Photosynthesis of H2O2 and O3 in ASW with ambient O2

Details Photosynthesis of H2O2 and O3 in ASW with ambient O2 Photosynthesis of H2O2 and O3 in ASW with ambient O2

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p21b1226s&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P21B-1226

Physics

[5422] Planetary Sciences: Solid Surface Planets / Ices

Scientific paper

Tenuous O2 atmospheres are known to exist around icy satellites such as Europa and Ganymede and around Saturn’s rings and satellites [1,2]. These faint atmospheres with pressures estimated in 10-3 to 10-8 mbar range [2] are thought to be produced from photolysis and radiolysis of surface ice. In efforts to simulate closely the conditions on icy satellites and grains, ion irradiation of laboratory ices have been performed while maintaining an ambient O2 pressure in recent experiments, which lead to the discovery of the ion-induced O2 adsorption [3]. In the experiments presented here, we investigated the effect of UV photolysis on the ice films in the presence of ambient O2 pressure. Water ice films vapor-deposited between 40 and 100 K were irradiated using 193 nm photons from an ArF Excimer laser, under a constant ambient gas pressure of 7x10-7 mbar. The changes in the film during laser irradiation were monitored using infrared spectroscopy and quartz crystal microgravimetry. At 50 K, we exposed our film to photolysis under ambient pressure of different gas species: Ar, N2 and O2. Interestingly we find enhanced gas absorption only in the presence of ambient O2. In addition, we find that the dangling bond IR absorption disappears, indicating the collapse of micropores. We note that the dangling bond absorption decreases only ~10% in an ice film subject to UV irradiation in the absence of ambient gas, which suggests that the enhanced O2 absorption is not due to gas trapping during compaction [3], but to photo-chemical reactions between OH and transiently adsorbed O2, forming H2O2 and O3, which we detected with IR spectroscopy. We observed that the formation of these two products increases below 50 K, which is explained by increased O2 adsorption at decreasing temperature. Interestingly, if we increase the ambient O2 pressure, then we can increase the maximum temperature for which we can make O3. O3 has not been identified in photolysis of laboratory ice and only traceable H2O2 [4] can be detected by photolysis of pure ice. Our experiments show that solar UV irradiation can enhance H2O2 and allow O3 to be photo-synthesized on icy bodies of the outer solar system, such as those on the surface of Ganymede, Dione and Rhea [5], in the presence of an oxygen exospheres. 1 D.T.Hall et al. (1998) APJ, 499, 475; R.Tokar et al (2005) GRL, 32, L14S04 2 D.T.Hall et al. (1995) Nature, 373, 677 3 J.Shi et al. (2009) PRB, 79, 235422 4 R.A.Baragiola et al., CH 3-3, Radiation Effects on Water Ice, The Science of Solar System Ices, Spinger, To be Published 5 K.S.Noll et al (1996) Science, 273, 341; (1997)Nature 388, 45

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