Geoscience Reference
In-Depth Information
distance from the Sun (or any other star in fact) where liquid water can
persist is known as the “habitable zone,” which is sometimes referred to
as the “Goldilocks Zone.”
3
But distance from the sun is only part of the story. Earth has an atmo-
sphere with greenhouse gases that contribute to surface warming. With-
out any greenhouse warming, and with surface albedo as it is,
4
Earth
would be frozen with a temperature of -15°C (5°F) or so. Therefore,
discovering the habitable zone of a planet is more involved than de-
scribed above. This requires some rather complex heat-budget calcula-
tions, which were first attempted decades ago; however, the most widely
referred to models were presented in 1993 by Jim Kasting of Penn State
University, along with his coworkers Daniel hitmire and Ray Reyn-
olds. Jim has been a leader in applying his detailed knowledge of at-
mospheric chemical dynamics to understanding the evolution of both
Earth's atmosphere and atmospheres beyond our own. To attack the
habitable zone issue Jim tried, through his model, to keep liquid water
on the planet by changing atmospheric CO
2
(carbon dioxide) levels, as
these control greenhouse warming. One can easily imagine that differ-
ent atmospheric CO
2
levels would be needed to maintain a habitable
zone in response to differences in solar luminosity, which is basically the
intensity of a star; and differences in solar luminosity are expected as
one travels either away from or toward the star, or in our case, the Sun.
With Jim's model, the outer reaches of the habitable zone are encoun-
tered when atmospheric CO
2
concentrations become so high that CO
2
clouds form. These clouds block solar radiation from reaching the
planet surface and thereby increase planetary albedo. The end result is
a frozen planet. There were other considerations in Jim's modeling that
I won't get into here, but in the end, Jim and colleagues concluded that
Mars probably lies just outside of the habitable zone. Likewise, Venus
also lies outside of the habitable zone. In this case, solar luminosity is
simply too high. Even with miniscule levels of atmospheric CO
2
sup-
plying minimal greenhouse warming, the planet surface becomes so hot
that water boils into the atmosphere. This situation generates a runaway
greenhouse and very high surface temperatures because water is also a
good greenhouse gas (and the most important on the modern Earth!).
5
By some of Jim's calculations, the inner edge of the habitable zone may
lie as close as 95% of the distance from the Sun to Earth. This is about
