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

Following our previous detection of ubiquitous H2O and O-2 absorption against the far-ultraviolet continuum of stars located near the nucleus of Comet 67P/Churyumov-Gerasimenko, we present a serendipitously observed stellar occultation that occurred on 2015 September 13, approximately one month after the comet's perihelion passage. The occultation appears in two consecutive 10-minute spectral images obtained by Alice, Rosetta's ultraviolet (700-2100 angstrom) spectrograph, both of which show H2O absorption with column density >10(17.5) cm(-2) and significant O-2 absorption (O-2/H2O approximate to 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 O-2 column densities decrease with increasing impact parameter, in accordance with expectations, but the O(2 )column decreases similar to 3 times more quickly than H2O. When combined with previously published results from stellar appulses, we conclude that the O-2 and H2O column densities are highly correlated, and O-2/H(2)Odecreases with the increasing H2O column.