Today I had planned on taking the time I usually put into (sort of) transcribing the MER mission press briefings and writing something up on "the hematite" that's the big news in Meridiani Planum. It's being reported by all the mainstream media outlets without any real context and I'd hoped to offer some of that. I got several paragraphs in when I ran across this good hematite primer, featuring Dr. Joy Crisp, right up front at the Mars Rover site. Well, needless to say, it covers a lot of the same ground I was trying to cover.
I do have a few notes I think will add to that story, though, a bit more about what hematite can tell us, and a brief note on what we've already learned from the surface. (note: I'm not a geologist, so please don't hold me to that standard.)
Now that we've got Opportunity on the surface with this awesome science payload, it's looking like we may have an ideal environment for a very productive study of the hematite at Meridiani Planum. Initial indications suggest that the hematite is probably strewn across Meridiani Planum but not wind-carried in from afar. In the floor of the small crater where Opportunity sits, we see the hematite in a dark, grayish medium gravel layer (grains probably about 1/12 to 1/2 inch diameter) sitting on top of a shallow, fine-grained, not-hematite, soil layer.
Even more exciting than the readily available hematite, in relatively large pieces (one fear of the mission scientists was that the hematite would be a finely ground up sand or silt and therefore difficult to study and to trace to its location of origin) is the possibly that Opportunity will have access to hematite in original beds sitting atop and possibly underneath the light bedrock layer within Opportunity's small crater.
Dr. Phil Christensen provided two very exciting pieces of information at the last press conference: first, that we may be able to trace the coarse-grained hematite to its in-place origin, and second, that the spectral signature we see from mini-TES points to a low-temperature formation process for the Hematite.
If there are exposed beds of hematite sitting on top of or below the light bedrock layer, we may be able to see physical characteristics like smooth layering, veins, cementation, differential erosion, or other physical indicator as to whether this hematite was precipitated in low-temperature waters in a lake or sea, precipitated from higher-temperature waters around hydrothermal vents, or we may learn that it wasn't precipitated at all but created from a weathered ashfall or lava bed, or some other drier mechanism. Close physical examination with the Microscopic Imager and the other Opportunity cameras should be able to help make these determinations.
In addition to the physical characteristics of the hematite material -- its size, shape and distribution, Opportunity is and will continue to collect spectral data. The temperature of formation, and the type of mineral precursors have an impact on the spectral signature of the hematite. So far, the available mini-TES spectral signatures seems to point to a low-temperature formation process with goethite as the precursor mineral. There are two basic formation mechanisms that the science team believe may be behind Mars' hematite, a low-temperature process and a high-temperature process. In the low-temperature goethite process, Christensen says, iron minerals "precipitate from water, at low-temperatures, forming a variety of amorphous iron materials -- an iron ooze, if you will -- that can then convert to a mineral called goethite which over time converts to hematite." A high-temperature formation process for Martian hematite would be where magnetite, a common volcanic material, is converted to hematite by high-temperature thermal oxidation.
TES data from orbit had previously indicated a higher likelihood for a lower temperature, goethite-based formation. The current mini-TES spectral data from the surface also indicate that the hematite likely came from goethite. With Opportunities great instrument payload, the science team can perform further tests to confirm this low-temperature hypothesis. One test, Christensen says, will be to look very closely at a sample of the hematite with the Mossbauer spectrometer, an instrument designed to determine with very high accuracy the composition of these iron-bearing minerals. If the Mossbauer identifies goethite remnants in or around the hematite, then we'll have a fairly strong case for a relatively cool, wet formation process. If, on the other hand, with the close-up instruments on Opportunity, they find spectral signatures for magnetite, that could suggest a higher temperature, volcanic formation process.
The data is flowing in and the science team's hard at work evaluating what it means. The jury's still out on how this hematite formed and what we've learned on the surface so far has, mostly, confirmed what we'd already learned from orbit -- but more important than that, what we've learned on the surface confirms the hope of all of the science team and everyone following this mission closely, that we'd land these tools, safely on the surface, in a location to go much, much further in our understanding of this hematite, where it came from, and how it formed. Solving this mystery, as Dr Crisp says, "will help us characterize the past environment and determine whether that environment was favorable for life."
update: For an alternate hypothesis, one that begins by challengeing the very existance of coarse gray hematite at Meridiani -- something that would moot just about everything I said above, check out this (PDF) article. An intro to this paper can be found at author Laurel Kirkland's site. And there's additional reasearch for your thermal spectroscopy reading pleasure at Kirkland's site too.Posted by asa at January 31, 2004 03:49 PM