Lecture 18 - Jupiter’s Interior, Magnetosphere, Moons and Ring

Jupiter's Interior

• No direct information is available about Jupiter’s interior.
• Its main components, hydrogen and helium, are well understood.
• Jupiter’s internal structure is deduced from:
  • Spacecraft measurements of its atmosphere
  • Theoretical modeling
• We know:
  • The pressure and temperature in Jupiter’s atmosphere
  • Pressure and temperature increase with depth

Atmosphere

• Radius: $71,500$ km
• Thickness: $100$ km
• Mostly Hydrogen and Helium gas ($H_2$ and $He$)
• At a depth of $100$ km:
  • Temperature ~ $300$ K
  • Pressure ~ $10$ atm
  • Note: $1$ atm = atmospheric pressure near Earth’s surface

Interior

• Below about $100$ km, the atmosphere gives way to Jupiter’s interior
• Pressure and temperature increase with depth
• If you compress a gas enough, it becomes a liquid
• If you compress it even more, it becomes a solid
• At a depth of a few thousand km, the pressure is high enough that the gas becomes a liquid
• At a depth of about $20,000$ km:
  • Pressure ~ $3$ million atm
  • Temperature ~ $11,000$ K
  • Hydrogen behaves like a liquid metal
  • A huge shell of metallic hydrogen with a radius of $50,000$ km exists under great pressure
• Deeper inside lies a rocky core of radius ~ $10,000$ km.
  • Composition similar to the terrestrial planets, but much larger ($5-10$ Earth masses)
  • At the center of the core:
    • Pressure ~ $60$ million atm
    • Temperature ~ $25,000$ K

Cometary Impacts

• July 1994: Comet Shoemaker-Levy 9, in fragments, struck Jupiter
  • This caused a series of explosions high in Jupiter’s atmosphere as each fragment struck Jupiter
• 2009: another comet impact
• $3$ more since then ($2$ in 2010, $1$ in 2012)
• It takes years for all the comet’s matter to settle into Jupiter’s interior
• Comet impacts are more common than once thought
• Can use them to study Jupiter’s atmosphere and interior.

Jupiter's Magnetosphere

• Jupiter’s metallic hydrogen interior and rapid rotation produce a very strong magnetic field (magnetic dynamo effect)
• Particles get accelerated and trapped by the magnetic field, acting as a source of radio emission
• Radio telescopes saw radiation leaking from Jupiter
• Pioneer, Voyager, and Galileo spacecraft established that Jupiter has a huge magnetic field
• Juno spacecraft currently studying magnetosphere in more detail
• Jupiter is surrounded by belts of charged particles, much like Earth’s Van Allen belts but much larger
  • Magnetosphere is $30$ million km across
  • Magnetic field strength is $20,000$ times that of Earth
• Flat current sheet of charged particles squeezed into the magnetic equatorial plane by the planet’s rapid rotation
• The plasma torus is a ring of charged particles associated with the moon Io
• The Solar wind is a stream of high-energy charged particles expelled by the Sun
  • The magnetosphere deflects the Solar wind particles
  • They are funneled through the openings in the magnetosphere near the poles
• When the Solar wind particles collide with the gas in Jupiter’s atmosphere, they excite the gas molecules, which then de-excite and cause it to glow The output are aurorae, much like on Earth
• Solar wind also “pushes” the magnetic field away from the Sun
  • The magnetic field on the side of Jupiter facing away from the Sun is “stretched” into a long teardrop shape
• The Pioneer 10 spacecraft did not detect any solar wind particles while moving far behind Jupiter in 1976
• Jupiter’s Magnetosphere can extend by $4$ AU, out to the orbit of Saturn

The Moons of Jupiter

• $79$ moons (as of 2021)
• The four largest are the Galilean moons, first observed by Galileo: Io, Europa, Ganymede, Callisto
• All others are small, less than $300$ km across
• Most are only a few km across and discovered since year 2000
• Most moons are in synchronous orbits
• Most are frozen solid
• The exception is Io, which has many active volcanoes!

Regular Satellites

• $8$ of them
• Prograde, nearly circular orbits of low inclination

Inner Satellites

• $4$ Moons: Metis, Adrastea, Amalthea, Thebe
• Orbit very close to Jupiter inside Io’s orbit
• Replenish and maintain Jupiter's faint ring system

The 4 Galilean Satellites

• Io, Europa, Ganymede, Callisto

Irregular Satellites

• Substantially smaller objects, most only a few km across
• More distant orbits (further out than Galilean Moons)
• More eccentric orbits
• Created when larger (but still small) parent bodies were shattered by impacts from asteroids captured by Jupiter's gravitational field

The Galilean Moons

• Form something like a “miniature Solar system” around Jupiter
• Have similarities to terrestrial planets:
  • Orbits have low eccentricity
  • Largest is somewhat larger than Mercury
• Their properties vary with the distance from the planet
• Their densities decrease as their distance from Jupiter increases
• This is called “differentiation” and it is similar to what we find in the Solar System
• Moving outward from Io to Callisto:
  • Densities steadily decrease
  • Composition and level of differentiation changes:
    • Rocky mantles and metallic cores in Io and Europa
    • Thick icy crust and smaller core in Ganymede
    • Almost uniform rock and ice mix in Callisto
• There is a trend towards more ice and less differentiation as we work our way out through the Galilean satellites:
  • Io: iron, rock, differentiated
  • Europa: icy crust, rock, differentiated
  • Ganymede: ice and rock, differentiated
  • Callisto: ice/rock mixture, undifferentiated

Io

• Innermost Galilean Moon
• Mass and radius similar to Earth’s Moon
• Densest of Jupiter’s moons. Iron core is half of Io’s radius.
• Most geologically active object in the solar system:
  • Many active volcanoes, some quite large
  • Can change surface features in a few weeks
  • No craters. They fill in too fast.
  • Surface is kept smooth and brightly colored by constant volcanism.
  • Io has the youngest surface of any solar system object
• Orange color is probably from sulfur compounds in the ejecta
• Cause of volcanism: Gravity!
  • Io is very close to Jupiter, which exerts huge tidal forces on Io.
  • The other Galilean Moons also exert forces on Io.
  • Ganymede, Europa and Io are in a 1:2:4 resonance. For every orbit of Ganymede, Europa orbits twice, Io orbits 4 times.
  • The huge tidal forces cause enormous stress on Io’s interior, causing it to heat up
  • Tidal heating provides the energy for Io’s volcanoes
• Lava ejected from Io’s volcanoes is so hot ($2000$ K) that its chemical elements are ionized (electrons separated from nuclei)
• The charged particles from the heavy ions are swept up by Jupiter’s rapidly rotating magnetic field.
• This creates the Io plasma torus, a doughnut-shaped region of heavy ions that track Io’s orbit
• Io’s plasma torus was detected from Earth from radio emission before satellite observations
• Spectroscopic analysis indicates that the torus is made mainly of sodium and sulfur atoms and ions

Europa

• Europa’s surface is water ice, possibly with liquid water below
• Europa has no craters, so its surface must be young. Some ongoing process is covering craters soon after they form.
• Tidal forces stress and crack the ice. Water flows, keeping surface relatively flat
• Liquid water upwells from the interior and freezes, filling in the gaps.
• Interactions of Europa with Jupiter’s magnetic field suggest salty ocean below the surface

Ganymede

• Ganymede is the largest moon in the solar system. Even larger than Mercury
• Has impact craters
• History similar to Earth’s Moon, but craters in water ice instead of lunar rock
• Systems of grooves and ridges, unlike Earth’s moon. May have been caused by a process similar to plate tectonics on Earth.
• Dark regions:
  • Oldest parts of the moon’s surface
  • May represent its original icy crust.
  • Darkening was caused by micro-meteorites.
  • Galileo Regio is $3200$ km wide
• Lighter regions:
  • Younger parts of the surface
  • Result of flooding and freezing that occurred within a billion years of Ganymede’s formation
  • The brightest-colored spots are recent impact craters.
• Ganymede has a weak magnetic field
  • Suggests some type of iron core, perhaps slushy water ice under surface
• Ganymede’s density is lower than Io’s or Europa’s, so its iron core must by relatively small

Callisto

• Callisto resembles Ganymede in overall composition (similar density)
• Callisto is more heavily cratered
• Callisto shows no signs of geological activity (no plate tectonics, no volcanism)
• Callisto has a concentric ring structure (Valhalla) arising from impact in the moon’s past
• Largest ring has $1500$ km radius
• Ridges formed when “ripples” from a large meteoritic impact froze before they could disperse completely
• Callisto is undifferentiated, never in a molten state
• Ganymede is differentiated and hence was molten at some point (perhaps tidal stressing in the recent past)

Jupiter's Ring

• Voyager discovered faint ring $50,000$ km above cloud tops
• Most of the ring is confined to a region a few thousand km across
• Made of dark fragments of rock and dust possibly chipped off the innermost moons by meteoritic impacts
• Jupiter’s 4 inner satellites help maintain the rings
  • Metis & Adrastea maintain the main ring
  • Amalthea & Thebe maintain other fainter rings
  • These are called “shepherd satellites”