From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: sci.geo.geology
Subject: Re: life, Moon and geology
Date: 19 Nov 92 00:23:05 GMT

In article <1992Nov11.175421.19955@mailer.cc.fsu.edu>, cain@geomag.gly.fsu.edu (Joe Cain) writes:
> 	First, we have read arguments that life would not have started
> or evolved on Earth at all if we did not have such a large Moon. That
> is, there would not have been tidal pools in which organics could
> concentrate, and the Earth's axis would not have been so stable.
> Because the moon's of Mars are such tiny captured asteroids its axis
> wobbled more and long term seasonal variations would have helped wipe
> any life that had started there during the period when it must have
> been much warmer and wetter. Any comments on this scenario?

One, mars has no life because its low gravity prevents it from having sufficient
atmosphere.  It is also far enough from the sun to be too cold.  Axis
stability is not a question here - having a moon would make the axis less
stable.  Axis stability is determined by the stability of the angular
momentum of the planet.  Having a moon can disrupt that.  Regarding
tidal pools - yes, a moon is required.  But mars likely had only a very
short period of time during which there was any liquid water (and mainly
due to volcanic activity).  Venus never had liquid water.  Having tide
pools don't help much if there's no water to make a tide.

> 	After life started on Earth so as to raise the oxygen level
> from near zero creatures evolved with shells so the carbon dioxide
> started to be captured in the shells that ended up as carbonate rocks.

No.  oxygen levels were already high by the time multi-cellular organisms
appeared.  3 billion years of algae produced oxygen by photosynthesis.
The carbon was most probably locked up as organic molecules.  Carbonate
deposits were probably inorganic.  Even stromatolitic limestones were
a pretty late development.

> greenhouse affect of all the water vapor and CO2 was diminished. (The
> increase in oxygen allowed ozone to form and organisms to survive in
> the sunshine filtered of its hard UV). Was this decrease of
> temperature a significant factor in allowing plate tectonics to
> develop? What would have been the temperature of Earth had the CO2 not
> been removed? 

Plate tectonics started as soon as the surface of the new earth was cool enough
that some magma could solidify.  If anything, it has diminished with time.
It is driven by the thermal gradient between the outer core and the surface.
The outer core is at 12000K.  It reall doesn't make much difference if the
earth's surface is at 0K or 350K, the outer core temperature dominates.
The presence of liquid water on the surface, however, does change some
of the surface characteristics of plate tectonics (mountains can't get very
high since they erode, volcanoes form on back arcs partly due to subducted
water, spreading ridges are mostly underwater, so the volcanism there is
pretty quite).

> 	An alternate question might be what would happen to the
> geologic processes and atmosphere and oceans on Earth if all life were
> suddenly removed? Would the atmosphere stay the same or would the oxygen
> oxidize the materials in the crust and the volcanoes start to rebuild
> the CO2 concentration, eventually leading to a runaway greenhouse like
> Venus? If the surface were warmed significantly, would this begin to

No runaway.  Greenhouse would raise the temperature, but nowhere near that
of Venus, we would still have liquid water.

> slow down the heat loss from the interior, raise mantle temperatures,
> and make subduction more difficult so Earth would resort to the blob
> tectonics that is seen on Venus to get rid of its heat? 

No, as explained, the temperature differential drives tectonics and
is not significantly affected by the surface temperature.  Tectonic
style on earth differs from that on Venus partly due to whole-planet
compositional differences, partly due to the presence of liquid water.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov