What's Inside Ganymede? Iron Snow Experiments Shed Light on the Magnetic Dance Within Jupiter's Largest Moon

NASA's Hubble Space Telescope has provided compelling evidence of an extensive underground saltwater ocean on Jupiter's largest moon Ganymede, estimated to be 60 miles thick, potentially harboring more water than Earth's surface. This discovery holds significance in the quest for habitable worlds and the search for life beyond Earth.

Recent research shows that Ganymede boasts an unexpectedly robust magnetic field, influenced by Jupiter's tidal forces that warm its core. However, the core's geological processes remain enigmatic. Recently, a pioneering experimental study examined one of the leading models of Ganymede's core dynamics: the 'iron snow' model.

The Iron Snow Model Experiment

The concept of the iron snow theory resembles a geological 'weather model' for a planetary core. It elucidates the process in which iron cools and crystallizes near the upper edge of the core, descends inwards, and subsequently melts back into the planet's liquid center.

Essentially, Ganymede's core can be envisioned as a molten metal snowglobe, subject to the gravitational influence of Jupiter. This cyclical movement of rising and falling iron induces motions in the liquid core, serving as the energy source for the generation of a magnetic field. Despite these insights, critical aspects of this mechanism remain elusive.

To probe these unknowns, researchers devised an experiment simulating the iron snow model in a laboratory setting. Unable to directly observe a planetary core, they used water ice as an analog for iron snow crystals. The experiment involved a tank of water cooled from below, with a salty layer representing the planetary mantle and a layer of fresh water above representing the liquid core.

Surprisingly, the crystals' behavior in this setup revealed sporadic bursts of activity followed by periods of inactivity, challenging the expected steady flow of crystallization, rising, and melting.

Insights Beyond Ganymede's Core Dynamics

Researchers explained that to initiate crystallization, the liquid must supercool, dropping below the typical freezing point of ice. This prompts a burst of snowflake formation, followed by a pause until temperatures once again permit new crystal generation.

This sporadic process has profound implications for planetary magnetic fields. Ganymede's magnetic fluctuations result from intermittent iron snow, occurring at different locations within its core. This dynamic magnetic field continuously shifts in strength, shape, and intensity.

Iron snow's influence extends beyond Ganymede, shaping core behavior in various small celestial bodies like the Moon, Mercury, Mars, and large metallic asteroids. Understanding this phenomenon is crucial for deciphering the dynamics of systems with known magnetic fields, such as Mercury and Ganymede.

In contrast, Earth's core operates differently. It isn't primarily governed by iron snow, as gravitational pressure and distinct material composition cause metals to solidify in the core's center, melting as they move outward. Recent studies propose the coexistence of both processes, introducing nuances to our comprehension of Earth's core dynamics.

The team published the full findings of their study, titled "A Laboratory Model for Iron Snow in Planetary Cores," in the journal Geophysics Research Letters.

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