Near-Infrared Spectral Monitoring of Triton with IRTF/SpeX II: Spatial Distribution and Evolution of Ices

This report arises from an ongoing program to monitor Neptune's largest moon Triton spectroscopically in the 0.8 to 2.4 micron range using IRTF/SpeX. Our objective is to search for changes on Triton's surface as witnessed by changes in the infrared absorption bands of its surface ices N2, CH4, H2O, CO, and CO2. We have recorded infrared spectra of Triton on 53 nights over the ten apparitions from 2000 through 2009. The data generally confirm our previously reported diurnal spectral variations of the ice absorption bands (Grundy & Young 2004). Nitrogen ice shows a large amplitude variation, with much stronger absorption on Triton's Neptune-facing hemisphere. We present evidence for seasonal evolution of Triton's N2 ice: the 2.15 micron absorption band appears to be diminishing, especially on the Neptune-facing hemisphere. Although it is mostly dissolved in N2 ice, Triton's CH4 ice shows a very different longitudinal variation from the N2 ice, challenging assumptions of how the two ices behave. Unlike Triton's CH4 ice, the CO ice does exhibit longitudinal variation very similar to the N2 ice, implying that CO and N2 condense and sublimate together, maintaining a consistent mixing ratio. Absorptions by H2O and CO2 ices show negligible variation as Triton rotates, implying very uniform and/or high latitude spatial distributions for those two non-volatile ices.Comment: 22 pages, 13 figures, 5 tables, to appear in Icaru

The geology and geophysics of Kuiper Belt object (486958) Arrokoth

The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, are primitive objects preserving information about Solar System formation. The New Horizons spacecraft flew past one of these objects, the 36 km long contact binary (486958) Arrokoth (2014 MU69), in January 2019. Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters diameter) within a radius of 8000 km, and has a lightly-cratered smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism

Global compositional cartography of Pluto from intensity-based registration of LEISA data

International audienceIn 2015 the New Horizons spacecraft reached the Pluto system and returned unprecedentedly detailed measurements of its surface properties. These measurements have already been integrated into global reectance, topography and narrow-band multispectral surface maps. However, analysis of the hyperspectral data from the Ralph/LEISA infrared spectrometer, which lets us analyse the surface composition, has until now been conned to the highresolution encounter hemisphere of Pluto. We use an innovative technique intensity-based registration to co-register this high-resolution data with lower-resolution measurements taken during the spacecraft's approach, and present the rst global qualitative composition maps for CH 4 , N 2 and H 2 O ice, and a tholin-like red material. We compare these maps with the other maps produced for Pluto and study the global extent of the previously-described latitudinal distribution of the surface components, which is relatively longitudinally constant with the exception of Sputnik Planitia. We also correlate these compositional components with geological features and propose physical interpretations, which include: CH 4-ice-rich dissected plateaus in high northern latitudes, CH 4-rich eroded terrain with N 2-rich inll in medium northern latitudes, CH 4-rich bladed terrain in low northern latitudes, and a red material belt overlaying H 2 O ice in low southern latitudes

Modeling glacial flow on and onto Pluto’s Sputnik Planitia

Observations of Pluto's surface made by the New Horizons spacecraft indicate present-day N2 ice glaciation in and around the basin informally known as Sputnik Planitia. Motivated by these observations, we have developed an evolutionary glacial flow model of solid N2 ice that takes into account its published thermophysical and rheological properties. This model assumes that glacial ice flows laminarly and has a low aspect ratio which permits a vertically integrated mathematical formulation. We assess the conditions for the validity of laminar N2 ice motion by revisiting the problem of the onset of solid-state buoyant convection of N2 ice for a variety of bottom thermal boundary conditions. Subject to uncertainties in N2 ice rheology, N2 ice layers are estimated to flow laminarly for thicknesses less than 400–1000 m. The resulting mass-flux formulation for when the N2 ice flows as a laminar dry glacier is characterized by an Arrhenius–Glen functional form. The flow model developed is used here to qualitatively answer some questions motivated by features we interpret to be a result of glacial flow found on Sputnik Planitia. We find that the wavy transverse dark features found along the northern shoreline of Sputnik Planitia may be a transitory imprint of shallow topography just beneath the ice surface suggesting the possibility that a major shoreward flow event happened relatively recently, within the last few hundred years. Model results also support the interpretation that the prominent darkened features resembling flow lobes observed along the eastern shoreline of the Sputnik Planitia basin may be the result of a basally wet N2 glacier flowing into the basin from the pitted highlands of eastern Tombaugh Regio

The CH 4 cycles on Pluto over seasonal and astronomical timescales

International audienceThese observations suggest that CH 4 on Pluto has a complex history, involving reservoirs of different composition, thickness and stability controlled by volatile processes occurring on different timescales. In order to interpret these observations, we use a Pluto volatile transport model able to simulate the cycles of N 2 and CH 4 ices over millions of years. By assuming fixed solid mixing ratios, we explore how changes in surface albedos, emissivities and thermal inertias impact volatile transport. This work is therefore a direct and natural continuation of the work by Bertrand et al. (2018), which only explored the N 2 cycles. Results show that bright CH 4 deposits can create cold traps for N 2 ice outside Sputnik Planitia, leading to a strong coupling between the N 2 and CH 4 cycles. Depending on the assumed albedo for CH 4 ice, the model predicts CH 4 ice accumulation (1) at the same equatorial latitudes where the Bladed Terrain Deposits are observed, supporting the idea that these CH 4-rich deposits are massive and perennial, or (2) at mid-latitudes (25°− 70°), forming a thick mantle which is consistent with New Horizons observations. In our simulations, both CH 4 ice reservoirs are not in an equilibrium state and either one can dominate the other over long timescales, depending on the assumptions made for the CH 4 albedo. This suggests that long-term volatile transport exists between the observed reservoirs. The model also reproduces the formation of N 2 deposits at mid-latitudes and in the equatorial depressions surrounding the Bladed Terrain Deposits, as observed by New Horizons. At the poles, only seasonal CH 4 and N 2 deposits are obtained in Pluto's current orbital configuration. Finally, we show that Pluto's atmosphere always contained, over the last astronomical cycles, enough gaseous CH 4 to absorb most of the incoming Lyman-α flux

Methane distribution on Pluto as mapped by the New Horizons Ralph/MVIC instrument

International audienceThe data returned from NASA's New Horizons spacecraft have given us an unprecedented, detailed look at the Pluto system. New Horizons' Ralph/MVIC (Multispectral Visible Imaging Camera) is composed of 7 independent CCD arrays on a single substrate. Among these are a red channel (540-700 nm), near-infrared channel (780-975 nm), and narrow band methane channel (860-910 nm). By comparing the relative reflectance of these channels we are able to produce high-resolution methane "equivalent width" (based on the 890 nm absorption band) and spectral slope maps of Pluto's surface. From these maps we can then quantitatively study the relationships between methane distribution, redness, and other parameters like latitude and elevation. We find Pluto's surface to show a great diversity of terrains, particularly in the equatorial region between 30°N and 30°S latitude. Methane "equivalent width" also shows some dependence on elevation (while spectral slope shows very little)

Prebiotic Chemistry of Pluto

International audienceWe present the case for the presence of complex organic molecules, such as amino acids and nucleobases, formed by abiotic processes on the surface and in near-subsurface regions of Pluto. Pluto's surface is tinted with a range of non-ice substances with colors ranging from light yellow to red to dark brown; the colors match those of laboratory organic residues called tholins. Tholins are broadly characterized as complex, macromolecular organic solids consisting of a network of aromatic structures connected by aliphatic bridging units (e.g., Imanaka et al.

The puzzling detection of x-rays from Pluto by Chandra

Using Chandra ACIS-S, we have obtained low-resolution imaging X-ray spectrophotometry of the Pluto system in support of the New Horizons flyby on 14 July 2015. Observations were obtained in a trial “seed” campaign conducted in one visit on 24 Feb 2014, and a follow-up campaign conducted soon after the New Horizons flyby that consisted of 3 visits spanning 26 Jul to 03 Aug 2015. In a total of 174 ksec of on-target time, in the 0.31 to 0.60 keV passband, we measured 8 total photons in a co-moving 11 × 11 pixel2box (the 90% flux aperture determined by observations of fixed background sources in the field) measuring ∼121,000 × 121,000 km[superscript 2](or ∼100 × 100 R[subscript Pluto]) at Pluto. No photons were detected from 0.60 to 1.0 keV in this box during the same exposures. Allowing for background, we find a net signal of 6.8 counts and a statistical noise level of 1.2 counts, for a detection of Pluto in this passband at > 99.95% confidence. The Pluto photons do not have the spectral shape of the background, are coincident with a 90% flux aperture co-moving with Pluto, and are not confused with any background source, so we consider them as sourced from the Pluto system. The mean 0.31 - 0.60 keV X-ray power from Pluto is 200 +200/-100 MW, in the middle range of X-ray power levels seen for other known Solar System emission sources: auroral precipitation, solar X-ray scattering, and charge exchange (CXE) between solar wind (SW) ions and atmospheric neutrals. We eliminate auroral effects as a source, as Pluto has no known magnetic field and the New Horizons Alice UV spectrometer detected no airglow from Pluto during the flyby. Nano-scale atmospheric haze particles could lead to enhanced resonant scattering of solar X-rays from Pluto, but the energy signature of the detected photons does not match the solar spectrum and estimates of Pluto's scattered X-ray emission are 2 to 3 orders of magnitude below the 3.9 ± 0.7 × 10[superscript −5] cps found in our observations. Charge-exchange-driven emission from hydrogenic and heliogenic SW carbon, nitrogen, and oxygen (CNO) ions can produce the energy signature seen, and the 6 × 10[superscript 25] neutral gas escape rate from Pluto deduced from New Horizons’ data (Gladstone et al. 2016) can support the ∼3.0 +3.0/-1.5× 10[superscript 24] X-ray photons/s emission rate required by our observations. Using the solar wind proton density and speed measured by the Solar Wind Around Pluto (SWAP) instrument in the vicinity of Pluto at the time of the photon emissions, we find a factor of 40 +40/-20 lower SW minor ions flowing planarly into an 11 × 11 pixel[superscript 2], 90% flux box centered on Pluto than are needed to support the observed emission rate. Hence, the SW must be somehow significantly focused and enhanced within 60,000 km (projected) of Pluto for this mechanism to work. Keywords: Pluto, atmosphere; Solar wind; Interplanetary medium; Spectroscop

Charon tectonics

New Horizons images of Pluto's companion Charon show a variety of terrains that display extensional tectonic features, with relief surprising for this relatively small world. These features suggest a global extensional areal strain of order 1% early in Charon's history. Such extension is consistent with the presence of an ancient global ocean, now frozen