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The moon Ganymede

GanymedeGanymede. © based on NASA images

Ganymede is the largest moon around the planet Jupiter. Ganymede is also the biggest moon in the solar system. Ganymede is roughly three thousand miles across, making it bigger than the planet Mercury. If Ganymede was orbiting the Sun, it would be considered a planet. Ganymede is large enough to be pulled into a sphere by its gravity.

Ganymede compared with other objects of Solar SystemGanymede compared with other objects of Solar System. © based on NASA images

Ganymede orbits Jupiter every 7.16 days. Its orbit has an average distance of 665,000 miles or a million kilometers from Jupiter. Its orbit is close to Jupiter’s equator. Ganymede is tidally locked, presenting the same face to Jupiter as it rotates. This means that Ganymede’s “day” is the same as its orbit, just over 7 days long.

Jupiter rising over Ganymede with Io and Europa
Jupiter rising over Ganymede with Io and Europa. © NASA

Ganymede has a very circular orbit around Jupiter. It also has minimal orbital inclination. Ganymede is one of the three moons of Jupiter that is in a stable Laplace resonance. For each orbit of Ganymede, Europa orbits twice while Io orbits four times.

Because we know that there is an ocean under Europa’s surface and its approximate volume, scientists have used the overall density of Europa to assume that there is a solid core under the ice that is half as wide as the moon’s total diameter. Since Ganymede is about 5,300 miles across, the solid core would be about 2,650 miles across.

History of Ganymede

Ganymede has areas on its surface as old as the solar system, up to 4.5 billion years old. The gradual erosion of its surface and odd patterns of grooves and cracks are what led scientists to discover its underground ocean.

Ganymede was one of the four moons of Jupiter discovered by Galileo in 1610. Fellow astronomer Simon Marius named it Ganymede for a Greek child who served as a cup-bearer to Zeus or Jupiter, king of the Gods. The moons of Jupiter were initially assigned numbers. The name Ganymede was unofficial from the start but became official in the early 1800s.

Geography of Ganymede

We know that Ganymede has a liquid water ocean under its icy exterior. Ganymede does not have a volcanic core heated by the decay of radioactive elements like Earth and Venus. Instead, Ganymede’s core is heated by the gravitational tug of war between Jupiter and the other moons around Jupiter. Ganymede is the only moon to our knowledge to have a magnetic field. This may be a sign of a molten magnetic core, albeit much smaller than Earth’s.

The surface of Ganymede is relatively smooth. It has craters from meteor impacts. However, anything large that breaches the frozen surface causes water to fill it.

Ganymede has an infinitesimal atmosphere. Saturn’s moon Titan has a full atmosphere composed of hydrocarbons, including ethane clouds and methane rain. Ganymede’s atmosphere contains traces of oxygen, probably the result of ice on the surface broken down into molecular hydrogen and oxygen. Hydrogen has a lower escape velocity, leaving the traces of oxygen behind. There are also traces of ozone, caused by particles crashed into Ganymede‘s atmosphere propelled by Jupiter’s magnetic field.

Internal structure of Io, Europa, Ganymede and CallistoInternal structure of Io, Europa, Ganymede and Callisto. © based on NASA images

Ganymede has areas that are up to 4.5 billion years old. And there are much younger areas marked by grooves and look like filled in cracks. This terrain may be the result of slow moving ice flows on the Ganymede surface. Or they may result from tidal forces causing seismic shifts similar to tectonic plates on Earth. Where a break or crack occurs, the frozen slush rises up and fills in the crack creating a relatively smooth groove that runs for hundreds of miles.

The primary landmark on Ganymede is a large crater called Anat. However, there are many smaller craters sprinkled across Ganymede. The older portions of Ganymede’s surface, those not filled in by ice water below, have more craters than the newer portion of the planet surface.

Craters that have been nearly completely worn away or filled in reveal that Ganymede’s surface is still changing with time. Ganymede has no large mountains caused by impact craters like Earth’s moon, since there was no hot, molten rock to be blasted into the atmosphere upon impact. Any mountains that did occur were worn down to low ridges.

Life on Ganymede?

Surface of Ganymede
Surface of Ganymede. © NASA

We do not know if Ganymede holds life under its thick icy exterior. However, it has many of the prerequisites we think life needs. Liquid water exists. The planet has an ocean that contains more water than Earth’s oceans, though at far greater pressures than even the bottom of the Mariana Trench in the Pacific Ocean. It has energy in the form of a molten interior. Ganymede is theoretically a more stable home for life, since it will be warmed by tidal heating by Jupiter long after the Earth’s core cools while the thick layers of ice protect any ocean life instead from asteroid strikes like the one that wiped out the dinosaurs. Ganymede has a rocky core and rock mixed in with the ice, providing elements similar to those available to undersea life. This means that Ganymede has the potential of evolving sea life similar to those that live at the bottom of Earth’s ocean, living off of undersea thermal vents. Life on Ganymede would evolve without needing sunlight, since none penetrates the thick ice of Ganymede. In this way, Ganymede is similar to Europa as a potential home for life in our solar system.

Exploration of Ganymede

JUICE will launch in June 2022 on an 11-year mission to explore Jupiter and several of its moons
JUICE will launch in June 2022 on an 11-year mission to explore Jupiter
and several of its moons. © NASA

Both Pioneer 10 and Pioneer 11 flew past Ganymede. Voyager 1 flew close to Ganymede on its journey through the solar system. Voyager 2 provided more detailed information about the moon’s size. It was the Voyager craft that proved that Ganymede was the largest moon. The Hubble space telescope in orbit around Earth was what discovered the thin oxygen atmosphere around Ganymede.

A Jupiter moon exploration mission was cancelled in 2005. The Europa/Ganymede penetrator is a proposed spacecraft. It would be carried by either the Jupiter Ganymede Orbiter or Jupiter Europa Orbiter and drop to Ganymede’s surface similar to the Titan Mare Explorer that gave us detailed views of Titan’s surface. It would be able to penetrate the surface of Ganymede about a meter.

The European Union Space Agency is designing an exploration craft to be sent to the moons of Jupiter. This craft will enter Ganymede orbit, but it is not yet known if it will drop probes to the surface. It is tentatively planned to launch in 2022. The craft would arrive at Jupiter about ten years later.