Ganymede Facts: 7 Interesting Facts About Jupiter’s Moon

Ganymede has a number of interesting characteristics that make it unique among the moons that we know of. Here are some of the fun and interesting facts about Ganymede, one of the four moons of Jupiter:

Ganymede, taken by NASA

Fact 1: It wasn’t always named Ganymede

Galileo did not give Ganymede and the other large satellites of Jupiter their names, however; he named the moons the Medician Stars after the Medici family who funded his experiments. Another astronomer, Simon Marius, independently discovered all four moons shortly after Galileo and decided they should be named after the mythological companions of the Roman lightning god Jupiter, or his Greek equivalent, Zeus. Ganymede was in charge of carrying cups of wine at the gods’ banquets. However, this naming convention did not stick until the mid-1900s. Astronomers preferred to refer to it as “Jupiter III” or “the third satellite of Jupiter” or even just “the largest satellite of Jupiter”, largely for shorthand and translation reasons.

Fact 2: It has its own magnetic field and aurora

One of the most intriguing aspects of Ganymede is its intrinsic magnetic field, making it the only moon in the solar system known to have such a feature. This magnetic field, generated by the moon’s iron core, helps to form a magnetosphere around Ganymede, which interacts with the charged particles of Jupiter’s magnetic field. This interaction has been key to understanding more about the moon’s internal structure and composition. Ganymede is the only moon known to possess its own, self-produced magnetic field. 

The awe-inspiring display of auroras is not confined to Earth alone. Ganymede, fascinatingly, is known to experience its own magnetic auroras. Ganymede’s extremely tenuous (less than 0.001% as thick as Earth’s) oxygen atmosphere combined with the moon’s magnetic field means that it can also host aurorae. However, these aurorae are extremely faint and are only visible in ultraviolet light, meaning you could not possibly see them even if you stood on Ganymede’s surface.

Speaking of standing on Ganymede’s surface, you really can’t. Ganymede receives about 1 rem of radiation a day from Jupiter, which means that in ten days you’d be exceeding the maximum amount of radiation NASA lets astronauts receive over their entire career. Unless you walked around in a lead box, you’d probably die of cancer or radiation sickness very quickly. Jupiter’s more distant moon Callisto is far more hospitable for life than Ganymede since it lies outside the most irradiated parts of the giant planet’s magnetic field. However, as mentioned earlier, for probes, the surface of Ganymede is indeed a less hostile place than Mars or Europa, which are far more popular targets for exploration despite the hazards.

Fact 3: It has an underground ocean, which may host life

Like its neighboring moons Europa and Callisto, Ganymede possesses a thick, salty ocean beneath its ice/rock crust. It is possible that this ocean is more than a hundred kilometers thick, which would make it the biggest ocean in the Solar System.

Ganymede is not as suitable for life as Europa for a number of reasons. Europa is subject to more tidal heating from Jupiter and its neighboring moons and is composed more of H2O than rocks (unlike Ganymede, which is just below 50/50), and Ganymede is lacking in organic compounds such as tholins, which are key ingredients for forming life. Ganymede also has less geological activity than Europa, which may also be influential.

Fact 4: It has an unusual chain of craters

Ganymede: Torn Comet - Crater Chain

Crater chains, or catenas, occur in various places throughout the Solar System, but most are very worn away and hard to recognize. Ganymede’s Enki Catena, however, is a fairly recent marker of an unusual impact event. Enki was probably formed when a loosely-bound asteroid or comet broke up due to a close pass by Jupiter, and the fragments struck Ganymede in succession. This chain is just over 100 miles long and comprises 13 craters in all.

Fact 5: It is in orbital resonance with Jupiter’s other large moons

The orbital resonance of Ganymede is a fascinating aspect of its celestial mechanics. Ganymede participates in a 1:2:4 orbital resonance with Jupiter’s moons Europa and Io, meaning that for every orbit Ganymede completes around Jupiter, Europa completes two, and Io completes four. This precise dance is a result of their gravitational influences on one another, maintaining their orbits in a stable configuration over time. This interaction has implications for the geological activity on these moons, including the possibility of tidal heating, which could contribute to maintaining subsurface oceans. Like the other Galilean moons and our own Moon, Ganymede is tidally locked to Jupiter, making one rotation around its axis every 7.154 days, or the same time it takes to complete one orbit. As such, the same side of Ganymede always faces Jupiter.

Fact 6: It will be the first moon other than Earth’s to be orbited by a spacecraft

Ideas for a Ganymede orbiter have been floating around for decades, but it has taken a while for anything to reach past the proposal stage and receive funding. Ganymede has largely been neglected by scientists in favor of volcanically active Io or potentially life-harboring Europa, despite its uniqueness, potential for life, easier conditions for spacecraft, and proximity. Numerous missions to Ganymede (albeit usually part of a broader program including Europa and/or other Jovian moons) have been proposed by NASA, ESA, and other space agencies, including JAXA and the Russian space agency Roscosmos. However, ESA’s JUICE spacecraft was the first to actually get off the drawing board.

The European Jupiter Icy Moons Explorer spacecraft (JUICE) launched in 2023 and will arrive at Jupiter in 2031. After numerous flybys of Europa, Ganymede, and Callisto, JUICE will enter orbit around Ganymede in 2034 and remain there for at least 12 months, after which it will either deorbit or (more likely) the mission will be extended.

But, over the decades, our understanding of Ganymede has been greatly enhanced by various space missions. Pioneer 10, in the early 1970s, provided the first close-up images of Ganymede. The Voyager missions in the late ’70s further refined our knowledge, discovering signs of the moon’s intrinsic magnetic field. In the late 1990s and early 2000s, the Galileo mission confirmed the existence of Ganymede’s magnetosphere and suggested the presence of a subsurface ocean. The New Horizons mission in 2007 provided more valuable data during its flyby en route to Pluto. The Juno mission, which started in 2011 and extended into the 2020s, has continued to explore the complex interactions between Ganymede and Jupiter, further enhancing our understanding of this captivating moon. NASA’s Europa Clipper will also fly by the icy moon several times in the 2030s.

Ganymede has been the subject of a number of proposals for a robotic landing mission, as it is further from the ionizing radiation of Jupiter, which would quickly fry anything touching down on Io and Europa, the inner Galilean satellites. Its magnetic field also provides some protection from Jupiter and the solar wind, while ice on the surface could even be refined into breathable air or rocket fuel. However, the only serious effort has been by the Russian space agency, whose Laplas-P probe (originally envisioned to hitch a ride with JUICE), was cancelled due to the failure of the Phobos-Grunt mission in 2011 and subsequent cuts to the Russian planetary sciences department, along with delays to the super-heavy variant of the Angara rocket that was to launch it.

Fact 7: Ganymede is the largest Moon in the Solar System

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME Magazine, National Geographic, Sky & Telescope, La Vanguardia, and The Guardian. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars.

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