Total Recall :D They just need to remember to plant some trees so everyone can breath!
I'm speculating the only reason no group of scientists have planted trees on Mars, is because it would change its atmospheric properties. I suppose in a way it could jeopardise any evidence of past or existing life.
Would I be right in thinking though that with enough trees, it would no longer be 'the red planet' but more like Earth, but considerably cooler?
I was going to poo-poo the idea, but according to the fountain of all knowledge (en.wikipedia.org/wiki/Mars) Mars atmosphere is 95% Carbon Dioxide and the soil contains the right mix of nutrients so a hardy bread of tree might actually survive there if we could find enough water for it. Now how many tree's do you think it'll take to terraform the planet?
Thinking about life on mars though (no I'm not having a 70's flash back), haven't they found evidence of water? If they have then some bacterial or algae life can't be beyond the realms of possibility (not quite the over lords I was hoping for (but contenders for Dr Who villains non the less)). Even if life isn't already there, it'd be interesting to see if some our own lower order organisms could be introduced...
But it is not just the lower gravity it is also a lack of geological activity in the core. Mars is smaller than Earth and it has lost heat to space more rapidly than Earth. Compare this with Earth: We see volcanoes as a result of very high temperatures at the core. Earth's outer core is liquid metal surrounding a solid metal inner core. This is where the Earth's geomagnetic filed originates and this protects our atmosphere from being blown into space by the solar wind.
Any more substantial that used to exist on Mars has been eroded by the solar wind.
Anything living exposed to the atmosphere on Mars would have to be able to tolerate high Carbon Dioxide concentrations, low atmospheric pressure [0.087 PSI vs Earth's at 14.69 PSI at sea level] and high levels of UV and cosmic radiation due to the lack of an Ozone layer and lack of geomagnetic field [as mentioned earlier]
And people would build bombs out of them.
And I wouldn't seriously buy a nuclear fission device someone at Vauxhall had made, because they can't even make you an EGR valve that doesn't fall off after 30 miles (right SirBendy?) :o