P. Bertelsen

Evidence from Opportunity's microscopic imager for water on Meridiani Planum

The Microscopic Imager on the Opportunity rover analyzed textures of soils and rocks at Meridiani Planum at a scale of 31 micrometers per pixel. The uppermost millimeter of some soils is weakly cemented, whereas other soils show little evidence of cohesion. Rock outcrops are laminated on a millimeter scale; image mosaics of cross-stratification suggest that some sediments were deposited by flowing water. Vugs in some outcrop faces are probably molds formed by dissolution of relatively soluble minerals during diagenesis. Microscopic images support the hypothesis that hematite-rich spherules observed in outcrops and soils also formed diagenetically as concretions.

Indication of drier periods on Mars from the chemistry and mineralogy of atmospheric dust

The ubiquitous atmospheric dust on Mars is well mixed by periodic global dust storms, and such dust carries information about the environment in which it once formed and hence about the history of water on Mars. The Mars Exploration Rovers have permanent magnets to collect atmospheric dust for investigation by instruments on the rovers. Here we report results from Mössbauer spectroscopy and X-ray fluorescence of dust particles captured from the martian atmosphere by the magnets. The dust on the magnets contains magnetite and olivine; this indicates a basaltic origin of the dust and shows that magnetite, not maghemite, is the mineral mainly responsible for the magnetic properties of the dust. Furthermore, the dust on the magnets contains some ferric oxides, probably including nanocrystalline phases, so some alteration or oxidation of the basaltic dust seems to have occurred. The presence of olivine indicates that liquid water did not play a dominant role in the processes that formed the atmospheric dust.

Magnetic enhancement on the surface of Mars

The magnetic properties experiments on the Viking missions and the Pathfinder mission indicate that the Martian soil and airborne dust are somewhat magnetic (average saturation magnetization, σS ∼ 4 A m2kg−1). While hematite, superparamagnetic or macrocrystalline, is not sufficiently magnetic to yield the results obtained, pyrogenetic titaniferous magnetite (TiMt) might conceivably be the cause. However, the σS of the dust is considerably higher than that in any of the known Martian meteorites, some of which may be representative of the bedrock from which the Mars soil formed. Furthermore if the reported TiO2 content of Mars soil (∼1% by weight) was entirely present as TiMt of composition Usp 60 (that typical of terrestrial ocean floor basalts), the calculated abundance (<4%) would yield σS of only 1.2 A m2kg−1. As the Pathfinder magnetic properties experiment results pertain only to the airborne dust particles on Mars, the likelihood of aeolian concentration of such TiMt grains is minimal. Ferrous iron in the bedrock silicates must have been converted to maghemite (γ-Fe2O3) by some unknown oxidative mechanism; this “magnetic enhancement” should be incorporated in any process envisioned for the origin of Martian soil.

Magnetic Properties Experiments on the Mars Polar Lander

The Mars Polar Lander carries an instrument package called the Magnetic Properties Experiments. This package consists of one magnet array, one tip-plate magnet, and three Thermal and Evolved Gas Analyzer (TEGA) magnets. The magnet array and the tip-plate magnet are identical to those flown on Mars Pathfinder and will be passively exposed to airborne dust. The TEGA magnets are a new addition to the package designed to study actively sampled material collected from three different depths in the trench that will be dug by the robotic arm soil sampler.