O2 Outgassing at Icy Satellites and Comets
Abstract
The production of atmospheric molecular oxygen beyond Earth has been an enigma for decades after being discovered at the icy Galilean satellites as well as the large icy Saturnian satellites (Hall et al. 1995, 1998; Teolis & Waite 2016). O2 was also observed in surprisingly abundant quantities in the comas of comet Halley (Rubin et al. 2015) and comet 67P/C-G (Bieler et al. 2015 ; Keeney et al. 2017). The recent observation and modeling of a diurnal O2 cycle on Europa and Ganymede suggest that radiolytically produced O2 is being thermally outgassed from O2 trapped in the ice at dangling hydrogen bonds with an activation energy 1.5 times that of pure O2 sublimation (Leblanc et al. 2017 ; Oza et al. 2018 ; Johnson et al. 2018). The O2 volatiles are also observed to be much more strongly trapped in bubbles in the irradiated icy surfaces of Europa and Ganymede (Spencer et al. 1995, 2002). Due to the much higher surface temperatures and several meters of mass loss per orbit (Keller et al. 2015), the in-situ observations of O2 at 67P/C-G by Rosetta/ROSINA and Rosetta/ALICE are not likely to be due to weakly bound O2, but rather due to the release of the O2 trapped in bubbles, observed in the solid phase in the low-temperature regoliths (< 150K) of the icy Galilean satellites. Because the radiolytic formation of O2 in the gas phase is inefficient, the modeling at Europa suggests that the ice grains in the solar nebula were likely irradiated forming O2 in the solid phase prior to their assembly in comets. Identifying the thermal mechanism of O2 outgassing is critical to being able to describe the observed O2 column density on any airless icy body.
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #50
- Pub Date:
- October 2018
- Bibcode:
- 2018DPS....5040303O