Toxic Ice Cloud Found in Titan’s Stratosphere

Planetary researchers with NASA’s Cassini mission have detected a cloud of toxic hybrid ice high above the south pole of Saturn’s hazy moon, Titan.

This view of Titan is among the last images NASA’s Cassini spacecraft sent to Earth before it plunged into the giant planet’s atmosphere. Image credit: NASA / JPL-Caltech / Space Science Institute.

This view of Titan is among the last images NASA’s Cassini spacecraft sent to Earth before it plunged into the giant planet’s atmosphere. Image credit: NASA / JPL-Caltech / Space Science Institute.

Located at an altitude of about 100 to 130 miles (160-210 km), the new cloud is far above the methane rain clouds of Titan’s troposphere, or lowest region of the atmosphere.

The cloud covers a large area near the south pole, from about 75 to 85 degrees south latitude.

It is invisible to the human eye and was detected at infrared wavelengths by Cassini’s Composite Infrared Spectrometer (CIRS).

“This cloud represents a new chemical formula of ice in Titan’s atmosphere,” said CIRS co-investigator Dr. Carrie Anderson, of NASA’s Goddard Space Flight Center.

“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.”

The new cloud has a distinctive and very strong chemical signature that showed up in three sets of Titan observations taken from July to November 2015.

Because Titan’s seasons last seven Earth years, it was late fall at the south pole the whole time.

The spectral signatures of the ices did not match those of any individual chemical, so Dr. Anderson and colleagues began lab experiments to simultaneously condense mixtures of gases.

Using an ice chamber that simulates conditions in Titan’s stratosphere, the researchers tested pairs of chemicals that had infrared fingerprints in the right part of the spectrum.

At first, they let one gas condense before the other. But the best result was achieved by introducing both hydrogen cyanide and benzene into the chamber and allowing them to condense at the same time.

By itself, benzene doesn’t have a distinctive 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 close match for the CIRS observations.

Additional studies will be needed to determine the structure of the co-condensed ice particles.

The scientists expect them to be lumpy and disorderly, rather than well-defined crystals.

Dr. Anderson and colleagues previously found a similar example of co-condensed ice in CIRS data from 2005. Those observations were made near the north pole, about two years after the winter solstice in Titan’s northern hemisphere. That cloud formed at a much lower altitude, below 93 miles (150 km), and had a different chemical composition: hydrogen cyanide and cyanoacetylene.

“We attribute the differences in the two clouds to seasonal variations at the north and south poles,” the scientists said.

“The northern cloud was spotted about two years after the northern winter solstice, but the southern cloud was spotted about two years before the southern winter solstice.”

“It’s possible that the mixtures of gases were slightly different in the two cases or that temperatures had warmed up a bit by the time the north polar cloud was spotted, or both.”

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D.E. Jennings et al. 2017. Composite infrared spectrometer (CIRS) on Cassini. Applied Optics 56 (18): 5274-5294; doi: 10.1364/AO.56.005274

Thomas Gautier et al. 2017. Environmental temperature effect on the far-infrared absorption features of aromatic-based Titan’s aerosol analogs. Icarus 281: 338-341; doi: 10.1016/j.icarus.2016.07.015

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