The Shape, Structure, and Dynamics of Volatile-Driven Exospheres on Airless Bodies Across the Solar System

The majority of objects in the Solar System lack thick atmospheres. Nevertheless, many of the nominally "airless" bodies in our Solar System can and do possess tenuous, often temporary surface-bound exospheres composed of common volatile materials. The shape, structure, and dynamics of these exospheres are sensitive to the physical properties, composition, and location of their host body. For example, gases that condense onto the nighttime surface of a refractory body (e.g., argon or water on the Moon) tend to form a narrow exosphere in the morning [1-3], that runs from pole to pole [3]. However, the location of this morning exosphere is sensitive to the thermal conditions of the body. Bodies with volatile-rich (i.e., icy) surfaces form global exospheres with a sublimative mass flux and surface vapor pressure that depend on local thermal conditions. Furthermore, photodestruction and ballistic escape eliminate exospheres at rates that depend on solar insolation conditions and physical properties of the object respectively.

Here we discuss three first-order physical quantities that describe how exospheric structure and density vary over the surface of a body: sublimation/desorption (which populate an exosphere), and photodestruction and ballistic escape (which permanently remove exospheric molecules). We neglect sputtering-induced exospheres, which, while important in the Jovian and Saturnian systems, are nevertheless outside of our thermal considerations. We consider exospheres created by four of the most common volatile species in the Solar System (H₂O, CO₂, CO, and N₂), and consider how the resulting exospheric structure, density, lifetime, and dynamic vary by species, heliocentric distance, and properties of the host body. Lastly, we consider how a body's exospheric properties mediate volatile migration across the surfaces of bodies and form stable albedo contrasts (both regional and global).

[1] Hodges, R.R. (1975) The Moon 14, 139

[2] Stern, S.A. (2007) Rev. of Geophys. 37, 453

[3] Stewart, B.D. et al. (2011) Icarus 215, 1