Titan is covered in a thick, methane-dominated atmosphere. Image: NASA/JPL/Space Science Institute |
Saturn’s largest
moon, Titan, is an intriguing, alien world that’s covered in a thick atmosphere
with abundant methane. With an average surface temperature of a brisk -300 F
(about 90 K) and a diameter just less than half of Earth’s, Titan boasts
methane clouds and fog, as well as rainstorms and plentiful lakes of liquid
methane. It’s the only place in the solar system, other than Earth, that has
large bodies of liquid on its surface.
The origins of
many of these features, however, remain puzzling to scientists. Now,
researchers at the California Institute of Technology (Caltech) have developed
a computer model of Titan’s atmosphere and methane cycle that, for the first
time, explains many of these phenomena in a relatively simple and coherent way.
In particular,
the new model explains three baffling observations of Titan. One oddity was
discovered in 2009, when researchers led by Caltech professor of planetary
science Oded Aharonson found that Titan’s methane lakes tend to cluster around
its poles—and noted that there are more lakes in the northern hemisphere than
in the south.
Secondly, the
areas at low latitudes, near Titan’s equator, are known to be dry, lacking
lakes and regular precipitation. But when the Huygens probe landed on Titan in
2005, it saw channels carved out by flowing liquid-possibly runoff from rain.
And in 2009, Caltech researchers discovered raging storms that may have brought
rain to this supposedly dry region.
Finally,
scientists uncovered a third mystery when they noticed that clouds observed
over the past decade—during summer in Titan’s southern hemisphere—cluster
around southern middle and high latitudes.
Scientists have
proposed various ideas to explain these features, but their models either can’t
account for all of the observations, or do so by requiring exotic processes,
such as cryogenic volcanoes that spew methane vapor to form clouds. The Caltech
researchers say their new computer model, on the other hand, can explain all
these observations—and does so using relatively straightforward and fundamental
principles of atmospheric circulation.
“We have a
unified explanation for many of the observed features,” says Tapio
Schneider, the Frank J. Gilloon Professor of Environmental Science and
Engineering. “It doesn’t require cryovolcanoes or anything esoteric.”
Schneider, along with Caltech graduate student Sonja Graves, former Caltech
graduate student Emily Schaller (PhD ’08), and Mike Brown, the Richard and
Barbara Rosenberg Professor and professor of planetary astronomy, have
published their findings in Nature.
Schneider says
the team’s simulations were able to reproduce the distribution of clouds that’s
been observed—which was not the case with previous models. The new model also
produces the right distribution of lakes. Methane tends to collect in lakes
around the poles because the sunlight there is weaker on average, he explains.
Energy from the sun normally evaporates liquid methane on the surface, but
since there’s generally less sunlight at the poles, it’s easier for liquid
methane there to accumulate into lakes.
But then why are
there more lakes in the northern hemisphere? Schneider points out that Saturn’s
slightly elongated orbit means that Titan is farther from the sun when it’s
summer in the northern hemisphere. Kepler’s second law says that a planet
orbits more slowly the farther it is from the sun, which means that Titan
spends more time at the far end of its elliptical orbit, when it’s summer in
the north. As a result, the northern summer is longer than the southern summer.
And since summer is the rainy season in Titan’s polar regions, the rainy season
is longer in the north. Even though the summer rains in the southern hemisphere
are more intense—triggered by stronger sunlight, since Titan is closer to the
sun during southern summer—there’s more rain over the course of a year in the
north, filling more lakes.
In general,
however, Titan’s weather is bland, and the regions near the equator are
particularly dull, the researchers say. Years can go by without a drop of rain,
leaving the lower latitudes of Titan parched. It was a surprise, then, when the
Huygens probe saw evidence of rain runoff in the terrain. That surprise only
increased in 2009 when Schaller, Brown, Schneider, and then-postdoctoral
scholar Henry Roe discovered storms in this same, supposedly rainless, area.
No one really
understood how those storms arose, and previous models failed to generate
anything more than a drizzle. But the new model was able to produce intense
downpours during Titan’s vernal and autumnal equinoxes—enough liquid to carve
out the type of channels that Huygens found. With the model, the researchers
can now explain the storms. “It rains very rarely at low latitudes,”
Schneider says. “But when it rains, it pours.”
The new model
differs from previous ones in that it’s three-dimensional and simulates Titan’s
atmosphere for 135 Titan years—equivalent to 3,000 years on Earth—so that it
reaches a steady state. The model also couples the atmosphere to a methane
reservoir on the surface, simulating how methane is transported throughout the
moon.
The model
successfully reproduces what scientists have already seen on Titan, but perhaps
what’s most exciting, Schneider says, is that it also can predict what
scientists will see in the next few years. For instance, based on the
simulations, the researchers predict that the changing seasons will cause the
lake levels in the north to rise over the next 15 years. They also predict that
clouds will form around the north pole in the next two years. Making testable
predictions is “a rare and beautiful opportunity in the planetary
sciences,” Schneider says. “In a few years, we’ll know how right or
wrong they are.
“This is
just the beginning,” he adds. “We now have a tool to do new science
with, and there’s a lot we can do and will do.”
