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Project Progress

We uniquely probed the physical environment in the inner coma of comet C/2012 S1 (ISON) during its first (and last) approach to the Sun since being emplaced in the Oort Cloud some 4.5 billion years ago. We extracted H₂O production rates and spatial profiles of gas rotational temperature and molecular column abundance (Figure 1), observed at heliocentric distances of 0.53 and 0.35 AU. Our observations revealed a comet evolving on various time-scales, both over hours and days. At 0.35 AU, ISON showed a considerable decrease in water production rate in less than 2 hr, likely declining after a major outburst.

The applied technique extends the state-of-the-art for spatial-spectral studies of coma physical parameters, revealed through long-slit studies of cometary water – the most abundant primary volatile in the coma. Our measured spatial distributions of temperature not only serve as a sensitive probe of the near-nucleus environment, but also are diagnostic for how water vapor is released in comets – directly from the nucleus versus via extended release from icy grains in the coma. These are fundamental questions in cometary science, relevant to understanding of both surface and interior evolution and critical for obtaining the most accurate abundances of water and prebiotic molecules.