Drake on Drake: ET Life A Certainty

astro writes "Frank Drake, Chairman of the Board of Trustees of the SETI Institute applies Occam's Razor to his own Drake equation: 'Life should appear very frequently on other Earth-like planets. There will be microbial life nearby the solar system.' The simplest scenario is that 'Not Life' has a nearly identical number of assumptions as 'Life.' The contrasting view is that experimentation can prove it--but how many times did life independently create itself while the Earth changed through the whole spectrum of what biological forces might conjure up elsewhere. A sample size of 1 is in fact an experimental sample size of many--just here during Earth's climatic history."

Comments

Timing

[Michael Woodhams]

We have a sample size of only one, and that this sample resulted in intelligent life is a given (else we wouldn't be here to make observations on it.) We do however have some timing information. From this we see:

1) Life evolved on Earth pretty much as soon as conditions were stable enough to allow it. This suggests that bacterial life is highly likely.

2) It took at least hundreds of millions of years to develop Eukariotic life (big cells with a nucleus, such as we are made of, as opposed to bacteria.) This means that this step might be rare.

3) It took about 3 billion years to evolve differentiated multicellular life. This means that this step could be exceedingly rare.

4) Multicellular life evolved into a vast array of designs in a just a few million years (the 'Cambrian explosion'.) This means that once multicellular life starts, it will quickly produce complex forms.

5) From the Cambrian explosion to us is something like 500 million years. This is an intermediate time scale that makes it hard to judge how likely intelligent life is.

Disclaimer: I'm not 100% sure of some of the timescales above. It is all from memory.

Disclaimer 2: The Edicara fauna complicate the picture above on the origin of multicellular life, depending on how you interpret them.

Disclaimer 3: All the above is merely probabilistic. E.g. if the evolution of bacterial life is very rare, there is still a 5% chance that it will have occurred during the first 5% of the available time. Therefore we can't strongly exclude the possiblity that the evolution of bacterial life is hard.

Re:1 != Many

[AJWM]

Good points about Venus and Mars which orbit at the edges of the "life zone".

One possibly critical datum missing from that analysis relates to Earth's history. Earth has a relatively thin crust and the combination of tectonic motion, subduction, and vulcanism recycles elements back to the atmosphere after they'd been locked up in rocks. (Actually you do hint at this in your last para.)

Mars seems to be small enough that it has solidified down far enough that there's no more cycling of the crust and thus the oxygen and hydrogen locked in the rocks stays there, also any free hydrogen (from the ultraviolet lysis of water) escapes to space before it can recombine, thus Mars now has a very thin atmosphere.

Venus may be large enough, although that isn't certain. It certainly seems to have continental masses but I don't know about any active volcanism. It's heavy enough to retain atmosphere though -- too much of it, as you point out.

It's possible that the reason Earth escaped Venus's fate has less to do with the distance from the Sun and more to do with the formation of our Moon. Current theory is that late in the formation of the solar system, the proto-earth was smacked by a Mars-size protoplanet which literally splashed a good chunk of the proto-Earth into space, some of which condensed to form the Moon. This has several implications. The lightest elements would have boiled away in significant quantity, so there's just less of them around to form a thick atmosphere (hence less runaway greenhouse). The medium light-weight elements (that form crust, particularly continental crust) were greatly reduced, some of them forming the Moon (so in one sense, the Moon is the 8th continent), meaning that tectonic circulation has an easier time of it. (The heat from that impact might also have some effect there, although I think that would be dissipated by now.)

All of which leads to the (somewhat depressing) conclusion that Earth is habitable only because of a really unlikely sequence of events (much more unlikely than merely forming at the right size in the right place). OTOH, observation of our own solar system and some of the very strange (to us) places on Earth that life survives and thrives indicate that there could be a lot of places that primitive life exists. Star Trek's "Class M" planets are probably pretty darned rare, though.

(Oh, BTW, Mercury isn't in "tidal lock" like the Moon is with Earth, but in a 3:2 tidal resonance with the Sun.)