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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updated 12/30/2008 11:35:16 AM ET 2008-12-30T16:35:16

Humans have pondered the mysteries of Mars for thousands of years, with one question eliciting particular interest: Is there life up there?

While there is no good evidence that life ever existed on Mars, the tantalizing possibility that the planet may once have been able to support life garnered some of the strongest support yet this year, as the orbiters, landers and rovers recently sent to investigate our rusty neighbor gathered key evidence of the planet's watery past, climatic shifts and other points of geology and chemistry that could have impacted the emergence of any potential Martian life.

Some of the missions that made key findings this year:

NASA's Phoenix Mars Lander studied the surface of the Martian arctic plains from its landing on May 25 until it lost power on Nov. 2. NASA's Mars Reconnaissance Orbiter (MRO) has used its high-resolution instruments to probe the planet's surface from orbit and just finished its two-year primary mission. NASA's Mars Exploration Rovers, Spirit and Opportunity, marked their fourth anniversary on the planet in January (the rovers will celebrate their fifth anniversary in January 2009).

Water, water everywhere?
While many conditions are considered important for life to arise on a planet or moon, the one that is essential for life as we know it is liquid water. So NASA scientists trying to answer the question of habitability have used the mantra "follow the water" for their missions.

Though Mars is bone dry today, scientists have known for some time that features including what look like gullies, river beds, and possibly lakebeds suggest that water once flowed on its surface.

But when that water flowed, whether or not it was hospitable to life, and whether it was the result of rainfall, melting ice or groundwater were still big questions — questions that Phoenix, MRO and other missions have helped to at least partially answer this year.

Several processes have been implicated in forming water-related surface features, including groundwater and potentially oceans (as evidenced by MRO images), a lake inside a crater (also found by MRO), possible hydrothermal springs (also MRO), massive flash floods, and possibly even rainfall (including some evidence from rovers).

Slideshow: Month in Space: January 2014

Large bodies of water that persisted for a long time are prime places for life to have potentially emerged that scientists can further explore.

Evidence had originally suggested that much of the precipitation, flooding and other actions by water ended after the first billion years or so of Mars' 4.5 billion year history, but one study from this year used MRO's HiRISE imager and found light-toned deposits that suggest the planet was wet for a billion years longer still. Opal deposits detected by MRO also suggest a longer wet period for Mars.

Widespread water
Another MRO study, which looked at a type of clay mineral called phyllosilicates (clays are formed in the presence of water), suggests that water was more widespread on Mars' surface than was previously thought.

Phoenix was able to confirm the presence of an underground layer of water ice in Mars' arctic plains (first suggested by observations from NASA's Mars Odyssey orbiter made in 2002). Scientists are now poring over the data from Phoenix's analysis of arctic dirt to see if that ice layer might once have been liquid during past periods when Mars' orbital tilt made the north pole a warmer place. Preliminary results suggest this may have been the case, as Phoenix detected the signatures of clays in the surface dirt.

Whether or not any of the water held in the polar ice caps turns to liquid below the surface is not known, but another MRO study this year found that the crust below the northern polar cap was exceptionally thick, which suggests that the planet is colder than previously thought, meaning that if that does turn to liquid, it's likely much deeper than anticipated.

Harsh or benign?
While water has clearly been present in Mars' past, it's less clear how hospitable that water would have been to any potential Martian microbes or other primitive life forms.

Opportunity data, newly analyzed this year, shows water in its roving area would have been very briny — while the finding doesn't rule out life entirely, even the toughest halophiles (salt-loving) organisms known on Earth would have trouble thriving in these salt-choked waters, scientists said.

Previous findings from the MER rovers also painted an unfriendly picture for life in another way: They detected acidic dirt with plenty of sulfates — indicative of volcanic activity — that suggest that the water that once existed in the areas of the planet they are exploring (closer to the equator) was acidic and therefore less hospitable to life.

But a recent analysis of MRO data found pockets of the surface that may have escaped the acid bath. The orbiter detected signatures of carbonates, which dissolve in acids; their presence indicates that some areas of the surface were less hostile to life and could have preserved signatures of any life that may have persisted there.

Phoenix also found signs of a more pleasant place for potential life, with its instruments indicating that the dirt at its landing site is alkaline (a surprise to scientists given the acidic nature of the surface elsewhere).

Samples of Martian dirt dug up by Phoenix also showed signs of perchlorate, a compound that could act as an energy source for Martian microbes.

MRO also detected a curious signature that could indicate life was once present on the surface: reduced iron. On Earth, reduced (or ferrous) iron is generally formed by microorganisms, though other processes, such as reaction with organic carbon brought by a comet impact could account for the signature.

Cycling climate
Understanding the climate of Mars, both past and present, is also important to gauging the planet's potential habitability, and various missions have contributed to the picture of Mars' climate this year.

It was known that Mars' climate changes as its orbital axis wobbles (changes angles), with the poles and equator alternately warming and cooling. Evidence of this showed up in new MRO images that suggest past equatorial glaciers and repeated episodes of glaciation on Mars. MRO has even found current glaciers outside of Mars' polar regions, buried beneath aprons of debris (which keep the ice from sublimating), which likely formed under a different climate regime than currently governs the planet.

Slideshow: Month in Space: January 2014 Variations in sediment layers observed by MRO also provide evidence for the effects that Mars' changing climate can have on the surface of the planet.

Above the surface, the European Space Agency's Mars Express orbiter detected dry ice (or frozen carbon dioxide) clouds in Mars' atmosphere that cast shadows on the surface below and affect regional weather and wind patterns.

Phoenix also observed clouds overhead, which increased as the landing site transitioned into northern hemisphere winter. The lander even observed snow falling from high in the air and frost forming at the surface.

Phoenix also monitored the cycling of water between the surface and the atmosphere from night to day. In warmer, wetter times, more water droplets may have stuck to the surface, creating a film of water that some microbes are capable of living in.

While these findings have filled in some pieces in the puzzle of Mars' past, there are still many holes left to fill in. The MER rovers, MRO and other orbiters and the upcoming Mars Science Laboratory (delayed until 2011 in November) and MAVEN missions will all aim to find more clues that will help scientists put together a bigger picture of Mars or Mars' past, especially its habitability.

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