THE WOODLANDS, TEXAS — The dark dunes of Titan, Saturn’s largest moon, may have come from space.
More than enough cometary material could have hit 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 mysterious drifts formed from objects coming from the primordial Kuiper Belt, a current source of comets beyond the orbit of Neptune. The suggested 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 thought about. Beneath the moon’s tangerine skies drift some 10 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 moved from where they were created 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.
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 black material on Iapetus is believed to have come from another place. to have come from another place, says Radebaugh, who was not part of the study. So it’s possible that Titan’s sands may also have come from beyond this world.
However, it’s uncertain if the material would stay on Titan’s surface after falling. Radebaugh suggests that ice volcanoes might be erupting or have erupted on Titan. These eruptions would cover up the old fallen debris over time.
NASA’s Dragonfly mission to Titan, planned to launch in 2028, could solve the mystery (SN: 6/27/19). “It’s a testable idea,” says Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who is involved in the mission. The instruments on the rotorcraft will be able to measure the compositions of the dune particles, she says.
And so one day, maybe, a flying machine will confirm that seas of shattered comets ripple on a faraway moon.
