Computer Models
Suggest Planetary and Extrasolar Planet Atmospheres
(22 June 2007) The world is abuzz with
the discovery of an extrasolar, Earth-like planet around the star Gliese 581
that is relatively close to our Earth at 20 light years away in the
constellation Libra.
Bruce Fegley, Jr., Ph.D., professor of
earth and planetary sciences in Arts & Sciences at Washington University in
St. Louis, has worked on computer models that can provide hints to what
comprises the atmosphere of such planets and better-known celestial bodies in
our own solar system.
New computer models, from both Earth-based
spectroscopy and space mission data, are providing space scientists compelling
evidence for a better understanding of planetary atmospheric chemistry. Recent
findings suggest a trend of increasing water content in going from Jupiter
(depleted in water), to Saturn (less enriched in water than other volatiles),
to Uranus and Neptune, which have large water enrichments. "The farther out you
go in the solar system, the more water you find," said Fegley.
Fegley
provided an overview of comparative planetary atmospheric chemistry at the
233rd American Chemical Society National Meeting, held March 25-29, 2007, in
Chicago. Fegley and Katharina Lodders-Fegley, Ph.D., research associate
professor of earth and planetary sciences, direct the university's Planetary
Chemistry Laboratory. "The theory about the Gas Giant planets (Jupiter, Saturn,
Uranus, and Neptune) is that they have primary atmospheres, which means that
their atmospheres were captured directly from the solar nebula during accretion
of the planets," Fegley said.
Gas Giants
He said that
Jupiter has more hydrogen and helium and less carbon, nitrogen and oxygen than
the other Gas Giant planets, making its composition closer to that of the
hydrogen- and helium-rich sun. The elements hydrogen, carbon and oxygen are
predominantly found as water, the gases molecular hydrogen and methane and in
the atmospheres of the Gas Giant planets. "Spectroscopic observations and
interior models show that Saturn, Uranus and Neptune are enriched in heavier
elements," he said. "Jupiter, based on observations from the Galileo Probe, is
depleted in water. People have thought that Galileo might just have gone into a
dry area. But Earth-based observations show that the carbon monoxide abundance
in Jupiter's atmosphere is consistent with the observed abundances of methane,
hydrogen and water vapour. This pretty much validates the Galileo Probe
finding."
The abundances of these four gases are related by the reaction
CH4+H20 = CO+3H2. Thus, observations of the methane, hydrogen and CO abundances
can be used to calculate the water vapour abundance. Likewise, Earth-based
observations of methane, hydrogen and carbon monoxide in Saturn's atmosphere
show that water is less enriched than methane. In contrast, observations of
methane, hydrogen and carbon monoxide in the atmospheres of Uranus and Neptune
show that water is greatly enriched in these two planets. Although generally
classed with Jupiter and Saturn, Uranus and Neptune are water planets with
relatively thin gaseous envelopes.
"On the other hand, the terrestrial
planets Venus, Earth and Mars have secondary atmospheres formed afterwards by
outgassing - heating up the solid material that was accreted and then releasing
the volatile compounds from it," Fegley said. "That then formed the earliest
atmosphere."
He said that by plugging in models he's done on the
outgassing of chondritic materials and using photochemical models of the
effects of UV sunlight, he and his collaborator Laura Schaefer, a research
assistant in the Washington University Department of Earth and Planetary
Sciences, can speculate on the atmospheric composition of Earth-like planets in
other solar systems. "With new theoretical models we are able to surmise the
outgassing of materials that went into forming the planets, and even make
predictions about the atmospheres of extrasolar terrestrial planets," he
said.
"Because the composition of the galaxy is relatively uniform, most
stars are like the sun - hydrogen-rich with about the same abundances of rocky
elements - we can predict what these planetary atmospheres would be like,"
Fegley said. "I think that the atmospheres of extrasolar Earth-like plants
would be more like Mars or Venus than the Earth." Fegley said that
photosynthesis accounts for the oxygen in Earth's atmosphere; without it, the
Earth's atmosphere would consist of nitrogen, carbon dioxide and water vapour,
with only small amounts of oxygen. Oxygen is 21 percent of Earth's atmosphere;
in contrast, Mars has about one-tenth of one percent made by UV sunlight
destroying carbon dioxide.
"I see Mars today as a great natural
laboratory for photochemistry; Venus is the same for thermochemistry, and Earth
for biochemistry," he said. "Mars has such a thin atmosphere compared to Earth
or Venus. UV light can penetrate all the way down to the Martian surface before
it's absorbed. That same light on Earth is mainly absorbed in the ozone layer
in the lower Earth stratosphere. Venus is so dense that light is absorbed by a
cloud layer about 45 kilometres or so above the Venusian
surface."
(source: Washington University in St. Louis)