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The primary objective of the Europa Clipper mission is to assess the habitability of Europa, an overarching goal that rests on improving our understanding of Europa’s interior structure, composition, and geologic activity. Here we describe the Gravity and Radio Science (G/RS) investigation. The primary measurement, the gravitational tidal Love number $k_{2}$ k 2 , will be an independent diagnostic of the presence of a global subsurface ocean, but G/RS will make a number of other key measurements related to Europa’s deep interior, silicate mantle-ocean interface, ice shell, ionosphere, and plasma environment. Although radio science is common to many missions, Europa Clipper’s orbit and spacecraft configuration during flybys present special challenges for the design of this experiment. The information obtained through G/RS will be complementary to the measurements by the other instruments onboard Europa Clipper, and their combined analysis will refine the geophysical understanding of Europa necessary to best assess its potential habitability.

Bierson and Nimmo [1] have proposed that, under certain circumstances, as Galilean-scale satellites accrete they may lose substantial (if not all) of their accreted water ice. This is not, however, a get out of jail free card for satellite accretion models or scenarios. Each satellite accretion model (of which there are many) must be evaluated against the parameters chosen in [1], and the modest ice content of Europa and the apparent anhydrous nature of Io accounted for. Here we evaluate published models in this light, and focus especially on the roles of background protojovian subnebula temperatures and pressures. Introduction. The formation of Jupiter in the core accretion–gas capture model inevitably ends with a circumplanetary accretion and/or decretion disk (CPD) around the planet. Numerous satellite formation models have been proposed [2-11], with different parameters for disk structure (surface density, temperature, opacity, viscosity). All rely on inferences and assumptions of mass and angular momentum inflow to Jupiter. Numerical gap opening calculations [e.g., 12] provided the basis for the gas-starved model [2-4]. Later 3D radiative-hydrodynamic inflow calculations prompted consideration of heliocentric planetesimal capture as the dominant source of satellite-building solids [13]. Hypotheses for dust and small pebble deposition range from distant infall [e.g., 14] to close-in infall and formation of an outflowing, decretion disk [15]. Multiple generations of satellites may form and be lost [3], Or maybe not, if a magnetospheric cavity opens up close to Jupiter and the innermost satellite, destined to become Io, stalls there [e.g., 16,17]. The thermal environment of Io and Europa’s formation could have been hot enough to largely devolatilize any small, accreting satellitesimals, including the ablated fragments of heliocentric planetesimals that encounter the CPD [8]. Some models call for accretion of Io and Europa in the cold outer disk followed by inward type I migration [e.g., 15,18]. Ultimate removal of major amounts of ice is a non-trivial requirement in such scenarios. Hydrodynamic Wind. We consider a water vapor atmosphere at the saturation vapor pressure (SVP) of a water ocean formed during accretion [1]. If the outflow wind speed u, which increases away from the surface, becomes transonic at the critical radius rc = GM/2c2 (where G is the gravitational constant, M the satellite mass, and c the isothermal sound speed), then the outflow can continue unimpeded [19,20]. For example, If Europa accreted with a 300 K water surface, the SVP would be 3.35 kPa, with a c = 372 m/s. The isothermal atmosphere is a favorable case for hydrodynamic escape, and because it admits simple analytic solutions for u(r), it is well-suited to this discussion. The critical distance rc = 7.4Re, where Re is Europa’s present radius. At the surface rs [20]: For the 300 K Europa case above, us ≈ 0.04 m/s, and for steady-state (mass flux conserved) outflow, both the thermodynamic and ram pressure at rc are ≈6 mPa. Such pressures are low compared with the CPD pressures in even gas-starved models [1,2], even far from Jupiter, which suggests that CPD details as well as accretional mass, radius and surface T, determine if blowoff is possible (e.g., Fig. 1). Figure 1. Escape of isothermal water vapor atmospheres from Io is suppressed by the back pressure of the CPD if the back pressure exceeds the wind's critical point pressure. E.g., for a nebular pressure of 1 Pa, an escaping wind begins to blow only for T>400 K. If the back pressure exceeds the critical pressure, the flow reverses sign and Io accretes nebular gas. Discussion. Ram and thermodynamic pressures at the critical, transonic radius are strong functions of satellite radius and accretional surface temperature. Larger values imply free flow to the background CPD, whereas smaller values (if below ambient) imply the wind is inhibited (the pressure boundary condition at rc cannot be met). Because saturation is assumed at the surface, the actual atmospheric pressure should follow the saturation vapor curve, implying even lower transonic pressures and more stringent limits [19,20]. For cold CPD conditions consistent with the condensation of water ice (2O loss will depend critically on accretion timescale and surface temperatures reached [1]. For accretion of icy satellitesimals in warm to hot CPD conditions, such as resulting from inward drift or direct capture from heliocentric orbit, the satellitesimals themselves should actively sublimate prior to accretion. Such comet-style loss would not in itself contribute to isotopic shifts [cf. 1]. Most importantly, however, the hydrodynamic loss model discussed here is an idealization, and implicitly assumes a static background gas subnebula (or lack thereof). The reality of satellite accretion involves a nebular headwind, which will act to sweep any water vapor atmosphere away from the growing satellite. This process is daunting to model, but will both act to accelerate water vapor loss (the ultimate limit being free sublimation) but at the same time increase sublimation cooling. Acknowledgement. WBM thanks NASA’s Europa Clipper project. References [1] Bierson, C. J. and F. Nimmo. 2020. ApJ, 897, L43. [2] Canup. R.M. and W.R. Ward. 2002. AJ, 124, 3404-3423. [3] Canup, R.M. and W.R. Ward. 2006. Nature, 441, 834-839. [4] Canup, R. M. and W. R. Ward. 2009. In Europa, 59-83. [5] Mosqueira, I., and P.R. Estrada. 2003. Icarus, 163, 198-231. [6] Mosqueira, I. and P.R. Estrada. 2003. Icarus, 163, 232-255. [7] Estrada, P. R., et al.. 2009. In Europa, 27-58. [8] Ronnet, T., et al. 2017. ApJ, 845, 92. [9] Ronnet, et al. 2018. AJ, 155, 224. [10] Ronnet, T. and A. Johansen. 2020. A&A, 633, 93. [11] Estrada, P.R., and I. Mosqueira. 2006. Icarus, 181, 486-509. [12] Lubow S. H., M. Seibert, and P. Artymowicz P. 1999. ApJ, 526, 1001-1012. [13] Schulik, M., et al. 2020. A&A, 642, A187. [14] Cilibrasi, M., et al. 2018. MNRAS, 480, 4355-4368. [15] Batygin, K. and A. Morbidelli. 2020. ApJ, 894, 143. [16] Sasaki, T., et al.. 2010. ApJ, 714, 1052-1064. [17] Batygin, K. 2018. ApJ, 155, 178. [18] Cilibrasi, M., et al. 2021. MNRAS, 504, 5455-5474. [19] Lehmer, et al. 2017. ApJ, 839, 32. [20] Zahnle, K.J. and D. C. Catling. 2017. ApJ, 843, 122.

Pluto and Charon are strikingly diverse in their range of geologies, surface compositions, and crater retention ages. This is despite the two having similar densities and presumed bulk compositions. Much of Pluto's surface reflects surface-atmosphere interactions and the mobilization of volatile ices by insolation. Abundant evidence, including past and present N2 ice glacial activity, implies that Pluto has undergone substantial climate evolution. An ancient impact basin contains a massive, convectively overturning N2 ice reservoir, whose position and surrounding tectonics suggest a subsurface ocean. Aligned blades of methane ice hundreds of meters tall, found only at high altitude, likely cover much of Pluto's low latitudes and may be a consequence of obliquity variation–driven volatile migration. Multikilometer-high possible cryovolcanic constructs and apparent fissure eruptions indicate relatively late endogenic activity on Pluto. Pluto's range of surface ages is extreme, whereas Charon's surface, while old, displays a large resurfaced plain and a globally engirdling extensional tectonic network attesting to earlier endogenic vigor. ▪ The vast N2 ice sheet Sputnik Planitia controls Pluto's atmosphere and climate, comparable in importance with the role of Greenland and Antarctica on the climate of Earth. ▪ Spectacular evidence for erosion such as now-unoccupied glacial valley networks implies a vigorous early climate, and more widespread N2 ice glaciation, on Pluto. ▪ Geological activity on both bodies requires or required sustained internal heat release and suggests a past (Charon) or present (Pluto) ammoniated, subsurface ocean. ▪ The varieties of geologic experience witnessed on Pluto and Charon should play out among the many and varied dwarf planets of the Kuiper belt.

The topography of Neptune’s large icy moon Triton could reveal important clues to its internal evolution, but has been difficult to determine. New global digital color maps for Triton have been produced as well as topographic data for

The topography of rifts on icy bodies can be used to probe their internal properties. Uplifted and curved rift flanks occur when the elastic thickness (te) of the crust is low, indicative of high heat flow out of the body. Stereo topography of Pluto shows no evidence of rift-flank uplift associated with large extensional graben to the west of Sputnik Planitia. Modeling the amount of topographic deflection expected from varying elastic thicknesses yields a conservative lower bound on te of 8 km at the time of deformation; the maximum implied heat flux since graben formation is 66–85 mWm−2. This upper bound is consistent with the predicted paleo-heat fluxes from radioactive decay (~6 mWm−2), but likely only marginally consistent with nearby apparently viscously-relaxed impact craters. Additionally, we also analyze the shear stresses on these faults and find that Pluto's faults only need to support low stresses in the range of 100–300 kPa to explain their dimensions.

Chaos terrains are characterized by disruption of preexisting surfaces into irregularly arranged mountain blocks with a chaotic appearance. Several models for chaos formation have been proposed, but the formation and evolution of this enigmatic terrain type has not yet been fully constrained. We provide extensive mapping of the individual blocks that make up different chaos landscapes, and present a morphological comparison of chaotic terrains found on Pluto, Jupiter's moon Europa, and Mars, using measurements of diameter, height, and axial ratio of chaotic mountain blocks. Additionally, we compare mountain blocks in chaotic terrain and fretted terrain on Mars. We find a positive linear relationship between the size and height of chaos blocks on Pluto and Mars, whereas blocks on Europa exhibit a flat trend as block height does not generally increase with increasing block size. Block heights on Pluto are used to estimate the block root depths if they were floating icebergs. Block heights on Europa are used to infer the total thickness of the icy layer from which the blocks formed. Finally, block heights on Mars are compared to potential layer thicknesses of near-surface material. We propose that the heights of chaotic mountain blocks on Pluto, Europa, and Mars can be used to infer information about crustal lithology and surface layer thickness., 45 pages, 3 tables, 9 figures, 5 appendices

We discuss in a thermodynamic, geologically empirical way the long-term nature of the stable majority ices that could be present in Kuiper Belt Object 2014 MU69 after its 4.6 Gyr residence in the EKB as a cold classical object. Considering the stability versus sublimation into vacuum for the suite of ices commonly found on comets, Centaurs, and KBOs at the average ~40K sunlit surface temperature of MU69 over Myr to Gyr, we find only 3 common ices that are truly refractory: HCN, CH3OH, and H2O (in order of increasing stability). NH3 and H2CO ices are marginally stable and may be removed by any positive temperature excursions in the EKB, as produced every 1e8 - 1e9 yrs by nearby supernovae and passing O/B stars. To date the NH team has reported the presence of abundant CH3OH and evidence for H2O on MU69s surface (Lisse et al. 2017, Grundy et al. 2020). NH3 has been searched for, but not found. We predict that future absorption feature detections will be due to an HCN or poly-H2CO based species. Consideration of the conditions present in the EKB region during the formation era of MU69 lead us to infer that it formed "in the dark", in an optically thick mid-plane, unable to see the nascent, variable, highly luminous Young Stellar Object-TTauri Sun, and that KBOs contain HCN and CH3OH ice phases in addition to the H2O ice phases found in their Short Period comet descendants. Finally, when we apply our ice thermal stability analysis to bodies/populations related to MU69, we find that methanol ice may be ubiquitous in the outer solar system; that if Pluto is not a fully differentiated body, then it must have gained its hypervolatile ices from proto-planetary disk sources in the first few Myr of the solar systems existence; and that hypervolatile rich, highly primordial comet C/2016 R2 was placed onto an Oort Cloud orbit on a similar timescale., Comment: 34 Pages, 5 Figures, 2 SOM Tables

A near-surface thermal model for Arrokoth is developed based on the recently released 105 facet model of the body. This thermal solution takes into account Arrokoth’s surface reradiation back onto itself. The solution method exploits Arrokoth’s periodic orbital character to develop a thermal response using a time-asymptotic solution method, which involves a Fourier transform solution of the heat equation, an approach recently used by others. We display detailed thermal solutions assuming that Arrokoth’s near-surface material’s thermal inertia  = 2.5 W/m−2 K−1 s1/2. We predict that at New Horizons’ encounter with Arrokoth, its encounter hemisphere surface temperatures were ∼57–59 K in its polar regions, 30–40 K in its equatorial zones, and 11–13 K for its winter hemisphere. Arrokoth’s orbitally averaged temperatures are around 30–35 K in its polar regions and closer to 40 K near its equatorial zones. Thermal reradiation from the surrounding surface amounts to less than 5% of the total energy budget, while the total energy ensconced into and exhumed out of Arrokoth’s interior via thermal conduction over one orbit is about 0.5% of the total energy budget. As a generalized application of this thermal modeling together with other Kuiper Belt object origins considerations, we favor the interpretation that New Horizons’ REX instrument’s 29 ± 5 K brightness temperature measurement is consistent with Arrokoth’s near-surface material being made of sub-to-few-millimeter-size tholin-coated amorphous H2O ice grains with 1 W/m−2 K−1 s1/2 <  < 10–20 W/m−2 K−1 s1/2 and which are characterized by an X-band emissivity in the range 0.9 and 1.

Proposed to pierce the heliopause and explore proximal interstellar space, the Interstellar Probe (IP) mission would have extraordinarily valuable flyby opportunities with other planetary bodies along its flight path. Gas and ice giant encounters are one kind of opportunity, but the most likely such flyby would be with Jupiter, New Horizons style. Jupiter and the Galilean satellites should be well covered scientifically in the years to come, but the jovian magnetosphere and especially the long magnetotail would be prime targets for IP instruments. Superior opportunities for scientific discovery would come with the Kuiper Belt (KB) and beyond, however, as reaching such distant, unexplored worlds remains challenging otherwise. Whatever the heliospheric community decides as the best and most scientifically advantageous outbound asymptote for IP, there will be multiple possible KB encounters, both with dwarf planets and smaller bodies, on either the chosen or very similar trajectories. Given the likely timeframe for the launch of IP (2030 or later), many more potential targets will have been discovered in the intervening years. The discoveries of New Horizons at the Pluto system (dwarf planet) and Arrokoth (cold classical KB object) have proven revolutionary [1,2]; IP would no doubt achieve the same, even before reaching the heliopause, but only if it carries the necessary instrumentation. The minimum would be a visible wavelength camera and a near-infrared imaging spectrometer. Fast flyby speeds are manageable; New Horizons proved that with the Arrokoth encounter. IP will likely be a spinning spacecraft, however, to maximize the capabilities of its primary fields and particle instrument suite, which poses a challenge to any KB object flyby. One strategy is simply to adapt to the spin rate, in the manner that Juno has at Jupiter (i.e., line scanning). Another would be for IP to have the ability to vary (slow) its spin rate, or even to go into 3-axis mode for encounters. In either case, a gimbal in the optical path would greatly reduce (or eliminate) the need for additional spacecraft repointings. Bodies in the so-called detached region of the KB, or inner Oort Cloud (OC), such as Sedna, would be especially tempting targets, as the likelihood of other missions to this region of the solar system are essentially zero. That is, unless there really is a major planet on a distant eccentric orbit shepherding the orbits of the detached/inner OC bodies. This is a big if of course, but if such a planet exists, the drive to visit it will be great, which alone could be used to leverage the launch of IP. Such an encounter would demand a more comprehensive remote sensing suite, in which case a daughter spacecraft to IP might be considered as an engineering option. A fully autonomous RTG-powered daughter craft would probably put too much of a lean on IP’s mass and C3, but a battery-powered 3-axis stabilized daughter craft could, potentially, carry out the critical rendezvous measurements and transmit the results to IP, which from there would be transmitted to the Earth. Finally, the dream of sending a full IP (1000 AU in 50 yr, which would require speeds of ~100 km/s) should not preclude thinking about tempering ambitions in the face of financial or engineering realities. Even a mission that “simply” doubles the asymptotic velocity of Voyager 1 (the fastest of the 5 interstellar precursor missions) could accomplish much of IP’s science in a similar timeframe. Plus there may be a speed which is “too fast,” even for fields and particles measurements. The technological ability to reach ~35 km/s would also allow us to rendezvous with, or even ride along with interstellar visitors (e.g., 1I/ 'Oumuamua and 2I/Borisov) as they head back into the depths of space. [1] Stern S. A., Grundy W. M., McKinnon W. B., Weaver H. A., and Young L. A. (2018) The Pluto system after New Horizons. Annu. Rev. Astron. Astrophys., 56, 357–392, doi: 0.1146/annurevastro-081817-051935. [2] Stern S. A., et al. (2019) Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object. Science, 364, eaaw9771, doi: 10.1126/science.aaw9771.

Cassini observations have led to several efforts to map the global topography of Enceladus [1-5]. The most prominent large-scale features include 100+ km scale basins (1), and the depressed south polar terrain (SPT) [2]. Here we present a revised and complete stereogrammetric map of Enceladus topography at 0.4-km scales (Fig. 1) including the north polar regions mapped at the end of the mission and that does not require interpolation. We reevaluate the distribution and origin of large-scale topographic deviations and compare north and south polar regions. Introduction Stereogrammetric digital elevation models (DEMs) of Enceladus were constructed from Cassini stereo pairs and mosaics using the same software used to map the topography of Pluto, Charon, and other icy bodies [6]. As each stereo pairing has a unique stereo geometry, the vertical precision (or uncertainty) is variable across Enceladus and ranges from Basins Schenk and McKinnon [1] discovered several oval-shaped depressions on the trailing hemisphere circa 150-km across and 1-1.5 km deep in their partial DEM. They found an isostatic explanation for these depressions to be most physically plausible, and concluded that basins could be sites of cold, dense, immobile (or downwelling), partially clathrated ice, or alternatively, manifestations of deeper convective upwellings (and porosity reduction), either now or in the past. Tajeddine et al. [4] extended the global topography map through a spherical harmonic representation (to degree 16) based on both limb profiles [2] and stereogrammetric control points, and reported that additional depressions occurred on the remaining surface and that some formed an apparent great circle; [4] further suggested the circle was offset due to true polar wander (TPW). Our new map is much denser in terms of stereo-derived elevations, yielding ~5.2×106 measurements, spaced every 0.4 km or so. We find that Enceladus’ surface topographic signature is both complex and subtle. Contrary to our expectations [1], no additional deep basins similar to those reported by [1] occur in the leading hemisphere. Without these basins the case for a great circle alignment (and by extension any TPW implied) is weakened. We also note that the average ice shell thickness of ~20-25 km implied by Enceladus’ libration [7] does not easily support a convective origin for the “E basins” proposed by [4]; the present-day shell is too thin [cf. 8]. The circular resurfaced zone that dominates the leading hemisphere [9] has a warped signature suggestive of large rounded polygons separated by elongate depressions. Some of the highest elevations occur in the southern part of this resurfaced zone. The topographic signature of the trailing hemisphere zone is different again. Neither exhibit the concentric rings that might be expected from topographic relaxation or thickening of a region similar to the current depressed SPT, if they were once active in a similar way and ceased thermal activity. The two resurfaced zones may have responded differently to regional stresses than the older cratered terrains between them. 2. North vs. South Polar Terrains A fundamental characteristic of Enceladus is that the SPT is vigorously resurfacing and active but the cratered northern polar terrains (NPT) are not despite models that show similar tidal heating at both poles [10]. The NPT were the last areas to be mapped by Cassini. Our global DEM confirms that the SPT (>70°S) is depressed ~400 m relative to the best-fit ellipsoid (confirming [2]), but the NPT (>70°N) are elevated ~0.7 km above. The north polar imaging confirms that tectonic features with fresh blue-ice signatures do cross the NPT. The apparent broad doming of the NPT might suggest that either the ice shell is relatively thick there (and perhaps more resistant to tidal heating relative to the original southern terrain) or that incipient resurfacing is preceded by doming before the crustal replacement cycle begins. 3. Global Aspects and Conclusions A global hypsogram of Enceladus (Fig. 2) using our new complete DEM (Fig. 1) shows that global topographic deviations from the first-order triaxial ellipsoid [2] are at the 95th percentile within ±1 km of the triaxial ellipsoid, confirming earlier incomplete maps [1,4]. Two ‘humps’ can be identified in the hypsogram at -0.4 km (representing the SPT) and at -1 km (representing the basins). This is possibly the narrowest global topographic signature in the Solar System for an ellipsoidal body [11], although it might be matched by Europa, Ganymede and Triton (work in progress), and is consistent with expected subdued topography of an active icy ocean world. The preservation of the deep 1-1.5 km deep basins is an unsolved problem [1,3], but might reflect patterns of heat flow from the core [12]. Enceladus’ topographic signature is complicated, with indications that older cratered terrains and the three younger resurfaced terrains responded differently to stresses. The distribution of large deep basins is enigmatic, and although they do not form a great circle, lower-lying topography is more prevalent at mid-northern latitudes. More complete analyses of higher harmonics are underway and may yield additional insights. References [1] Schenk, P., and McKinnon, W., GRL 36, L16202, 2009. [2] Thomas, P.C, et al., Icarus 190, 573–584, 2007. [3] Nimmo, F., et al., JGR 116, E11001, 2011. [4] Tajeddine, R., et al., Icarus 295, 46-60, 2017. [5] Bland, M., et al., 4th Planetary Data Workshop, abs. #7048, 2019. [6]. Schenk, P.M., et al., Icarus, 314, 400-433, 2018a. [7] Thomas, P.C, et al., Icarus 264, 37-47, 2016. [8] Barr A.C., and McKinnon, W.B., GRL 34, L09202, 2007. [9]. Crow-Willard, E., R. Pappalardo, JGR., 120, 928-950. [10] Nimmo, F., et al., in Enceladus and the Icy Moons of Saturn, UA Press, pp. 79-94, 2018. [11] Schenk, P., et al., in Enceladus and the Icy Moons of Saturn, UA Press, pp. 237-265, 2018. [12] Čadek, O., et al., Icarus 319, 476-484, 2019.

Examining Arrokoth The New Horizons spacecraft flew past the Kuiper Belt object (486958) Arrokoth (also known as 2014 MU 69 ) in January 2019. Because of the great distance to the outer Solar System and limited bandwidth, it will take until late 2020 to downlink all the spacecraft's observations back to Earth. Three papers in this issue analyze recently downlinked data, including the highest-resolution images taken during the encounter (see the Perspective by Jewitt). Spencer et al. examined Arrokoth's geology and geophysics using stereo imaging, dated the surface using impact craters, and produced a geomorphological map. Grundy et al. investigated the composition of the surface using color imaging and spectroscopic data and assessed Arrokoth's thermal emission using microwave radiometry. McKinnon et al. used simulations to determine how Arrokoth formed: Two gravitationally bound objects gently spiraled together during the formation of the Solar System. Together, these papers determine the age, composition, and formation process of the most pristine object yet visited by a spacecraft. Science , this issue p. eaay3999 , p. eaay3705 , p. eaay6620 ; see also p. 980

We used existing data from the New Horizons LORRI camera to measure the optical-band ($0.4\lesssim\lambda\lesssim0.9{\rm\mu m}$) sky brightness within seven high galactic latitude fields. The average raw level measured while New Horizons was 42 to 45 AU from the Sun is $33.2\pm0.5{\rm ~nW ~m^{-2} ~sr^{-1}}.$ This is $\sim10\times$ darker than the darkest sky accessible to the {\it Hubble Space Telescope}, highlighting the utility of New Horizons for detecting the cosmic optical background (COB). Isolating the COB contribution to the raw total requires subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection-limit within the fields, and diffuse Milky Way light scattered by infrared cirrus. We remove newly identified residual zodiacal light from the IRIS $100\mu$m all sky maps to generate two different estimates for the diffuse galactic light (DGL). Using these yields a highly significant detection of the COB in the range ${\rm 15.9\pm 4.2\ (1.8~stat., 3.7~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 18.7\pm 3.8\ (1.8~stat., 3.3 ~sys.)~ nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $\mu$m. Subtraction of the integrated light of galaxies (IGL) fainter than the photometric detection-limit from the total COB level leaves a diffuse flux component of unknown origin in the range ${\rm 8.8\pm4.9\ (1.8 ~stat., 4.5 ~sys.) ~nW ~m^{-2} ~sr^{-1}}$ to ${\rm 11.9\pm4.6\ (1.8 ~stat., 4.2 ~sys.) ~nW ~m^{-2} ~sr^{-1}}$. Explaining it with undetected galaxies requires the galaxy-count faint-end slope to steepen markedly at $V>24$ or that existing surveys are missing half the galaxies with $V< 30.$, Comment: 32 pages, 20 figures. Accepted for publication in ApJ

Planetary impact events eject large volumes of surface material. Crater excavation processes are difficult to study, and in particular the details of individual ejecta fragments are not well understood. A related, enduring issue in planetary mapping is whether a given crater resulted from a primary impact (asteroid or comet) or instead is a secondary crater created by an ejecta fragment. With mapping and statistical analyses of six lunar secondary crater fields (including Orientale, Copernicus, and Kepler) we provide three new constraints on these issues: 1) estimation of the maximum secondary crater size as a function of distance from a primary crater on the Moon, 2) estimation of the size and velocity of ejecta fragments that formed these secondaries, and 3) estimation of the fragment size ejected at escape velocity. Through this analysis, we confirmed and extended a suspected scale-dependent trend in ejecta size-velocity distributions. Maximum ejecta fragment sizes fall off much more steeply with increasing ejection velocity for larger primary impacts (compared to smaller primary impacts). Specifically, we characterize the maximum ejecta sizes for a given ejection velocity with a power law, and find the velocity exponent varies between approximately -0.3 and -3 for the range of primary craters investigated here (0.83-660 km in diameter). Data for the jovian moons Europa and Ganymede confirm similar trends for icy surfaces. This result is not predicted by analytical theories of formation of Grady-Kipp fragments or spalls during impacts, and suggests that further modeling investigations are warranted to explain this scale-dependent effect., Comment: Including main manuscript and supplement: 47 pages, 20 Figures, 5 Tables

The Pluto-Charon system provides a broad variety of constraints on planetary formation, composition, chemistry, and evolution. Pluto was the first body to be discovered in what is now known as the Kuiper belt, its orbit ultimately becoming a major clue that the giant planets underwent substantial orbital migration early in Solar System history. This migration has been linked to an early instability in the orbits of the giant planets and the formation of the Kuiper belt itself, from an ancestral trans-Neptunian planetesimal disk that included Pluto. Pluto-Charon is emblematic of what are now recognized as small or dwarf planets. Far from being a cold, dead, battered icy relic, Pluto displays evidence of a complex geological history, with ongoing processes including tectonism, cryovolcanism, solid-state convection, glacial flow, atmospheric circulation, surface-atmosphere volatile exchange, aeolian processes, and atmospheric photochemistry, microphysics, and haze formation. Despite Pluto's relatively modest scale, the combination of original accretional heat, long-term internal radiogenic heat release, and external solar forcing, when combined with sufficiently volatile (and thus mobile) materials, yields an active world. Pluto may have inherited a large organic mass fraction during accretion, which may responsible, in part, for its surface and atmospheric volatiles. Charon, Pluto's major moon, displays evidence of extensive early tectonism and cryovolcanism. Dwarf planets are thus truly planetary in terms of satellite systems and geological and atmospheric complexity (if not ongoing activity). What they may lack in mass is made up in number, and the majority of the Solar System's dwarf planets remain undiscovered.

The 2015 New Horizons flyby through the Pluto system produced the first high-resolution topographic maps of Pluto and Charon, the most distant objects so mapped. Global integrated mosaics of the illuminated surface of Pluto's large icy moon Charon have been produced using both framing camera and line scan camera data (including four-color images at up to 1.47 km pixel scales), showing the best resolution data at all areas of the surface. Digital elevation models (DEMs) with vertical precisions of up to ∼0.1 km were constructed for ∼40% of Charon using stereo imagery. Local radii estimates for the surface were also determined from the cartographic control network solution for the LORRI framing camera data, which validate the stereo solutions. Charon is moderately cratered, the largest of which is ∼250-km across and ∼6 km deep. Charon has a topographic range over the observed hemisphere from lowest to highest of ∼19 km, the largest topographic amplitude of any mid-sized icy body (including Ceres) other than Iapetus. Unlike Saturn's icy moons whose topographic signature is dominated by global relaxation of topography and subsequent impact cratering, large-scale tectonics and regional resurfacing dominate Charon's topography. Most of Charon's encounter hemisphere north of the equator (Oz Terra) is broken into large polygonal blocks by a network of wide troughs with typically 3–6 km relief; the deepest of these occur near the illuminated pole and are up to 13 km deep with respect to the global mean radius, the deepest known surfaces on Charon. The edge of this terrain is defined by large tilted blocks sloping ∼5° or so, the crests of which rise to 5 or 6 km above Charon mean, the highest known points on Charon. The southern resurfaced plains, Vulcan Planitia, consist of rolling plains, locally fractured and pitted, that are depressed ∼1 km below the mean elevation of the disrupted northern terrains of Oz Terra that comprise much of the northern hemisphere (but ∼2–2.5 km below the surfaces of the blocks themselves). These plains roll downward gently to the south with a topographic range of ∼5 km. The outer margins of Vulcan Planitia along the boundary with Oz Terra form a 2-3-km-deep trough, suggesting viscous flow along the outer margins. Isolated massifs 2–4 km high, also flanked by annular moats, lie within the planitia itself. The plains may be formed from volcanic resurfacing of cryogenic fluids, but the tilted blocks along the outer margins and the isolated and tilted massifs within Vulcan Planitia also suggest that much of Charon has been broken into large blocks, some of which have been rotated and some of which have foundered into Charon's upper “mantle”, now exposed as Vulcan Planitia, a history that may be most similar to the disrupted terrains of Ariel.

The 2015 New Horizons flyby has produced the first high-resolution maps of morphology and topography of Pluto and Charon, the most distant objects so mapped. Global integrated mosaics of Pluto were produced using both LORRI framing camera and MVIC line scan camera data, showing the best resolution data obtained for all areas of the illuminated surface, ∼78% of the body. A unique feature of the Pluto imaging data set is the observation of terrains illuminated only by light scattered from atmospheric haze, allowing us to map terrains in the southern hemisphere that would otherwise have been in darkness. MVIC 4-color data were combined with the panchromatic map to produce full color global maps. Digital elevation models (DEMs) over ∼42% of Pluto were produced using combinations of MVIC hemispheric scans and LORRI mosaics, from which slopes at scales of ∼1 km can be determined. Pluto can be divided into regions each with distinct topographic signatures, corresponding with major physiographic terrain types. Large areas of Pluto are comprised of low-relief moderately cratered plains units. Deeply pitted and glaciated plains east of Sputnik Planitia are elevated ∼0.7 km. The most dominant topographic feature on Pluto is the 1200-by-2000-km wide depression enclosing the bright Sputnik Planitia ice sheet, the surface of which is 2.5-to-3.5 km deep (relative to the rim) and ∼2 km deep relative to the mean radius. The partial ring of steep-sided massifs, several of which are more than 5 km high, along the western margins of Sputnik Planitia produce some of the locally highest and steepest relief on Pluto, with slopes of 40–50°. The second major topographic feature is a complex, eroded, ridge-trough system ∼300–400 km wide and at least 3200 km long extending north-to-south along the 155° meridian. This enormous structure has several kilometers of relief. It may predate the large impact event forming the basin, though some post-Sputnik Planitia deformation is evident. The large depressed, partially walled plain, Hyecho Palus, lies due southwest of Sputnik Planitia. Near the center of Hyecho Palus lie the circular constructional edifices Wright and Piccard Montes. Wright Mons rises 4.5 km above these plains, with a central depression ∼4.5 km deep, whereas Piccard Mons, best observed in haze-light, rises ∼5.5 km above the plains but has a bowl-shaped central depression ∼5.5 km below the plains for a total relief of up to 11 km, the greatest observed on Pluto. Both of these features are interpreted as constructional (volcanic?) in nature. Additional prominent topographic features include a 2–3 km high and ∼600 km wide dome centered on the illuminated IAU pole and the amoeboidal plateaus of “bladed” terrains in the equatorial region, which rise 2–5 km above local terrains and are the highest standing geologic units on the encounter hemisphere. The mean elevations in the integrated DEM for the two radio occultation areas are consistent with the 5–6 km difference in elevation as determined independently by the radio experiment, and a limb profile near the egress point confirms the presence of elevated bladed terrains in that area. Local relief of 3–5 km at massifs, troughs and pits supports conclusions that the icy shell of Pluto is relatively rigid. Numerous examples of topographic control of ice or frost deposition occur across Pluto, including the distinct coloration of the polar dome, the elevated terrains of eastern Tombaugh Regio, and along the ridge-trough system, where ridge tops and fossae rims are covered in different ices than at lower elevations. The topographic hypsogram of Pluto's encounter hemisphere is strongly bimodal due to the large Sputnik Planitia depression. Otherwise the topographic signature of Pluto is controlled by deviations from the otherwise dominant low plains, including elevated bladed terrain plateaus and the depressed volcanic province including Wright and Piccard Montes.

Observations by the New Horizons spacecraft have determined that Pluto has a larger bulk density than Charon by 153 ± 44 kg m − 3 (2σ uncertainty). We use a thermal model of Pluto and Charon to determine if this density contrast could be due to porosity variations alone, with Pluto and Charon having the same bulk composition. We find that Charon can preserve a larger porous ice layer than Pluto due to its lower gravity and lower heat flux but that the density contrast can only be explained if the initial ice porosity is ≳ 30%, extends to ≳100 km depth and Pluto retains a subsurface ocean today. We also find that other processes such as a modern ocean on Pluto, self-compression, water-rock interactions, and volatile (e.g., CO) loss cannot, even in combination, explain this difference in density. Although an initially high porosity cannot be completely ruled out, we conclude that it is more probable that Pluto and Charon have different bulk compositions. This difference could arise either from forming Charon via a giant impact, or via preferential loss of H2O on Pluto due to heating during rapid accretion.

Viscously relaxed craters provide a window into the thermal history of Ganymede, a satellite with copious geologic signs of past high heat flows. Here we present measurements of relaxed craters in four regions for which suitable imaging exists: near Anshar Sulcus, Tiamat Sulcus, northern Marius Regio, and Ganymede's south pole. We describe a technique to measure apparent depth, or depth of the crater with respect to the surrounding terrain elevation. Measured relaxation states are compared with results from finite element modeling to constrain heat flow scenarios [see companion paper: Bland et al. (2017)]. The presence of numerous, substantially relaxed craters indicates high heat flows—in excess of 30–40 mW m−2 over 2 Gyr, with many small (

Ganymede’s bright terrain formed during a near-global resurfacing event (or events) that produced both heavily tectonized and relatively smooth terrains. The mechanism(s) by which resurfacing occurred on Ganymede (e.g., cryovolcanic or tectonic), and the relationship between the older, dark and the younger, bright terrain are fundamental to understanding the geological evolution of the satellite. Using a two-dimensional numerical model of lithospheric extension that has previously been used to successfully simulate surface deformation consistent with grooved terrain morphologies, we investigate whether large-amplitude preexisting topography can be resurfaced (erased) by extension (i.e., tectonic resurfacing). Using synthetically produced initial topography, we show that when the total relief of the initial topography is larger than 25–50 m, periodic groove-like structures fail to form. Instead, extension is localized in a few individual, isolated troughs. These results pose a challenge to the tectonic resurfacing hypothesis. We further investigate the effects of preexisting topography by performing suites of simulations initialized with topography derived from digital terrain models of Ganymede’s surface. These include dark terrain, fresh (relatively deep) impact craters, smooth bright terrain, and a viscously relaxed impact crater. The simulations using dark terrain and fresh impact craters are consistent with our simulations using synthetic topography: periodic groove-like deformation fails to form. In contrast, when simulations were initialized with bright smooth terrain topography, groove-like deformation results from a wide variety of heat flow and surface temperature conditions. Similarly, when a viscously relaxed impact crater was used, groove-like structures were able to form during extension. These results suggest that tectonic resurfacing may require that the amplitude of the initial topography be reduced before extension begins. We emphasize that viscous relaxation may be the key to enabling tectonic resurfacing, as the heat fluxes associated with groove terrain formation are also capable of reducing crater topography through viscous relaxation. For long-wavelength topography (large craters) viscous relaxation is unavoidable. We propose that the resurfacing of Ganymede occurred through a combination of viscous relaxation, tectonic resurfacing, cryovolcanism and, at least in a few cases, band formation. Variations in heat flow and strain magnitudes across Ganymede likely produced the complex variety of terrain types currently observed.

A search for temporal changes on Pluto and Charon was motivated by (1) the discovery of young surfaces in the Pluto system that imply ongoing or recent geologic activity, (2) the detection of active plumes on Triton during the Voyager 2 flyby, and (3) the abundant and detailed information that observing geologic processes in action provides about the processes. A thorough search for temporal changes using New Horizons images was completed. Images that covered the same region were blinked and manually inspected for any differences in appearance. The search included full-disk images such that all illuminated regions of both bodies were investigated and higher resolution images such that parts of the encounter hemispheres were investigated at finer spatial scales. Changes of appearance between different images were observed but in all cases were attributed to variability of the imaging parameters (especially geometry) or artifacts. No differences of appearance that are strongly indicative of a temporal change were found on the surface or in the atmosphere of either Pluto or Charon. Limits on temporal changes as a function of spatial scale and temporal interval during the New Horizons encounter are determined. The longest time interval constraint is one Pluto/Charon rotation period (~6.4 Earth days). Contrast reversal and high-phase bright features that change in appearance with solar phase angle are identified. The change of appearance of these features is most likely due to the change in phase angle rather than a temporal change. Had active plumes analogous to the plumes discovered on Triton been present on the encounter hemispheres of either Pluto or Charon, they would have been detected. The absence of active plumes may be due to temporal variability (i.e., plumes do occur but none were active on the encounter hemispheres during the epoch of the New Horizons encounter ..., Accepted for publication in Icarus

We show that Ceres’ measured degree-2 zonal gravity, J 2 , is smaller by about 10% than that derived assuming Ceres’ rotational flattening, as measured by Dawn, is hydrostatic. Irrespective of Ceres’ radial density variation, as long as its internal structure is hydrostatic the J 2 predicted from the shape model is consistently larger than measured. As an explanation, we suggest that Ceres’ current shape may be a fossil remnant of faster rotation in the geologic past. We propose that up to ∼7% of Ceres’ previous spin angular momentum has been removed by dynamic perturbations such as a random walk due to impacts or a loss of satellite that slowed Ceres spin as it tidally evolved outward. As an alternative, we also consider a formal degree-2 admittance solution, from which we infer a range of possible non-hydrostatic contributions to J 2 from uncompensated, deep-seated density anomalies. We show that such density anomalies could be due to low order convection or upwelling. The normalized moments-of-inertia derived for the two explanations – faster paleospin and deep-seated density anomalies – range between 0.353 ± 0.009 and 0.375 ± 0.001 for a spherically equivalent Ceres, which can be used as constraints on more complex Ceres interior models.

Bladed Terrain on Pluto consists of deposits of massive CH 4 , which are observed to occur within latitudes 30° of the equator and are found almost exclusively at the highest elevations (> 2 km above the mean radius). Our analysis indicates that these deposits of CH 4 preferentially precipitate at low latitudes where net annual solar energy input is lowest. CH 4 and N 2 will both precipitate at low elevations. However, since there is much more N 2 in the atmosphere than CH 4 , the N 2 ice will dominate at these low elevations. At high elevations the atmosphere is too warm for N 2 to precipitate so only CH 4 can do so. We conclude that following the time of massive CH 4 emplacement; there have been sufficient excursions in Pluto's climate to partially erode these deposits via sublimation into the blades we see today. Blades composed of massive CH 4 ice implies that the mechanical behavior of CH 4 can support at least several hundred meters of relief at Pluto surface conditions. Bladed Terrain deposits may be widespread in the low latitudes of the poorly seen sub-Charon hemisphere, based on spectral observations. If these locations are indeed Bladed Terrain deposits, they may mark heretofore unrecognized regions of high elevation.

NASA's New Horizons mission within the New Frontiers program; NASA's Mars Data Analysis Program [NNX15AM48G]

International audience; During its departure from Pluto, New Horizons used its LORRI camera to image a portion of Pluto's southern hemisphere that was in a decades-long seasonal winter darkness, but still very faintly illuminated by sunlight reflected by Charon. Recovery of this faint signal was technically challenging. The bright ring of sunlight forwardscattered by haze in the Plutonian atmosphere encircling the nightside hemisphere was severely overexposed, defeating the standard smeared-charge removal required for LORRI images. Reconstruction of the overexposed portions of the raw images, however, allowed adequate corrections to be accomplished. The small solar elongation of Pluto during the departure phase also generated a complex scattered-sunlight background in the images that was three orders of magnitude stronger than the estimated Charon-light flux (the Charon-light flux is similar to the flux of moonlight on Earth a few days before first quarter). A model background image was constructed for each Pluto image based on principal component analysis applied to an ensemble of scattered-sunlight images taken at identical Sun−spacecraft geometry to the Pluto images. The recovered Charon-light image revealed a high-albedo region in the southern hemisphere. We argue that this may be a regional deposit of N 2 or CH 4 ice. The Charon-light image also shows that the south polar region currently has markedly lower albedo than the north polar region of Pluto, which may reflect the sublimation of N 2 ice or the deposition of haze particulates during the recent southern summer.

One of the clearest but unresolved questions for Europa is the thickness of its icy shell. Europa's surface is resplendent with geological features that bear on this question, and ultimately on its interior, geological history, and astrobiological potential. We characterize the size and topographic expression of circular and subcircular features created by endogenic thermal and tectonic disturbances on Europa: pits, uplifts, and small, subcircular chaos. We utilize the medium-resolution Galileo regional maps (RegMaps), as well as high-resolution regions, digital elevation models derived from albedo-controlled photoclinometry, and in some cases stereo-controlled photoclinometry. While limited in extent, the high-resolution images are extremely valuable for detecting smaller features and for overall geomorphological analysis. A peak in the size-distribution for all features is found at ~5–6 km in diameter and no pits smaller than 3.3 km in diameter were found in high resolution images. Additionally, there is a trend for larger pits to be deeper, and larger uplifts to be higher. Our data support a diapiric or intracrustal sill interpretation (as opposed to purely non-intrusive, melt-through models) and place a lower limit on ice shell thickness at the time of feature formation of 3-to-8 km, assuming isostasy and depending on the composition of the ice and underlying ocean.

New Horizons images of the illuminated nonencounter hemisphere (far side) of Charon display geomorphic features that are consistent with features observed in the highest-resolution images of the encounter hemisphere. Scarps, ridges, craters, and one area of smooth plains are identified. These features support previous hypotheses of global expansion and large-scale faulting across this icy world.

Measurement of crater depths in Ganymede’s dark terrain have revealed substantial numbers of unusually shallow craters indicative of viscous relaxation [see companion paper: Singer, K.N., Schenk, P. M., Bland, M.T., McKinnon, W.B., (2017). Relaxed impact craters on Ganymede: Regional variations and high heat flow. Icarus, submitted]. These viscously relaxed craters provide insight into the thermal history of the dark terrain: the rate of relaxation depends on the size of the crater and the thermal structure of the lithosphere. Here we use finite element simulations of crater relaxation to constrain the heat flux within the dark terrain when relaxation occurred. We show that the degree of viscous relaxation observed cannot be achieved through radiogenic heating alone, even if all of the relaxed craters are ancient and experienced the high radiogenic fluxes present early in the satellite’s history. For craters with diameter ≥ 10 km, heat fluxes of 40–50 mW m − 2 can reproduce the observed crater depths, but only if the fluxes are sustained for ∼1 Gyr. These craters can also be explained by shorter-lived “heat pulses” with magnitudes of ∼100 mW m − 2 and timescales of 10–100 Myr. At small crater diameters (4 km) the observed shallow depths are difficult to achieve even when heat fluxes as high as 150 mW m − 2 are sustained for 1 Gyr. The extreme thermal conditions required to viscously relax small craters may indicate that mechanisms other than viscous relaxation, such as topographic degradation, are also in play at small crater diameters. The timing of the relaxation event(s) is poorly constrained due to the sparsity of adequate topographic information, though it likely occurred in Ganymede’s middle history (neither recently, nor shortly after satellite formation). The consistency between the timing and magnitude of the heat fluxes derived here and those inferred from other tectonic features suggests that a single event caused both Ganymede’s tectonic deformation and its crater relaxation. Future observations should permit more robust determination of the relative timing of the heating event that caused viscous relaxation.

The highlands region north and east of Sputnik Planitia can be subdivided into seven terrain types based on their physiographic expression. The northern rough uplands are characterized by jagged uplands and broad troughs, and it may contain a deeply-eroded ancient mantle. Dissected terrain has been interpreted to have been eroded by paleo-glaciation. The smooth uplands and pits terrain contains broad, rolling uplands surrounding complexes of pits, some of which contain smooth floors. The uplands are mantled by smooth-surfaced deposits possibly derived from adjacent pits through low-power explosive cryovolcanism or through slow vapor condensation. The eroded smooth uplands appear to have originally been smooth uplands and pits terrain modified by small-scale sublimation pitting. The bright pitted uplands features intricate texturing by reticulate ridges that may have originated by sublimation erosion, volatile condensation, or both. The bladed terrain is characterized by parallel ridges oriented north–south and is discussed in a separate paper. The dark uplands are mantled with reddish deposits that may be atmospherically deposited tholins. Their presence has affected long-term landform evolution. Widespread pit complexes occur on most of the terrain units. Most appear to be associated with tectonic lineations. Some pits are floored by broad expanses of ices, whereas most feature deep, conical depressions. A few pit complexes are enclosed by elevated rims of uncertain origin.

International audience; The geology and stratigraphy of the feature on Pluto informally named Sputnik Planitia is documented through geologic mapping at 1:2,000,000 scale. All units that have been mapped are presently being affected to some degree by the action of flowing N2 ice. The N2 ice plains of Sputnik Planitia display no impact craters, and are undergoing constant resurfacing via convection, glacial flow and sublimation. Condensation of atmospheric N2 onto the surface to form a bright mantle has occurred across broad swathes of Sputnik Planitia, and appears to be partly controlled by Pluto's obliquity cycles. The action of N2 ice has been instrumental in affecting uplands terrain surrounding Sputnik Planitia, and has played a key role in the disruption of Sputnik Planitia's western margin to form chains of blocky mountain ranges, as well in the extensive erosion by glacial flow of the uplands to the east of Sputnik Planitia.

Fields of pits, both large and small, in Tombaugh Regio (Sputnik Planitia, and the Pitted Uplands to the east), and along the scarp of Piri Rupes, are examples of landscapes on Pluto where we conclude that sublimation drives their formation and evolution. Our heuristic modeling closely mimics the form, spacing, and arrangement of a variety of Tombaugh Regio's pits. Pluto's sublimation modified landforms appear to require a significant role for (diffusive) mass wasting as suggested by our modeling. In our models, the temporal evolution of pitted surfaces is such that initially lots of time passes with little happening, then eventually, very rapid development of relief and rapid sublimation. Small pits on Sputnik Planitia are consistent with their formation in N2-dominated materials. As N2-ice readily flows, some other ``stiffer'' volatile ice may play a role in supporting the relief of sublimation degraded landforms that exhibit several hundred meters of relief. A strong candidate is CH4, which is spectroscopically observed to be associated with these features, but the current state of rheological knowledge for CH4 ice at Pluto conditions is insufficient for a firm assessment.

Pluto is known to have undergone thousands of cycles of obliquity change and polar precession. These variations have a large and corresponding impact on the total average solar insolation reaching various places on Pluto's surface as a function of time. Such changes could produce dramatic increases in surface pressure and may explain certain features observed by New Horizons on Pluto's surface, including some that indicate the possibility of surface paleo-liquids. This paper is the first to discuss multiple lines of geomorphological evidence consistent with higher pressure epochs in Pluto's geologic past, and it also the first to provide a mechanism for potentially producing the requisite high pressure conditions needed for an environment that could support liquids on Pluto. The presence of such liquids and such conditions, if borne out by future work, would fundamentally affect our view of Pluto's past climate, volatile transport, and geological evolution. This paper motivates future, more detailed climate modeling and geologic interpretation efforts in this area.

New Horizon's accurate determination of the sizes and densities of Pluto and Charon now permit precise internal models of both bodies to be constructed. Assuming differentiated rock-ice structures, we find that Pluto is close to 2/3 solar-composition anhydrous rock by mass and Charon 3/5 solar-composition anhydrous rock by mass. Pluto and Charon are closer to each other in density than to other large (≳1000-km diameter) Kuiper belt bodies. Despite this, we show that neither the possible presence of an ocean under Pluto's water ice shell (and no ocean within Charon), nor enhanced porosity at depth in Charon's icy crust compared with that of Pluto, are sufficient to make Pluto and Charon's rock mass fractions match. All four small satellites (Styx, Nix, Kerberos, Hydra) appear much icier in comparison with either Pluto or Charon. In terms of a giant impact origin, both these inferences are most consistent with the relatively slow collision of partly differentiated precursor bodies (Canup, Astrophys. J. 141, 35, 2011). This is in turn consistent with dynamical conditions in the ancestral Kuiper belt, but implies that the impact precursors themselves accreted relatively late and slowly (to limit 26 Al and accretional heating). The iciness of the small satellites is not consistent with direct formation of the Pluto–Charon system from a streaming instability in the solar nebula followed by prompt collapse of gravitationally bound “pebble piles,” a proposed formation mechanism for Kuiper belt binaries (Nesvorný et al., Astron. J. 140, 785–793, 2010). Growth of Pluto-scale bodies by accretion of pebbles in the ancestral Kuiper belt is not ruled out, however, and may be needed to prevent the precursor bodies from fully differentiating, due to buried accretional heat, prior to the Charon-forming impact.

NASA's New Horizons flyby mission of the Pluto-Charon binary system and its four moons provided humanity with its first spacecraft-based look at a large Kuiper Belt Object beyond Triton. Excluding this system, multiple Kuiper Belt Objects (KBOs) have been observed for only 20 years from Earth, and the KBO size distribution is unconstrained except among the largest objects. Because small KBOs will remain beyond the capabilities of ground-based observatories for the foreseeable future, one of the best ways to constrain the small KBO population is to examine the craters they have made on the Pluto-Charon system. The first step to understanding the crater population is to map it. In this work, we describe the steps undertaken to produce a robust crater database of impact features on Pluto, Charon, and their two largest moons, Nix and Hydra. These include an examination of different types of images and image processing, and we present an analysis of variability among the crater mapping team, where crater diameters were found to average ± 10% uncertainty across all sizes measured (∼0.5–300 km). We also present a few basic analyses of the crater databases, finding that Pluto's craters' differential size-frequency distribution across the encounter hemisphere has a power-law slope of approximately –3.1 ± 0.1 over diameters D ≈ 15–200 km, and Charon's has a slope of –3.0 ± 0.2 over diameters D ≈ 10–120 km; it is significantly shallower on both bodies at smaller diameters. We also better quantify evidence of resurfacing evidenced by Pluto's craters in contrast with Charon's. With this work, we are also releasing our database of potential and probable impact craters: 5287 on Pluto, 2287 on Charon, 35 on Nix, and 6 on Hydra.

One of the giants of planetary science, H. J. Melosh, died unexpectedly on 11 September 2020 at age 73. Through his students, postdocs and collaborators, he brought a high level of physical rigour to the growing field of planetary geology.