Where to look for signs of life on Titan

New discoveries, published in the journal Astrobiology suggest that large craters are the preferred places to find the bricks of life on Saturn's largest moon, Titan.

Titan is an icy expanse covered with organic molecules, with liquid methane lakes shrouded by a thick, misty atmosphere of nitrogen and methane that begs the question: why life on this world strangely similar to Earth does not exist? Maybe that's the mild temperature of -179 degrees Celsius (-300 degrees Fahrenheit) on the surface that would likely prevent any biochemical reaction from taking place. But is there a place on Titan where there could be hope that biomolecules, such as amino acids, could be formed?

Using images and data from the Cassini spacecraft and the Huygens spacecraft, scientists led by Dr. Catherine Neish, a global scientist specializing in impact craters at the University of Western Ontario, have been looking for better places to search for biological molecules on Titan's surface. Life, as we know it, is carbon-based and uses liquid water as a solvent. The surface of Titan has abundant carbon-rich molecules (hydrocarbons) that have been found to form amino acids, the building blocks of the proteins necessary for life, when exposed to liquid water in laboratory simulations

. much too cold for the liquid water to be present on the surface. Although this is not a favorable scenario for the formation of living molecules, there is hope.

Crater Erase

Cassini's radar measurements, orbiting Saturn for 13 years, have been able to optically thicken Titan's atmosphere, revealing the terrain of this enigmatic world. What was revealed was unexpected – Titan is active. The Cassini radar instrument unveiled lakes, dunes, mountains, river valleys, and few craters, indicating that there are recurring processes that resurface Titan and fill or erode old craters. The discovery of a world similar to Earth on nine times its distance from the Sun was monumental.

With a landscape so familiar to Earth, where would be the best places to look for signs of life? Although methane lakes may have seemed like the obvious choice, Neish and his colleagues instead found craters and cryovolcans (areas where liquid water gushes under Titan's icy surface) to be the two most enticing. Both features are promising for melting Titan's ice crust in liquid water, a necessary step to form complex biomolecules.

Morgan Cable, a technologist in NASA's Jet Propulsion Laboratory Instrument Concepts and Implementation Unit in Pasadena, California, is an expert on "tholins" (organic products). subject to cosmic radiation). She commented, "When we mix tholines with liquid water, we make amino acids very quickly. Any place where there is liquid water on the surface of Titan or near its surface could generate the precursors of life – biomolecules. we know it, and it's really exciting. "

The craters are the best

With both cryovolcanoes and craters as literal hotspots to melt on Titan, what is the function on which you should bet? For Neish, the answer is unequivocal, although there is not as much on Titan as on our Moon.

"The craters appeared clearly as the winner for three main reasons," says Neish at Astrobiology magazine. "First, we are pretty sure that there are craters on Titan.

The crater is a very common geologic process and we see circular features that are almost certainly craters on the surface," he says. she.

The craters would likely produce more molten material than a cryovolcano, which means that "they take longer to freeze, so that [the water] will remain liquid longer," says Neish, adding that liquid water is essential for complex chemical reactions.

"The last point is that impact craters should produce water at a higher temperature than a cryovolcan," says Neish. Warmer water means faster chemical reaction rates, which is promising for the creation of life-giving molecules.

"The water could remain liquid in these environments for thousands of years or even longer," Cable says

Cryovolcanoes, on the other hand, are not so hot. "When a cryovolcan bursts, it typically bursts just at the melting temperature of the ice, and we think that any 'lava' on Titan would be heavily doped with ammonia, which removes the freezing point a little cold lava, "says Neish.

To put the last nail in the coffin of these frozen volcanoes, cryovolcanism is a more obscure and elusive process. Imagine the ice, less dense than the water, floating in a glass of water. "Trying to get water at the top of the ice is quite difficult when you have a density contrast like that," says Neish. "Cryovolcanism is the hardest thing to do and there is very little evidence on Titan."

In fact, cryovolcanism might not even be real on Titan. "Sotra Facula [a mountainous feature on Titan that appears to have a caldera-like depression] is perhaps the best and the only example we have of a cryovolcano on Titan." Neish adds. "So it's a lot rarer, if it exists."

In situ measurements

Sinlap (112 kilometers / 70 miles in diameter), Selk (90 kilometers / 56 miles) and Menrva Craters (392 kilometers / 244 miles), which are the most large, fresh craters on Titan, are prime places to look when we finally have the capabilities to search for biomolecules in these craters. A probe would need to land on Titan and take steps to make such a discovery. But are these targets the next candidates for a future Titan mission? Not everyone is convinced.

"We do not know where to look, even with results like this," says Dr. David Grinspoon, Senior Scientist at the Planetary Science Institute. "I would not use it to guide our next mission to Titan, it's premature."

Instead, Grinspoon wants to sniff more places on Titan. "Because there are so few things we know about the planet, it makes more sense to characterize a series of environments first," he says.

Nevertheless, although Titan is puzzled, has to start somewhere and the result of this research does not give us one, but three potential candidates to know where to start this research, with much hope to come.

Learn more:
Changes in Titan's surface brightness point to cryovolcanism

More information:
Catherine D. Neish et al. Strategies for detection of biological molecules on Titan, Astrobiology (2018). DOI: 10.1089 / ast.2017.1758