June 23, 2018 : A Team Of Researchers Involving Xinting Yu Has Analyzed The Mechanical Properties Of Key Sands To Try To Determine The Origin Of The Sand Found In The Equatorial Or Tropical Dunes On Titan

A new study entitled  Where Does Titan Sand Come From: Insight from Mechanical Properties of Titan Sand Candidates  and proposed by a group of researchers involving Xinting Yu unveils the mechanical properties of potential sand grains in order to try to determine the origin of the sand found in the linear and parallel dunes extending over long distances at relatively low latitudes on Titan. Does the sand of those remarkable dunes come from the polar regions where lakes, seas and rivers can be observed ? Is the accumulation of sand the outcome of an ancient sea or ocean ? Is the sand related to the meteorology of Titan ? Is the sand related to molecules produced in the atmosphere or in the haze of Titan ? Several molecules or grains have been analyzed by Xinting Yu and her collaborators. The mechanical properties of tholin, water ice, some simple organics, silicate sand and white gypsum sand were studied and compared in this work. The surface of Saturn's largest moon may be rich in tholin or organics. That's why scientists focus their attention on organics or hydrocarbons.

Some researchers had put forward the hypothesis that the linear and parallel dunes of Titan, which can be compared to the Seif Dunes located in the Namib Desert on Earth, may be closely related to a migration of sand produced in the polar areas where lakes and seas are found. Lakes, seas and rivers on Titan are mostly concentrated in the high latitudes of the northern hemisphere. A major pool of liquids known as Ontario Lacus can also be found in the high latitudes of the southern hemisphere. One can imagine that liquid methane and liquid ethane engender the formation and the development of sand over time due to strong erosional processes. But the study of Xinting Yu and her collaborators suggests that organics or tholins are too brittle or not stiff enough to migrate from the polar regions to the Equatorial or Tropical regions where the linear and parallel dunes have been observed via radar data acquired from the Cassini orbiter during its mission. In fact, tholin, water ice and some simple organics appear to be too soft compared to silicate sand or white gypsum sand to come from the lakes or seas dominated by methane or ethane in the high latitudes of the Orange Moon.

The exact nature of the sand found at relatively low latitudes on the giant moon of the Gas Giant Saturn is still unknown. However, researchers believe that the dunes are dominated by organics or tholins. We would be surprised if the dunes of Titan's low latitudes were mostly composed of silicon dioxide (SiO2) or gypsum (CaSO4-2H2O). The dunes of Titan are likely exotic in terms of composition. On our planet, one can encounter dunes made of silicate sand or dunes made of white gypsum. The dunes of the Sahara are likely the outcome of an ancient sea. Liquid water is a powerful agent in terms of erosion. The solvent can sculpt the landscape and dissolve rocks engendering sand grains. Does liquid methane have the same type of action in the environment of Titan where the geology can be very different from that of the Earth ? The team of researchers resorted to nanoindentation to analyze the mechanical properties of several molecules or grains of sand which are encountered on Earth or which can be encountered on the Opaque Moon. The scientists measured the elastic modulus, the hardness and the fracture toughness of the key materials in order to predict their behavior, their resistance and their dynamics. Silicate sand and white gypsum sand are clearly more resistant or stiffer than tholin, simple organics or water ice.

Several worlds in the Solar System unveil dunes. In fact, the presence of dunes is closely related to the presence of an atmosphere. Venus, The Earth and Mars have dunes thanks to their atmosphere which engenders winds. On our planet, winds, snow, rainfall, rivers and pools of liquid water play a key role in the formation and the development of dunes. On Mars, liquid water can't be present on the surface because the atmospheric pressure on the surface is too low to allow the presence of pools of liquid water even if the environmental temperature is above zero degree Celsius. However, one can regularly encounter dust storms on Mars and winds can sculpt or erode the landscape and engender the formation and the development of dunes. The dunes found in the Outer Solar System may be more exotic than the dunes we can find on Mars or on the Earth because the worlds beyond Mars and the Asteroid Belt tend to be particularly rich in water ice, hydrocarbons like methane or ethane or nitrogen. Some dunes have been found on Titan, Neptune's moon Triton and the Dwarf Planet Pluto. The atmospheric pressure on Titan's surface is higher than the atmospheric pressure on the surface of our planet. The atmosphere of Triton and Pluto is particularly thin but the density of the air is high enough to enable the formation and the development of dunes on both worlds.

On Triton, some wind streaks had been observed. Those wind streaks may be composed of dark complex hydrocarbons. The dunes identified on Pluto, in the plain of bright ice dominated by nitrogen close to mountains, may be composed of methane ice. The dunes of Titan may be closely related to the photochemistry occurring in the haze of its deep atmosphere. Ultraviolet light from the Sun engenders a soup of hydrocarbons, organics or exotic compounds in the upper atmosphere of the Opaque Moon. If the molecules become too heavy, they fall to the soil and can accumulate to form dune fields. The radar views obtained with the Radar Mapper of the Cassini orbiter clearly show the influence of prevailing winds on the shape of the dunes at relatively low latitudes. The crust of Saturn's largest moon may be rich in water ice. That's why some researchers believe that the dunes of the Orange Moon may be composed of grains of water ice mixed with organics or hydrocarbons. Let's point out that the environmental temperature is around minus 179 degrees Celsius on the surface of Titan. Therefore, water can only appear in its solid form on the surface of the giant moon.

The Huygens probe had recorded an increase in the level of methane during the landing phase at a low latitude in the Shangri-La/Adiri region on January 14, 2005. Aerial views from the parachuted probe had revealed the presence of bright hills composed of dark drainage channels. A contrast between those bright hills and a dark or red plain resembling a sea had been clearly noticed. The images obtained from the ground had revealed the presence of eroded stones or pebbles suggesting the presence of an ancient stream or brook on the landing site. If there are periodic rainfall events or monsoon events in the area implying strong precipitation processes and strong evaporation processes, one can imagine that the soil will generate sand over time under the action of erosion exerted by liquid methane and winds. The infrared or near-infrared views taken from the Cassini spacecraft had clearly shown the contrast between dark areas and bright areas at low or mid-latitudes. Prior to the touchdown of the Huygens probe, some researchers had advanced that the dark areas found at low or mid-latitudes may correspond to seas of methane or ethane. We know today that the dark areas found at low or mid-latitudes are dominated by the famous  Cat Scratches  which are Seif Dunes or linear and parallel dunes extending over long distances.

The diameter of the sand particles on Titan may be around 300 m on the basis of hypotheses regarding wind speed, the density of the sand particles and the level of cohesion between particles. If the density of the particle is lower or if the cohesion level between particles is higher, the diameter of sand particles may reach approximately 600m. The cohesion forces of tholin appear to be stronger than those of silicate sand. Researchers are surprised to notice that aerosol compounds produced in the haze of Titan's atmosphere and which are about 1 m wide can aggregate into largely bigger molecules. Four mechanisms have been proposed to account for that remarkable transformation that is to say sintering, lithification and erosion, flocculation and evaporation. The mechanical behavior of sand particles was studied by the group of Xinting Yu via the method of nanoindentation which allows researchers to analyze the behavior of small volumes of materials in order to understand or infer large-scale phenomena. Scientists have envisaged the hypothesis of the presence of evaporites composed of acetylene, ethylene or butane within the sand. However, we have to determine their solid mechanical properties in the harsh environment of Titan.

Researchers have also imagined that the sand particles may be dominated by water ice with a thin layer of organics unveiling a brown or dark color. If the sand particles are mainly composed of organics, they may not be as stiff as the common sand we find on Earth. We have to determine their interactions with winds, liquid methane and liquid ethane. Are they dense ? Are they porous ? The group of Xinting Yu analyzed multiple materials including materials that we may encounter on Titan like Naphthalene, Biphenyl, Phenanthrene, Coronene, Adenine and Melamine. The specialists also analyzed natural sand from carbonate sand to silicate beach sand and white gypsum sand. And they studied materials used in planetary wind tunnels like chromite, basalt or quartz sand. Since the sand of Saturn's largest moon is thought to be mainly composed of organics or hydrocarbons which can't be as stiff or as resistant as silicate sand, researchers advance that the origin of the sand is in the area rather than in the polar regions dominated by lakes, seas and rivers. Are the dark areas of Shangri-La, Fensal and Aztlan dried-up seas which have engendered a desert of sand like in the Sahara Desert for instance ?

The image above shows Titan's disk and reveals the well-known contrast between bright areas and dark areas on the surface of the Opaque Moon. The view, here, clearly shows Belet, a dark area of Titan located at a relatively low latitude. Belet marks a sharp contrast with the bright regions around it. One can notice in particular Adiri, a bright region found in the right part of the disk. The dark areas located at low latitudes are generally dominated by linear and parallel dunes extending over long distances. The image was obtained with the Narrow-Angle Camera of the Cassini spacecraft on June 23, 2011 with a spectral filter sensitive to wavelengths of near-infrared radiation centered at 938 nanometers. Image Credit: NASA/JPL-Caltech/Space Science Institute.

- To get further information on that news, go to: https://arxiv.org/abs/1806.08056.



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