Thursday, April 18

Titan’s dark dunes could be made from comets

THE WOODLANDS, TEXAS — The dark dunes of Titan, Saturn’s largest moon, could have fallen from space.

More than enough cometary material may have struck Titan to have formed its vast dune fields, planetary scientist William Bottke reported March 12 at the Lunar and Planetary Science Conference. Computer simulations suggest that the enigmatic drifts formed from objects hailing from the primordial Kuiper Belt, a modern source of comets beyond the orbit of Neptune. The proposed scenario could also explain the presence of similar material observed on other worlds, said Bottke, of the Southwest Research Institute in Boulder, Colo.

The nature of Titan’s sand has long been pondered. Beneath the moon’s tangerine skies drift some 15 million square kilometers of dusky dunes (SN: 5/23/06). These waves of sand are about as big as the massive dunes found in the United Arab Emirates, says planetary geologist Jani Radebaugh of Brigham Young University in Provo, Utah. Those earthbound mounds are also where the recent Dune films were shot (SN: 3/1/24).

The popular hypothesis contends that Titan’s undulating sands consist of organic particles produced by solar irradiation of its hazy atmosphere (SN: 2/1/19). After these micron-sized particles fall to the surface, they somehow grow larger into sand-sized grains that can form dunes. But it’s not clear how exactly that growth occurs. What’s more, laboratory tests show that the organic particles may break apart too easily to endure being buffeted into dunes, Bottke said.

He and his colleagues propose another scenario, one that begins early in the history of the solar system, roughly 4 billion years ago.

One of the most popular theories for the solar system’s evolution says that the giant planets migrated from where they formed to their current positions (SN: 10/20/23). During this time, those planets are thought to have passed through the Kuiper Belt. That grand reshuffling would have led to the bombardment of Titan and other moons by comets. But many comets would have also smashed together, pulverizing them into tiny particles.

We know a surprising amount about these particles, Bottke said, because many have struck spacecraft and Earth. They’re resilient enough to survive passing through our atmosphere. And they’re dark and often around 200 microns wide, just the right size to build dark dunes on Titan.

Black and white radar image of dunes on Saturn’s moon Titan
This radar image of dunes on Saturn’s moon, Titan, was captured in 2009 by NASA’s Cassini spacecraft.JPL/NASAThis radar image of dunes on Saturn’s moon, Titan, was captured in 2009 by NASA’s Cassini spacecraft.JPL/NASA

Bottke and his colleagues ran computer simulations on how Saturn, Jupiter and their moons evolved during this chaotic period, tracking how much pulverized comet dust and how many large impactors fell on Titan and other moons of Saturn and Jupiter.

Both the dust and the impactors could have delivered more than enough material to account for Titan’s dunes, the team found. “We have two sources that can potentially do this,” Bottke said.

What’s more, the simulations showed that much of the material also struck Jupiter’s moons Callisto and Ganymede and the Saturnian moon Iapetus, all of which are known to have large patches of dark material.

The dark material on Iapetus is thought to have hailed from elsewhere, notes Radebaugh, who was not involved in the research. So it’s plausible that Titan’s sands could have otherworldly origins too, she says.

Nonetheless, it’s unclear whether the material would remain on Titan’s surface after falling. Ice volcanoes may be erupting, or have erupted, on Titan, Radebaugh says. “If you’re resurfacing through volcanism, that would create a problem for this [story].” Eruptions would subsume and bury the old, fallen debris over time.

NASA’s Dragonfly mission to Titan, slated for launch in 2028, could solve the mystery (SN: 6/27/19). “It’s a testable hypothesis,” says Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who works on the mission. Instruments onboard the rotorcraft will be able to take measurements of the compositions of the dune particles, she says.

And so one day, perhaps, a flying machine will confirm that seas of shattered comets ripple on a distant moon.