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Science

The Many Faces of Pluto and Charon

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Reposted from https://blogs.nasa.gov/pluto/2016/02/12/the-many-faces-of-pluto-and-charon/.

Today’s blog post is from Kimberly Ennico, a member of the New Horizons’ Composition Theme Team and one of the deputy project scientists. She works at NASA’s Ames Research Center in Moffett Field, California, and has been on detail to the Southwest Research Institute in Boulder, Colorado.

No one can doubt the beauty of Pluto and Charon—amazing worlds revealed by the images from NASA’s New Horizons mission. From Pluto’s mountains, glaciers, ice-volcanoes, blue skies, and layered colorings to Charon’s vast tectonic structures and enigmatic red-colored pole, these pictures and associated spectra are rich puzzles waiting to be solved.

The July 14, 2015 Pluto flyby gave us an initial look at one side of Pluto, with its iconic heart-shaped feature. But I’m interested in the full planetary perspective, finding the “other sides” of Pluto to be every bit as fascinating as the encounter hemisphere. We must remember that a flyby is a moment in time lasting a few hours. In contrast, Pluto and Charon each rotate about its axis every 6.4 Earth days. This means that when New Horizons flew through the Pluto system it captured one hemisphere of each body in incredible detail.

What do we know about the “other sides” of Pluto and its largest moon? In the three weeks before the flyby, the Long Range Reconnaissance Imager (LORRI) and Multispectral Visible Imaging Camera (MVIC) imaged Pluto and Charon every day, sometimes two or three times a day to gather as much coverage across the bodies as New Horizons closed in. LORRI is New Horizons’ primary camera, an 8-inch telescope outfitted with an unfiltered charge-coupled device (CCD) – like you’d find in your own digital camera – sensitive to visible light. MVIC is a separate instrument with multiple CCDs, for which several are outfitted with color filters. The highest resolution images of the “other sides” of Pluto and Charon were observed 3.2 Earth days earlier, around July 10-11.

Working with a subset of the data (as not all these images have been sent to Earth from New Horizons yet), we’ve received our first glimpse of these “non-encounter” hemispheres below.

nh-pluto_four_faces_withwhitebox

Four faces of Pluto in black-and-white and color. From left to right, the central sub-observer longitudes are ~180, 240, 360 and 60 degrees East Longitude. The Pluto “Encounter Hemisphere” (indicated by the white box) is most recognizable by the “heart” feature of the informally-named Tombaugh Regio. This is also the hemisphere that today never faces Charon, as Charon is “tidally locked” to Pluto, similarly to how the Earth only sees one face of our moon. Pluto’s “Charon-facing” side is the second column from the right. Pluto’s north pole is up in all these images. The top row contains LORRI grey-scale images taken on July 13, July 12, June 27 and July 3rd, when Pluto was 620, 189, 24 and 36 LORRI pixels across, respectively. The bottom row shows MVIC “enhanced-color” images made by combining the near infrared, red and blue filters. They were taken on July 13, July 12, July 10 and July 9, when Pluto was 163, 56, 26 and 21 MVIC color pixels across, respectively. All these images surpass what we had previously seen from Hubble Space Telescope imagery where Pluto’s disk was only about 12 pixels across. Of course, New Horizons was only millions of miles from Pluto—Hubble is over 3 billion miles away! Credits: NASA/JHUAPL/SwRI

nh-charon_six_faces_box

Six faces of Charon. Central sub-observer longitudes: top, from left to right, 350 (B&W), 2 (color), 32 (color); Bottom, from left to right, 67 (color), 86 (B&W), and 180 (color) degrees East Longitude. The side that faces Pluto is highlighted by the inset box. From left to right, the top row images were taken July 14, 14 and 13, 2015, with Charon spanning 523 (LORRI), 81 (MVIC), and 43 (MVIC) pixels. The bottom row images were captured from July 12, 12 and 10, 2015, with Charon spanning 28 (MVIC), 96 (LORRI), and 13 (MVIC) pixels. Charon remains a mainly neutral greyish color all around, with a distinct red northern polar cap appearing from all sides. Credits: NASA/JHUAPL/SwRI

What strikes me most about the new Pluto color images is that the latitudinal (horizontal) banding identified on the encounter hemisphere is evident all around Pluto. Specifically, the northern polar region has a distinctive color from adjacent latitudes. The darkest region, which spans the equator, also appears to continue around Pluto, showing distinct variations on the side facing Charon, which have yet to be understood.

Why is this interesting? Coloring on Pluto is thought to have been the result of hydrocarbons called tholins that have formed in the atmosphere and have been “raining” down on Pluto’s surface over the millennia. We’re investigating whether Pluto’s colored terrains are primarily due to changes in or movements of its surface ices, specifically whether they have been undergoing seasonal effects –changing in temperature over time from the amount of cumulative sunlight – which could display itself as horizontal banding. The presence of that vast reservoir of methane, nitrogen and carbon monoxide ices in Pluto’s “heart” complicates the picture and could serve as a visible marker to trace changes.

Over the next few months, as more of this late-approach imagery gets downlinked from the spacecraft’s recorders, we will continue to piece together this colorful story of Pluto and Charon – from all sides.

Science

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

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Reposted from https://blogs.nasa.gov/mission-ames/2013/07/26/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.

europa_tyre

The Tyre crater on Europa (one of Jupiter’s moons) fromhttp://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 Vmin, minimum velocity.  And if that velocity is lower than the escape velocity for an object (the speed above which would allow the object to escape the gravity well and go into orbit), you can create secondary craters. For ice, Vmin ~150-250 m/s. Pluto has Vescape ~1180 m/s; Charon has Vescape ~550 m/s. So we expect to see secondary craters on Pluto & Charon. Sesquinary craters might form when Vmin>> Vescape and the ejecta does escape the surface and then falls back onto its surface or onto another body. These are expected to be rare, but it’s possible you can have a crater formed on Pluto from a secondary ejected from Charon.

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

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

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 wastingis 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 modelshave 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.

Science

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

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Reposted from https://blogs.nasa.gov/mission-ames/2013/07/26/its-more-than-skin-deep-interiors-of-pluto-and-charon-a-discussion/.

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

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

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 3rd Law, Bulk density, Moment of Inertia, Gravity, Magnetism, Radar, Seismology, Tides. For New Horizons, as the fly-by is not that close, Gravity is not a viable method; the lack of a magnetometer aboard rules out Magnetism, and Seismology requires the spacecraft to land, also not possible. He described their approach  that will use Shape Modeling to measure a Tidal Bulge. Both Pluto and Charon may each raise a tidal bulge on the other. He cautioned that we may not be able to determined the existence of an ocean using a shape model.

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

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

Science

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

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Reposted from https://blogs.nasa.gov/mission-ames/2013/07/26/some-insights-into-charon-and-what-roles-laboratory-work-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 (July23rd, 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 C22Hm 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.

Science

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

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Reposted from https://blogs.nasa.gov/mission-ames/2013/07/25/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, 2013th 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 4th 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 CO2features 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.

long_ch4_co_n2_ice

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 sub-millimeter 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!