April 21, 2020 : The Environmental Conditions On Titan May Allow The Potential Presence Of Dust Devils Likely To Engender Dunes According To A New Study Involving Brian Jackson

A new research work entitled « Dust Devils on Titan », published in AGU's journal Geophysical Research Letters and proposed by a team involving Brian Jackson suggests that Saturn's largest moon Titan may have the right environmental conditions to allow the potential presence of dust devils like on the Earth or Mars. Titan is known to contain vast systems of linear and parallel dunes extending over long distances. The shape of those dunes is closely related to prevailing winds. Planetologists imagine the potential influence of dust devils on the dynamics or the shape of the dunes in the relatively dry environment found at low or mid-latitudes on that enigmatic moon of the Ringed Planet. In the opaque and dense atmosphere of Titan, the power of winds can engender a strong erosion of the soil. The nature and the dynamics of the sand of Titan represent key topics for planetologists today. On the basis of radar data obtained from the Cassini spacecraft during its long mission in the Saturn System from 2004 to 2017, they can deduce or evaluate the potential influence of whirlwinds on the concentration of dust at the level of the soil.

The huge amount of data acquired from the Cassini orbiter has allowed us to determine that dunes on Titan are mostly found in the low or mid-latitudes of the giant moon and represent, approximately, 30 percent of the surface. The specialists have been in a position to identify a large dust storm during the Cassini-Huygens mission in the Saturn System. The sand grains and the dust of the dunes may be the outcome of a type of snowfall of hydrocarbons or organics from the haze as Brian Jackson, who is a planetary scientist at Boise State University in Idaho and who is the lead author of the research work, suggests. The heavier molecules of the haze may fall toward the surface to form the dunes we observe in radar images from the orbiter. The appearance of the dunes is a mystery but some researchers imagine that they may have a plasticky texture or that they may be quite different from the dunes one can encounter on the Earth or on the Red Planet Mars. We all have in mind the big dust storms encountered in Australia or the relatively recent global storm of Mars but, surprisingly, the dust devils can generate more dust than the typical dust storms on our planet. The dynamics of the soil that they engender is different in each configuration.

On Earth, winds can be strong because the atmosphere is relatively dense and because there are strong potential constrasts between the level of energy of the ocean and the level of energy of the continents in particular. Some researchers advance that winds are more influential on Earth than on Mars or Titan. Brian Jackson doesn't believe that winds are strong in a typical day on Titan. He advanced : « Winds at the surface of Titan are usually very weak. Unless there is a big storm rolling through, there's probably not that much wind, and so dust devils may be one of the main dust transport mechanisms on Titan - if they exist. » The team of researchers has never obtained any evidence of dust devils on the surface of Titan but the researchers advance that they may take shape from time to time on the basis of their meteorological models or simulations applied to data obtained from the Huygens probe on January 14, 2005 during its journey from the upper limits of Titan's atmosphere to the soil where the presence of methane had been clearly identified. The module had landed onto a field of exotic pebbles or eroded stones implying the potential presence of an ancient brook or river.

On Earth, dust devils take shape in dry and calm environments where the radiations from the Sun warm the soil and the air near the ground engendering an upward movement of warm air that generates vortices. Those vortices or whirls can be clearly discerned due to the presence of sand and dust inside those structures. A parallel can be drawn between dust devils and tornadoes because those phenomena are related to local instabilities and because they represent vortices but dust devils appear in dry environments whereas tornadoes appear in humid environments. Moreover, dust devils don't have the destruction power of tornadoes and they can't become as large as tornadoes in their process of development. Researchers don't fully understand the way dust devils work. Brian Jackson pointed out : « When we plug the numbers in for how much dust the dust devil ought to lift based on the wind speeds we see, they seem to be able to lift more dust than we would expect. There may be some other mechanism which is helping them pull this dust - or the equations are just wrong. » Like tornadoes, dust devils have their secrets.

Brian Jackson and his students have led exploration campaigns upon dust devils across southeastern Oregon's Alvord Desert by resorting to small drones mobilizing meteorological instruments and flying to the areas of the whirls in order to try to see and to analyze the interior of those unusual structures. The particularly dry environment of Mars can engender dust devils during the Summer season even if ambient temperatures are relatively low. The size of the Martian dust devils can become extremely high, potentially reaching an altitude of 8 kilometers or 5 miles. However, the power of dust devils on the Red Planet remains relatively limited due to the fact that the atmosphere is very thin compared to our atmosphere. One can advance that winds reaching speeds of 200 miles per hour would not have the power to uproot any potential tree. The dust devils of Mars can lift the dust of the soil and can trigger significant movements of sand across the planet. The Opportunity rover had not survived a recent global dust storm because relatively significant amounts of sand had probably invaded its solar panels.

Unlike the Earth, Mars is devoid of any significant concentration of water at low or mid-latitudes so that dunes can develop like in the low or mid-latitudes of Titan. Brian Jackson pointed out : « We can watch dust devils skitter across the surface of Mars and see what their internal structure is like, but that doesn't tell us how much dust they are lifting. Mars' atmosphere is really, really dusty and dust plays an important role in the climate. Dust devils are probably, if not the dominant mechanism, one of the most important mechanisms for lofting the dust. » On Titan, dust devils may also take shape from time to time in the dry areas of the low or mid-latitudes. Radar data obtained from the Cassini orbiter have clearly shown that the low-albedo areas or the dark areas of the low or mid-latitudes of Saturn's largest moon are dominated by linear and parallel dunes extending over long distances. Those dunes undergo the influence of prevailing winds in a particularly dense and harsh environment. Brian Jackson believes that the potential dust devils of those regions are relatively weak due to the combination of the relatively high atmospheric pressure and the relatively low gravity.

The gravity of the Opaque Moon is weaker than the gravity of the Moon and represents about one seventh of the gravity of our planet. Yet, Titan is larger than our natural satellite but its mean density is lower than the mean density of the Moon. The Huygens probe had recorded a surface pressure of 1467 hPa on January 14, 2005. Thus, the atmospheric pressure at sea level on Titan is about one and a half times higher than the atmospheric pressure of the Earth at sea level even if our planet is much more massive than the Orange Moon. Can we say that the weight of Titan's atmosphere prevents any tornado or dust devil from being highly destructive ? Brian Jackson advanced : « It's just this enormous, puffy atmosphere. When you've got that much air it's hard to get it churning. So you just don't usually get big winds on the surface of Titan so far as we know. » Yet, the dunes found at low or mid-latitudes seem to be shaped by regular winds which seem to blow relatively strongly in the same direction to engender a regular topographic pattern. Those dunes may be dominated by organics or hydrocarbons resulting from particles or molecules from the haze. Many landscape features or atmospheric phenomena of Titan are reminiscent of landscape features or atmospheric phenomena of the Earth.

Researchers often say that the environment of Titan looks like the environment of the Early Earth. Titan's atmosphere is dominated by molecular nitrogen like our atmosphere but oxygen is absent or almost absent in that harsh environment. Like on Earth, there is a meteorological cycle on Saturn's largest moon. That meteorological cycle is based on methane. Methane can form lakes, seas and rivers on Titan like water can form lakes, seas and rivers on Earth. There are evaporation processes, condensation processes, cloud formations and precipitation processes on Titan like on Earth. Curiously, the pools of liquids are mostly found in the high latitudes of Titan. The lower latitudes or the tropical or equatorial regions appear particularly dry. That's why we believe that the dry and harsh environment of the low or mid-latitudes is conducive to the development of dust devils. Are there tornadoes or phenomena resembling waterspouts in the lakes or seas found in the high latitudes of the northern hemisphere of the giant moon ? The team of Brian Jackson will have the opportunity to check whether there are dust devils when the Dragonfly drone explores the land of dunes from the start of the mission on Titan in 2034. Dust devils may be too weak to potentially damage the big octocopter of the Dragonfly mission.

The image in the upper part of the table corresponds to a raw view of the disk of Saturn's largest moon Titan. The view whose file name is N00194383.jpg was obtained with the CL1 filter and the MT3 filter of the Cassini orbiter on August 30, 2012. The snapshot had not been validated or calibrated at the time of the observation and a validated or calibrated image was going to be archived with the Planetary Data System proposed by NASA. A vortex can be discerned above the south polar area of the giant moon in particular. The view in the lower part of the table represents a colorized version of the original view. Credit for the raw image: NASA/JPL-Caltech/Space Science Institute. Credit for the colorization of the original view: Marc Lafferre, 2020.

- To get further information on that news, go to: https://blogs.agu.org/geospace/2020/04/16/dust-devils-may-roam-hydrocarbon-dunes-on-saturns-moon-titan and https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JE006238.



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