Malcolm writes:
> Furthermore, we
> have observational evidence that the temperature differences among earth,
> venus and mars are better explained by their greenhouse gas levels than by
> their distance from the sun.
There is a grain of truth in that statement, but unfortunately only a grain.
Solar distances *are* the primary determinant of a terrestrial planet's
history. The "habitable zone," the hypothetical region allowing persistent
presence
of liquid water around a host star, is determined primarily by the solar flux
(IR, visible, and UV) at a that distance, along with the planetary body's
size. Whatever greenhouse gases might exist in the planet's atmosphere at any
one
time are a complex result of that distance, the planet's internal heat and the
origination of life on its surface and its adopted biochemistries.
Venus, Earth and Mars all had oceans at the time of their formations, but
Venus and Mars lost theirs within the first 500 Ma to 2 Ga for two different
reasons, as it is estimated that the Earth will also in 500 Ma to 1 Ga.
The moderator of the list requested that I stop posting the Lecture of the
Week notices because of its current emphasis on astrobiology. I have absolutely
no complaint about that. It is never my intention to offend. However, I do
disagree a bit about the irrelevance of the lectures to ecology. Geology was
the
science that informed the evolution of evolutionary ecological thought during
Darwin's time; comparative planetology will be the science that will do the
same during ours in the coming years.
Last week's lecture in the series was by David Grinspoon (SwRI, Boulder) on
the planetary history of Venus's oceans and atmosphere. This week's lecture is
by Matt Golombek (NASA JPL), the geologist most responsible for the site
selections of the current Mars rovers, on the Spirit Rover's traverse of the
Gusev
Crater. For the first time, we have geologists on Mars, albeit automated and
slow as the devil, but they're doing a magnificient job. Spirit's results in
the ancient cratered highlands, which Golombek describes, indicate that Mars
died geologically 3.5 Ga ago, and that whatever water existed on Mars was gone
by
then.
Similarly, next week's lecture is by Steven Squyres (Cornell), the PI for the
rovers, and he will talk about Opportunity's traverse of the Meridiani
Planum, on the other side of Mars, a region that was demonstrably once deep in
water. The following week's lecture will be by David Catling (U. Washington) on
the
evolution of the three planetary atmospheres (Venus, Earth and Mars).
While these lectures look at the complete histories of the planetary
volatiles on the interior terrestrial planets and are only of ancillary value
to
conversations about global warming, they should still quite interesting to
listen
to by any ecologist and do set the philosophical stage for discussions about
global atmospheric stabilities.
The lectures are available at:
http://aics-research.com/lotw/
Wirt Atmar
