Swapna Krishna

Feb 14th 2022

Why Ganymede Is Unique Among the Solar System’s Moons


As we continue to discover exoplanets and worlds outside our immediate celestial neighborhood, it’s easy to forget that our own solar system still holds undiscovered wonders. Ganymede, one of Jupiter’s four Galilean moons (so named because they were discovered by Galileo), is an icy world and the largest moon in our solar system. It’s even larger than Mercury.

This, along with many other features, makes this an incredibly unique moon.

The Moon’s Magnetic Field and Thin Atmosphere

In 1996, the Galileo spacecraft made a stunning discovery about Ganymede — that the largest moon in the solar system actually has its own magnetic field. We don’t know of any other moons that have one, and this suggests that the moon has an iron core similar to Earth’s.

What’s more, that same year, the Hubble Space Telescope detected evidence of an atmosphere on the moon. It’s way too thin to support life as we know it, but these discoveries suggest that the moon is more Earth-like than most planets in the solar system, possibly with the exception of Mars.

The European Space Agency’s JUICE mission, which is scheduled to launch in 2022, will study Ganymede, Callisto and Europa with an eye toward understanding how Ganymede formed by testing its atmosphere and studying its magnetosphere.

Details About the Moon’s Subsurface Ocean

We know that this moon has a thick, icy surface and a rocky mantle that likely protects the liquid iron core. It’s unclear whether the surface is pure ice or has some rock mixed in, but what scientists did discover in 2015 — thanks to the Hubble Space Telescope — is that there may be water on Ganymede’s surface, too.

The largest moon in the solar system may harbor a salty ocean that resides under the ice we’ve detected on the surface. In fact, it’s so much water that, according to, NASA officials have said that the moon’s oceans may harbor more water than all of the oceans on Earth combined.

Scientists uncovered this clue by looking at the moon’s auroras, which are created by its magnetic field. Because the magnetic field is contained within the massive one around Jupiter, the interaction between the two can tell researchers a lot about what’s happening on and under this moon’s surface.

When the scientists discovered that the interaction between the two wasn’t going quite as they expected, they looked to see what on the moon’s surface could be the culprit — and theorized that it’s actually a giant subsurface ocean causing the discrepancy.

Water Vapor in the Atmosphere

The most recent exciting discovery about this unique moon came in July 2021, when scientists found the first evidence of water vapor in the moon’s atmosphere. The discovery actually came from analyzing previous data from the Hubble Space Telescope, as the scientists theorized that the oxygen within the large moon’s atmosphere was actually water vapor. Their findings are outlined in an academic paper from Nature Astronomy.

Water vapor forms as ice or water, turns to gas and escapes through the atmosphere. But because the moon is so cold, scientists knew it couldn’t be liquid water escaping beneath the ice crust. Therefore, the vapor had to be coming from the crust itself. Scientists discovered that around noon each day, it is warm enough at the equator for the sun to briefly chip away at the icy crust and for that ice to turn into water vapor.

What’s Next for the Largest Moon in the Solar System

As previously mentioned, the JUICE (JUpiter ICy moons Explorer) probe will be the next major mission to study the largest moon in the solar system. It will arrive among the Jovian moons in 2029 and has a three-year mission to investigate three of Jupiter’s largest moons: Ganymede, Europa and Callisto.

JUICE will be able to shed light on and answer many questions we still have about one of Jupiter’s most fascinating moons, from analyzing the contents of its atmosphere to confirming the presence of an ocean on each of these worlds. JUICE’s main mission is to help scientists understand how these worlds evolved and what factors go into habitable environments. It will be exciting to watch these developments unfold and learn more about this special moon.

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