When we think of moons with oceans of liquid water locked under shells of ice, Europa and
Enceladus immediately come to mind though there are others.
The reason for that is simply that those two moons show the most evidence of such oceans,
including fissures that spray water into space.
But there is also a fair amount of evidence for another ocean moon, almost conclusive
evidence.
It also happens to be the largest moon in the solar system, in fact it has a larger
diameter than the planet Mercury.
It's Ganymede, and it not only offers the possibility of liquid water and life, but
several different environments for it.
Ganymede is a very unique body in the solar system.
For one, it's the only moon known to have a magnetic field, though it's vastly less
extensive than that of earth.
This is because Ganymede has a convecting liquid core.
This makes it's interior more like Earth than a typical moon, but also Ganymede is thought
to be fully differentiated, meaning that has layers in its internal structure such as a
rocky mantle like earth and indirect evidence of a subsurface ocean, such as certain effects
on its magnetic field that essentially confirm the presence of liquid water.
But the similarities to earth end there.
This ocean would be very different from earth, and indeed different from all of the other
ice shell ocean moons in the solar system.
In a 2014 paper, link below, along with a NASA press release, Steve Vance and colleagues
presented research that suggests that Ganymede doesn't just have one ocean, but multiple
oceans stacked one on top of the other separated by different stages of ice shell.
This is actually good news for the potential for life on Ganymede.
The previous model was problematic, in it the ocean sat between the surface ice and
a layer of a different phase of ice at the bottom of the ocean kept that way by pressure.
That would not favor life, the interaction of liquid water and rock are important for
providing mineral nutrients for life and that doesn't easily happen if there is a layer
of ice separating the two.
This state of affairs was suspected because Ganymede's ocean was thought to be hundreds
of miles deep, with extreme pressures at the bottom.
But there also seemed to be salts evident on the surface.
But in the new model, the salts cause the water's density to change.
Add that with how different phases if ice form under high pressure, and you get the
layered effect, with the lowest ocean in contact with the rocky subsurface, with all of the
oceans perhaps interacting through breaks in the ice layers.
As an aside, this model also revealed a bizarre possible phenomenon in these oceans.
It would snow in one layer.
A form of ice, known as Ice III could form in the water itself.
The salts would precipitate out and fall downward, but the ice itself would float upwards, meaning
that it might snow upside down in this ocean.
So chalk Ganymede up as a potential abode for life along with its other Galilean companions.
But in the case of Ganymede, any ecosystem could be unlike anything else in the solar
system, with layers of life adapted specifically for which oceanic shell they live in and the
respective pressures they have to deal with.
But life under high pressure is possible.
In fact, here on earth, there is an entire class of organism termed the barophiles which
thrive under high pressures, and in fact species such as the bacteria Halomonas salaria can't
survive without the high pressures.
If the very deepest, high pressure places in our oceans can host life, then perhaps
Ganymede might too.
Thanks for listening!
I am futurist and science fiction author John Michael Godier and be sure to check out my
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