UV Induced Surface Chemistry

Group leader: Svend Knak Jensen

Heterogeneous oxidation of Fe(II) in a Mars-like environment

Background

Pictures of the tracks left by the previous Pathfinder Rover as it moved around on the dusty Martian surface showed that while the surface layer was red the material below was dark. These colors are interpreted as originating from Fe(III)- and Fe(II) compounds, respectively. It would be straightforward to explain the oxidation of Fe(II) to Fe(III) if water was abundant on Mars. However, Mars is a very dry planet and the question arises as to how Fe(II) may be oxidized in the Martian environment without involving water.

Project

We have theorized that the dominant gas in the atmosphere, carbon dioxide CO2, along with the abundant UV radiation reaching the surface due to the absence of ozone in the atmosphere are important factors. We have proposed the following heterogeneous reaction mechanism leading to oxidation of Fe(II) to Fe(III).

It is known from in situ experiments that the Martian soil is highly oxidizing. The detailed chemical composition of the surface is not known, so there is some freedom in selecting a chemical species in a high oxidation state for the mechanism. We have chosen nitrate, NO3-. Laboratory experiments show that UV radiation - with wavelengths similar to those present on the Martian surface - will isomerize nitrate to peroxynitrite, ONOO-

NO3- → ONOO-

Structure of the nitrate ion Structure of the
peroxynitrate ion

Peroxynitrite will react with carbon dioxide to produce an adduct

ONOO- + CO2 → [ONOO, CO2]-

Structure of the adduct of the peroxynitrite ion and carbon dioxide

This adduct may break up into two highly reactive radicals

[ONOO, CO2]- → CO3- + NO2

The carbonate radical, CO3-, may oxidize Fe(II) to Fe(III)

CO3- + Fe(II) → CO32- + Fe(III)

Thermodynamic data for a homogeneous medium, like water, show that the final reaction is feasible for that medium. Similar data are not available for minerals likely to be found on Mars and we are therefore performing suitable experiments in the Surface chemistry environmental chamber to investigate the feasibility of the above heterogeneous reaction scheme.

Personnel

Svend Knak Jensen

Per Nørnberg

Jonathan P. Merrison

Haraldur Páll Gunnlaugsson

Martha E. C. Ventura