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Gray Hematite for a Red Planet

You have probably heard a lot about a mineral called hematite. Opportunity’s landing site was chosen mainly because spacecraft in orbit around Mars found that Meridiani Planum is rich in gray crystalline hematite. Scientists are interested in gray hematite because it usually, but not always, forms in association with liquid water. One of the goals of the Mars Exploration Rover mission is to search for evidence of ancient water on Mars – water that may have given rise to conditions that could have been suitable for life.

Hematite is an iron oxide (iron and oxygen) mineral formed in a wide variety of environments here on Earth. Its color changes dramatically with the coarseness or fineness of its particles (think of sugar as opposed to flour). In a coarse-grained deposit, hematite is gray to black in color. In a fine-grained deposit, hematite is bright red.

The red color of Mars is a dead giveaway that there are iron oxides in the soils, but the red dust that blankets the planet is not pure hematite. Chances are a number of different iron oxide minerals contribute to the rusty hue. On the other hand, gray hematite is seen in only a few places on Mars. The largest of these deposits is at Meridiani Planum and scientists are using Opportunity’s science instruments to learn if water may have played a role in its formation.

Hypotheses about the origin of gray hematite on Mars fall into two categories – those that involve water and those that do not. On the water side are three possibilities: (1) a standing body of water was so supersaturated with iron that the iron precipitated from the water, leaving hematite behind when the water dried up, (2) iron-rich water that percolated through the ground was heated by volcanic activity that left veins of gray hematite behind, or (3) at some point in the history of Mars, a veneer of gray hematite formed on rocks from weathering processes in the presence of a small amount of liquid water. On the dry side is the theory that the gray hematite on Mars is the result of processes having to do with iron-rich lava or volcanic ash.

To test these different hypotheses scientists are sorting through data from Opportunity. They are looking to see if the gray hematite deposits appear to have formed at their present location or if they have been carried by the wind. They are looking for landforms (eroded volcanic vents, shoreline features, or concentration in dunes) that might suggest how the mineral formed. Is the gray hematite in the form of a cement in a sedimentary rock? Thin coatings on grains? Or in veins filling rock fractures? Textures (arrangement of grains, grain sizes and shapes), and minerals in addition to hematite in the spectra from Mini-TES and Moessbauer can also reveal whether hot, cold, wet or dry conditions were present when the coarse hematite formed. The presence of materials such as clays and carbonates would indicate there had been water in the area. Olivine and pyroxene would point to a gray hematite that probably formed from volcanic activity.

As deposits of hematite are sniffed out by the Opportunity rover, scientists are looking at their location, their texture, and the kinds of associated minerals the deposits contain to determine if gray hematite at Meridiani Planum is the result of a wet or dry martian environment from long ago.

Web content editor/writer: Pamela R. Smith

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