[ExI] Life must be everywhere!

Anders Sandberg anders at aleph.se
Mon Apr 16 15:43:57 UTC 2012

I did a quick-and-dirty simulation of the heat diffusion in spherical 
rock. I don't trust the quantitative results but the qualitative picture 
looks a bit like I thought: a wave of high temperature diffusing into 
the rock, with a fairly rapid cooling of the surface in space. Small 
pebbles get fried before they cool, large ones dissipate the wave before 
it gets too deep.

I also re-read the paper that initiated the whole discussion, and I 
found it to be severely lacking. The authors seem to have relied very 
strongly on the pro-panspermia community for impact physics, producing 
very high estimates. However, there is a decent literature of proper 
simulations with less of an axe to grind. In particular, "Impact Seeding 
and Reseeding in the Inner Solar System "
suggests that the timescale for particles in the inner system from 
launch to impact is about 30,000 years, which is good news for local 
panspermia. It also includes fragment calculations based on Melosh, H J. 
(1985) Ejection of rock fragments from planetary bodies. Geology 13, 
144–148. that seem to imply a power law.

On 16/04/2012 07:48, Kelly Anderson wrote:
> By sequential breakups, do you mean big rocks that get broken into 
> littler pieces later? 

Yup. One can run the above calculation backwards: heating spreads into 
an initially frozen impactor as it descends. If it is too small, 
everything gets fried. If it is larger a core survives, but it might 
still get vaporised on impact. But if the rock is slowed by friction and 
then blows up, then cold fragments might now find themselves in the 
lower atmosphere where their terminal velocity doesn't heat them up too 
much. Unfortunately this kind of cascades are hard to analyse and little 
constrained by data.

> I think that it would be safe to estimate boulder size with a Poisson
> distribution. The other really big problem is that smaller stuff seems
> likely to travel faster (F=ma and all) and thus have a better chance
> of escaping the solar system.

Not sure if the sizes are going to be Poisson or power law - I suspect 
the later, since fragmentation processes typically give power laws.

> On the biology side of things though... Bacteria are MUCH less 
> susceptible to radiation damage than Eukaryotes. Our DNA has the 
> protection of the nuclear sack, and so it has lost some of the ability 
> that prokaryotic cells have for ongoing DNA repair. So the take home 
> message here is that prokaryotes are much more likely to have come 
> from outer space than eukaryotes. And since it did take a long time 
> for the eukaryotes to emerge on earth, that is consistent with the Law 
> of Accelerating returns whereas the sudden appearance of the 
> prokaryotes seems rather sudden.

However, D radiodurans is pretty extreme by prokaryote standards too. I 
would not trust the LoAR in biology. Plenty of observation bias and all 

Anders Sandberg,
Future of Humanity Institute
Philosophy Faculty of Oxford University

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