Source: http://io9.com/kim-stanley-robinson-says...

Kim Stanley Robinson's Mars trilogy filled us all with hope that we could terraform Mars in the 21st century, with its plausible description of terraforming processes. But now, in the face of what we've learned about Mars in the past 20 years, he no longer thinks it'll be that easy.

Talking to SETI's Blog Picture Science podcast, Robinson explains that his ideas about terraforming Mars, back in the 1990s, were based on three assumptions that have been called into question or disproved:

1) Mars doesn't have any life on it at all. And now, it's looking more likely that there could be bacteria living beneath the surface. "That's going to be very hard to disprove," says Robinson. "We could be intruding on alien life."

2) There would be enough of the chemical compounds we need to survive. In particular, we need a lot of nitrogen — and scientists had expected there to be a lot, based on the ordinary distribution of elements in planetary accretion. But there's much less nitrogen on Mars than we'd hoped.

3) There's nothing poisonous to us on the surface. In fact, the surface is covered with perchlorates, which are highly toxic to humans, and the original Viking mission did not detect these. We could use bacteria to dispose of them, but it would be a very long-term process.

"It's no longer a simple matter," Robinson says. "It's possible that we could occupy, inhabit and terraform Mars. But it's probably going to take a lot longer than I described in my books."