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We can, it's fair to say, be confident that there's life on Earth ? but proving it is a different matter. In December 1990, the Galileo spacecraft pointed its sensors back at Earth before setting off for Jupiter. The probe reported an atmosphere abundant in oxygen and unusually rich in methane. It also detected a mysterious pigment that was unlikely to be of mineral origin: something earthlings call chlorophyll. Yet the astrophysicist Carl Sagan and his colleagues were still cautious in their conclusions based on these results. "Together, these are strongly suggestive of life on Earth," they wrote.

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The original samples from the Viking landers on Mars oh so many years ago could have been interpreted as finding materials that indicated a presence or past presence of life. Unfortunately, those findings could also be interpreted as originating from non-life sources also. Not unlike the little thingies found inside the Mars rock that fell to Earth in Antarctica.

Even the signs of water erosion on Mars have alternative explanations that do not require water to be present now or at any time on Mars. But the wieght of evidence seems to be pointing more and more towards water.

You know, the "crystal sphere", earth centered model of the solar system actually served quite well as a predictor of planetary movements. Good enough for determining Easter many years in advance. Capernicus and Gallileo had no real proof of the heliocentric theory. Capernicus just offered it up as a simpler model for the church to determine Easter, and Gallileo's pamphlet offered no empirical evidence. Just the persuasive idea that a simpler model was more likely to be true than a complicated one.

It wasn't until Kepler came along that we really understood, empirically, that we were not the center of the solar system. And even at that, we had to wait until after Kepler's death, when Halley's comet reapeared at exactly the time and point relative to the earth that Kepler's laws predicted.

Until we find a fossil or an actual beastie on Mars, scientists will be very cautious. It's and extraordinary claim, and as Sagan liked to point out, it requires extraordinary evidence (and sceptical rigor) in order to be accepted as a provisional "fact".

Advances in telescopes might detect oxygen and methane on extra-solar planets (a pretty powerful smoking gun indicating life, we don't know of any chemistry that accounts for such an atmosphere without life being involved) before we find enough evidence on Mars to declare life is or was present there.

Which opens up the next question. Earth must be quite the beacon for inteligent life not much more advanced technologically than we are.

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How come they haven't dropped by, or even left a message?

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Heh. Heh. Heh.