The Phoenix Mars Polar Lander is slated to land on Mars' north pole on May 25th - very soon now! Phoenix is intended to look for signs of microbial life, and there's a case that evidence may be found on Mars.
The first piece of evidence is purported microfossils found in an Antarctic meteorite in 1997 - this Shergottic meteorite ALH84001, estimated to be 4.5 billion years old, is still the subject of some controversy. There are some who say that the micro structures are evidence of biotic contamination from its long tenure on Earth, others maintain hope that it's evidence of a link to life on the Mars.
Martian life has a long held a fascination for astronomers - for one, it's a bright object in the sky, and for two, after the development of the telescope in the 1600s, it showed the most obvious color changes of any object in the sky. From Schiapiarelli to Lowell to Wells, the prospect of Martian life has held the imagination, even as the scientific evidence mounted that such life would be nothing at all like we'd expect.
The case for life on Mars is reinforced by the exposure of bacteria on Lunar missions - samples went out, and came back, and were able to survive the harsh Solar and Van Allen radiation belts - even some of the plasma and thermal changes of reentry through the Earth's atmosphere. So life is remarkably hardy and capable of surviving in the vicious environment of space. Achaeobacteria and tube worms living on volcanic vents show that life can survive and thrive wherever there's a source of energy to exploit, even down in the depths of the ocean where sunlight isn't even a memory.
However, the question of life on Mars has a few more complications. For one, it's a cold case -whether there was life in the past, Mars' surface conditions have changed over the last 5 billion years. There are definite epochs in Martian geology (called areology), where Mars speculation shows Mars having a thicker atmosphere than now, and receding (and advancing) surface water levels. Mars' current climate cannot support liquid water on the surface - the temperature is too low, and the atmospheric pressure is too low; if you took a tray of ice cubes out on the surface, they'd slowly evaporate by sublimation, the way dry ice does on Earth.
What caused these changes in Martian climate? Plate tectonics, or rather, the lack of them. Earth's biosphere is driven in large part by plate tectonics, which serve to bury carbon (in the form of limestone) caused by sedimentation. The driving force on plate tectonics is the decay of radioactive elements in the Earth's core.
Venus also shows evidence of having had plate tectonics in the past; its plate tectonics appear to have stopped due to the lack of water in the subsurface crust - the water in the oceans is far from being the majority of it on the planet; most of the rest is seepage down into the Earth's mantle, where it acts as a lubricant.
On Mars, due to short range radio surveys by the Mars Reconnaissance Orbiter, we now show that Mars' crust isn't as flexible as Earth's - it's not being pressed down as much as it should be by the mass of the Martian polar ice cap. This is indicative that Mars is tectonically dormant.
Why does tectonic dormancy matter for the case for life on Mars? Tectonic activity is the likely driving energy source for any Martian microbes out there, and it's the only candidate presently known to make long standing seeps of liquid water, which is necessary for life as we know it.
Phoenix will be landing at the Martian north pole to give us some answers - but even if it finds nothing, there's no reason to give up hope. Mars has a surface area equal to all of Earth's dry land put together. Phoenix will only be able to sample an area comparable to a ring of soil around an office cubical.
Phoenix' other mission profile is to give climate data on Martian weather patterns and observational data on the polar winter, and it's slated to provide data for at least six months, with optional extensions for years. (The Spirit and Opportunity rovers are now approaching the fourth anniversary of the beginning of their 90 day survey...)
Phoenix live commentary from NASA begins at at 3:30 pm USA Pacific time on the 25th as the craft prepares for its decent into the Martian atmosphere, or about 9:30 on the morning of the 26th for Eastern Australia.
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