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By NASA // September 24, 2018
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Artist concept of a dust storm on Titan. (Image by IPGP / Labex UnivEarthS / Paris Diderot University – C. Epitalon & S. Rodriguez)
(NASA) – NASA's Cassini probe data revealed what appears to be giant dust storms in the equatorial regions of Titan, Saturn's moon.
The discovery, described in an article published on September 24 in Nature Geoscience, makes Titan the third body of the solar system, in addition to Earth and Mars, where dust storms have been observed.
Observation helps scientists better understand the fascinating and dynamic environment of Saturn's largest moon.
"Titan is a very active moon," said Sebastien Rodriguez, astronomer at Paris Diderot University, France, and senior author of the newspaper.
"We already know it for its geology and its exotic hydrocarbon cycle. Now we can add another analogy with Earth and Mars: the active dust cycle, in which organic dust can be lifted from large dune fields around Titan's equator.
Titan is an intriguing world, very similar to Earth. In fact, it is the only moon in the solar system with a substantial atmosphere and the only celestial body other than our planet where it is known that stable bodies of surface liquid still exist.
This animation, based on the images captured by NASA's Cassini Mission Visualization and Infrared Mapping (VIMS) spectrometer during several Titan flyovers in 2009 and 2010, shows clear points that have been interpreted as dust storms. . (Image of NASA)
There is, however, a big difference: on Earth, rivers, lakes and seas are filled with water, while on Titan, it is mainly methane and ethane that pass through these liquid reservoirs. In this unique cycle, the hydrocarbon molecules evaporate, condense in clouds and the rain returns to the ground.
The time it does on Titan also varies from one season to another, like on Earth. In particular, around the equinox – the moment the sun crosses Titan's equator – massive clouds can form in the tropics and cause powerful methane storms. Cassini observed such storms during several of her Titan flybys.
When Rodriguez and his team spotted three unusual equatorial thinnings in Cassini infrared imagery around the northern equinox of the moon in 2009, they thought they might be of the same type as the methane clouds; however, an investigation revealed that they were 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" said Rodriguez. "The convective methane clouds that can develop in this area and during this period would contain huge droplets and should be at a very high altitude – much higher than the 10 kilometers that the modeling tells us. "
The researchers were also able to rule out the features on the surface of Titan as frozen methane rain or frozen lava. Such surface stains would have a different chemical signature and would remain visible much longer than the luminous characteristics of this study, which were only visible from 11 hours to 5 weeks.
This compilation of images of nine Titan Cassini flyovers in 2009 and 2010 captures three cases where clear points suddenly appeared in the images taken by the spacecraft's visual and infrared mapping spectrometer. (Image of NASA)
In addition, modeling has shown that the characteristics must be atmospheric but always close to the surface, most likely forming a very thin layer of tiny solid organic particles. Since they were just above the dune fields around Titan's equator, the only explanation left was that the spots were actually dust clouds from the dunes.
Organic dust is formed when organic molecules, formed by the interaction of sunlight with methane, grow sufficiently to fall to the surface. Rodriguez said that although it is the first sighting of a dust storm on Titan, the result is not surprising.
"We believe that the Huygens probe, which landed on Titan's surface in January 2005, generated a small amount of organic dust on arrival because of its strong aerodynamic wake," Rodriguez said.
"But what we have seen here with Cassini is on a much larger scale. The wind speed near the surface required to raise such a large amount of dust should be very strong – about five times higher than the average wind speed estimated by near surface Huygens measurements and climate models. . "
The existence of such strong winds generating massive dust storms means that the underlying sand can also be set in motion and that the giant dunes covering the equatorial regions of Titan are still active and in constant evolution.
The winds could carry dust from the dunes over long distances, contributing to the overall cycle of organic dust on Titan and causing effects similar to those observed on Earth and Mars.
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