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Comment by Robotbeat | Hacker News Reader

Sure, but that’s not a huge effect. The Sun hits us with 125-150Petawatts of solar energy, was about 15% cooler when life developed (~20PW), and the amount of heat the Earth emitted from radioactive decay was at most about a factor of 1000 times less (~120TW) around the time life developed.

Even the energy released by the Earth cooling and solidifying over 100 million years still only comes down to like 1-2 Petawatts except immediately. And to try to make up another 10PW from bombardment heating would require About 1000 cubic kilometers of infalling asteroids per year (more than a Moon’s worth of mass over 100 million years), so Earth should’ve become pretty frigid soon after solidifying… UNLESS there was significantly more greenhouse effect back then (which is most likely!).

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Sure, but the amount of 70F water that lasted for more than a decade might have been 1000x higher because of increased geysers, volcanoes, deep sea vents etc. Sure it might have been a vanishingly small fraction of the earth's surface and oceans, but still have an impact on the evolution of life while the earth was on average quite cool.

All this assumes that life had to form on the surface. But that's not the only possibility. If life formed deeper in the Earth and migrated to the surface later, the fainter sun might not have been an issue. Geothermal energy is small on the surface, but not deeper down.

My reading of the article is that scientists are puzzled by the the evidence for large amounts of liquid water, not just life. If you look back in the past and see ice, but also life, geothermal is relevant. But they see rivers and oceans, so you have to focus on bigger solutions.

Those numbers look right, thanks. Looks like about 3x the radiogenic heat from the major nuclides producing heat today (not taking into account nuclides that may have decayed away... but I don't think there were any substantial ones). There's also the possibility of fission chain reactions, as U235 enrichment was very high, but I doubt these came near the PW range. Atmospheric CO2 looks like the primary factor.

Is that 15% of it's current value, or 15% lower? Because 15% lower its more like 100petawatts.

Mind you, the bigger question is where the entropic difference was. The earth, including the water would all still be warmer while cooling down, and it needs to drop that heat somewhere for life to do it's entropic increases

That slight increase in earth's internal heating can have an outsized effect. It would have meant more and different volcanism, which means a different atmosphere which, perhaps, was better at absorbing and retaining heat.

Yeah, the atmosphere is by far the biggest effect. Geothermal heating (especially by radioisotopes) is a rounding error in this case.

It’s also directly meaningful difference because ice floats and provides insulation. Even a runaway greenhouse atmosphere provides nowhere near as much insulation as a 1km thick ice sheet. Seemingly minimal amounts of geothermal heat should provided thousands of cubic km worth of oceans for life to live in during the three snowball earths believed to have happened.

This is also why Europa’s −171 °C surface temperature is believed to give way to liquid oceans.

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