Charon – Mission: Ames

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.

How can you form Pluto and Charon? Let me just count the ways.

On the afternoon July 23, 2013 at the Pluto Science Conference continued, we switched gears from atmospheres to small satellites. This blog entry is about the formation theories for Pluto and Charon.

Hal Levison (SwRI) started the afternoon with a talk entitled “Unraveling the Early Dynamical Evolution of the Outer Solar system.” The “Nice Model” (Gomes, Levison, Morbidelli, Tsiganis) was devised to introduce possible models that could produce the Outer Solar System as we know it and preserve the Inner Solar System as we know it. The authors have been updating it with planets in resonances (Morbidelli et al 2007), put Pluto-objects in the disk (Levinson et al 2011), restricted the models to “save the Earth” by making sure Jupiter does not encounter an ice giant planet (Brasser et al 2009), and added a third ice giant (Nesvorny & Morbidelli 2011).

To learn more information about the Nice Model, check out a good entry at http://en.wikipedia.org/wiki/Nice_model.

The Nice model has told us a lot of good things. It predicts the right number and range or orbits for Jupiter and Saturn, predicts the right number and orbits for Trojans (things in 1:1 resonance with primary body) and reproduces the Late Heavy bombardment of Moon. However, it comes short of explaining the Kuiper Belt.

So, what does this all mean for the Kuiper Belt? The Kuiper Belt is a rich structure. Observationally the sum of all the mass in the Kuiper Belt is <= 0.1 Mass_Earth. In order to get objects the size of Pluto to grow in the timescales of our Solar System, you need a lot more mass. So we need find this missing mass.

Above is a Comparison of updated 2008 Nice Model (green ) vs. Kuiper Belt Data (blue dots). It qualitatively shows similarities however it cannot reproduce the “kernel” (described in Brett Gladman’s talk from July 22nd) nor objects with high inclination (large i) nor objects in the “Cold Classical Population.”

Cold Classical Kuiper Belt Objects have orbits much like the planets; nearly circular, with an orbital eccentricity of less than 0.1, and with relatively low inclinations up to about 10° (they lie close to the plane of the Solar System rather than at an angle). They have characteristics similar to an undisturbed protoplanetary disk. Often the term ‘primordial’ is used when describing Cold Classicals. They tend to be in binaries and have “red” colors.

He then ended his talk by sharing an recent update with his work on Outer Solar System modeling, hoping to explain high inclination Kuiper Belt Object formation, by looking at the formation of Jupiter and Saturn with and without a gas disk present. Jupiter and Saturn, when they are forming, are scattering objects outward. Then if there is a gas disk present, these objects get into what is known as Kozai resonances, where bodies exchange eccentricity for inclination. As the gas disperses, a population at high inclinations in the Kuiper Belt region (30-50 AU) are caught. In the models, if you vary the outside extent of the disk, you spread out the populations. However, this is not in agreement with our solar system (we don’t see those types of objects). Their conclusion is that you needed to have the gas disk truncated and this modification of the Nice Model can explain high inclination KBOs.

Hal Levinson stated strongly “We definitely need New Horizons to visit a cold classical object!”

Anders Johansen (Lund University, Sweden) in “Accretion of Kuiper Belt Objects” stepped us through the two models of major planet formation: Planetesimal (coagulation) vs. Pebble (steaming instability).

He asked, could Pluto be formed by planetesimal accretion? This will require a cold disk of km-size planetesimals (Kenyon & Bromley 2012) where a key prediction of the planetesimal accretion model gives a differential size distribution that is in agreement with observations (i.e. lots of smaller objects). But, there is a problem this this approach since to make kilometer size objects beyond 20 AU as it would taken 100 Myr which is much longer than the life-time of the gas disk (Lambrechts & Johnansen, in prep). Thus, to make a Pluto-size (few 1000s km size) object would taker longer than the age of the Solar System. (Pluto orbit is 29 AU at closest to Sun to 49 AU, furthest from Sun). However adding streaming instability can speed up the planetesimal growth timeframe.

Could Pluto have been formed by pebble accretion? Pebbles are accreted very efficiently by planetesimals (Lambrechts & Johnansen, 2012; Ormel & Jlahr 2010). This shapes the distribution (makes it steeper) and brings it more into agreement with asteroid and KBO populations.

What new data will New Horizons shed? If data from New Horizons reveals the presence of Aluminum 26, this will imply a formation age for Pluto. Formation time data can be fed back into planet-forming models, be they planetesimal or pebble accretion, and those models can be used to help explain other systems, such as observed proto-planetary disks or exoplanet systems around other stars.

Robin Canup (SwRI) talked about the “Origin of Pluto’s Satellites.” Massive Charon and four very tiny outer moons make up Pluto’s satellites. All of these satellites are co-planar (they are moving in the same plane) and prograde with respect to Pluto’s rotation (they revolve about Pluto in the same direction as Pluto’s rotation). However, Pluto’s rotation is retrograde (in the motion opposite) to its orbit.

It is thought that Charon was formed by a giant impact that could have preserved a lot of angular momentum in the system. Her models (Canup 2011) predict a grazing impact was needed to match the system angular momentum and produce a Charon-mass object. Achieving a Charon-mass object requires an extreme case, as most of them like to create a companion that is 6-8% of mass of the primary object.

She also modeled cases where there is an undifferentiated impactor, and those systems can form “intact-moons.” In many scenarios, Charon-mass objects are created. And “the Charon that was created” forms entirely from impactor material. She postulates that this is the more probable explanation for Charon’s formation.

What about the origin of the tiny moons? The models do create a disk, which has enough mass to form tiny moons. The challenge is that the disk that is made too compact compared to the current existence of Nix & Hydra. Could these moons have been transported out? But no model has been able to drive them out via “resonant transport.”

The alternative theory for the formation of the smaller moons is by capture, but it’s rather very low probability. Plus that could imply far more irregular satellites and Pluto’s smaller moons are more regular. So this opens up the path for other theories. Collisional spreading? Collisional dampening? Preferential re-accretion?

This is an active area of study.

How New Horizons can help. By providing better constraints on masses and densities of Pluto & Charon, compositions of the tiny moons, any information about the differentiation shape of Pluto & Charon, and presence of distance satellites can better constrain these origin model.

More predictions about Pluto’s changing atmosphere. And Charon may have a few surprises of its own.

Blog series continues. These are summaries of talks presented on July 23, 2013 at the Pluto Science Conference. The New Horizons mission will fly by the Pluto System on July 14, 2015, a place that has never been explored before by any other spacecraft. Many questions about the Pluto System remain unanswered. For more information about NASA’s intrepid explorer to the Solar Systems’ Third Zone go to http://pluto.jhuapl.edu/ and https://www.nasa.gov/mission_pages/newhorizons/main/index.html.

Richard French (Wellesley College) presented a talk on “A Comparison of Models of Tides in Pluto’s Atmosphere and Stellar Occultation Observations.”

We have come to understand that Pluto’s atmosphere is cold & tenuous, has a long radiative time constant, shows weak diurnal variations, indicates seasonal transport of volatiles with long term variations of atmospheric mass, and seems to be convectively stable. Current Pluto general circulation models (GCMs) predict smooth T(P) profiles reveal mean circulation and thermal structure. But there are problems. GCMs predictions (with these smooth T(P) profiles) are inconsistent with stellar occultation data, which imply much more complex T(P) profile. The other challenge to this mystery is that stellar occultations are spatially constrained (i.e., map across a particular lat/long swath of Pluto surface at the time of event).

Are there waves in Pluto’s atmosphere? This is one proposition to explain the structures (spikes) seen in the Pluto occultation data. Tidal models they have built make predictions for large scale and small-scale structures. Also they can predict temperature profiles with altitude. Next steps are to apply this model to other occultation geometries. Richard French showed a comparison of a tidal model (Toigo et al 2010) against occultation data from an event on Aug 21, 2002 and they showed qualitative agreement.

Richard French’s predictions for New Horizons fly-by: When New Horizons provides a true frost pattern, they can input this into their models and generate large-scale and small-scale structures for comparison with actual New Horizons atmosphere measurements. Their tidal models do generate regionally variable, latitude dependent thermal changes. If this is what New Horizons observers, their model can help constrain parameters.

Bruno Sicardy (Observatoire de Paris, France) next took us on a rich tour of “Pluto’s Atmospheric Pressure From Stellar Occultations: 2002-2012.”

There was a dual Pluto & Charon occultation event on 4 June 2011. Pluto and Charon each pass in front of the star (at different times). Look at curve shapes. Charon’s curve sharply drops, indicative of no atmosphere, unlike Pluto’s curve, which has not-as-steep ingress/egress that indicates the presence of an atmosphere.

Using the light curve data, Sicardy and his team use a temperature vs. altitude model to fit the light curve depth, width and ingress/egress slope. Then with the temperature, they can derive a pressure. He presented results from the most recent Pluto occultation that was observed May 4, 2013. Good data and good fit. Next were shown the derived pressure (at 1215km) for occultation events observed from 1988 to 2013.

Occultation results show the Pluto atmosphere is increasing over the past few years. There is some notable evolution and implies a regular expansion. But a question from the audience stressed caution as we could be seeing just the northern pole facing the Sun with that contributing to the expansion but it could be a localized phenomena.

Bruno Sicardy’s predictions for New Horizons fly-by: Atmosphere will be present for the fly-by.

Michael Person (MIT) next described “Trends in Pluto’s Atmosphere From Stellar Occultations.” He started his talk with the advantages of occultation measurements: you get spatial resolution (~1 km at Pluto) with direct measurements of atmospheres (temperature, pressure, number profiles). MIT has collected data sets from 1988 through 2013. Their group tends to separate the upper vs. lower atmosphere when they fit their data. He next showed a light curve comparison over. Are we seeing a gradually decrease lower atmosphere slope? Is there a gradual lowering of the separation boundary?

“Haze or No Haze? That is the question.” Best evidence of haze is from the occultation event of 2002, where there is a distinct change in brightness as a function of wavelength (Elliot et al 2003). Attempt to look for haze in the 2011 occultation event with SOFIA in three bands was not successful. The main question is why does the haze come and go, and what is causing it?

What Mike Person is looking forward to: New Horizons will finally provide the size of Pluto! Knowing where the Pluto surface really is, or equivalently, the size of Pluto, is a key data point, as all these interpretations of occultation light curves and interpretations to atmosphere assumes a Pluto size.

Alex Dias de Oliveira (Observatoire de Paris, France).“Pluto’s Atmosphere from Jul 18, 2012 stellar occultation.” This is his PhD work and he provided an updated status of the steps taken from prediction of the event, the observation data collected, various calibration items, and first attempts to invert the light curve to get a temperature profile. He observed this Pluto occultation event with the ESO VLT (8m telescope in Chile) with the NACO instrument in the H band (1.65 microns). Comparison with the June 12, 2006 AAT event showed that spikes seen in the light curves were repeated in the July 18, 2012 event he observed wit the VLT in Chile.

Cathy Olkin (SwRI) presented results from “The May 4^th, 2013 Stellar Occultation by Pluto and Implications for Pluto Atmosphere in 2015.” This was an event where Pluto passed in front of a R=14.4 mag star with a slow shadow velocity of 10.6 km/s. The event was observed from the southern hemisphere, from Cerro Tololo in Chile.

Erika Barth (SwRI). “Is Methane Supersaturation Consistent with the Presence of Haze Particles in Pluto’s Atmosphere?” She asked the question: If you put haze particles into Pluto’s atmosphere how do they interact with the methane in Pluto’s atmosphere?” She developed a model to ingest haze particles into a supersaturated environment and this predicts the growth of clouds and condensation of methane. Then when methane condenses out, that reduces the amount of observable methane. Her model requires the existence of a troposphere (which we learned earlier in Emmanuel Lellouch’s talk today that there is no evidence for this, but its existence could explain some phenomena, some observed to date, other predicted) and also predicts a thick troposphere as well. She created a Pluto version of CARMA, the Community Aerosol and Radiation Model for Atmospheres.

Jason Cook (SwRI) next spoke about his “Analysis of High Resolution Spectra of Pluto: A Search for Cold Gaseous Methane Layer, and Spatial Variation in Methane Column Abundance.” Occultations have told us that Pluto’s upper atmosphere (above 1195 km) is pretty warm (100 K). But 2.15 micron N₂ ice measurements of Pluto’s surface tells us the surface is ~40 K. So this implies there needs to be a cold-layer in the atmosphere. To investigate a search for this “cold layer of air” they took NIR (near infrared) spectra with NIRSPEC on Keck with R=35,000 in 2011. They need to move to a two-temperature model to help constrain the observed data (i.e. measured methane line depths from the high-res NIR spectra), but the hot/cold ratio of the two temperatures is an unresolved topic (pun intended).

They also took spectra of Pluto over several nights to probe the different longitudes of Pluto (Pluto has a rotational period of 6.4 days) and they got a fairly consistent number except near 180 deg longitude where gaseous CH₄ was not easily detected. They would like more data to probe this temporal measurement.

Selection of high-resolution NIR spectra of Pluto obtained over several days. This series probes a range of Pluto rotations and show how methane lines (Q-branch) vary.

Eliot Young (SwRI) spoke about “Evidence for Recent Change in Pluto’s Haze Abundance.” Hazes have been observed on Titan (photolysis products from higher up in the atmosphere) and Triton (condensates from surface). The August 21, 2002 occultation showed evidence for haze (change in brightness with color, Elliot et al 2003), but 2007 (0.51 & 0.76 micron) and June & July 2011 occultation events in different bands (I & K bands) showed no change in color.

Occultations can only probe down to a certain depth, so they are limited. We don’t really how close you got to the Pluto surface. If you have a special case where you can have a central flash or sets of flash spikes, you can derive more information. By applying a new technique on the 2007 Mt John light curves, he proposed they can determine amount of haze by evaluating the attenuation in those parts of the light curve.

Central Flash Description: A central-flash occultation is visible when the observer is located near the center of the shadow path of the object. It is here where the atmosphere near the edges of the occulting body (for Pluto occultations, this is Pluto) refracts extra star light (from the background star) directly opposite from the star, forming a “brightening” in the middle of the deep light curve.

Mark Gurwell (Harvard CfA) provided a talk entitled “Atmosphere CO on Pluto: Limits from Millimeter-Wave Spectroscopy.” Carbon monoxide (CO) is expected based upon the presence as an ice on its surface. The first direct detection of CO was done in the NIR with the VLT (Lellouch et al 2011). Then JCMT (Greaves et al 2011) revealed a CO(2-1) line in the submillimeter, but this line had not been there a few years back, leaving a mystery. There is still mystery in fitting the CO abundance based on the measured submillimeter width and strength of this line. He did show that Pluto had been in the fore-ground of a galactic emission during the JCMT observations. He supposes that they had contamination. They did their own observations using the SMA sub-millimeter telescope multiple times and did not detect the CO(2-1) line in the spectra (they have upper limits). So he is excited about using the ALMA array that has 30-50x SMA sensitivity to really address the CO, nitriles and isotopes.

And the final talk of the morning Atmosphere session just could not leave Charon out of it.

Alan Stern (SwRI) “Cometary Impact Produce Transient Atmospheres on Charon.” Most scientists have come to accept that Charon does not have an atmosphere (see earlier posting in Bruno Sicardy’s talk showing the dual occultation event for Pluto and Charon in Jun 2011.) But he postulates what about impacts? Coming from the Kuiper Belt, impactors (assuming cometary-level amounts of volatiles) could provide volatiles to the surface to Charon and therefore creating a “nanobar” atmosphere on Charon. Similar events could lead to atmospheres on Kuiper Belt objects. Their modeling (Trafton & Stern 2008) predicts presence of N₂, CO, Ar, CH₄, with CO dominating after impacts, and N₂ being the dominate species (in terms of amount).

Predictions for New Horizons. Duty cycle would be short lived so it will be rare if New Horizons catches this event. However, with smaller impactor sizes, there could be a possibility that those events could have occurred within the “photoionization time” (before the molecule breaks down or escapes) or resulting implanted molecules by the time New Horizons gets there. Alan Stern coyly stated, “could be as much as a 25% chance” to see an nano-bar atmosphere on Charon.

A good question regarding surface volatiles that are revealed by impacts got the crowd excited. After all, when you describe an atmosphere you can categorize things as sources and sinks. Sources bring material to the system and here they could be not only the KBO impactor but also the materials that are revealed from the impacted-body after the impact.

On July 14, 2015 New Horizons will be doing a very sensitive experiment via the observations of the Charon occultation (Charon passing in front of our Sun as viewed from New Horizons).

Charon may indeed hold a few surprises of her own!