Journey to All the hidden oceans of the Solar system
Try to imagine a huge reservoir capable of holding
more water than the Atlantic and Pacific Oceans combined.
It sounds like science fiction, but this reservoir really exists
and is located 800 million kilometers away from us,
under the surface of Ganymede, the largest of Jupiter's moons.
It is an ocean of saltwater over 100 km deep, buried under a blanket of ice 150 km thick.
A discovery that dates back only a couple of decades ago,
thanks to the unforgettable Galileo probe. But continuing exploration has now revealed
compelling evidence for large volume watery oceans on at least five ice-covered
moons of our outer solar system with perhaps as many as 10-20 when all candidate moons and dwarf
planets are considered. Amazing, isn't it?
You might say: OK, we believe it on trust, but how is it possible to discover on such
distant celestial objects the existence of subterranean and therefore undetectable oceans?
Well, the ability of a planet to have oceans on its surface is dependent on various factors,
including a narrow range of distances from its star - the habitable zone - where temperatures
could allow liquid water to exist. As we have found in our own solar system, however,
interior water ocean worlds can exist at a much wider range of distances. How? In the case of
some ocean moons, for example, the gravitational tugging from their parent planets, like Jupiter
and Saturn, and probably Uranus and Neptune as well, can create enough heat, along with perhaps
radioactive decay, which also produces heat, to allow water to remain liquid in their interiors.
Their surfaces are still frozen since they are exposed to space with little to no atmospheres,
but below the ice crust, oceans can exist. That something can exist does not mean that
it really exists. So how can astronomers be reasonably sure of what they are saying?
Well, methods to infer the actual existence of an ocean do exist, and they are very varied.
Ganymede's ocean, for example, has been hypothesized by studying the movements of
the great moon's polar auroras. The satellite is in fact the only satellite in the Solar System
to possess a magnetic field. Its core, however, alone would not be able to produce a magnetic
field so strong ... Astronomers are therefore almost sure that it is the ocean of saltwater,
through the continuous friction with the nucleus,
to generate a field capable of influencing the oscillations of the auroras.
In other cases, the presence of an ocean was instead deduced from small
oscillations of the axis of rotation, due to the movement of the mass of water inside it.
The existence of an underground ocean is then evident when on the surface
one is lucky enough to observe geysers in action,
which are the sign of underground activity. And this was the case with Europa and Enceladus.
But each alleged ocean hypothesized by astronomers is a case in itself, and it is,
therefore, necessary to review every single celestial body to really understand something.
From nearest to farthest,,, let's try to start this long journey, shall we?
Ceres: average distance from the Sun 414 million kilometers; diameter of 946 km
The largest asteroid and smallest dwarf planet in the solar system could be home to liquid water,
sitting deep underground. Ceres, a dwarf planet that sits between Mars and Jupiter,
was studied by Dawn probe from orbit from 2015 to 2018. Scientists are still unpacking and analyzing
that data, but tantalizing studies in the past few years suggest there’s an ocean sitting 40
km below the surface, and could stretch for hundreds of miles. It would almost certainly be
extremely salty - which would keep the water from freezing even well below 0°C. Dawn even found
evidence of organic compounds on Ceres that could act as raw materials for life.
There’s no shortage of new proposals for future missions to study the dwarf planet,
including ones that would even attempt a sample return mission. But nothing is going up soon.
Europa: average distance 628 million kilometers; diameter of 3122 km
Europa is Jupiter's fourth-largest moon and the smoothest of all the celestial bodies. There are
almost no craters, and despite a dense network of cracks and ridges covering this moon, none are
higher or deeper than a few hundred meters. This suggests that Europa's surface is
geologically young and possibly floating on a liquid mantle. The Hubble Space Telescope has
also spotted plumes of water vapor spewing 200 km into the air from the south pole.
This lends weight to the idea that Europa has a subsurface saltwater ocean deep 60-150 km,
covered by a layer of ice that may be just a few kilometers thick.
Quindi, mentre Europa ha solo un quarto del diametro della Terra, il suo oceano può
contenere il doppio di acqua rispetto a tutti gli oceani del nostro pianeta messi insieme.
Tidal flexing and friction from the gravitational interaction with Jupiter generate enough heat to
keep the interior ocean liquid, but because it is so far from the sun, the surface remains frozen.
Luckily, we’re set to study Europa in great detail.
The Jupiter Icy Moons Explorer (JUICE), an interplanetary spacecraft in development by
the European Space Agency, will study three of Jupiter's Galilean moons:
Ganymede, Callisto, and Europa all of which are thought to have significant bodies of
liquid water beneath their surfaces, making them potentially habitable environments.
The spacecraft is scheduled to launch in April 2023 and will reach Jupiter in July 2031
But the marquee mission on the books is Europa Clipper, a spacecraft that would conduct
low-altitude flights that would attempt to study and characterize the surface, and investigate the
subsurface environment as best it can. Clipper launches in 2024 and will reach Europa in 2030.
Ganymede: average distance 628 million kilometers; diameter 5268 km
In the 1970s, NASA scientists first suspected that Ganymede - the largest moon to orbit Jupiter,
and simply the largest moon in the solar system - has a thick ocean between two layers of ice,
one on the surface and one beneath a liquid ocean and atop the rocky mantle.
In the 1990s, Galileo's mission flew by Ganymede and found indications of such a subsurface
ocean. An analysis published in 2014, taking into account the realistic thermodynamics for water and
the effects of salt, suggests that Ganymede might have a stack of several ocean layers
separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle.
Water–rock contact may be an important factor in the origin of life.
In March 2015, scientists reported that measurements with the Hubble Space Telescope
of how the aurorae moved confirmed that Ganymede has a subsurface ocean.
A large salt-water ocean affects Ganymede's magnetic field, and consequently, its aurora.
The evidence suggests that Ganymede's oceans might be the largest in the entire Solar System.
Callisto: average distance 628 million kilometers; diameter of 4821 km
Callisto is Jupiter's second-largest moon, and was thought to be a "dead world" by many;
however, new data could suggest otherwise.
The strange thing about Callisto is that the surface is completely saturated with craters,
with no breaks or smooth plains caused by geological processes
below. E questo fino a poco tempo fa faceva pensare all'assenza di un oceano sotterraneo.
The possibility that Callisto may have an ocean underneath its icy surface was first
proposed in 1998. Subsequent observations in 2001 added credence to this theory.
Ultimately, readings collected by the sensors aboard the Galileo Spacecraft
showed that the moon’s magnetic field fluctuates as Jupiter rotates on its Axis.
An underground ocean of salt water was the most plausible theory - since saltwater is a great
conductor of electricity, and it can interact with Jupiter’s magnetic field causing the fluctuations,
it seemed to be the most likely candidate. However, initially, we thought that Callisto’s
ocean was frozen. Scientists thought that this was the case because the heat from the
satellite’s core was hypothesized to escape through the icy surface,
which means that it wouldn’t be able to keep the ocean underneath in liquid form.
Yet, according to a more recent theory, the moon’s icy surface may hold heat better than
previously estimated, which could keep the underground ocean in liquid form.
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Enceladus: average distance 1272 million kilometers; diameter 505 km
Enceladus is the sixth-largest moon of Saturn. It is mostly covered by fresh,
clean ice, making it one of the most reflective bodies of the Solar System. Consequently, its
surface temperature at noon only reaches −198 °C, far colder than a light-absorbing body would be.
Despite its small size, Enceladus has a wide range of surface features,
ranging from old, heavily cratered regions to young, tectonically deformed terrains.
In 2005, the Cassini probe observed plumes of water vapor erupting near the south pole of the
moon. Because the gravity on Enceladus is only 1% of Earth's, the ice crystals are easily flung into
orbit, and we now know they are responsible for most of the material in Saturn's E Ring. Enceladus
has a rocky core around 370 km across, surrounded by a 10 km deep ocean under an icy crust.
Initially, scientists thought the ocean was only present as an underground lake at the south pole,
as that's where the plumes have all been seen. But measurements of Enceladus' slight wobble
in his orbit show that the rocky core is likely completely detached from the crust.
This means that the ocean envelopes the moon and probably accounts for 40%
of its volume. The reason that the plumes only occur at the south pole is that the surface ice
is believed to be much thinner - just 5 km thick,
compared with 20-45 km thick surface across the rest of Enceladus.
Dione: average distance 1272 million kilometers; diameter 1122 km
Dione is the 15th largest moon in the Solar System. It is also Saturn's fourth-largest
moon. Based on its density, Dione’s interior is likely a combination of silicate rock and
water ice in nearly equal parts by mass. Shape and gravity observations collected by
Cassini suggest a roughly 400 km radius rocky core surrounded by a roughly 160 km envelope of water,
mainly in the form of water ice, but some models suggest that the lowermost
part of this layer could be in the form of an internal liquid saltwater ocean deep 35-95 km.
There is also evidence from Cassini which has hinted at the moon having plume activity.
One of the moon's mountains may be responsible for these plumes, where they send water gushing
out into space. This is called "cryovolcanism" and takes place through a subsurface ocean.
There is another instance with Cassini that also points to Dione having an ocean.
If a spacecraft is flying over a solid body, gravity will
not have much of an effect on its trajectory. However, if there is a liquid ocean beneath a
layer of ice on the surface of that body, then the spacecraft will exhibit small,
but noticeable deviations with the pull due to the mass of the liquid.
Titan: average distance 1272 million kilometers; diameter 5150 km
Titan is unusual because it is the only body in the Solar System, besides Earth,
that has a substantial atmosphere and bodies of surface liquids. Titan's surface temperature is
-292 degrees Fahrenheit (-180 degrees Celsius), so it's far too cold for liquid water on the surface,
but it's just about right for liquid methane and ethane.
With its organic dunes, lakes, channels, and mountains,
Titan has one of the most diverse, active, and Earth-like surfaces in the Solar System,
but nothing until 15 years ago suggested that this large moon also hides a liquid ocean inside.
Everything changed between October 2005 and May 2007, when the team of the Cassini mission used
the data of the first radar observations to establish the positions of 50 reference
points on the surface of the satellite, verifying then in the following years that
those same points had shifted from the expected positions of up to 30 km! A systematic shift is
difficult to explain unless we assume that the icy crust of the moon floats above an internal ocean!
Nowadays it is believed that about 100 km below the surface
there is an internal ocean of liquid water, very salty and mixed with ammonia, up to 200 km deep.
The upcoming NASA Dragonfly mission, which should be launched in 2027,
will surely give us a confirmation or denial of these hypotheses.
Triton: average distance 4338 million kilometers; diameter 2707 km
Triton is the largest moon of Neptune. It is slightly larger than Pluto
and has almost the same composition. A close pass by the Voyager 2 spacecraft in 1989 showed
that the satellite's surface is composed mainly of water ice, nitrogen, methane, and carbon dioxide.
Its incredible retrograde orbit - unique in the solar system for such a large celestial body - is
a sign that it must not have formed in Neptune's system. In fact, likely, it was gravitationally
captured by swapping places with Pluto itself. When it was captured, its initial orbit must
have been very eccentric, and this should have generated a lot of tidal heating, enough to melt
the icy interior and form a mantle of liquid water and a solid crust of water and nitrogen.
Eventually, after a billion years, Triton's orbit became circular enough to lose most of
its tidal heating, but it still receives energy from the radioactive elements in the core.
Computer models show that a small amount of impurities dissolved in the water,
such as ammonia, would be enough to lower the freezing point and keep Triton's ocean liquid.
Pluto: average distance 5900 million kilometers; diameter 2377 km
Last, but certainly not least, we're going to talk about everyone's favorite (dwarf) planet, Pluto.
Based on data from the New Horizons mission in 2015, also Pluto may contain a subsurface ocean.
We are obviously always in the field of hypotheses and computer simulations,
but according to some planetologists, the ocean would be located below the famous heart-shaped
basin around the equator, called Sputnik Planitia. In correspondence to this area the icy crust of
Pluto is thinner and according to researchers an ocean deep 100-180 km would be wrapped and
protected by a layer of methane hydrates that would isolate it thermally from its icy surface.
The temperature of the surface of Pluto is in fact about 225 degrees below zero.
Final Thoughts The discovery of these subsurface oceans
within our solar system is rapidly changing the way we think about planetary science.
It shows that liquid water may be more common than previously thought;
and while this does not imply that life resides on all of these objects,
there is growing hope that extremophile organisms may survive in those icy, dark waters.
The presence of water makes any world a prime candidate for life, which is why
NASA has made "follow the water" its battle cry in the search for extraterrestrial life.