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! A K/T-sized
event would occur every 100,000 years, not every 100 million years.
Knowing all these things will be vital to our future discussions
of creation and survival of life on other planets.