Stellar Occultation by Comet 67P/Churyumov–Gerasimenko Observed with Rosetta's Alice Far-ultraviolet Spectrograph
2019
Following our previous detection of ubiquitous H2O and O2 absorption against the far-UV continuum of stars located near the nucleus of Comet 67P/Churyumov-Gerasimenko, we present a serendipitously observed stellar
occultationthat occurred on 2015 September 13, approximately one month after the comet's perihelion passage. The
occultationappears in two consecutive 10-minute
spectral imagesobtained by Alice, Rosetta's ultraviolet (700-2100 A) spectrograph, both of which show H2O absorption with column density $>10^{17.5} \mathrm{cm}^{-2}$ and significant O2 absorption ($\mathrm{O2/H2O} \approx 5$-10%). Because the projected distance from the star to the nucleus changes between exposures, our ability to study the H2O column density profile near the nucleus (
impact parameters$<1$ km) is unmatched by our previous observations. We find that the H2O and O2 column densities decrease with increasing
impact parameter, in accordance with expectations, but the O2 column decreases $\sim3$ times more quickly than H2O. When combined with previously published results from stellar
appulses, we conclude that the O2 and H2O column densities are highly correlated, and O2/H2O decreases with increasing H2O column.
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