July 21, 2018 : A New Study Suggests That We Should Look For Life In Large Impact Craters On Titan

A new study entitled « Strategies for Detecting Biological Molecules on Titan », appearing in the journal Astrobiology in May 2018 and proposed by a team led by Doctor Catherine Neish, a planetary scientist who is an expert in impact cratering at the University of Western Ontario, reveals that large impact craters on Saturn's largest moon Titan may be the best places to look for life, biological molecules or prebiotic chemistry. Many researchers believe that Titan may host the building blocks of life and its impact craters deserve our whole attention. The giant moon of Saturn may have all the ingredients for the development of complex organics. Its deep and thick atmosphere is dominated by nitrogen like our atmosphere and is composed of a significant concentration of methane. There is a haze rich in organics and hydrocarbons in Titan's atmosphere. That haze or smog prevents us from discerning surface features from outer space in the visible spectrum. Some lakes, seas and rivers of methane and ethane have been identified in the polar regions of the Orange Moon. There is also a meteorological cycle involving methane. Therefore, there are similarities between Titan and the Earth. But can there be life in the exotic environment of Saturn's largest moon ?

To a certain extent, the landscape of Titan looks like the landscape of the Earth but let's recall that the environmental temperature on Titan is around -179 degrees Celsius that is to say approximately -300 degrees Fahrenheit. As a result, chemical reactions must be particularly slow on Titan, preventing the development of a biochemistry. Yet, are there locations on that enigmatic moon where we may find biomolecules or building blocks of life such as amino acids ? The team of Catherine Neish used data acquired from the Cassini orbiter and from the Huygens probe to identify key places that may harbor complex organics or even biomolecules. We know that the organisms on Earth are mainly based on carbon and water. Life has developed thanks to liquid water which is a perfect solvent for life. Liquid water can't be present on Titan's surface due to the extremely low environmental temperatures. However, there are liquid hydrocarbons on the surface of Titan and hydrocarbons are rich in carbon and can engender more complex molecules. We know that amino acids can be formed from hydrocarbons if those molecules which are the building blocks of proteins are exposed to liquid water in laboratory experiments.

The radar data acquired from the Cassini spacecraft during its long mission from 2004 to 2017 have allowed us to identify remarkable surface features on Titan. We know, now, that the Opaque Moon is an active world with lakes, seas, rivers, meteorological phenomena and linear and parallel dunes extending over long distances. Like on Earth, there are hills, mountains, canyons, tectonic features, and a few craters on that enigmatic world. The limited amount of craters or impact craters implies that there are active processes on Titan's surface. Winds and rainfall of methane or ethane engender erosion on Titan. That's why there are eroded craters and that's why the number of craters on that strange world is particularly low. The craters can be filled by the sand likely rich in hydrocarbons or they can progressively disappear via typical erosional processes. Planetologists didn't expect to find such a world so far away from the Sun, at about 1.4 billion km from the Sun. What are the key places to explore on Titan if we want to find signs of life or a complex organic chemistry ? Could an exotic lifeform develop in the seas, lakes or rivers of methane or ethane ? That can't be ruled out but Catherine Neish and her collaborators focused their attention on craters and potential cryovolcanoes on the Orange Moon.

Craters on Titan can be related to meteorites or internal activity. If the craters are the outcome of internal activity, they represent cryovolcanoes. If they have been formed via the impact of a meteorite, they represent impact craters. In each case, a lot of energy is involved. Cryovolcanoes can take the shape of domes or calderas and they are likely related to reservoirs of liquid water beneath the crust. There may be geysers or cryovolcanoes of liquid water on Titan. The lava of those exotic volcanoes may be radically different from the typical lava on our world. The impact of meteorites is likely to engender the melting of the presumed icy crust rich in liquid water. Thus, if water can remain liquid long enough, it may allow the development of complex organic molecules or even complex biomolecules such as amino acids or proteins. Titan is believed to contain a large amount of tholins on its surface. Tholins represent complex organics engendered by the interactions between molecules, ions or particles in Titan's upper atmosphere and the radiations from the Sun. Ultraviolet light from the Sun may play a key role in that photochemical process.

The Doctor Morgan Cable who is a technologist in the Instrument Systems Implementation and Concepts Section at Nasa's Jet Propulsion Laboratory, in Pasadena, California and who is a specialist of tholins pointed out : « When we mix tholins with liquid water we make amino acids really fast. So any place where there is liquid water on Titan's surface or near its surface could be generating the precursors to life – biomolecules – that would be important for life as we know it, and that's really exciting. » Catherine Neish and her colleagues believe that craters on Titan, which are quite rare as opposed to the craters of the Moon, are more interesting than the presumed cryovolcanoes of Titan in terms of exobiology because of the level of energy they have engendered in particular. She pointed out : « Craters really emerged as the clear winner for three main reasons. » She advanced : « One, is that we're pretty sure there are craters on Titan. Cratering is a very common geologic process and we see circular features that are almost certainly craters on the surface. » The second reason is that craters related to impact events would likely produce more melt than a cryovolcano, implying that « they take longer to freeze so [the water] will stay liquid for longer. » She insisted on the fact that liquid water is a major factor for the development of complex chemical reactions.

Catherine Neish concluded : « The last point is that impact craters should produce water that's at a higher temperature than a cryovolcano. » An increased level of water temperature implies an increased level of energy and faster chemical reactions or more chemical reactions during the same amount of time. Complex organics like amino acids, proteins or self-replicating molecules could take shape in a solvent with a warmer environment. Could we encounter pools of liquid water on Titan's surface or near Titan's surface for long periods of time from the impact events ? Morgan Cable advanced : « Water could stay liquid in those environments for thousands of years, or even longer. » Cryovolcanoes which spew water, ammonia or hydrocarbons may produce much less energy than the collision events between a meteorite and the soil of the Opaque Moon. Catherine Neish argued : « When a cryovolcano erupts, it typically erupts right at the melting temperature of the ice, and we think any 'lava' [in this case, a slushy form of water] on Titan would be heavily doped with ammonia, which suppresses the freezing point quite a bit so that would make the lava pretty cold. » In fact, planetologists don't know the exact composition of the presumed cryolava of Titan.

Cryovolcanism which has been observed on the tiny moon of Saturn Enceladus as well as on Neptune's moon Triton is quite mysterious to us. Water ice is known to be less dense than liquid water which implies that it naturally floats on liquid water and that typical icebergs float on the ocean. Catherine Neish argued : « Trying to get the water up on the top of the ice is quite difficult when you have a density contrast like that. » She added : « Cryovolcanism is the harder thing to do and there is very little evidence of it on Titan. » A few landscape features of Titan may represent ice volcanoes or cryovolcanoes but researchers are not sure about it. The difference between an impact crater and a caldera is not always obvious. Catherine Neish advanced : « Sotra Facula [a mountainous feature on Titan that appears to have a caldera-like depression] is perhaps the best and only example that we have of a cryovolcano on Titan. » She pointed out : « So it's much rarer, if it exists at all. » However, there may be geysers erupting from fractures like in the south polar region of the bright icy moon Enceladus but the existence of typical cryovolcanoes on Titan has to be proven.

The planetologists believe that the Sinlap crater that is 112 kilometers or 70 miles in diameter, the Selk crater that is 90 kilometers or 56 miles in diameter and the Menrva crater that is 392 kilometers or 244 miles in diameter which represent significant and fresh craters on the Opaque Moon are key places to study in our search for biomolecules or complex organics. We would have to send a probe in situ in order to find organisms, amino acids, proteins, sugars or biomolecules. Currently, there are no plans to send probes to those specific locations and the choice for future missions to the surface of the giant moon of Saturn is difficult. The Doctor David Grinspoon who is a Senior Scientist at the Planetary Science Institute pointed out : « We don't know where to search even with results like this. » He added : « I woudn't use it to guide our next mission to Titan. It's premature. » He wants to discover more about Titan because Titan's landscape is very diverse and complex. He advanced : « Because there is so little that we actually know about the planet, it makes more sense to characterize a range of environments first. » The team of Catherine Neish thinks that we clearly have, on the basis of the outcome of the analysis of the group, three major candidates for the exploration of Titan and the search for exobiological phenomena. The study was in part supported through the Planetary Data Archiving, Restoration, and Tools (PDART) program. NASA Astrobiology brings resources for the analytical work and other Research and Analysis programs within the Science Mission Directorate (SMD) of NASA that gathers or encourages proposals related to astrobiology studies.
 

- To get further information on that news, go to: https://www.astrobio.net/news-exclusive/where-to-search-for-signs-of-life-on-titan and https://www.liebertpub.com/doi/abs/10.1089/ast.2017.1758?journalCode=ast.

 

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