NASA's Cassini Finds Monstrous Ice Cloud in Titan's South Polar Region

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http://www.nasa.gov/feature/goddard/monstrous-ice-cloud-in-titans-south-polar-region

NASA's Cassini Finds Monstrous Ice Cloud in Titan's South Polar Region
November 11, 2015

New observations made near the south pole of Titan by NASA's Cassini spacecraft
add to the evidence that winter comes in like a lion on this moon of Saturn.
November 11, 2015

Scientists have detected a monstrous new cloud of frozen compounds in
the moon's low- to mid-stratosphere - a stable atmospheric region above
the troposphere, or active weather layer.

[Image]
As winter sets in at Titan's south pole, a cloud system called the south
polar vortex (small, bright "button") has been forming, as seen in this
2013 image.
Credits: NASA/JPL-Caltech/Space Science Institute

Cassini's camera had already imaged an impressive cloud hovering over
Titan's south pole at an altitude of about 186 miles (300 kilometers).
However, that cloud, first seen in 2012, turned out to be just the tip
of the iceberg. A much more massive ice cloud system has now been found
lower in the stratosphere, peaking at an altitude of about 124 miles (200
kilometers).

[Image]
This 2012 close-up offers an early snapshot of the changes taking place
at Titan's south pole. Cassini's camera spotted this impressive cloud
hovering at an altitude of about 186 miles (300 kilometers). Cassini's
thermal infrared instrument has now detected a massive ice cloud below
it.
Credits: NASA/JPL-Caltech/Space Science Institute

The new cloud was detected by Cassini's infrared instrument - the Composite
Infrared Spectrometer, or CIRS - which obtains profiles of the atmosphere
at invisible thermal wavelengths. The cloud has a low density, similar
to Earth's fog but likely flat on top.

For the past few years, Cassini has been catching glimpses of the transition
from fall to winter at Titan's south pole - the first time any spacecraft
has seen the onset of a Titan winter. Because each Titan season lasts
about 7-1/2 years on Earth's calendar, the south pole will still be enveloped
in winter when the Cassini mission ends in 2017.

"When we looked at the infrared data, this ice cloud stood out like nothing
we've ever seen before," said Carrie Anderson of NASA's Goddard Space
Flight Center in Greenbelt, Maryland. 'It practically smacked us in the
face."

Anderson is presenting the findings at the annual Meeting of the Division
of Planetary Sciences of the American Astronomical Society at National
Harbor, Maryland, on Nov. 11.

The ice clouds at Titan's pole don't form in the same way as Earth's familiar
rain clouds.

For rain clouds, water evaporates from the surface and encounters cooler
temperatures as it rises through the troposphere. Clouds form when the
water vapor reaches an altitude where the combination of temperature and
air pressure is right for condensation. The methane clouds in Titan's
troposphere form in a similar way.

However, Titan's polar clouds form higher in the atmosphere by a different
process. Circulation in the atmosphere transports gases from the pole
in the warm hemisphere to the pole in the cold hemisphere. At the cold
pole, the warm air sinks, almost like water draining out of a bathtub,
in a process known as subsidence.

The sinking gases - a mixture of smog-like hydrocarbons and nitrogen-bearing
chemicals called nitriles - encounter colder and colder temperatures on
the way down. Different gases will condense at different temperatures,
resulting in a layering of clouds over a range of altitudes.

Cassini arrived at Saturn in 2004 - mid-winter at Titan's north pole.
As the north pole has been transitioning into springtime, the ice clouds
there have been disappearing. Meanwhile, new clouds have been forming
at the south pole. The build-up of these southern clouds indicates that
the direction of Titan's global circulation is changing.

"Titan's seasonal changes continue to excite and surprise," said Scott
Edgington, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory
(JPL) in Pasadena, California. "Cassini, with its very capable suite of
instruments, will continue to periodically study how changes occur on
Titan until its Solstice mission ends in 2017."

The size, altitude and composition of the polar ice clouds help scientists
understand the nature and severity of Titan's winter. From the ice cloud
seen earlier by Cassini's camera, scientists determined that temperatures
at the south pole must get down to at least -238 degrees Fahrenheit (-150
degrees Celsius).

The new cloud was found in the lower stratosphere, where temperatures
are even colder. The ice particles are made up of a variety of compounds
containing hydrogen, carbon and nitrogen.

Anderson and her colleagues had found the same signature in CIRS data
from the north pole, but in that case, the signal was much weaker. The
very strong signature of the south polar cloud supports the idea that
the onset of winter is much harsher than the end.

"The opportunity to see the early stages of winter on Titan is very exciting,"
said Robert Samuelson, a Goddard researcher working with Anderson. "Everything
we are finding at the south pole tells us that the onset of southern winter
is much more severe than the late stages of Titan’s northern winter."

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European
Space Agency) and the Italian Space Agency. JPL manages the mission for
NASA's Science Mission Directorate in Washington. The CIRS team is based
at Goddard.

For more information about Cassini, visit:

http://www.nasa.gov/cassini
http://saturn.jpl.nasa.gov

Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.
301-614-5438
elizabeth....@nasa.gov

JPL Media Contact:
Preston Dyches
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-7013
preston...@jpl.nasa.gov