Europa's Atmosphere: Regolith Effects

Details Europa's Atmosphere: Regolith Effects Europa's Atmosphere: Regolith Effects

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.6407c&link_type=abstract

American Astronomical Society, DPS meeting #38, #64.07; Bulletin of the American Astronomical Society, Vol. 38, p.613

Astronomy and Astrophysics

Astronomy

Scientific paper

The surfaces of 'airless' bodies in our solar system are covered by porous regoliths; granular, porous surfaces generated by impacts. Europa's tenuous neutral atmosphere (composed primarily of O2) is generated within this regolith.
Hubble Space Telescope (HST) observation by McGrath et al. (2004) suggested that Europa's O2 atmosphere is spatially nonuniform. Previous simulations (e.g. Shematovich et al., Icarus, 2005), which assumed that O2 could only be lost by pickup and escape, found that a spatially nonuniform source could not produce the morphology suggested by the HST observations. It is shown here that a nonuniform surface/O2 interaction, treated here as surface reactivity, results in a spatially nonuniform O2 atmosphere with the highest column density above the regions of lowest reactivity. We can reproduce the observed nonuniformity even with a very slight reactivity in the sulfur rich, reddened regions.
A slight reactivity can make a significant difference in the atmospheric distribution because of the many interactions an O2 molecule experiences with the surface during its lifetime caused by two effects: (1) the many returns to the surface experienced by an O2 (desorption followed by ballistic arc followed again by desorption and so on); (2) the residence time in the porous regolith porosity involving many interactions with the surfaces of grains (Cassidy and Johnson, Icarus, 2005). Thus, in addition to the atmosphere above the surface, there is a substantial amount of gas in the porous regolith. This "regolith atmosphere" may also resolve a question surrounding CO2? condensed on Europa's surface: it should have a large vapor pressure but no CO2 vapor has been detected.
This work is supported by a GSRP fellowship through the NASA Langley Research Center.

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