Things needed for life on Mars...
Surface :
Warmth - Gets upto 86 degrees F at the eguator around noon (mars time ofcourse). Plenty warm enough, life would just have to hibernate when it gets to cold.
Liquid Water - Areas where the martian atmosphere are higher then normal go above the 'tripoint', where liquid water can exist, though in a smaller tempreture range then earth, in these areas, water can remain liquid between 0 - 10 Centigrade. Areas where the atmostphere would be thicker include deep craters, and massive canyons.
Light - Mars has plenty of light on the surface
Underground:
Liquid Water - Mars is covered in a permafrost that goes very deep into the planet. this permafrost is mostly ice. Mars seems geothermally dormant, but looks can be decieving, unlike earth which 4.8 billion years ago got smacked by a small planetoid, which knocked by a rather sizeable chunk into orbit, and provides enough stress on our crust to keep it thin. Mars has a normal thick crust like most other terrestrial planets. But that does not mean it's asleep, it just means that events happen less often, but on a much larger scale, thus Olympus Mons. So in the deeper parts of the martian crust it is likely warmer, possible with huge ungerground rivers, lakes, and maybe even oceans.
Warmth - see above
Light - we already know that life does not need light
Under the icecaps:
liquid water - liquid water needs 2 things, pressure and heat. the more pressure you have, the less heat it takes.
The martian icecaps are huge, and express a great deal of pressure on the lower parts of the icecap. It is not inconcievable that the pressure under the ice could be great enough, to compress the ice into liquid water, and if theres even one tiny geothermal dome or vent under the ice, it's pretty much assured.
Warmth - having warth is well and good, but life does not actually need it. Seeing as how life lives on or in every continent on earth. Some forms of algae actually live IN artic ice, melting it as they need it, by excreting antifreezes.
Light - life does not need light, only energy, which can come so many ways.
Summary:
Despite Mars' harsh conditions, and the obvious fact that the surface is not infested with martian life, the conditions are there for life to exist across large expanses of the planet. Some are more likely then other to harbor life. Most will be microbial, but maybe some will be large enough to see, and maybe even crawl across the surface. The trick to finding this life, is finding where the conditions exist for the life, and waiting to see them wake up.
AfterThought: It is often stated, that mars is too small to sustain a sizable atmosphere neccessary to sustain a living planet. I believe this to be wholey untrue, but rather, that mars had it's atmosphere destroyed by a very massive impact, after most of the free material in the solar system that could replinish it's atmosphere had dissipated. the evidence is in the southern hemisphere, a crater 6 miles deep. The majority of all life on mars would have parished shortly after this impact, but the oceans would take a long time to evaporate and snow down onto the poles. Life during this time would have been adapting to a collapsing ecosystem, finding nitches where ever they lie. These lifeforms could possibly still be there today. They would likely be so fragile, they the very act of discovering them could kill them. We must tred cautious in our exploration of Mars, and hope we find life, but also that it remains living after we find it.
