Water on Mars? Maybe.
Is Gusev Crater the site of an ancient Martian lake? Scientists analyzing data from NASA's Spirit rover may not have a final answer for several more weeks, but today they announced that they had uncovered one tantalizing clue.
|This graph, consisting of data from the Mars Exploration Rover Spirit's mini-thermal emission spectrometer, shows the light, or spectral, signatures of carbonates - minerals common to Earth that form only in water.
"We came [to Gusev] looking for carbonates," said Phil Christensen, payload instrument lead for Spirit's mini-thermal emission spectrometer (Mini-TES). "And we have found carbonates." They are present, however, in only trace amounts - 1 to 2 percent of the surface soil.
Mini-TES works in much the same way as a night-vision camera. Unlike human eyes, it "sees" in infrared; it detects heat. Because different minerals radiate heat at slightly different temperatures, an infrared spectrometer like Mini-TES can be used to analyze the mineral content of Mars's rocks and soil.
Mini-TES is a miniature version of TES, the thermal emission spectrometer on onboard Mars Global Surveyor (MGS). The TES science team, which Christensen heads, reported several months ago in the journal Science that they had detected trace amounts of carbonates over large areas of the Martian surface, including in Gusev Crater.
This hard-won finding - the TES team had been trying unsuccessfully for several years to find carbonates on Mars - was confirmed by Mini-TES, which detected a similar carbonate signature in the surface material at Gusev.
Mini-TES did not find evidence of carbonates when it looked at rocks separate from the soil, but because most of the rocks at the Spirit landing site are small and Mini-TES samples a relatively large area at a time, to date the Mini-TES team has been able to acquire data on only a handful of larger rocks.
The carbonates detected, both from orbit and by Spirit, are magnesium carbonates, a group of substances that are typically white and powdery. You probably have some magnesium carbonate in your bathroom at home: it's often used in making toothpaste and cosmetics.
Carbonates require water to form. But, as rover scientists explained today, Spirit could be seeing nothing more than a dusty coating of carbonates, which could have formed through an interaction between water vapor in the atmosphere and rocks on the surface. If this proved true, it would not provide any evidence of the role of water in Gusev's past.
On the other hand, the carbonates could be deposits left behind by an ancient lake. Many scientists believe that a lake once existed in Gusev Crater. Its lakebed could now be covered over by a thck layer of dust and ash. But Gusev, like much of Mars, has been relentlessly pounded by impacts from asteroids and comets. So it's possible that even deeply buried materials have been thrown up near the surface.
|Topological rendering of Gusev, oblique view of crater, delta to the south, Ma'adim Vallis, and volcano to the north. Top banner image shows the martian terrain through the eyes of the Mars Exploration Rover Spirit's mini-thermal emission spectrometer, an instrument that detects the infrared light, or heat, emitted by objects. The different colored circles show a spectrum of soil and rock temperatures, with red representing warmer regions and blue, cooler. Credit: rendering space4case.com
To find them, Spirit will have to dig deeper. If carbonates are present below the surface, especially if their concentration increases as Spirit burrows down into the soil, there's a good possibility that they were formed by some watery process in Gusev's past.
Spirit has two ways of digging in. Even though it hasn't yet left its lander, it has already inadvertently begun trenching operations. As the lander bounced and rolled across the surface, it scraped away some of the overlaying dust, exposing the soil underneath. All Spirit needs to do to see below the surface is to roll up to one of these exposed patches, point Mini-TES at it and take a look.
But the rover can also use one of its wheels as a trenching device. By locking five of the rover's wheels and spinning the sixth one, Spirit's operators can dig a trench about 20 centimeters (about 8 inches) deep. Spirit then simply backs up a few feet and peers in.
Spirit is currently scheduled to roll off its lander onto the surface next Wednesday or Thursday, and the science team is ready and eager to get started.
"We have a long ways to go to understand the nature of this carbonate," said Christensen, "but it's incredibly exciting."
If Gusev once held liquid water, it may also have been a habitat for life.
Related Web Pages
|Dr. Phil Christensen, payload instrument lead, ASU.
Spirit's Sol images and slideshow
Alpha Proton X-ray Spectrometer
Mars Rover: The Owner's Manual