Hi, comicbook friends. Any fan of Warren Ellis out there? Me, I like Chris Sprouse as well, so it is only natural I enjoyed Ocean. Set 100 years from now, the story focuses on a UN special weapon inspector sent to Cold Harbor, a UN research station in orbit around Europa. Deep, under the ice surface of that moon, there is an ocean and a set of nonhuman artifacts have been discovered. I’m not going further into the plot, in case anyone wants to read it spoiler-free and also because I don’t need to. It was only natural at the time that Ellis thought of Europa as a background for his plot: the smoothness of the surface has led people to think there might be a water ocean beneath its icy crust, kept liquid by tidal acceleration. Nasa also reported detection of water vapor plumes in 2013.
However, it turns out it’s its bigger sister Ganimede who might be proven to yield an ocean first. NASA’s Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter’s largest moon. The subterranean ocean is thought to have more water than all the water on Earth’s surface.
And, because science is cool like that, they figured it out by observing Ganymede’s aurorae borealis. The idea behind this statement is pretty simple. Aurorae appear when plasma from a solar storm reaches Earth. Our electromagnetic field basically works like a shield that deflects particles from the storm.
The shape of the magnetic field lets some particles reach our magnetic poles, both when it moves past us and backwards,sort of like a wave in the sea that hits you when the water retreats. An excellent explanation of the full process can be watched in the following 4 minutes video.
So far, so good, but what does this have to do with Ganymede? Easy. Ganymede is the only moon in our system that has its own magnetic field and, hence, aurorae in regions circling the north and south poles of the moon. However, since it is so close to Jupiter, its own magnetic field interacts with Jupiter’s. As a result, its aurorae “rock” back and forth. The plot below, for example, shows how our geomagnetic field changes depending on Earth orbital motion due to interaction with the interplanetary magnetic field brought to you by courtesy of the solar wind (see full explanation on A Hitchhiker’s Guide to Space and Plasma Physics).
According to calculations, given the proximity of Jupiter and Ganymede, aurorae should rock up to 6 degrees. However, if a saltwater ocean were present, Jupiter’s magnetic field would create a secondary magnetic field in the ocean1 that would counter Jupiter’s field. That second field would actually suppress the rocking of the aurorae. And, ta-daaa, indeed they rock only 2 degrees according to estimations!
Scientists estimate the ocean is 60 miles (100 kilometers) thick — 10 times deeper than Earth’s oceans — and is buried under a 95-mile (150-kilometer) crust of mostly ice.
1. Dissolved salts in seawater conduct electricity, and as ocean currents move within the planet main magnetic field, they generate their own secondary magnetic field (see here).