Saturn's strange moon Titan hides many of its secrets behind layers and layers of thick haze, but scientists have now peered through the haze in a new way — and spotted a massive stretch of water ice to boot.
That ice block stretches across nearly half of Titan's girth. The feature was a surprise companion to the patches of water ice scientists expected to find, and they aren't positive precisely what sort of geologic feature it might indicate. The research is based on data gathered by NASA's Cassini spacecraft, which spent 13 years studying the Saturn system and made more than 100 flybys of the massive moon before self-destructing in September 2017.
"It's a good example of how we're doing really well at continuing to mine these amazing Cassini data for new results," Jani Radebaugh, a planetary scientist at Brigham Young University in Utah who wasn't involved in the new research, told Space.com. "We're far from being finished with understanding Titan to the degree we can with Cassini."
Scientists want to understand Titan because its features provide an eerie twist on the planet we know and love so well. The moon has a thick, nitrogen-rich atmosphere, and liquids rain down onto its surface to fill lakes and seas. But those liquids are organic compounds falling onto a surface made, in part, of water ice. It's weird. It's also a struggle to study, since all those weird things get in the way of each other.
"What we're curious about is, beyond that global gentle snowfall of organics, what's happening?" Radebaugh said. "It can be really hard to see through that layer to be able to see what's going on."
That's where the technique used in the new paper comes in. The new research uses a statistical approach called principal component analysis, which lets the scientists look past the most dominant features in the data — like the haze — and pick up on much smaller signatures that otherwise get missed — like water ice. In the new study, the scientists applied the technique only to the swath of Titan between 30 degrees north and south latitude.
"The idea was to say 'Let's try to get a global sense of what the distribution of ice is on the surface compared to the amount of organics on the surface,'" lead author Caitlin Griffith, a planetary scientist at the University of Arizona, told Space.com. "We needed a method for measuring these very, very weak features and try to pull out information from these features."
Principal component analysis allowed Griffith and her colleagues do precisely that. "We're looking for very subtle features that are hidden behind bigger features," Griffith said. "That works like a charm actually, so that allowed us to get very detailed information on these very weak features" — like where water ice was visible at the surface.
Some of the regions where the team found water ice were what scientists expected — around what they believe is a large cryovolcano that once erupted liquid water across Titan's surface and around impact craters where meteorites dug into the moon's surface. But no one expected the massive belt of water ice — 3,900 miles (6,300 kilometers) long — that also turned up in the same analysis.
"There's really nothing that tells us that it should be made of water ice, even just as you look at the landscape," Radebaugh said. "There's nothing that really says this is different from everything else."
Griffith said she isn't sure yet what the massive line of ice, which she compares to a scar, represents — it will take more research to pin down what left the ice uncovered on the surface of Titan. "It's a big feature that tells us something about the way that Titan was in the past, but we don't know really what it is," she said. "I think right now it's basically telling us that it's complicated, the surface is fairly complicated."
Radebaugh said that from her perspective outside of the team, it seems like the feature could be the result of a massive faulting — cracking — event that hefted a stretch of water-ice bedrock upward and left it uncovered. If that turns out to be the case, the feature could offer a window into a layer of Titan that has not yet been in the spotlight.
"Tectonism on Titan in some ways has taken a little bit of a back seat just because we see all of these surface processes — the fluid flow, the erosion, the wind deposits, and all of those kinds of things," Radebaugh said. "These surface processes, those things are sort of in your face."
But just because something is hidden doesn't mean it deserves to remain that way, she added. "We need to remember that there's a very interesting, a possibly active, lithosphere beneath that as well."
The research is described in a paper published today (April 29) in the journal Nature Astronomy.
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