What leads scientists to suspect that there is liquid water beneath Ganymede's surface?

The discovery of a subsurface ocean on Ganymede is primarily attributed to the detection of a magnetic field. When the Galileo spacecraft observed distinct magnetic fields around Ganymede, it suggested a conducting layer below the surface.

This finding is significant due to the fact that liquid saltwater is an excellent conductor of electricity. As the icy moon orbits Jupiter, the movement of this saltwater generates a magnetic field, much like how Earth's liquid outer core creates our planet's magnetic field. This phenomenon is known as the dynamo effect. In Ganymede's case, the dynamo effect indicates a vast ocean beneath the crust, interacting with and affected by Jupiter's powerful magnetic field, leading to further interesting characteristics such as auroras, which are also indicative of magnetic interactions.

Ganymede's surface presents a historical record of its geological activity and potential evidence for a subsurface ocean. By examining the contrasting terrains, scientists interpret the moon's geological history.

The dark terrain consists of highly cratered and older areas, implying that this surface has been stable for extended periods, allowing for a greater accumulation of impacts. In contrast, the brighter regions exhibit patterns suggesting geological activity, such as ridges and grooves likely formed through tectonic processes.

Understanding Tectonic Activity

Such tectonic activity may be driven by tidal heating, which is the result of Ganymede's gravitational interaction with Jupiter. This relationship can generate heat due to the deformation of the moon's interior, which might help keep a subsurface ocean in a liquid state over geological timescales - a crucial factor for the potential harboring of extraterrestrial life.

Auroras observed on Ganymede are another fascinating aspect that supports the existence of a subsurface ocean. Normally, auroras are caused by charged particles from the sun interacting with a planet’s magnetic field, but Ganymede’s celestial light shows also hint at an internal magnetic process.

The Role of Auroras

The auroras on Ganymede 'rock' back and forth due to the push and pull of Jupiter's magnetic field on the moon's own. If Ganymede were composed entirely of solid rock, the auroras would not exhibit such movement. However, the detection of rocking auroras suggests a salty, conductive ocean beneath the icy crust, which interacts with and alters the local magnetic field.

Tidal Forces at Play

Tidal forces arise from the gravitational interaction between Ganymede and Jupiter. As the moon is stretched and squeezed by these forces, frictional heating occurs, which could be enough to maintain a subsurface ocean in its liquid state. This phenomenon further evidences the interconnectedness between astronomical bodies and how external forces can influence the internal dynamics of moons and planets in our solar system.