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SYFY WIRE Bad Astronomy

Another Ocean in Space: Jupiter’s Moon Ganymede

By Phil Plait

Hot on the heels of the announcement that hydrothermal vents have been found on Saturn’s moon Enceladus, we get another watery alien world announcement: Indirect evidence has been found for a salty ocean under the surface of Jupiter’s moon Ganymede!

Ganymede is a huge moon, bigger than Mercury, and edging in on Mars-sized territory (Ganymede is 5,270 kilometers in diameter; Mercury is 4,880 and Mars 6,780). Its density is low, meaning it probably is a mixture of rock and ice—that’s common for moons in the outer planets. In the 1990s, the Galileo spacecraft flew past the moon several times, and careful measurements of its magnetic field indicated it might have an ocean of liquid water deep under its surface.

New observations of the moon using Hubble support this. Ganymede has a weak magnetic field, and, like on Earth, this generates an aurora—the glow created when high-speed subatomic particles slam into the extremely thin atmosphere. This glow is brightest in ultraviolet, and so astronomers used the Space Telescope Imaging Spectrograph (my old camera!) on Hubble to observe Ganymede. STIS is quite sensitive to UV and detected the aurora.

Now this part is a bit tricky: Jupiter has a powerful magnetic field as well, which interacts with Ganymede’s. As they do, the aurora changes position over time, moving up and down in latitude. However, the observations show that the aurorae do not change nearly as much as expected if Ganymede were solid. The best way to explain this is if the moon has a salty ocean under its surface. The ocean would have its own magnetic field and would resist the influence of Jupiter’s magnetic field, which in turn keeps the aurora steadier.

This ocean is probably located about 150 km deep under Ganymede’s surface and may extend another 100 km deeper. If so, then it contains more water than all the oceans on Earth! That’s astonishing.

Speculation time: Interestingly, older observations indicate the presence of various salts (like magnesium sulfate) on the surface. So we have a salty ocean, and salt on the surface … that makes me wonder if the ocean is interacting with the surface in some way, maybe leaking up through cracks. Given how deep the ocean is that seems unlikely, but geez, given all these new and delightful discoveries I wouldn’t rule it out just yet.

Surface/subsurface interactions are important because the radiation from Jupiter can interact with the surface too, creating simple organic molecules. If those can get down into the water … well, like I said, this is all speculation. But it’s interesting.

And clearly an investigatory path we should follow. This is all very new, and so we’re taking our first steps of discovery here. With oceans under Enceladus and Jupiter’s moon Europa, as well as possibly under Saturn’s giant moon Titan as well, it seems that outer space is loaded with watery worlds. They may not be what we first expected, but hey, nature is smarter than we are. Science is all about learning what it’s trying to show us.

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