Researchers using data from NASA’s Cassini orbiter have found evidence for massive dust storms in Titan’s equatorial regions. The discovery, reported in the journal Nature Geoscience, implies that Titan — like Earth and Mars — has an active dust cycle.
An artist’s concept of a dust storm on Titan. Image credit: NASA / ESA / IPGP / Labex UnivEarthS / University Paris Diderot.
“Titan is a very active moon. We already know that about its geology and exotic hydrocarbon cycle,” said Dr. Sebastien Rodriguez, a researcher at the Université Paris Diderot, France.
“Now we can add another analogy with Earth and Mars: the active dust cycle, in which organic dust can be raised from large dune fields around Titan’s equator.”
Titan is the only moon in the Solar System with a substantial atmosphere and the only celestial body other than Earth where stable bodies of surface liquid are known to still exist.
On our planet such rivers, lakes and seas are filled with water, while on Titan it is primarily methane and ethane that flows through these liquid reservoirs. In this unique cycle, the hydrocarbon molecules evaporate, condense into clouds and rain back onto the ground.
The weather on Titan varies from season to season as well, just as it does on Earth.
In particular, around the equinox — the time when the Sun crosses Titan’s equator — massive clouds can form in tropical regions and cause powerful methane storms. Cassini observed such storms during several of its Titan flybys.
When Dr. Rodriguez and colleagues first spotted three unusual equatorial brightenings in infrared images taken by Cassini around the moon’s 2009 northern equinox, they thought they might be the same kind of methane clouds; however, an investigation revealed they were something completely different.
This compilation of images from nine Cassini flybys of Titan in 2009 and 2010 captures three instances when clear bright spots suddenly appeared in images taken by Cassini’s Visual and Infrared Mapping Spectrometer. The brightenings were visible only for a short period of time — between 11 hours to five Earth weeks — and cannot be seen in previous or subsequent images. Image credit: NASA / JPL-Caltech / University of Arizona / University Paris Diderot / IPGP.
“From what we know about cloud formation on Titan, we can say that such methane clouds in this area and in this time of the year are not physically possible,” Dr. Rodriguez said.
“The convective methane clouds that can develop in this area and during this period of time would contain huge droplets and must be at a very high altitude — much higher than the 6 miles (10 km) that modeling tells us the new features are located.”
The team was also able to rule out that the features were actually on the surface of Titan in the form of frozen methane rain or icy lavas.
Such surface spots would have a different chemical signature and would remain visible for much longer than the bright features in this study, which were visible for only 11 hours to five weeks.
In addition, modeling showed that the features must be atmospheric but still close to the surface — most likely forming a very thin layer of tiny solid organic particles.
Since they were located right over the dune fields around Titan’s equator, the only remaining explanation was that the spots were actually clouds of dust raised from the dunes.
Organic dust is formed when organic molecules, formed from the interaction of sunlight with methane, grow large enough to fall to the surface.
“While this is the first-ever observation of a dust storm on Titan, the finding is not surprising. We believe that the Huygens probe, which landed on the surface of Titan in January 2005, raised a small amount of organic dust upon arrival due to its powerful aerodynamic wake,” Dr. Rodriguez said.
“But what we spotted here with Cassini is at a much larger scale.”
The discovery makes Titan the third Solar System body, in addition to Earth and Mars, where dust storms have been observed.
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S. Rodriguez et al. Observational evidence for active dust storms on Titan at equinox. Nature Geoscience, published online September 24, 2018; doi: 10.1038/s41561-018-0233-2
