There's another world out there where it rains in the summer. It's a world where, like Earth, liquid shapes the landscape, and you can find puddles and ponds and lakes and seas.
This is Titan, Saturn's largest moon. Besides Earth, it's the only other planetary object scientists know of in the solar system with a cycle that forms clouds, rain and surface liquid, says Jason Barnes, an associate professor at the University of Idaho who's been studying Titan for years.
"Titan," Barnes says, "is the coolest place in the solar system."
And yet, it's also completely different from Earth. Instead of water, the liquid falling from the sky is methane, which in the frigid temperatures of Titan can be a liquid, solid or a gas. Instead of lava, it's water ice that forms the crust and then oozes out onto the surface.
Now, thanks to research from a team led by Barnes and Rajani Dhingra, one of his students this past year, scientists are understanding more about this world. In January, they published a paper showing evidence of changing seasons and rain on Titan's North Pole, which scientists had previously predicted but were unable to find evidence of. And now, Barnes is waiting for approval to send a spacecraft called Dragonfly to Titan to explore the Earth-like surface.
Because it's not just the weather and climate that's so fascinating about Titan.
It's the possibility of life.
Dhingra used to research Earth's moon before she became interested in Titan. It wasn't just the name "Titan" that was more exciting than our moon, simply called "the moon." Dhingra, like any scientist studying Titan, was drawn to the Earth-like hydrologic process.
"There are so many planets out in the solar system, and then there's Earth. It's special: It has a hydrologic cycle, rain and clouds. And there is nothing like Earth in the solar system," she says, "except Titan."
But there was something scientists still didn't understand about Titan. They knew there were clouds and rain on southern parts of Titan. And they predicted the same on the North Pole — but there wasn't any evidence of rain on the North Pole yet.
"People were calling it the curious case of missing clouds on the North Pole," says Dhingra, who completed her doctorate from the University of Idaho earlier this month.
So Dhingra studied an image from the spacecraft Cassini that was taken in 2016. She noticed a brightly reflected region, which didn't show up in other pictures before. When the team led by her and Barnes analyzed it, they determined the brightness was due to the fact that it had recently rained.
It's the "wet sidewalk effect," Barnes says, because it's kind of like pavement just after it rains. When it's wet, you get a reflection from the surface. That's essentially what happened on Titan, and it was a way for the team to prove it rained on the North Pole, something that was challenging to prove due to Titan's thick atmosphere.
It's beneath that thick atmosphere that Barnes hopes to one day uncover secrets about potential life on Titan.
The haze particles of Titan's atmosphere are made up of organic compounds. If the water ice oozes onto the liquid service, it's possible that water mixing with the organics can form life — much like life was formed on Earth long ago.
It's not impossible that it's forming life currently, Barnes says. There could be water-based life, or the liquid methane in Titan's seas can perform the same role as water when mixed with organics. At this point, neither can be ruled out.
"We want to send a spacecraft to measure the progression," Barnes says. "When you mix organics and water in an abiotic environment, how close do you get to forming life molecules? Were there molecules that might have been precursors to life on Earth?"
It's the kind of thing that's difficult to simulate in a lab. So essentially, the composition of Titan's surface can serve as a sort of planetary laboratory, potentially capable of informing scientists on how life can form.
Barnes says he hopes to hear back soon whether the NASA Dragonfly mission will be approved. If it is approved, then it would launch in 2025, and it wouldn't arrive at Saturn's moon until 2034. Outer space missions take plenty of long-term planning.
And in many ways, the excitement about Titan all starts with the rain.
"That rainfall-based erosion is the most powerful force that governs landscapes on Earth," Barnes says. "And it's one we can't explore anywhere else but Titan." ♦
