Saturn’s moon Enceladus wears a thick blanket of snow

The fluff suggests the moon’s plume of water vapor was once more active than it is today

A chain of craters on Enceladus looks like a Saturnian snowman.

This chain of craters on Enceladus looks like a Saturnian snowman. It’s actually made from snow draining into cracks underneath.

JPL-Caltech/NASA, Space Science Institute

Saturn’s moon Enceladus is blanketed in a thick layer of snow. In some places, the downy stuff would be deep enough to bury nearly any skyscraper on Earth, new research suggests.

“It’s like Buffalo, but worse,” says planetary scientist Emily Martin, referring to the famously snowy city in New York. The snow depth suggests that Enceladus’ dramatic plume of water vapor and other ingredients may have been more active in the past. Martin was part of a team that reported its new findings in the March 1 Icarus.

Planetary scientists have been fascinated by geysers on Enceladus since the Cassini spacecraft spotted them in 2005. The spray probably comes from a salty ocean beneath an icy shell.

Some of that water goes to form one of Saturn’s rings. Most of it falls back onto the moon’s surface as snow, Martin says. Scientists want to understand the properties of that snow — its thickness and how dense and compact it is. This could help reveal Enceladus’ history. It might also lay groundwork for future missions to this moon.

“If you’re going to land a robot there, you need to understand what it’s going to be landing into,” says Martin. She works at the National Air and Space Museum in Washington, D.C.

To figure out how thick that snow on Enceladus is, Martin’s team looked to Earth — specifically, Iceland. The island nation hosts geological features called pit chains. These lines of pockmarks in the ground form when loose rubble such as rocks, ice or snow drains into a crack below. Similar features show up all over the solar system, including on Enceladus.

Previous work suggested a way to use geometry and the angle at which sunlight hits the surface to measure the depth of the pits. That measurement can then reveal the depth of the material the pits sit in. A few weeks of fieldwork in Iceland back in 2017 and 2018 convinced Martin and her colleagues that the same technique would work on Enceladus.

A person walks near pit chain craters in Iceland.
Planetary scientist Emily Martin and colleagues traveled to Iceland to explore pit-chain craters. The pits, like those shown here, helped the scientists verify that they could measure the depth of craters on Enceladus.E. Martin

So much snow, so little time

Using images from Cassini, the researchers found that snow’s thickness varies across the surface of Enceladus. It is hundreds of meters deep in most places and 700 meters (2,300 feet) deep at its thickest. That would bury the second tallest skyscraper on Earth.

Martin says it’s hard to imagine how all that snow got there, though. If the plume’s spray was always what it is today, it would take 4.5 billion years to blanket the surface with that much snow. And that number has to be wrong: That’s the entire age of the solar system. Even then, the snow would have had to be especially fluffy.

It seems unlikely that the plume switched on the moment the moon formed and never changed, Martin says. And even if it did, later layers of snow would have compressed the earlier ones, making it much less deep than it is today.

“It makes me think we don’t have 4.5 billion years to do this,” Martin says. Instead, the plume must have been much more active in the past. To do it in a much shorter timeframe, she explains, “you need to crank up the volume on the plume.”

The technique was clever, says planetary scientist Shannon MacKenzie. She works at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. Without rovers or astronauts on the ground, there’s no way to scoop up the snow and see how far down it goes. “Instead, the authors are very cleverly using geology to be their rovers, to be their shovels.”

MacKenzie was not involved in the new work, but she led a mission-concept study for an orbiter and lander that might one day visit Enceladus. One of the major questions in that study was where a lander could safely touch down. Researchers had to ask themselves, “What do we expect the surface to be?” MacKenzie says. The new paper could help “identify the places that are too fluffy to land in.”

Lisa Grossman is the astronomy writer at Science News. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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