Ganymede, Jupiter’s largest moon, stands out as a remarkable celestial body. It is the largest moon in our solar system, surpassing even Mercury in size. This massive moon possesses its own tenuous atmosphere. The presence of an atmosphere on such a distant and cold world invites closer examination of its formation and unique properties.
How Ganymede’s Atmosphere Was Discovered
The Hubble Space Telescope played a central role in discovering Ganymede’s atmosphere, providing data in the ultraviolet spectrum. In 1995, Hubble detected specific light emissions, known as airglow, from atomic oxygen. This airglow is produced when molecular oxygen is broken apart by electron impacts, indicating a neutral atmosphere composed mainly of O2 molecules.
Further ultraviolet images from Hubble in 1998 showed distinct auroral bands, similar to those seen on Earth, suggesting Ganymede possessed a magnetic field. Initially, scientists interpreted these as evidence of atomic oxygen. However, a re-examination of Hubble data in 2018 revealed very little atomic oxygen, leading to a new explanation for the observed ultraviolet auroras. This re-analysis ultimately pointed to the presence of water vapor, which sublimates from the icy surface, as a factor in the emissions.
What Ganymede’s Atmosphere is Made Of
Ganymede’s atmosphere is primarily composed of molecular oxygen (O2), with recent evidence also indicating the presence of water vapor. This oxygen is not a sign of life, but a product of a geological process. Ganymede’s surface, rich in water ice, is constantly bombarded by charged particles from Jupiter’s powerful magnetosphere.
This bombardment leads to radiolysis, where energetic particles strike water ice molecules. During radiolysis, water molecules (H2O) are split into hydrogen and oxygen. The lighter hydrogen atoms readily escape Ganymede’s weak gravitational pull. The heavier oxygen molecules remain, gradually accumulating to form the moon’s thin atmosphere. Water vapor forms when ice from Ganymede’s surface sublimates, directly turning from a solid into a gas.
The Thin Atmosphere and Its Auroras
Ganymede’s atmosphere is exceptionally tenuous, characterized as an “exosphere” where particles are so sparse they rarely collide. The surface pressure is incredibly low, roughly a hundred billion times less than Earth’s atmospheric pressure.
Despite its sparse nature, Ganymede’s atmosphere is visible through spectacular auroras. These auroras result from the interaction between Ganymede’s intrinsic magnetic field and Jupiter’s magnetosphere. Ganymede is the only moon in our solar system known to possess its own internally generated magnetic field, which creates a protective magnetic bubble around it.
As Ganymede orbits Jupiter, its magnetic field interacts with charged particles from Jupiter’s magnetosphere. This interaction funnels energetic electrons and ions towards Ganymede’s polar regions, where its magnetic field lines connect to Jupiter’s. When these charged particles collide with the oxygen and water vapor in Ganymede’s thin atmosphere, they excite the atmospheric gases, causing them to emit light. These emissions appear as glowing ribbons, similar to Earth’s Northern and Southern Lights, but on Ganymede.
The auroras form bright spots or ovals near the moon’s north and south poles and have been observed to be stable over many years. The oscillation of these auroras can also provide clues about Ganymede’s interior, including its subsurface ocean where the moon’s magnetic field is generated.