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The Pluto System After New Horizons

John R. Spencer Southwest Research Institute, Suite 300, 1050 Walnut St, Boulder, CO 80302, USA

William M. Grundy Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001, USA

Francis Nimmo Department of Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064, USA

Leslie A. Young Southwest Research Institute, Suite 300, 1050 Walnut St, Boulder, CO 80302, USA

Chapter for The Trans-Neptunian Solar System, Submitted November 21st 2018, Revised April 30th 2019 Keywords: Pluto; Charon; New Horizons.

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Abstract Pluto and its moons are by far the best-characterized and understood Trans-Neptunian objects, thanks to Pluto’s size and brightness, its long history of telescopic observations, and especially the 2015 flyby by the New Horizons spacecraft. Pluto displays complex and active geology, an enormous range of surface ages, great compositional variety, and a dynamic atmosphere intimately linked to the surface. Charon is geologically and compositionally very different from Pluto, and the four small moons form a satellite system unlike any previously visited by spacecraft. Knowledge of Pluto Before New Horizons Physical studies of Pluto began with determination of its lightcurve, establishing its rotation period and the presence of albedo features on its surface (Walker and Hardie 1955). Spectroscopic studies found CH4 on its surface (Cruikshank et al. 1976), and the discovery of Charon (Christy and Harrington 1978) allowed determination of the system mass. In the 1980s, mutual eclipses and occultations between Pluto and Charon allowed determination of the diameters of both bodies, initial mapping of their albedo features (Buie et al. 1992), and established that Charon’s surface was dominated by water ice (Marcialis et al 1987). Pluto’s atmosphere was discovered by stellar occultation (Elliot et al. 1989). The atmospheric scale height (Yelle and Lunine 1989), and detection of nitrogen, in addition to CH4 and CO on the surface (Owen et al. 1993), established that the atmosphere was dominantly N2. The high amplitude of the lightcurve, and direct imaging of the surface with HST, revealed very high contrast albedo features on Pluto’s surface (Buie et al. 2010), and rotationally-resolved spectroscopy revealed large longitudinal variations in surface composition (Grundy et al. 2013). Thermal observations also revealed large surface temperature contrasts (Lellouch et al. 2000). Additional stellar occultations between 2002 and 2015 revealed, surprisingly, that Pluto’s atmosphere was increasing in density as Pluto receded from the sun (Elliot et al. 2007, Dias-Oliveira et al. 2015). Two additional small moons, Nix and Hydra, were discovered in 2005 (Weaver et al. 2006), followed by the smaller Styx and Kerberos in 2011 and 2012 (Showalter et al. 2015) (Table 1). Meanwhile, starting in 1992, the discovery of additional objects beyond Pluto, including many that shared Pluto’s 3:2 mean motion orbital resonance with Neptune, and some that approached Pluto itself in size, finally revealed Pluto’s true place in the solar system. Table 1: Bodies in the Pluto System

Semi-major axis

Eccen-tricity

Inclina-tion, degrees

Period, days

Period relative to Charon

Pluto 0.07 1188a 1854a 0.57c 6.387d 0 2,125d,e 0.0000d 0.00d 6.3872d 1.00Charon 0.15 606a 1701a 0.36c 6.387d 0 17,448d,e 0.0000d 0.00d 6.3872d 1.00Styx 3.13 16 x 9 x 8b - 0.65b 3.24b 82b 42,656f 0.0058f 0.81f 20.1616f 3.16Nix 0.30 50 x 35 x 33b - 0.56b 1.83b 131b 48,694f 0.0020f 0.13f 24.8546f 3.89Kerberos 1.97 19 x 10 x 9b - 0.56b 5.31b 96b 57,783f 0.0033f 0.39f 32.1676f 5.04Hydra 1.14 65 x 45 x 25b - 0.83b 0.43b 117b 64,738f 0.0059f 0.24f 38.2018f 5.98

Orbit Relative to Barycenter

Body

Highest-Resolution

NH Imaging, km/pix Radius, km

Density, kg m-3

Geo-metric Albedo

Rotation Period, days

Spin Pole Obliquity, degrees

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References: aNimmo et al. 2017; bWeaver et al. 2016; cBuie et al. 1997, adjusted to true radii; dBuie et al. 2012; eBrozovic et al. 2015, fShowalter and Hamilton 2015. The New Horizons Encounter with Pluto The New Horizons mission was launched on January 19th 2006, and encountered the Pluto system on July 14th 2015. Its payload (Table 2, and Weaver et al. 2008) provided a scientifically rich portrait of the Pluto system. Table 2: New Horizons Payload

The encounter was designed to satisfy multiple constraints (Young et al. 2008). Flying through Pluto and Charon’s shadows enabled Earth and solar occultation studies of Pluto’s atmosphere at radio and UV wavelengths, and placed upper limits on Charon’s atmosphere. This geometry also enabled observations of Pluto’s highest-contrast hemisphere near closest approach. Closest approach distance to Pluto, 12,500 km, was a compromise between maximizing spatial resolution and, probing the atmosphere and plasma interaction, and providing observations at a wide range of viewing geometries.

Instrument Angular resolution

Field of View

Wavelength / Energy Range

Best Resolution at Pluto, km

Notes

Ralph/MVIC 19.8 µrad/pix 99.0 mrad

400 – 550 nm; 540 – 700 nm; 780 – 975 nm; 860 – 910 nm; 400 – 975 nm

0.33Color + Pan pushbroom camera, Pan framing camera

Ralph/LEISA

60.8 µrad/pix

15.6 x 15.6 mrad 1.25 – 2.5 µm 2.83

Near-IR imaging spectrometer: λ/Δλ = 240

LORRI 4.96 µrad/pix

5.08 x 5.08 mrad 350 – 850 nm 0.07 Framing camera

Alice 1700 µrad/pix

35 x 35 mrad; 1.7 x 70 mrad

47 – 188 nm 23UV imaging spectrometer: spectral resolution 0.18 nm

REX 20 mrad 20 mrad 4.2 cm 240Doppler tracking of uplink DSN signals; Passive radiometry

SWAP 200° x 10° 200° x 10° 0.25 – 7.5 keV N/A Low-energy plasma spectrometer

PEPSSI 25° x 12° 160° x 12° 1 – 1000 keV N/A High-energy plasma spectrometer

SDC n/a 180° x 180° Particle mass > 10-12 g

N/A Dust detector

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Figure 1. The Pluto encounter sequence, with instruments identified for each observation: frequently more than one instrument was used at a time. Highest-resolution observations are indicated with an asterisk. Lines extending from observation mid-points along the trajectory indicate instrument pointing (except for Nix and Hydra observations, which pointed out of the plane of this graphic). NIR = near-infrared, UV = ultraviolet. The spacecraft traveled from upper left to lower right. In situ observations with PEPSSI, SWAP, and SDC were made throughout the flyby. Approach observations of Pluto began in January 2015, with periodic LORRI images for lightcurve studies and optical navigation. By May 2015 the resolution of the LORRI images exceeded that of the best HST images. From 64 to 13 days before closest approach, a series of deep image mosaics were obtained to search for potentially hazardous rings, or new moons capable of generating hazardous debris, though no new moons or other hazards were found. The close approach sequence is illustrated in Figure 1, and the best resolution obtained on Pluto by the different instruments is shown in Table 2. Figure 2 shows the best global color images of Pluto and Charon. Departure observations focused first on Alice and REX occultations by Pluto and Charon, and then high phase remote sensing of Pluto, Charon, and the small satellites, continuing for 2 weeks after the flyby corresponding to roughly two rotation periods of Pluto and Charon. Data downlink was completed in Fall 2016. Note that many of the feature names referred to in this chapter are informal.

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Figure 2. The best color images of Pluto (left) and Charon (right). Colors are enhanced relative to those visible to the human eye. Pluto north is up. Light-colored Sputnik Planitia occupies most of the center of the Pluto image. Pluto Interior The interior of Pluto remains poorly understood. The New Horizons spacecraft provided an accurate radius measurement (1188 km; Nimmo et al. 2017), allowing determination of Pluto’s density (1854 kg m-3). This value suggests that Pluto is roughly two-thirds rock and one-third ices (McKinnon et al. 2017). Although there are no measurements of Pluto’s moment of inertia, it is generally assumed that Pluto is differentiated. This is partly because ices dominate the surface spectra (Grundy et al. 2016) but also because an undifferentiated Pluto would have experienced high-pressure ice phase transformations, leading to surface compression – which is not observed in its surface geological record (McKinnon et al. 2017). Pluto’s high rock fraction and relatively large size mean that radioactive heat production would be sufficient to melt a conductive ice shell and maintain a global, subsurface ocean to the present day (Hammond et al. 2016, Bierson et al. 2018). However, if the ice shell were convective, heat would be removed rapidly enough that an ocean would never develop (Robuchon et al. 2011). Therefore, if there is a present-day ocean, its presence would suggest a cold and rigid ice shell, with little communication between the ocean and the surface. A freezing ocean would generate extensional surface stresses, which are a likely explanation for the large and apparently young normal faults observed on Pluto’s surface (Moore et al. 2016). Freezing would also pressurize the ocean, perhaps helping produce the putative “cryovolcanic” features observed (Moore et al. 2016). The location of the nitrogen-filled Sputnik Planitia basin near the anti-Charon point suggests that it caused true polar wander, and thus represents a mass excess (Keane et al. 2016, Nimmo et al. 2016). One plausible way of generating such a mass

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excess is if the ice shell beneath the basin were thinned and replaced with denser water (Nimmo et al. 2016). The ice shell must be cold to avoid rapid relaxation of the basin; this might suggest a cold, ammonia-rich ocean beneath. Figure 3 shows a plausible interior structure.

Figure 3. Cartoons of plausible interior structure for Pluto and Charon. Cross-sections are to scale, except that the depth of Sputnik Planitia has been exaggerated for clarity. Surface Geology Pluto’s geology is remarkably varied (Figures 2, 4, and Moore et al. 2016), and shows us that the expected few mW m-2 of radiogenic heat is capable of powering ongoing varied geological activity on volatile-rich ice worlds. Impact crater densities vary from near-saturation to negligible (Singer et al. 2019), implying surface ages from ~ 4 Ga to < 10 Ma (Moore et al. 2016). The oldest preserved feature on Pluto’s surface is the basin containing Sputnik Planitia (Figure 5), probably an ancient impact basin (Moore et al. 2016). Much of the encounter hemisphere west of Sputnik is covered by moderately cratered smooth planes of unknown origin, cut by graben suggesting global expansion (Figure 4a), and by scarps that may result from sublimation erosion (Moore et al. 2017). North and east of Sputnik the terrain is much more complex, with what appear to be several generations of mantling material, several kilometers thick with steep-sided pits and marginal scarps (Figure 4b and Howard et al. 2017a, Umurhan et al. 2017). Extensive valley erosion is seen in many places (Figure 4b, 4f). A distinctive high-

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standing methane-rich unit is extensively eroded, probably by sublimation, into penitente-like “bladed terrain” (Moore et al. 2018).

Figure 4. Illustration of the diversity of terrain types on Pluto: coordinates are those of the image centers. A. Elliot, a relatively pristine impact crater with a central peak, an apparent nitrogen ice deposit occupying its floor, and a graben cutting part of its rim (141 E, 12N). B. Eroded mantles northeast of Sputnik Planitia (195 E, 59 N); C. Glacial flow into Sputnik Planitia from the highlands to the east (193 E, 2 N); D. Convection cells, and neighboring chaotic mountain blocks, near the west margin of Sputnik Planitia (174 E, 6 N); E. Wright Mons, a structure of unknown origin, possibly a cryovolcanic construct (172 E, 20 S); F. Dendritic channels and other erosional features southwest of Sputnik Planitia (158 E, 20 S). Portions of the MVIC image taken near close approach, with a resolution of 330 m/pixel. Scale bar is 50 km long in all panels. The most remarkable known terrains on Pluto occur in and around Sputnik Planitia. The planitia itself appears to be the surface of a nitrogen-rich deposit, several km thick, partially filling the original impact basin. All of the Planitia except the southernmost part features polygonal cells strongly suggesting active convection (Figure 4d): convection of solid N2 is plausible for expected radiogenic heat fluxes (McKinnon et al. 2016). Clusters of hills at the margins of the convection cells appear to be buoyant material rafted to those locations by the convection (White et al. 2017). The northern margin is depressed (Figure 5) and shows evidence of flow toward the margin, perhaps driven by sublimation of the northern margin (Bertrand et al. 2018a, Protopapa et al. 2017). The southern portion is marked by complex patterns of pits, probably sublimation-generated. The west margin of Sputnik features rugged mountain blocks up to 6.5 km high with a chaotic jumbled appearance (Figure 4d, and Schenk et al. 2018a). It is likely that these are

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fragments of the water ice bedrock, perhaps undermined and transported by denser, more fluid, nitrogen ice. On the east side of Sputnik, active N2 glaciers (Figure 4c) flow down from pitted highlands into the Planitia, perhaps resupplied by nitrogen condensed out of the atmosphere (Howard et al. 2017b). To the south-west of Sputnik, a pair of large mountains with central depressions, Wright Mons (Figure 4e) and Piccard Mons (175 E, 35 S), may be cryovolcanic constructs, unlike any seen elsewhere in the solar system.

Figure 5. Global map of Pluto, colored by elevation using stereo images and limb profiles where available (Schenk et al. 2018a). Sputnik Planitia is the depression near the center of the image. “+” and “x” mark the locations of the radio occultation ingress and egress respectively: see Fig. 8. Surface chemistry Pluto’s surface materials can be divided into two categories: volatile ices that are mobilized through sublimation and condensation and non-volatile materials that are inert at Pluto surface temperatures (30 to 60 K). The volatile ices are N2, CO, and CH4 (Figure 6). N2 is the most volatile and dominates Pluto’s lower atmosphere and the seasonal migration of ice, driven by changing patterns of sunlight. New Horizons detected N2 at mid-northern latitudes, where it preferentially occupies low lying areas like the floors of craters and valleys where atmospheric pressure is highest (Figure 6c and Schmitt et al. 2017). N2 ice is mostly absent from the north pole, currently experiencing continuous summer sunlight. It partly fills the Sputnik Planitia basin, along with CO and CH4, both of which are soluble in N2 ice (Protopapa et al. 2017). The distribution of CH4 ice (Figure 6d) contrasts with that of N2, due to varying relative abundances of the CH4-rich and N2-rich phases of the N2/CH4 solid solution. CH4 is abundant at high northern latitudes, and also at high elevations at lower latitudes, such as the rims of craters and crests of mountains, and in the bladed terrain (Earle et al. 2018; Moore et al. 2018). Being much

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less volatile, CH4 migrates more slowly, on the million-year timescales of the pole precessional cycle (e.g., Bertrand et al. 2018b).

Figure 6. Distribution of color (approximately natural), and absorption band strengths for H2O, N2, and CH4 ices on the encounter hemisphere of Pluto. The yellow line marks the Charon-facing meridian. The inert bedrock underlying Pluto’s surface is H2O ice. It is exposed at the surface in several places, mostly at mid to low latitudes (Figure 6b). It appears in the floors of numerous craters, in rugged mountains in and adjacent to Sputnik Planitia, and in the broken belt of dark red maculae typified by Cthulhu (20 E – 150 E, 20 S – 10 N) and Krun (190 E – 220 E, 25 S – 5 S) that encircles Pluto along its equator (e.g., Grundy et al. 2016a; Cook et al. 2019). It is mostly associated with dark, reddish regions, but there are a few regions showing comparatively clean,

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bright, H2O ice, most notably in Supay Facula (215 E, 25 N, Figure 7). The red coloration (Figure 6a) mostly originates in complex organic chemistry in Pluto’s upper atmosphere, where solar ultraviolet radiation drives processing of CH4 and N2 into complex macromolecules known as tholins. These form aerosol particles that eventually settle to the surface (e.g., Grundy et al. 2018). Additional radiolytic and photolytic chemistry could occur directly in Pluto’s surface ices (e.g., Cruikshank et al. 2015). Smaller, but still inert molecules such as C2H6 and CH3OH have been tentatively detected in eastern Cthulhu (Cook et al. 2019). They could be produced from CH4 plus other Pluto materials through energetic radiation. Surface/atmosphere Interactions Like Mars and Triton, Pluto has an atmosphere whose density is controlled by vapor pressure equilibrium with surface ices, and which is global in extent, because subsonic winds are able to transport gas from regions of net sublimation to regions of net condensation (Trafton and Stern 1983). The transport of latent heat by wind means that the N2 ice surface is virtually isothermal regardless of local insolation. The ice temperature, and thus (by vapor pressure equilibrium) the atmospheric pressure, is controlled by the global balance between absorbed insolation, thermal radiation of the nitrogen ice, and subsurface heat flow, and thus by the spatial distribution, albedo, thermal inertia, and emissivity of the ice, as well as the distribution of incident sunlight. Because about a meter of ice can be transported between hemispheres over a seasonal cycle, complex feedback is possible between ice distribution and atmospheric pressure over a Pluto year and longer (precessional) timescales (Hansen and Paige 1996, Young 2013, Bertrand et al. 2018a,b). Stellar occultations show a tripling in atmospheric pressure between 1988 and 2013, consistent with many model predictions (Olkin et al. 2015). These models also predict that pressure will soon begin to fall, though the degree of atmospheric collapse at aphelion is still unclear. Because condensation is enhanced at low elevation due to higher atmospheric pressure, there is a tendency for nitrogen to migrate to depressions, which likely explains the presence and probable long-term stability of kilometers of nitrogen in the basin of Sputnik Planitia (Figure 7, and Bertrand et al. 2018a). The permanent reservoir of N2 in Sputnik Planitia has a major influence on the seasonal and longer-term distribution of volatiles on Pluto, and the variations in atmospheric pressure (Bertrand and Forget 2016, Bertrand et al. 2018a). For simple static volatile models, perihelion surface pressure could exceed Martian values (~6 mbar) at times in the precessional cycle when solstice coincides with perihelion (Stern et al. 2016). Figure 7 shows how the compositional provinces seen in Figure 6 may be understood in terms of Pluto’s volatile cycles.

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Figure 7. Composite image of the distribution of CH4, N2, and H2O on Pluto (from Figure 6), annotated to show the major compositional provinces and tentative interpretations. Atmospheric structure and composition

Figure 8. Overview of Pluto’s vertical atmospheric structure at the time of the New Horizons flyby, at three different magnifications. Below 30 km, black indicates the dawn atmosphere over

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the southern tip of Sputnik Planitia (radio ingress, “+” symbol on Fig. 5), while gray indicates the dusk atmosphere over an uncertain terrain type near the center of the Charon-facing hemisphere (radio egress, “x” symbol on Fig. 5). Above 30 km, ingress and egress are similar in both the radio and UV occultations. The surface at radio ingress was at planetary radius = 1,187.4 ± 3.6 km, while at egress was at radius = 1,192.4 ± 3.6 km. The altitude scale is relative to a reference radius of = 1,190 km. Figure and caption adapted from Young et al. (2018). The pressure and temperature of Pluto’s atmosphere (Figure 8) was probed by New Horizons at altitudes below 100 km by the REX radio occultation (Hinson et al. 2017) and at altitudes near 1000 km by the Alice ultraviolet solar occultation (Young et al. 2018). It is constrained at intermediate altitudes by ground-based stellar occultations (Sicardy et al. 2017), and hydrostatic equilibrium (see Young et al. 2018). The temperature is surprisingly cold at the highest altitudes measured by New Horizons; CH4, CO, HCN, C2H6, atmospheric hazes, and H2O may all be important in the energy budget and temperature of Pluto’s atmosphere (Strobel and Zhu 2017, Yelle et al. 2017, Zhang et al 2017, Krasnopolsky 2018). Thermal conduction via a strong thermal gradient connects the warm middle atmosphere to the colder surface (Hinson et al. 2017). The surface pressure at 1189.9 km radius was 11.5 ± 0.7 microbar in 2015. Over the N2-filled Sputnik Planitia basin, at lower elevation (radius 1187.4 km), the surface pressure is 12.8 ± 0.7 microbar, in vapor-pressure equilibrium with N2 ice at 37.1 K (Fray and Schmitt, 2009). From the Alice ultraviolet solar occultation (Young et al. 2018) we find: (1) the atmosphere is primarily N2 below 1200 km altitude above the surface, (2) the CH4 mixing ratio increases from 0.3% at 80 km to 25% at 1200 km altitude, and should dominate over N2 at the exobase, at ~1700 km altitude, and (3) the simple hydrocarbons C2H2, C2H4, and C2H6 have mixing ratios near 0.1% at ~550 km altitude, decreasing to ~1x10-6, 8x10-6, and 4x10-6 at 200 km altitude for C2H4, C2H2, and C2H6 respectively (Young et al. 2018), likely due to condensation on haze (Cheng et al. 2017). The Atacama Large Millimeter Array measured CO with a mixing ratio of 5x10-4, and HCN supersaturated in Pluto’s atmosphere with a mixing ratio of ~1.5x10-5 to 4 x 10-5 that varies with altitude (Lellouch et al. 2017). Solar Wind Interaction Due to the unexpectedly cold upper atmosphere, the escape rate derived from analysis of the Alice data is 100 times smaller than pre-encounter estimates (6x1025 CH4 s-1, Young et 2018, vs. the expected 3.5x1027 N2 s-1, Zhu et al. 2014), which leads to lower mass loading and a smaller obstacle to the solar wind. Moreover, the solar proton flux at the time of the flyby was unusually strong, about 4 times larger than the predicted mean (Bagenal et al. 2016), further shrinking the interaction region. Analysis of data from the SWAP instrument (McComas et al. 2016), supported by the PEPSSI instrument (Bagenal et al. 2016), paints a picture of a small bow shock at the time of the flyby, only 4.5 Pluto radii (RP) from Pluto center, and a wake or tail that extends at least 100 RP. New Horizons entered the bowshock region in its flanks, and detected a 20% slowing of the solar wind. Plutogenic species fill the wake of the interaction region: PEPSSI detected suprathermal particles (Bagenal et a. 2016), and SWAP detected heavy ions, likely CH4+ (Zirnstein et al. 2017). Charon

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Interior Charon is about half the size of Pluto (Nimmo et al. 2017) and about 10% less dense (1701 kg m-3), suggesting it is about three-fifths rock (Figure 3). Like Pluto, the observed surface ices and lack of compression suggest a differentiated interior. Modeling shows that, although Charon could have developed a subsurface ocean via radioactive heating mid-way through its history, such an ocean would not survive to the present day (Desch & Neveu 2017, Bierson et al. 2018). The imaged hemisphere of Charon shows a wide range of extensional tectonic features (Figures 2, 9), implying a total strain of order 1% (Beyer et al. 2017). This strain is what one would expect from refreezing of an ocean ~40 km deep, consistent with thermal evolution models. Charon also has extensive smooth plains that apparently post-date extensional scarp formation, perhaps indicative of an episode of cryovolcanism and/or crustal overturn (Beyer et al. 2019). The lower density of Charon compared to Pluto is unlikely to be due to differences in porosity alone (Bierson et al. 2018), and indicates a slightly lower rock mass fraction. As with Pluto, Charon’s rock core most likely underwent significant hydrothermal alteration (Desch and Neveu 2017, Malamud et al. 2017). Charon also differs from Pluto in that it may have experienced an early episode of tidal heating and stress generation if its orbit was initially non-circular (Rhoden et al. 2015).

Figure 9. Topography of the best-imaged part of Charon, showing the contrast between the smooth plains of Vulcan Planitia (lower right) and the polygonally fractured rugged terrain (upper left). Total displayed elevation range is -6 km (black) to +6 km (white). “M” indicates examples of the moat at the north and west margin of Vulcan Planitia, and “m” indicates the moated mountains within Vulcan. From Schenk et al. (2018b).

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Surface Geology Charon’s surface is on average older than that of Pluto, as expected for a smaller, less volatile-rich, world. Crater counts suggest that most of the surface has an age of ~4 Ga (Moore et al. 2016). Two dominant terrain types are visible in the New Horizons images (Figures 2, 9). The oldest, occupying more than half of the encounter hemisphere, is a heavily cratered rugged terrain broken into polygonal blocks up to several 100 km in size by canyons up to at least 12 km deep (Beyer et al. 2017, Schenk et al. 2018b), providing evidence for global expansion likely due to freezing of a subsurface ocean. The other terrain type, which is has similar crater density but is younger based on superposition relationships, is a smooth plains unit embaying the rugged terrain (Beyer et al. 2019). The margins of the plains units abutting the rugged terrain form a “moat” that is depressed by 1 – 3 km (“M”, Figure 9), and several mountains partially embayed by the smooth plains show similar moats (“m’, Figure 9, also visible near the terminator in Figure 2). These margins suggest that the flows had high viscosity, emplaced either in the solid state or as a liquid/solid mixture (Beyer et al. 2019). Surface chemistry New Horizons spectral observations show ubiquitous H2O ice across Charon (Grundy et al. 2016a). A band at 1.65 µm indicates the ice is predominantly crystalline rather than amorphous. Exposure to space radiation amorphizes cold H2O ice over time. If warmed, radiation-damaged ice can recrystallize, but Charon’s surface temperature was thought to be too cold for that, triggering speculation about recent cryovolcanic activity (Cook et al. 2007). The New Horizons images show no evidence of recent cryovolcanism, casting doubt on the extrapolation from laboratory to geological timescales. A band at 2.21 µm is seen across Charon (Dalle Ore et al. 2018). It had been attributed to ammonia or ammonia hydrate, but these materials are likewise thought to be readily destroyed by space radiation, so some other, more stable ammoniated compound is suspected (e.g., Cook et al. 2018). Much higher concentrations in the ejecta of a few craters suggests a localized subsurface reservoir. Charon’s dark polar caps are among its most visually striking features. They are hypothesized to slowly form through cold-trapping CH4 escaping from Pluto’s atmosphere onto Charon’s frigid winter pole, where it is radiolytically processed into heavier hydrocarbons and ultimately dark, reddish, tholin-like material (Grundy et al. 2016b). Small satellites Dynamics The orbits of Pluto’s four small satellites (Table 1, Figure 10) are almost coplanar with Charon’s orbit, and are close to, but not exactly in, the 1:3, 1:4, 1:5, and 1:6 mean-motion resonances with the orbit of Charon. Their rotation is non-synchronous with their orbits, and is much faster than that of Pluto or Charon. The fastest period, that of Hydra (10.3 hours), is typical for small bodies in heliocentric orbit, suggesting that it simply is too far from Pluto for tides to have slowed its rotation- the fact that the closer satellites spin more slowly may indicate that they have been

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subject to some tidal despinning. The obliquities of the spin axes are extreme, perhaps having been tilted by resonances between spin precession and mean motion (Quillen et al. 2017).

Figure 10. Montage of New Horizons images of Pluto’s small satellites, to scale, compared to the limb of Charon. Physical Properties The masses of the small moons have not been directly determined, though there is hope that mutual perturbations can eventually provide useful information. Albedos are higher than that of Charon, which is surprising if, as is likely, they formed from the same circum-Pluto debris disk following the Charon-forming impact. Their shapes are irregular, and both Kerberos and Hydra have a bi-lobed shape suggesting formation from the coalescence of two smaller bodies. Craters are visible on Hydra and Nix, with a density slightly higher than that of Charon (Weaver et al. 2016). The largest crater imaged on Nix is associated with both bright spots and dark reddish regions, due either to excavation of contrasting subsurface material, or contamination by the impactor. Surface chemistry New Horizons near-infrared spectroscopy of Nix, Hydra, and Kerberos revealed deep H2O ice absorption bands, consistent with clean coarse-grained ice (Cook et al. 2018). Styx is likely similar, as it shares the others’ high albedos. As on Charon, the 1.65 µm band indicates the ice is predominantly crystalline. These small bodies are clearly not cryovolcanically active, so some other mechanism must keep their ice crystalline. Nix and Hydra have prominent 2.21 µm absorptions tentatively attributed to the same ammoniated species as on Charon (Cook et al. 2018).

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System origin and evolution The high Charon/Pluto mass ratio is reminiscent of the Earth-Moon system. As with the latter system, the leading explanation for Pluto-Charon is that Charon was formed during a collision between Pluto and another KBO (Canup 2011). Although Charon would have initially orbited closer to Pluto, tidal dissipation within the two bodies drove them apart until they reached their current, doubly-synchronous state (Dobrovolskis et al. 1997). Assuming that the small satellites also formed during the giant impact, the outwards evolution of Charon also presumably pushed these bodies out into their present near-resonant configuration (Showalter & Hamilton 2015), although how this happened in detail is not currently understood (e.g. Walsh & Levison 2015). The survival of the small satellites also places constraints on how the Pluto-Charon system as a whole migrated outwards in response to Neptune’s outwards motion (Pires et al. 2015). If proto-Pluto had been fully differentiated, then Charon would consist almost entirely of ice, which is not the case (Canup 2011). On the other hand, a completely undifferentiated Pluto is hard to reconcile with the apparently icy nature of the four small moons (at least at their surfaces – density constraints are currently lacking). So the most likely proto-Pluto structure is a partially-differentiated body, with an icy shell but a rock-ice mixture in the interior (McKinnon et al. 2017). An only partially-differentiated proto-Pluto suggests that it cannot have formed while 26Al was still live, i.e. within the first 3 Myr of solar system history (McKinnon et al. 2017). Such a timescale makes it difficult to have formed the Pluto-Charon system via pebble accretion (Johansen et al. 2015), which requires nebular gas to still be present. Pebble accretion also has difficulty in explaining the (apparently) different bulk compositions of the small satellites compared to Pluto and Charon. Conversely, forming Pluto and Charon via more conventional coagulation (Kenyon & Bromley 2012) does not suffer from these problems and currently seems the more likely explanation. Conclusions The New Horizons data has revealed the Pluto system to be even more diverse and spectacular than anticipated. We already have at least plausible explanations for many aspects of the system, including the high Charon/Pluto mass ratio; the origin and nature of Sputnik Planitia; the distribution of many of Pluto’s other compositional provinces; the composition and structure of Pluto’s lower atmosphere; the extensional features and smooth plains of Charon; and Charon’s dark poles. However, many mysteries remain, such as the origin of many of Pluto’s peculiar endogenic and erosional features; the low temperature of Pluto’s upper atmosphere; and the origin and orbits of its small satellites. There is much still to learn from continued analysis of existing data, from future telescopic observations and, eventually, from future missions to the Pluto system.

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