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SPUTTERING AND MOLECULAR SYNTHESIS INDUCED BY 100 keV PROTONS IN CONDENSED CO2 AND RELEVANCE TO THE OUTER SOLAR SYSTEM

We present results on sputtering and radiation chemistry of CO2 films induced by 100 keV H+ at 25 and 50 K. Using a quartz crystal microbalance, we measure a sputtering yield (SY) between ∼10 and 20 CO2 equivalent per ion at 25 K. The yield at 50 K is similar to that at 25 K at low fluences, but increases to ∼2400 by mid-1014 H+ cm−2 and declines at higher fluence. Irradiation to 1 × 1015 H+ cm−2 depletes ∼85%–90% of the initial film mass at 50 K, compared to 3% at 25 K. In both cases, mass spectrometry shows that CO is the dominant constituent in the sputtered flux, followed by O2, O, and CO2. Using infrared spectroscopy, we monitor the depletion of CO2 and the accumulation of CO and O2 and minor species as O3 and CO3. We determine G(−CO2) = 2.6 ± 0.3, the number of CO2 destroyed per 100 eV at 25 K. A significant fraction of the radiolyzed CO and O2 are retained in the film at 25 K; only those near the surface are removed during irradiation, contributing to a smaller SY. At 50 K, CO and O2 are unstable along the “hot” ion track and are expelled possibly from the entire depth of the film. Our results, and the lack of detection of CO in the exospheres around Rhea and Dione, show that the CO2 does not originate from sputtering, since otherwise the exosphere would be dominated by CO, the main molecule in the sputtered flux. We suggest that the exospheric CO2 is thermally released from an endogenic source.