Search

The soil on Mars is known to contain a ferrimagnetic mineral, the unequivocal identification of which mineral will contribute to understanding the origin of the Martian soil. Mossbauer spectroscopy of magnetically separated dust can give essential information on the iron mineralogy and magnetic properties of the dust. The Mars Surveyor missions in 2001 and 2003 will carry a Mossbauer spectrometer and several permanent magnets, which together should be able to achieve these goals.

The significance of the element titanium for the study of the magnetic mineral in the surface dust of Mars is described.

A Mossbauer spectrometer is included in the preliminary payload of a rover to be placed on the surface of Mars in the Soviet mission to the planct in 1996/1,2/. In counection with the American planctary program it has also been suggested to construct a Mossbauer spectrometer to be landed on Mars /3, 4/. The objective is to study the iron compounds of the Martian soil and rocks by backscattering Mossbauer spectroscopy. The paper describes the significance of the element iron in the study of the evolution of the planetary system and what we might expect to learn from Mossbauer spectroscopy of the surface materials of Mars. The study of Mars is expected to expand substantially in the coming decades, probably culminating with a manned flight to the planet. The international Mossbauer community may contribute significantly to the preparation of these events.

Both the USSR and US have plans of performing rover missions and sample return missions to the planet Mars in the coming decade. There may be possibilities of performingin situ Mossbauer spectroscopy on Mars or on fresh samples returned to a space station.

[1] The Mars Exploration Rovers each carry a set of Magnetic Properties Experiments designed with the following objectives in mind: (1) to identify the magnetic mineral(s) in the dust, soil and rocks on Mars, (2) to establish if the magnetic material is present in the form of nanosized (d < 10 nm) superparamagnetic crystallites embedded in the micrometer sized airborne dust particles, and (3) to establish if the magnets are culling a subset of strongly magnetic particles or if essentially all particles of the airborne dust are sufficiently magnetic to be attracted by the magnets. To accomplish these goals, the Mars Exploration Rovers each carry a set of permanent magnets of several different strengths and sizes. Each magnet has its own specific objective. The dust collected from the atmosphere by the Capture magnet and the Filter magnet (placed on the front of each rover) will be studied by the Mossbauer spectrometer and the Alpha Particle X-ray Spectrometer, both of which are instruments located on the rover's Instrument Deployment Device. The captured dust particles will also be imaged by the Pancam and Microscopic Imager. The Sweep magnet will be imaged by Pancam and is placed near the Pancam calibration target. The four magnets in the Rock Abrasion Tool (RAT) are designed to capture magnetic particles originating from the grinding of Martian surface rocks. The magnetic particles captured by the RAT magnets will be imaged by Pancam.

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 (S 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.

The Martian moons Phobos and Deimos are relatively small (r ∼ 104 m) irregularly shaped objects that represent a large class of small Solar System Bodies. Based on spectral reflectance measurements and comparison with meteorites the Martian moons are believed to be of Carbonaceous Chondrite composition /1/. The Carbonaceous Chondrites are characterized by a very high average oxidation state of the iron in them compared to that of all other types of meteorites. By use of Mossbauer Spectroscopy Carbonaceous Chondrites of the classes CI, CM, CV and CO are easily distinguished from each other and from other classes of meteorites. The possibility of identifying the Martian moons as distinct types of Carbonaceous Chondrites based on Mossbauer spectra of the surface is investigated.

Locating a rock on the surface of Mars that bears unambiguous evidence of the existence—prior or present—of life on that planet is, understandably, the “Holy Grail” of NASAs sample return missions. Remote recognition of such a rock on Mars will not be easy. We do know, however, that present in the Martian crust—especially in the “Southern highlands”—is rock carrying strong natural remanent magnetization (NRM). Characterization of such magnetized rock has profound implications for adding to our knowledge about the origin and early evolution of the Martian interior, lithosphere, atmosphere, and possibly even Martian life forms [Ward and Brownlee, 2000]. Moreover, it should be possible to recognize such rocks by use of a simple magnetic compass mounted on a Rover.

JSC Mars-1 is a recently announced simulant for the Mars soil “available for scientific, educational study” [Allen et al., 1998]. It is a somewhat altered volcanic ash from a Mauna Kea, Hawaii, cinder cone that yields a reflectance spectrum remarkably similar to the Olympus-Amazonis bright region on Mars [see Allen et al., 1998,Figure 1].The comparability in terms of other chemical and physical properties of the simulant to what is considered to be known of the properties of Mars soil is outlined, and an acceptable similarity is implied. We wish, however, to point out several shortcomings in this Martian analog, to caution researchers and teachers that “looks can be deceiving,” and that they should not conclude that because of the spectral similarity, Mars soil must necessarily be “altered volcanic ash.” It may well prove to be ash in some form but this sample is not the answer.

Viking mission soil analyses and the constitution of the SNC group of meteorites have indicated that the Martian regolith is derived from basalt. On Earth such material often weathers to yield a highly magnetic soil (spontaneous magnetization σS ≥ 1 J T−1 kg−1), the magnetic constituent of which is fully oxidized titanomaghemite. We propose that titanomaghemite is likewise responsible for the magnetism of the soils on Mars.

The article starts with a brief review of ideas on the origin of the solar system, with emphasis on the cosmic history of the element iron. Some examples of the application of Mossbauer spectroscopy to the study of iron compounds from various types of meteorites, which represent different stages of the evolution of the planetary system, then follow.

Mossbauer investigations of three meteorites of the type SNC-achondrites are reported. The results are compared with Mossbauer spectra of other meteorites, and selected terrestrial and lunar rocks. Through the oxidation state of iron in the pyroxenes and magnetic oxides studied, the work confirms that the mantle of the Earth is highly oxidized compared with the mantle of asteroidal bodies and lunar basalts. The investigations show that at least one of the SNC meteorites (Nakhla) comes from a highly oxidized parent body, believed to be the planet Mars. Based on the results obtained, a somewhat general conclusion about the oxidation state of iron in planetary bodies is proposed.

The meteorites called carbonaceous chondrites are the least altered samples we have of the material which formed our solar system. Using57Fe Mossbauer spectroscopy we show that the carbonaceous chondrite Murchison contains a compound exhibiting superparamagnetic relaxation below 80 K.