February 19, 2022: A New Study Reveals That Craters Whose Diameter Is Smaller Than 50 Kilometers Tend To Completely Disappear Due To Erosion Over Geologic Time Scales On Titan

A new research work entitled "Crater production on Titan and surface chronology", proposed by N.L. Rossignoli, R.P. Di Sisto and M.G. Parisi and published in Arxiv reveals the key action of erosion on relatively small craters on Saturn's largest moon. Big craters whose diamater is over 50 kilometers can persist or can be well discerned over the Solar System age whereas smaller craters tend to completely vanish due to the erosion processes related to winds, rainfall events or even snowfall events. Titan is the only world evolving around the Gas Giant Saturn that contains a significant atmosphere implying a relatively strong action of erosion on its surface. The other moons of Saturn contain numerous craters that are likely to allow us to determine the potential age or history of those worlds. Worlds like Tethys, Dione or Rhea are rich in impact craters on their bright surface likely rich in water ice. The researchers can analyze those craters in order to deduce the age of the crust or surface. They are in a position to determine constraints on the population of the impactors that can be comets or asteroids. Regarding the impactors, the planetologists focus their attention on centaur objects or objects that evolve in the area of the Gas Giants.

During the long Cassini-Huygens mission in the Saturn System, from 2004 to 2017, the Cassini orbiter has captured a huge amount of data regarding the surface of the giant moon. We have collected images obtained in the infrared or near-infrared spectrum as well as radar images so that the surface could be discerned despite the presence of a completely opaque atmosphere in the visible spectrum. Researchers have had the opportunity to evaluate the concentration of potential impact craters on that exotic world and it turns out that impact craters are remarkably scarce despite a relatively dry environment in the low or mid-latitudes. The relatively big craters can appear strongly degraded or eroded. The planetologists mobilized the previous research works on impact cratering rates on the various moons of the Ringed Planet Saturn and modeled the cratering process on the Orange Moon to constrain the surface chronology of that world and to evaluate the role of centaur objects that are considered the main impactors of Saturn's largest moon. The Outer Solar System is far from being empty in terms of asteroids, comets or even Dwarf Planets. Numerous objects can be identified between Jupiter and Neptune.

A theoretical model that had already been developed was mobilized to calculate the crater production on the Opaque Moon on the basis of the hypothesis that the centaur objects are the main impactors and on the basis of two different slopes for the size-frequency distribution (SFD) of the smaller objects of their source population. The researchers apply a simple model for the atmospheric shielding effects in the process of cratering on the Hazy Moon. Their outcome is then compared with other synthetic crater distributions and observational crater counts which have been updated. The analytical comparison is then mobilized to calculate the crater retention age for each crater diameter on that dynamic world. The planetologists observe that the cumulative crater distribution generated by the SFD or size-frequency distribution with a differential index of s2 = 3.5 appears to consistently anticipate large craters whose diameter is over 50 kilometers on the surface of the giant moon while that cumulative crater distribution overestimates the number of smaller craters. The modeled distribution and observed distribution of craters tend to flatten for craters whose diameter is lower than about 25 kilometers due to atmospheric shielding.

The difference between the modeled and the observed distribution can appear as a rule or proxy for the level of erosion on the surface of the giant moon in the long history of the Solar System. The analytical work led by the team of researchers allows the planetologists to conclude that craters whose diameter is over 50 kilometers can persist over geologic time scales whereas smaller craters tend to be erased by erosional processes engendered by winds, rainfall events or snowfall events in the long run. Tectonic forces are also likely to engender cryovolcanoes or geysers that can strongly contribute to modify the surface. For instance, the tiny moon Enceladus contains major fractures in its south polar region where geysers or cryovolcanoes spewing water ice or organics can be observed. Thus, a portion of the surface of the bright moon Enceladus appears remarkably young geologically speaking. Are there geysers or cryovolcanoes on Titan as well ? That's a major question we can't answer yet. Some landscape features of Saturn's largest moon suggest the potential presence of active cryovolcanoes. Bright sinuous channels can be observed in various areas. Are they relateled to cryovolcanoes ?

Several major craters can be identified on Titan. One can mention Menrva, the largest crater on that giant moon. The diameter of Menrva is almost 400 kilometers. The giant crater is found at a relatively low latitude since it can be found at 20.1 degrees north latitude and at 87.2 degrees west longitude. The crater appears quite eroded since there is no clear distinct central peak inside that crater. The Dragonfly rotorcraft is expected to explore another major crater of Titan. That crater is Selk, a crater that is found at 7 degrees north latitude and 199 degrees west longitude. The diameter of that crater that appears to be an impact crater is around 90 kilometers. Selk appears relatively young geologically speaking. Among the major craters of Titan, one can mention the Ksa crater as well. Ksa is located at 40 degrees north latitude and at about 65.4 degrees west longitude. The diameter of Ksa is around 29 kilometers. The edges of Ksa and its central peak are well defined in the radar data. Ksa appears to be a relatively young crater because the erosional processes appear relatively limited. The craters found in the low or middle latitudes tend to take shape in relatively dry areas compared to the polar regions.

The polar regions of Titan appear relatively humid since they contain seas, lakes or rivers likely dominated by methane or ethane. Any impact crater in those areas is likely to disappear relatively rapidly. Some lakes found in the high latitudes of the northern hemisphere tend to be relatively round or circular. Are those lakes related to meteorological phenomena or are they related to cryovolcanism ? Any meteorite or comet crashing into the soil of the north polar region may interact with a potential subsurface layer of methane or ethane. The crust must be dynamic in the high latitudes of the northern hemisphere. The craters found in the low or middle latitudes must undergo heavy rainfall events from time to time during the long Titanian year which lasts almost 30 Terrestrial years. Therefore, their shape can strongly change over geologic time scales. The radar images of the low or middle latitudes have clearly shown that the dark areas tend to be dominated by Seif dunes that can invade any impact crater. Some craters must be buried beneath the dune fields of the low-albedo areas. The haze of Titan can bring a type of snow rich in hydrocarbons or organics to the potential impact craters.

Some craters of Titan may represent cryovolcanoes rather than impact craters. But today, we have no clear sign of cryovolcanism on Titan. Some researchers have advanced that there may be a subsurface layer rich in liquid water beneath the icy crust. Any cryovolcano is likely to spew water ice, methane, ethane or organics. If there is a biosphere in the hypothetical subsurface ocean dominated by liquid water, any cryovolcano can spew organics or basic lifeforms that we will be in a position to study on the surface in some particular areas in the near future. The meteorites or comets that crash into the surface of Titan can reveal captivating minerals or molecules present beneath the icy crust. The dark or brown areas of Saturn's largest moon may contain a type of mud or sludge rich in organics and hydrocarbons. Can prebiotic molecules emerge from that type of mud or sludge known as tholin ? That's a major question that the Dragonfly rotorcraft will be in a position to answer in the 2030s. In the harsh environment of Titan, water can't appear in its liquid form on the surface. Water can be as hard as rock on the surface of Titan. However, methane can play the same role on Titan as liquid water on Earth.

There is a meteorological cycle of methane on Titan. A parallel can be drawn between the methane cycle of Titan and the water cycle of the Earth. On Titan, methane can fall as rain, can condense and form clouds and can evaporate from any sea, lake or river. There may be subsurface pockets or layers of liquid methane beneath the external crust. The north polar region which appears to be the most humid area on Titan may contain a subsurface layer rich in methane and ethane. Planetologists try to understand why Titan's atmosphere contains such a concentration of methane. The concentration of methane in the atmosphere is higher close to the surface. The atmosphere of Saturn's largest moon is dominated by molecular nitrogen like the atmosphere of the Earth. The deep and dense atmosphere of Titan represents a relatively strong shield against any asteroid or comet. The atmospheric shield of Titan is in fact stronger than the atmospheric shield of the Earth. The speed, the size and the mean density of the meteorite or comet must be taken into account in order to evaluate the potential characteristics of any impact crater on Saturn's largest moon. Titan is clearly a dynamic world !

The image above reveals a portion of Ganesa Macula, a landscape feature that may represent an impact crater or even a cryovolcano. The view represents a portion of a radar swath of Saturn's largest moon Titan obtained during the flyby of the giant moon performed on October 26, 2004 by the Cassini-Huygens spacecraft. Each side of the view represents about 100 kilometers. In the upper left part of the image, one can notice, in particular, a relatively round feature that appears relatively bright. That feature may represent the central peak of the presumed impact crater. Credit for the original view: NASA/JPL/Cassini Radar Team/Jason Perry. Montage credit: Marc Lafferre, 2022.

- To get further information on that news, go to: https://arxiv.org/abs/2202.04712.



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