Roaming the Rust Belt
Pasadena, Opportunity mission Sol 9
For the Mars’ site selection team, the Opportunity site at Meridiani was interesting because of its hematite. The site is sometimes referred to as ‘the prime’. Hematite is a rust-like iron-oxide, that gives parts of Mars their blood-red color. No one knows precisely why these rust-belts on Mars are hematite-rich but the mineral’s existence in high concentration is part of the ongoing martian water mystery. At least for the Opportunity rover, the mission theme is not just to ‘follow the water’, but also to follow the iron-oxides.
The Meridiani iron-rich spot has been compared to the size of Oklahoma. Like Oklahoma and Texas clay, the red soil is the first clue that iron has oxidized or ‘rusted’. Other notable places on Earth for rich deposits include England, Mexico, Brazil, Australia and the Lake Superior region, particularly Michigan and parts of Minnesota. "There are some huge deposits in the north-central US," said Buffalo volcanologist Tracy Gregg, "(think about the Minnestota and Michigan mining histories). Certainly on Earth they only form in the presence of large volumes of water."
|Key spectral indicator of hematite found at Meridiani is the ‘W’ signature at right in the the infrared part of this mini-TES example taken while still atop its landing platform Image Credit: ASU/NASA/JPL|
Hematite is an important iron ore and its blood red color (in powdered form) serves as a pigment and dye. Hematite gets its name from a Greek word meaning blood-like because of this maroon color. Ancient superstition held that large deposits of hematite formed around battlefields and the subsequent blood that flowed into the ground.
Among the scientists who first saw hematite from orbital spectra, Arizona State’s Phil Christensen has witnessed the ore discovery raise the stakes for what exactly happened to any ancient martian water. "Honestly, I am not surprised about the hematite signature, but many of my colleagues seem to be", said Christensen. "One of my worst fears was that the hematite would be in the fine dust, which would make it difficult to track its source. I believe [finding] the hematite in [Meridiani's] coarse-material will help us trace it back to where it started, to its origin."
Christensen confirmed that so far, surface spectra show hematite in gray, granular pebbles and also near deposited layers atop the light bedrock, but not on the bedrock itself. Hematite, when mixed with other mineral types, can appear steel or silver gray to black in some forms and red to brown in soil. The complex ‘squiggly lines’ in a spectra that Christensen presented, show two infrared peaks well beyond what the human eye can see as red. The twin peaks are shaped somewhat like the letter ‘W’ in the spectra, and also point to a mixture of other minerals in combination with hematite at shorter wavelengths. Christensen said: "The ‘W’ marks the hematite signature."
That hematite origin, if not part of the dusty fines, may mean that unlike most other places on Mars, wind probably did not just carry the hematite from some other deposit. On Mars particularly, this theory is considered sound too, because there are few hematite-rich zones, and wind would tend to carry dust globally, not concentrate it only in a couple rust belt zones. Principal investigator Steve Squyres discussed the different ways that hematite can form – many of them involve water – and how the science team plans to learn about the processes that formed the hematite in Meridiani.
"There’s lots of different ways you can make hematite. One is you can have these massive deposits like you form in deep-standing liquid water bodies. Then you’d expect to see the hematite uniformly through the rock".
"Another is something hydrothermal. So suppose you got hot water percolating through rocks, through cracks, and you precipitate the hematite, and we see it in veins, in fractures running through the rock. That tells a different story".
"You can form hematite as coatings on the outside of rocks, just by a thin film of liquid water. So suppose we see a hematite-bearing rock, we RAT it [using the Rock Abrasion Tool], we look underneath and there’s no hematite. That’s a coating."
|The mineral hematite (shown above) is abundant on Mars.|
Credit: Amethyst Galleries, Inc.
"I can’t tell you that this is a place where there was ever water. You can take a magnetite-bearing lava and you can oxidize it at high temperature – you get hematite, no water involved."
"The way you tell [how the hematite formed] is by asking yourself what other minerals are present. It’s those other minerals, the configuration in which you find it".
Christensen seconded the importance of understanding the ‘full context’ of how a mineral is formed. If the Meridiani hematite had a mineral precursor called goethite, he said, the reaction of geothite to form hematite may ‘not run to completion. When we put the other instruments, particularly the Mossbauer and APXS spectrometers, near the hematite, we will understand’ what might have preceeded its formation.
Before the Belt Got Rusty
If the other minerals found with hematite will decide the question of ancient water, Christensen says the early spectra from Meridiani can already rule out some candidates. Important combined minerals with hematite include jasper (a variety of quartz) in banded iron formations (BIF or Tiger Iron), dipyramidal quartz, rutile, and pyrite among others. Christensen noted that the squiggly lines to the left of hematite’s ‘W’ spectral signature gives one clue: "Hematite is not in combination with quartz, it is not in carbonates. We’re in the right place."
|Geothermal activity (Volcano Island, Italy)|
"A way to form hematite is precipitation in water at low temperatures," said Christensen. "That’s consistent with the data, that is one scenario. The other is to take magnetite, a volcanic material, and convert it to hematite at high-temperature. I hope Mossbauer will help us see granules of either magnetite or goethite"–a more clear indicator of high vs. low temperature formation.
What preceeded hematite may be magnetite or titanium magnetite, which would not point as clearly to aqueous formation compared to goethite. If hematite is rust-red, goethite is multispectral in color and often irridescent when found in rainbow stalactites on Earth. Goethite can be found near iron mines around the world, often times near other iron oxides like hematite. It is recognizable by a duller earth tone, ranging from yellow, brown, brownish red to black, but also metallic like hematite when found in rarer crystalline forms. Amateur geologists recognize such minerals in the field using what is known as a streak test, which scratches another surface like porcelain with the unknown rock type to uncover the abrasive color with less influence from other minerals in combination. The streak test can be compared to chalking with a rock sample. Goethite streaks in shades of brown with hints of yellow and orange.
If goethite with hematite seals an aqueous origin, magnetite with hematite would point to more volcanic or high-temperature formation of a weathered basalt. Magnetite, as its name implies, is a natural magnet. Even the dust itself is conductive on Mars, and as a consequence a magnet onboard the spacecraft routinely looks to pick up and sample what is blown around in the fines. But magnetite in larger rocks points to to places where some geothermal activity has transformed this metallic mineral. Magnetite is black, without the earth-tones expected from water effects. Magnetite can be found with talc, pyrite and chlorite, with notable rich deposits in South Africa, Germany, Russia and many hematite-rich, hot springs in the US like Yellowstone Park or Hot Springs, Arkansas. When magnetite is used in a streak test, the color is black too, but magnetism itself is as good an indicator if hematite is around.
While the Rust Belt states in the US include northern states betwen Indiana and New Jersey, on Mars the Meridiani rust is about 150 km by 500 km almost exactly at the equator (0 degrees latitude) along the prime meridian (0 degrees longitude). A first pass over the region years before the current orbiting constellation could map this region, it was considered rather bland, slightly younger than average and streaked by what was thought then to be wind action. While the mineralogy of Meridiani continues to surprise team members, it may well turn out that the fossil crater that is now home to the Opportunity rover may give the only relief from this flat plain. If the Meridiani plain is slightly depressed (as indicated by laser altimeters in orbit), there is a chance this region was once a very old area that could pool water or lava. A second perhaps more telling clue is that the hematite seems what is called ‘platy’ hematite, meaning it runs horizontally and may have been compacted into a stratum or layer.
On Mars, what was once thought dull is now full of discoveries yet to come. The site selection team now has a few reasons to celebrate finding their prime meridian.