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We used New Horizons LORRI images to measure the optical-band ($0.4\lesssim\lambda\lesssim0.9{\rm\mu m}$) sky brightness within a high galactic-latitude field selected to have reduced diffuse scattered light from the Milky Way galaxy (DGL), as inferred from the IRIS all-sky $100~\mu$m map. We also selected the field to significantly reduce the scattered light from bright stars (SSL) outside the LORRI field. Suppression of DGL and SSL reduced the large uncertainties in the background flux levels present in our earlier New Horizons COB results. The raw total sky level, measured when New Horizons was 51.3 AU from the Sun, is $24.22\pm0.80{\rm ~nW ~m^{-2} ~sr^{-1}}.$ Isolating the COB contribution to the raw total required subtracting scattered light from bright stars and galaxies, faint stars below the photometric detection-limit within the field, and the hydrogen plus ionized-helium two-photon continua. This yielded a highly significant detection of the COB at ${\rm 16.37\pm 1.47 ~nW ~m^{-2} ~sr^{-1}}$ at the LORRI pivot wavelength of 0.608 $\mu$m. This result is in strong tension with the hypothesis that the COB only comprises the integrated light of external galaxies (IGL) presently known from deep HST counts. Subtraction of the estimated IGL flux from the total COB level leaves a flux component of unknown origin at ${\rm 8.06\pm1.92 ~nW ~m^{-2} ~sr^{-1}}.$ Its amplitude is equal to the IGL., Comment: Accepted for publication in the Astrophysical Journal Letters

The Lucy Mission is a NASA Discovery-class mission to send a highly capable and robust spacecraft to investigate seven primitive bodies near both the L4 and L5 Lagrange points with Jupiter: the Jupiter Trojan asteroids. These planetesimals from the outer planetary system have been preserved since early in solar system history. The Lucy mission will fly by and extensively study a diverse selection of Trojan asteroids, including all the recognized taxonomic classes, a collisional family member, and a near equal-mass binary. It will visit objects with diameters ranging from roughly 1 km to 100 km. The payload suite consists of a color camera and infrared imaging spectrometer, a high-resolution panchromatic imager, and a thermal infrared spectrometer. Additionally, two spacecraft subsystems will also contribute to the science investigations: the terminal tracking cameras will supplement imaging during closest approach and the telecommunication subsystem will be used to measure the mass of the Trojans. The science goals are derived from the 2013 Planetary Decadal Survey and include determining the surface composition, assessing the geology, determining the bulk properties, and searching for satellites and rings.

We report observations of the Jupiter Trojan asteroid (3548) Eurybates and its satellite Queta with the Hubble Space Telescope and use these observations to perform an orbital fit to the system. Queta orbits Eurybates with a semimajor axis of 2350 ± 11 km at a period of 82.46 ± 0.06 days and an eccentricity of 0.125 ± 0.009. From this orbit we derive a mass of Eurybates of 1.51 ± 0.03 × 1017 kg, corresponding to an estimated density of 1.1 ± 0.3 g cm−3, broadly consistent with densities measured for other Trojans, C-type asteroids in the outer main asteroid belt, and small icy objects from the Kuiper Belt. Eurybates is the parent body of the only major collisional family among the Jupiter Trojans; its low density suggests that it is a typical member of the Trojan population. Detailed study of this system in 2027 with the Lucy spacecraft flyby should allow significant insight into collisional processes among what appear to be the icy bodies of the Trojan belt.

Light curves produced from color observations taken during New Horizons’ approach to the Pluto-system by its Multi-spectral Visible Imaging Camera (MVIC, part of the Ralph instrument) are analyzed. Fifty seven observations were analyzed, they were obtained between 9th April and 3rd July 2015, at a phase angle of 14.5° to 15.1°, sub-observer latitude of 51.2 °N to 51.5 °N, and a sub-solar latitude of 41.2°N. MVIC has four color channels; all are discussed for completeness but only two were found to produce reliable light curves: Blue (400–550 nm) and Red (540–700 nm). The other two channels, Near Infrared (780–975 nm) and Methane-Band (860–910 nm), were found to be potentially erroneous and too noisy respectively. The Blue and Red light curves show that Charon's surface is neutral in color, but slightly brighter on its Pluto-facing hemisphere. This is consistent with previous studies made with the Johnson B and V bands, which are at shorter wavelengths than that of the MVIC Blue and Red channel respectively.

Pluto energies of a few kiloelectron volts and suprathermal ions with tens of kiloelectron volts and above. We measure this population using the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on board the New Horizons spacecraft that flew by Pluto in 2015. Even though the measured ions have gyroradii larger than the size of Pluto and the cross section of its magnetosphere, we find that the boundary of the magnetosphere is depleting the energetic ion intensities by about an order of magnitude close to Pluto. The intensity is increasing exponentially with distance to Pluto and reaches nominal levels of the interplanetary medium at about 190

Acquiring and processing astrometric measurements of a spacecraft’s target using on-board images, generically referred to as optical navigation, is an integral function of the orbit determination and navigation of NASA’s New Horizons spacecraft. Since New Horizons’ reconnaissance of the Pluto system in July 2015, many preparations have been completed to further enhance the optical navigation system and prepare for the reconnaissance of New Horizons’ next target, Kuiper Belt Object (486958) 2014 MU69 (unofficially nicknamed Ultima Thule). Due to its low relative brightness compared to most planetary exploration targets, Ultima Thule presents several unique challenges to the optical navigation system. The optical navigation system design, imaging schedule, and technical algorithms that were developed and tailored to these challenges are explored in detail. Additionally, several operational readiness tests, simulation methods, and test results are presented and analyzed to assess the optical navigation system performance and implications to flight operations. Lastly, a first look at Ultima as viewed from the New Horizons LORRI imager is presented.

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.

One component of the REX instrument on NASA's New Horizons spacecraft was an investigation of the radio continuum radiation from Pluto and Charon during the flyby on 14 July 2015. The planetary thermal emission was recorded at a wavelength of 4.17 cm (7.18 GHz) during approach, departure, and specifically on the non-illuminated hemispheres of Pluto and Charon during the respective intervals between occultation ingress and egress. We derive the brightness temperatures for these disk-resolved and unresolved observations. The mean values and 1σ deviations of brightness temperature for the unresolved sunlit disk are 33.2 ± 1.4 K and 47.2 ± 5.3 K for Pluto and Charon, respectively, consistent with the global albedos of the two bodies as well as with previous ground-based estimates at smaller wavelengths. A slightly colder temperature of 29.0 ± 2.5 K was determined for the disk-integrated nightside of Pluto and a larger drop in temperature was observed for Charon (40.9 ± 0.9 K), implying a smaller thermal inertia for Charon than Pluto. The measured brightness temperature of Pluto across the nightside diametric scan reached a maximum of 29.0 ± 1.5 K in the center of the disk. The profile shape is attributed to an emissivity effect, which favors thermal emission toward higher elevation angles. As a first approximation, the effective emissivity for thermal emission is calculated for the case when Pluto and Charon are uniformly smooth homogenous spheres. Under this assumption, the effective emissivity for these observations is close to unity for all probable surface constituents, implying that the effective temperature of the Pluto subsurface is only a few percent higher than the observed brightness temperature. A considerably lower subsurface emissivity is implied, however, if the higher atmospheric temperatures near the surface determined from the REX occultation measurements are also valid for the subsurface.

The Student Dust Counter (SDC) is an in-situ dust detector aboard the New Horizons spacecraft observing the distribution of interplanetary dust particles (IDPs) with mass > 10 − 12 g or approximately 0.5 µm in radius. New Horizons was launched on January 19th, 2006 and performed a fly-by of the Pluto system on July 14th, 2015. SDC has nearly continuously mapped the dust density distribution along the trajectory of New Horizons, and it continues to operate providing measurements of the IDP in the Edgeworth–Kuiper Belt (EKB). We present results of the dust density distribution from 1 to 38 AU and compare these measurements to existing theoretical models.

We investigate what can be learned about a population of distant Kuiper Belt Objects (KBOs) by studying the statistical properties of their light curves. Whereas others have successfully inferred the properties of individual, highly variable KBOs, we show that the fraction of KBOs with low amplitudes also provides fundamental information about a population. Each light curve is primarily the result of two factors: shape and orientation. We consider contact binaries and ellipsoidal shapes, with and without flattening. After developing the mathematical framework, we apply it to the existing body of KBO light curve data. Principal conclusions are as follows. (1) When using absolute magnitude H as a proxy for the sizes of KBOs, it is more accurate to use the maximum of the light curve (minimum H) rather than the mean. (2) Previous investigators have noted that smaller KBOs tend to have higher-amplitude light curves, and have interpreted this as evidence that they are systematically more irregular in shape than larger KBOs; we show that a population of flattened bodies with uniform proportions, independent of size, could also explain this result. (3) Our method of analysis indicates that prior assessments of the fraction of contact binaries in the Kuiper Belt may be artificially low. (4) The pole orientations of some KBOs can be inferred from observed changes in their light curves over time scales of decades; however, we show that these KBOs constitute a biased sample, whose pole orientations are not representative of the population overall. (5) Although surface topography, albedo patterns, limb darkening, and other surface properties can affect individual light curves, they do not have a strong influence on the statistics overall. (6) Photometry from the Outer Solar System Origins Survey (OSSOS) survey is incompatible with previous results and its statistical properties defy easy interpretation. We also discuss the promise of this approach for the analysis of future, much larger data sets such as the one anticipated from the upcoming Vera C. Rubin Observatory.

International audience; We have compared spectroscopic data of Sputnik Planitia on Pluto, as acquired by New Horizons’ Linear Etalon Imaging Spectral Array (LEISA) instrument, to the geomorphology as mapped by White et al. (2017) using visible and panchromatic imaging acquired by the LOng-Range Reconnaissance Imager (LORRI) and the Multi-spectral Visible Imaging Camera (MVIC). We have focused on 13 of the geologic units identified by White et al. (2017), which include the plains and mountain units contained within the Sputnik basin. We divided the map of Sputnik Planitia into 15 provinces, each containing one or more geologic units, and we use LEISA to calculate the average spectra of the units inside the 15 provinces. Hapke-based modeling was then applied to the average spectra of the units to infer their surface composition, and to determine if the composition resulting from the modeling of LEISA spectra reflects the geomorphologic analyses of LORRI data, and if areas classified as being the same geologically, but which are geographically separated, share a similar composition. We investigated the spatial distribution of the most abundant ices on Pluto’s surface - CH4, N2, CO, H2O, and a non-ice component presumed to be a macromolecular carbon-rich material, termed a tholin, that imparts a positive spectral slope in the visible spectral region and a negative spectral slope longward of ~1.1 μm. Because the exact nature of the non-ice component is still debated and because the negative spectral slope of the available tholins in the near infrared does not perfectly match the Pluto data, for spectral modeling purposes we reference it generically as the negative spectral slope endmember (NSS endmember). We created maps of variations in the integrated band depth (from LEISA data) and areal mass fraction (from the modeling) of the components. The analysis of correlations between the occurrences of the endmembers in the geologic units led to the observation of an anomalous suppression of the strong CH4 absorption bands in units with compositions that are dominated by H2O ice and the NSS endmember. Exploring the mutual variation of the CH4 and N2 integrated band depths with the abundance of crystalline H2O and NSS endmember revealed that the NSS endmember is primarily responsible for the suppression of CH4 absorptions in mountainous units located along the western edge of Sputnik Planitia. Our spectroscopic analyses have provided additional insight into the geological processes that have shaped Sputnik Planitia. A general increase in volatile abundance from the north to the south of Sputnik Planitia is observed. Such an increase first observed and interpreted by Protopapa et al., 2017 and later confirmed by climate modeling (Bertrand et al., 2018) is expressed geomorphologically in the form of preferential deposition of N2 ice in the upland and mountainous regions bordering the plains of southern Sputnik Planitia. Relatively high amounts of pure CH4 are seen at the southern Tenzing Montes, which are a natural site for CH4 deposition owing to their great elevation and the lower insolation they are presently receiving. The NSS endmember correlates the existence of tholins within certain units, mostly those coating the low-latitude mountain ranges that are co-latitudinal with the tholin-covered Cthulhu Macula. The spectral analysis has also revealed compositional differences between the handful of occurrences of northern non-cellular plains and the surrounding cellular plains, all of which are located within the portion of Sputnik Planitia that is presently experiencing net sublimation of volatiles, and which do not therefore exhibit a surface layer of bright, freshly-deposited N2 ice. The compositional differences between the cellular and non-cellular plains here hint at the effectiveness of convection in entraining and trapping tholins within the body of the cellular plains, while preventing the spread of such tholins to abutting non-cellular plains.

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

Lucy is NASA's 13th Discovery-class mission and it will be the first mission to the Jupiter Trojan asteroids. Over the course of 12 years, the mission will encounter 7 Trojan asteroids in a series of 5 flybys. The time critical nature of the asteroid flybys coupled with the long-duration mission drive design considerations for redundancy in the implementation of the mission. This paper focusses on the redundancy in the science planning and instrument designs. A risk-based assesment informed decisions about instrument selection, instrument design, and science timeline considerations. The mission carries three scientific instruments, a high resolution panchromatic imager, a combination panchromatic/color visible imager and near-infrared spectral imager, and a thermal infrared spectrometer. Additionally, a component of the guidance and control subsystem, the terminal tracking cameras, will contribute to the scientific data collected on the mission. Redundancy in the science instruments includes redundant electronics and data storage separate from the data storage on the spacecraft. The science observation sequence also has redundancy built into it. An example of this is backup observations to provide more than one opportunity to accomplish critical science objectives. These design considerations are achievable in a Discovery-class mission and reduce risk during Lucy's 12-year operations phase to achieve the mission's science objectives.

The Lucy mission, which is part of NASA's Discovery Program, is the first reconnaissance of the Jupiter Trojans, objects that hold vital clues to deciphering the history of the Solar System. Due to an unusual and fortuitous orbital configuration, Lucy will perform a comprehensive investigation that visits seven of these primitive bodies, covering both the $\mathbf{L}_{4}$ and $\mathbf{L}_{5}$ swarms and all the known taxonomic types. In particular, Lucy will perform flybys of 3548 Eurybates and its satellite Queta ( $\mathbf{L}_{4}$ , C-type), 15094 Polymele ( $\mathbf{L}_{4}$ , P-type), 11351 Leucus ( $\mathbf{L}_{4}$ , D-type), 21900 Orus ( $\mathbf{L}_{4}$ , D-type), and the 617 Patroclus-Menoetius binary ( $\mathbf{L}_{5}$ , P-types). Eurybates and the binary are of particular interest. Eurybates is the largest member of the only major disruptive collisional family in the Trojan swarms. The Patroclus-Menoetius binary, which consists of two roughly equal mass objects in a circular mutual orbit, is likely a leftover from the first generation of planetesimals in the Solar System.

NASA's Lucy mission is the 13th Discovery-class mission, and it will be the first to visit the Jupiter Trojan asteroids. Over the course of 12 years, the mission will encounter 7 Trojan asteroids in a series of 5 flybys. Each of these flyby occurs over a short period of time, such that careful planning of time-critical observations must be performed to ensure that the mission's Science Requirement are achieved. Additionally, various sources of uncertainty in the spacecraft's attitude and pointing accuracy must be thoroughly tested and constrained to determine how they impact instrument operation and the resulting data. In order to develop and test observations sequences, we have developed the Science Encounter Simulation Software (SESS). This powerful software allows one to simulate the spacecraft's encounter with each of Lucy's target, and to perform a series of commands to adjust the spacecraft's attitude and trigger data acquisitions with the various instruments of the Lucy payload. The software outputs a variety of information that allows one to construct resolution maps which can be used to determine whether a given observation sequence produces adequate data that meet the Science Requirements. The code also performs numerous checks to ensure that the spacecraft, its instrument pointing platform, and the L'Ralph scan mirror, are operated within their capabilities.

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.

Observing dwarf planets and other large Kuiper Belt objects (KBOs) from vantage points between 8 and 47 au from the Sun, NASA’s New Horizons spacecraft has found diversity in the shapes of their solar phase curves. Here we extend solar phase angle coverage of dwarf planets (136199) Eris, (136472) Makemake, and (136108) Haumea; large KBOs (28978) Ixion, (50000) Quaoar, (307261) 2002 MS4, and (556416) 2014 OE394; and Neptune’s satellite Triton to phase angles as high as α = 94° using New Horizons data and fit the resulting solar phase curves to the Hapke photometric model. When accounting for sparse α sampling, these fits yield large uncertainties in the Hapke parameters; however, opposition effect parameters are generally well constrained and suggest a significant range of regolith maturation ages among these bodies. The expanded range in α enables evaluation of Bond albedos, phase integrals, rotation curves at high α, and comparisons of the surface scattering properties of these objects with those of others in the solar system. The dwarf planets with surface compositions dominated by hypervolatiles, Eris and Makemake, and Triton (a likely former KBO) have shallower solar phase curve slopes (i.e., lower phase coefficients, higher phase integrals, and Bond albedos) than objects with volatile-poor surfaces. The total amplitude of Haumea’s rotation curve at α = 48° is Δm = 0.6 ± 0.2 mag, nearly twice that of its rotation curve measured from Earth at low phase angles. Bond albedos range from 0.037 ± 0.007 for Ixion to 0.99 − 0.09 + 0.01 for Eris.

The Lucy Mission is a NASA Discovery class mission to send a highly capable and robust spacecraft to investigate seven Jupiter Trojan asteroids; a class of stable, primitive bodies near both the L4 and L5 Lagrange points with Jupiter. It is believed that Jupiter Trojan asteroids are leftover planetesimals from the outer planetary system that have been preserved since early in Solar System history, and represent the last of all of the stable populations of the Solar System to be visited by spacecraft. Lucy is slated to launch in October 2021, reach its first Trojan asteroid in 2027, and have its final encounter in 2033. During its lifetime, Lucy will perform five Trojan encounters closely studying at least seven objects (one encounter is of a nearly equal mass binary and another is an asteroid with a known satellite). The science goals include determining the surface composition, assessing the geology, determining the bulk properties and searching for satellites around all of Lucy’s targets. The payload suite consists of a color camera and infrared imaging spectrometer, a high resolution panchromatic imager, and a thermal infrared spectrometer. Additionally, two spacecraft subsystems will also contribute to the science investigations: the terminal tracking cameras and the telecommunication subsystem to measure the mass of the Trojan asteroids. Lucy’s Trojan targets include one C-type (Eurybates, 64 km in diameter), three P-types (Menoetius, Patroclus, and Polymele; 105, 114, 21 km in diameter, respectively), and two D-types (Leucus and Orus; 41 and 52 km in diameter, respectively), thereby covering a wide range of spectral types and sizes. Lucy will be the first spacecraft to observe the largest remnant of a catastrophic collision up close (Eurybates), and the first to visit a near-equal mass binary (Patroclus and Menoetius). In addition, on its way to L4, Lucy will fly by DonaldJohanson in 2025, a 4 km in diameter Main Belt asteroid named in honor of the discoverer of the Lucy fossil. In this talk, recent results from an international observational campaign of some of the Lucy’s targets will be presented, including the detection of a small satellite (1-2 km) orbiting Eurybates, and a detailed characterization of Leucus’s shape and rotational axis orientation.

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

International audience; A prominent fossa trough (Uncama Fossa) and adjacent 28-km diameter impact crater (Hardie) in Pluto’s Viking Terra, as seen in the high-resolution images from the New Horizons spacecraft, show morphological evidence of in-filling with a material of uniform texture and red-brown color. A linear fissure parallel to the trough may be the source of a fountaining event yielding a cryoclastic deposit having the same composition and color properties as is found in the trough and crater. Spectral maps of this region with the New Horizons LEISA instrument reveal the spectral signature of H2O ice in these structures and in distributed patches in the adjacent terrain in Viking Terra. A detailed statistical analysis of the spectral maps shows that the colored H2O ice filling material also carries the 2.2-μm signature of an ammoniated component that may be an ammonia hydrate (NH3⋅nH2O) or an ammoniated salt. This paper advances the view that the crater and fossa trough have been flooded by a cryolava debouched from Pluto’s interior along fault lines in the trough and in the floor of the impact crater. The now frozen cryolava consisted of liquid H2O infused with the red-brown pigment presumed to be a tholin, and one or more ammoniated compounds. Although the abundances of the pigment and ammoniated compounds entrained in, or possibly covering, the H2O ice are unknown, the strong spectral bands of the H2O ice are clearly visible. In consideration of the factors in Pluto’s space environment that are known to destroy ammonia and ammoniawater mixtures, the age of the exposure is of order

We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by the Stratospheric Observatory for Infrared Astronomy with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 August 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 ± 0.0005 hr. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies. © 2020. The American Astronomical Society. All rights reserved., This project would have been impossible without support from many groups and individuals. We had exceptional support from the US State Department (John Fazio, US Embassy in Argentina; James Garry, Heath Bailey, and Cheikh Oumar Dia, US Embassy in Senegal), CONAE (Felix Menicocci and Stan Makarchuk), and SOFIA (William Reach, Anil Dosaj, Paul Newton, James Less, Stephen Koertge, Kimberly Ennico, and Karina Leppik), SOFIA/FPI+ (Enrico Pfuller, Jurgen Wolf, and Manuel Wiedemann). Special thanks go to Tony Barry for rapid turnaround updates to his SEXTA reader for timing verification, QHY for updated camera firmware, and Robin Glover for updates to the SharpCap software. The discovery and subsequent astrometric images were made possible by the Hubble Space Telescope and the wonderful staff at the Space Telescope Science Institute (STScI). Extracting high-precision astrometry from the HST data was made possible by early release of Gaia DR2 data. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC,.https://www.cosmos.esa.int/web/gaia/dpac/consortium).Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The local logistical support from Argentina, especially from the municipality of Comodoro Rivadavia, was critical to the success of the observations on 2017 July 17. Support from the Government of Senegal and the Ministry of Higher Education, Research and Innovation (MESRI) was critical to the success of the observation on 2018 August 4. J. Desmars acknowledges the funding from the European Research Council under the European Community's H2020 (2014-2020/ERC grant Agreement No. 669416 "LUCKY STAR"). The French participants were supported by the French Space Agency (CNES), the 2014-2020/ERC grant Agreement No. 669416 "LUCKY STAR" and the French National Research Institute for Sustainable Development. S.S. acknowledges support from the Africa Initiative for Planetary and Space Science (http://africapss.org) and the Uranoscope de France (https://uranoscope.org). P.S.S. acknowledges financial support by the European Union's Horizon 2020 Research and Innovation Programme, under grant agreement No. 687378, as part of the project "Small Bodies Near and Far" (SBNAF) and also acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). This paper is based in part on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministerio da Ciencia, Tecnologia, Inovacaos e Comunicacaoes (MCTIC) do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). At SOAR, data acquisition was performed with a Raptor camera (visitor instrument) funded by the Observatorio Nacional/MCTIC. Based in part on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NNA17BF53C, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart. Financial support for the SOFIA observations was provided by NASA through award NAS297001 issued by USRA. This paper utilizes public domain data obtained by the MACHO Project, jointly funded by the US Department of Energy through the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48, by the National Science Foundation through the Center for Particle Astrophysics of the University of California under cooperative agreement AST-8809616, and by the Mount Stromlo and Siding Spring Observatory, part of the Australian National University. Overall funding for this project was from NASA's New Horizons project via contracts NASW-02008 and NAS5-97271/TaskOrder30 and special appreciation is due to Dr. James Green and Dr. Lori Glaze at NASA Headquarters for their support.

International audience; Pluto's atmosphere is mainly nitrogen and is in solid-gas equilibrium with the surface nitrogen ice. As a result, the global nitrogen ice distribution and the induced nitrogen condensation-sublimation flows strongly control the atmospheric circulation. It is therefore essential for Global Climate Models to accurately account for the global nitrogen ice distribution in order to realistically simulate Pluto's atmosphere. Here we present a set of new numerical simulations of Pluto's atmosphere in 2015 performed with a Global Climate Model using a 50-km horizontal resolution (3.75◦ × 2.5◦) and taking into account the latest topography and ice distribution data, as observed by the New Horizons spacecraft. In order to analyze the seasonal evolution of Pluto's atmosphere dynamics, we also performed simulations at coarser resolution (11.25◦ × 7.5◦) but covering three Pluto years. The model predicts a near-surface western boundary current inside the Sputnik Planitia basin in 2015, which is consistent with the dark wind streaks observed in this region.We find that this atmospheric current could explain the differences in ice composition and color observed in the northwestern regions of Sputnik Planitia, by significantly impacting the nitrogen ice sublimation rate in these regions through processes possibly involving conductive heat flux from the atmosphere, transport of dark materials by the winds, and surface albedo positive feedbacks. In addition, we find that this current controls Pluto's general atmospheric circulation, which is dominated by a retrorotation, independently of the nitrogen ice distribution outside Sputnik Planitia. This exotic circulation regime could explain many of the geological features and longitudinal asymmetries in ice distribution observed all over Pluto's surface.

International audience; A multiwavelength regionally dependent photometric analysis of Pluto's anti-Charon-facing hemisphere using images collected by New Horizons' Multispectral Visible Imaging Camera (MVIC) reveals large variations in the absolute value and spectral slope of the single-scattering albedo. Four regions of interest are analyzed: the dark equatorial belt, Pluto's north pole, nitrogen-rich regions, and the mid-latitude terrains. Regions dominated by volatile ices such as Lowell Regio and Sputnik Planitia present single-scattering albedos of ~0.98 at 492 nm, almost neutral across MVIC's visible wavelength range (400–910 nm), indicating limited contributions from tholin materials. Pluto's dark equatorial regions, informally named Cthulhu and Krun Maculae, have single-scattering albedos of ~0.16 at 492 nm and are the reddest regions. Applying the Hapke radiative transfer model to combined MVIC and Linear Etalon Imaging Spectral Array (LEISA) spectra (400–2500 nm) of Cthulhu Macula and Lowell Regio successfully reproduces the spectral properties of these two regions of dramatically disparate coloration, composition, and morphology. Since this model uses only a single coloring agent, very similar to the Titan-like tholin of Khare et al., to account for all of Pluto's colors, this result supports the Grundy et al. conclusion that Pluto's coloration is the result of photochemical products mostly produced in the atmosphere. Although cosmic rays and extreme ultraviolet photons reach Pluto's surface where they can drive chemical processing, observations of diverse surface colors do not require different chemical products produced in different environments. We report a correction scaling factor in the LEISA radiometric calibration of 0.74 ± 0.05.

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

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU69) has been largely undisturbed since its formation. We studied its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through irradiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, which suggests that Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29 ± 5 kelvin. ©2020, NASA (contract no. NASW-02008), NASA (grant no. NAS5-97271 / TaskOrder30)

During the New Horizons spacecraft's encounter with Pluto, the Alice ultraviolet spectrograph conducted a series of observations that detected emissions from both the interplanetary medium (IPM) and Pluto. In the direction of Pluto, the IPM was found to be 133.4$\pm$0.6R at Lyman $\alpha$, 0.24$\pm$0.02R at Lyman $\beta$, and, Comment: 27 pages, 8 figures

The LOng Range Reconnaissance Imager (LORRI) is a panchromatic (360--910 nm), narrow-angle (field of view = 0.29 deg), high spatial resolution (pixel scale = 1.02 arcsec) visible light imager used on NASA's New Horizons (NH) mission for both science observations and optical navigation. Calibration observations began several months after the NH launch on 2006 January 19 and have been repeated annually throughout the course of the mission, which is ongoing. This paper describes the in-flight LORRI calibration measurements, and the results derived from our analysis of the calibration data. LORRI has been remarkably stable over time with no detectable changes (at the 1% level) in sensitivity or optical performance since launch. By employing 4 by 4 re-binning of the CCD pixels during read out, a special spacecraft tracking mode, exposure times of 30 sec, and co-addition of approximately 100 images, LORRI can detect unresolved targets down to V = 22 (SNR=5). LORRI images have an instantaneous dynamic range of 3500, which combined with exposure time control ranging from 0ms to 64,967 ms in 1ms steps supports high resolution, high sensitivity imaging of planetary targets spanning heliocentric distances from Jupiter to deep in the Kuiper belt, enabling a wide variety of scientific investigations. We describe here how to transform LORRI images from raw (engineering) units into scientific (calibrated) units for both resolved and unresolved targets. We also describe various instrumental artifacts that could affect the interpretation of LORRI images under some observing circumstances., Comment: Accepted by PASP, January 2020

The July 2015 encounter of the Pluto system by the NASA New Horizons spacecraft has facilitated the study of Pluto’s origin, surface processes, volatile transport cycles, and the energetics and chemistry of its atmosphere in an unprecedented level of detail. Earle et al. (2018b) presented the highest spatial resolution composition maps of Pluto using data from the Ralph/MVIC instrument and provided a global interpretation of the maps. Here we build upon that work and leverage MVIC’s high spatial resolution to study the volatile distribution in and around craters to better understand how small scale topography affects volatile transport. We find that the compositional morphology in and around craters in our study can be divided into four different latitudinal bands, where differences are found for distribution trends in nitrogen, methane, and organic signatures in crater floors, walls, and surrounding slopes. We summarize the compositional characteristics of a “typical” crater in each latitude band, provide some possible explanation for the distribution based on current volatile transport models, and highlight some questions to be addressed by ongoing models.

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.

Pluto’s surface is geologically complex because of volatile ices that are mobile on seasonal and longer time scales. Here we analyzed New Horizons LEISA spectral data to globally map the nitrogen ice, including nitrogen with methane diluted in it. Our goal was to learn about the seasonal processes influencing ice redistribution, to calculate the globally averaged energy balance, and to place a lower limit on Pluto’s N 2 inventory. We present the average latitudinal distribution of nitrogen and investigate the relationship between its distribution and topography on Pluto by using maps that include the shifted bands of methane in solid solution with nitrogen (which are much stronger than the 2.15- μ m nitrogen band) to more completely map the distribution of the nitrogen ice. We find that the global average bolometric albedo is 0 . 83 ± 0 . 11 , similar to that inferred for Triton, and that a significant fraction of Pluto’s N 2 is stored in Sputnik Planitia. We also used the encounter-hemisphere distribution of nitrogen ice to infer the latitudinal distribution of nitrogen over the rest of Pluto, allowing us to calculate the global energy balance. Under the assumption that Pluto’s nitrogen-dominated 11.5 μ bar atmosphere is in vapor pressure equilibrium with the nitrogen ice, the ice temperature is 36 . 93 ± 0 . 10 K, as measured by New Horizons’ REX instrument. Combined with our global energy balance calculation, this implies that the average bolometric emissivity of Pluto’s nitrogen ice is probably in the range 0.47–0.72. This is consistent with the low emissivities estimated for Triton based on Voyager results, and may have implications for Pluto’s atmospheric seasonal variations, as discussed below. The global pattern of volatile transport at the time of the encounter was from north to south, and the transition between condensation and sublimation within Sputnik Planitia is correlated with changes in the grain size and CH 4 concentration derived from the spectral maps. The low emissivity of Pluto’s N 2 ice suggests that Pluto’s atmosphere may undergo an extended period of constant pressure even as Pluto recedes from the Sun in its orbit.

International audience; Pluto’s landscape is shaped by the endless condensation and sublimation cycles of the volatile ices covering its surface. In particular, the Sputnik Planitia ice sheet, which is thought to be the main reservoir of nitrogen ice, displays a large diversity of terrains, with bright and dark plains, small pits and troughs, topographic depressions and evidences of recent and past glacial flows. Outside Sputnik Planitia, New Horizons also revealed numerous nitrogen ice deposits, in the eastern side of Tombaugh Regio and at mid-northern latitudes.These observations suggest a complex history involving volatile and glacial processes occurring on different timescales. We present numerical simulations of volatile transport on Pluto performed with a model designed to simulate the nitrogen cycle over millions of years, taking into account the changes of obliquity, solar longitude of perihelion and eccentricity as experienced by Pluto. Using this model, we first explore how the volatile and glacial activity of nitrogen within Sputnik Planitia has been impacted by the diurnal, seasonal and astronomical cycles of Pluto. Results show that the obliquity dominates the N 2 cycle and that over one obliquity cycle, the latitudes of Sputnik Planitia between 25 °S-30 °N are dom- inated by N 2 condensation, while the northern regions between 30 °N and -50 °N are dominated by N 2 sublimation. We find that a net amount of 1 km of ice has sublimed at the northern edge of Sputnik Planitia during the last 2 millions of years. It must have been compensated by a viscous flow of the thick ice sheet. By comparing these results with the observed geology of Sputnik Planitia, we can relate the formation of the small pits and the brightness of the ice at the center of Sputnik Planitia to the subli- mation and condensation of ice occurring at the annual timescale, while the glacial flows at its eastern edge and the erosion of the water ice mountains all around the ice sheet are instead related to the as- tronomical timescale. We also perform simulations including a glacial flow scheme which shows that the Sputnik Planitia ice sheet is currently at its minimum extent at the northern and southern edges. We also explore the stability of N 2 ice deposits outside the latitudes and longitudes of the Sputnik Planitia basin. Results show that N 2 ice is not stable at the poles but rather in the equatorial regions, in particu- lar in depressions, where thick deposits may persist over tens of millions of years, before being trapped in Sputnik Planitia. Finally, another key result is that the minimum and maximum surface pressures ob- tained over the simulated millions of years remain in the range of milli-Pascals and Pascals, respectively. This suggests that Pluto never encountered conditions allowing liquid nitrogen to flow directly on its sur- face. Instead, we suggest that the numerous geomorphological evidences of past liquid flow observed on Pluto’s surface are the result of liquid nitrogen that flowed at the base of thick ancient nitrogen glaciers, which have since disappeared.

On 14 July 2015 New Horizons performed a radio occultation (RO) that sounded Pluto’s neutral atmosphere and ionosphere. The solar zenith angle was 90.2° (sunset) at entry and 89.8° (sunrise) at exit. We examined the data for evidence of an ionosphere, using the same method of analysis as in a previous investigation of the neutral atmosphere (Hinson et al., 2017). No ionosphere was detected. The measurements are more accurate at occultation exit, where the 1-sigma sensitivity in integrated electron content (IEC) is 2.3 × 1011 cm − 2 . The corresponding upper bound on the peak electron density at the terminator is about 1000 cm − 3 . We constructed a model for the ionosphere and used it to guide the analysis and interpretation of the RO data. Owing to the large abundance of CH4 at ionospheric heights, the dominant ions are molecular and the electron densities are relatively small. The model predicts a peak IEC of 1.8 × 1011 cm − 2 for an occultation at the terminator, slightly smaller than the threshold of detection by New Horizons.