A Layered Ocean within Ganymede?

by Paul Gilster on May 2, 2014

Remember as you ponder NASA’s Request for Information about a Europa mission that the agency is contributing three instruments to the European Space Agency’s JUpiter ICy moons Explorer (JUICE) mission, to be operational in Jupiter space in the 2030s. The goal here is to explore Europa, Callisto and Ganymede through numerous flybys, with the craft finally settling into orbit around Ganymede. This would be the first serious look at multiple Jupiter moons by a visiting spacecraft since the Galileo mission, which explored the system from 1995 to 2003.

The large Jovian moons have always been of interest, with not just Europa but Callisto and Ganymede also thought to have deep oceans beneath their icy crusts. Galileo, in fact, found evidence for salty seas within Ganymede, probably containing magnesium sulfate. At the Jet Propulsion Laboratory, a team led by Steve Vance is offering new research showing that what we may have on Ganymede is more than a simple sea between two layers of ice. Using computer modeling of the processes involved, the scientists are talking about regions of ice and oceans stacked in layers. “Ganymede’s ocean might be organized like a Dagwood sandwich,” says Vance.

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Image: This artist’s concept of Jupiter’s moon Ganymede, the largest moon in the solar system, illustrates the “club sandwich” model of its interior oceans. Scientists suspect Ganymede has a massive ocean under an icy crust. Previous models of the moon showed the moon’s ocean sandwiched between a top and bottom layer of ice. A new model, based on experiments in the laboratory that simulate salty seas, shows that the ocean and ice may be stacked up in multiple layers, more like a club sandwich. Credit: NASA/JPL.

Larger than Mercury, Ganymede may have 25 times the volume of water found on Earth. The JPL work, reported in a new paper in Planetary and Space Science, suggests that the sea bottom in the various layers may not be coated with ice but with salty water. Earlier models had shown the probability of dense ice at the bottom of a huge ocean with enormous pressures, but Vance’s team added salt to its models and found that liquid dense enough to sink to the sea floor was the result. Salt, as it turns out, makes the ocean denser, particularly under the extreme pressure conditions found within Ganymede and other moons.

Oceanic pressure on Ganymede would be high, for the moon’s oceans may be up to 800 kilometers deep. The deepest, densest form of ice thought to exist on Ganymede is known as Ice VI. If ordinary refrigerator ice (Ice I) floats in a glass of water, heavier forms of ice produced by extreme pressures show much more compact crystalline structure, with the molecules packed more tightly together, which is why some forms of ice can fall to the bottom of the ocean.

That would seem to produce an icy ocean floor, making potentially life-producing chemical interactions between water and rock unlikely. But the team’s models show up to three ice layers and a rocky seafloor, with the lightest ice on top and salty, dense liquid at the bottom. Moreover, the layers in the diagram above account for odd phenomena, with Ice III in the uppermost ocean layer being formed in the seawater and salts precipitating out. As the heavier salts begin to settle to the bottom, the lighter ice would float upward, perhaps melting again before it reaches the top of the ocean or leaving slush layers in the Ganymede ocean. When it’s snowing on (or within) Ganymede, in other words, the snow floats up, not down.

This JPL news release offers more, including the notion that the ‘club sandwich’ structure the researchers describe varies over time, sometimes returning to a single ocean found below a layer of Ice I and existing on top of regions of different high-pressure ices. In any case, the idea of chemical interactions where water and rock meet is intriguing from an astrobiological viewpoint, suggesting that a wet seafloor could produce the necessary conditions for life. The salts the Vance team added to its model can produce a sea bottom with the needed dense liquids.

The paper is Vance et al., “Ganymede׳s internal structure including thermodynamics of magnesium sulfate oceans in contact with ice,” Planetary and Space Science published online 12 April 2014 (abstract). Also of interest: Vance et al., “A Passive Probe for Subsurface Oceans and Liquid Water in Jupiter’s Icy Moons,” submitted to Icarus (preprint).

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Lionel May 2, 2014 at 12:51

Thanks for this science update on one of the most important (read: habitable) large bodies in the solar system

If these little robots that we are talking about elsewhere are little enough and ready by the 30s, then maybe some could be airbagged onto the surface of Ganymede? I’m probably dreaming to think that by then self-replicating little robots could piggy-back on JUICE and start up a self-sustaining and growing science and industry station on Ganymede… but Dreams is half the name of this blog :)

Michael May 3, 2014 at 0:57

@Lionel

Nothing wrong with dreaming I think it is humanities greatest gift.

There is nothing wrong with a lander mission to Ganymede, it has all the features of Europa, see high resolution images,

http://photojournal.jpl.nasa.gov/targetFamily/Jupiter?start=800

but appears to be covered in a dust layer which could also protect organisms. There is evidence of glaziers flowing, these glaziers could have brought organic material or organisms up from the lower seas. The radiation is also much lower, about 1/60 of Europa’s and it is not so far in the gravity well of Jupiter as Europa.

Ashley Baldwin May 3, 2014 at 8:50

Yes. Thanks Paul. Admid all the excitement over exoplanets its easy to forget what’s on the doorstep. The release of water from Europa was almost like a “Oi, don’t forget me” call. A Galilean moons missions would clearly offer a substantial science return. I just wish we could get the probes there quicker . The Voyagers were “gold plated” and had the old fashion heavy weight rocket launchers for a short, direct flight , but nowadays we have to rely on multiple Earth / Venus flyby route. The Wright brothers could get you there quicker. 2030 for JUICE! That’s forever. SLS could provide quick passage but is eye wateringly expensive , especially when as we have seen you have a very limited budget, effectively ruling it out. I’m hoping that SpaceX will develop some large launch vehicles that will do the job at a manageable price .

Raffaele Antonio Tavani May 3, 2014 at 17:59

Ma la presenza di un campo magnetico, all’interno di Ganimede, che implicazioni potrebbe avere, per le eventuali forme di vita esistenti, negli oceani sotterranei?

Un saluto a voi tutti, da Antonio Tavani

And via Google Translate:

But the presence of a magnetic field, within Ganymede, which could have implications for any existing forms of life in the oceans underground?

Greetings to you all, by Antonio Tavani

Ashley Baldwin May 4, 2014 at 9:48

The downside to this “onion skin” model , of course , is that although we have water/rock contact to potentially allow tidally induced heat to percolate out on the ocean floor to create hypothetical ecosystems , its even less accessible from space or the surface than on Europa . We’ve only just got into lake Vostok under Antarctica and that’s not even a fraction as deep and on our own planet . Not a reason to give up trying though…..

Michael May 4, 2014 at 16:32

Greetings to you Antonio as well

‘But the presence of a magnetic field, within Ganymede, which could have implications for any existing forms of life in the oceans underground?’

I am not quite sure if the magnetic field is produced by Jupiter’s field sweeping through Ganymede (induced) or circulating salts within its seas are doing it. Now if there is a magnetic field then surely electric currents are flowing, could they be used by living organisms? I am tending towards no as they would be small and spread over a wide region.

@Ashley Baldwin May 4, 2014 at 9:48

‘The downside to this “onion skin” model , of course , is that although we have water/rock contact to potentially allow tidally induced heat to percolate out on the ocean floor to create hypothetical ecosystems , its even less accessible from space or the surface than on Europa.’

If you look at pictures of Ganymede you will notice that it is very dirty, and most likely there will be rocks/organics mixed in with the top ice layer to great depths. Due to Jupiter’s great gravity there should be plenty of rocks and energy to penetrate into the ice to provide nutrients to organisms. This with the natural abundance of oxygen at Ganymede’s poles and thermally conducted heat from below may provide an abode for organisms negating the need for a bottom (vent) up approach to keeping life going on these worlds. Life could be just as likely just below the surface as at the sea/floor interface.

Ashley Baldwin May 5, 2014 at 12:09

More than enough reason to go and look as far as I’m concerned .

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