Comet 67P/Churyumov-Gerasimenko’s Increasing Atomic Sulfur Abundance Observed by Rosetta Alice

Alice, NASA’s lightweight and low-power far-ultraviolet (FUV) imaging spectrograph onboard ESA’s comet orbiting spacecraft Rosetta (Stern et al. 2007), is continuing its characterization of the nucleus and coma of the Jupiter family comet 67P/Churyumov-Gerasimenko (C-G) as it approaches and recedes from perihelion. With a spectral range from 700-2050 Å, Alice has the ability to detect the atomic sulfur multiplets at 1429 Å, 1479 Å and 1814 Å. Sulfur in C-G’s coma is most likely a dissociation product of CS₂ and OCS, but could also be produced after a secondary dissociation from H₂S and SO₂, all molecular species measured in C-G’s coma by ROSINA, the Rosetta orbiter’s mass spectrometer.Due to low abundances, Alice did not detect sulfur atoms at C-G until May 2015 when the comet was at ~1.7 AU and still 3 months from perihelion. Now, sulfur is ubiquitous in Alice observations above the limb of the nucleus. There is evidence that there is not a strong dependence of the abundance of sulfur on the distance from the nucleus in the pre-perihelion radial profiles of the gas, which may be indicative of the parent molecule and its distribution. This will be investigated further. The evolution of the presence of the three sulfur multiplets, their relative abundances and excitation processes, and behavior pre- and post-perihelion will be presented.