Flashback: What we know now about Mars

By Andrea Thompson

updated 11:35 a.m. ET Dec. 30, 2008

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).

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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.

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