LRO-LAMP Observations of the Preperihelion Coma of Comet C/2013 A1 (Siding Spring)

The Lyman-Alpha Mapping Project (LAMP) UV spectrograph on board the Lunar Reconnaissance Orbiter observed comet C/2013 A1 (Siding Spring) from lunar orbit at closest approach. On 2014 September 5, Siding Spring came within ∼0.89 au of the Earth–Moon system and provided an opportunity for LAMP to contribute to the broader campaign of observations. The comet coma was imaged during two consecutive orbits, approximately 2 hr apart. Coma emissions of atomic oxygen were resolved within LAMP observations at distances up to ∼1.4 × 106 km from the comet nucleus. We report an atomic production rate for the observed oxygen of 9.42 ± 0.22 × 1027 s−1. We additionally place upper limits on the column densities of candidate species including noble gases and primary transitions. Leveraging this, we place upper limits on the production of hydrogen and carbon of <3.59 × 1026 s−1 and <8.41 × 1025 s−1, respectively. Additionally, we derive OH and H2O production rates and estimate an upper limit on the production of CO. The H2O (8.17 ± 2.64 × 1027 s−1) and OH (7.53 ± 2.244 × 1027 s−1) production rates are found to be in general agreement with previous studies when production rates are derived utilizing oxygen observations, branching ratios, and empirical formulations. Similarly, the upper limit on the production of CO (<1.33 × 1028 s−1) is found to be in good agreement with previous studies (within ∼10%) when we utilize the upper limit on CO Fourth Positive group emissions. © 2022. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Lunar exospheric argon modeling

International audienceArgon is one of the few known constituents of the lunar exosphere. The surface-based mass spectrometer Lunar Atmosphere Composition Experiment (LACE) deployed during the Apollo 17 mission first detected argon, and its study is among the subjects of the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) and Lunar Atmospheric and Dust Environment Explorer (LADEE) mission investigations. We performed a detailed Monte Carlo simulation of neutral atomic argon that we use to better understand its transport and storage across the lunar surface. We took into account several loss processes: ionization by solar photons, charge-exchange with solar protons, and cold trapping as computed by recent LRO/Lunar Orbiter Laser Altimeter (LOLA) mapping of Permanently Shaded Regions (PSRs). Recycling of photo-ions and solar radiation acceleration are also considered. We report that (i) contrary to previous assumptions, charge exchange is a loss process as efficient as photo-ionization, (ii) the PSR cold-trapping flux is comparable to the ionization flux (photo-ionization and charge-exchange), and (iii) solar radiation pressure has negligible effect on the argon density, as expected. We determine that the release of 2.6 × 1028 atoms on top of a pre-existing argon exosphere is required to explain the maximum amount of argon measured by LACE. The total number of atoms (1.0 × 1029) corresponds to ∼6700 kg of argon, 30% of which (∼1900 kg) may be stored in the cold traps after 120 days in the absence of space weathering processes. The required population is consistent with the amount of argon that can be released during a High Frequency Teleseismic (HFT) Event, i.e. a big, rare and localized moonquake, although we show that LACE could not distinguish between a localized and a global event. The density of argon measured at the time of LACE appears to have originated from no less than four such episodic events. Finally, we show that the extent of the PSRs that trap argon, 0.007% of the total lunar surface, is consistent with the presence of adsorbed water in such PSRs

Ganymede’s tenous atmosphere

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Surface, Subsurface and Atmosphere Exchanges on the Satellites of the Outer Solar System

The surface morphology of icy moons is affected by several processes implicating exchanges between their subsurfaces and atmospheres (if any). The possible exchange of material between the subsurface and the surface is mainly determined by the mechanical properties of the lithosphere, which isolates the deep, warm and ductile ice material from the cold surface conditions. Exchanges through this layer occur only if it is sufficiently thin and/or if it is fractured owing to tectonic stresses, melt intrusion or impact cratering. If such conditions are met, cryomagma can be released, erupting fresh volatile-rich materials onto the surface. For a very few icy moons (Titan, Triton, Enceladus), the emission of gas associated with cryovolcanic activity is sufficiently large to generate an atmosphere, either long-lived or transient. For those moons, atmosphere-driven processes such as cryovolcanic plume deposition, phase transitions of condensable materials and wind interactions continuously re-shape their surfaces, and are able to transport cryovolcanically generated materials on a global scale. In this chapter, we discuss the physics of these different exchangeprocesses and how they affect the evolution of the satellites’ surfaces

Ganymede’s tenous atmosphere

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