The standard solar model predicts a young Sun which was too faint to sustain liquid water on the Earth, unless there was an extreme greenhouse effect at the time, which seems to contradict the geochemical record. It seems to be almost impossible to get liquid water on Mars under the standard solar model with any plausible early Mars atmosphere.
Here is an interesting article on an old problem...
Astrobiology Magazine picked this up through the actual reporter finding it out by reading proposal abstracts. Which is impressive but scare:
The basic idea is to look at solar models in which the young Sun was a little bit more massive, and got to its current mass through an enhanced stellar wind over the first few hundred million years.
This is an old idea, well explored by Sackmann and Boothroyd (2003), but for various reasons it may be timely to look at it again in detail.
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To plug something here in Baltimore, there is a Faint Young Sun workshop at the Space Telescope Institute in the spring...
It's often struck me that planets and even dust in the early solar system would be relatively flush with short-lived radioactive isotypes (Al-26 for instance -- I see Wikipedia agrees; perhaps C-14, Fe-60, Ti-44, maybe even some U-235). Flush by standards 4.6 billion years later, of course. Possibly these would have generated some amount of heat, and by examining samples to analyze the ages and quantities of decay products, we might get some guesstimate of radioactive heating in the FYS era.
Has this been done, with useful results? (Wikipedia mentions the possibility of melting asteroids, but it wasn't clear if this has been rigorously argued, or is a handwaving argument.)
Interesting. While i've always seen Sagan et al. cited on this, I'd presumed that astronomers had this problem done and dusted. Surely they've observations of enough early G dwarfs to confirm / refute FYS? Whats the current state of the art, observations-wise?
What is the status of the idea that the young Earth's atmosphere had a larger mass of N2 than it does today? This was proposed in 2009 as a way to warm the planet without requiring too much CO2.