Photodesorption and Photosynthesis in Ice Mantles

BARAGIOLA, RAÚL A.; BAHR, D. A.;; VIDAL, R. A

Estes Park, Colorado, EEUU

Simposio; Astrophysics of Dust; 2003

University of Toledo, EEUU

 The stability of ice mantles in interstellar grains exposed depends on photodesorption by Lyman-alpha radiation, which may be more important than thermal desorption at low temperatures. We have previously reported on the photodesorption of water ice and its astrophysical implications (1,2) and included evidence for the formation of molecular oxygen with a strong temperature dependence (2). Here we report new measurements on photodesorption from solid water, oxygen and carbon dioxide, which includes absolute total desorption yields and mass spectra of desorbed species, performed under ultrahigh vacuum conditions. During irradiation, we determined total desorption yields from the mass lost by the ice samples using a microbalance, and partial desorption yields of atoms and molecules by using mass spectrometry. In contrast to the case of water ice deposited on an optically smooth surface (1,2), the photodesorption yield of ice grown on rough surfaces does not show a fluence dependence and, in addition, is about three times larger. For O2 and CO2 ices, the yields depend on fluence due to the formation of radicals and the photosynthesis of new species (3). For carbon dioxide, we observe enhanced thermal desorption of CO, O2 and O2 when the photolyzed ice is warmed to temperatures below which thermal evaporation is important. We attribute this enhanced thermal desorption to thermally activated, exothermic reactions involving radicals. Ozone is photosynthesized in solid oxygen and carbon dioxide but is undetected in water ice. (1) Photodesorption from Low Temperature Water Ice in Interstellar and Circumsolar Grains. M. S. Westley, R. A. Baragiola, R. E. Johnson, and G. Baratta, Nature 373, 405-407 (1995) (2) Ultraviolet Photodesorption from Water Ice, M. S. Westley, R. A. Baragiola, R. E. Johnson, and G. A. Baratta, Planetary and Space Science 43, 1311 (1995) (3) P. A. Gerakines, W. A. Schutte, and P. Ehrenfreund, Ultraviolet Processing of Interstellar Ice Analogs, Astron. Astrophys. 312, 289-305 (1996)