And today, Hubble Space Telescope researchers announced they've found one more.
They have evidence of a subsurface saltwater ocean on ice-covered Ganymede, Jupiter's largest moon — which is also the largest moon in the solar system. If their calculations are correct, Ganymede could have more water than Earth's entire surface does.
This much water on Ganymede would primarily be exciting because it'd raise the possibility of life evolving there. But whether or not there's life on Ganymede, this string of discoveries shows that there's far more liquid water in our solar system — and perhaps across our galaxy — than we ever realized.
The intriguing evidence for Ganymede's ocean
Scientists have suspected for decades that Ganymede might have an ocean. Recently, a group of researchers led by Joachim Saur of the University of Cologne in Germany used Hubble telescope observations to provide the best evidence so far.
They did so by carefully watching a pair of aurorae present on Ganymede's northern and southern hemispheres. Just like the aurora borealis on Earth, these phenomena result from charged particles approaching Ganymede from space and interacting with its magnetic field.
But Ganymede is quite close to Jupiter, which has a much more powerful magnetic field. So as Ganymede orbits Jupiter, the two fields interact, causing the aurorae to rock back and forth slightly.
The scientists, though, realized that a third factor might also impact this movement: a saltwater ocean. Because of the way it would conduct charged particles and create its own, smaller magnetic field, an ocean on Ganymede would work against Jupiter's magnetic field, slightly reducing the rocking of the aurorae over time.
Observations of the aurorae, captured using ultraviolet light sensors, confirmed that this was indeed happening. Calculations indicate that the ocean is buried under 95 feet of ice and is roughly 60 miles deep — 10 times deeper than the oceans on Earth.
Could there be life on Ganymede?
There's a big reason why scientists are so excited about the newly discovered oceans on Europa, Enceladus, and Ganymede. "If we've learned anything about life on Earth, it's that where you find the liquid water, you find life," Kevin Hand, a NASA planetary scientist, told me for a recent article on Europa.
Still, there are thought to be two other big requirements for life: organic compounds (carbon-based molecules that serve as the building blocks for all life on Earth) and an energy source.
On the one hand, Ganymede might seem like a promising candidate for life because it's so big. An Earth-size ocean could theoretically provide ample opportunity for a suitable environment to occur. (By comparison, the ocean on Saturn's moon Enceladus is merely the volume of Lake Superior.)
But Enceladus and Europa may have a big advantage over Ganymede. They both have liquid oceans that are thought to be in contact with rock, increasing the chance of organic compounds and energy migrating into the water. Yesterday's finding that there are hydrothermal vents on Enceladus, for instance, makes it seem quite plausible that all three of the key ingredients for life are right there in that moon's oceans.
The scientists' model for Ganymede, however, suggests that the ocean is trapped between multiple layers of ice. If that's true, it might effectively seal off the ocean, preventing any sort of energy source or carbon-based molecules from entering it.
If that's the case, though, this finding could still be pretty exciting for an entire different reason. As we study our solar system, we're finding that liquid oceans are surprisingly abundant, raising the possibility that life might evolve at relatively far distances from the Sun.
It could mean that there are lots of ice-covered oceans all around the galaxy — greatly increasing the chance that we're not alone in the universe.