Ancient lake on Titan could have lasted thousands of years
Saturn’s large moon Titan is similar in many ways to Earth a few billion years ago, when life first emerged. One site of interest is what we call Selk Crater on Titan. It’s one of the places that NASA’s upcoming Dragonfly mission – a plan to send a robotic rotorcraft to the surface of Saturn’s moon – intends to explore. Researchers have suggested that liquid water might have existed in the crater, following the impact that created it. And now a new study from researchers at MIT in Cambridge, Massachusetts, suggests that liquid water might have existed in Selk Crater for a longer time than previously thought, perhaps for as long as tens of thousands of years. It might have been there long enough for some of the building blocks of life to begin to take form.
Kiona Smith wrote in Inverse on March 1, 2023, about the intriguing findings. As Smith pointed out, the new timeline for liquid water in Selk Crater:
… dramatically increases the odds that Dragonfly, whose mission is to search for the chemical ingredients for life, could find something interesting.
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How long did Selk Crater lake on Titan last?
Planetary scientist Shigeru Wakita of MIT led the research team. The new work – which is not yet peer-reviewed – was published as a preprint on February 22, 2023.
Selk Crater is about 50 miles (80 km) across. It’s just north of Titan’s equator, not too far northwest of the landing site of ESA’s Huygens probe, which successfully landed on Titan in 2005.
Dragonfly will explore this crater as a key part of its mission. Why the interest? Titan has an icy crust. And the impact might be from an icy comet. It’s possible that water from either or both of these sources filled the crater after the impact.
But a key question has been, how long did the water last?
2 possible types of icy crust
Researchers think other places on Titan might have had liquid water lakes from comet impacts. How long the water flowed there depends in part on the composition of Titan’s crust. The ice on Titan’s surface is mostly hydrocarbons, like methane. But underneath, in the bedrock, there’s thought to be more water ice.
But is it only water ice, or is it a combination of water ice and hydrocarbons? Scientists don’t know which it is for sure yet. And it makes a big difference in terms of how long the water could have remained liquid.
Ultimately, how long Titan’s Selk Crater lake remained liquid comes down to the type of crust beneath it. If the crust only contained frozen water, then scientists estimate the lake would have lasted several centuries before freezing.
But what if the crust also contained methane clathrates? A methane clathrate is composed mostly of water ice. But, it also has molecules of methane within its crystalline structure. You might know of it by the names fire ice or burning ice. And, indeed you can set methane clathrates on fire! The methane will burn while the water ice melts.
If the crust contained methane clathrates when the comet struck Titan, to form the crater, then the resulting lake could have persisted much longer than it would have otherwise. The crystalline structure of methane clathrates is stronger than that of pure water ice. The resulting crater – and lake – would also have been deeper and of a different shape than it would have been if Titan’s crust contained water ice only.
Insulated lake on Titan could stay liquid for thousands of years
But not only are methane clathrates stronger than water ice, they are also good insulators. Wakita told Inverse:
There are two interesting works on methane clathrate. One indicated that methane clathrate is stronger than water ice, and another showed methane clathrate can be an insulator.
Both are important for the crater formation.
Why is this significant? It means that, due to the increased insulation of the water inside the crater, that the water would have remained liquid for a longer period of time. Wakita and his team estimated that Selk Crater contained a pool of water six miles (10 km) across and two miles (three km) deep. This crater lake could have lasted anywhere from 5,000 to 90,000 years.
Formation of the building blocks of life
The lake, and any others like it, would have been an oasis compared to the rest of Titan’s hostile surface. In fact, if the lake survived long enough, then chemical reactions might have formed complex organic molecules within the lake … not life, but the building blocks of life, such as amino acids.
Afterwards, eventually, the amino acids might have bonded together to create proteins. The paper pointed out:
Melt pools associated with impact craters like Selk provide environments where liquid water and organics can mix and produce biomolecules like amino acids. It is partly for this reason that the Selk region has been selected as the area that NASA’s Dragonfly mission will explore and address one of its primary goals: to search for biological signatures on Titan.
If a thick layer of methane clathrate existed on Titan’s surface when Selk was formed, there was ample time for complex prebiotic chemistry to occur in the melt pool, increasing the likelihood of producing molecules of biological interest.
Measurements by the Dragonfly Mass Spectrometer instrument on Dragonfly (DraMS) will be able to confirm these predictions.
What will Dragonfly find?
Dragonfly will be designed to determine just what kinds of complex organic molecules still exist on Titan today, if any do exist, including those within Selk Crater. The fact is, Titan is literally covered in simpler organics called tholins. They form dunes on the surface and fill the atmosphere with an organic orange-ish smog (which is why we can’t see the moon’s surface directly). Even the lakes and seas that do still exist on Titan today are composed of liquid hydrocarbons – methane and ethane – instead of water.
Is there any chance those first complex organics could have evolved even further, into life itself? It’s possible, although it would be quite different than any life on Earth. Titan’s bone-crushing freezing temperatures would make it difficult for any life to exist today.
But, for now, we don’t know what’s possible.
Bottom line: Researchers at MIT say that a former liquid water lake on Titan – Saturn’s large moon – might have existed for as long as tens of thousands of years. The crater is a target of NASA’s upcoming Dragonfly mission.
Source: Modeling the formation of Selk impact crater on Titan: Implications for Dragonfly