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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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updated 5/19/2008 12:23:26 PM ET 2008-05-19T16:23:26

With the dozens of orbiters, landers and rovers that scientists have sent to Mars, you would think we'd have a good handle on just what makes our planetary neighbor tick. But even with all the pictures snapped of its rocky terrain, all the measurements taken from orbit and the soil samples scooped up, we've still barely begun to solve the puzzle that is the Red Planet.

The sheer amount of surface area of the planet left to investigate speaks to how difficult the project is. Mars' diameter is only half that of Earth and its mass is only a tenth of Earth's. But it has no oceans. Its surface area is equivalent to all of the continents on Earth, "so it's going to take a while to understand," said Ray Arvidson of Washington University in St. Louis, chairman of the Phoenix mission's landing site working group.

In the long list of questions that make up this puzzle, two related queries have long stood out in the minds of scientists and the public alike: Is there liquid water on Mars? And does this seemingly barren planet harbor some kind of life?

NASA's Mars Phoenix Lander, slated to touch down in the northern polar region of Mars on Sunday, will aim to help answer these two key questions as it surveys a tiny piece of the planet.

Where's the water?
Scientists have been keen to find evidence of water on Mars because it is essential to life as we know it, and having an "on-site" source of H2O would be crucial to any future manned missions to the planet.

"Liquid water is the holy grail on Mars. Where is it? Does it exist at all?" said Phoenix principal investigator Peter Smith of the University of Arizona.

For the first half of the 20th century, it was thought that liquid water sloshed around all over the surface of Mars, in dark patches assumed to be seas covering portions of the planet's surface (not to mention astronomer Percival Lowell's infamous canals, later shown to be optical illusions). Mariner 4's 1965 flyby, which returned the first images of the planet's surface, dashed hopes of finding any Martian seas: The surface looked as inactive and pockmarked with craters as the moon.

Mariner 9, however, found signs that liquid water had once flowed across the Martian landscape through ancient river beds, as well as evidence of water erosion. Other missions, including two current rovers, Spirit and Opportunity, have found ample evidence that water once flowed through rivers, pooled in lakes and spewed from hydrothermal vents.

But this liquid water flowed mostly in very ancient times, when conditions on Mars were much different than they are today. Now, the planet's atmospheric pressure is too low (about 1/100th of Earth's) for liquid water to last on the surface. The only place on the surface where water exists is at the poles, and there it is found only in its frozen form.

In February 2002, NASA's Mars Odyssey orbiter extended the known regions of water on Mars when it detected the signature of water ice just under the surface of the Martian arctic regions, and lots of it.

"It's not just a little bit that you might expect to get frozen into the ground from the atmosphere, but it's like 70 to 80 percent of the upper meter of the surface is ice," Smith said. "The amount of ice was a huge surprise."

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Because the arctic regions of the planet haven't been explored from the surface and the underground ice has so far only been detected indirectly, this subsurface arena is "all of a sudden this mystery zone in my opinion," Smith said.

Exactly how these substantial subsurface layers of water ice formed is unknown. Some scientists say it could be a remnant of an ancient northern sea that has been theorized to have existed when Mars was much warmer. It may also have formed as water vapor froze out of the atmosphere, which is partly how the polar caps on Mars form today. But this deposition typically only creates a small amount of soil-trapped ice.

"So you wonder how you can get 70 percent water in just the pore spaces [between soil grains], it doesn't make any sense!" Smith said. "So there must be some other way that you're getting this large amount of water in that area."

Phoenix will aim to shed some light on the origin of this ice, which is expected to be so cold that it will be as hard as concrete, and to characterize it by chipping away pieces for analysis. Smith and the other Phoenix investigators also hope the lander will help determine whether or not the ice periodically melted and wet the Martian soil to create a habitable zone that could have possibly supported some form of Martian life.

"You know there's ice there now, it's probably too cold to melt — the question is, in the last million or couple million years has there been a 'wet zone' up there, if you like, where you really did get liquid water, you wet the soil, and would that be a habitable zone on Mars?" Smith told Space.com.

Hunt for a habitable zone
The idea of "little green men" living on Mars has captured the human imagination in one form or another for decades. Countless science fiction books and movies, such as H.G. Wells' "The War of the Worlds," have featured Earth invasions by menacing Martian neighbors. Even Percival Lowell thought his canals were dug by an intelligent Martian civilization.

But when Viking 2 landed on the surface of the Red Planet in 1976 and sampled the ruddy soils, it found no signs of life: no microbes, no organic materials and certainly no "little green men" with laser guns.

The barren surface, frigid temperatures, thin atmosphere and lack of liquid water decrease the likelihood that Martian life might exist or have survived if it once existed, at least in any biological form we're familiar with. But recent discoveries of microbes living in extreme environments on Earth renewed speculation that life could be hiding away somewhere on Mars.

The soil at the surface of Mars — the type sampled by the Viking mission — is actually dust that gets blown all over the planet and is constantly bombarded by ultraviolet radiation from the sun, which is known to be harmful to life on Earth.

But the subsurface ice layer discovered by Odyssey is under about 2 to 4 inches (5 to 10 centimeters) of dust, so it would be protected from UV rays (though not from DNA-damaging cosmic rays), so the soil-ice layers would be more likely to have organics, Arvidson said — if they even exist.

Smith noted that there are microbes on Earth that freeze during the polar winter, but survive and thaw out again come spring, "like nothing happened." Similarly, Mars experiences much warmer periods when the wobble of its axis sets it at a different angle with respect to the sun — but this happens on much longer time scales than Earth seasons.

"Can that [microbe thawing] happen over 500,000-year periods? I don't know. Who knows? Life can evolve to all kinds of extreme environments and this one would be especially extreme," Smith added.

If life did thrive on Mars during more hospitable times, it could perhaps have left some telltale sign of organic compounds in the Martian soil. Phoenix is designed to analyze the soil near the subsurface ice layers for traces of organics; finding these chemicals wouldn't prove the existence of life on Mars, but it would bolster plans to probe the planet further and to find an answer to this longstanding question.

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