12.2 Galilean Moons of Jupiter

Galileo Spacecraft (1996-1999): Conducted repeated close encounters with the large Galilean moons.

Juno Spacecraft: More recently, a Jupiter orbiter that has provided some close observations of Ganymede and Europa.

Comparison with Titan: Data on Titan (Saturn's largest moon) obtained from the Cassini spacecraft and its Huygens probe (which landed on its surface) is included for comparative purposes.

Basic Facts of Large Moons (Summary from Table 12.1):

• Moon: Diameter 3476 km, Mass 1.0 (Earth's Moon = 1), Density 3.3 g/cm3, Reflectivity 12%

• Callisto: Diameter 4820 km, Mass 1.5 (Earth's Moon = 1), Density 1.8 g/cm3, Reflectivity 20%

• Ganymede: Diameter 5270 km, Mass 2.0 (Earth's Moon = 1), Density 1.9 g/cm3, Reflectivity 40%

• Europa: Diameter 3130 km, Mass 0.7 (Earth's Moon = 1), Density 3.0 g/cm3, Reflectivity 70%

• Io: Diameter 3640 km, Mass 1.2 (Earth's Moon = 1), Density 3.5 g/cm3, Reflectivity 60%

• Titan: Diameter 5150 km, Mass 1.9 (Earth's Moon = 1), Density 1.9 g/cm3, Reflectivity 20

Orbital Characteristics:

• Callisto is the outermost of the large Galilean moons, approximately 2 million kilometers from Jupiter.

• It completes an orbit around Jupiter in 17 days.

• Like Earth's Moon, Callisto exhibits synchronous rotation, meaning its rotation period is the same as its orbital period, so it always presents the same face toward Jupiter. Its day therefore equals its month, lasting 17 days.

Surface Environment:

• The noontime surface temperature on Callisto is extremely cold, only about 130 K (approximately -140 °C).

• Due to these frigid temperatures, water ice is stable on its surface year-round and does not evaporate.

Physical Composition and Structure:

• Diameter: Callisto has a diameter of 4820 kilometers, which is nearly the same as the planet Mercury.

• Mass: However, its mass is only about one-third as great as Mercury's, implying a significantly lower overall density.

• Density: With a density of 1.8 g/cm3, Callisto must be an icy body throughout much of its interior, containing far less rocky and metallic material compared to the inner planets.

• Differentiation (Lack Thereof):

  • Callisto has not fully differentiated (separated into distinct layers of materials based on density).

  • Evidence from the Galileo spacecraft's gravitational measurements indicates that Callisto lacks a dense core.

  • This observation surprised scientists, as differentiation is typically expected in large icy bodies because the low melting temperature of ice should make the process easier (allowing rock and metal to sink while slushy ice floats).

  • It is deduced that Callisto froze solid before its differentiation process could complete.

Surface Features and Geological Activity (Addressing Learning Objective):

• Major Observable Features on Callisto:

  • Heavily Cratered Surface: Callisto's surface is extensively covered with impact craters, much like the lunar highlands. The survival of these craters over billions of years indicates that icy objects in this cold environment can retain impact features.

  • Icy Spires: High-resolution images show unique icy spires, typically 80 to 100 meters tall.

  • Surface Erosion and Dark Material: As the ice slowly erodes, darker, ice-poor material slides down and accumulates in lower-lying areas.

  • Bright and Dark Patches: Bright areas are believed to be predominantly ice, while darker areas consist of more eroded, ice-poor material.

• Deductions from These Features:

  • Geologically Dead World: Callisto stands out among planet-sized solar system objects for its apparent absence of internal forces driving geological change.

  • It is considered geologically 'stillborn' and has remained geologically dead for over 4 billion years, implying a very primitive and ancient surface.

  • Behavior of Ice: At the extremely low temperatures prevailing in the outer solar system, ice on Callisto behaves more like rock

    —it is nearly as hard and rigid, unlike the deforming or flowing ice found in Earth's glaciers.

Surface Features and Age:

• Cratering: Ganymede, the largest moon in the solar system, shows a great deal of cratering.

  • About one-quarter of its surface is as old and heavily cratered as Callisto.

  • The remaining surface is younger, evidenced by sparser impact craters and their relative freshness.

  • The fresher terrain is estimated to be 2 to 3 billion years old, younger than the lunar maria or Martian volcanic plains based on crater counts.

• Terrain Contrast: Darker regions are older, heavily cratered areas, while lighter areas are younger.

  • The brightest spots represent geologically recent impacts.

Internal Structure and Geological Activity (Addressing Learning Objective: Explain the evidence for tectonic and volcanic activity on Ganymede):

• Differentiation: Ganymede is a differentiated world, similar to terrestrial planets, unlike Callisto.

  • Gravitational measurements indicate that rock sank to form a core approximately the size of Earth's Moon.

  • A mantle and crust of ice 'float' above this core.

• Magnetic Field: The Galileo spacecraft discovered that Ganymede possesses a magnetic field.

  • This is a clear sign of a partially molten interior.

  • It suggests the very likely presence of liquid water trapped within its interior.

• Geological Activity: Ganymede is an intermittently geologically active world, powered by an internal heat source, rather than being geologically dead.

  • Some surface features could be as young as a few hundred million years, similar to the surface of Venus.

• Evidence of Tectonic and Volcanic Forces:

  • The younger terrain was formed by tectonic and volcanic processes.

  • Cracking and Flooding: In some areas, the crust seemingly cracked, leading to craters being flooded with water from the interior.

  • Mountain Ranges: Extensive mountain ranges formed from crustal compression, creating long ridges with parallel valleys spaced a few kilometers apart.

  • Split Craters: Older impact craters were observed to be split and pulled apart, indicating crustal movement.

  • Plate Tectonics: There are indications of large-scale crustal movements resembling Earth's plate tectonics.

Comparison with Callisto and Tidal Heating:

• Divergence from Callisto: The significant geological differences between Ganymede and Callisto might stem from a slight difference in size and internal heating between the two, leading to divergent evolution.

• Role of Jupiter's Gravity: More likely, Jupiter's gravity is responsible for Ganymede's ongoing geological activity.

  • Tidal Forces: Ganymede is close enough to Jupiter for tidal forces from the giant planet to have episodically heated its interior.

  • Tidal heating is caused by the unequal gravitational pull on two sides of a body, leading to gravitational flexing or kneading in the moon's interior, generating heat.

  • This tidal heating is believed to have triggered major convulsions on Ganymede's crust.

  • The influence of Jovian tides becomes more pronounced for moons closer to Jupiter, as will be seen with Europa and Io.

General Characteristics and Comparison:

• Io is the innermost of Jupiter

’s Galilean moons, sharing nearly the same size and density as Earth’s Moon.

• Despite physical similarities to Earth's Moon, Io's appearance and geological history are vastly different.

Discovery and Extent of Volcanism:

• Io has the highest level of volcanism in the solar system, significantly exceeding that of Earth.

• Its active volcanism was first discovered by the Voyager spacecraft:

  • Voyager 1 observed eight volcanoes erupting in March 1979.

  • Six of these were still active four months later when Voyager 2 passed.

• The Galileo spacecraft (during 1997 alone) detected more than 50 eruptions using improved instruments.

Nature of Volcanic Eruptions:

• Many eruptions produce graceful plumes extending hundreds of kilometers into space.

• An observed bluish plume rose about 140 kilometers above a volcano’s surface.

• The Prometheus plume was seen rising about 75 kilometers from Io’s surface.

• Most volcanism involves hot silicate lava, similar to Earth’s volcanoes.

• When hot silicate lava encounters frozen sulfur and sulfur dioxide deposits:

  • The sudden heating results in massive eruptive plumes, far larger than terrestrial volcanic ejecta.

  • As these rising plumes cool, sulfur and sulfur dioxide recondense into solid particles.

  • These particles fall back as colorful 'snowfalls' extending up to a thousand kilometers from the vent.

Surface Changes and Composition:

• Major new surface features were observed to appear between Galileo orbits.

• An eruption at Pillan Patera created a dark deposit approximately 400 kilometers across.

• This new dark deposit was later covered by reddish material from the Pele volcano.

• Another unnamed volcano also erupted, leaving a dark deposit and a yellow ring.

• Maps of Io reveal over 100 recently active volcanoes.

• Huge lava flows from these vents cover about 25% of the moon’s total surface with still-warm lava.

• The bright surface colors of Io are due to a thin layer of sulfur compounds.

• The underlying volcanism is driven by eruptions of molten silicates.

Exploration Challenges:

• Approaching Io is dangerous due to intense belts of atomic particles trapped in Jupiter’s magnetic environment near Io’s orbit.

• The Galileo spacecraft absorbed damaging radiation beyond its design levels during its first pass by Io.

• To maintain operation, controllers had to modify or disable fault-protection software.

• Despite difficulties, Galileo achieved four successful flybys, obtaining high-resolution photos and spectra.

Explaining Io's Volcanism (Learning Objective):

• Io's extreme volcanic activity, despite its small size, is primarily driven by tidal heating caused by Jupiter's massive gravitational pull.

• Jupiter, being over 300 times more massive than Earth, exerts immense tidal forces on Io.

• These forces deform Io into an elongated shape, creating a bulge on its surface that points towards Jupiter.

Mechanism of Tidal Heating:

• While a constantly facing, perfectly circular orbit would not generate heat, Io's orbit is slightly eccentric due to gravitational perturbations from Europa and Ganymede.

• This eccentricity causes Io to twist and flex as it moves closer to and farther from Jupiter during each orbit.

• The continuous internal flexing and kneading generate significant frictional heat within Io's interior, similar to how bending a wire repeatedly causes it to heat up.

Long-Term Effects on Io:

• Over billions of years, this constant heating has vaporized and driven off most of Io's water (H2O), carbon dioxide (CO2), and other highly volatile gases.

• Consequently, sulfur and its compounds are the main remaining volatile materials, contributing to its colorful surface.

• Io's entire interior is believed to be molten, with its crust being continuously recycled through its vigorous volcanic activity.

Gradient of Geological Activity Across Galilean Moons:

• A clear trend of increasing geological activity and internal heating is observed as one moves closer to Jupiter among the Galilean moons:

  • Callisto: The outermost, displaying an ancient, heavily cratered, geologically 'dead' surface, suggesting minimal internal heating.

  • Ganymede: Exhibits intermittent geological activity with a mix of old, cratered terrain and younger, tectonically and volcanically modified regions. Its activity is powered by episodic tidal heating.

  • Europa: (As seen in Figure 12.11) Possesses a remarkably smooth, relatively crater-free surface, crisscrossed by cracks and ridges, indicating significant and ongoing geological resurfacing, likely due to a subsurface ocean and strong tidal heating.

  • Io: The innermost, showcasing the most extreme volcanism in the solar system, a direct result of the most intense tidal heating.

Distance as a Key Evolutionary Factor:

• The distance a moon maintains from a giant planet like Jupiter plays a crucial role in its composition and geological evolution.

• This is predominantly because of the varying intensity of internal heating caused by the giant planet's unrelenting tidal forces.

Jupiter Icy Moons Explorer (JUICE) Mission (Future Exploration):

• Launched: By the European Space Agency (ESA) in 2023.

• Arrival: Expected in the Jupiter system in 2031.

• Focus: To conduct detailed exploration of Europa, Ganymede, and Callisto.

• Mission Profile: Will perform multiple flybys of these icy moons and eventually enter a prolonged orbital phase around Ganymede.