Scientists with NASA’s Cassini mission discovered evidence of a deadly hybrid ice in a wispy cloud high over the south pole of Saturn’s largest moon, Titan.
The finding is a new demonstration of the complex chemistry taking place in Titan’s atmosphere—in this case, cloud formation in the massive moon’s stratosphere—and part of a collection of processes that finally helps deliver a smorgasbord of organic molecules to Titan’s surface.
Invisible to the human eye, the cloud was detected at infrared wavelengths by the Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft. Situated at an altitude of around 100 to 130 m (160 to 210 km), the cloud is way above the methane rain clouds of Titan’s troposphere, or lowest region of the atmosphere. The new cloud covers a large area close to the south pole, from around 75 to 85 degrees south latitude.
Laboratory experiments were used to discover a chemical mixture that was in line with the cloud’s spectral signature -- the chemical fingerprint measured by the CIRS instrument. The experiments established that the unusual ice in the cloud is a combination of the basic organic molecule hydrogen cyanide along with the large ring-shaped chemical benzene. The two chemicals seem to have condensed at the same time to develop ice particles, instead of one being layered on top of the other.
This cloud represents a new chemical formula of ice in Titan’s atmosphere. What’s interesting is that this noxious ice is made of two molecules that condensed together out of a rich mixture of gases at the south pole.
Carrie Anderson, CIRS co-investigator, Goddard Space Flight Center, NASA, Greenbelt, Maryland
Earlier, CIRS data helped detect hydrogen cyanide ice in clouds over Titan's south pole, as well as other poisonous chemicals in the moon's stratosphere.
In Titan’s stratosphere, a universal circulation pattern pushes a current of warm gases from the hemisphere where it is summer to the winter pole. This circulation changes direction when the seasons change, leading to an accumulation of clouds at whichever pole is going through winter. Soon after its arrival at Saturn, Cassini discovered evidence of this phenomenon at Titan’s north pole. Later, close to the end of the spacecraft’s 13 years in the Saturn system, a similar cloud accumulation was identified at the south pole.
The basic way to consider the cloud structure is that different types of gas will condense into ice clouds at various altitudes, virtually like layers in a parfait dessert. Precisely which cloud condenses where is influenced by how much vapor is there and the temperatures, which become colder and colder at lower altitudes in the stratosphere. The reality is more complex, however, because each type of cloud develops over a range of altitudes, so it is possible for some ices to condense concurrently, or co-condense.
Anderson and colleagues employ CIRS to sort through the intricate set of infrared fingerprints from many molecules in Titan’s atmosphere. The instrument splits up infrared light into its component colors, similar to raindrops creating a rainbow, and measures the strengths of the signal at the varying wavelengths.
CIRS acts as a remote-sensing thermometer and as a chemical probe, picking out the heat radiation emitted by individual gases in an atmosphere. And the instrument does it all remotely, while passing by a planet or moon.
F. Michael Flasar, the CIRS Principal Investigator, Goddard
The new cloud, which the researchers term the high-altitude south polar cloud, has a characteristic and very strong chemical signature that appeared in three sets of Titan observations captured from July to November 2015. Since the duration of Titan’s seasons is seven Earth years, it was late fall at the south pole the entire time.
The spectral signatures of the ices did not match those of any specific chemical, so the team initiated laboratory experiments to concurrently condense mixtures of gases. Using an ice chamber that mimics conditions in Titan’s stratosphere, they analyzed pairs of chemicals that had infrared fingerprints in the exact part of the spectrum.
In the beginning, they allowed one gas to condense before the other. But the ideal result was attained by adding both benzene and hydrogen cyanide into the chamber and allowing them to condense at the same time. By itself, benzene does not have a distinguishing far-infrared fingerprint. When it was allowed to co-condense with hydrogen cyanide, however, the far-infrared fingerprint of the co-condensed ice was a near match for the CIRS observations.
Further studies will be required to establish the structure of the co-condensed ice particles. The researchers imagine them to be lumpy and unsystematic, rather than well-defined crystals.
In CIRS data from 2005, Anderson and colleagues earlier found a similar example of co-condensed ice. Those observations were captured near the north pole, approximately two years after the winter solstice in Titan’s northern hemisphere. That cloud formed at a lot lower altitude, below 93 m (150 km), and had a diverse chemical composition: hydrogen cyanide and cyanoacetylene, one of the more multifaceted organic molecules found in Titan’s atmosphere.
Anderson links the differences in the two clouds to seasonal disparities at the north and south poles. The northern cloud was identified about two years after the northern winter solstice, but the southern cloud was identified about two years before the southern winter solstice. It is possible that the combinations of gases were somewhat different in the two cases or that temperatures had warmed up a little by the time the north polar cloud was identified, or both.
“One of the advantages of Cassini was that we were able to flyby Titan again and again over the course of the thirteen-year mission to see changes over time,” said Anderson. “This is a big part of the value of a long-term mission.”
The Cassini spacecraft finished its Saturn mission on September 15, 2017.
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, handles the mission for NASA’s Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter.