"Perplexed in Peoria"  wrote in message
news:c8l78j$1o6a$1{at}darwin.ediacara.org...
>
> "irr"  wrote in message
> news:c8iitu$soq$1{at}darwin.ediacara.org...
> > [...] Aside from a spurt of activity around the Moon-forming impact
> > 3.8 billion years ago, it is quite possible that the extent of the
earlier
> > era of Hadean bombardment may have been greatly exaggerated.
>
> The moon forming impact was 4.5Gya, not 3.8Gya.  After that,
> impacts were heavy until around 4.2Gya, with a spurt called
> the late heavy bombardment taking place around 3.8Gya.  The
> whole surface of neither the earth nor the moon has been molten
> since around 4.4Gya.
> http://pubs.usgs.gov/gip/geotime/age.html
>
> Tom, I believe, is correct that the earth's surface was once
> very hot (molten), then less hot (100C), then even less hot
> still.  However, he may be overestimating how long it took
> to cool.  It can go from molten to 100C in 100My and from
> 100C to 0C in another 100My even while being periodically
> reheated locally by impacts.  0.1Gy is a LONG time.
>
> Tom does have a good question as to how Mars could be
> warm enough to have liquid water if Earth was a snowball.
> My guess:  Mars had liquid water only episodically and
> locally.  After a comet impact or a volcanic eruption, there
> was some water locally for a few thousand years or so,
> but then everything froze over again, and the water either
> escaped to space or was adsorbed by the soil.

Thanks for the correction -- en route to fingers, two different partial
thoughts (Moon-forming event vs. lunar cataclysm) merged into one incorrect
statement!  I think you're right on in that the devil is in the details of
the length of time from the post-bombardment era presumably akin to Tom's
OOL scenarios, to Earth being a nearly frozen place desperately in need of a
greenhouse.  As with recent evidence for liquid water on modern mars (e.g.
methane detection), it's reasonable to assume there were local geothermal
hotspots even on what might have been a near-frozen Earth.  And things might
have in fact been lukewarm to balmy back then, its just the modelers don't
have a full suite of constraints to plug into their equations yet.
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