Jupiter’s moon Ganymede is the largest moon in our solar system, bigger than the planet Mercury and dwarf planet Pluto. NASA’s Hubble Space Telescope has found the best evidence yet for an underground saltwater ocean on Ganymede. The ocean is thought to have more water than all the water on Earth’s surface. Ganymede’s ocean is estimated to be 60 miles (100 kilometers) thick – 10 times deeper than Earth’s ocean – and is thought to be buried under a 95-mile- (150-kilometer-) thick crust of mostly ice. Identifying liquid water is crucial in the search for habitable worlds beyond Earth, and in the search for life as we know it.
Ganymede is the only moon known to have its own magnetic field – a discovery made by NASA’s Galileo spacecraft in 1996. The magnetic field causes auroras, which are ribbons of glowing, hot, electrified gas, in regions circling the north and south poles of the moon. Because Ganymede is close to Jupiter, its magnetic field is embedded in, or lies within, Jupiter’s magnetic field.
When Jupiter’s magnetic field changes, the auroras on Ganymede also change, “rocking” back and forth. It was by watching the rocking motion of the two auroras, that a team of scientists led by Joachim Saur of the University of Cologne in Germany came up with the idea of using the Hubble space telescope to learn more about the inside of the moon.
Discovery and Name
Ganymede was discovered by Italian astronomer Galileo Galilei on Jan. 7, 1610. The discovery, along with his discovery of three other large moons around Jupiter, was the first time a moon was discovered orbiting a planet other than Earth. The discovery eventually led to the understanding that planets in our solar system orbit the Sun, instead of our solar system revolving around Earth. (Jupiter now has 53 named moons and 26 provisional moons awaiting confirmation of discovery).
In mythology, Ganymede (“GAN uh meed”) was a beautiful young boy who was carried to Olympus by Zeus (the Greek equivalent of the Roman god Jupiter) disguised as an eagle. Ganymede became the cupbearer of the Olympian gods.
Potential for Life
A computer model of Ganymede’s interior created in 2014 supported the idea that the development of primitive life might be possible there. The model indicated the icy moon’s rocky sea bottom might be in contact with salt water. Scientists think water and rock interacting are key for the development of life.
Size and Distance
Ganymede has a radius of 1,635 miles (2,631 kilometers) and is the largest moon in our solar system. It’s bigger than Mercury and Pluto. Ganymede is about 665,000 miles (1.07 million kilometers) from Jupiter, which orbits about 484 million miles (778 million kilometers) from the Sun. Jupiter is 5.2 astronomical units away from the Sun. One astronomical unit (abbreviated as AU) is the distance from the Sun to Earth. From this distance, it takes sunlight 43 minutes to travel from the Sun to the Jovian system.
Orbit and Rotation
Ganymede orbits Jupiter at a distance of 665,000 miles (1,070,000 kilometers), making it third in distance from Jupiter among the Galilean satellites. Ganymede completes an orbit around Jupiter about every seven Earth days (7.155). Ganymede orbits the Sun along with Jupiter and its other satellites every 12 Earth years.
Formation
Ganymede and Jupiter’s other large moons (Io, Europa, and Callisto) likely formed from leftover material after Jupiter condensed out of the initial cloud of gas and dust surrounding the Sun, early in the history of our solar system. Ganymede is likely about the same age as the rest of the solar system – about 4.5 billion years old.
Atmosphere
Astronomers using the Hubble Space Telescope have found evidence of a thin oxygen atmosphere on Ganymede. The researchers think the oxygen is coming from Ganymede’s icy surface. Ganymede is much colder than Earth, with daytime surface temperatures ranging from -297 to -171 degrees Fahrenheit (90 to 160 Kelvin). Jupiter and its moons receive less than 1/30th the amount of sunlight that the Earth does, and Ganymede doesn’t have a dense atmosphere to trap heat.