August 11, 2021: Can There Be A Prebiotic Chemistry Or A Complex Chemistry In The Low Or Mid-Latitudes Of Titan ?

The Cassini spacecraft as well as the Huygens probe have allowed us to determine that the landscape of Titan is remarkably diverse. Saturn's largest moon is far from being a uniform world dominated by impact craters. Titan has a deep, thick and dense atmosphere that tends to act as a shield against the potential asteroids, meteorites or comets. That opaque atmosphere composed of a complex haze engenders the typical erosional processes that one can encounter on Venus, on Earth or on Mars. For instance, the radar views obtained from the Cassini orbiter have clearly revealed the presence of linear and parallel dunes extending over long distances in the low or mid-latitudes of the giant moon. The infrared or near-infrared views acquired from the Cassini spacecraft unveil a sharp contrast between relatively bright areas and relatively dark areas. The linear and parallel dunes which are reminiscent of the Seif Dunes in the Namib Desert on Earth in particular can be found in the relatively dark areas of the low or mid-latitudes. Prior to the Cassini-Huygens mission in the Saturn System, some planetologists had imagined that the relatively dark regions of the low or mid-latitudes might be seas or oceans dominated by methane and ethane.

The Huygens probe was going to land at a low latitude in the southern hemisphere of Titan. Was it going to land on a desert ? Was it going to land on a sea or ocean rich in hydrocarbons ? The parachuted descent took shape on January 14, 2005. A multitude of panoramic views was captured from the Huygens probe during that breathtaking dive. The views of the landscape during the parachuted descent unveiled bright hills marking a sharp contrast with a brown or dark plain. At first sight, some commentators had interpreted the brown or dark plain as a sea or ocean. But the view obtained from the surface clearly showed that the Huygens module had not landed on a sea or ocean. However, the presence of eroded stones or pebbles in the field of view implies that the probe may have landed on an ancient river, brook or stream. The aerial views have clearly shown striking landscape features in the bright hills. A network of dark channels can be identified in those hills. That network of dark channels likely corresponds to a network of drainage channels closely related to meteorological phenomena. At the time of the observation, the dark channels were probably completely empty or dry.

The remarkable landscape features unveiled in the aerial views demonstrate that it must rain from time to time in the area. Those dark channels fuel our imagination, our reflection or our theories regarding the dynamics or the nature of the meteorology of Saturn's largest moon. Thanks to the huge amount of data taken from the Cassini spacecraft during its long mission in the Saturn System from 2004 to 2017, we have now a better level of understanding of Titan's meteorology. We know, in particular, that the lakes, seas or rivers are mostly found in the high latitudes of the giant moon. The low or mid-latitudes of Titan tend to be relatively dry. But from time to time, during the long Titan year which represents almost 30 Earth years, large cloud systems can develop in the low or mid-latitudes. Those clouds likely dominated by methane can produce strong rainfall events. The Huygens probe found close to the equator may receive rain from time to time due to seasonal factors. Flooding events can be imagined in the low or mid-latitudes from time to time. However, the floods may be particularly ephemeral due to relatively high evaporation processes of liquid methane in the low or mid-latitudes. Does the soil of the low or mid-latitudes act as a sponge against rainfall events of liquid methane ?

The linear and parallel dunes extending over long distances in the low or mid-latitudes of Titan may have been generated or shaped by the erosion related to prevailing winds, rainfall events, floods or even ancient seas or oceans. Researchers believe that those exotic dunes are rich in organics and hydrocarbons. They may regularly receive particles or compounds falling from the relatively complex haze that prevents us from discerning the surface from outer space. A parallel in terms of dynamics can be drawn between those dunes and the Seif Dunes on Earth. However, the nature of the dunes which dominate the relatively dark areas of the low or mid-latitudes on Titan appears really completely different from the various types of dunes one can encounter on our planet. The dunes of Saturn's largest moon have taken shape in a really harsh environment where the ambient temperature evolves around -179 degrees Celsius, -290 degrees Fahrenheit or 94 Kelvin. Planetologists or exobiologists are wondering what type of molecule can be found in those dunes. Can there be complex organics or hydrocarbons on those exotic dunes ? Those dunes may contain molecules like benzene (C6H6) or acetylene (C2H2).

Soon, in the 2030s, a rotorcraft known as Dragonfly will explore the low or mid-latitudes of Titan. The probe will be in a position to study several areas for a relatively long time. Dragonfly will study the dunes and interdunes in the equatorial area. It will be in a position to gather samples and to analyze them. Researchers expect to find organics, hydrocarbons as well as water ice. They also hope to find relatively complex organics or hydrocarbons. Will they discover a prebiotic chemistry ? Will the analysis of the chemistry of organics on Titan help us better understand the rules of nature or the chemistry of life on Earth ? The Dragonfly mission is really ambitious. The rotorcraft is also expected to explore the Selk Crater, a crater located at a latitude of 7 degrees in the northern hemisphere. That crater that is about 80 kilometers wide may contain water ice and organics. A research work entitled "Science Goals and Objectives for the Dragonfly Titan Rotorcraft Relocatable Lander", involving Jason W. Barnes and published in The Planetary Science Journal on July 19, 2021 unveils the multiple goals of the Dragonfly mission. The researchers point out that the Dragonfly mission can potentially be fruitful in terms of exobiology.

Can we find amino acids on Titan ? The amino acids are the building blocks of the typical life we know on Earth. That's why the identification of that type of molecule beyond the Earth appears crucial. On Titan, chemical reactions are expected to be extremely slow due to the particularly low environmental temperature. There can't be stable pools of liquid water on the surface of the Opaque Moon. Thus, the typical lifeform based on liquid water can't be envisaged on the surface of the giant moon. Planetologists envisage, however, a layer dominated by liquid water beneath the external crust. Therefore, a lifeform based on liquid water like on Earth can be potentially imagined in that presumed subsurface ocean rich in liquid water. Planetologists hope to find geysers or cryovolcanoes spewing water molecules. The presumed subsurface ocean rich in liquid water may fuel some cryovolcanoes or geysers on Titan. Therefore, the compounds of the soil close to the hypothetical geysers or cryovolcanoes can tell us a lot regarding the composition of the presumed subsurface ocean or regarding the potential presence of a biological system or a prebiotic chemistry in the hypothetical subsurface ocean rich in liquid water.

Planetologists or exobiologists also envisage the potential presence of a biological system based on liquid methane from the external crust to the upper atmosphere of that giant moon of the Ringed Planet. The atmosphere of the Orange Moon is dominated by molecular nitrogen like the atmosphere of our planet. But, Titan's atmosphere is devoid or almost devoid of oxygen unlike the atmosphere of the Blue Planet. In fact, the second most abundant compound of Titan's atmosphere is methane. Methane can form clouds, fall as rain and form lakes, seas or rivers in the high latitudes of Titan. Therefore, can methane represent a solvent for an exotic lifeform based on methane ? Planetologists and exobiologists are willing to bring significant clues to that question soon or to simply answer that question soon. Life on Earth is mostly composed of water and contains a relatively high concentration of carbon. One can notice that the pools of liquid water on the surface of the Earth have likely played a key role in the emergence and in the development of life on Earth. Thus, one could advance that the emergence and the development of a biosphere imply the stable presence of a liquid largely mobilized by the biosphere on the surface.

The identification of seas or lakes dominated by methane or ethane in the high latitudes of the northern hemisphere of the Opaque Moon makes us realize that liquid methane could play the same type of role on Titan as liquid water on Earth. However, on Titan, liquid methane is much less abundant than liquid water on Earth and chemical reactions are much slower on Titan than on Earth. Two types of ecosystem could potentially be present on Titan and inside Titan. The Dragonfly rotorcraft is not expected to explore the lakes, seas or rivers found in the high latitudes of the northern hemisphere but it may unveil major clues regarding the potential prebiotic chemistry or biological chemistry of Titan. Planetologists hope to gather significant clues regarding the chemistry of organics and hydrocarbons on Titan and regarding the climate or the meteorology of Titan. They are particularly interested in the cycle of methane on that moon. Are there subsurface pockets of methane beneath the external crust or beneath the icy crust ? Is the soil of the exploration site saturated with methane ? Are there long-term cycles in the movement of methane between the south polar region and the north polar region ?

The image above represents a portion of a radar swath obtained from the Cassini orbiter during the T120 Flyby performed on June 7, 2016. Each side of the portion represents about 100 kilometers (around 62 miles). One can notice linear and parallel dunes as well as bright patches marking a significant contrast with those dunes. Planetologists and exobiologists are eager to determine the exact composition of those exotic dunes. Credit for the original image: NASA/JPL/Cassini Radar Team/PDS Image Atlas. Montage credit: Marc Lafferre, 2021.

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