October 6, 2020 : A New Study Of Impact Craters On Titan Reveals The Potential Presence Of Different Materials Like Tholin Or Water Ice

A new study entitled « The chemical composition of impact craters on Titan », published by Astronomy & Astrophysics and involving Catherine Neish reveals that the impact craters of Saturn's largest moon unveil different compounds and bring clues regarding the composition of the crust. The group of researchers resorted to data obtained from the Visual and Infrared Mapping Spectrometer of the Cassini spacecraft to analyze the potential composition of several impact craters. The impact craters found in the equatorial areas where dune fields are widespread tend to be rich in tholin or dark material whereas the impact craters found at higher latitudes reveal what may be water ice as well as other compounds like tholin, hydrocarbons or organics. Some researchers advance that the recent or fresh impact craters can reveal some compounds related to an ancient ecosystem. The Titanian impact craters can unveil major clues regarding the nature of the crust or the nature of the sediments. Due to the thick atmosphere of the giant moon, most meteorites don't reach the surface because they disintegrate in the air and any impact crater tends to undergo a relatively strong erosion over time like on Earth.

Planetologists have used the infrared and near-infrared data obtained with the VIMS in particular to study surface features on Titan because the atmosphere is completely opaque from outer space in the visible spectrum. The low or mid-latitudes of the Hazy Moon appear relatively dry and unveil vast areas composed of linear and parallel dunes extending over long distances. In the low-albedo areas of the low or mid-latitudes where sand is widespread, any impact crater will likely be rapidly buried with the sand driven by prevailing winds. Therefore, any impact crater can appear relatively uniform. At higher latitudes, the logic is different because the environment tends to be wetter or less dry so that any meteorite or comet will not engender the same type of impact crater. Some craters unveil bright materials which may correspond to the signature of water ice. The crust of Titan is probably rich in water ice like the crust of the other major moons of Saturn. If the crust of Saturn's largest moon is unstable with fractures or cryovolcanoes for instance, one can imagine that any material or compound of the presumed subsurface ocean can go up toward the surface. That's why impact craters are potentially interesting exobiologically speaking.

The haze composed of hydrocarbons or organics is likely to engender snowfall events. The dunes present in the low or mid-latitudes may be regularly fuelled by the presumed exotic snow engendered by that haze. One can easily imagine that the impact of any meteorite or comet will be relatively hard to discern in the low or mid-latitudes. Prevailing winds clearly influence the shape of the Titanian dunes which look like the famous Seif dunes found in the Namib Desert in Africa on Earth. Heavy rainfall events can also be encountered from time to time in the low or mid-latitudes. Winds, snow and rain of methane or ethane must play a key role in the erosion of any impact crater. At higher latitudes, the concentration of sand may be much more limited so that any impact crater will be easier to discern. The team of Catherine Neish has identified what may be fresh water ice in some impact craters of the giant moon where the concentration of sand is lower and where the environment is wetter. The Huygens probe had clearly identified, during its atmospheric descent, the presence of dark channels in the bright hills of the area at a low latitude. Those types of streams are likely to erode any impact crater.

The deep and dense atmosphere of Titan engenders familiar phenomena via cloud formation, precipitation phenomena or erosion. Catherine Neish pointed out : « It's wild. There's no other place like Titan in the solar system. There's more sand on Titan per area than anywhere else. » She added : « And Titan has weather. It's not unlike the Earth in that way. It's just that the ingredients are all wrong. It has methane rain and streams cutting through the surface and organic sand getting blown around. It's still very active just like it is here on Earth. » The other major moons of Saturn are devoid of any significant atmosphere and are generally dominated by impact craters. Their surface doesn't appear to be very active. There are no erosional processes related to atmospheric phenomena on those worlds. Their surface generally appears bright due to the presence of a high concentration of water ice. The tiny moon Enceladus is an exception among those worlds because a portion of its surface is relatively young. There are very active geysers in the fractures of its south polar region. The geysers spew water ice as well as more complex molecules like relatively complex organics.

The geysers of Enceladus imply that there may be pockets of liquids beneath the icy crust or a subsurface sea or ocean. Enceladus clearly demonstrates that the other major moons of the Ringed Planet may also contain pockets of liquids or subsurface seas or oceans beneath their icy crust. If there is a subsurface ocean beneath the external crust of Saturn's largest moon, one can assume that the impact craters of that world can tell us a lot regarding the chemistry of the subsurface ocean. Moreover, some craters of Titan may in fact correspond to calderas or cryovolcanoes. Therefore, they may contain a relatively recent material which has been brought from the presumed subsurface ocean to the surface. The craters appear very interesting for planetologists or exobiologists who can better study the nature or the dynamics of the crust. Some researchers imagine the possibility of ancient ecosystems located on the floor of some impact craters. If the remnants of life are completely frozen, the ancient lifeform may likely be based on liquid water because water appears in its solid form on the soil of Titan. In the interior of the giant moon, at a higher depth, the ambient temperature will tend to increase like inside the Earth. Thus, a subsurface ocean dominated by liquid water is clearly a possibility in that world.

The drone of the Dragonfly mission whose launch from our planet is scheduled for the year 2027 will explore the land of dunes and will have the opportunity to study an impact crater where potential signs of life can be found. Catherine Neish is convinced that we can learn more by exploring beyond the Red Planet Mars. Worlds like Ceres, Io, Europa, Ganymede, Enceladus, Titan, Triton or Pluto can tell us a lot regarding the secrets of chemistry in the universe. Today, the Earth appears to be the « center of gravity » of organic chemistry. But in the future, we may realize that there are other worlds where very complex chemical reactions involving carbon or organics take shape or develop. Mars is dry in the low or mid-latitudes at least whereas Titan is relatively humid in its polar areas. Titan contains lakes, seas and rivers in its high latitudes. Researchers want to know whether the pools of hydrocarbons of Titan can engender complex chemical reactions like on Earth. Are there prebiotic molecules on Titan ? Are there amino acids, proteins, sugars, lipids or even more complex molecules in the harsh environment of the Hazy Moon ?

We probably know very little about the Solar System today. Many worlds deserve more attention. That's the case for Venus, a planet which looks like the Earth in terms of size and mean density. Venus contains a captivating atmosphere where some microorganisms may potentially exist. Recently, phosphine may have been detected in the dense and opaque atmosphere of Venus. Phosphine contains phosphorus, a key element of the lifeform we know on Earth. That's a very motivating finding that encourages us to continue exploring Venus even if the surface is a hell due to strong greenhouse effets. Many worlds in the Outer Solar System deserve landers, rovers or drones to explore them in a better way or more efficiently than orbiters. Catherine Neish argued : « I think more and more, we're seeing a false equivalency between life and Mars. The recent findings about Venus and all the new things we're learning about it once being an ocean world is another game-changer. » She concluded : « Finally, people are saying, in our search for life in the universe, we really need to focus on a lot more places, and not just Mars. And that includes NASA sending the Dragonfly mission to Titan. »

Studying some impact craters of Titan is clearly a way for Catherine Neish and her team to prepare the next ambitious mission to Titan. Catherine Neish who is a member of Western's Institute for Earth and Space Exploration (Western Space) has studied nine impact craters of the Opaque Moon in the prospect of getting new elements or clues about their composition. She has resorted to VIMS data and she has used a radiative transfer code to deduce the surface albedo of the impact craters analyzed. The team focused its attention on the crater floor and on the ejecta blanket of the various craters. The craters found in the equatorial region like Selk, Ksa and Guabonito turn out to be purely composed of organic material. The nature of the compound may be closely related to the sand dunes of the area which are likely to send a lot of sand inside the craters. The mid-latitude plain craters represented by Afekan, Soi and Forseti as well as Menrva and Sinlap turn out to be rich in water ice within a mixture of materials containing organics. The group of researchers did not identify the presence of ammonia ice (NH3) or carbon dioxide ice (CO2) in those landscape features. Tholin, water ice, hydrocarbons and organics seem to be the key compounds of Titan's landscape.

The image above represents a radar view of a portion of what seems to be a giant impact crater on Saturn's largest moon Titan. The diameter of the presumed crater is around 440 kilometers or 273 miles. The presumed crater may have been generated by a comet or a meteorite whose diameter likely represented tens of kilometers. Some bright sinuous channels at the level of the edges may be related to the presumed impact event. The view is part of a radar swath obtained with the Radar Mapper of the Cassini spacecraft during the Cassini-Huygens mission in the Saturn System on February 15, 2005. Image credit: NASA/JPL-Caltech/ASI.

- To get further information on that news, go to: http://astrobiology.com/2020/10/titan-has-the-ingredients-for-life.html and https://www.aanda.org/articles/aa/abs/2020/09/aa37866-20/aa37866-20.html .



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