Newly discovered craters on Ganymede were previously unknown

Science & Technology

We’ve spent so many years wandering 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.

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The dark side of Ganymede was obtained by Juno’s Stellar Reference Unit navigation camera during its June 7, 2021, flyby of the moon.Credit to NASA/JPL-Caltech/SwRI

What is Ganymede?

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.

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.

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.

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An artist's depiction fo the Juno spacecraft observing Ganymede during the flyby on June 7, 2021.Credit to NASA / JPL-Caltech.

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. This brings us back again to the craters on Ganymede.

Unknown Craters:

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 anyone 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.

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Twenty five years ago something funny happened around Mars.Credit to NASA / JPL-Caltech.

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 Callisto Moon, 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.

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 keeps 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.

Measuring Ganymede's Magnetic Field:

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 an 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.

Conclusions:

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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