September 25, 2018 : Dust Storms At Low Or Mid-Latitudes On Titan Identified In The Infrared Spectrum Thanks To Cassini Data

A new research work unveiled recently in Nature Geoscience, entitled « Observational evidence for active dust storms on Titan at equinox » and led by Sébastien Rodriguez who is an astronomer at the University Paris Diderot in France, reveals that dust storms can take shape from time to time at low latitudes on Saturn's largest moon Titan. Researchers have been in a position to identify dust storms on the basis of infrared data obtained from the Cassini orbiter with the Visual and Infrared Mapping Spectrometer. Titan appears to be the third world known where dust storms can form. We know that dust storms can take shape and develop on Earth and on the Red Planet Mars. Impressive dust storms have been observed in Australia for instance and recently a global dust storm has taken shape on Mars causing some trouble for the Opportunity Rover. The observations of dust storms on the Opaque Moon are likely to help us better understand the climate, the meteorology and the dynamics of Titan's complex environment.

Radar data obtained with the Radar Mapper of the Cassini spacecraft during its long mission in the Saturn System ranging from mid-2004 to the second half of 2017 had revealed the presence of « Cat Scratches », Seif dunes or linear and parallel dunes extending over long distances in the dark areas of the low or mid-latitudes on Titan. Therefore, we knew that prevailing winds can shape the landscape and engender linear and parallel dunes over long distances on the Orange Moon. Now, we realize the potential power of winds at low or mid-latitudes on that enigmatic world. There can be large-scale dust storms at low or mid-latitudes on Titan which implies that the landscape of dunes can be remarkably dynamic. Let's note that the air is particularly dense on the surface of the giant moon. The atmospheric pressure on Titan's surface is higher than that of our planet at sea level. Sébastien Rodriguez pointed out : « Titan is a very active moon. » He added : « We already know that about its geology and exotic hydrocarbon cycle. Now we can add another analogy with Earth and Mars: the active dust cycle. »

The Cassini-Huygens mission has been really fruitful regarding the exploration of Saturn's largest moon. We suspected the presence of oceans, seas or lakes of methane or ethane prior to the Cassini-Huygens mission because Titan has the right ingredients and environmental conditions for that kind of configuration. The opaque and dense atmosphere of Saturn's largest moon is dominated by molecular nitrogen like the atmosphere of our planet and it contains a significant concentration of methane. Therefore, a methane cycle involving evaporation processes, condensation processes or cloud formation and precipitation processes can take shape. The surface of Titan can't be discerned in the visible spectrum from outer space due to the haze or smog rich in organics found in Titan's atmosphere. Titan is in fact the only moon in the Solar System covered by a significant atmosphere. Ganymede, the largest moon of Jupiter which is the largest moon in the Solar System, is devoid of any significant atmosphere. Triton, the largest moon of Neptune, contains a thin atmosphere but that atmosphere is much thinner than that of Mars. The Dwarf Planet Pluto also has a very thin atmosphere but that intriguing world unveils bright dunes which demonstrate the influence of winds.

Thanks to infrared or near-infrared data and radar data acquired from the Cassini orbiter, we have been in a position to clearly identify lakes, seas and rivers in the high latitudes of Saturn's largest moon. The low-albedo areas contrasting with bright areas at low or mid-latitudes don't represent seas or oceans of methane or ethane. The radar data obtained with the Radar Mapper of the Cassini spacecraft have clearly shown that those dark areas are dominated by Seif dunes or linear and parallel dunes extending over long distances under the influence of prevailing winds. The bodies of surface liquids turn out to be mostly concentrated in the polar regions. The north polar area appears to be the most humid area on Titan today. Why ? That's a big question but the dichotomy in the distribution of surface liquids may be related to seasonal and orbital phenomena. A parallel can be drawn between the water cycle of the Earth and the methane cycle of Titan which is the only extraterrestrial world known to contain stable pools of liquid on its surface.

The atmosphere of Titan is rich in organics and several hydrocarbons can appear in their liquid form on the surface of the giant moon. One can potentially encounter liquid methane, liquid ethane or liquid propane. Methane can evaporate, condense, form clouds and fall as rain on Titan like water on the Blue Planet. One may also find a snow made of organics falling from the organic haze of Titan's atmosphere. The relatively complex molecules which fall from the sky may form dunes and may be light enough to be transported by winds to generate new landforms. Researchers have been in a position to study the weather of the Opaque Moon for a relatively long period of time thanks to the Cassini orbiter but the journey inside the Saturn System only represented less than a Titanian year since a Titanian year represents approximately 30 Terrestrial years. However, we have monitored the evolution of the weather during three Titanian seasons from the Winter season in the northern hemisphere to the Summer season in the northern hemisphere. We can conclude that the dynamics of Titan's atmosphere is closely related to seasonal factors.

We have noticed in particular that massive cloud systems can take shape at low or mid-latitudes around the Equinox. Those massive cloud systems representing methane storms are likely to produce heavy rainfalls. The Cassini orbiter had imaged such cloud systems during several flybys of the Orange Moon. Around the Equinox of the Northern hemisphere in 2009, the Cassini orbiter had spotted three unusual bright patches in the Equatorial area. Sébastien Rodriguez and his collaborators had first believed that those bright patches represented methane clouds. However, a meticulous analysis revealed that they paradoxically represented something else. Sébastien Rodriguez advanced : « 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. » He added : « 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 10 km that modelling tells us the new features are located. »

The group of Sébastien Rodriguez managed to determine that the bright features didn't represent surface features such as patches of frozen methane rain or patches of icy lavas. The chemical signature of the bright patches was not in line with the hypothesis of frozen methane rain or icy lavas. Furthermore, if the bright features had represented bright surface deposits, they would have remained there for a longer period of time. The research work of Sébastien Rodriguez and his team showed that the bright features were only present or visible for 11 hours to five weeks. Modelling also revealed that the bright patches must be atmospheric and near the ground or soil. They likely correspond to a thin layer of tiny organic molecules which appear in their solid form. The planetologists noticed that the bright patches were found right over the dune fields of the Equatorial or Tropical region. Therefore, they concluded that the only remaining hypothesis was that the bright features represented clouds of dust emerging from the dunes.

Sébastien Rodriguez is not really surprised by that phenomenon which is regularly encountered on Earth or Mars. He pointed out : « 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. » Obviously, that was almost nothing compared to what we saw from the Cassini probe in 2010. He argued : « But what we spotted here with Cassini is at a much larger scale. The near-surface wind speeds required to raise such an amount of dust as we see in these dust storms would have to be very strong – about five times as strong as the average wind speeds estimated by the Huygens measurements near the surface and with climate models. » The Huygens probe which landed at a low latitude of Titan on January 14, 2005 recorded a surface wind of less than one meter per second prior to the touchdown. That's particularly weak. In fact, like on Earth, the meteorology on Titan can significantly change from time to time.

Sébastien Rodriguez advanced : « For the moment, the only satisfactory explanation for these strong surface winds is that they might be related to the powerful gusts that may arise in front of the huge methane storms we observe in that area and season. » Is the absence of stable lakes or seas in the Equatorial or Tropical area of Titan related to strong prevailing winds which stimulate evaporation processes ? The phenomenon of gusts evolving close to huge methane storms is called « haboob ». That particular configuration can be encountered on our planet in arid regions where big dust clouds can take shape just before storms. The observations of those dust storms in the area of dunes on Titan inform us about the properties of the sand which forms the exotic dunes of the giant moon. The molecules must be light enough to undergo the influence of gusts or prevailing winds. The Titanian dunes which are likely composed of organics or hydrocarbons and which extend over long distances must be clearly dynamic like the dunes of the Namib Desert for instance.

The mosaic of images of Titan's disk above, obtained with the Visual and Infrared Mapping Spectrometer of the Cassini orbiter from May 22, 2009 to September 24, 2010, shows the evolution of surface and atmospheric features on the giant moon. One can observe in particular transient bright features at a relatively low latitude over low-albedo areas in the view of January 12, 2010 and in the view of June 21, 2010. The bright features appearing over areas which are rich in dune fields may correspond to dust storms rather than methane storms according to Sébastien Rodriguez and his team. Image credit: NASA/JPL-Caltech/University of Arizona/University Paris Diderot /IPGP/S. Rodriguez et al. 2018.

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