Saturn’s moon Enceladus is covered in a thick layer of snow. According to new research, in some places the depth of the plume reaches 700 meters.
“It’s like Buffalo, but worse,” says planetary scientist Emily Martin, referring to the famously snowy city in New York. The depth of the snow suggests that Enceladus’ dramatic plume could have been more active in the past Martin and colleagues report in Icarus from March 1.
Planetary scientists have been fascinated by Enceladus’ geysers, which consist of water vapor and other ingredients , after the Cassini spacecraft spotted them in 2005. The spray probably comes from a salty ocean beneath the icy shell.
Part of this water goes to formation of one of Saturn’s rings. But most falls back to the moon’s surface as snow, Martin says. Understanding the properties of this snow—its thickness, its density, and its compactness—could help unravel Enceladus’ history and lay the groundwork for future missions to the moon.
“If you’re going to put a robot in there, you need to understand where it’s going to land,” says Martin of the National Air and Space Museum in Washington, DC.
To determine how thick the snow is on Enceladus, Martin and his colleagues looked to Earth, namely Iceland. The island country contains geological features called pit chains , which are lines of rippled spots in the ground that form when loose debris, such as rocks, ice, or snow, flows into a crack below. Similar features appear throughout the Solar System, including Enceladus.
Previous work proposed a way to use geometry and the angle at which sunlight hits the surface to measure the depth of pits. This measurement can then reveal the depth of the material in which the pits sit. Several weeks of fieldwork in Iceland in 2017 and 2018 convinced Martin and her colleagues that the same technique would work on Enceladus.
Using images from Cassini, Martin and his colleagues found that the thickness of snow on the surface of Enceladus varies. In most places, its depth is hundreds of meters, and the thickest is 700 meters.
It’s hard to imagine how all that snow got there, Martin says. If the spray plume had always been as it is now, it would have taken 4.5 billion years—the entire age of the Solar System—to deposit that much snow on the surface. Even then, the snow should be especially fluffy.
It seems unlikely that the plume turned on at the time the moon formed and hasn’t changed, Martin says. And even if that were the case, the later layers of snow would have compressed the earlier ones, compacting the entire layer and making it much smaller than it is today.
“It makes me think we don’t have 4.5 billion years to do it,” Martin says. Instead, the plume may have been much more active in the past. “We have to do it in a much shorter time. We need to turn up the volume on the loop.”
The technique was clever, says planetary scientist Shannon McKenzie of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. Without rovers or astronauts on the ground, it is impossible to scoop snow and see how far it falls. “Instead, the authors very cleverly use geology as their rovers, as their shovels.”
McKenzie was not involved in the new work, but she led research into a mission concept for an orbiter and lander that could one day visit Enceladus. One of the main questions in this study was where the lander could land safely. “A key point in these discussions was what kind of surface do we expect?” she says A new paper could help “identify places that are too fluffy to land on.”
