Post by Cathy Weitz and Melissa Lane Planetary Science Institute, Tucson, Arizona, USA.
Fine-grained red hematite is a common mineral on the surface of Mars and explains much of the reddish color for martian soils and rocks. However, hematite can also be gray in color if it is coarser grained. Gray, crystalline hematite has been identified by the Thermal Emission Spectrometer (TES) at several sites on Mars , including: Meridiani Planum, Aram Chaos, Valles Marineris, Aureum Chaos, and Iani Chaos (Image 1) [Christensen et al., 2000; 2001; Glotch and Christensen, 2005; Glotch and Rogers, 2007; Noe Dobrea et al., 2007; Weitz et al., 2007]. At Meridiani, Aram Chaos, Iani Chaos, and Aureum Chaos the hematite units are confined to a specific layer or fairly continuous unit [e.g., Christensen et al., 2001, Glotch and Christensen, 2005]. Whereas, in Valles Marineris the gray hematite is more patchy in distribution and scattered in separate troughs [Weitz et al., 2007; Le Deit et al., 2008].
Several origins for the gray hematite were proposed by Christensen et al. [2000a; 2001]. Their favored interpretations were 1) the direct precipitation of fine-grained hematite from Fe-rich circulating fluids of hydrothermal origin. Or 2) a low-temperature precipitation of fine-grained Fe-oxides/hydroxides from standing, oxygenated, Fe-rich water, followed by subsequent alteration (coarsening) to gray hematite.
It was the detection of gray hematite in Meridiani Planum that led scientists to select this location as the landing site for one of the Mars Exploration Rovers (MER). The rover Opportunity landed in Meridiani Planum on January 24, 2004. After driving on the surface for several sols, instruments on the rover allowed scientists to determine that millimeter-size spherules (informally termed ‘blueberries’ by the MER science team) contained the gray hematite [Christensen et al., 2004; Squyres et al., 2004]. The hematite-rich spherules are postulated to have formed by secondary alteration of the sulfate-rich outcrop as water permeated through the rocks and produced concretions [McLennan et al., 2005]. Hematite-cemented concretions formed by groundwater flow have been found on Earth in the Jurassic Navajo Sandstone of southern Utah [Chan et al., 2004], although they are dominated by quartz.
The size of the spherules ranges from about 1-4.5 mm [Weitz et al., 2006; Calvin et al., 2008]. In bedrock outcrops, the spherules make up only a few volume percent of the rocks so their signature is not strong enough to be detected in the rocks themselves from orbit. It is only because erosion of the sulfate-rich host rock has concentrated the more resistant hematite spherules in larger abundances as a lag deposit in the soils that the signature at Meridiani Planum was strong enough to be detected from orbit. Hence, other sulfate-rich rocks with hematite spherules may exist on Mars, but the hematite may not be concentrated at high enough abundances on the surface to be detected from orbit.
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