[ExI] Double-Earth (Was: kepler study says 8.8e9 earthlike planets)

[Anders Sandberg]

On 2013-11-17 16:59, Tomaz Kristan wrote:
> This deep vents life we know on Earth, needs oxygen. It gets it from 
> green plants in our case. Worth to remember.

No problem really, since either you could have an oxidizing atmosphere 
courtesy of surface algae, or a reducing environment with some 
alternative electron transport solutions.

For example Anammox bacteria turn ammonia into nitrogen without any 
oxygen need (they use nitrites instead), Thiobacillus denitrificans turn 
sulphur into sulphates using nitrates, hydrogen bacteria turn hydrogen 
into water using sulphates, while phosphite bacteria convert phosphite 
into phosphate using sulphate, metanogens turn hydrogen into water using 
carbon dioxide, and the carboxydotropic bacteria convert carbon monoxide 
into carbon dioxide while turning water into hydrogen. If you have a 
good, strong flow of volcanic chemicals you can build a pretty 
functional ecosystem even without any oxygen.

This is why I think dry Double-Earth will be a nicer place for life than 
wet Double-Earth: the heavier volcanism will really add a proper 
chemical outflow that will last for a long time no matter how the 
surface looks. While wet Double-Earth will stop continental drift after 
about 3 gigayears and then have the ice-crust become increasingly inert. 
Without heavy volcanism the sea will become very stratified and the ice 
will insulate it from the rocks, making the mineral content low. The 
total sea volume is also much bigger (at least a factor of 10), making 
it even harder to get any heavier atoms.

Note that life might still thrive on Wet. Even if one thinks it is 
unlikely to originate there it could spread through panspermias, and 
once established on the surface it can use photosynthesis to grab CHON 
into structures that catch the rare heavier atoms needed. (e.g. 
http://en.wikipedia.org/wiki/Siderophore ) But it will be largely 
limited to the surface layer: just four times Earth's area, and well 
mixed so that there will be fewer species. Dry at the very least can run 
two near-independent ecosystem layers plus some stuff in-between. 
Intelligence evolution... well, I don't think I know enough to say 
anything.

(In Charles Stross' "Neptune's Brood" there is Shin-Tethys, a waterworld 
relatively similar to Wet, although the higher amount of radioactives in 
the rocky core plays a fun role in the ecosystem/economy. However, when 
we chatted he admitted he did not do any elaborate calculations for the 
planet. His ecosystem also seems to be at least partially escaped 
nanotech. I suspect that one could tweak my model to have a much denser 
and more radioactive core and get something more like it. )

-- 
Dr Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University