Though I was tempted to review an article I mentioned briefly earlier last week about the slowing of the Atlantic Thermo/Haline circulation (i.e. the Gulf Stream), I decided it was time to toss in a little variety. So, this week I’ve chosen an article in Nature that reveals some of the first major results from the European Space Agency’s OMEGA imaging spectrometer.
Citation: (view online at CiteULike.org)
Poulet, F. and others (2005). Phyllosilicates on Mars and Implications for Early Martian Climate. Nature 438(7068), pp. 623-627.
Synopsis:
The OMEGA imaging spectrometer provides unambiguous evidence that clay minerals are present in a variety of locations across the surface of Mars. The minerals are detected by the absorption of certain frequencies of infrared light that are known from laboratory studies. These clay minerals are of the smectite variety, which is associated with abundant liquid water here on Earth. These clays are not simply the coatings on rock surfaces that have been observed in Antarctica, either. Thus they indicate that abundant water once flowed over the surface of Mars.
The figure to the left (image credit: ESA) is a perspective view of the Marwth Vallis outflow channel. The serpentine indentation is the outflow channel itself, while the blue indicate the presence of clay minerals not in the channel, but on its sides where flood waters briefly but violently eroded the rocks above the clay minerals. The one below and to the right is an example spectrum indicating the presence of clays in the OMEGA data (from figure 1 of Poulet and others, 2005). 
The horizontal axis on this plot is the wavelength of light observed by the spectrometer, while the vertical axis gives the relative reflectance value (i.e. how much sunlight is reflected back to the satellite). The curves labeled “a” and “b” correspond to data, and their ratio “d” provides the final observed spectrum. This is compared to the laboratory study “c.” Notice that the lab result is much cleaner, with distinct absorbtion bands. The data from OMEGA reveal all of the same bands, but they are weaker than the lab sample. This is to be expected as the martian data represent a region that is not composed entirely of clay, but is “contaminated” with dust as well as other minerals.
These clays are present, at least in one location (Syrtis Major) in rock outcrops of Noachian age. The Noachian epoch extended from the formation of Mars to somewhere between 3.8 and 3.5 billion years ago. Thus, these findings support the “Early-Wet Mars” hypothesis. Furthermore, the OMEGA instrument detected sulfate-rich minerals in layers lying on top of the Noachian materials. These, too, required water though not in such great abundance.
Context:
The debate between the various hypothesis of Martian wetness has an interesting history. Prior to the first Mariner images of Mars, it was thought to possess water in some abundance, perhaps even plant life. But, the desert-like images that early spacecraft sent back, and the bone-dry Viking landing sites changed the prevailing view to a super-dry Mars that perhaps never had water in great quantity. Now, with more data from Mars than can even be fully analyzed, Martian planetary geologists are starting to assemble a more cohesive and realistic picture of a formerly wet planet that desiccated slowly over hundreds of millions of years. Researchers have speculated for years that much of this water sought refuge beneath enormously thick ice-layers deep within the Martian crust, but, until last week they had no confirmation of this hypothesis.
General Explanations:
Spectroscopy is one of the key tools of planetary exploration and astronomy. It’s used to identify minerals, search for life, understand the composition of stars and galaxies, and estimate the rate of expansion of the universe, among many many other applications. All spectroscopy relies upon one basic fact: individual elements and molecules absorb or emit electromagnetic radiation (light) in a way that is often unique to that element or molecule. Electromagnetic radiation is composed continuous spectrum of frequencies extending well below AM radio range all the way up to high-energy cosmic rays. Absorption or emission at a certain frequency can then identify a molecule. In this case, the specific chemical bond between Iron (Fe) and Hydroxyl (OH) was detected, along with other absorption frequencies characteristic of clays.
