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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