Evolution of life [was Re: Why Would Aliens Hide?]

Robert J. Bradbury (bradbury@www.aeiveos.com)
Thu, 25 Nov 1999 12:11:08 -0800 (PST)

On Thu, 25 Nov 1999, Charlie Stross wrote:

> Interesting corollary:
> As successive generations of nucleosynthesis increases the proportion of
> things like lead and uranium in the mix from which planetary systems
> are formed, can we anticipate the closure of the "window" within which
> terrestrial planets provide conditions suitable for the evolution
> of carbon-chemistry based life?

Not clear. It is clear, at least for us, that the Earth got lucky in that it got enough Uranium in the core to keep us hot enough to allow plate tectonics to do carbon recycling (CO2 -> CaCO3 on the ocean bottom, gets subducted and rereleased as CO2 into the atmosphere), if you are smaller, you lose H2O from the atmosphere faster (less gravity) and you lose plate tectonics faster (less uranium & less insulating material preventing heat radiation), so the CaCO3 gets deposited in sedimentary rocks and stays there (so you lose C from the atmosphere and thus plant life). On the other side of the coin if you get too hot for oceans to remain around, the CO2 remains in the atmosphere and you get a runaway greenhouse effect. Mars & Venus provide us with excellent examples of near-misses on chances for life to evolve.

So, as you accumulate Uranium, you are going to have planets that remain volcanic for longer which is probably a good thing. Life can clearly evolve to be radiation tolerant, so the radiactivity isn't really a show stopper (within limits). I suspect the increasing uranium abundance provides an interesting sliding window of planetary sizes and "planetary life zones" (insolation+atmosphere composition leading to liquid water). More uranium, probably gives smaller planets a longer window to crank up life.

We know we have planets (27 outside the solar system, is the most recent number I think), what we don't know is the size distribution and orbits because we have a selection effect for big planets with small orbits.

> So we may not only be nearly the first
> kids off the starting block, but not that far from the last generation
> before the neighbourhood turns into a toxic waste dump.
It sounds speculative to me. I'd place my bets on the evolution of life being willing to take advantage of *any* free energy resource and Uranium certainly provides that. It could be that really "hot" planets allow you to do away with the star entirely!