9 Mysterious Images of Mars

The fourth planet from the sun, Mars is one of our closest neighbors yet it remains a mystery. There is strong evidence that water once flowed on the planet, but what happened to make it disappear? Is there life there? So many questions, and our quest to learn more about the red planet is just beginning. As NASA's Mars rovers continues to feed our curiosity about the red planet, we can admire it from afar.

Mars maintains a bright red-orange hue because of the rust-colored, iron-rich minerals in the soil. This landscape is named from the 1913 science-fiction classic, "The Gods of Mars" by Edgar Rice Burroughs. "Barsoom" was the local name for the red planet in Burroughs' series of books featuring John Carter. This photo was taken by the Mars Rover Spirit inside the Gusev Crater in 2005. (Those tracks belong to the rover.)

This images shows what appears to be a horde of spiders crawling across the Martian surface, but these aren't long-sought-after signs of life. Called araneiform terrain, these are mounds that form when the carbon dioxide in the Martian ice cap heats up and is released. Eventually, this gas builds up enough pressure to break through the ice as a dust-erupting jet. The loss of the carbon dioxide leaves behind these spidery smudges on the surface.

This image was taken from the HiRISE camera on the Mars Reconnaissance Orbiter. These mystical trails are the work of surface dust devils that weave across the planet as they do on Earth. The dust devils whip up the red soil, leaving the darker sand underneath visible. These dust devils have been credited with cleaning the solar panels on the Mars rovers, according to NASA.

The rover Spirit caught this sunset on May 19, 2005, on the 489th Martian day of the rover's journey. The photo was taken in the Gusev crater on Mars. The planet typically experiences a long twilight for up to two hours after sunset because of an abundance of dust scattered across the night sky that reflects sunlight.

This is another snapshot from the HiRISE camera aboard the Mars Reconnaissance Orbiter. Taken on Feb. 9, 2009, this image shows dunes and ripples in the sand. Experts speculate that the dunes are composed of basaltic sand, which comes from volcanic rock, thus making them darker than the surrounding sand ripples.

This false-color photo was taken by the Mars rover Opportunity on Oct. 29, 2007. Cape Verde can be seen jutting from the Victoria Crater in the Meridiani Planum, a plain located two degrees south of Mars' equator. Cape Verde is named for the places Ferdinand Magellan visited in his voyage around the world in the 16th century on a ship named Victoria.

This false-color photo was taken by the Mars Phoenix Lander on Aug. 14, 2008. Soon after the photo was taken, the frost disappeared from the heat of the rising sun. Mars has a gravitational pull that is about 38 percent that of Earth's, though Earth is about 10 times more massive than Mars. There is no evidence that the planet has magnetic poles now, but there are indications that they once existed.

The arm in this photo belongs to the Spirit rover, which touched Mars in September 2005. Spirit was the first of two rovers sent to the red planet in 2004. It has been roaming the surface since then, though no communications have been received since March 2010. In May 2009, the rover became mired in some soft soil, where it remains.