Methane clouds were already known to exist in Titan's troposphere,
the lowest layer of the atmosphere. Like rain and snow clouds on Earth,
those clouds form through a cycle of evaporation and condensation, with
vapor rising from the surface, encountering cooler and cooler
temperatures and falling back down as precipitation. On Titan, however,
the vapor at work is methane instead of water.
The newly identified cloud instead developed in the stratosphere, the
layer above the troposphere. Earth has its own polar stratospheric
clouds, which typically form above the North Pole and South Pole between
49,000 and 82,000 feet (15 to 25 kilometers) -- well above cruising
altitude for airplanes. These rare clouds don't form until the
temperature drops to minus 108 degrees Fahrenheit (minus 78 degrees
Celsius).
Other stratospheric clouds had been identified on Titan already,
including a very thin, diffuse cloud of ethane, a chemical formed after
methane breaks down. Delicate clouds made from cyanoacetylene and
hydrogen cyanide, which form from reactions of methane byproducts with
nitrogen molecules, also have been found there.
But methane clouds were thought unlikely in Titan's stratosphere.
Because the troposphere traps most of the moisture, stratospheric clouds
require extreme cold. Even the stratosphere temperature of minus 333
degrees Fahrenheit (minus 203 degrees Celsius), observed by Cassini just
south of the equator, was not frigid enough to allow the scant methane
in this region of the atmosphere to condense into ice.
What Anderson and her Goddard co-author, Robert Samuelson, noted is
that temperatures in Titan's lower stratosphere are not the same at all
latitudes. Data from Cassini's Composite Infrared Spectrometer and the
spacecraft's radio science instrument showed that the high-altitude
temperature near the north pole was much colder than that just south of
the equator.
It turns out that this temperature difference -- as much as 11
degrees Fahrenheit (minus 12 degrees Celsius) -- is more than enough to
yield methane ice.
Other factors support the methane identification. Initial
observations of the cloud system were consistent with small particles
composed of ethane ice. Later observations revealed some regions to
be clumpier and denser, suggesting that more than one ice could be
present. The team confirmed that the larger particles are the right size
for methane ice and that the expected amount of methane --
one-and-a-half percent, which is enough to form ice particles -- is
present in the lower polar stratosphere.
The mechanism for forming these high-altitude clouds appears to be
different from what happens in the troposphere. Titan has a global
circulation pattern in which warm air in the summer hemisphere wells up
from the surface and enters the stratosphere, slowly making its way to
the winter pole. There, the air mass sinks back down, cooling as it
descends, which allows the stratospheric methane clouds to form.
"Cassini has been steadily gathering evidence of this global
circulation pattern, and the identification of this new methane cloud is
another strong indicator that the process works the way we think it
does," said Michael Flasar, Goddard scientist and principal investigator
for Cassini's Composite Infrared Spectrometer (CIRS).
Like Earth's stratospheric clouds, this methane cloud was located
near the winter pole, above 65 degrees north latitude. Anderson
and Samuelson estimate that this type of cloud system -- which they call
subsidence-induced methane clouds, or SIMCs for short -- could develop
between 98,000 to 164,000 feet (30 to 50 kilometers) in altitude above
Titan's surface.
"Titan continues to amaze with natural processes similar to those on
the Earth, yet involving materials different from our familiar water,"
said Scott Edgington, Cassini deputy project scientist at NASA's Jet
Propulsion Laboratory (JPL) in Pasadena, California. "As we approach
southern winter solstice on Titan, we will further explore how these
cloud formation processes might vary with season."
The results of this study are available online in the journal Icarus.
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL, a division of
the California Institute of Technology, Pasadena, manages the mission
for NASA's Science Mission Directorate in Washington. The CIRS team is
based at Goddard. The radio science team is based at JPL.
More information about Cassini is available at the following sites: http://www.nasa.gov/cassini - http://saturn.jpl.nasa.gov
Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Maryland
301-614-5438
elizabeth.a.zubritsky@nasa.gov
Preston Dyches
NASA's Jet Propulsion Laboratory, Pasadena, California
818-354-7013
preston.dyches@jpl.nasa.gov