12.2 The Galilean Moons of Jupiter

Jupiter's Galilean moons are a celestial wonder, each with unique characteristics. From Callisto's ancient, cratered surface to Io's fiery volcanoes, these moons showcase diverse geological processes and potential for habitability.

Tidal forces from Jupiter play a crucial role in shaping these moons. They drive Io's volcanic activity, create Europa's icy cracks, and influence the orbital dynamics of all four moons, highlighting the complex interplay between Jupiter and its satellites.

Galilean Moons

Key characteristics of Callisto

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• Outermost and largest of the Galilean moons orbiting Jupiter
• Heavily cratered surface pockmarked with impact scars
  • Craters are ancient, dating back billions of years, indicating a lack of recent geological activity or resurfacing events
• Low density suggests a composition of roughly equal parts rock and ice, with a less differentiated interior compared to other Galilean moons
• Thin atmosphere composed primarily of carbon dioxide, likely generated by sublimation or sputtering processes
• Subsurface ocean likely exists beneath the icy crust, potentially harboring liquid water and creating conditions for possible habitability

Tectonic activity on Ganymede

• Largest moon in the Solar System, even larger than the planet Mercury
• Surface consists of dark, heavily cratered regions and lighter, grooved terrain
  • Grooved terrain indicates past tectonic activity, where the icy crust has been deformed and reshaped
    • Formed by the movement and deformation of the moon's icy crust, likely due to internal stresses and heat
  • Absence of current volcanic activity suggests that the moon has cooled and become less geologically active over time
• Magnetic field suggests the presence of a liquid iron core, which may help generate and sustain the field
• Thin oxygen atmosphere, likely formed by the breakdown of water ice on the surface by radiation and charged particles
• Evidence of a subsurface saltwater ocean, which may be sandwiched between layers of ice (ice shell and high-pressure ices)

Surface features of Europa

• Smoothest surface among the Galilean moons, with relatively few impact craters
  • Lacks substantial craters, indicating a geologically young surface that has been recently resurfaced or renewed
• Crisscrossed by a network of linear cracks and ridges, resembling a cracked eggshell
  • Likely caused by tidal forces from Jupiter, which flex and deform the icy surface
    • Tidal heating keeps the interior warm and the surface ice relatively young, allowing for the formation of unique surface features
• Reddish-brown streaks and splotches on the surface, adding color to the otherwise icy exterior
  • Possibly caused by mineral salts or organic compounds, potentially originating from the subsurface ocean
• Strong evidence for a subsurface liquid water ocean beneath the icy crust
  • Potential habitability for microbial life, as the ocean could provide the necessary ingredients (water, energy, and nutrients)
• Potential for cryovolcanism, where water and other volatiles erupt through the icy surface

Volcanic nature of Io

• Most volcanically active body in the Solar System, with intense and ongoing eruptions
  • Hundreds of active volcanoes across its surface, constantly reshaping the moon's appearance
  • Volcanic plumes can reach heights of several hundred kilometers, ejecting material into space
• Surface is covered in volcanic sulfur and silicate deposits, creating a unique and colorful landscape
  • Creates a variety of colors, including red (sulfur), yellow (sulfur dioxide), white (sulfur dioxide frost), and black (silicate lava)
• Volcanic activity is driven by tidal heating from Jupiter's gravitational pull, which causes the moon to flex and deform
  • Generates heat in Io's interior, melting rock and driving volcanism, leading to the formation of magma chambers and lava flows
• Io's volcanic activity contributes to the formation of a plasma torus around Jupiter, a ring-shaped region of ionized gas and particles

Tidal forces on Europa and Io

• Tidal forces are caused by the gravitational pull of Jupiter and the other Galilean moons, leading to significant effects on Europa and Io
• Europa:
  1. Tidal forces create flexing and stretching of the moon's icy surface, causing it to rise and fall by tens of meters
  • Leads to the formation of cracks, ridges, and other linear features, which are visible as intricate patterns on the surface
  1. Tidal heating keeps the subsurface ocean liquid and the ice crust relatively thin (a few kilometers to tens of kilometers thick)
  • Allows for the exchange of materials between the surface and the ocean, potentially providing nutrients and energy for potential life
• Io:
  1. Tidal forces cause extreme heating in Io's interior, generating tremendous amounts of heat and energy
  • Melts rock and generates magma, which rises to the surface and fuels the moon's extensive volcanic activity
  1. Tidal heating is the primary driver of Io's extensive volcanic activity, with the constant flexing and deformation of the moon's interior
  • Responsible for the constant resurfacing of Io, erasing impact craters and creating new volcanic features (lava flows, calderas, and mountains)
  1. Tidal forces also contribute to the formation of mountains on Io's surface, as the crust is pushed and pulled by the gravitational interactions
  • Some of these mountains can reach heights of up to 17 kilometers, making them some of the tallest in the Solar System

Orbital dynamics and interactions

• Orbital resonance exists among Io, Europa, and Ganymede, with their orbital periods in a 1:2:4 ratio
• All four Galilean moons are tidally locked to Jupiter, always presenting the same face to the planet
• Jupiter's powerful magnetosphere interacts with the Galilean moons, particularly Io, influencing their environments and atmospheric processes