Introduction:

The Opportunity Mars Rover discovered evaporite deposits on Meridiani Planum, Mars. By observing the analogue Mars environment around the Mars Desert Research Station (MDRS), evaporite minerals, primarily gypsum, can be collected and observed. Evaporite minerals on Earth have the capacity to entrap many particles important to astrobiologists and geologists alike (Wilkins, 2003). The inclusions (entrapped particles) can consist of fluids present at the time the mineral precipitated, biomolecules such as amino acids and hydrocarbons and microbial life. This project's aim is to describe the potential of evaporites; in this case gypsum, to preserve the abovementioned particles and to describe which form of gypsum is the best candidate to analyze for these particles, as there are several.

At MDRS, samples of gypsum and calcite were collected and will be imaged and analyzed by scanning electron microscopy (SEM) in the near future. Imaging is most important to identify which minerals have inclusions and chemical analysis will verify the mineral's identity.

Geology:

The evaporite field studies focused on the Summerville Formation due to its reputable abundance in gypsum. In Candor Chasma; a box canyon, the Summerville is well exposed in cross section and consists of channel sandstones, siltstones, mudstones and three forms of gypsum: bedded gypsum, gypsum nodules and gypsum veins (fig. 1). The bedded gypsum is found intermixed with green or reddish siltstone and in selenite form. The gypsum nodules are dusty brown on their weathered surface and consist of light pink mm-scale crystals on a fresh surface. They vary in size, from a few millimeters to several meters in diameter and display a brain-like shape. The larger nodules are massive and bulky and tend to disturb the overall bedding. The gypsum veins are observed up to 50 cm thick and crosscut the stratigraphy throughout most of the outcrop.

The veins are secondary precipitations and therefore younger than the bedded and nodular gypsum. The Summerville Formation is Jurassic in age and is interpreted to be formed in a tidal flat environment during high evaporation rates. For a more complete description of the Summerville Formation near MDRS, see Battler, 2006 and Stokes, 1986 for a summary of Utah's geology.

Figure 1: Bedded, nodular and veined gypsum in Candor Chasma. Field book (19 cm x 12 cm) for scale.

Mudflats; one just to the west of MDRS and another in Tank Wash were rich in flat selenite crystals up to 30 cm long. This gypsum was also sampled though the outcrops of origin could not be described. Several crystals are interpreted to be aragonite, found overlying outcrops in Lith canyon and various areas surrounding MDRS, with no evidence of a source outcrop. These crystals are striated, slightly off white with thin reddish veins.

The sediment in the MDRS area is carbonate-rich. Calcite cobbles were identified overlying the mudflats West of MDRS. The crystals are euhedral, translucent and 5 cm wide on average. The origin of this calcite was likely evaporitic, given the purity and shape of the crystals. Nothing more of the outcrop can be described apart from the proximity of the calcite deposit to the mudflat gypsum deposit.

Methods:

Field notes and sampling were completed systematically: samples were collected in new translucent bags, labeled according to location, identified by waypoints and digital photographs (table 1). The stratigraphy, textures and environmental interpretation were briefly but clearly described. Clear inclusions were identified in one of the selenite crystals though any further identification will need a higher powered microscope. Thin sections and/or grain mounts will be made to suit the mineral, and SEM imaging and analysis will further describe the minerals collected. For details of these samples, Click Here to download the sample file, (Adobe Acrobat format).

Discussion:

Inclusions within the evaporites will likely be observed predominantly in the larger selenite form gypsum crystals because they were formed by slow evaporation. The inclusions will be easier to identify and access if any research conducted on the inclusions themselves were to be completed. The finer grained gypsum, such as in the nodules, will be difficult to analyze.

The evaporite mineral group demonstrates great potential in identifying not only the presence of liquid water on Mars, but its composition and anything that may entail. This could give rise to a new dimension of Mars geology and geochemistry.

To improve the MDRS experience, samples of the gypsum, calcite, chert and petrified wood were labeled and left in the Hab geology lab for future geologists and cross-training crew members. Hopefully this will aid mineral identification on EVA. It would be great if a paleontologist participated in one of the research teams at the Hab, to locate and describe the dinosaur bones reported to have been found in Lith Canyon.

References:

Battler, M., Clarke, J., and Coniglio, M. "Possible Analog Sedimentary and Diagenetic Features for Meridiani Planum Sediments near Hanksville, Utah: Implications for Martian Field Studies" in "Mars Analog Research," American Astronautical Society Science and Technology Series special publication, vol. 109, 2006.

Stokes, W.L. "Geology of Utah," Utah Museum of Natural History, University of Utah and Utah Geological and Mineral Survey Department of Natural Resources, Utah, 1978.

Wilkins, A.D. et al. "Model Crystals to Test Techniques in Astrobiological Exploration of Evaporites on Mars," Lunar and Planetary Science XXXIV, 2003.