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Cassini detects dust storms on Titan for the first time

Data from the international Cassini-Huygens mission, which explored Saturn and its moons between 2004 and 2017, have revealed what appear to be dust storms in Titan's equatorial regions. The discovery, described in a paper published today in Nature Geoscience, makes Titan only the third body in the solar system where dust storms have been observed, after Earth and Mars.

Cassini detects dust storms on Titan for the first time

Publication date: 25/09/2018

Press, Research

Related themes : Origins

Researchers believe that huge quantities of dust can be kicked up on Titan, Saturn's largest moon, by strong gusts of wind that occur during powerful methane storms. Previously observed in images from the Cassini probe, these violent methane storms can form at equinox time in Titan's equatorial regions, where vast dune fields are located. (© IPGP/Labex UnivEarthS/Université Paris Diderot - C. Epitalon & S. Rodriguez)

This observation helps scientists to better understand the fascinating and dynamic environment of Saturn’s largest moon. “Titan is a very active moon,” says Sébastien Rodriguez, astrophysicist at Université Paris Diderot (France) and lead author of the paper. “We already knew this from its geology and hydrocarbon cycle. Now we can add another analogy with Earth and Mars: an active dust cycle. On Titan, this dust would in fact consist of organic aerosol particles produced in the atmosphere, which would fall to accumulate on the ground. What we would have observed with Cassini is that this organic dust could be lifted into gigantic clouds just above Titan’s large equatorial dune fields, just as can happen on Earth.”

Titan is an exotic and intriguing world – in some ways very similar to Earth. In fact, it’s the only moon in the solar system with a dense atmosphere, and the only body other than our planet where stable expanses of liquid exist on the surface. But there’s a big difference: while on Earth rivers, lakes and seas are filled with water, on Titan it’s mainly methane that flows through these liquid reservoirs. In this unique methane cycle, hydrocarbon molecules evaporate, condense into clouds and then rain down to earth. Titan’s active meteorology varies from season to season, just as it does on Earth. Particularly around the equinox, when the Sun crosses Titan’s equator, massive clouds can form in tropical regions, triggering severe methane storms. Cassini has observed this type of event during several of its Titan flybys.

When Sébastien and his team spotted brief bursts of brightness near Titan’s equator in infrared images from the VIMS (Visual and Infrared Mapping Spectrometer) instrument on board Cassini, at the time of the spring equinox between 2009 and 2010, they initially thought it might be the same methane clouds. Further investigation, however, revealed that it was something completely different. “From what we know about cloud formation on Titan, we can say that such methane clouds, in this region and at this time of year, are not physically possible,” says Sébastien. Convective methane clouds that could form in this region and at this time of year would be very opaque, made up of much larger particles, and would have to be located at a much higher altitude than we know from modeling the infrared signal from these structures.”

Modeling of their infrared signal has shown that, if these singular events are indeed of atmospheric origin, they appear to be confined very close to the surface (less than ten kilometers above sea level). What’s more, their chemical signature seems to indicate that they are more likely to be a tenuous layer of tiny solid organic particles in suspension. As these were just above Titan’s vast seas of organic sand, there was only one explanation left: these events were in fact gigantic clouds of organic dust lifted up from the dunes.

Although this is the very first observation of a dust storm on Titan, the discovery comes as no surprise to Sébastien. “We think that the Huygens probe, which landed on Titan’s surface in January 2005, raised a small amount of organic dust when it arrived, and that, given the atmospheric origin of this dust, Titan’s surface should be very largely covered with it,” explains Sébastien. “But what we’ve seen here with Cassini is on a much larger scale. Near the surface, wind speeds must be very strong to kick up a quantity of dust such as we see in these storms – about five times stronger than the average wind speed estimated by Huygens measurements near the surface and predicted by climate models.”

This compilation of images from nine Cassini flybys of Titan in 2009 and 2010 captures the three cases where bursts of brightness suddenly appeared in images taken by the visible and infrared spectro-imager on board Cassini. These brightenings were only visible for a short period of time - between 11 hours and five Earth weeks - and cannot be seen on previous or subsequent images. By analyzing the infrared spectra of these images, the researchers discovered that these bursts were most likely clouds of organic dust originating from Titan's dune fields. This is the first time that dust storms have been observed on Titan, making this intriguing moon of Saturn only the third body in the solar system with an active dust cycle, after Earth and Mars (© NASA/JPL-Caltech/University of Arizona/University Paris Diderot/IPGP/S. Rodriguez et al. 2018)

Huygens made only one direct measurement of surface wind speed just before landing on Titan, and at that time it was very low, less than 1 meter per second. Even at the time of the spring equinox, a few years later, surface winds were not expected to reach such speeds. “For now, the only satisfactory reason to explain such strong surface winds is that they could be linked to the powerful gusts that can occur at the front of the huge methane storms we observe in this region and this season,” concludes Sébastien. “On Earth, this phenomenon is called ‘haboob’: it generates giant dust storms just ahead of violent thunderstorms and is well known in desert regions. Seeing it happen on Titan was less expected! But it provides us with first-hand information on the climatic and geological activity of this moon, which has not finished surprising us.”

The existence of the violent winds generating these dust storms, even if transient, implies that the sand just below can also be set in motion, and that the dunes covering Titan’s equatorial regions are still active and continuing to evolve. Such winds could transport the dust raised by the dunes over great distances, contributing to the global cycle of organic dust and therefore carbon on Titan, and could cause effects similar to those observed on Earth and Mars.

Find out more :

  • Observational evidence for active dust storms on Titan at equinox, S. Rodriguez et al., Nature Geoscience ; DOI: 10.1038/s41561-018-0233-2
  • The results were obtained using Cassini’s VIMS visible and infrared imaging spectrometer.
  • The Cassini-Huygens mission is an international cooperation project between NASA, ESA and the Italian Space Agency.
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