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.

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). 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 Phoenixdetected 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.

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.