How soon was the Earth Habitable?
Evidence from the Moon suggests >200 km objects where still hitting
it up to and including 3.8 b.y.a., but possibly as recent as 3.5 b.y.a.
A >300 km impact object would heat the Earth to 2000 K, boiling the
oceans, then sinking the Earth into an extreme greenhouse effect,
reaching a final temperature of 3000 K. Nothing could survive.
The Earth would be utterly sterilized.
A 150-200 km impact would evaporate the oceans down to 200 m,
the depth which sunlight can penetrate. If life was photo-synthetic,
then even this smaller impact could destroy all present life.
Smaller impacts were more common and possibly went on for
even a longer time. Amazingly, there is evidence that life existed
at very early times, 3.5 - 3.8 b.y.a. This begs the question:
1) Did life form and die multiple times between these events?
- If so, life is extremely quick to come about, and
re-create time and again.
2) Did life form once and cling to existence within hydrothermal
environments in the safety of the deep ocean?
- It is difficult to imagine the creation of life again and again,
but in this scenario life had near equally incredible survival ability!!
3) Perhaps something in between..
- Life may have formed just 2 or 3 times,
and just barely survived the other many life-threatening impact
events.
--Finally, thank goodness for Jupiter and Saturn. They have
kept our impact rate reduced by a factor of 10,000x!
Knowing all these things will be vital to our future discussions
of creation and survival of life on other planets.