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Following our previous detection of ubiquitous H2O and O2 absorption againstthe far-UV continuum of stars located near the nucleus of Comet67P/Churyumov-Gerasimenko, we present a serendipitously observed stellaroccultation that occurred on 2015 September 13, approximately one month afterthe comet's perihelion passage. The occultation appears in two consecutive10-minute spectral images obtained 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 ($\mathrm{O2/H2O} \approx5$-10%). Because the projected distance from the star to the nucleus changesbetween exposures, our ability to study the H2O column density profile near thenucleus (impact parameters $<1$ km) is unmatched by our previous observations.We find that the H2O and O2 column densities decrease with increasing impactparameter, in accordance with expectations, but the O2 column decreases $\sim3$times more quickly than H2O. When combined with previously published resultsfrom stellar appulses, we conclude that the O2 and H2O column densities arehighly correlated, and O2/H2O decreases with increasing H2O column.

Stellar Occultation by Comet 67P/Churyumov-Gerasimenko Observed with Rosetta's Alice Far-Ultraviolet Spectrograph