by Duncan Lunan

A story reprinted from August has brought more good news for Europe’s Jupiter Icy Moons Explorer, which successfully completed a Venus flyby on August 31^st and has a second Earth-Moon flyby to come, in January 2029, before setting off to reach the Jupiter system in July 2031  (Fig. 1). 

It turns out that in its first Earth-Moon flyby, in August 2024  (Figs. 2 & 3), JUICE not only tested its high-resolution cameras in images of both from various distances, and studied Earth’s Van Allen radiation belts on the way through  (Fig. 4), but also performed an important calibration of its Radar for Icy Moon Exploration (RIME), like the one performed over Mars in April by NASA’s Europa Clipper.  (Brandon Specktor, ‘Scientists scan famous ‘Earthrise’ crater on mission to find alien life in our solar system’, Space.com, August 2^nd 2025.) 

As the headline implies, the chosen target was the crater Pasteur T, prominent in the foreground of the famous Apollo 8 photograph, renamed “Anders’ Earthrise” in 2018  (Fig. 5), in honour of the astronaut who took the first shots.  The calibration was performed using data from NASA’s Lunar Orbiter Laser Altimeter (LOLA), successfully, and like Europa Clipper’s, JUICE’s radar is now ready for action on arrival at Jupiter’s moons.  Coming from top right to bottom left in Fig. 4, JUICE might have been able to recapitulate the Apollo 8 image, but perhaps didn’t because the other instruments were turned off at that point, not to interfere with the radar calibration.

Jupiter’s four large moons  (Fig. 6), discovered by Galileo Galilei, orbit the planet in the plane of the equator, tightly constrained by Jupiter’s gravitational field.  Their orbital periods are in resonance, and would evolve into collision courses but that their pulls on one another are constantly moving them inward and outward.  Their densities decrease in order outward, implying that they were affected during formation by heat coming from the planet as it contracted, though it wasn’t massive enough to begin fusion reactions and become a companion star to the Sun.  Their sizes range from Io and Europa, which bracket the size of Earth’s Moon  (Fig. 7), to Ganymede, the largest moon in the Solar System, which is larger than Mercury.  All of them are larger than Pluto, and might be considered worlds in their own right if they orbited freely around the Sun.

As I pointed out in ‘Life on Other Worlds’  (ON, January 9^th, 2022), the general rule in searching for such life is ‘Follow the water’, and Jupiter’s moons are considered to be prime targets.  The innermost of the Galileans, Io, might seem very unpromising with it surface dominated by sulphur erupted from hundreds of volcanoes – 266 of them, as estimated in the Juno flyby of Io on its 43rd approach to Jupiter  (Fig. 8). 

Previously it had been assumed that there was a deep magma ocean within Io  (Fig. 9), and from Fig. 8 it was estimated to be 50 km deep and 50 km thick, from which lava made its way upwards through cracks as it does on Earth  (Fig. 10). 

But in February 2024, on the 58th perijove pass, Io’s dimensions were very accurately measured and the distortion to be expected, if there was a liquid ocean, was not found  (Fig. 11).  It becomes possible that the volcanic activity comes from individual magma chambers in the crust, heated by tidal stress, and renews an earlier idea that Io’s interior may be more like the composition of the other moons, with liquid water – though presumably less of it, given Io’s greater density.

If there is any, the chances that it sustains life are remote, and the prospect of reaching it is no more likely than the ideas for deep probes into the Earth that crop up from time to time.  (The idea of sinking a probe within a giant blob of molten iron has always rather appealed to me, though I remember a cartoon of a scientist comparing the melting pot with a mechanical mole and saying, “Admittedly, I know which one looks more practical.”)  The same probably applies to Callisto, the outermost of the Galileans, where rock and ice are thoroughly mixed and the gravitational stresses slowly melt the impact craters until they’re flush with the landscape  (Fig. 12).  If there is any liquid water inside, it will be hard to get to  (Fig. 13). 

At first glance, it looked as if Ganymede’s might be a great deal easier to reach.  The first blurred imagery  (Fig. 14)  and long-distance mass estimate from Pioneer 10 in 1974 suggested a density so close to water’s that the moon, despite its size, might be a single drop of liquid water with a thin rocky or ice crust  (news release, NASA Ames Research Center, 4^th May 1976).  I remember the late artist Ed Buckley exclaiming “Good God!” as he read that.  The Voyager images and measurements 5 years later showed that there was a lot of ice on the surface and inside, and although there had been a great deal of crustal movement in the past  (Fig. 15), it might all be frozen solid now. 

The Galileo mission of 1996-97, and the subsequent New Horizons flyby, established that Ganymede has a thin atmosphere and a magnetic field, whose properties indicate internal water, possibly an internal ocean layer  (Figs. 16 & 17), with exotic forms of ice below  (Fig. 18).  Juno’s more recent flybys have done little to change that, though there are suggestions that although there’s evidently a large body of water, deep down, it may be a giant lake rather than a world-spanning ocean.  Either way, it’s out of reach unless we go back to that giant blob of sinking iron.

Europa, however, is an entirely different kettle – maybe even of fish.  The Voyager images revealed a moon which was entirely covered in ice, thick enough to withstand impacts  (Fig. 19), but thin enough to be cracked like terrestrial pack ice, but on a much larger scale, due to tidal stresses on a huge ocean below  (Fig. 20). 

On closer examination, the edges of the cracks were lined with orange, sulphur from Io  (Fig. 21), like the deposit which has covered the spaceship Discovery in 2010, Odyssey 2  (Figs. 22 & 23). 

Very quickly the idea arose that sunlight could penetrate the cracks and life, started perhaps in seafloor volcanic vents as it may have done in Earth’s ‘black smokers’  (Fig. 24), might adapt to colonise those cracks, make use of the sulphur and migrate with them as they open and close.

So could we get down there to find out?  The overall ice crust may be 10 miles thick or more, and the ice in the cracks perhaps a mile thick, so it’s not going to be that easy, even if much easier than on the other Galileans.  A further development was that in 2012-14, and again in 2016, the Hubble Space Telescope detected plumes of water vapour from the south pole of Europa, 125 miles high, like those from Saturn’s ice moon Enceladus  (Figs. 25-27), and re-examining the Galileo data, it turned out that the spacecraft had flown through one of them. 

But for fresh ice deposits from them, Europa would be as thickly covered in sulphur as the Discovery was.  It’s possible that they might provide entry to the waterworld below, but a first question is, do the vents go all the way down, as on the left of Fig. 28, or only to sub-surface lakes and pools like the ones on the right  (Fig. 29)? 

They may be coming from ‘chaotic terrain’ at the south pole of Europa  (Figs. 30-32), and the crevasses there may make the ones in Antarctica look trivial  (Fig. 33). 

Perhaps impact sites might provide easier entry points, if we can find ones recent enough not to have frozen over  (Fig. 34).  Or then again, there are models of the interior which suggest it might just be ice all the way down  (Fig. 35).

A new story in Universe Today suggests that lasers could drill down through the ice, if we can find a suitable spot to land  (Andy Tomaswayk, ‘Lasers Can Melt Through Extraterrestrial Ice Efficiently’, September 13^th 2025).  In one of Sydney Jordan’s Lance McLane stories, he had a megawatt laser which, from orbit. could drill a hole a mile down into a ‘snowball Earth’. big enough to fly a 2-person lander down  (Fig. 36).  It seemed unlikely to me! 

Years earlier, in my If story ‘How to Blow Up an Asteroid’, I had a laser which could drill far enough into ice to plant a shaped charge which could excavate a shaft – then repeat till you got to the depth you want.  But it was nothing like as far as a mile of glacial ice, let alone the possible 10 miles of ice on Europa.  The laser would have to be pulsed to allow the evaporated material to escape from the shaft, and if you’re trying to drill through miles of ice on Europa, the pulses will have to get further and further apart.  My guess is that at some depth, the vaporised material will condense in the shaft on the way up, and block it, so it would have to be re-vaporised and allowed to escape to regain the original depth.  Altogether it seems to me that the method isn’t going to be much good except to implant near-surface probes:  it might have worked on Mars where the INSIGHT drill failed  (see ‘Sky Above You Updates’. ON, 7^th September 2025).  But the same result could probably be achieved more easily with penetrators.  Japan’s HITEN probe was to have tried that on the Moon in 1990  (Fig. 37), but didn’t succeed, and it was also to be tried by CRAF, the Comet Rendezvous Asteroid Flyby mission  (Fig. 38), sister ship to Cassini, which was cancelled. 

So the technique hasn’t yet been used off-planet, though Barnes Wallis used it to good effect with his Tallboy and Grand Slam bombs – see the description of them in the later chapters of Paul Brickhill’s The Dam Busters.  Astrium has designed a penetrator for Europa, initially just to embed a probe in the surface  (Fig. 39), but with the technique established, much more powerful devices could be sent below  (Figs. 40-44). 

We’re still a long way from the underwater tourism of Fig. 45. but maybe it will happen someday.

Duncan Lunan’s recent books are available from bookshops and through Amazon;  details are on Duncan’s website, www.duncanlunan.com.