Week in Geek: Water, water everywhere

Interiors of icy bodies in the solar system (as of 2010) (Credit: Doug Ellison, Emily Lakdawalla, and Bob Pappalardo/CC BY 3.0)

We have seen a flood (pun intended!) of news lately about water elsewhere in the Solar System, both past and present. There is compelling scientific evidence now for a massive ocean on Mars at one time, hydrothermal activity on Saturn’s moon Enceladus, and a subterranean saltwater ocean on Jupiter’s moon Ganymede. While the first two are more like updates on water already known, the latter is BIG NEWS. First hinted at by measurements of Ganymede’s magnetic field made by the Galileo probe in 2002, new observations by the Hubble Space Telescope of aurorae on the moon strongly support the theory of an underground ocean.

Artist’s cut-away representation of the internal structure of Ganymede. Layers drawn to scale. (Kelvinsong/CC BY SA 3.0)

Ganymede is the largest of Jupiter’s moons (over 3,000 miles in diameter) and is one of the four Galilean moons originally discovered by Galileo himself in the early 1600’s. It orbits Jupiter about once every seven days at a distance of over 650,000 miles, almost three times the distance between us and our moon. Ganymede’s iron core means it generates its own magnetic field which can result in aurorae being observed around the north and south pole, just like here on Earth. However, Jupiter has its own magnetic field, the largest and most powerful in the Solar System (save the Sun), which extends well beyond Ganymede’s orbit. As a result, any measurements of Ganymede’s magnetic field are in fact a combination of the magnetic fields of both the moon and its planet.

Here comes the cool part.

NASA Hubble Space Telescope images of Ganymede’s auroral belts (colored blue in this illustration) are overlaid on a Galileo orbiter image of the moon. The amount of rocking of the moon’s magnetic field suggests that the moon has a subsurface saltwater ocean. (Image Credit: NASA/ESA)

Jupiter’s magnetic field causes aurorae on Ganymede to oscillate up and down over an average latitude. The extent of the oscillations can be observed and then compared with what we would expect based on Ganymede’s composition. If Ganymede had a solid rocky exterior, the oscillations would be larger (approximately +/- 6 degrees in latitude) than if it had a saltwater ocean beneath its outer crust (closer to +/- 2 degrees in latitude). This is because the component of Ganymede’s magnetic field generated by the existence of an ocean would “fight” against Jupiter’s magnetic field and dampen its overall influence. The new results from Hubble suggest this is exactly what is observed.

Kudos to the German scientists from the University of Cologne, led by Joachim Saur, who came up with the idea of using aurorae as a way to probe the moon’s interior structure:

“I was always brainstorming how we could use a telescope in other ways,” said Saur. “Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior.”

Here’s some non-Jovian geek from the week:

Keep on geeking!

@Summer_Ash, In-house Astrophysicist


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Week in Geek: Water, water everywhere