June 6, 2022: A New Study Brings A New Light On The Potential Formation Mechanisms Of Sand Dunes On Venus, Mars, Titan, Triton Or Pluto
A new research work entitled "Conditions for aeolian transport in the Solar System", published in Nature Astronomy and proposed by Andrew Gunn and Douglas J. Jerolmack reveals the various potential formation mechanisms of the sand dunes encountered on several worlds of the Solar System. Worlds like Venus, the Earth, Mars, Titan or Pluto unveil systems of dunes whose formation mechanisms can be different due to physical parameters or local conditions. The gravity, the composition of the crust or the density of the air at sea level can have a significant impact on the topography or on the nature of the soil. Relatively strong winds that exceed threshold speed can engender sand dunes if the grains are sufficiently strong to keep their shape in any collision event. Most moons of the Solar System are devoid of any significant atmosphere so that the surface is dominated by impact craters. The Terrestrial worlds or the icy worlds that contain a relatively significant atmosphere must have sand dunes. That's the case for Venus, the Earth, Mars, Titan and Pluto. Triton, the largest moon of the Gas Giant Neptune, may also contain vast fields of sand dunes but the resolution of the surface obtained thanks to data captured from the Voyager 2 probe is too low to confirm that strong hypothesis.
The threshold theories that are available today tend to neglect physical processes. Yet, physical processes must be taken into account for extraterrestrial worlds. Therefore, the theories on the mechanics of sand dune formation must be revised or refined on the basis of other parameters and on the basis of the new clues we regularly obtain in planetary exploration. The planetologists analyzed the results in contact, rarefied gas, statistical and adhesion mechanics in order to propose a theory for the fluid and impact thresholds of aeolian movement that is in line with the various environmental configurations encountered in the different worlds of the Solar System. The predictions of the theory are in line with available experimental threshold observations and show that these thresholds are closely related to local environmental conditions on the various extraterrestrial worlds found in our planetary neighborhood. The outcome of the research work reveals that the dunes found on Titan must be locally sourced and that the high threshold of the Red Planet Mars makes its dunes less unstable or more resistant to movement. The large-scale views tell us a lot regarding the dynamics of the dunes.
The study allows us to better understand the potential dynamics of the dunes and to better understand the atmospheric dynamics of the various worlds thanks to the analysis of the dunes. Planetologists can have a better idea upon the composition of the soil, the composition of the surface sediments or the composition of the dunes. The researchers will have to perform new studies on the basis of parameters or factors that have been neglected so far. We must take into account complex processes that we don't understand very well at the present time. The nature of the sand can be remarkably diverse in the Solar System. On Earth, there are several types of dunes whose composition can significantly vary from one place to another place. Erosion along the sea side can engender giant dunes like the "Dune du Pilat" in France along the Atlantic Ocean. The Dune du Pilat is rich in quartz in particular. The desert "White Sands" in New Mexico is rich in gypsum (CaSO4.2H2O). Thus, there is a remarkable diversity of dune types whose composition can significantly vary on our own planet. In the Outer Solar System, the composition of the dunes found on icy worlds like Titan, Triton or Pluto can be fundamentally different from the composition of the dunes we regularly encounter on Earth.
The largest moon of Saturn, Titan, is the only world of the Outer Solar System where a probe has been sent to analyze the surface and the environment in situ. The Huygens probe landed at a low latitude in the southern hemisphere of the giant moon, in the region of Adiri and Shangri-La on January 14, 2005. The probe did not land onto a mountain, onto a hill or onto a dune. The module landed in an area where eroded stones or pebbles could be seen close to the probe. The Huygens probe had apparently landed onto an ancient stream, brook or river. The low or middle latitudes of Titan can undergo heavy rainfall events from time to time during the long Titanian year that lasts almost 30 Earth years. Those heavy rainfall events can engender transient rivers, brooks or streams. The liquid likely dominated by methane (CH4) is likely to significantly erode the surface. Over geological time scales, there must be a relatively high accumulation of sand in some areas. That's the case in the relatively dark areas found in the low or middle latitudes of the Opaque Moon where Seif dunes or linear and parallel dunes extending over long distances have been clearly identified from the Radar Mapper of the Cassini spacecraft in its long mission in the Saturn System from 2004 to 2017.
The linear and parallel dunes extending over long distances in the relatively dark areas of the low or middle latitudes reveal the potential dynamics or strength of prevailing winds in the low or middle latitudes. One can imagine that the composition of the surface in the relatively bright areas of the low or middle latitudes is significantly different from the composition of the surface in the relatively dark areas of the low or middle latitudes. One can advance that the bright areas of the low or middle latitudes of Titan are probably richer in water ice and that the dark areas of the low or middle latitudes of the giant moon are probably rich in organics or hydrocarbons like methane, ethane or benzene. The haze of Titan is likely to generate a type of snow or heavy molecules that will tend to regularly fuel the dunes. The grains of the dunes are light enough to move under the influence of prevailing winds. Planetologists can evaluate the potential size of the grains of the dunes on Titan on the basis of the density of the atmosphere at sea level, on the basis of the potential strength of prevailing winds and on the basis of the potential composition of the dunes. The analysis is far from being simple !
The planetologists focused their attention on several worlds where dunes have been identified or must be present. They analyzed Venus, Earth, Mars, Titan, Triton as well as Pluto. Those worlds contain atmospheres that imply the presence of winds whose strength can be significantly different from one place to another place. The atmosphere of Mars and Venus is dominated by carbon dioxide whereas the atmosphere of the Earth, Titan, Triton and Pluto is dominated by molecular nitrogen. The atmosphere of Venus is extremely heavy with an atmospheric pressure of about 93 Bar on the surface compared to only about 1 Bar on the surface of the Earth at sea level. Thus, the environmental temperature at the level of the surface of Venus is extremely high and much higher than the environmental temperature of the Earth at sea level due to the significant greenhouse effects on Venus where the infrared light from the Sun is trapped in the atmosphere. Due to the relatively high density of the air at sea level on Venus, any wind must be relatively strong. On Titan, at sea level, the density of the air is higher than the density of the air at sea level on Earth. Therefore, the potential strength of prevailing winds on Titan must be relatively high.
On Mars, the density of the air at sea level is much lower than the density of the air of the Earth at sea level so that any wind at the level of the surface is generally relatively weak. Yet, there can be global storms of dust from time to time so that the sand can move and form significant dunes. The variations of the environmental temperature between the day and the night on Mars imply relatively significant erosional processes even if Mars is devoid of any liquid water on its surface. Winds in a thin atmosphere play their role in erosional processes so that significant dunes can take shape. That's the case on the Red Planet ! On Triton, the largest moon of Neptune, large dune fields are probably present even if the atmosphere is extremely thin and much thinner than on Mars. Curiously, the Dwarf Planet Pluto contains a type of dune in the bright area of Tombaugh Regio. Yet, the atmosphere of Pluto is extremely thin with a very low density of the air at sea level. The dunes found in Tombaugh Regio appear bright. That's clearly not the type of dune we regularly encounter on Earth. We know that Sputnik Planitia is rich in molecular nitrogen and carbon monoxide. So, the local dunes must contain those molecules.
One of the goals of the study was to evaluate the minimum of the winds to move sediments on the surface of each planetary body. Are the grains of the dunes stable over time ? The planetologists mobilized a large amount of experimental data as well as other studies to perform their predictions on the characteristics or the dynamics of the dunes on the worlds studied. In their research work, they took into account gravity, atmospheric composition, surface temperature as well as the strength of sediments. We can speculate on the potential characteristics of the grains found in the dunes on each world. Regarding the dunes of Titan, the researchers believe that the grains must contain a mixture of organics from the haze and ice so that the grain remains stable over time or does not disintegrate easily. The dunes of Pluto found in Tombaugh Regio may represent sublimation waves because prevailing winds are relatively weak. Can prevailing winds really move methane ice or nitrogen ice on Pluto ? The surface of Triton also unveils an irregular surface but the resolution of the views of Triton obtained from the Voyager 2 spacecraft in 1989 is too low to have a good idea upon the nature and the dynamics of the surface. We only know that the surface is remarkably dynamic with geysers or cryovolcanoes. We also know that the landscape is far from being uniform.
The image above reveals a portion of the surface of Saturn's largest moon Titan. One can clearly discern linear and parallel dunes extending over long distances as well as bright patches which seem to represent topographic obstacles that can influence the direction of the dunes. Each side of the view represents approximately 100 kilometers (about 62 miles). The original view was obtained during the T-120 Flyby performed on June 7, 2016. Credit for the original view: NASA/JPL/Cassini Radar Team/PDS Image Atlas. Montage credit: Marc Lafferre, 2022.
- To get further information on that news, go to: https://phys.org/news/2022-05-venus-pluto-sand-dunes-clues.html and https://www.nature.com/articles/s41550-022-01669-0.