It turns out that we've already sent life to Mars, and it may be still living there, according to a researcher who has studied the decontamination methods used by NASA. Only our early Viking landers were fully heat sterilized. Subsequent missions have most likely been contaminated with bacteria from Earth. This, in my opinion, is a very good thing. In fact I would advocate raising some private money now to deliberately send pods of Earth life to Mars as soon as possible -- bacteria, spores, seeds, and perhaps other lifeforms that can survive on Mars. Humanity could easily wipe ourselves out, as well as the rest of our planetary ecosystem -- this would at least help to increase the chances that Earth life would live on somewhere else. Perhaps after billions of years, we could even re-evolve on Mars from the bacteria we send up. Of course we could speed things up a lot by sending up higher lifeforms, like lizards etc. -- but we would also need to send something for them to eat. I have in the past proposed sending up little "ecopods" -- ecosystems in a pod -- that are designed to fall to the surface of Mars and seed ecosystems -- they are self-contained ecosystems that can survive on Mars, and also have opening so that things from inside them can go in and out at will. Ideally these pods would be designed to replicate themselves -- perhaps by spreading seeds and bacteria around themselves, or by containing things like ant colonies that have a way of spreading. It's an interesting design challenge to try to figure out what would be the optimal (lightest weight, most durable) "ecopod" technology that could spread life on Mars.