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Project Progress

Additional reactions were add to and modifications in the ozone cascade sequence were made to our existing in-house Monte Carlo grain surface chemistry code. No changes to our early results were detected. Traditionally, it has been assumed that CO₂ ice is formed by atomic O reacting with CO. However, laboratory studies do not support this formation mechanism. An important finding from this study is that CO + OH can reasonably account for the observed CO and CO₂ ice abundances. If O + CO is responsible for CO₂ then trace species with appreciable abundances, including NH₂CHO and H₂O₂ are formed. The O₂/O gas phase ratio is large so that N preferentially reacts with HCO (as opposed to O) and, as a result of the large build-up of O2 on the surface, H + O₂ becomes a dominant reaction. On the other hand, if OH + CO forms CO₂ then we find HCOOH and NH₂CHO but not H₂O₂ . In this case, there is more atomic O in the gas phase that, once it is accreted, can react with the HCO radical. A concerted effort should be made to search for signatures of these trace molecules which will then better constrain the chemical reactions that occur on grain surfaces.