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The Planetary Society Blog

By Emily Lakdawalla

Catching up on some reading

Dec. 28, 2005 | 06:15 PST | 14:15 UTC

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I'm writing this from a long plane flight, a post-Christmas trip to see my extremely aged grandparents in Florida. Painful though traveling is it does give me a chance to catch up on some of the reading I don't usually have time to do. I brought two recent issues (from September and October) of the Journal of Geophysical Research - Planets (otherwise known as "JGR") with me today. This is a scientific journal, in which researchers publish peer-reviewed papers about their work, and it's the main one I read besides Science and Nature. There are other important journals in planetary science but this is the key one if you are interested in the geology of planets and moons -- their surfaces, their interiors, and their histories. And it's much less expensive for me to subscribe to than other important journals like Icarus, which I just can't afford.

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Martian meteorite MIL03346
MIL03346 is a Martian meteorite (a nakhlite) that was found in the Miller Range of Antarctica. The original rock was 10 by 6 by 5.5 centimeters in size and weighed 715.2 grams. Source Credit: NASA JSC

One reason I save JGR for airplanes is that the reading is heavy slogging, the more so with every year that elapses from the end of my formal education. I was very pleased to come across one paper on which Darby Dyar was the first author: "MIL03346, the most oxidized Martian meteorite: A first look at spectroscopy, petrography, and mineral chemistry." (There were five coauthors: Allan Treiman, Carlé Pieters, Takahiro Hiroi, Melissa Lane, and Vanessa O'Connor). Darby is a friend of mine who teaches at Mount Holyoke College, which is just down the road from my alma mater, Amherst College. She is a petrologist (one who studies the chemical composition and origin of rocks), and her research group's specialty is extremely detailed laboratory analysis of rock samples on Earth (either Earth rocks or Mars meteorites) for comparison to all of the data being acquired by the orbiters and landers on Mars.

All that being said, if Darby weren't a friend of mine I probably wouldn't be able to make it through her papers, which are extremely dense with mineral names and petrological terms such as "augite," "cumulate," "nakhlite," "pigeonite," "exsolution lamellae," and one of my all-time favorite mineral names, "smectite." (Sounds rude, doesn't it?) These words frustrate me because I know that just a few years ago each of these words would have called to mind not only the appearance of the mineral in question but also its chemical composition and its likely origin. To petrologists, mineral and rock names form a complicated vocabulary full of significance and meaning, but I only have a hazy memory of it all. It's a foreign language that I learned in my classes in mineralogy, petrology, and planetary geology, but I'm losing it with disuse.

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Thin section of Mars Meteorite MIL03346
A polished thin section of Martian meteorite MIL03346, viewed through a petrographic microscope under polarized light, contains lots of stubby grains of a mineral called augite (a pyroxene). Source Credit: NASA JSC

Which is sad because I really liked studying mineralogy and petrology, in the laboratory at Amherst with the tall glass-fronted cabinets full of tray after tray of mineral samples organized as to chemical families (elements, oxides, sulfides, and so on up to all the different classes of silicates). Learning all of those minerals and rocks allowed me, briefly, to look at a rock and tell a whole long story about its origin and its evolution. I can still do that to some extent but I have to rely on the Internet to remind me that augite is in fact the most common form of the common rock-forming mineral pyroxene. (For those of you who enjoy mineral names as much as little kids enjoy dinosaur names, here are some other names of pyroxenes, all of which have basically the same crystal structure but which can have different metal ions occupying the chemical makeup: diopside, hedenbergite, pigeonite, johannsenite…)

Anyway, I digress from the topic of Darby's paper. Her team performed a pile of different analyses of a Martian meteorite and found that it must have formed from an originally oxidized (that's the opposite of reduced) source magma, which is important because it suggests change over Mars' history or at least local variation in magma composition across Mars, because Mars is a highly reducing chemical environment today. What impressed me about the paper was the amount of analyses performed on a truly tiny sample of rock. They began with a sample of rock amounting to 528 milligrams (0.0186 of an ounce), of which they set aside a 100 milligram chip for thermal emission spectroscopy, and ground up 100 milligrams into a very fine powder for Mössbauer spectroscopy. The remaining 328 milligrams -- tiny amount! -- were "gently crushed by hand" so that they could look at the component mineral crystals. The rovers have access to uncountable numbers of different rocks to sample and study, but here on Earth we have only a tiny precious few samples of rock from Mars to subject to the full array of possible mineralogical analysis.

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Aram Chaos, Mars
Viking mosaic of the Aram Chaos region on Mars. Aram Chaos is a 280-kilometer crater centered at 2.5°N, 228.5°E. Aram Chaos is connected on its eastern rim to the Ares Vallis outflow channel by a 15-kilometer-wide, 2.5-kilometer-deep channel. Ares Vallis curves around Aram to the east and north. Credit: NASA / JPL / PDS Map-a-Planet

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Hematite in Aram Chaos, Mars
Using the THEMIS imaging spectrometer on 2001 Mars Odyssey, scientists Tim Glotch and Phil Christensen produced a map of gray hematite abundance within the Aram Chaos region on Mars. The hematite is similar to that seen at Meridiani Planum, Opportunity's landing site, but the hematite in Aram Chaos must have formed approximately a billion years later. Source Credit: Glotch and Christensen, Journal of Geophysical Research, 2005

Another paper I enjoyed reading was by Tim Glotch and Phil Christensen, on "Geologic and mineralogic mapping of Aram Chaos: Evidence for a water-rich history." Aram Chaos is a Martian crater 280 kilometers in diameter located near Mars' equator and 20 or 25 degrees to the west of Opportunity's landing site. It's connected to one of the great big outflow channels, called Ares Vallis. "Chaos" is a general term for a landscape found in many places on Mars where the ground appears to be fractured and broken into immense blocks that appear to have separated, tilted, and sometimes sunk downward with respect to the surrounding terrain, and they're usually connected with an outflow channel. The most common interpretation is that the catastrophic emptying of either water or lava from a confined region in the subsurface caused the fracturing and collapse of the ground above it.

There were several things I found surprising and interesting about this paper. The main surprise was the depth and complexity of the geologic history that Glotch and Christensen could read from the images, spectral data, and topographic information across the region. Most surprising to me was the fact that the disruption of the ground into chaos terrain wasn't the most recent thing that happened -- instead, it was one of the first events in the story that they told, predated only by the formation of the original crater. The story goes: impact crater happened -- crater got filled with sedimentary or volcanic material -- then there were (probably) multiple catastrophic outflow events, which cause the formation of the chaotic terrain. After that, the crater was infilled again with more layers of sediment and/or lava. One of the many different layers of infilling material contained a lot of hematite like that found at Meridiani, or at least it contained some iron-rich mineral that was later converted to hematite. Then after that there was a tectonic event that caused the whole interior of the crater to dome upward in the center, tilting the layers to the edges, and the pile of domed sediments was cut and eroded into sharp-edged landforms.

This is an awfully complicated story being told about one relatively small province on Mars. The conclusions made about the geologic history of Aram Chaos in this paper do have some bearing on the rest of Mars, but at their core they are a story about one small and unique locality on the planet, like the Atacama Desert or Greenland ice sheet or Hawai'ian islands here on Earth. It is getting to the point now where there is so much data covering Mars in so much depth that scientists can do this kind of research, exploring the deep past of one small region through its interlinked topography, mineralogy, stratigraphy, and structure. I think there's only two other place in the solar system where that is true, and that's Earth and the Moon. For all other places in the solar system, scientists' research is most often of a whole-planet view: the atmosphere of Titan, coronae on Venus, stress patterns on Europa, that kind of thing.

One last little note: in reading the October issue of JGR, I came across a paper by Karen Stockstill and four coauthors, containing a report of her exhaustive work developing a computer program that mines the Mars Global Surveyor TES data set to search for evidence of carbonate minerals in Martian craters. Many Mars craters look like they've had rivers flowing through them and appear from space to be likely spots to search for evaporite minerals and other hallmarks of ancient lake deposits. But Karen found no evidence for carbonates in any of them -- including Aram Chaos. If the carbonates are there, she and her coauthors concluded, they are not exposed to view from Mars Global Surveyor above a 5 or 8 percent level.