Topic Name: Titan's icy climate mimics Earth's tropics
Category: Geo sciences & technology
Research persons: Raymond T. Pierrehumbert, Jonathan Mitchell
Location: 5801 South Ellis Ave., Chicago, IL 60637 773-702-1234, United States
If space travelers ever visit Saturn's largest moon, they will
find a tropical world where temperatures plunge to minus 274 degrees Fahrenheit,
methane rains from the sky and dunes of ice or tar cover the planet's most arid
regions. These conditions reflect a cold mirror image of Earth's tropical
climate, according to scientists at the University of Chicago.
"You have all these things that are analogous to Earth. At the
same time, it's foreign and unfamiliar," said Ray Pierrehumbert, the Louis Block
Professor in Geophysical Sciences at
Titan, one of Saturn's 60 moons, is the only moon in the solar
system large enough to support an atmosphere. Pierrehumbert and
Mitchell, who recently completed his Ph.D. in
Astrophysics at Chicago, have been comparing observations of Titan collected
by the Cassini space probe and the Hubble Space Telescope with their own
computer simulations of the moon's atmosphere. Their study of the dynamics
behind Titan's methane clouds have appeared in the Proceedings of the
National Academy of Sciences. Their
continuing research on Titan's climate focuses on the moon's deserts.
"One of the things that attracts me about Titan is that it has a
lot of the same circulation features as Earth, but done with completely
different substances that work at different temperatures," Pierrehumbert said.
On Earth, for example,
water forms liquid
and is relatively active as a vapor in the atmosphere. But on Titan, water is a
"It's not more volatile on Titan than sand is on Earth."
Methane-natural gas-assumes an Earthlike role of water on Titan.
It exists in enough abundance to condense into rain and form puddles on the
surface within the range of temperatures that occur on Titan.
"The ironic thing on Titan is that although it's much colder
than Earth, it actually acts like a super-hot Earth rather than a snowball
Earth, because at
Titan temperatures, methane is more volatile than water vapor is at Earth
temperatures," Pierrehumbert said.
Pierrehumbert and Mitchell even go so far as to call Titan's
climate tropical, even though it sounds odd for a moon that orbits Saturn more
than nine times farther from the sun than Earth. Along with the behavior of
methane, Titan's slow rotation rate also contributes to its tropical nature.
Earth's tropical weather systems extend only to plus or minus 30 degrees of
latitude from the equator. But on Titan, which rotates only once every 16 days,
"the tropical weather system extends to the entire planet," Pierrehumbert said.
Titan's tropical nature means that scientists can observe the
behavior of its clouds using theories they've relied upon to understand Earth's
tropics, Mitchell noted.Titan's atmosphere produces an updraft where surface
winds converge. This updraft lifts evaporated methane up to cooler temperatures
and lower pressures, where much of it condenses and forms clouds.
"This is a well-known feature on Earth called an ITCZ, the
inter-tropical convergence zone," Mitchell said. Earth's oceans help confine
the ITCZ to the lowest latitudes. But in some scenarios for oceanless Titan, the
ITCZ in Mitchell's computer simulations wanders in latitude almost from one pole
to the other. Titan's clouds should also follow the ITCZ.
Titan's orange atmospheric haze complicates efforts to observe
the moon's clouds. "This haze shrouds the entire surface," Mitchell said. "It
pretty much blocks all visible light from reaching us from the surface or from
the lower atmosphere."
Nevertheless, infrared observations via two narrow frequency
bands have recently revealed that clouds are currently confined to the
hemisphere, which is just now emerging from its summer season.
"There should be a very large seasonality in these cloud
features," Mitchell said. "Cassini and other instruments might be able to tell
us about that in the next seven to 10 years or so, as the seasons progress."
Mitchell and Pierrehumbert's next paper will describe how
oscillations in Titan's atmospheric circulation dry out the moon's midsection.
Over the course of a year, Mitchell explained, "this oscillation in the
atmosphere tends to transport moisture, or evaporated methane, out of the low
latitudes and then deposit it at mid and high latitude in the form of rainfall.
This is interesting, because recent
observations of the surface suggest that the low latitudes are very dry."
Cassini images show dunes of ice or tar covering these low-latitude regions that
correspond to the tropics on Earth. When ultraviolet light from the sun
interacts with methane high in Titan's atmosphere, it creates byproducts such as
ethane and hydrogen.
These byproducts become linked to chains of hydrocarbon
molecules that create Titan's orange haze. When these molecules coalesce into
large particles, they settle out as a tar-like rain. "Titan is like a big
petrochemical plant," Pierrehumbert said. "Although this is all happening at a
much lower temperature than in a petroleum refinery, the basic processes going
on there are very closely allied to what people do when they make fuel."
Raymond T. Pierrehumbert
Louis Block Professor in Geophysical Sciences and the College
Areas of Expertise:
Climate: Climate change, climate simulation
Phone: (773) 702-8366
the Louis Block Professor in Geophysical Sciences. Pierrehumbert was a lead
author of the IPCC’s Third Assessment Report, released in 2001. He studies the
physics of climate, especially regarding the long-term evolution of the climates
of Earth and Mars. He directs the Climate Systems Center, which develops
software for rapidly conducting advanced climate simulations. Last December, he
was co-author of an article describing how methane clouds affect atmospheric
circulation on Titan, a moon of Saturn.
Pierrehumbert was a member of the National Research Council’s Panel on Abrupt
Climate Change and its Societal Impacts (2000-2001). He currently serves on
Chicago Mayor Daley’s Task Force on Climate Change, and on the National Oceanic
and Atmospheric Administration’s Panel on Abrupt Change. He was a Guggenheim
Fellow in 1996-97.
Advisor: Raymond Pierrehumbert (Geophysics Department)
Location: HINDS 571
Planetary Atmospheres and Climate
Research Fields: Solar System Astronomy
Some Important Notes:
Astronomy is the scientific study of celestial objects (such as stars,
planets, comets, and galaxies) and phenomena that originate outside the Earth's
atmosphere (such as the cosmic background radiation). It is concerned with the
evolution, physics, chemistry, meteorology, and motion of celestial objects, as
well as the formation and development of the universe.
Astronomy is one of the oldest sciences. Astronomers of early civilizations
performed methodical observations of the night sky, and astronomical artifacts
have been found from much earlier periods. However, the invention of the
telescope was required before astronomy was able to develop into a modern
science. Historically, astronomy has included disciplines as diverse as
astrometry, celestial navigation, observational astronomy, the making of
calendars, and even astrology, but professional astronomy is nowadays often
considered to be identical with astrophysics. Since the 20th century, the field
of professional astronomy split into observational and theoretical branches.
Observational astronomy is focused on acquiring and analyzing data, mainly using
basic principles of physics. Theoretical astronomy is oriented towards the
development of computer or analytical models to describe astronomical objects
and phenomena. The two fields complement each other, with theoretical astronomy
seeking to explain the observational results, and observations being used to
confirm theoretical results.
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