"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.
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