectrometer covers the 1.25-2.5micron spectral band, with resolution R~240, and a mode of R~550 between 2.10-2.25 microns, making it ideal for identifying solid features. It’s proximity to Pluto during the July 2015 fly-by provides unprecedented spatial resolution. Compared to ground-based & Hubble spectral measurements which can only provide full-disk (~1500km/pix) measurements (because Pluto appears only in a few pixels), New Horizons’ LEISA will provide the true “first look” at the composition of Pluto at 6.0km/pix (global) with some patches at 2.7 km/pixel.

Images in this blog entry show flux (measure of amount of light) or albedo (measure of reflectance) versus wavelength.

Pluto’s near infrared spectrum (Grundy et al 2013) is rich in identifiable diagnostic solid materials, nitrogen (N₂), methane (CH₄) and carbon monoxide (CO). A comparison with Triton’s spectrum over the same wavelength is shown. Carbon dioxide (CO₂) is suspiciously absent from Pluto’s atmosphere.

Pluto’s mid-infrared (Protopapa et al 2008) show a series of methane bands. The gap at 4.2 microns is due to CO₂ absorption from the Earth’s atmosphere.

Pluto’s UV Spectrum from HST (Stern et al 2012) also indirectly supports the presence of organics.

What do we know about the surface of Pluto? The major surface ice components are methane (CH₄), nitrogen (N₂) and carbon monoxide (CO). Some of the CH₄ is pure, and some may be dissolved in N₂. N₂has been seen in two crystalline phases and the thickness should be at least a few centimeters. CO, may or may not be dissolved in N2. Ethene (C₂H₆) has also been detected (De Meo et al 2010). Suspected species, not yet detected, are Hydrogen Cyanide (HCN) and Carbon Dioxide (CO₂). Predicted species include those from atmospheric chemistry, surface chemistry and other radicals.

There are tantalizing hints that HCN and other nitriles (where you have a carbon with three bonds to a nitrogen molecule with the 4^th bond to another atom or group) are potentially present (Protopapa et al 2008). If confirmed, the presence of HCN opens up a series of chemistry pathways that enable Pluto to be a pretty complex place.

HCN Chemistry pathways. HCN has not been confirmed to exist on Pluto, but suggested. If present, a whole set of possible chemistry becomes possible.

These ices are white but Pluto has a colored surface. It’s actually quite red. The coloring on Pluto is hypothesized to be due to the presence of tholins, a complex organic molecule formed by ultraviolet irradiation of simple organic compounds.

Geometric albedo (measure of reflectivity) of Pluto as a function of wavelength. See how red it looks?

The Surface of Charon. Charon has an intriguing different kind of surface than Pluto. There is water (H₂O) ice, perhaps crystalline ice, and ammonia (NH₃) hydrate. But there are no CO, CO₂, N₂ or CH₄, all which are present (or predicted) for Pluto. The nature and source of the ammonia is under debate. Could it come from below the surface and diffuse up or come from cryo-volcanism?

Predictions for New Horizons. It will be hard to find HCN with LEISA due to its spectral resolution as there is a strong methane band nearby. Dale Cruikshank thinks it will be challenging as well to find alkenes.

The mystery of the missing CO₂ on Pluto remains. Carbon dioxide is seen on Triton (see above), whose spectra is very similar to Pluto. Dale Cruikshank looks to NASA’s JWST (James Webb Space Telescope, a 6.5 m diameter visible infrared space telescope) as the proper tool to make this detection. New Horizons LEISA instrument has probably to low a resolution to detect CO₂ features around 2 microns.

Will Grundy (Lowell Observatory) talked next on the “Distribution and Evolution of Pluto’s Volatile Ices from 0.8-2.4 micron spectra.” He reported on an IRTF (3.5 m telescope) SpeX (spectrometer) Pluto monitoring program spanning 10 years. The SpeX instrument provides R~1000 NIR spectroscopy over 0.8-2.4 microns. A recent paper on their findings can be found at http://adsabs.harvard.edu/abs/2013Icar..223..710G (Grundy et al 2013 Icarus 223, 710-721).

They would obtain disk integrated hemisphere spectra because Pluto fills the SpeX slit, but during the course of this long monitoring they probed a variety of longitudes. Below are the longitudes on the Pluto that they probed. Each green point is the center of a particular pointing. This is overlaid on the best albedo (reflectance) vs. longitude surface map of Pluto from Marc Buie. In the coordinate system shown in this image, 0 deg longitude is facing Charon, with 180 deg longitude anti-Charon.

Species abundance (measured as equivalent width) as a function of Pluto longitude. They have found that max CO amount is correlated with the 180 E region (anti-Charon), whereas largest amounts of CH₄ is in the 270 E region. Equivalent width is a calculation of the depth of an absorption feature with respect to the absence of the feature at nearby wavelengths (continuum). When plotted against time, or in the case above, against spatial location (longitude location on Pluto), it can tell you something about the abundance variations of that molecular species.

In summary, they have found that ice distributions seem heterogeneous (mottled, not smooth). The NIR spectra show intriguing parallels between Pluto and Triton. From this 10 year period of observations they find that CH₄ is increasing but CO and N₂ is decreasing. They have also observed non-uniformities in both time and longitude.

With a 10 year Earth program they have only observed ~5% of a Pluto year, so perhaps they will start seeing seasonal changes? For a more lengthy discussion on suggested Pluto seasons, see this later blog post entry.

Predictions for New Horizons. Will Grundy is eagerly awaiting New Horizons LEISA’s infrared spectral data. The instrument will have much higher spatial resolution than these “global hemisphere” maps with the SpeX instrument. The spacecraft’s closest approach geometry will be the anti-Charon hemisphere (180 deg E). This will be ideal for probing the strongest CO signatures.

Noemi Pinilla-Alonso (University of Tennessee) provided a talk on “IRAC/Spitzer Photometry of the Pluto/Charon System.” With warm Spitzer/IRAC they took images in four bands probing the 3-5 micron range. Their intent was to look for the mid-infrared spectral signatures of N₂, CO, CH₄ ices and tholins, all which had discovered in the near-infrared (1-2.5 microns). They covered 8 longitudes with their observation set.

Results. Their data confirms the surface heterogeneity that was measured by HST (Marc Buie). They also found their “slopes in color with wavelength” do have a longitude dependence and fall into two groups 160-288 deg Longitude and 234-110 deg Longitude. Both N₂ and CO are also found to be strong at 180 deg Longitude at mid-IR wavelengths. This agrees with Will Grundy’s measurements at shorter wavelengths from the IRTF (see above, this blog entry).

Jason Cook (SwRI) presented a talk on “Observations of Pluto’s Surface and Atmosphere at Low Resolution.” Intrigued by the ethane (C₂H₆) detection (De Meo et al 2010), he got the new idea to look for it this in old data he took in 2004 using the Gemini-N NIRI instrument, with R~700 (low resolution) spectroscopy. In his analysis, he had to include the C₂H₆ ice contribution to make a fit of ice abundances to the data. He was able to fit multiple methane bands and derive comparable amounts that agrees with other published methane detections at higher resolution.

Implications for New Horizons. The big take-away is that low resolution spectra with high signal precision are capable of detecting Pluto’s atmosphere. New Horizons LEISA spectra has R~500 so this data example is an excellent comparative data set. He is eager to talk with others who have low-resolution spectra of Pluto or Charon to apply the new analysis techniques.

Next, Emmanuel Lellouch (Observatoire de Paris, France) gave a talk on “Pluto’s Thermal LightCurves as seen by Herschel.” He ended his talk sharing tantalizing science on TNO temperatures from thermal measurements with Herschel and optical measurements used together to measure the diameter, albedo, and thermal inertia. They derive that TNOs have low thermal inertia (2.5 +/- 0.5 MKS), lower than Saturn’s satellites (5-20MKS), Pluto (20-30MKS), and Charon (10-20 MKS). More details can be found at http://meetingorganizer.copernicus.org/EPSC2012/EPSC2012-590-3.pdf.

Moving further out beyond the Spitzer/Herschel far infrared, into the sub-millimeter range, Bryan Butler (NRAO) talked about “Observations of Pluto, Charon and other TNOs at long wavelengths.” As you go to longer wavelengths, you are less affected by solar reflection. You become dominated by the thermal emission from the body itself. But the emission at these wavelengths will be weak such that building highly sensitivity instruments is key, such as ALMA (in Chile) or updated VLA, called the EVLA (in New Mexico). They have been using ALMA and EVLA to observe Pluto and Charon in 2010-2012 and they had to remove the background contribution as Pluto had been moving through the galactic plane in this period.

The path of Pluto is shown with the green line that appears to make loops. This is the path of Pluto projected against the sub-millimeter. The enhanced horizontal signal is strong submillimeter thermal emission from the plane of the Milky Way. This caused an undesired extra background signal that needed to be removed from data taken in the 2010-2012 time frame.

What’s Next? They wish to use ALMA to study Pluto & Charon and also attempt to detect Nix & Hydra, if they fall on the larger size. ALMA will be used to observe TNOs and will have the capability to resolve the largest TNOs like Eris (size ~2400 km diameter). They predict they can make high-SNR images of Pluto, but barely resolve Charon within a short observation time. To get high-SNR images of Charon would take more observatory time than they think would be awarded for a single object.

Switching away from the infrared and sub-millimeter and moving back to the ultraviolet Eric Schindhelm (SwRI) gave the final talk in this session entitled “FUV Studies of Pluto and its Satellites: From IUE to New Horizons.” IUE took the first UV spectra of Pluto in 1987-1988. This was confirmed with HST using the FOS (Faint Object Spectrograph) instrument in 1992. After 17 years, the HST COS (Cosmic Origins Spectrograph) instrument was used to observe two different longitudes, and they found some differences between the two data sets. The COS data indicated an absorption feature at 2000-2500 Angstroms (see Dale Cruikshank talk summary above), and it was suggested this is a hydrocarbon creating this feature.

Eric Schindhelm next described New Horizon’s Alice instrument measurements and predictions for the Pluto and satellites during the New Horizons fly-by. He also summarized that more lab H₂O, NH₃ and CO₂ ice FUV reflectance spectra is needed for interpretation of these data sets.

Predictions for New Horizons. Pluto’s UV reflectance spectra will be limited due to faint signal and atmosphere absorption. Nix and Hydra will be barely detectable in FUV. Charon’s albedo for wavelength longer than 1200 angstroms should be detectable and they expect to get albedo, color and composition. They also expect to distinguish between different mixing ratios of the ices (ratios of H₂O to NH₃, H₂O to CO₂, etc.) with the UV spectra obtained by New Horizons.

Although the predictions for detecting Pluto’s surface composition in the UV with New Horizons’ Alice instrument are expected to be limited, the Alice instrument will also be measuring Pluto Atmosphere (and searching for an atmosphere around Charon), which is its main purpose and directly addresses a prime Group 1 science goal.

Bring on the spectra!

Playing Marbles at Pluto. Looking at the Dynamic Dust Environment. Generators, Sweepers, and Sweet-Spots.

From the July 24, 2013 morning session at the Pluto Science Conference.

Simon Porter (Lowell Observatory) began this morning’s session with “Ejecta Transfer within the Pluto System.” He asked, “Where does the short lived dust go?” Having small satellites is not unusual in the solar system. Both Jupiter & Saturn have low number-density rings formed from short-lived dust particles ejected from small satellites.

Their Hypothesis: Dust ejected from the small satellites is swept up by Pluto and Charon. Their Experiment: Simulate dust trajectories in a computer (N-body computation) starting randomly in the system (but constrained within the orbits of the small satellites) and map where they impact Pluto & Charon. Repeat this 10,000 times for a combination of parameters. Their Results: Dust particles do hit all the bodies in the Pluto System. For the Charon impacts, smaller particles survive longer, and those that hit Charon tend to have speeds around 50 m/s (like fastball pitcher). If a particle were to hit Pluto, it would be happen with speeds in the 50-200 m/s range and occur much quicker (due to the fact that Pluto has a larger gravity mass than Charon). They found that lower speed particles would hit the Pluto’s trailing side, whereas the higher speed particles hit the Pluto’s leading side. They also found a slight northern preference for smaller particles due to radiation pressure. And they made an intriguing observation that the impacts they computed correlate well to bright albedo areas (high reflectivity) on the Pluto surface. Coincidence?

Implications for New Horizons. New Horizons will provide datasets from the Student Dust Counter instrument, plus updated albedo maps from image data, to test their computational model.

David Kaufman (SwRI) next talked about “Dynamical Simulations of the Debris Disk Dust Environment of the Pluto System.” He was interested in modeling where debris dust would exist in the Pluto System. The motivation was to evaluate the probability of whether New Horizons would encounter a large enough dust particle that could be catastrophic for the spacecraft. He described the dynamics: the Pluto System can be approximated by a “circular restricted three-body (Pluto-Charon-particle) problem,” but it’s far from simply three bodies. There are features such as the Charon Instability Strip, where the moon Charon sweeps away material. The Lagrange points are unstable. And the outer moon can significantly perturb (change) trajectories that cross their orbits. He mentioned that “unusual type orbits” can be sustained by the unique gravity and motion characteristics of the Pluto System. He’s done numerical simulations following the particles, governed by physics principles for the system, over a time period of 500 years, and derived that the debris disk is an expended three-dimensional and stable. The inner debris disk recreated the instability strip.

Silvia Giuliatti Winter (UNESP, Brazil) talked about “The Dynamics of Dust Particles in the Pluto-Charon System.” She is interested in the orbital evolution of small particles ejected form the surface of Nix and Hydra and what happens to them when dust particles from interplanetary meteoroids impact these satellites. The goal is to place constraints on predictions for a ring in the Pluto System. They model 1 micron and 5-10 micron “dust particles” and track where they travel.

Conclusions: Particles released from the surfaces of Nix and Hydra temporarily form a ring. Collisions with the massive bodies remove 30% of the 1micron size particles in 1 year. The ring that was formed is very faint (optical depth tau=4×10^-11).

Implications for New Horizons: For such a faint disk, it will be a challenge for New Horizons to detect. However, if there is forward scattering it could be bright enough to be detected. (The models provided did not include a phase function, that is, a geometric indication of where the sun-light could illuminate the particles).

What’s optical depth? Optical depth is a measure of transparency. If the optical depth is large (tau >> 1), we say the region is optically thick — light is readily absorbed. If the optical depth is small (tau << 1), the region is optically thin, and light passes through easily.

Othon Winter (UNESP Brazil) spoke about “On the Relevance of the Sailboat Island for the New Horizons Mission.” In investigating where particles would find stable orbits, their modeling predicted a region where there was a cluster of orbits characterized by high eccentricity (e= 0.2 to 0.8) and located around 0.6 Pluto-Charon semi-major axis (i.e. between Pluto and Charon). They nicknamed it “Sailboat Island’ because on a eccentricity vs. distance from Pluto plot it looked like a sailboat. This population of “stable orbits” had not been predicted from previous work.The figure above is taken from Giuliatti Winter et al 2010 where they describe a family orbits called S-type that are stable. The plots are in d vs. e. where d, on the x axis is the Pluto-centric semi-major axis (how far from the Pluto barycenter) and e, on the y axis is the eccentricity. The “white” areas are orbit solution that were found to be stable. Area ‘1’ is the “Sailboat Island” described in the talk. Left are prograde (inclination=0) orbits, right are retrograde (inclination=180 degrees) orbits.

Example of a particular family of orbits from the “Sailboat Island” parameter space in the full-family of stable orbits.

Implications for New Horizons: Opportunity for discovery to look for these objects in the Pluto-Charon system.

Andrew Poppe (UC Berkeley) on “Interplanetary dust influx to the Pluto System: Implications for the Dusty Exosphere and Ring Production.” The three previous talks addressed what happened to particles in the Pluto system with time (i.e., their lifetime, where they impacted objects, what stable orbits they achieved). Here he asked, could the source of the dust come from interplanetary sources? For example, come from the Kuiper Belt being dragged into the Sun.

Because Pluto’s orbit is highly inclined but our Solar Systems Kuiper Belt dust disk is mainly in the ecliptic plane and Pluto periodically passes through the thickest part of the dust disk. (EKB = Edgeworth–Kuiper belt)

Computation of the dust flux (in particles/m²/s) for Pluto over one Pluto orbit. The peaks are when Pluto crosses the ecliptic (expected). New Horizon’s July 2015 Pluto fly-by (shown by the red dashed line) will be close to an ecliptic crossing.

Implications for Rings. They turn their “mass influx models” and do calculations on where rings could form. They predict optical depth tau < 10^-7 (in backscatter). They are working to refine their models to include larger grains.

Open questions. We still do not really have a good handle on the amount of dust generated by “the Kuiper Belt residents”. This is an active area of study.

Henry Throop (SwRI at large) talked about putting “Limits on Pluto’s Ring System from the June 12, 2006, Stellar Occultation.” You can search for rings by direct limited (e.g., using HST) or using stellar occultations. Direct imaging is 2D but at coarse scales whereas stellar occultation give 1 D cuts at higher spatial resolution. He saw that although the June 12, 2006 occultation event was 61 seconds in duration, about 3 hours of data was taken over the entire event, so he started to look outside the main events in search for rings that would appear as shallower drops in the light curve.

Three hours of data taken around the June 12, 2006 Pluto occultation even. They did not see any rings or debris with this data set. Looking back at the timing they realized that Nix was just missed by 1000 km or so. So had their been a cosmic coincidence that this occultation caught Nix, Nix would have been discovered 10 years earlier.

Implications for New Horizons. This null results combined with other searchers for rings (e.g. recent HST observations) it put limits on ring detection, but this dataset is the only data set looking for rings at scales < 1500 km, the spatial resolution on HST.

The New Horizons spacecraft on its fly-by through the Pluto system in July 2015 should detect a ring with its Student Dust Counter instrument, if such a ring exists.