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

## TL;DR

This study reports on a serendipitous stellar occultation observed by Rosetta's Alice spectrograph, revealing detailed measurements of water and oxygen absorption near Comet 67P, and analyzing their spatial distribution and correlation.

## Contribution

First detection of O2 absorption in a stellar occultation by Comet 67P, providing high-resolution data on H2O and O2 distribution close to the nucleus.

## Key findings

- O2 and H2O column densities decrease with impact parameter.
- O2 decreases faster than H2O with distance from nucleus.
- O2/H2O ratio decreases as H2O column density increases.

## Abstract

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 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 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.