When looking for life outside Earth, Jupiter's moon Europa, with its hypothesized underground ocean, is one of the strongest candidates in the Solar System. Understanding this fascinating moon could prepare us for future missions there, so scientists are carefully looking at how Europa might have gotten its ocean.
One issue to be considered is how this ocean was kept as a liquid, but a team of astronomers from the U.S. has now suggested that Jupiter's gravitational attraction has more of a heating effect than thought. Their prediction is an order of magnitude larger than previous estimates, and it better models the condition this (and maybe other) natural satellites go through.
The results, published in Earth and Planetary Science Letters, focused on the ice grains found on Europa. The researchers ran experiments to recreate the conditions the ice is subjected to, and they discovered that the main source of heat comes from defects in the crystal ice structure, which affects how heat is actually dissipated across the ice shell.
“The beauty of this is that once we get the physics right, it becomes wonderfully extrapolative,” said co-author Reid Cooper from Brown University in a statement.
“Those physics are first order in understanding the thickness of Europa’s shell. In turn, the thickness of the shell relative to the bulk chemistry of the moon is important in understanding the chemistry of that ocean. And if you’re looking for life, then the chemistry of the ocean is a big deal.”
The complex surface of Europa is due to the tidal stress the planet is subjected to. NASA/JPL/University of Arizona/University of Colorado
The first indication of Europa being an active world came from images taken by the Voyager probes in 1979. The satellite was not the smooth, icy ball that was expected, but was instead striped and cracked.
Europa is the sixth moon of Jupiter and orbits the planet in just over three days. It is tidally locked with Jupiter, meaning one face always points towards the gas giant, and it is in orbital resonance with two other moons, Io and Ganymede. The three moons' periods are whole integer ratios of each other. That is to say, Ganymede's revolution is twice as long as Europa’s, which is twice as long as Io’s.
This combination of celestial mechanics, added to the size of Jupiter, stretches and compresses the moons. Io has active volcanoes, and Europa and Ganymede, potentially, liquid oceans.
“[Scientists] had expected to see cold, dead places, but right away they were blown away by their striking surfaces,” said Christine McCarthy, lead author of the research, in the statement.
“There was clearly some sort of tectonic activity – things moving around and cracking. There were also places on Europa that look like melt-through or mushy ice.”
The only way to have tectonics is through heat, and the only way to have heat in such a small object far away from the Sun is through gravitational force. Perhaps this research will bring us closer to understanding just how Europa's ocean is maintained.