Geoscience Reference
In-Depth Information
whose formation is best explained by the action of water. Indeed, the
Curiosity
rover recently landed on the Mars surface and is, as I write,
exploring the surroundings of its landing site, which appear to be an
ancient river bed! All of this is in addition to spectroscopic observations
of water just at and below the soil surface. So, Mars demonstrates that
liquid water may be found, at least occasionally, somewhat outside of
the habitable zone. By contrast with Earth, however, any life on Mars,
if it exists at all, is not obvious and is seemingly restricted in its abun-
dance and occurrence. Therefore, Mars does not and cannot support
the magnitude of life that we find on our planet.
Buried in the discussion of Jim Kasting's habitable zone calculations
is the idea that over long time scales, Earth actually regulates its own
temperature. This idea was first raised by the cosmologist Carl Sagan.
Sagan contributed greatly to our understanding of the composition of
planetary atmospheres, and he helped frame the discussion about the
search for life in the universe. He was an enormous inspiration to those
interested in science through his PBS (Public Broadcasting System)
program COSMOS, which was originally broadcast in 1980. However,
of more importance here, he and his colleague George Mullen asked
why Earth didn't freeze early in its history when the Sun was much less
luminous than today.
6
Geological evidence points to the more or less
continuous presence of liquid water for as far as back as 4.2 billion years
ago. Yet, with the present abundance of greenhouse gases in Earth's at-
mosphere, the planet should have been frozen under the reduced lumi-
nosity of the early Sun. This is famously known as “The Faint Young
Sun Paradox.” Sagan and Mullen argued that this paradox could be
solved with a high concentration of greenhouse gases like ammonia
and methane; these gases are unstable in our present oxygenated atmo-
sphere but could have been present in the oxygen-poor atmosphere of
early Earth. It was soon pointed out, however, that ammonia would be
photochemically unstable, even in an oxygen-free atmosphere. This gen-
erated a serious problem for the model. However, in a true intellectual
quantum leap, Jim Walker, Paul Hays, and Jim Kasting recognized that
CO
2
may well have been the greenhouse gas mitigating against an early
frozen Earth. Okay, CO
2
, big deal. But there is much more to this pro-
posal, because Walker, Hays and Kasting also demonstrated a mecha-
nism that actually regulates surface temperature.
