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Astronomy

Moons of the Large Jovian Planets

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Weblecture

Jupiter and Saturn: Focus on the Atmosphere

Jupiter and Saturn Moons: Fire, Stone, Ice, and Air

Introduction

Ac primo cum Iouem consimilibus interstitiis modo consequantur, modo praeeant, ab eoque, tum versus ortum, tum in occasum anguistissimis tantu diuaricationibus elongentur, eundemque retrogradum pariter, atque directum concomitetur, quin circa illium suas consiciant conuersiones, interea dume circa mundi centrum omnes una duo decenales periodos absoluunt, nemini dubium esse potest.

And, in the first place, since they are sometimes behind, sometimes before Jupiter, at like distances, and withdraw from this planet towards the east and towards the west only within very narrow limits of divergence, and since they accompany this planet alike when its motion is retrograde and direct, it can be a matter of doubt to no one that they perform their revolutions about this planet, while at the same time they all accomplish together orbits of twelve years’ length about the centre of the world.

— Galileo Galilei, The Sidereal Messenger, 1610

Variation in Surface Features

We've now looked at enough different bodies that we can start classifying the forces creating surface features, and group actual physical features for comparison. Study the tables below. What characterizes moons of large mass? What characteristics would you look for in moons formed as part of the gravitational collapse of the planet? What characteristics would you look for in moons captured after the period of planetary formation?

Force Description Factors Examples
Planetary heat of formation Heat generated by gravitational collapse of planet. Pressure from outer layers compresses core. Core state depends on composition, pressure, and temperature and planetary size. A given substance can have different melting points and boiling points at different pressures. Total planetary mass and density of core determine individual core characteristics. Small planets and moons lose heat faster than large planets, which have a smaller surface-to-volume ratio (the greater the surface area available per internal volume unit, the faster heat can dissipate). The gas giants Jupiter and Saturn actually generate more heat than they receive from the sun, due to internal heat flows. Mercury and Earth's moon should be internally inert (the Moon is); all heat due to planetary collapse has dissipated. Where there appears to be heat in the core (as with Mercury), other mechanisms must account for it. [See tidal forces].
Radioactive Elements Heat is generated when energy is released as part of the fission event. Composition and the amount of fissionable material in the crust, mantle, and core determine whether a planet can generate enough heat to "fund" the energy necessary for geological activity. Smaller planets do not have enough radioactive material to generate heat more rapidly than normal thermal radiation can get rid of it. Earth has radioactive elements in sufficient quantity to produce more heat than can be dissipated through the surface.
Surface Tectonics Tectonics covers the deformation of the crust due to heat forces from below and cooling from above. Lava flow from fissures in the surface will cover existing surface features. In rare cases (the Earth), the crust may fracture into multiple plates, but in most cases, the plates remain intact, but can be ruptured at weak points. Hot areas in the mantle under weak spots in the crust can result in eruptions. Rills, scarps, and rifts can also occur when the crust shrinks after solidifying. Lava flows may fill in fissures or crates. Earth: multiple plates in friction with each other result in earthquakes and volcanic activity along plate boundaries.
Mars, Venus: weak spots in crust result in shield volcanoes
Mercury, Mars: shrinkage forms rift valleys, scarps
Earth's moon exhibits large flat Mare covering craters.
Tidal Forces Gravitational forces on the near and far sides of the object may cause internal deformation, generating heat sufficient to drive geological activity such as quakes and volcanic eruptions, or may create liquid mantle oceans of water or methane below frozen crust surfaces.. Such forces can also create spin:orbit coupling (rotation is a whole-number ratio of the orbital period), or orbit-orbit coupling (two moons have orbital periods that are related in whole number ratios). The secondary body must be large enough and close enough to its primary for there to be a significant difference in the forces exerted by the primary body. Earth's moon, all of Jupiter's large moons, several of Saturn's small moons exhibit 1:1 spin-orbit coupling. Mercury exhibits a 2:3 spin-orbit coupling with respect to its own rotation and orbital period around the sun. Io and Mercury both appear to have geological activity or liquid layers internally (as evidenced by magnetic fields) due to tidal friction forces.
Wind Erosion Erosion is the breakdown of surface features due to "wind" or liquid or ice action. As a result, planets with erosion forces have young surfaces: their older surface features have been erased by erosion. Planets must have atmospheres or substances in a liquid state for erosion to occur. Venus, Earth, Mars, Titan, Triton all have atmospheres where gas winds can blow and gases change state seasonally to liquid or ice. As a result, their surface features are constantly erased and replaced.
Ice Ice deposits can melt and reform inside crevices, expanding to break the surrounding rock. Ice surfaces can reflect solar energy back into space. Planets and moons must have surface temperatures below 0 °C to retain ice, and sources of oxygen to form water ice. Earth and Mars have polar caps. Mercury and the Moon may have ice deposits in polar craters shielded from direct sunlight. Jupiter's moons (other than Io) have ice-covered surfaces that may hide liquid water oceans. Saturn's mid-size moons may be primarily composed of ice compressed at sufficient high pressure but low temperature to form "plastic water".
Impact Events Impact craters are formed when meteors strike planetary or lunar surfaces. Over time, as planets aggregate the matter along their orbital path, the frequency of impact events has decreased. Highly cratered surface regions are generally considered older than less cratered surfaces.
Atmospheres may protect planets by causing incoming objects to burn up (due to friction with gas molecules in the atmosphere) before the object can strike the surface. If the surface is malleable (thin crust, easy lava flows, or icy crusts that can melt and reform), craters may not form or last long. 		Mercury, the Earth's Moon, Mars, and gas giant moons that lack atmospheres or tidally-generated geological forces are heavily cratered. Venus (thin crust), Io (geologically active), Europa and Ganymede (icy crusts) have fewer crater features. Earth lack's craters due to both its atmosphere and erosion forces.

Characteristics of Jupiter's Moons

Having identified the factors that can influence and determine surface features, let's turn now to the moons of the gas giants and explore the diversity we find.

Io Europa Ganymede Callisto

Naked Eye Observation
All four moons are bright enough to be visible to the naked eye, but close enough to Jupiter to be lost in the glare. Can be easily viewed through 8X telescopes, but surface features not visible from Earth-based telescopes. Spectroscopic studies revealed sulfur dioxide on Io and water ice on Europa.

Exploration
Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 (1979), Voyager 2 (1979), and Galileo (1995-2003) Galileo (1995-2003) Voyager 1 (1979), Voyager 2 (1979), and Galileo (1995-2003)
Pioneer 10 Orbit

Source: NASA

Orbital Characteristics
1.769 d (1:1 spin-orbit coupling)
Elliptical orbit and elongated shape under tidal stress from Jupiter, modified by periodic tidal counter forces from Europa and Ganymede. 		3.551 d (1:1 spin-orbit coupling)
Elliptical orbit and elongated shape under tidal stress from Jupiter, modified by periodic tidal counter forces from Io and Ganymede. 		7.155 d (1:1 spin-orbit coupling) 16.689 d (1:1 spin-orbit coupling)
Io:Europa:Ganymede synchronized 1:2:4
Callisto is too distant to be synchronized by the gravitational pull of the other three large moons.

Magnetosphere
Apparently has its own dynamo, with a magnetic field roughly equivalent to Earth's. None. However, movement through Jupiter's magnetic field creates induced current. Magnetic field equal to Mercury's; has its own magnetosphere. None. Movement through Jupiter's magnetic field creates induced current in sub crust layer, possibly water/ammonia ocean.

Surface
No impact craters. Most active volcano system in the solar system, with geysers spewing sulfur and sulfur dioxide, as well as slower lava flows. Ice marked by fractures, creating ridges and floes. Dark cratered, furrowed areas (older), and light furrowed areas with fewer craters. Craters marked by ice ejection rays. Larger craters evident; small craters apparently eroded. Covered with dark material (rock dust from impact?)
Comparison of Jupiter's Galilean moons

Source: NASA

Core
Oscillation as a result of tidal forces between Jupiter, Ganymede, and Europa cause tidal heating. Thin rocky crust floating on global liquid magma "ocean" mantle. Mountain ranges due to uplift of crustal blocks breaking and sinking. Magnetic field suggests iron and iron sulfide solid core. Density studies show Europa's interior is 90% rocky. Warm interior (possibly due to tidal forces from Jupiter, weaker than those on Io). Ice crust and liquid water above rock mantle and solid metallic core. Magnetic field indicates warm layered interior. Ice crust, rocky mantle, and metallic core. Variations in magnetic field indicate possible internal liquid saltwater layer between ice crust and rocky mantle. Internal heat possibly result of previous tidal forces and orbital relocation. Density indicates very large mixed ice-rock core covered with thin ice/ocean mantle and ice crust.

Rings
Ejection of ionized gases forms torus in orbital plane and complex electrical flow system.

Lunar formation
The generally accepted theory for the origin of Jupiter's Galilean moons is a gravitational collapse similar to that which is thought to have occurred to create the Solar System planets. Irregularly formed smaller moons, particularly those in retrograde orbits, are assumed to be captured asteroids or cometary cores.

Characteristics of Saturn's Moons

Titan Mimas Enceladus Tethys Dione Rhea Iapetus

Naked Eye Observation
Spectroscopic studies revealed methane presence, no surface features visible. No discernible features from Earth-based observations.
Saturn's Satellites

Source: NASA

Exploration
Voyager 1 (1980) Voyager 2 (1981); Cassini/Huygens (2004-2008) Cassini/Huygens (2004-2008)

Orbital Characteristics
1:1 spin-orbital coupling. Prograde orbit circular in Saturn's equatorial plane with little tidal stress from Saturn. 1:1 spin-orbit coupling. Prograde, in Saturn's equatorial plane. Enceladus and Dione share a 1:2 coupling. Mimas and Tethys share a 1:2 coupling.

Magnetosphere
No discernible magnetospheres.

Atmosphere
Pressure: 1.5X Earth
Composition:Nitrogen: 95%, methane: < 5%, other hydrocarbons in aerosol suspension.
Some rainfall possible. 		None.

Surface
Water ice and polymer sand dunes, some volcanoes. Liquid ethane lakes only near north pole. Heavily cratered, with single recent very large crater. Crater-free regions; possible recent geologic activity? High albedo, missing dust and rock. Ice geysers create torus around Saturn. Linear stress fractures. Heavily cratered; Ithaca Chasm rift valley, possibly due to impact (crater opposite) or crustal shrinkage. Surface dichotomy: lead surface is heavily cratered, trailing hemisphere showing stress fractions due to geological activity. Heavily cratered. Leading hemisphere black (low albedo); trailing hemisphere highly reflective.

Core
Current state rocky core, partially melted liquid rock mantle, liquid water mantle, and ice crust Rock-ice conglomerations. Some tidal stresses possible on Enceladus (internal heat suggested by geysers). Tethys and Dione large enough to have internal heat sources, including radioactive decay, pressure, and retention from formation.

Lunar formation
The generally accepted theory for the origin of Titan and the mid-sized moons is a gravitational collapse similar to that which is thought to have occurred to create the Solar System planets. Irregularly formed smaller moons, particularly those in retrograde orbits, are assumed to be captured asteroids or cometary cores.

Characteristics of Uranus's Moons

Uranus has 27 known moons, all of them named after characters in the plays of William Shakespear and Alexander Pope (thus does literature inform astronomy).

Miranda Ariel Umbriel Titania Oberon

Naked Eye Observation
Moons are not visible to the naked eye; the two largest (Titania and Oberon) were first viewed by William Herschel in 1787.

Exploration
Voyager 2 returned photographs of 13 moons in 1986, although one (Perdita) was not identified until 1999. Observations are currently limited to Earth-based telescopes and the Hubble Space Telescope.

Orbital Characteristics
Most moons lie in the equatorial plane of Uranus, along with very sparse rings. Thirteen small inner moons are shepherds for ring particles. Nine moons lying outside the orbit of Oberon have high inclinations to Uranus' equatorial plane, and appear to be gravitational captures. All but one of these are in retrograde orbits (orbiting Uranus in the direction opposite to its rotation). Uranus Moons

Source: NASA

Magnetosphere
No directly measured information is available from the Voyager 2 flyby, but given the assumption of the moons' composition and their sizes, no magnetic dynamos are likely within these bodies.

Atmosphere
None of the moons of Uranus have atmospheres.

Surface
Fewest craters; youngest surface? A past 3:1 orbital resonance
between Ariel and Umbriel, and a past 4:1 resonance between Ariel
and Titania, may explain surface features on Ariel that indicate
internal heat temperatures higher than the moon could generate
without tidal forces. 		Most craters, oldest surface?
Uranus Moons

Source: NASA

Core
Moons are mixed rock/ice conglomerates in roughly equal proportions, with the rocky material concentrated in the core, but from orbital studies, astronomers have determined that Miranda is primarily ice.

Planetary formation
Moons in the equatorial plane probably formed as the planet coalesced. Moons in high-elliptical orbits are likely to be captured asteroids or comets.

Characteristics of Neptune's Moons

Triton Nereid Proteus

Naked Eye Observation
None of Neptune's moons are visible as naked-eye objects. Triton and Nereid are visible from Earth-based telescopes.

Exploration
Six moons were discovered during the Voyager 2 flyby in 1989.

Orbital Characteristics
Retrograde Highly elliptical. Extremely close to planet.

Magnetosphere
None measured for any of the moons.

Atmosphere

Pressure:


1/100000 Earth's atmosphere at sea level.

Composition:

Nitrogen with traces of methane 		None. None.

Surface
Nitrogen ice (55%), water ice (15-35%), carbon dioxide ice (10-20%). Distinguishing surface features not visible from Earth or Voyager 2 flyby photographs. Cratered, irregular shape (just under mass limit, above which bodies form spherical shapes under their own weight), no geological reformation.

Core
30-40% water ice mantel and crust surrounding rocky core. Possible ice crust. Core compositiona and mantle unknown. Likely rock-ice conglomerate..

Planetary formation
Based on density and high water ice components, Triton appears to be a gravity capture from the Kuiper belt. Nereid's surface indicates a different composition; it may have formed as part of Neptune's planetary collapse Proteus is hexagonal in shape and appears to have formed after Neptune from small planetesimals, probably from debris formed when Triton was captured.

Discussion Questions

Optional Web Reading

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