We’ve spent so many years wondering about the secrets of our own moon that, until very recently,
we’ve not paid much attention to the moons of other planets. But some moons out there are
phenomenally interesting places, and finally, we have the technology and the drive to investigate
them - and we’re making a lot of discoveries. This is Unveiled, and today we’re answering
the extraordinary question: what are Ganymede’s massive, mysterious craters?
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Ganymede is the largest moon of Jupiter and the largest moon in the entire solar system – it’s
even larger than the planet Mercury. But even though it’s visible from Earth,
because it’s so far away, we know very little about it. It’s an extremely intriguing place,
however. Although to the untrained eye, Ganymede looks like another barren moon – much like
Earth’s – scientists have growing evidence that Ganymede is actually covered in ice,
and that it could have an underground ocean beneath all of that icy exterior. Callisto and
Europa, two more of Jupiter’s four Galilean moons, are also thought to have large deposits of ice and
potentially even oceans. Like Ganymede, Callisto has been ravaged by impact events, making it the
single-most “cratered” celestial body in our solar system. However, it’s the craters of Ganymede
that take center stage for today’s video. Thanks to images from the NASA space probe Juno,
we now know that the craters on Ganymede are far more numerous and larger than what had been
originally thought - there’s even one that’s sixty-eight miles wide. The Juno space probe
was launched back in 2011, and it entered Jovian orbit five years later. It’s been orbiting Jupiter
ever since, sending back stunning images of the gas giant and its many moons. But of course,
Juno wasn’t the first space probe to see Jupiter; Voyager 1 and Voyager 2, the two oldest artificial
objects still operating in the solar system, both saw Jupiter as well. Both were launched in 1977,
and as of writing are still functioning, decades after their original missions ended.
Importantly though, the Voyager spacecraft didn’t enter orbit around Jupiter. They continued going
so that they could provide data on the furthest parts of our solar system, and are now billions of
miles away from us here on Earth. The New Horizons probe, launched in 2006, also did flybys of
Jupiter so that it could perform a gravity assist, but its primary goal remained the exploration of
Pluto - and so it headed there without stopping by Jupiter for too long. It’s been providing a
wealth of data about trans-Neptunian objects and the Kuiper Belt ever since. The fact that
the Juno probe remains in orbit around Jupiter (and will remain until it stops functioning,
which could be many years from now) is what sets it apart. It means that it can keep sending back
to us a series of stunning, high-quality images. Which brings us back again to the craters on
Ganymede. Among the images sent from Juno are some that have led to the discovery
of never-before-seen marks on the surface of Ganymede. Upon analyzing the latest pictures,
scientists have explained that while the craters were obviously there before,
we just couldn’t see them because the previous images we had of Ganymede, taken by Voyager,
just weren’t good enough. Indeed, capturing a single shot is difficult enough.
Depending on what angle the camera is at and where Ganymede is in relation to the sun,
the topography of the moon can look completely different. The same is really true for any other
object, including the human face, which can look significantly different depending on where a light
source is coming from. But it means that any one image of this alien world taken by one spacecraft,
at one point in time… could potentially produce all kinds of strange perspectives. And it’s only
when you have additional images that you can really make sense of what you’re seeing. One
particularly famous example of this from elsewhere in the solar system is the “Face on Mars”. The
original black-and-white “Face on Mars” image was taken by the Viking 1 probe in 1976. It’s on a
region of the Red Planet called “Cydonia”, and has been a magnet for people trying to find proof of
intelligent life away from Earth. However, more detailed, more contemporary images show that the
Face on Mars is actually just a rock feature. It was the particular shadows captured in the
original image that had made it so evocative - but they’re obviously not a constant. Similarly, the
lighting just wasn’t right in the original images from Voyager for us to properly see the massive
craters on Ganymede’s surface. But now we can. More interestingly, many of the newly identified
Ganymede features aren’t thought to be impact craters at all. Unlike on Callisto, which is
covered with impact craters and has minimal, or potentially even no, evidence of volcanic
activity, Ganymede is volcanically active. Some of the large depressions we see, then, are actually
calderas caused by volcanic eruptions. These weren’t the first calderas found on Ganymede
and it’s also not completely conclusive that that’s what’s in the latest images…
but scientists are generally confident that these depressions were caused by cryovolcanoes.
Yes, on Earth the word “volcano” is synonymous with fire and lava, but on other planets,
there are volcanoes that blast out massive amounts of ice or vapor, like geysers - and that’s what’s
happening here. We do have geysers in volcanically active parts of Earth, too, such as in Iceland,
but these are nothing like an alien cryovolcano. The off-Earth volcanoes - like those on
Ganymede - are able to spew frozen material sometimes miles into the air and out of the
atmosphere. And the ice isn’t always made of H2O, either. It’s also frozen ammonia and methane.
Both volcanoes and cryovolcanoes are found throughout the solar system. Traditional volcanoes
like the ones on Earth have also been found on Mars, Venus, Mercury, and the Jovian moon Io,
as well as many more. Cryovolcanoes, on the other hand, likely exist on Ganymede, Europa, Titan, and
potentially on the dwarf planets Ceres and Pluto. They also exist on Saturn’s moon Enceladus, and
possibly a Neptunian moon, Triton, though further study is required. For cryovolcanoes to exist,
there needs to be enough tectonic activity, but also a cold enough atmosphere or surface
temperature to solidify the materials the volcano ejects, called “cryolava”. We’re also not exactly
sure of the mechanisms at work in the outer solar system that keep these moons and dwarf planets
warm enough to support volcanic activity. A large part of it is certainly to do with
“tidal heating”, where heat is generated from the push-and-pull motion between a planet and its
satellite - explaining the existence of subsurface oceans within some moons. But it may also be aided
by the atmosphere, if it’s particularly dense and has enough gasses to trap heat.
Away from volcanoes, we know that Ganymede has subsurface oceans thanks to its magnetic field.
Based on models of the moon, scientists have suspected the existence of such bodies
of water since the 1970s. New evidence came from analysis of Ganymede’s magnetic field,
discovered by the space probe Galileo in 1996. Ganymede is the only moon in the solar system
known to have a magnetic field, and its behavior indicates the presence of salty seas within.
Researchers further confirmed this idea in the 2010s using images of Ganymede’s auroras,
produced by the Hubble Telescope. Ganymede’s magnetic field generates the auroras,
a feature also seen on Earth and the other planets in the solar system
(save for Mercury). Scientists believe that the behavior of the auroras on Ganymede, which should
“rock” back and forth more in response to changes in Jupiter’s magnetic field,
confirms the presence of liquid water below the surface. Juno has also been able to image these
auroras in unprecedented detail, offering more insights, on top of those linked to the craters.
But soon, we won’t just have Juno to rely on for data on these moons. In April 2023,
the European Space Agency is scheduled to launch the Jupiter Icy Moons Explorer, better known as
JUICE. JUICE is a large spacecraft that is specifically designed to gather information
on three of the four Galilean moons, rather than Jupiter itself. Those moons are Callisto,
Europa, and Ganymede… the ones we believe to have subsurface oceans. The cutting edge probe is going
to be equipped with all kinds of instruments to study the chemical makeup of these bodies,
their atmospheres, how they’re affected by Jupiter’s gravity, and much more beyond.
Unfortunately, even if JUICE launches on time it’s still not going to make it to Jupiter until 2031,
and won’t enter orbit around Ganymede itself until 2034. But we may have learned much more
from Juno in that time, and it’s at least inspiring to know that we’ll get the data
we want on the Galilean moons eventually. The 2030s are set to be a great decade for
studying alien moons in general, but particularly in the Jovian system. This
once mysterious part of the solar system is set to reveal many of its secrets, and these images
might just be the start of that. And that’s how scientists have just discovered massive
(previously unknown) craters on Ganymede - and why it’s a big deal in the science of space.
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