Wide view of sunset over Gusev Crater taken by NASA’s Spirit Rover in 2005. Both blue aureole and pink sky are seen. Because of the fine nature of Martian dust, it can scatter blue light coming from the Sun forward towards the observer.
"This is one of the most significant findings by the rovers," said Steve Squyres of Cornell University a principal investigator for the Mars twin rovers, Spirit and Opportunity. "A substantial carbonate deposit in a Mars outcrop tells us that conditions that could have been quite favorable for life were present at one time in that place."
Spirit inspected rock outcrops, striking a bonanza at one the NASA scientists named Comanche, along the rover's route from the top of Husband Hill to the vicinity of the Home Plate plateau. Magnesium iron carbonate makes up about one-fourth of the measured volume in Comanche. That is a tenfold higher concentration than any previously identified for carbonate in a Martian rock.
"We used detective work combining results from three spectrometers to lock this down," said Dick Morris, a member of a rover science team at NASA's Johnson Space Center in Houston."The instruments gave us multiple, interlocking ways of confirming the magnesium iron carbonate, with a good handle on how much there is."
Massive carbonate deposits on Mars have been sought for years without much success. Numerous channels apparently carved by flows of liquid water on ancient Mars suggest the planet was formerly warmer, thanks to greenhouse warming from a thicker atmosphere than exists now.
Shown above is a National Geographic artist's concept of how Gusev Crater might have appeared billions of years ago. The ancient, dense Martian atmosphere was probably rich in carbon dioxide, because that gas makes up nearly all the modern, very thin atmosphere. It is important to determine where most of the carbon dioxide went. Some theorize it departed to space.
Others hypothesize that it left the atmosphere by the mixing of carbon dioxide with water under conditions that led to forming carbonate minerals. That possibility, plus finding small amounts of carbonate in meteorites that originated from Mars, led to expectations in the 1990s that carbonate would be abundant on Mars. However, mineral-mapping spectrometers on orbiters since then have found evidence of localized carbonate deposits in only one area, plus small amounts distributed globally in Martian dust.
Spirit's Alpha Particle X-ray Spectrometer instrument detected a high concentration of light elements, a group including carbon and oxygen, that helped quantify the carbonate content.
The Daily Galaxy via NASA/JPL-Caltech