May 2, 2014
Ganymede Could Be Home To A ‘Club Sandwich’ Of Ice And Water
redOrbit Staff & Wire Reports - Your Universe Online
While astronomers have long believed that Jupiter’s moon Ganymede was home to a thick ocean sandwiched between two layers of ice, a new NASA-funded research project now claims that the largest natural satellite in our solar system could actually have several layers of ice and ocean stacked one upon another.
Writing in the journal Planetary and Space Science, Steve Vance of the Jet Propulsion Laboratory (JPL) in Pasadena, California and his colleagues explain that Ganymede’s ocean could be compared to a club sandwich or a “Dagwood sandwich,” a multi-tiered snack made famous by a character in the Blondie comic strip.
According to NASA, this so-called club sandwich model was first proposed in 2013, but the newly published study has uncovered theoretical evidence to support this notion. The findings are also said to strengthen the idea that the icy moon could once have been home to primitive life, as regions where water and rock are said to be vital to the development of life.
Previously, the rocky sea bottom of Ganymede was believed to have been coated with ice instead of liquid, which would have hindered the emergence of living creatures. However, the new model of its inner structure suggests that the first layer above the moon’s rocky core could actually be salt water, the US space agency explained.
“This is good news for Ganymede,” said Vance, a research scientist with JPL’s planetary chemistry and astrobiology group. “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.”
NASA scientists have reportedly suspected that Ganymede could be home to an ocean since the 1970s, and the presence of the large body of water was confirmed roughly two decades later by the Galileo mission. They found that the ocean is hundreds of miles deep, and there was also evidence suggesting it contained magnesium sulfate.
[ Watch the Video: Ganymede May Harbor ‘Club Sandwich’ Of Oceans And Ice ]
Earlier models of the moon’s oceans assumed that the presence of salt had little impact on the properties of liquids under pressure. However, laboratory experiments conducted by Vance and his associates demonstrated that the salt actually increased the density of liquids under the extreme conditions present on satellites such as Ganymede.
“It may seem strange that salt can make the ocean denser, but you can see for yourself how this works by adding plain old table salt to a glass of water,” the US space agency explained. “Rather than increasing in volume, the liquid shrinks and becomes denser. This is because the salt ions attract water molecules.”
When various forms of ice are added to the mix, things become a little more complicated, according to NASA. The type of ice that typically floats in beverages is the least dense form of ice. It is lighter than water, but in high pressure conditions, the ice crystal structures become more compact. While that specific type of ice is known as “Ice I,” the ice believed to persist on Ganymede is called “Ice VI” and can become so dense that it is heavier than water.
Vance’s team created computer models of these processes, and came up with an ocean that was sandwiched between as many as three different ice layers, as well as the rocky seafloor. The lightest ice is on top, while the saltiest liquid finds its way to the bottom. Furthermore, they discovered a bizarre phenomenon which can cause oceans to essentially “snow upwards.”
“As the oceans churn and cold plumes snake around, ice in the uppermost ocean layer, called ‘Ice III,’ could form in the seawater,” NASA explained. “When ice forms, salts precipitate out. The heavier salts would thus fall downward, and the lighter ice, or ‘snow,’ would float upward. This ‘snow’ melts again before reaching the top of the ocean, possibly leaving slush in the middle of the moon sandwich.”
“We don’t know how long the Dagwood-sandwich structure would exist,” noted Christophe Sotin, who like Vance is a member of JPL’s Icy Worlds team. “This structure represents a stable state, but various factors could mean the moon doesn't reach this stable state.”