Radiolysis and Photolysis of Icy Satellite Surfaces: Experiments and Theory

Abstract

The transport and exchange of material between bodies in the outer solar system is often facilitated by their exposure to ionizing radiation. With this in mind we review the effects of energetic ions, electrons and UV photons on materials present in the outer solar system. We consider radiolysis, photolysis, and sputtering of low temperature solids. Radiolysis and photolysis are the chemistry that follows the bond breaking and ionization produced by incident radiation, producing, e.g., O 2 and H 2 from irradiated H 2 O ice. Sputtering is the ejection of molecules by incident radiation. Both processes are particularly effective on ices in the outer solar system. Materials reviewed include H 2 O ice, sulfur-containing compounds T. Cassidy et al. (such as SO 2 and S 8 ), carbon-containing compounds (such as CH 4 ), nitrogen-containing compounds (such as NH 3 and N 2 ), and mixtures of those compounds. We also review the effects of ionizing radiation on a mixture of N 2 and CH 4 gases, as appropriate to Titan's upper atmosphere, where radiolysis and photolysis produce complex organic compounds (tholins).

A Hypothesis for the Color Bimodality of Jupiter Trojans

“…Irradiation studies of sulfurbearing ices have found that among the stable irradiation products are long-chained sulfur molecules (e.g., Moore 1984;Cassidy et al 2010), which produce a dark residue and have been interpreted, for example, as the cause of the darkened, intensely red deposits found on Io (Carlson et al 2007). Within the framework of our hypothesis, we propose that the retention of H 2 S on the surface of objects in the outer primordial disk contributed to a significant additional reddening, leading to the VR colors.…”

Exobiology and Planetary Protection of icy moons

“…The resulting organics which are produced through high energy photon and electron chemistry, include low molecular organic species, several of which are of prebiotic interest, such as HCN and HC 3 N, and higher molecular compounds, tholin-like products (cf. Cassidy et al 2010, this issue). The later probably form the aerosol particles present in the lower parts of the atmosphere.…”

“…Atmospheric chemistry in Titan allows the continuous transformation of methane and dinitrogen into complex organic compounds from the high atmosphere to the surface (see Coustenis et al 2010, this issue;Cassidy et al 2010, this issue).The recent Cassini data, especially from the INMS and CAPS instruments, indicates that the processes start in the ionosphere (Waite et al 2005(Waite et al , 2007. The resulting organics which are produced through high energy photon and electron chemistry, include low molecular organic species, several of which are of prebiotic interest, such as HCN and HC 3 N, and higher molecular compounds, tholin-like products (cf.…”

Planetary space weather: scientific aspects and future perspectives

“…The interactions between the Galilean satellites and the plasma of the Jovian magnetosphere lead to local changes in the charged particle populations through either additions of new ions or through changes of the momentum that produce plasma heating or cooling. Plasma-surface interactions at Europa and Ganymede have been shown to generate neutral environments around these moons through either direct ion sputtering or radiolysis, followed by sputtering of the recombined water-dissociation products (Shematovich & Johnson 2001;Shematovich et al 2005;Cassidy et al 2010Cassidy et al , 2013. These exospheres have been demonstrated to be spatially non-uniform (Plainaki et al 2012(Plainaki et al , 2013, depending both on the moons' illumination by the Sun and on Jupiter's magnetospheric plasma properties.…”

“…Indirectly: The impact of energetic ions on an icy satellite surface is followed by the release of surface material to the body's exosphere through direct sputtering and/or radiolysis (for an example of how these release mechanisms work see: Johnson et al 2004;Cassidy et al 2010;Teolis et al 2010;Plainaki et al 2012). The generated exospheres are furthermore subject to loss processes either due to gravity escape and particle sticking to the surface or due to photon or plasma-induced ionization/dissociation of their constituents.…”