October 27, 2024: A Deep Layer Of Methane Clathrate Explaining Relatively Shallow Impact Craters On Titan ?
A new study entitled "Rapid Impact Crater Relaxation Caused by an Insulating Methane Clathrate Crust on Titan", published by the American Astronomical Society on September 30, 2024 and proposed by a team of researchers mobilizing Lauren R. Schurmeier and Gwendolyn E. Brouwer in particular reveals that the relatively limited depth of the impact craters of Titan may be related to the action of a relatively deep layer of methane clathrate ice. So far, planetologists have been in a position to identify only 90 craters on Saturn's largest moon. Most craters may not be the outcome of a meteoritic impact or a cometary impact. The deep and thick atmosphere of Titan implies resurfacing events or strong erosional processes. That's why a particularly limited amount of craters has been identified on the surface of that intriguing world. The planetologists have performed a comparison between the craters of Titan and the craters of the giant moon of Jupiter Ganymede. Thus, they managed to determine that the presumed impact craters of Titan are hundreds of meters shallower than expected on the basis of similar-sized craters on the largest moon in the Solar System. How can we explain that difference ?
Several processes have been put forward to account for the rapid evolution of the craters of the Opaque Moon. In the harsh environment of Titan where the air is particularly dense at sea level, winds can be relatively strong in the low or middle latitudes and erode the impact craters, the cryovolcanic craters or the calderas. The winds can displace the exotic sand that will tend to fill the craters. Therefore, the appearance of any crater will tend to change relatively quickly over geological time scales. Some craters of Titan may not be discernable due to relatively strong erosional processes or due to the migration of sand that can partially or completely fill any impact crater or cryovolcanic crater. Rainfall events mobilizing methane or ethane can engender transient rivers that can invade or fill the craters implying relatively strong erosional processes like rainfall events mobilizing water on Earth. Topographic relaxation engendered by insulating sand infill can also be considered to account for the change in the appearance of the craters of that giant moon of Saturn. The team Of Lauren R. Schurmeier proposes another potential mechanism to account for the alteration or the change in the appearance of the Titanian craters.
The potential mechanism proposed by the group of researchers mobilizing the research associate Lauren Schurmeier, Gwendolyn Brouwer who is a doctoral candidate and Sarah Fagents who is associate director and researcher in the Hawai'i Institute of Geophysics and Planetology (HIGP) in the UH Manoa School of Ocean and Earth Science and Technology (SOEST) represents topographic relaxation related to an insulating methane clathrate crustal layer in the upper ice shell of the giant moon. Titan is believed to be a differentiated planetary body with several types of layer from the core to the external crust. There is the rocky core. Above it, there is a layer of high-pressure ice. Above the high-pressure ice, there is an ocean. Above that ocean layer, there is a convecting ice shell. Above that convecting ice shell, there is a methane clathrate crust. Above that methane clathrate layer or crust, there is the surface probably rich in water ice, hydrocarbons and organics. And above the surface, there is that surprising atmosphere rich in molecular nitrogen and methane that is relatively deep and dense at sea level. That structure represents the presumed structure of Saturn's largest moon according to the team of researchers.
The planetologists resorted to finite element modeling to check whether a clathrate crust or layer 5, 10, 15 or 20 kilometers thick could warm the ice layer and relax the craters to account for the current depth of the craters or to account for their current level of alteration. In the model, they evaluated the viscoelastic evolution of crater diameters 120, 100, 85 and 40 kilometers on the basis of two initial depths determined on the basis of depth-diameter trends of the craters of the giant moon Ganymede. They concluded that all the thicknesses of the clathrate crust engender a rapid topographic relaxation even if the environment at sea level and the external crust are extremely cold. They note that the crust of clathrate that has a depth or thickness of 5 kilometers is in line with the configuration of the observed shallow depths for the craters in most cases. In many cases, the process can take less than 1000 years. The configuration of a crust of clathrate that has a thickness or a depth of 10 kilometers is in line with the configuration of the observed depths of the larger craters if we envisage geologic timescales. Therefore, the presumed thickness of the hypothetical clathrate layer may be between 5 and 10 kilometers.
The hypothesis on the thickness of the presumed crust of methane clathrate ice is based on the hypothesis that relaxation is the primary cause of the shallow craters. Topographic relaxation can play a major role in the alteration of the crater but it can't be the only factor for the disappearance of the craters. Features such as crater rims or flexural moats will tend to remain over time. The scientists advance that multiple processes must be combined to engender the complete disappearance of the craters and to reproduce the observed biased crater distribution on the Opaque Moon. The airless moons of the Solar System tend to be heavily cratered because the absence of atmosphere on those bodies significantly limits any erosional processes. That's not the case on Titan where the atmosphere is particularly deep and dense at sea level. Lakes, seas and rivers have been clearly identified in the high latitudes of that world. Methane and ethane can evaporate, produce clouds and engender rainfall events that will tend to erode the landscape or to interact with the soil to engender new materials or compounds. The shape and the appearance of the craters found in the high latitudes will tend to be significantly modified over time due to the erosional processes related to those liquids.
The difficulty for researchers is to evaluate the impact of each factor. The haze, the winds, the rainfall events, the dynamics of the external crust, the dynamics of the internal crusts or the convection phenomena in the upper layers can play a significant role in the shape and in the appearance of the craters we have identified. On Earth, impact craters in humid areas tend to disappear or to progressively disappear relatively rapidly due to rainfall events and due to the accumulation of sediments over time. However, in dry areas, major craters can remain in a relatively stable state over time due to the fact that rainfall events are infrequent. Meteor Crater in Arizona in the United States is a well-known example of a preserved impact crater on Earth. That crater has a diameter of 0.737 miles or 1186 meters and a depth of 170 meters (560 feet). That impact crater is 50000 years old. The craters of Titan have been identified thanks to infrared or near-infrared data and radar data acquired from the Cassini orbiter during its long mission in the Saturn System from 2004 to 2017. The Radar Mapper of the spacecraft has allowed us to identify a limited amount of relatively significant craters but we may have not been in a position to identify relatively small craters due to the limited resolution of the Radar Mapper.
Like the atmosphere of the Earth, the atmosphere of Titan is likely to slow down any potential asteroid, meteorite or comet. The impact speed will tend to be lower for a planetary body containing an atmosphere than for a planetary body devoid of any atmosphere. The impact speed of the comet or meteorite will tend to be lower in the configuration in which the atmosphere is dense and deep than in the configuration in which the atmosphere is thin and shallow. The atmospheric factor must be taken into account for the analysis of the shape, the size and the depth of any impact crater. In principle, the depth of any medium impact crater on Titan is expected to be lower than the depth of any medium impact crater on the other moons of Saturn which are devoid of any atmosphere. However, the nature of the soil or the nature of the crust can also play a major role in the size, the depth and the appearance of the impact crater. What will the characteristics of the impact crater be if the soil is dominated by water ice and what will the characteristics of the impact crater be if the soil is dominated by silicon dioxide, hydrocarbons or organics ? The analysis is far from being simple !
The researchers of the study have imagined a deep crust of methane clathrate ice beneath the soil. Methane clathrate ice represents a type of solid water ice containing methane gas trapped inside the crystal structure of the material. In their simulation, they noticed that the methane clathrate crust or layer warms the interior of the moon and engenders surprisingly rapid topographic relaxation so that impact craters become shallow at a relatively rapid pace. The hypothetical ice shell rich in methane that may be relatively thick or deep beneath the layer of sediments, hydrocarbons, water ice and organics may regularly fuel the atmosphere with methane molecules and may help us better understand the methane cycle of Titan between the underground, the soil and the atmosphere. The cycle of methane on Titan can tell us a lot regarding our climate. A parallel could be drawn between methane clathrate ice on Titan and the methane clathrate hydrates we have on Earth in the permafrost of Siberia or below the arctic seafloor. The relatively shallow depth of the craters of Titan implies a relatively dynamic interior or a relatively warm interior. The clathrate crust or layer tends to insulate the interior where convection processes can occur. If there is an internal ocean beneath that crust and if there are active cryovolcanoes, we may be in a position to identify biomarkers in a future mission like the Dragonfly mission.
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The image above reveals a portion of a radar swath of Titan obtained from the Cassini orbiter during the T56 Flyby of June 6, 2009. The file name of the radar swath is BIUQI36S166_D195_T056S01_V02_part2.jpg. Each side of the view represents about 100 kilometers (approximately 62 miles). A relatively dark circular feature surrounded by a relatively bright material can be identified in particular. Does it represent an impact crater or a cryovolcanic feature ? Linear features that may represent dunes can be noticed inside the circular feature. Credit for the original view: NASA/JPL/Cassini Radar Team/Jason Perry. Montage credit: Marc Lafferre, 2024. |
- To get further information on that news, go to: https://www.soest.hawaii.edu/soestwp/announce/news/titan-insulating-methane-rich-crust/ and https://iopscience.iop.org/article/10.3847/PSJ/ad7018 .