Rosetta-Alice Observations of Exospheric Hydrogen and Oxygen on Mars

TL;DR

The Rosetta spacecraft's Alice instrument observed hydrogen and oxygen emissions in Mars' exosphere during a fly-by, providing data on atmospheric composition and escape fluxes that align with some past measurements but challenge recent models.

Contribution

This study presents the first in situ measurements of Mars' exospheric hydrogen and oxygen using a spacecraft not dedicated to Mars, offering new insights into atmospheric escape processes.

Findings

Hydrogen density at 200 km exobase derived from observations.

Hydrogen escape flux consistent with previous Mariner data.

Oxygen scale height measurements challenge recent escape models.

Abstract

The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on 25 February 2007, closest approach being at 01:54UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000 A spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta emissions from exospheric hydrogen out beyond 30,000 km from the planet's center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal oxygen atoms by the dissociative recombination of O2+.