[ExI] Life must be everywhere!

[Anders Sandberg]

Ì wasn't too convinced by the paper because it mostly counted rocks 
rather than cells. The authors were much more interested in how many 
pebbles could get from point A to B than what was in them.

Let's assume 90 kg/m^2 of biomass (double a tropical forest; I am 
assuming just as much biomass in the lithoautotropic ecosystem as on the 
surface to be optimistic). They suggest that ejecta amounting to 30% of 
mass of the impactor (sounds *very* optimistic!). So some 
0.3*pi*5000^2*90=2.1e9 kg of biomass ends up in space (5000 from the 
radius of the impact site). The ejected total mass is 3*10^14 kg, so the 
fraction that is biomass is 7e-6 - that would indeed allow a few hundred 
million bacteria on each 1 cm pebble.

However, much of this material will be subjected to denaturating 
temperatures (it was a 96 teraton explosion), and worse, the 
distribution of cells is uneven: the vast majority of material will be 
from deep crust and the impactor itself, both which are likely 
cell-free. I don't think anybody knows how to calculate the denaturating 
effects, but they are likely severe. The uneven distribution on the 
other hand might be roughly approximated: if we assume the ejecta is an 
equal mix of impactor and the same volume of Earth, that only the top 
kilometer of rock is life-bearing, and that pebbles keep together, the 
fraction of ejecta that is from the life layer is (pi*5000^2*1000) / 
(4*pi*5000^3/3)=0.15 - just 15% of the pebbles will have cells, the rest 
are from impactors or deep crust.

I certainly cannot rule out panspermia this way; life can be amazingly 
hardy, and it is enough to just get a few cells into a survivable 
environment for them to spread. But I suspect the denaturation of impact 
is a massive factor that reduces the viability of launched pebbles even 
before they are subjected to space conditions. If the denaturation 
reduces the number of viable pebbles by just two orders of magnitude 
(which sounds eminently likely), then a thousand pebbles reaching Gliese 
581 will not transfer life. The numbers within the solar system still 
seem to be high enough to allow transfer (and KT was just one recent big 
impact), so if it is possible I expect local panspermia to have occurred.


-- 
Anders Sandberg
Future of Humanity Institute
Oxford University