It also reinforces the value of infrared data acquired by remote sensing from orbit by Mars Global Surveyor and Mars Odyssey. Data from the MGS's Thermal Emission Spectrometer and Odyssey's THEMIS have allowed Christensen and his colleagues to analyze Mars' geology with great accuracy and to predict many of the findings that the rovers have begun to confirm, including the presence of an ancient lakebed in the otherwise flat landscape of Meridiani.

The finding of jarosite in the bedrock formation at Eagle is important because the mineral is only known to form in two ways - in the presence of standing water or in volcanic vent environments.

"The only way you can have this concentration on earth, is to dissolve it in water and precipitate it out," said MER Rover Principal Investigator Steve Squyres. On earth, jarosite is frequently formed when pyrite reacts with water.

There have been a variety of contending theories for what may have caused the unusual mineralogy at Meridiani, including volcanic causes. This new evidence further supports the hypothesis prominently proposed by Christensen that the minerals were formed in a past standing body of water.

Meridiani Planum is a plain in the middle of Mars' Terra Meridiani region, an area of Mars that has long intrigued planetary geologists - particularly Christensen, who is Principal Investigator for four Mars instruments that are either currently orbiting Mars or on the rovers. Meridiani, Christensen frequently notes, is a "layered" area, where erosion has exposed the martian rock and a number of clearly distinct geological layers.

One of Christensen's instruments, the Thermal Emission Imaging System (THEMIS) on the Mars Odyssey orbiter, has taken infrared and visible light images of the Meridiani area that show several clear layers of rock with distinctly different thermal characteristics. The distinct differences in temperature of these layers implies strongly that they are different in mineral or structural composition.

The finding that the layers are different in turn implies that there were differences in the local martian environment at the times these layers were formed. In the way geologists look at things, exposed layers of different kinds of rock are chapters in the history of the planet (and perhaps of the planet's climate).

Christensen's other Mars-orbiting instrument, the Thermal Emission Spectrometer (TES) on Mars Global Surveyor, detected a clear signal of the mineral hematite in one of Meridiani's layers. Hematite, a gray mineral mainly composed of iron, generally only forms in the presence of liquid water, so the implication is that Meridiani's Hematite Formation was once associated with water in some way.

The minerals at Meridiani Planum are almost certainly remnants of a place in Mars' past where there once was water, but the question remains with regard to what kind of water. Here, theories have varied widely, with some scientists arguing that the hematite could have been formed by a subsurface hydrothermal system, by an above- or below-ground lake, or by volcanic activity causing a deposit of iron-rich ash. Christensen argues convincingly, however, that the hematite signature detected by TES does not match the signature of a mineral that developed from volcanic deposits and thus must have been left by liquid water.

In an article that Christensen has recently submitted for publication (and presented in talks at the American Geophysical Union national conference), he argues further that other evidence seen by THEMIS points to the conclusion that the Meridiani Hematite Formation was once the site of an ancient martian lake. He shows that the hematite layer is thin, which is consistent with it being deposited by a lake, and that it appears to have flowed in and around the older channels and craters in the area. If the hematite had been deposited by dust or air-born volcanic ash, it would be spread evenly across all features in the landscape. The fact that the layer only occurs in the lowest areas points to it being deposited by standing water which would have pooled in those places.