Liquid water is suspected on some of Jupiter’s moons. For a long time, astronomers believed that the giant planet itself would “knead” these moons with its gravitational pull and ensure that the water under the surface remains liquid. But now it turns out that it is not Jupiter – it is the moons themselves.
Every six hours. The water comes; the water goes. Ebb and flow.
“Most of us are familiar with rising and falling sea levels. The oceans on the Earth’s surface are attracted to the Moon. This ‘water bulge’ travels around our planet. It is always oriented toward the moon as it does so.”
Antony Trinh of the University of Arizona’s Lunar and Planetary Lab explains tidal friction. It doesn’t just work with our moon and the oceans on Earth. Other moons in the solar system also exert an attraction on everything near them – for example, the moons of the gas giant Jupiter, adds planetary scientist Hamish Hay of California’s Jet Propulsion Laboratory.
“How strong such tidal forces are depends on the mass of the body causing them. After Jupiter itself, the most massive bodies in the Jovian system are the four Galilean moons.”
Ganymede is even larger than Mercury
And those are – from the inside out – Io, Europa, Ganymede and Callisto. These are not small moons. They’re about the size of Earth’s moon – or larger. Ganymede is even larger than the planet Mercury. And yet:
“We had not thought that moons that have 10 to 40,000 times less mass than Jupiter could have such an impact. But according to our studies, it appears that the tidal forces that Jupiter’s moons exert on each other can keep possible oceans liquid beneath their surfaces.”
Antony Trinh and other geologists believe that an even thicker ocean of liquid salt water lies beneath the kilometer-thick ice shell of these moons. To prevent this from freezing over as well, an external energy supply is needed. Until now, scientists thought that Jupiter’s gravitational pull would be sufficient. But is that true?
“The forces that Jupiter exerts are uniform. They don’t change. The moons, on the other hand, orbit Jupiter. So their positions change. Sometimes they are close together; at other times they are far away, on the opposite side of the planet. Whenever two moons come close, the mutual attraction between them reaches a maximum. For example, Ganymede kicks its neighboring moon Europa every time the two pass each other.”
“Europa then looks like a rugby ball”.
This “kick” raises and lowers the ice crust, the water ocean below, the ocean floor and even the rock layer inside the moon. Such momentary lifting and lowering releases heat. And that heat keeps the ocean liquid, Hamish Hay also believes.
“The tidal forces emanating from Ganymede on Europa are really pulling Europa apart. It then looks like a rugby ball. The ocean and the ice layer above it also deform. And the moon responds to that deformation.”
By releasing heat that keeps the water liquid. Only a momentary gravitational influence from a passing celestial body is thought to be capable of doing this. Despite its size – Jupiter itself is not capable of generating such heat waves, Antony Trinh also believes.
“Oceans tens of kilometers deep can only be set in motion by a force that occurs quickly and only for a short time. Jupiter’s perpetual, constant gravitational pull could only set in motion water masses that are shallower, perhaps a few hundred meters.”
In a few years, U.S. and European space probes will explore whether life might even exist in these liquid water oceans. They are currently being prepared for missions to the Jupiter system.