# H$_2$O and O$_2$ Absorption in the Coma of Comet 67P/Churyumov-Gerasimenko Measured by the Alice Far-Ultraviolet Spectrograph on Rosetta

## TL;DR

This study used the Alice spectrograph on Rosetta to detect and measure H$_2$O and O$_2$ absorption in the coma of Comet 67P, revealing higher O$_2$/H$_2$O ratios than previous measurements.

## Contribution

First independent detection of O$_2$ absorption in the comet's coma using stellar occultations, providing new insights into gas abundances near the nucleus.

## Key findings

- O$_2$ absorption confirms ROSINA detection
- O$_2$/H$_2$O ratio is higher than previous measurements
- Column densities agree with VIRTIS-H data

## Abstract

We have detected H$_2$O and O$_2$ absorption against the far-UV continuum of stars located on lines of sight near the nucleus of Comet 67P/Churyumov-Gerasimenko using the Alice imaging spectrograph on Rosetta. These stellar appulses occurred at impact parameters of $\rho=4$-20 km, and heliocentric distances ranging from $R_h=-1.8$ to 2.3 AU (negative values indicate pre-perihelion observations). The measured H$_2$O column densities agree well with nearly contemporaneous values measured by VIRTIS-H. The clear detection of O$_2$ independently confirms the initial detection by the ROSINA mass spectrometer; however, the relative abundance of O$_2$/H$_2$O derived from the stellar spectra (11%-68%, with a median value of 25%) is considerably larger than published values found by ROSINA. The cause of this difference is unclear, but potentially related to ROSINA measuring number density at the spacecraft position while Alice measures column density along a line of sight that passes near the nucleus.