by Duncan Lunan

For much of my life we thought Jupiter had just twelve moons, eventually pushed up to 13:  the inner moon Amalthea, the four large Galilean satellites, and two groups of four in highly inclined, crisscrossing orbits, thought to be the remains of two captured asteroids.  (The outer four are in retrograde orbit.)   But after multiple spacecfraft missions, by 2003 the count was around 60, and it’s now reached 79.  

The four large Jupiter moons, discovered by Galileo and called the Galilean satellites in his memory, are Io, Europa, Ganymede and Callisto, in order out from the planet.  If they were orbiting the Sun they would be considered to be planets, and but for the glare of Jupiter, they would be easily visible to the naked eye.  The four Galilean moons are like a Solar System in miniature:  Io has intense volcanic activity and a dominant sulphur chemistry, like Venus, Europa has an ice crust covering an ocean of liquid water like Earth’s, Ganymede’s crust is part ice, part rock, like Mars’s, and Callisto has a more thorough ice/rock mixture may tell us what the asteroid planet could have been like if Jupiter hadn’t prevented it from forming.  The analogy between the moons of Jupiter and the inner planets was striking even for Victorian astronomers.  In Other Worlds than Ours  (1887), R.A. Proctor wrote,

“Once when an assistant had unluckily broken a slide showing the orbits of Mercury, Venus, Earth and Mars about the Sun, I, for the occasion, substituted a slide showing the orbits of Jupiter’s four moons about their central planet.  No one noticed the change.”  

The energy requirements for transfers between them are comparable to interplanetary missions   (Jerry Pournelle, ‘Those Pesky Belters and their Torchships’, in A Step Father Out, W.H. Allen, 1980).

The Galilean moons would follow circular orbits, forced into them by Jupiter’s pull, but for the gravitational effects they have on each other.  The moons orbit Jupiter with periods which resonate almost exactly with one another – a situation which would be dynamically unstable but for the perturbations which their respective pulls impose.  The orbital periods of Jupiter’s moons are ‘commensurate’, causing the patterns they take up to recur periodically, and one of the respects in which they resemble the inner planets of the Solar System is that they’re in what the late Prof. Archie Roy called ‘statistical stability’, avoiding configurations which would make the whole system unstable.  For all four moons to be in line on the same side of the planet is one of the forbidden configurations, and it isn’t possible for the four moons to be in line with Jupiter and the Sun – one of the clues at the end of 2001, a Space Odyssey that what we’re seeing is not as it appears.

The resonances were known long before spacecraft reached Jupiter, but what nobody realised was how powerful the resulting tides would be.  They generate violent volcanic activity on Io, maintain a planet-wide ocean under the icy surface of Europa, and warm the crusts of Ganymede and Callisto enough to form more liquid water within them.  On Io, the volcanoes emit hydrogen sulphide and sulphur dioxide which are disassociated by sunlight, forming a very thin hydrogen atmosphere;  they also emit salt  (perhaps dissolved in water)  which sputters under the bombardment of Jupiter’s radiation belts and laces the atmosphere with sodium, making it electrically conducting. 

Io’s sulphur cloud extends far into space, coating the surfaces of Europa and Amalthea  (the small moon next closer to the planet).  That’s why in the film of 2010 the Discovery is coated in sulphur when the recovery party reaches it, though the film never explains it.  Io’s electrically active atmosphere envelops Jupiter, expanding to form a torus around the orbit, and envelop the planet over the poles.  An ionised flux tube flows continuously between Io and the planet, conducting multi-megaton lightning bolts to and from Jupiter when the moon’s conducting core cuts across the equatorial sheet of Jupiter’s off-centre magnetic field, causing decametric radio bursts every 21 hours which were first detected in the 1950s.  The scientific view is that the multimegaton strike would be spread over the hemisphere of the moon facing the planet, but ‘I hae ma doots’ about that:  natural or artificial circumstances on the moon could localise it, to cause a ‘Thor’s Hammer’ effect, and I’d be surprised if that has nothing to do with triggering the volcanoes.  With all that and the ‘supralethal’ radiation flux, I reckon Io qualifies as ‘the most dangerous place in the Solar System’!

The surface of Europa has virtually no craters or mountains, because the ice is smoothed out by tidal activity and there’s little surface rock if any.  Instead, a complex network of cracks forms, and there could even be life in Europa’s ocean where sunlight penetrates into the water, as Arthur C. Clarke portrays in the novel 2010 – an extended sequence with a Chinese landing on Europa which didn’t occur in the film.  Volcanic vents below, driven by tidal energy like Io’s volcanoes, could sustain life without light, as in Earth’s ‘black smokers’ on the deep ocean floor.  

Under Antarctica, Lake Vostok, 4000 metres below the ice, is as large in area as Lake Ontario but three times as deep, at 500 metres.  It’s been buried for at least five million years and possibly for 30-40 million years, since the ice-cap formed.  Yet there are indications that it may still have bacterial life, if not more, and the international scientific community demanded great efforts to ensure that it wouldn’t be contaminated when the Russians drill down to it and send in robots.   From 1999 to 2006, drilling was halted 120 metres above the water, and it was finally penetrated in 2011, when little or no trace of life was found.  However, in 2012 drilling into briny slush at the bottom of Lake Vidal in the McMurdo dry valleys found living microorganisms, after more than 2,800 years of isolation.  Techniques being developed for Lake Vostok and others may later be used to explore the ocean on Europa.

In Yucutan there are huge circular wells in the limetone rock, geologically termed cenotes.  The cenotes are giant wells, where rising fresh water has cut away the limestone from below:  the Maya used them for sacrifices, both of treasure and of people.  There’s spectacular footage of one in the film of Erich von Däniken’s Chariots of the Gods, where he suggests that it’s been formed by the exhaust of a space vehicle taking off.  It was indeed caused by a visitor from outer space.  The cenotes lie along the stress rings, concentric with the impact point offshore, produced by the Chicxulub impact which killed the dinosaurs.  The offshore ones are a danger to shipping, especially submarines, because the rising fresh water is less dense than the seawater around it. 

In an episode of Seaquest DSV, at the time when Robert Ballard was still scientific advisor to the programme, the submarine inadvertently entered one of the freshwater columns and because it was trimmed for salt water, it plunged through the seafloor into the aquifer below.  The first explorer to dive in a cenote was Edward Herbert Thompson, at Chichen Itzá in 1904-07  (E.H. Thompson, ‘Into the Sacred Well’, extract from People of the Serpent, Houghton Mifflin, 1932, reprinted by Robert Silverberg in Great Adventures in Archaeology, Penguin, 1985.)  In 2007 British divers claimed a record in exploring the caves below the cenotes to a distance of seven miles, and the Stone Aerospace company of Texas has used them for tests of the DEPTHX vehicle, a possible prototype for exploring the oceans below the ice of Europa.

The active surfaces of Io and Europa erase impact marks, but the two outer Galilean moons, Ganymede and Callisto, are both composed of mixtures of ice and rock, and heavily cratered.   Ganymede is the largest moon in the Solar System, just 50 miles bigger in diameter than Saturn’s moon Titan, but both are larger than Mercury.  Ganymede has now been found to have a very thin atmosphere, and both Ganymede and Callisto have internal water oceans, kept liquid by tidal heating.

Comet Shoemaker-Levy  (SL-9)  passed close by Jupiter in July 1992, and split into 22 fragments which were discovered in March the following year.  They hit Jupiter in July 1994, on the far side of the planet from Earth, but the flares were seen by the Galileo space probe and over the rim of Jupiter by the Hubble Space Telescope.  As Jupiter’s rotation brought the impact points into view, optical and infra-red telescopes saw features the size of the Earth forming in Jupiter’s bitterly cold cloud layers. 

Even at Airdrie Observatory, where I was Assistant Curator at the time, with a six-inch refracting telescope we could clearly see the impact scars a week later.  The whole sequence was much more spectacular than expected – with correspondingly scary implications about past and future impacts on Earth.  Crater chains on Callisto and Ganymede may be due to SL-9-type events –   Callisto has 12 or 13 of these and Ganymede three.  They were previously ascribed to secondary cratering, where debris ejected by an impact falls back, but no ‘parents’ could be identified.  Our Moon has secondary chains, e.g. from the craters Davy and Humboldt, but they begin next to their ‘parents’.  Callisto is the most cratered moon we know in the Solar System, with a crust of thoroughly mixed rock and ice.  One hemisphere is dominated by Valhalla, a huge multi-ringed impact feature, “rippled as if a continent had struck it”, to quote the late Prof. Edwin Morgan’s poem sequence The Moons of Jupiter.   (In his anthologies below and the anthology I edited, Starfield, Science Fiction by Scottish Writers, Orkney Press, 1989, revised and reprinted by Shoreline of Infnity, 2018).  

Of the four large Galilean moons, only Callisto is outside the radiation belts.  That’s why it was planned to be the base for constructing the interstellar probe in the British Interplanetary Society’s ‘Project Daedalus’  (reprinted as K.F.Long and P.R. Galea, eds, Project Daedalus, Demonstrating the Engineering Feasibility of Interstellar Travel, BIS, ), and why Nigel Calder’s Spaceships of the Mind foresaw future conflict between the Earth and Callisto over the resources of the Asteroid Belt  (BBC, 1978).   

Voyager 2 photographed all four Galilean moons and also the tiny moon Amalthea, discovered by Barnard in 1892 and designated ‘Jupiter V’ until its name was ratified by the International Astronomical Union during the Pioneer 10 flyby in 1974.  Amalthea is elongated and coloured red by sulphur deposited on it from volcanic eruptions on Io.   It was thought that Amalthea might have been stretched during its formation by tidal forces exerted on it by Jupiter, and Edwin Morgan imagined them still in force in his poem sequence ‘The Moons of Jupiter’.  (Star Gate, Science Fiction Poems by Edwin Morgan, Third Eye Centre  (Glasgow), 1979, Collected Poems, Carcanet, 1990).

The Galileo spacecraft reached Jupiter in November 2005 and entered orbit in December after releasing an entry probe into the atmosphere.  In September 2003 Galileo was deliberately de-orbited into Jupiter to prevent it from contaminating any of the planet’s moons in the future.  A mysterious, unidentified painting on the cover of the January 2003 Analog is actually a detail from an artist’s impression of the probe’s end, which appeared in full in the Astronomy Now 2003 Yearbook.

Galileo’s instruments detected large empty spaces within Amalthea, and NASA scientists suggest the new findings mean that Amalthea is not solid, but just an aggregation of boulders.  They’re overlooking Arthur C. Clarke’s suggestion that it may be a giant spaceship, which he described in his story ‘Jupiter Five’, written before Amalthea’s name was ratified by the International Astronomical Union  [in his collection Reach for Tomorrow  (1956)  and illustrated in The Sentinel (Panther, 1985).  It wouldn’t be the first time his predictions have been correct…

Amalthea was not to remain Jupiter’s ‘innermost satellite’ for very long.  Voyager 1 passed its closest approach to Jupiter on March 5^th 1979, and Voyager 2 passed Jupiter soon afterwards.  On March 7^th scientists at the Jet Propulsion Laboratory announced the discovery of a narrow ring around Jupiter, composed of dust ejected by the volcanoes on Io, themselves identified only on March 8^th by Linda Morobito.  In a Voyager 1 photo of March 5^th and a Voyager 2 image of July 8^th two new satellites were found, Adrastea and Thebe, shepherding the ring, and subsequently Metis was found within the ring itself.   (David Morrison and Jane Samz, Voyage to Jupiter, NASA SP-439, US Government Printing Office, 1980;  Michael Hanlon, The Worlds of Galileo, Constable, 2001).

At the time of the Pioneer 10 mission to Jupiter in 1974, the eleventh known moon out from the planet, also the eleventh to be discovered, was informally named ‘Pan’.  Jupiter VII was discovered by Perrine in 1905, and informally named ‘Hestia’.  Jupiter X was named ‘Demeter’.  But for some unknown reason, the International Astronomical Union refused to recognise the names allocated to any of Jupiter’s moons after the four big ‘Galilean’ satellites  (which he had wanted to name after his patrons, the Medici). 

It was analogous to the situation with compass points on the Moon, where by convention, Moon maps were drawn with south at the top, but with east on the left and west on the right to correspond with the terrestrial horizons they were nearer to.  The convention is the same with constellations, for very good reasons which nevertheless cause vast confusion.  In the case of the Moon, it meant that the Sun rose in the west even though the Moon rotates on its axis in the same direction that the Earth does – and that the huge impact basin of Mare Orientale, ‘the Eastern Sea’, of which Patrick Moore was a co-discoverer, is actually on the western limb of the Moon as truly defined with reference to Sinus Medii, the Central Bay, which marks the Moon’s zero longitude.  

But during the 1960s, the US Air Force mapped the Nearside of the Moon in hitherto unprecedented detail, by radar, in advance of the Moon landings.  Those maps were drawn for use by the astronauts in situations where confusion might be fatal, so they were drawn with north at the top and east and west where they should be.  Because they were the best maps available, lunar observers immediately began to use them, and to their chagrin, the IAU found that what should have been their decision had been bypassed.

So the moons of Jupiter had a very good set of names which it was fashionable not to use, instead using numbers which, unlike the systems for any of the other planets, numbered the moons in order of discovery instead of sequence outwards from the planet.  It was a load of old nonsense and when Pioneer 10 was approaching Jupiter in 1973, NASA would have none of it and identified each moon whose orbit the probe crossed by its unofficial name.

“Not again you don’t!” cried the IAU.  If NASA got away with abrogating the IAU’s powers a second time, their credibility would be shot forever.  The IAU responded by rushing out a new set of names for Jupiter’s moons.  By that time it had become common to refer to Jupiter V as ‘Amalthea’, so they let that stand and renamed all the rest of the ‘unofficially’ named ones, declaring that for consistency all those in direct orbit would have new names ending in ‘a’, like Amalthea, while the outer ones in retrograde orbit would have new names ending in ‘e’.  Jupiter VII was renamed ‘Himalia’.  Counting outwards, Himalia is the third of the direct-orbit group.  ‘Jupiter X’, Lysisthea, is the next one inward, because they’re still numbered in order of discovery.  ‘Pan’ became Carme, because its orbit is retrograde – almost certainly a captured asteroid.  But in the rush, the IAU forgot that the largest moon in the Solar System is Ganymede, in direct orbit, whose name ends in ‘e’.

Like all the moons of Jupiter except Amalthea and the Galileans, Pan/Carme has never been photographed in detail by spacecraft.   When everything that has been photographed has turned out to be surprising, that may yet prove to be a major omission.

NASA’s Juno spacecraft was launched in 2011, and after Earth flybys to gain speed in 2012 and 2013, it reached Jupiter in 2016.  The most distant spacecraft to be powered by solar panels, to date, Juno went into orbit over the poles and returned spectacular images showing that the polar regions were much more active and storm-filled than had been supposed – possibly putting paid to the idea that they might be the home of airborne life. 

In December 2016 Juno was intended to descend to a lower orbit over the poles and eventually to plunge into the planet, to avoid any chance of contaminating the Galilean moons, especially Europa.  But the retrofire was cancelled due to a problem with the propulsion system, and the mission was extended to July 2022, then in January 2021 to September 2025 at least.  Meanwhile the elliptical orbit has been precessing due to the pull of the planet’s equatorial bulge, allowing close flybys of the Galilean moons for the first time in 20 years.  Ganymede was passed on the 34^th orbit in June 2021, Europa will be passed on the 45^th in December 2022, followed by two flybys of Io in 2024, centred on the 45^th orbit.  But all three moons will be passed at greater distances meantime, and if the spacecraft remains operational beyond that, no doubt there will be more surprises to come. 

Interested in space exploration and astronomy? check out our other articles by Duncan Lunan – including this: The Sky Above You – September 2021