NASA Scientists Find 'Impossible' Cloud on Titan -- Again

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NASA Scientists Find 'Impossible' Cloud on Titan -- Again
Jet Propulsion Laboratory
September 20,2 016

The puzzling appearance of an ice cloud seemingly out of thin air has
prompted NASA scientists to suggest that a different process than previously
thought -- possibly similar to one seen over Earth's poles -- could be
forming clouds on Saturn's moon Titan.

Located in Titan's stratosphere, the cloud is made of a compound of carbon
and nitrogen known as dicyanoacetylene (C4N2), an ingredient in the chemical
cocktail that colors the giant moon's hazy, brownish-orange atmosphere.

Decades ago, the infrared instrument on NASA's Voyager 1 spacecraft spotted
an ice cloud just like this one on Titan. What has puzzled scientists
ever since is this: they detected less than 1 percent of the dicyanoacetylene
gas needed for the cloud to condense.

Recent observations from NASA's Cassini mission yielded a similar result.
Using Cassini's composite infrared spectrometer, or CIRS -- which can
identify the spectral fingerprints of individual chemicals in the atmospheric
brew -- researchers found a large, high-altitude cloud made of the same
frozen chemical. Yet, just as Voyager found, when it comes to the vapor
form of this chemical, CIRS reported that Titan's stratosphere is as dry
as a desert.

"The appearance of this ice cloud goes against everything we know about
the way clouds form on Titan," said Carrie Anderson, a CIRS co-investigator
at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead
author of the study.

The typical process for forming clouds involves condensation. On Earth,
we're familiar with the cycle of evaporation and condensation of water.
The same kind of cycle takes place in Titan's troposphere -- the weather-forming
layer of Titan's atmosphere -- but with methane instead of water.

A different condensation process takes place in the stratosphere -- the
region above the troposphere -- at Titan's north and south winter poles.
In this case, layers of clouds condense as the global circulation pattern
forces warm gases downward at the pole. The gases then condense as they
sink through cooler and cooler layers of the polar stratosphere.

Either way, a cloud forms when the air temperature and pressure are favorable
for the vapor to condense into ice. The vapor and the ice reach a balance
point -- an equilibrium -- that is determined by the air temperature and
pressure. Because of this equilibrium, scientists can calculate the amount
of vapor where ice is present.

"For clouds that condense, this equilibrium is mandatory, like the law
of gravity," said Robert Samuelson, an emeritus scientist at Goddard and
a co-author of the paper.

But the numbers don't compute for the cloud made from dicyanoacetylene.
The scientists determined that they would need at least 100 times more
vapor to form an ice cloud where the cloud top was observed by Cassini's
CIRS.

One explanation suggested early on was that the vapor might be present,
but Voyager's instrument wasn't sensitive enough in the critical wavelength
range needed to detect it. But when CIRS also didn't find the vapor, Anderson
and her Goddard and Caltech colleagues proposed an altogether different
explanation. Instead of the cloud forming by condensation, they think
the C4N2 ice forms because of reactions taking place on other kinds of
ice particles. The researchers call this "solid-state chemistry," because
the reactions involve the ice, or solid, form of the chemical.

The first step in the proposed process is the formation of ice particles
made from the related chemical cyanoacetylene (HC3N). As these tiny bits
of ice move downward through Titan's stratosphere, they get coated by
hydrogen cyanide (HCN). At this stage, the ice particle has a core and
a shell comprised of two different chemicals. Occasionally, a photon of
ultraviolet light tunnels into the frozen shell and triggers a series
of chemical reactions in the ice. These reactions could begin either in
the core or within the shell. Both pathways can yield dicyanoacteylene
ice and hydrogen as products.

The researchers got the idea of solid-state chemistry from the formation
of clouds involved in ozone depletion high above Earth's poles. Although
Earth's stratosphere has scant moisture, wispy nacreous clouds (also called
polar stratospheric clouds) can form under the right conditions. In these
clouds, chlorine-bearing chemicals that have entered the atmosphere as
pollution stick to crystals of water ice, resulting in chemical reactions
that release ozone-destroying chlorine molecules.

"It's very exciting to think that we may have found examples of similar
solid-state chemical processes on both Titan and Earth," said Anderson.

The researchers suggest that, on Titan, the reactions occur inside the
ice particles, sequestered from the atmosphere. In that case, dicyanoacetylene
ice wouldn't make direct contact with the atmosphere, which would explain
why the ice and the vapor forms are not in the expected equilibrium.

"The compositions of the polar stratospheres of Titan and Earth could
not differ more," said Michael Flasar, CIRS principal investigator at
Goddard. "It is amazing to see how well the underlying physics of both
atmospheres has led to analogous cloud chemistry."

The findings are published in the journal Geophysical Research Letters.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European
Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory,
a division of Caltech in Pasadena, manages the mission for NASA's Science
Mission Directorate, Washington. JPL designed, developed and assembled
the Cassini orbiter. The CIRS instrument was built by Goddard.

For more information about Cassini, visit:

http://www.nasa.gov/cassini

http://saturn.jpl.nasa.gov

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

Written by Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.

2016-244