Stellar Occultation by Comet 67P/Churyumov-Gerasimenko Observed with Rosetta's Alice Far-ultraviolet Spectrograph
Parker, Joel Wm.; Bertaux, Jean-Loup; Stern, S. Alan; Weaver, Harold A.; Feldman, Paul D.; Steffl, Andrew J.; A'Hearn, Michael F.; Feaga, Lori M.; Keeney, Brian A.; Knight, Matthew M.; Noonan, John; Vervack, Ronald J., Jr.; Medina, Richard A.; Pineau, Jon P.; Schindhelm, Rebecca N.; Versteeg, M.
United States, France
Abstract
Following our previous detection of ubiquitous {{{H}}}2{{O}} 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 Å) spectrograph, both of which show {{{H}}}2{{O}} absorption with column density >1017.5 cm-2 and significant {{{O}}}2 absorption ({{{O}}}2/{{{H}}}2{{O}} ≈ 5%-10%). Because the projected distance from the star to the nucleus changes between exposures, our ability to study the {{{H}}}2{{O}} column density profile near the nucleus (impact parameters <1 km) is unmatched by our previous observations. We find that the {{{H}}}2{{O}} and {{{O}}}2 column densities decrease with increasing impact parameter, in accordance with expectations, but the {{{O}}}2 column decreases ∼3 times more quickly than {{{H}}}2{{O}}. When combined with previously published results from stellar appulses, we conclude that the {{{O}}}2 and {{{H}}}2{{O}} column densities are highly correlated, and {{{O}}}2/{{{H}}}2{{O}} decreases with the increasing {{{H}}}2{{O}} column.