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• Eris' eccentric orbit results in 2 atmospheric regimes: a global atm near perihelion and a local atm near aphelion. • A coupled thermal-transport numerical model to simulate thermal and volatile evolution in the LCSA regime is presented. • Volatile transport by local, collisional, sublimation atms (LCSA), even at solar distances of 100 AU, can be significant. • Uniform collapse of a global, nitrogen atmosphere likely cannot explain Eris' anomalous albedo in the present epoch. • Changes of Eris' albedo or color from nitrogen transport may be observable. Kuiper belt object (KBO) Eris is exceptionally bright with a greater visible geometric albedo than any other known KBO. Its infrared reflectance spectrum is dominated by methane, which should form tholins that darken the surface on timescales much shorter than the age of the Solar System. Thus one or more ongoing processes probably maintain its brightness. Eris is predicted to have a primarily nitrogen atmosphere that is in vapor pressure equilibrium with nitrogen-ice and is collisional (not ballistic). Eris's eccentric orbit is expected to result in two atmospheric regimes: (1) a period near perihelion when the atmosphere is global (analogous to the atmospheres of Mars, Triton, and Pluto) and (2) a period near aphelion when only a local atmosphere exists near the warmest region (analogous to the atmosphere of Io). A numerical model developed to simulate Eris's thermal and volatile evolution in the local atmosphere regime is presented. The model conserves energy, mass, and momentum while maintaining vapor pressure equilibrium. It is adaptable to other local, collisional, sublimation atmospheres, which in addition to Io and Eris, may occur on several volatile-bearing KBOs. The model was applied for a limiting case where Eris is fixed at aphelion and has an initial nitrogen-ice mass everywhere equal to the precipitable column of nitrogen in Pluto's atmosphere during the New Horizons encounter (the resultant mass if the Pluto atmosphere collapsed uniformly onto the surface). The model results indicate that (1) transport of nitrogen in the local, collisional, sublimation atmosphere regime is significant, (2) changes of Eris's albedo or color from nitrogen transport may be observable, and (3) uniform collapse of a global, nitrogen atmosphere likely cannot explain Eris's anomalous albedo in the present epoch. Seasonal volatile transport remains a plausible hypothesis to explain Eris's anomalous albedo and geologic processes that renew Pluto's brightest surfaces, such as convection and glaciation, may also be operating on Eris. [ABSTRACT FROM AUTHOR]

The region of Titan’s hydrocarbon sea, Ligeia Mare, where transient bright features were previously discovered, was anomalously bright in the first of two more recent Cassini RADAR observations but not the second. Another transient bright feature in a different region of Ligeia Mare was also discovered in the first of the new observations. Here we present all the high-resolution observations of the regions containing these transient features and the quantitative constraints that we derived from them. We argue that these features are unlikely to be SAR image artifacts or permanent geophysical structures and thus their appearance is the result of ephemeral phenomena on Titan. We find that the transient features are more consistent with floating and/or suspended solids, bubbles, and waves than tides, sea level change, or seafloor change and based on the frequency of these phenomena in terrestrial settings, we consider waves to be the most probable hypothesis. These transient features are the first instance of active processes in Titan’s lakes and seas to be confirmed by multiple detections and demonstrate that Titan’s seas are not stagnant but rather dynamic environments. [ABSTRACT FROM AUTHOR]

Abstract: We model Titan’s lakes and seas as methane–ethane–nitrogen systems and model the buoyancy of solids in these systems assuming thermodynamic equilibrium. We find that ice will float in methane-rich lakes for all temperatures below the freezing point of pure methane and that ice will also float in ethane-rich seas provided the ice has an air porosity of greater than 5% by volume. [Copyright &y& Elsevier]

At least two active plumes were observed on Neptune's moon Triton during the Voyager 2 flyby in 1989. Models for Triton's plumes have previously been grouped into five hypotheses, two of which are primarily atmospheric phenomena and are generally considered unlikely, and three of which include eruptive processes and are plausible. These hypotheses are compared, including new arguments, such as comparisons based on current understanding of Mars, Enceladus, and Pluto. An eruption model based on a solar-powered, solid-state greenhouse effect was previously considered the leading hypothesis for Triton's plumes, in part due to the proximity of the plumes to the subsolar latitude during the Voyager 2 flyby and the distribution of Triton's fans that are putatively deposits from former plumes. The other two eruption hypotheses are powered by internal heat, not solar insolation. Based on new analyses of the ostensible relation between the latitude of the subsolar point on Triton and the geographic locations of the plumes and fans, we argue that neither the locations of the plumes nor fans are strong evidence in favor of the solar-powered hypothesis. We conclude that all three eruption hypotheses should be considered further. Five tests are presented that could be implemented with remote sensing observations from future spacecraft to confidently distinguish among the eruption hypotheses for Triton's plumes. The five tests are based on the: (1) composition and thickness of Triton's southern hemisphere terrains, (2) composition of fan deposits, (3) distribution of active plumes, (4) distribution of fans, and (5) surface temperature at the locations of plumes and/or fans. The tests are independent, but complementary, and implementable with a single flyby mission such as the Trident mission concept. We note that, in the case of the solar-driven hypothesis, the 2030s and 2040s may be the last chance for approximately a century to observe actively erupting plumes on Triton. • Locations of Triton's plumes/fans don't strongly favor solid-state greenhouse model. • Solid-state greenhouse model for Triton's plumes is no longer leading hypothesis. • 5 remote sensing tests for distinguishing among published models are presented. • Models for plumes can likely be distinguished with another single-flyby mission. • 2030s & 2040s are opportune time: possibly last chance for ≈ century to see plumes. [ABSTRACT FROM AUTHOR]

On January 1st 2019, the New Horizons spacecraft flew by the classical Kuiper belt object (486958) Arrokoth (provisionally designated 2014 MU69), possibly the most primitive object ever explored by a spacecraft. The I/F of Arrokoth is analyzed and fit with a photometric function that is a linear combination of the Lommel-Seeliger (lunar) and Lambert photometric functions. Arrokoth has a geometric albedo of p v = 0.21 −0.04 +0.05 at a wavelength of 550 nm and ≈0.24 at 610 nm. Arrokoth's geometric albedo is greater than the median but consistent with a distribution of cold classical Kuiper belt objects whose geometric albedos were determined by fitting a thermal model to radiometric observations. Thus, Arrokoth's geometric albedo adds to the orbital and spectral evidence that it is a cold classical Kuiper belt object. Maps of the normal reflectance and hemispherical albedo of Arrokoth are presented. The normal reflectance of Arrokoth's surface varies with location, ranging from ≈0.10–0.40 at 610 nm with an approximately Gaussian distribution. Both Arrokoth's extrema dark and extrema bright surfaces are correlated to topographic depressions. Arrokoth has a bilobate shape and the two lobes have similar normal reflectance distributions: both are approximately Gaussian, peak at ≈0.25 at 610 nm, and range from ≈0.10–0.40, which is consistent with co-formation and co-evolution of the two lobes. The hemispherical albedo of Arrokoth varies substantially with both incidence angle and location, the average hemispherical albedo at 610 nm is 0.063 ± 0.015. The Bond albedo of Arrokoth at 610 nm is 0.062 ± 0.015. • Geometric albedo > than mean but consistent with distribution of cold classical KBOs. • Maps of normal reflectance & hemispherical albedo of Arrokoth's surface are presented. • Normal reflectance of Arrokoth varies with location with ≈Gaussian distribution. • Normal reflectance distributions of Arrokoth's two lobes are very similar. • Hemispherical albedo varies substantially with both incidence angle and location. [ABSTRACT FROM AUTHOR]

We obtained laboratory measurements of nitrogen solubility in mixed solutions of ethane and methane at temperatures and pressures relevant to Titan's lakes and seas. Our results show that nitrogen solubility is increased at higher methane concentration, lower temperature, and higher pressure. We developed an empirical fit that agrees well with our measurements. We show that significant volumes of nitrogen gas will be released from Titan lake fluids on heating, and that significant volumes of nitrogen gas will be absorbed by Titan lake fluids on cooling. The densities of the lake fluids will be affected by nitrogen dissolution. We also show that mixing of two cryogenic fluids of different composition can lead to the release of large amounts of nitrogen gas. This has implications for lake fluids, bubble formation, geological phenomena, and also for future landed missions on the surface of Titan. We find in particular that methane-rich lakes at lower temperatures on Titan will be the most sensitive to changes in surface conditions, either cooling, heating, or compositional mixing. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ethane is expected to be the dominant photochemical product on Titan’s surface and, in the absence of a process that sequesters it from exposed surface reservoirs, a major constituent of its lakes and seas. Absorption of Cassini’s 2.2 cm radar by Ligeia Mare however suggests that this north polar sea is dominated by methane. In order to explain this apparent ethane deficiency, we explore the possibility that Ligeia Mare is the visible part of an alkanofer that interacted with an underlying clathrate layer and investigate the influence of this interaction on an assumed initial ethane–methane mixture in the liquid phase. We find that progressive liquid entrapment in clathrate allows the surface liquid reservoir to become methane-dominated for any initial ethane mole fraction below 0.75. If interactions between alkanofers and clathrates are common on Titan, this should lead to the emergence of many methane-dominated seas or lakes. [ABSTRACT FROM AUTHOR]

From orbit, the visibility of Titan's surface is limited to a handful of narrow spectral windows in the near-infrared (near-IR), primarily from the absorption of methane gas. This has limited the ability to identify specific compounds on the surface—to date Titan's bulk surface composition remains unknown. Further, understanding of the surface composition would provide insight into geologic processes, photochemical production and evolution, and the biological potential of Titan's surface. One approach to obtain wider spectral coverage with which to study Titan's surface is by decreasing the integrated column of absorbers (primarily methane) and scatterers between the observer and the surface. This is only possible if future missions operate at lower altitudes in Titan's atmosphere. Herein, we use a radiative transfer model to measure in detail the absorption through Titan's atmosphere from different mission altitudes, and consider the impacts this would have for interpreting reflectance measurements of Titan's surface. Over our modeled spectral range of 0.4–10 μ m , we find that increases in the width of the transmission windows as large as 317% can be obtained for missions performing remote observations at the surface. However, any appreciable widening of the windows requires onboard illumination. Further, we make note of possible surface compounds that are not currently observable from orbit, but could be identified using the wider windows at low altitudes. These range from simple nitriles such as cyanoacetylene, to building blocks of amino acids such as urea. Finally, we discuss the implications that the identifications of these compounds would have for Titan science. • Titan's infrared spectral windows broaden considerably in the lower atmosphere. • Onboard illumination is needed to utilize widened windows in the lower atmosphere. • Enhanced spectral coverage improves the understanding of Titan's prebiotic potential. [ABSTRACT FROM AUTHOR]