21.03.2021

Saturn’s frigid moon Titan has long intrigued scientists searching for life in the Solar System. Its surface is coated in organic hydrocarbons, and its icy crust is thought to cover a watery ocean. An asteroid or comet slamming into the moon could theoretically mix these two ingredients, according to a new study, with the resulting impact craters providing an ideal place for life to get started.

The idea is “very exciting,” says Léa Bonnefoy, a planetary scientist and Titan expert at the University of Paris. “If you have a lot of liquid water creating a temporary warm pool on the surface, then you can have conditions that would be favorable for life,” she says. And, “If you have organic material cycling from the surface into the ocean, then that makes the ocean a bit more habitable.”

Scientists have believed an ocean sits about 100 kilometers below Titan’s crust ever since 2012, when NASA’s Cassini mission measured sight variations in the moon’s tides. Alvaro Penteado Crósta, a planetary geologist at the University of Campinas, knew the moon was pocked with many large impact craters. He wondered whether any of the impacts were big enough to pierce the crust and churn up the surface’s organic material with the water below. That may have produced “a primordial soup that you would need for life to develop,” Penteado Crósta says.

To find out, he and his colleagues modeled the impact for the moon’s largest crater, 425-kilometer-wide Menrva, thought to have formed 1 billion years ago. The model suggested the crater resulted from a 34-kilometer-wide space rock hitting the surface at 7 kilometers per second.

The heat of the impact would have created a lake in the crater, according to the model, which the team presented this week at the Lunar and Planetary Science Conference. The lake would likely only have existed for 1 million years before freezing over in Titan’s frosty temperatures. But Penteado Crósta says this may have been enough time for microbes to evolve, taking advantage of liquid water, organic molecules, and heat from the impact. “That’s pretty good for bacteria.”

Although the team’s research focused on Menrva, Penteado Crósta says it is possible that smaller impacts were sufficient to break through Titan’s ice shell, perhaps even at Selk—a 90-kilometer-wide crater about 5000 kilometers away. Selk is thought to be much younger than Menrva, perhaps just a few hundred million years old, which would mean any evidence of life there would be fresher. “Selk may have more chance to have some sort of fossilized bacteria preserved in the ice,” Penteado Crósta says.

Selk is the planned landing site for NASA’s Dragonfly mission, a $1 billion autonomous and nuclear-powered drone set to launch in 2027 and arrive on Titan 2036. If the impact did break the ice crust here, the mission could find out.

But Elizabeth Turtle, principal investigator for the Dragonfly mission at the Johns Hopkins University Applied Physics Laboratory, isn’t so sure it did. “There isn’t strong evidence to suggest you actually had puncturing,” she says.

Still, Dragonfly could visit other craters in an extended mission. And although Menrva might be too distant, it could be an intriguing landing site in future, Penteado Crósta says.

Quelle: AAAS