A massive collision billions of years ago may have dramatically reoriented Ganymede, Jupiter鈥檚 largest moon.

at Kobe University, Japan, and his colleagues studied Ganymede鈥檚 extensive furrow system, a series of concentric troughs believed to be remnants of the largest impact structure in the outer solar system.

The centre of the furrow system aligns closely with Ganymede鈥檚 tidal axis 鈥� the imaginary line running to Jupiter from the centre of the moon鈥檚 side that always faces its planet. This led the researchers to suggest that the impact that formed the furrows caused a significant redistribution of mass that reoriented the moon.

Through simulations, the researchers determined that the impactor responsible probably had a diameter of about 150 kilometres 颅鈥� significantly larger than the one that caused the extinction of the dinosaurs on Earth, which is estimated to have had a diameter of about 10 kilometres.

at the University of Illinois Chicago says that if an asteroid like that hit Earth, 鈥渋t would be a global sterilising event, a bad day鈥�.

Upon impact, this asteroid would have breached Ganymede鈥檚 icy crust into the liquid oceans below, creating a transient crater and hurling vast amounts of material across the moon鈥檚 surface.

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As this settled, it would have formed a thick blanket of ejecta around the impact site, creating a region where gravity is stronger due to the extra mass. Over time, this anomaly would cause Ganymede to reorient, aligning the impact site with its tidal axis, the simulation showed.

Hirata鈥檚 team compared this process with an event on Pluto, where a large impact created a basin called Sputnik Planitia, leading to a reorientation of the dwarf planet.

However, although it is likely that the Ganymede impact significantly affected the moon鈥檚 early history, estimating the size of the object that hit it is complicated because we lack good data on the gravity and topography of this frigid world, says Hirata.

Dombard says the model used in the paper doesn鈥檛 account for some of the complexities of Ganymede鈥檚 unique icy structure. 鈥淚 think it is very good for establishing that this process could occur, but I don鈥檛 necessarily trust the numbers,鈥� he says.

Later this year, NASA’s Europa Clipper will launch, travelling to Jupiter’s fourth-largest moon, Europa, an icy world with an ocean up to 100 kilometres deep. This, in addition to hints of geophysical activity and complex chemistry, is why Europa has captivated astronomers searching for habitable worlds beyond our pale blue dot. So could Europa have the conditions for life? The probe has some of the instruments to find out. And this is where lab work on Earth can help.

As missions like this progress, learning about distant environments and the processes at play, lab work can simulate those environments and test specific reactions to better understand what is going on. “On a mission, you see a signature on a planet, but you don’t necessarily know where it came from,” says Barge. Efforts back home can help interpret that signature. “By comparing to the mission data, you can get a better understanding of what we’re observing on these other worlds,” she says.

The Jupiter Icy Moons Explorer (JUICE) spacecraft is stopping by home in what the European Space Agency (ESA) is calling 鈥渁 double world first鈥�. It will fly past Earth and the moon on 19 and 20 August, marking the first of several complex manoeuvres along its looping path to Jupiter 鈥� and it may be possible to spot the spacecraft in the sky as it hurtles by.

JUICE launched on 14 April 2023, and since then it has been circling the sun roughly alongside Earth. But on 19 August, its journey will start to get interesting. It will pass within 700 kilometres of the surface of the moon, slowing it down slightly聽in what is called a gravity assist. Then, the next day, it will dip less than 7000 kilometres from Earth鈥檚 surface, bending its trajectory even more. That is when it may be possible for eagle-eyed skywatchers in South-East Asia to catch a glimpse of the spacecraft.

All of this is necessary because if JUICE were to fly in a straight line to Jupiter, it would need more than 60,000 kilograms of fuel to get there, and even more to slow down and enter orbit once it arrives. The mass of the entire spacecraft without fuel is 2420 kilograms, so that is not remotely feasible. Instead, the upcoming flyby of Earth and the moon will send the craft towards Venus, where it will start to speed up, and then it will make two more passes by Earth to gain just the right amount of speed to get to Jupiter in 2031.

The first manoeuvre may be the most complicated of the entire mission because JUICE must pass through precisely the right point in relation to both Earth and the moon to catch a ride to Venus. 鈥淚t鈥檚 like passing through a very narrow corridor, very, very quickly: pushing the accelerator to the maximum when the margin at the side of the road is just millimetres,鈥� said ESA鈥檚 in a . A double-flyby of Earth and the moon has never been attempted before, nor has any double gravity assist.

If all goes well, JUICE won鈥檛 just be on its way to Venus and then Jupiter 鈥� it will also perform the first real tests of its scientific instruments. For some of them, this is the only chance they will get to iron out any kinks while observing a planetary surface before the spacecraft arrives in orbit around Jupiter and begins its mission there: searching for hints that the giant world鈥檚 moons could host life.

Jupiter鈥檚 moon Io has been continuously remodelled by volcanic eruptions for billions of years, possibly since it first formed.

Io is the most volcanically active body in the solar system, spewing plumes of sulphurous material from its many volcanoes, which can be seen from Earth. Astronomers know that this is currently driven by so-called tidal heating as the gravity of Jupiter and nearby moons deforms Io, but it was unclear if that was always the case or whether there had been a calmer past.

Now, at the California Institute of Technology and her colleagues have found that Io has probably been blasting out lava for almost its entire history. They did this by measuring the ratio of two isotopes of sulphur in its atmosphere.

Sulphur鈥檚 most common stable form contains 16 protons and 16 neutrons in each atom, but a heavier stable form called sulphur-34 has two extra neutrons. On Io, volcanoes are constantly spewing both isotopes into its atmosphere and onto its surface. The very top layer of its atmosphere, which contains more of the lighter sulphur atoms, is lost to space as the moon travels around Jupiter, which changes the ratio of these isotopes.

De Kleer and her colleagues used observations from the Atacama Large Millimeter/submillimeter Array (ALMA), a set of radio telescopes in Chile, to measure the ratio in Io鈥檚 atmosphere. Then, by modelling how much sulphur Io might be losing each year, the team could work back to find out when Io鈥檚 sulphur ratio looked like the rest of the solar system. Although they can鈥檛 say exactly how long it has been volcanically active, it appears to have been erupting for between 2.5 and 4 billion years.

Because Io鈥檚 volcanism is down to tidal heating by Jupiter and its other moons, like Europa and Ganymede, the results can also be used to infer the arrangement of the Jovian system billions of years ago. 鈥淚o鈥檚 longevity of volcanism directly reflects how long this orbital configuration has been present,鈥� says de Kleer.

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If Io has been consistently volcanic for billions of years, then this also means it will have recycled its deeper geological layers many times over, says at Lancaster University in the UK.

This presents a rare opportunity to find out about the chemical makeup of Io鈥檚 deeper layers, such as the mantle that lies below its outer crust, by sampling the material that is blasting out, he says. 鈥淚f these volcanoes have been erupting for the whole solar system history, essentially, then it鈥檚 safe to look at the composition of what鈥檚 coming out and know that that鈥檚 really a snapshot of the entire mantle of Io,鈥� says Wilson.