The Ocean and Aurora of Ganymede

When most people think of oceans, they think of Earth’s oceans, waves eternally lapping their shores and water accessible from the surface. In other parts of the solar system, however, oceans are below the surface, covered by a layer of ice. Jupiter’s moon Ganymede has been considered as one such world with a possible subsurface ocean, but data has been inconclusive.
Ganymede, pictured below, is the largest moon in the solar system and has its own magnetic field within the magnetic field of Jupiter. Aurora have been noted. Like other aurora, they are influenced by the magnetic field environment they are in. The clever authors of today’s papers decided to use these auroras to see if Ganymede has a subsurface ocean under its layer of ice.
A color-enhanced image of Ganymede. Image credit Galileo Project, DLR, JPL, NASA. Astronomy Picture of the Day on September 20th, 2009.
A color-enhanced image of Ganymede. Image credit Galileo Project, DLR, JPL, NASA. Astronomy Picture of the Day on September 20th, 2009.
Today’s article is “The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals” by Joachim Saur, Stefan Duling, Lorenz Roth, Xianzhe Jia, Darrell F. Strobel, Paul D. Feldman, Ulrich R. Christensen, Kurt D. Retherford, Melissa A. McGrath, Fabrizio Musacchio, Alexandre Wennmacher, Fritz M. Neubauer, Sven Simon, and Oliver Hartkorn. It was published in the Journal of Geophysical Research: Space Physics in 2015.
The idea in this paper is that a saline, electrically conductive subsurface ocean would interact with the magnetic field and change the location of the aurora. The location of the aurora are expected to change in concert with Ganymede’s orbit around Jupiter, but the ocean would be noticeable because the amount these aurora moved would be lessened by the presence of an ocean.
The authors looked at two sets of Hubble Space Telescope data, each observing session lasting about seven hours. The size of the auroral ovals is greater than the difference in location expected for the scenario with an ocean and without, so a simple visual inspection would not answer the question. The Hubble data was combined with modelling of the magnetic field environment. They looked at the rocking angle, an angle which allows a determination of how much the auroral ovals moved. Without an ocean, a shift by 8 to 10 degrees was expected; with an ocean, the shift was expected to be around 2.2 degrees, give or take 1.3 degrees. The rocking angle found by observation was 2 degrees, implying that there very much is a subsurface ocean on Ganymede, and it is electrically conductive enough to have an impact on the aurora’s location. Most likely, the ocean is between 150 and 250 kilometers (93 to 155 miles) below the surface. The estimate of the depth is determined by the probable conductivity of the ocean and the magnitude of its effect on the magnetic field.
An artist's impression of the JUICE spacecraft. Image credit ESA/ATG medialab.
An artist’s impression of the JUICE spacecraft. Image credit ESA/ATG medialab.
There are of course inherent uncertainties in this method. Everything comes with an error; it’s the nature of mathematics and statistics, and science cannot be without it. The European Space Agency is planning the JUpiter ICy moons Explorer (JUICE) mission, above, for a 2022 launch and 2030 arrival at its destination; the instruments on board this craft would help minimize the uncertainties here. Even so, this method of looking at aurora to determine facts about a subsurface ocean can be applied to other objects, aside from Ganymede.
It may well turn out to be that the solar system, and beyond, is filled with water. There is, however, no rule that all oceans must be on the surface as are the oceans that are familiar to the inhabitants of Earth.

 

Bibliography
Astronomy Picture of the Day. “APOD: 2009 September 20 – Ganymede Enhanced.” N.p., 20 September 2009. Web.
The image of Ganymede that appears in the post.
European Space Agency. “JUICE.” N.p., n.d. Web. 4 Sept. 2015.
The European Space Agency’s page for the upcoming JUpiter ICy moons Explorer mission. The artist’s impression image at the end of today’s post comes from their multimedia gallery.
National Aeronautics and Space Administration. “Ganymede.” N.p., n.d. Web.
NASA’s page about Ganymede; more information under the tabs on the right-hand navigation bar.
Saur, Joachim, et al. “The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals.” Journal of Geophysical Research: Space Physics 120.3 (2015): 1715-1737.
Today’s main paper.
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