The cold interstellar medium is replete with dust particles surrounded by icy mantles ranging in size up to approximately 100 monolayers with water ice the dominant species. Other abundant molecules in the ice include CO, CO<sub>2</sub>, and methanol. The production of water-dominated ice starts on bare dust particles of silicates or amorphous carbon. This production occurs via chemical reactions on the dust particles in addition to accretion from the interstellar gas, where an active chemistry is also taking place. As the water ice covers the dust, the subsequent surface chemistry occurs mainly on and in the ice mantles. The details of the low temperature ice mantle chemistry are still poorly understood, however, and a number of exotic mechanisms in addition to the standard ones in surface science have been suggested. One basic unsolved problem is that so-called “complex” interstellar organic molecules (COMs) are produced much more efficiently in the laboratory and on cold interstellar dust than calculated with chemical simulations using standard reaction mechanisms. Because the molecules are mainly detected in the gas phase, an additional problem emerges: how can molecules produced on and in solid mantles desorb into the gas at all at temperatures as low as 10 K? The problems with ice chemistry are sufficiently severe that some investigators have returned to the older view that the complex molecules detected in the gas are actually produced in the gas, a view that had been rejected after it was concluded that complex molecules in warm regions associated with star formation are more likely produced on grains and subsequently thermally desorbed into the gas. But there are problems with this view as well. The current situation regarding these contrasting views will be discussed with emphasis on the role of water ice as a universal “catalyst."

Chemistry on mantles of water ice surrounding interstellar dust particles

2019 Fall National Meeting

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