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Rings and Natural Satellites: 

Rings and Natural Satellites

Planetary rings: 

Planetary rings

Saturn’s rings: 

Saturn’s rings Main structures: A and B rings, separated by the Cassini Division (2:1 resonance with satellite Mimas) The outer part of the A ring hosts the Encke Division, which is cleared by satellite Pan The C and D rings are broad, faint structures interior to the B ring (D ring unobservable from Earth) The E ring is very wide and diffuse, fed by volcanic ejecta from satellite Enceladus The F and G rings are very narrow; the F ring is shepherded by satellites Prometheus and Pandora

Co-orbiting satellites: 

Co-orbiting satellites An object B orbiting very close to another object A about the same planet in nearly circular orbits performs a horseshoe orbit due to the mutual gravitational attraction Example: Saturn’s co-orbiting satellites Janus and Epimetheus

Fine structure of the rings: 

Fine structure of the rings All the major ring components exhibit a fine pattern of radial density variation with rather high contrast, giving them the appearance of a gramophone record Voyager 2 false-color picture of Saturn’s rings

Apparent repulsion: 

Apparent repulsion - a small particle B orbiting near a larger object A experiences a hyperbolic deflection when passing near A. - This leads to loss or gain of angular momentum, causing the orbit of B to be “repelled” from A

Gap clearing & shepherding: 

Gap clearing & shepherding Satellite Pan orbiting inside the Encke Division Satellites Prometheus and Pandora orbit on the inner resp. outer side of the F ring

Jupiter’s rings: 

Jupiter’s rings Even the Main Ring is very faint All rings are strongly forward scattering and consist of very small particles The Halo is inside the main ring, and the two Gossamer rings are outside All the inner satellites are connected to the ring structures

Jupiter’s main ring in forward scattering: 

Jupiter’s main ring in forward scattering Voyager picture taken in the direction of the Sun

Jupiter’s inner moons: 

Jupiter’s inner moons Metis (diam. 40 km) is embedded in the main ring Adrastea (diam. 20 km) is at the main ring’s outer edge Amalthea (diam. 190 km) is at the outer periphery of the inner Gossamer ring Thebe (diam. 100 km) is near the outer periphery of the outer Gossamer ring

Uranus’ rings & inner moons: 

Uranus’ rings & inner moons The rings were discovered during a stellar occultation in 1977 They are dark and narrow, situated mostly rather close together The outermost rings are connected with the system of small, inner satellites

Uranus’ rings: 

Uranus’ rings The rings are bright in forward scattering, and the intermediate regions also prove not to be void of material The outer, bright and relatively broad  ring is shepherded by satellites Cordelia and Ophelia

Neptune’s rings & inner moons: 

Neptune’s rings & inner moons Data mainly from stellar occultations and Voyager 2 imaging Main rings: LeVerrier and Adams; broader features in between: Galle, Arago and Lassell 5 satellites orbit inside the Adams ring; 3 inside the LeVerrier ring

Neptune’s ring arcs: 

Neptune’s ring arcs Stellar occultation measurements indicated asymmetric ring features Voyager 2 pictures revealed arcs (clumps of material) in the Adams ring: Fraternité, Egalité, Liberté

The Roche limit: 

The Roche limit Repulsive, tidal acceleration: Mutual attraction: Ft=Fg

Rings and Roche limits: 

Rings and Roche limits Jupiter: the RL is in the Gossamer region Saturn: the RL is in the A-B ring region Uranus: the RL is outside the  ring, in the region of the outer rings Neptune: the RL is near the Adams ring Indication: collisional shattering of small, inner moons and dispersion of material inside the RL may have caused, and still be causing the rings

Planetary satellite systems: 

Planetary satellite systems The terrestrial planets have few satellites, while the giant planets have a multitude In some respects the giant planet satellite systems resemble the Solar System in miniature, but each system is highly unique The giant planet satellites may be arranged in three broad categories corresponding to an inner, a central and an outer zone with respect to the planet

Giant planet satellites: 

Giant planet satellites The inner satellites are always small and have equatorial, circular orbits (“regular orbits”) The central zone contains all the large, classical satellites, and in the case of Saturn also some small ones. All except Neptune’s have regular orbits All the outer satellites are irregular (high inclinations to the equator) and small; nearly all are recent discoveries

Origin of the satellites: 

Origin of the satellites The inner, small satellites orbit within or near the Roche Limit and ring system. They appear to be eroded remnants of tidal disruption or collisional fragmentation The central, regular satellites were formed by solid accretion in a circumplanetary gas/dust disk that may have been the result of gas capture from the solar nebula The outer, irregular satellites have orbits that are perturbed by the Sun more than by the equatorial flattening of the planet; they were captured when the planets were still young

Collisional captures: 

Collisional captures Triton Somewhat smaller than Europa but larger than Pluto Comparable to other large satellites with respect to distance from the planet Orbit is circular but retrograde! Collisional capture also expelled Nereid into its highly elliptic orbit, and ejected other original satellites Irregular satellites may also be collisionally captured but their parents were smaller and may have been fragmented

Jupiter’s Galilean satellites: 

Jupiter’s Galilean satellites Discovered by Galileo in 1610 Europa is slightly smaller than the Moon; Callisto and Ganymede are larger than Mercury Io has a rocky composition; Europa is mostly rocky; Ganymede and Callisto are 50% rock and 50% ice Tidal heating effects are important for Io and Europa

Tidal heating of satellites: 

Tidal heating of satellites The tidal force from the planet raises bulges on the planet-facing and planet-opposing sides of the satellite The orbits of Io and Europa around Jupiter are eccentric due to mutual gravitational forces of the 4:2:1 resonance Io-Europa-Ganymede triplet The orbital eccentricity causes flexing of the satellite due to (1) varying distance from Jupiter; (2) varying angular velocity while the rotational velocity is constant This picture illustrates the tidal lag of a planet that rotates faster than the orbital motion of the satellite

Io’s volcanism (1): 

Io’s volcanism (1) Io’s tidal heating causes a constant volcanism heat flux is 40 times greater than for Earth tidal heat is too large to be removed by conduction or solid-state convection  melting of the subsurface and volcanic eruptions over 200 volcanic calderas, generally over 20 km in size volcanic flows hundreds of km long indicate low viscosity similar to terrestrial basalt lavas resurfacing rate estimated to 1-10 cm/year all geologic features related to volcanism; no impact craters

Io’s volcanism (2): 

Io’s volcanism (2) Io’s surface is dominated by S-bearing species: light SO2 frosts, elemental S and coloured S compounds Two classes of volcanic plumes are concentrated in the equatorial region: Prometheus-type and Pele-type Pele-type plumes are higher and bigger, short-lived with darker deposits, higher temperatures Prometheus-type eruptions are probably driven by vaporization of SO2 in contact with molten S Pele-type eruptions may be driven by liquid S heated by molten silicates at several km depth: phase change to gaseous S drives the volcano Some very small hot spots are extremely hot (>1700 K) and probably correspond to ultramafic, highly fluid magmas

Europa (1): 

Europa (1) Slightly smaller than the Moon, mostly rocky composition, tidally heated H2O crust ~ 100 km thick; the lower part is certainly liquid Weak magnetic field, induced by a conducting liquid (salty water?) moving in Jupiter’s magnetic field Very bright surface; spectral features of nearly pure water ice Extremely flat, topography < 300 m; few impact craters indicate young surface (10-100 Myr)

Europa (2): 

Europa (2) Global network of dark ridges, up to > 1500 km long Appears to have broken up the ice into plates ~ 30 km in size; lateral movements have occurred Some evidence of geyser- or volcanic-like activity along ridges; active resurfacing?

Ganymede’s tectonic features: 

Ganymede’s tectonic features • Old, cratered icy surface • Regionally extensive, bright and dark areas like on the Moon • But, unlike the Moon, the dark areas are oldest, most heavily cratered • Very complex geology with tectonic features in the younger terrain • Parallel ridges and grooves up to 10 km wide, ~ 100 m high • Ridges are probably tensional grabens


Enceladus Very bright, icy object Smooth surface with few impact craters Hot spot near the south pole Geyser-like activity due to tidally induced cryovolcanism Source of the material in Saturn’s E ring

Saturn’s E ring: 

Saturn’s E ring This Cassini picture shows part of the E ring, and the bright spot is Enceladus, from where the material originates


Titan Visual appearance from a distance: orange, featureless Dense atmosphere: ps≈1.5 bar, N2 and minor CH4 Optically opaque, dense upper layer of photochemical smog: hydrocarbons, nitriles Aerosols precipitate out of the gas as 0.2-1 m particles, accumulate into larger aggregates and fall to the surface

Titan’s atmosphere: 

Titan’s atmosphere Surface temperature ≈ 90 K; very small greenhouse effect N2 and CH4 condense into clouds at ≈ 20-30 km height; precipitation may occur Detached haze layer at ≈ 300 km height; main haze is at < 100 km height

Titan’s photochemistry: 

Titan’s photochemistry Solar uv and particle radiation dissociate N2 molecules at >1000 km height N atoms react with methane, producing H (escaping into space), HCN, hydrocarbons and C-N compounds These react further, producing stable species that sink into lower layers, eventually precipitating onto the surface This is a sink of methane (minor atmospheric constituent), which needs to be resupplied from the surface of Titan

Results from Huygens landing on Titan: 

Results from Huygens landing on Titan Geologically young surface evidence of flow around ”islands” deposits and rocks of water ice drainage channels which may have been created by methane springs few craters dark, extensive lowlands Landing occurred in liquid-saturated ”mud” No extensive liquid methane-rich hydrocarbon ocean at the surface, but lakes occur Possible cryovolcanism releases methane into the atmosphere


Iapetus Extreme albedo contrast between the leading and trailing hemispheres Equatorial ridge of about 10 km height, encircling the object Icy composition with very dark material powdered on the surface Impact craters are abundant


Miranda Very complex despite its small size some areas very old and heavily cratered other regions endogenic and crater poor, consisting of white and dark bands and highly fractured scarps and ridges models of origin include tidal heating due to Uranus’ vicinity incomplete differentiation and convection patterns disruption by impact followed by reaccretion localized late accretion of heavy core material


Triton Somewhat smaller than the Moon, extremely cold Tenuous atmosphere of N2 with trace CH4 Very bright surface made of N2 and CH4 ice with trace NH3 Trailing-leading hemispheric dichotomy Cryo-volcanoes of liquid N2 in polar regions with constant insolation carry particles into the atmosphere

Irregular satellites (1): 

Irregular satellites (1) Orbits are contained within the Hill radius Moderate to high eccentricities Separation into prograde and retrograde classes Groupings are evident mostly for jovian satellites

Irregular satellites (2): 

Irregular satellites (2) Similar colours tend to be observed for members of the same dynamical group This supports an origin by collisional fragmentation Collisions are part of some capture models, where a temporary capture is made permanent by dissipative forces: - Increase of the planetary mass by accretion - Gas drag through a planetary envelope or circumplanetary disk - Collision or close encounter with another satellite - Dynamical friction from a huge number of small objects orbiting in the vicinity


Phoebe The largest irregular satellite (≈220 km diameter) Imaged by the Cassini probe orbiting Saturn: intensively cratered Spectra show abundant water ice, hydrous minerals, CO2, organics, nitriles, cyanide compounds Composition similar to comets; density of 1.6 g/cm3 indicates compact object like Pluto and Charon

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