Rosetta-Alice observations of exospheric hydrogen and oxygen on Mars

Abstract

The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swingby of Mars on 25 February 2007, closest approach being at 01:54 UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000Å 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-α and Lyman-β 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 1304Å 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 1 oxygen atoms by the dissociative recombination of O + 2 .

Solar wind interaction with the Martian upper atmosphere: Crustal field orientation, solar cycle, and seasonal variations

“…For the neutral atmosphere densities, however, we use an extrapolation based upon the hydrostatic assumption which assumes the neutral atmosphere densities decrease exponentially with altitude, i.e., n = n 0 exp(−dz/H s ), where dz is the altitude change and H s is the scale height (which depends on the gravity, neutral temperature, and neutral species mass). Technically speaking, the hydrostatic assumption may not be accurate enough to describe the cold oxygen component in the Martian exosphere, which should dominate the hot component up to 600 km in altitude [Feldman et al, 2011] Planets, 2015) and ALICE/Rosetta observations of the OI 1304 Å brightness [Feldman et al, 2011] shows good agreement with each other on the transition altitude from cold to hot oxygen (∼600 km), indicating that our extrapolation approach is reasonable. It is noteworthy that the cold oxygen component also plays an important role in the solar wind-Mars interaction, especially below 600 km.…”

“…The cold exospheric oxygen component [e.g., see Ma et al, 2004, Figures 1 and 2] also plays an important role in the solar wind interaction with the Martian upper atmosphere, especially below 600 km [Feldman et al, 2011]. In order to reproduce a realistic asymmetric corona of hot species from observations, a 3-D global kinetic exosphere model is required, especially above the exobase (Knudsen number, K n ≈ 1) where the fluid assumption usually fails [Lee et al, 2013].…”

Modeling of Venus, Mars, and Titan

“…The profile of the 130 nm oxygen emission observed by ALICE onboard Rosetta during its Mars flyby present a change in slope, indicating two oxygen populations (Feldman et al 2011). The cold oxygen population is dominant below 500 km while the hot oxygen population is dominant above 500 km (Feldman et al 2011). In the last decades, several models have been developed to study the Martian exosphere.…”

Influence of Martian crustal magnetic anomalies on the emission of energetic neutral hydrogen atoms

“…This approximation is valid for the current study because (1) this difference is less than the WANG ET AL. scale heights of atomic hydrogen (∼ 600 km) and hot atomic oxygen (∼ 240 km) [Feldman et al, 2011] which are the major neutrals to produce charge exchange ENAs, (2) the variation of the IMB altitude during the whole observation period is larger than this difference [Dubinin et al, 2006], and (3) the resultant ENA source intensity map is not sensitive to the altitude of IMB. We made a test assuming different reference sphere altitudes (e.g., 0.1 or 0.3 R M ), the results do not change at current resolution (20 • × 20 • ), meaning that the backtracing method is robust under current resolution against different IMB altitudes in a realistic range.…”