In a combination of the astronomical and the culinary, scientists at NASA’s Jet Propulsion Laboratory (JPL) say that Jupiter’s largest moon, Ganymede, may not have a single large ocean, but instead may be built like a club sandwich with alternating layers of ice and water. The claim is based on computer models of how salt water acts under the high pressures that may exist beneath Ganymede’s global ice pack, and may improve the chances of finding life elsewhere in the Solar System.
Ganymede is not only Jupiter’s largest moon, but the largest in the Solar System with a diameter of 5,268 km (3,273 mi). At that size, it’s not only larger than the Moon, but even slightly larger than the planet Mercury. Its surface is covered entirely by ice.
Since the first Pioneer robotic space probes flew by in the 1970s, scientists have speculated that the gravitational tides caused by the pull of Jupiter on the moon warmed it sufficiently to melt the ice below the surface, producing a gigantic subterranean ocean. The Galileo orbiter spacecraft’s findings seem to confirm that some sort of ocean is beneath the ice, but the question remains, what is it like?
The usual idea is that Ganymede is like a giant frozen lake with a layer of ice perhaps 800 km (497 mi) thick floating on it. For those scientists seeking life on other planets, this seemed a reason for optimism. Over the years, there’s been a great deal of speculation about whether the Ganymedean ocean supported primitive life, but then scientists realized that the water could be 100 km (60 mi) deep with a volume five times that of Earth’s combined oceans. Life needs more than water survive. It also needs minerals, which on Earth are supplied by runoff from the land or vented from undersea volcanoes. However, on Ganymede these minerals are only available from the sea bottom. That put a spanner into the works because while ice normally floats, its properties change under extreme pressure. The ice crystals are squeezed together, making the ice denser than the surrounding water and instead of floating, it sinks to the bottom. The result is a thick layer of ice on the seabed, sealing off the vital minerals.
Club sandwichWhat the NASA team did was take a closer look at how water might work on Ganymede, based on its composition. Through laboratory experiments and computer modelling, they found that adding salt made the water denser; much like the party trick of making an egg float in a tumbler of water by emptying a salt shaker into the glass.
In the ocean of Ganymede with its extreme depth, the team says that the water sorts itself into different layers. "Ganymede’s ocean might be organized like a Dagwood sandwich," says Steve Vance of JPL, referring to the “Blondie” cartoon character’s trademark giant sandwiches.
These layers are based on salinity, with the most salty on the bottom and the least at the top. These layers form different types of ice, which floats to the top of each layer in a process the team compares to “snowing upwards” to form a cap of ice or slush, resulting in alternating layers of water and slush, similar to that of a complicated sandwich.
Life on a sandwichAccording to NASA, for the perspectives of life on Ganymede, the club sandwich model is a reprieve because there may be a liquid layer on the bottom, freeing the minerals. "This is good news for Ganymede," says Vance. "Its ocean is huge, with enormous pressures, so it was thought that dense ice had to form at the bottom of the ocean. When we added salts to our models, we came up with liquids dense enough to sink to the sea floor."
According to the team, the club sandwich model could mean the top ocean layer may also be salty and mineral-rich, including magnesium sulfate. Whether this layered structure actually exists at the moment is another question. It could be that Ganymede goes through many periods of layering and unlayering.
“We don’t know how long the Dagwood-sandwich structure would exist," says JPL scientist Christophe Sotin. "This structure represents a stable state, but various factors could mean the moon doesn't reach this stable state.”
The team’s findings were recently published in Planetary and Space Science.
The video below shows the hypothetical structure of the “moonwich”