Back on Friday, news that evidence had been found of a subsurface ocean on Ganymede, Jupiter’s largest satellite, was widely propagating. Centauri Dreams’ Paul Gilster (““Evidence Mounts for Ganymede’s Ocean”) was one of the first bloggers on my RSS feed to report this.
Yesterday’s discussion of hydrothermal activity inside Saturn’s moon Enceladus reminds us how much we can learn about what is inside an object by studying what is outside it. In Enceladus’ case, Cassini’s detection of tiny rock particles rich in silicon as the spacecraft arrived in the Saturnian system led to an investigation of how these grains were being produced inside Enceladus through interactions between water and minerals. If correctly interpreted, these data point to the first active hydrothermal system ever found beyond Earth.
Now Ganymede swings into the spotlight, with work that is just as interesting. Joachim Saur and colleagues at the University of Cologne drew their data not from a spacecraft on the scene but from the Hubble Space Telescope, using Ganymede’s own auroral activity as the investigative tool. Their work gives much greater credence to something that has been suspected since the 1970s: An ocean deep within the frozen crust of the moon.
The early work on a Ganymede ocean grew out of computer models of the interior, but the Galileo spacecraft was able to measure the moon’s magnetic field in 2002, offering enough evidence for an ocean to keep the idea in play. The problem was that the Galileo measurements were too brief to produce an overview of the field’s long-term cyclical activity.
It was Saur’s idea to look at the idea afresh. Given that Ganymede is deeply embedded in Jupiter’s magnetic field, the aurorae that are produced in its polar regions are going to be influenced by any changes to that field, changes that produce a ‘rocking’ movement in the aurorae. These movements, Saur reasoned, would be a useful marker, one that, like the silica grains near Enceladus, could tell a story about activity deep below the surface. Says Saur:
“I was always brainstorming how we could use a telescope in other ways. 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.”
The ‘rocking’ of the aurorae on Ganymede depends upon what’s inside the moon, and by the researchers’ calculations, a saltwater ocean would create a secondary magnetic field that would act against Jupiter’s field, tamping down the motion of the aurorae. The Hubble data show us that this is happening, for Saur’s models indicate the auroral activity is reduced to 2 degrees as opposed to the 6 we would expect if an ocean were not present. Ganymede thus joins Europa and Enceladus as an outer planet moon with increasing evidence for an ocean.
In “An internal ocean on Ganymede: Hooray for consistency with previous results!“, the Planetary Society Blog’s Emily Lakdawalla noted that this discovery fits with our understanding of Ganymede’s inner workings.
The Hubble results don’t precisely pin down the size and depth of an ocean at Ganymede, but the range of possible ocean thicknesses and depths is small. The observed behavior of the aurorae at Ganymede are consistent with a liquid ocean that is a relatively small fraction of Ganymede’s radius. Of course, Ganymede is a very big world, so “relatively small” in this case can mean an ocean up to about 100 kilometers deep. According to the paper, some possible oceans that are consistent with the results include, but are not limited to:
•A liquid layer from 150 to 250 kilometers depth with relatively low salinity
•A liquid layer from 190 to 210 kilometers depth with relatively high salinity
•A liquid layer below 330 kilometers depth with relatively high salinity
Water in one form or another at Ganymede goes from the surface to a depth of about 720 kilometers, at which point you reach Ganymede’s rocky outer core. So I think we can generalize these all to say: there is an ocean of less than 100 kilometers thickness located within a few hundred kilometers of the surface of Ganymede. In all possible cases, the liquid layer would be perched within the solid icy mantle, with solid ice above it of the form we have on Earth, and ice below it in a high-pressure crystalline form. [. . .]
That is a lot of liquid water, but it’s in a very different place within Ganymede than the liquid water that we think exists at Europa and Enceladus. Europa has a much higher proportion of rock to ice than Ganymede does, and is also warmer because of greater tidal friction; the same physics that predicted Ganymede’s perched-ocean-within-an-icy-mantle predicts that Europa’s liquid water ocean is in direct contact with its warm rocky core. Warm liquid water percolating among warm rocks is, by definition, hydrothermal activity. Hydrothermal zones are places that exobiologists imagine life might happen, because you have lots of energy and you have rich chemistry created in all that warm liquid water eating away at rocks. Earlier this week, one of the Cassini instrument teams announced that they had detected rock particles from just such an environment that originated within Enceladus.
At Discover‘s Out There blog, Corey S. Powell noted in “Looking for Life in All the Wrong Places?” that the existence of liquid water oceans has implications for the search for extraterrestrial life generally, and for the dispatch of space probes more specifically.
The story used to be all about Mars. Now it is clear that most of the water, most of the organic chemistry, and by extension most of the potentially habitable territory in the solar system resides on or in ice moons. If that’s true in our solar system, there’s a good chance it’s true around other stars across our galaxy and beyond.
Currently there are five orbiters and two surface robots exploring Mars. Here are the equivalent numbers for the four moons: Europa, 0. Ganymede, 0. Enceladus, 0. Titan, 0. It seems like we may have been looking for life in all the wrong places.
That’s the bad news. Now that good part. The Cassini spacecraft orbiting Saturn is performing local observations and occasional flybys of Enceladus and Titan. Archived information from NASA’s Galileo probe, along with new data from the Hubble Space Telescope, are deepening our understanding of Enceladus and Ganymede. Europe is working on a spacecraft called JUICE, which will examine Ganymede in detail. And the Obama administration is poised to approve the Europa Clipper, the first mission dedicated entirely to one of these icy moons; it could launch as early as 2022. In short, our explorations are starting to catch up with our fast-changing knowledge.
Getting to the next stage of understanding won’t be easy. The icy moons are far away, making them time-consuming and costly to reach. A trip to Mars takes about 8 months. Galileo needed 6 years to reach Jupiter, and Cassini’s voyage to Saturn was a 7-year undertaking. NASA also has a whole planetary-science bureaucracy built around the exploration of Mars. There are a lot of careers tied to the Red Planet.
At the same time, it’s hard to ignore the contrast. This past week, a group of researchers reported that the Red Planet probably had a vast ocean covering its northern hemisphere. It was an encouraging discovery, one that got quite a bit of news coverage. That ocean on Mars dried up about 4 billion years ago, however. The oceans of Enceladus and Europa are calling to us right now. If we want answers—if we want to find life, or the processes leading up to life—those are the places where we have to go.