October 1st, 2022: A New Study Reveals The Potential Topography And The Potential Properties Of The Terrain In The Selk Crater Area On Titan

A new research work entitled "Composition, Roughness, and Topography from Radar Backscatter at Selk Crater, the Dragonfly Landing Site", published on August 29, 2022 in The Planetary Science Journal and proposed by a group of researchers involving Léa E. Bonnefoy and Alexander G. Hayes reveals the potential characteristics of the terrain in the area of the Selk Crater on Titan thanks to the large amount of radar data obtained from the Cassini spacecraft during its long mission in the Saturn System from 2004 to 2017. The Dragonfly mission will involve a rotorcraft that will explore the area of the Selk Crater in the 2030s. The touchdown in the area of the Selk Crater is expected to occur in 2034 since the launch of the probe from Earth is scheduled to take shape in 2027. A major goal for mission planners is to make sure that there is a perfect landing phase. That's why an in-depth analysis of the potential topography of the terrain and of the potential properties of the surface must be performed by planetologists. Researchers can analyze infrared or near-infrared data as well as radar data acquired from the Cassini spacecraft during its long mission in the Saturn System in order to anticipate the potential configuration of the landing site.

The Dragonfly rotorcraft is expected to land at a relatively low latitude in the area of the Selk Crater. The researchers know that the equatorial area unveils an hummocky terrain or a knoll-like landscape. They can base their analyses on the Cassini radar images that can bring major clues regarding the topography, the nature or the composition of the surface. Thanks to the Radar Mapper of the Cassini orbiter, we have realized that the relatively dark areas of the low or middle latitudes tend to be dominated by linear and parallel dunes extending over long distances. The rotorcraft of the Dragonfly mission will have the opportunity to explore those exotic dunes. The team of Léa Bonnefoy analyzed radar reflectivity and angled shadows to deduce the potential properties of the surface. The planetologists anticipate that the probe will evolve in an environment composed of sand dunes and of broken-up ice. Léa Bonnefoy who is a post-doctoral researcher pointed out: "Dragonfly - the first flying machine for a world in the outer solar system - is going to a scientifically remarkable area." What types of hydrocarbons or organics will we find on the surface or in the air ?

The rotorcraft of the Dragonfly mission will not be the first probe to land on Titan. The surface of Saturn's largest moon has already been visited or explored by the Huygens probe. The Huygens probe performed a parachuted descent in the atmosphere of that enigmatic world on January 14, 2005. The landing phase was ideal. The aerial views revealed bright hills composed of a network of dark channels as well as a brown or dark plain. The color view obtained from the surface showed eroded stones or pebbles implying that the probe may have landed onto an ancien brook, stream or river. The landing site was at a particularly low latitude in the southern hemisphere, in the area of Shangri-La and Adiri. We know, now, that the low or middle latitudes appear relatively dry but they can undergo heavy rainfall events from time to time during the long Titanian year. The probe of the Dragonfly mission will also land in a relatively low latitude. Léa Bonnefoy pointed out: "Dragonfly will land in an equatorial, dry region of Titan - a frigid, thick-atmosphere, hydrocarbon world." Let's recall that the atmospheric pressure at the level of the surface of that giant moon is higher than that on Earth at sea level.

The atmosphere of Titan is dominated by molecular nitrogen like the atmosphere of our planet. The second most abundant gas in the atmosphere of Titan is methane and methane can form clouds that can produce rainfall events like on Earth. Léa Bonnefoy advanced: "It rains liquid methane sometimes, but it is more like a desert on Earth - where you have dunes, some little mountains and an impact crater. We're looking closely at the landing site, its structure and surface. To do that, we're examining radar images from the Cassini-Huygens mission, looking at how radar signal changes from different viewing angles." The resolution of the radar images can vary so that it is quite complicated to interpret what we are seeing. Léa Bonnefoy pointed out: "The radar images we have of Titan through Cassini have a best-resolution of about 300 meters per pixel, about the size of a football field and we have only seen less than 10% of the surface at that scale." Thus, the level of detail is quite limited and any brook won't be identified on the basis of the radar data obtained from the Cassini spacecraft obviously. Léa Bonnefoy argued: "This means there are probably a lot of small rivers and landscapes that we couldn't see."

The network of dark channels observed in the bright hills from the Huygens probe during its atmospheric descent on January 14, 2005 could not be identified in the radar images captured from the Cassini orbiter due to the limited resolution. The team of Léa Bonnefoy mobilized the radar views to map six terrains in the exploration area of the Dragonfly rotorcraft, characterized the landscape and evaluated the height of the rim of Selk Crater. The shape of Selk Crater is likely to tell us a lot regarding the geology of the area. Will the rotorcraft be in a position to land inside the Selk Crater ? What is the composition of the surface at the bottom of Selk Crater ? Is the crater strongly eroded ? Are there streams or brooks of methane from the rims to the bottom of the crater ? Many questions must be answered regarding Selk Crater. Selk Crater can also tell us a lot regarding the structure of the external crust. The mission of the Dragonfly rotorcraft in the environment of Titan may last three years. That's much more than the mission of the Huygens probe on the surface of that intriguing world. The rotorcraft will have to withstand extremely low environmental temperatures.

In the harsh environment of Titan, at the level of the surface, the ambient temperature evolves around -179 degrees Celsius, - 290 degrees Fahrenheit or 94 Kelvin. Water can only be present in the form of ice on the surface of that enigmatic moon. However, methane, ethane or propane can be present in their liquid form on the surface of the Opaque Moon. The touchdown of the Huygens probe had engendered a relatively strong release of methane implying that the soil may be relatively rich in methane in the low latitudes. Researchers are particularly interested in the methane cycle on Titan. Are there subsurface reservoirs of liquid methane beneath the external crust ? Are there geysers of methane in the low latitudes ? Can the potential lakes, seas or rivers be relatively stable over time in the low or middle latitudes ? The various radar images acquired from the Cassini orbiter have clearly shown that the most humid areas on Titan are found in the high latitudes of each hemisphere. The radar images as well as the infrared or near-infrared images taken from the Cassini orbiter during its long mission in the Saturn System had clearly shown that the most humid area on Titan was the north polar region.

The Dragonfly rotorcraft won't be in a position to explore Kraken Mare, Ligeia Mare or Punga Mare but we hope that the probe will be in a position to identify ponds or small lakes in its long mission in the environment of Titan. Strong rainfall events engendering transient lakes or rivers can occur from time to time in the low or middle latitudes. The pools of that enigmatic world that looks like the Earth to a certain extent are likely to engender relatively complex organics or hydrocarbons. Are there prebiotic molecules on the surface or in the pools or rivers of Titan ? The chemistry of the soil of Titan is likely to tell us a lot regarding the chemistry of life or the chemistry of organics in general. The dunes found in the relatively dark areas of the low or middle latitudes may be rich in hydrocarbons and organics. The rotorcraft of the Dragonfly mission will be in a position to gather major clues regarding the composition and the dynamics of those exotic dunes. The radar views obtained from the Cassini spacecraft clearly show that the shape of the Titanian dunes found in the low or middle latitudes is closely linked to prevailing winds. The dunes can be deflected by bright topographic obstacles.

A parallel between the major dune fields on Titan and the fields of Seif dunes on Earth can be drawn. However, the composition of the Titanian dunes must be completely different from the composition of the dunes on Earth. Alexander G. Hayes pointed out: "Over the next several years, we are going to see a lot of attention paid to the Selk crater region." He added: "Lea's work provides a solid foundation upon which to start building models and making predictions for Dragonfly to test when it explores the area in the mid-2030s." The dense air at the level of Titan's surface makes any flight easier for a rotorcraft. Let's point out that the air is about 4 times denser than on Earth at sea level close to the surface on Titan. The configuration is very different than on Mars since the air of Mars at the level of the surface is extremely thin so that any rotorcraft or helicopter must turn its blades much more rapidly on Mars than on Titan in order to keep its altitude. Léa Bonnefoy concluded: "Dragonfly is going to finally show us what the region - and Titan - looks like." The study was financed by the Habitability of Hydrocarbon Worlds: Titan and Beyond/NASA Astrobiology Institute and the Centre National d'Etudes Spatiales (CNES). The rotorcraft of the Dragonfly mission will probably make us perform a great leap forward regarding our knowledge of the chemistry of organics and hydrocarbons.

The image above reveals a portion of a radar swath acquired during the T95 Flyby performed by the Cassini orbiter on October 14, 2013. The file name of the original radar swath is BIFQI09N199_D253_T095S01_V03.jpg. One can notice a large portion of Selk Crater, a crater that will be explored by the Dragonfly rotorcraft in the 2030s. Each side of the view is about 100 km long. Credit for the original image: NASA/JPL/Cassini Radar Team/PDS Image Atlas. Montage credit: Marc Lafferre, 2022.

- To get further information on that news, go to: https://news.cornell.edu/stories/2022/09/scientists-depict-dragonfly-landing-site-saturn-moon-titan and https://iopscience.iop.org/article/10.3847/PSJ/ac8428/meta .



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