Exospheres, the tenuous gas environments surrounding planets, planetary satellites, and cometary comae, play a significant role in mediating the interactions of these astronomical bodies with their surrounding space environments. This paper presents a comprehensive review of both analytical and numerical methods employed in modeling exospheres. The paper explores analytical models, including the Chamberlain and Haser models, which have significantly contributed to our understanding of exospheres of planets, planetary satellites, and cometary comae. Despite their simplicity, these models provide baselines for more complex simulations. Numerical methods, particularly the Direct Simulation Monte Carlo (DSMC) method, have proven to be highly effective in capturing the detailed dynamics of exospheres under non-equilibrium conditions. The DSMC method's capacity to incorporate a wide range of physical processes, such as particle collisions, chemical reactions, and surface interactions, makes it an indispensable tool in planetary science. The Adaptive Mesh Particle Simulator (AMPS), which employs the DSMC method, has demonstrated its versatility and effectiveness in simulating gases in planetary and satellite exospheres and dusty gas cometary comae. It provides a detailed characterization of the physical processes that govern these environments. Additionally, the multi-fluid model BATSRUS has been effective in modeling neutral gases in cometary comae, as discussed in the paper. The paper presents methodologies of exosphere modeling and illustrates them with specific examples, including the modeling of the Enceladus plume, the sodium exosphere of the Moon, the coma of comet 67P/Churyumov-Gerasimenko, and the hot oxygen corona of Mars and Venus.

The extremely hot and dense environment on Venusian surface will degrade almost any material via atmosphere-surface interactions, therefore the exploitation of its soil and atmosphere is very challenging. Exploring rovers are designed with mostly mechanical parts, and the knowledge of the effect of Venusian exposure on mechanical and tribological systems is very important for the longevity of the missions. Herein, we studied the effect of 3-day Venusian exposure on selected interfaces. It was found that diamond-like carbon (DLC) and Ti-doped molybdenum disulfide (TiMoS2) experienced negligible morphological changes, whereas polycrystalline diamond (PCD) and PS400 (plasma sprayed Ni-alloy) formed few-microns thick sulfur-containing reacted surface layers after exposure. Also, PCD retained its structural integrity, while the mechanical properties of DLC deteriorated the most, manifested as 49% decrease in hardness. The hardness of PS400 and TiMoS2 degraded to a lesser degree, with 8 and 26% decrease, respectively. The above coatings could be candidate materials to coat structural and bearing systems in the rovers, probes, and drills for future missions to Venus. (c) 2024 COSPAR. Published by Elsevier B.V. All rights reserved.

Venus and Mars likely had liquid water bodies on their surface early in the Solar System history. The surfaces of Venus and Mars are presently not a suitable habitat for life, but reservoirs of liquid water remain in the atmosphere of Venus and the subsurface of Mars, and with it also the possibility of microbial life. Microbial organisms may have adapted to live in these ecological niches by the evolutionary force of directional selection. Missions to our neighboring planets should therefore be planned to explore these potentially life-containing refuges and return samples for analysis. Sample return missions should also include ice samples from Mercury and the Moon, which may contain information about the biogenic material that catalyzed the early evolution of life on Earth (or elsewhere). To obtain such information, science-driven exploration is necessary through varying degrees of mission operation autonomy. A hierarchical mission design is envisioned that includes spaceborne (orbital), atmosphere (airborne), surface (mobile such as rover and stationary such as lander or sensor), and subsurface (e.g., ground-penetrating radar, drilling, etc.) agents working in concert to allow for sufficient mission safety and redundancy, to perform extensive and challenging reconnaissance, and to lead to a thorough search for evidence of life and habitability.