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Photos: The greatest hits from Mars

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  1. The face of Mars

    The Hubble Space Telescope focuses on the full disk of Mars, with a head-on view of a dark feature known as Syrtis Major. Hubble astronomers could make out features as small as 12 miles wide. (AURA / STSCI / NASA) Back to slideshow navigation
  2. Red, white and blue planet

    Two decades before Pathfinder, the Viking 1 lander sent back America's first pictures from the Martian surface. This 1976 picture shows off the lander's U.S. flag and a Bicentennial logo as well as the planet's landscape. (NASA) Back to slideshow navigation
  3. Grand canyon

    This is a composite of Viking orbiter images that shows the Valles Marineris canyon system. The entire system measures more than 1,875 miles long and has an average depth of 5 miles. (NASA) Back to slideshow navigation
  4. Red rover

    A mosaic of eight pictures shows the Pathfinder probe's Sojourner rover just after it rolled off its ramp. At lower right you can see one of the airbags that cushioned Pathfinder's landing on July 4, 1997. (NASA) Back to slideshow navigation
  5. Twin Peaks at their peak

    The Pathfinder probe focuses on Twin Peaks, two hills of modest height on the Martian horizon. Each peak rises about 100 feet above the surrounding rock-littered terrain. (NASA) Back to slideshow navigation
  6. Blue horizon

    A Martian sunset reverses the colors you'd expect on Earth: Most of the sky is colored by reddish dust hanging in the atmosphere, but the scattering of light creates a blue halo around the sun itself. (NASA / JPL) Back to slideshow navigation
  7. Two-faced Mars

    The image at left, captured by a Viking orbiter in the 1970s, sparked speculation that Martians had constructed a facelike monument peering into space. But the sharper image at right, sent back in 1998 by Mars Global Surveyor, spoiled the effect. (NASA) Back to slideshow navigation
  8. Put on a happy face

    The "Happy Face Crater" - officially named Galle Crater - puts a humorous spin on the "Face on Mars" controversy. This image was provided by the Mars Global Surveyor orbiter. (MSSS / NASA) Back to slideshow navigation
  9. A monster of a mountain

    Mars' highest mountain, an inactive volcano dubbed Olympus Mons, rises as high as three Everests and covers roughly the same area as the state of Arizona. Mars Global Surveyor took this wide-angle view. (MSSS / NASA) Back to slideshow navigation
  10. Pockmarked moon

    Mars Global Surveyor snapped this picture of Phobos, the larger of Mars' two potato-shaped moons. Phobos' average width is just 14 miles. The image highlights Phobos' 6-mile-wide Stickney Crater. () Back to slideshow navigation
  11. From Mars with love

    This valentine from Mars, as seen by Mars Global Surveyor, is actually a pit formed by a collapse within a straight-walled trough known in geological terms as a graben. The pit spans 1.4 miles at its widest point. (MSSS / NASA) Back to slideshow navigation
  12. Sandy swirls

    An image taken by Mars Global Surveyor shows a section of the northern sand dunes on Mars' surface. The dunes, composed of dark sand grains, encircle the north polar cap. (JPL / NASA) Back to slideshow navigation
  13. Curls of clouds

    Global Surveyor focuses on a storm system over Mars' north polar region. The north polar ice cap is the white feature at the top center of the frame. Clouds that appear white consist mainly of water ice. Clouds that appear orange or brown contain dust. (MSSS / NASA) Back to slideshow navigation
  14. Swiss cheese

    Global Surveyor captured images of a frost pattern at Mars' south polar ice cap that looks like Swiss cheese. The south polar cap is the only region on the Red Planet to contain such formations. (NASA / JPL / Malin Space Science) Back to slideshow navigation
  15. Purple Planet

    A false-color image from the Opportunity rover, released Feb. 9, 2004, accentuates the differences between a green-looking slab of Martian bedrock and orange-looking spheres of rock. Scientists likened the "spherules" to blueberries embedded within and scattered around muffins of bedrock. The spherules are thought to have been created by the percolation of mineral-laden water through the bedrock layers. (NASA / JPL / Cornell University) Back to slideshow navigation
  16. Dunes of Mars

    A false-color view from NASA's Opportunity rover, released Aug. 6, 2004, shows the dune field at the bottom of Endurance Crater. The bluish tint indicates the presence of hematite-containing spherules ("blueberries") that accumulate on the flat surfaces of the crater floor. (NASA / JPL / Cornell University) Back to slideshow navigation
  17. Alien junkyard

    The Opportunity rover looks at its own heat shield, which was jettisoned during the spacecraft's descent back in January 2004, on Dec. 22, 2004. The main structure from the heat shield is at left, with additional debris and the scar left by the shield's impact to the right. The shadow of the rover's observation mast is visible in the foreground. (NASA / JPL) Back to slideshow navigation
  18. Devil on Mars

    This image shows a mini-whirlwind, also known as a dust devil, scooting across the plains inside Gusev Crater on Mars, as seen from the Spirit rover's hillside vantage point on April 18, 2005. (NASA / JPL) Back to slideshow navigation
  19. Rub al Khali

    The tracks of NASA's Opportunity rover are visible in a panoramic picture of a desolate, sandy stretch of Martian terrain in Meridiani Planum, photographed in May 2005 and released by NASA on July 28. "Rub al Khali" (Arabic for "Empty Quarter") was chosen as the title of this panorama because that is the name of a similarly barren, desolate part of the Saudi Arabian desert on Earth. (NASA / JPL / Cornell University) Back to slideshow navigation
  20. Double moons

    Taking advantage of extra solar energy collected during the day, NASA's Spirit rover spent a night stargazing, photographing the two moons of Mars as they crossed the night sky. The large bright moon is Phobos; the smaller one to its left is Deimos. (NASA / JPL / Cornell / Texas A&M) Back to slideshow navigation
  21. Mars in the round

    A 360-degree panorama shows a stretched-out view of NASA's Spirit rover and its surroundings on the summit of Husband Hill, within Mars' Gusev Crater. The imagery for the panorama was acquired in August, and the picture was released on Dec. 5. (NASA) Back to slideshow navigation
  22. Fossil delta

    Scientifically, perhaps the most important result from use of the Mars Orbiter Camera on NASA's Mars Global Surveyor has been the discovery in November 2003 of a fossil delta located in a crater northeast of Holden Crater. (NASA / JPL / MSSS) Back to slideshow navigation
  23. Underneath the ice

    This view taken in January 2005 shows sharp detail of a scarp at the head of Chasma Boreale, a large trough cut by erosion into the Martian north polar cap and the layered material beneath the ice cap. (NASA / JPL / MSSS) Back to slideshow navigation
  24. Celestial celebration

    Controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., cheer on Friday after hearing that Mars Reconnaissance Orbiter successfully made it into orbit around the Red Planet. (Phil McCarten / Reuters) Back to slideshow navigation
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By
updated 5/29/2008 6:07:54 PM ET 2008-05-29T22:07:54

In 2004, NASA's Opportunity rover found evidence in Martian soils that water had once flowed across the surface there, buoying hopes that the red planet may once have supported primitive life.

But a new study throws some cold water, and a big pinch of salt, on those hopes.

"Liquid water is required by all species on Earth and we've assumed that water is the very least that would be necessary for life on Mars," said study team member Nicholas J. Tosca, a Harvard University postdoctoral researcher. "However, to really assess Mars' habitability we need to consider the properties of its water. Not all of Earth's waters are able to support life, and the limits of terrestrial life are sharply defined by water's temperature, acidity and salinity."

Tosca and his team analyzed salt deposits in the 4-billion-year-old Martian rock investigated by Opportunity (and by spacecraft orbiting the planet). The new analysis shows that the water that would have flowed across these ancient Martian rocks may have been exceedingly briny.

"Our sense has been that while Mars is a lousy environment for supporting life today, long ago it might have more closely resembled Earth," said Andrew H. Knoll, also of Harvard and on the study team. "But this result suggests quite strongly that even as long as four billion years ago, the surface of Mars would have been challenging for life. No matter how far back we peer into Mars' history, we may never see a point at which the planet really looked like Earth."

The research was presented in February at the annual meeting of the American Association for the Advancement of Science in Boston.

Halophiles, or organisms that can tolerate high-salinity waters, are known to exist in places on Earth, but they likely evolved from organisms that lived in purer waters, scientists think, making it unlikely that life would actually arise initially in extremely briny waters.

The high salinity, however, "doesn't rule out life forms of a type we've never encountered," Knoll added, "but life that could originate and persist in such a salty setting would require biochemistry distinct from any known among even the most robust halophiles on Earth."

Knoll and Tosca also say the finding doesn't rule out the possibility that less salty waters once flowed on the planet, though Meridiani Planum, where the Opportunity rocks were found, is believed to have been one of the wetter, more hospitable places on the planet.

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