Moons of the Solar System ❑ The moons of the various planets show a huge variety of surfaces ❑ Many are large enough to be considered 'worlds' of their own right. Mercury, Venus, Earth ❑ Mercury and Venus : no moons ❑ Proposed that Venus had a moon which tides eventually brought down to the surface ❑ (might explain slow rotation of Venus) ❑ Earth's Moon is unusually large (mass 1/81 of Earth), only Charon/Pluto bigger ❑ Our Moon likely produced via giant impact. Mars has two tiny moons ❑ Asteroid­sized ❑ Rocky, but low density (1.9,1.8 g/cm3) ❑ Deimos just outside synchronous orbit, but Phobos INSIDE ❑ So, tides cause Phobos to spiral towards Mars. ❑ Estimate will hit surface in ~40 Myr. ❑ Currently inside Roche limit (!?) ❑ How is this possible? Phobos ❑ Spacecraft show a rocky surface with many craters + regolith. ❑ Looks very similar to a C­type asteroid ❑ Phobos and Deimos are likely captured asteroids, although no idea how this works! The galilean satellites ❑ Galileo discovered 1610 ❑ Four massive satellites in orbit around Jupiter, periods 2­17 days Regular vs irregular satellites ❑ The galilean satellites are examples of REGULAR satellites, with nearly­circular orbits in the plane of the planet's equator ❑ In contrast, irregular satellites orbit :

1) Further from the planet

2) On orbits that are inclined (vs. equator)

3) On orbits with large eccentricities (usually) We will not discuss the irregular satellites The 4 galilean moons are regular ❑ In very low­eccentricity orbits near planet ❑ Except Europa, are bigger than our Moon ❑ Densities decrease with distance to planet...

Density pattern argues for formation in a 'mini'­nebula ❑ Just as there was a 'condensation sequence' for planets in the solar nebula, moons forming in a nebula around Jupiter also show the pattern of more ice with greater distance. ❑ This implies that the young Jupiter was very hot! (release of gravitational energy). Io ❑ Highest density galilean moon ❑ Pre­Voyager 1, realized jovian tidal stress on Io is huge

❑ Io's orbit not perfectly circular (see next slide) ❑ This produced massive internal heating (100 trillion watts!) ❑ Scientists (three days before Voyager 1 !) proposed surface should be volcanic ❑ What causes the heating? ❑ Tidal flexing. ❑ Io's orbit is very slightly eccentric ❑ Varying distance causes varying height of tidal bulge (Jupiter) producing FLEXING ❑ Flexing heats moon ❑ (Eccentricities are maintained by orbital resonance between Io, Europa & Ganymede) Io ❑ Indeed, Voyager found a smooth surface with no impact craters ❑ But then...! Io ❑ Is the only place in the solar system with observed...

❑ ACTIVE VOLCANOES! ❑ Plumes >200 km above Io's surface ❑ Volcanic volatile is sulfer dioxide; eruptions are more like geysers than volcanos Io

❑ Lava curtain in this image is erupting sheet >1 km high! Io Io Torus ❑ Jupiter's strong magnetic field interacts with ions coming off Io, which spread out into a torus around orbit ❑ Current flows along the magnetic field lines, and there is a 'flux tube' of charged particles between Io and Jupiter (5 million amps!) ❑ This radiation environment very dangerous to spacecraft and has destroyed instruments on the Galileo probe Europa ❑ Crater­poor surface of almost pure water ice ❑ Many fractures criss­ cross surface ❑ Mild tidal heating causes continual upwelling of water to surface where it freezes into new crust ❑ Density of 3 g/cm3 implies planet is mostly rock inside, but... Europa ❑ Complex ridge pattern due to cracks opening in crust, filling with water and freezing ❑ Tidal flexing around an orbit causes cracks to open San Francisco to same scale and close. Europa

❑ Details suggest there is a water OCEAN tens of km deep under surface ❑ Water + heat source may be enough for LIFE !?!? ❑ NASA mission proposed ❑ Icy worlds with cratered surfaces ❑ Comparable to Mercury! ❑ Ganymede: ❑ Weak magnetic field ❑ Dichotomy. Ganymede ❑ Ancient dark terrain Callisto ❑ Bright young terrain ❑ Callisto: ❑ Missing small craters ❑ Valhalla impact basin Jupiter's inner satellites

❑ All interior to Io, difficult to approach! ❑ These moons interact with Jupiter ring, and may produce it! Saturn has 30 known moons

❑ Three categories:

1) Ring moons

2) Regular satellites

3) Irregular satellites Saturn's inner satellites shape ring Saturn's 6 icy medium moons ❑ Icy bodies with cratered surfaces ❑ Maybe reassembled several times ❑ Iapetus : dark front hemisphere

Best pre­Cassini photo Recent high­ Iapetus resolution Cassini spacecraft image

The moon has an incredible equatorial ridge about 20 km high that extends around the satellite. The Cassini Enceladus... orbiter discovered geysers

Here, one sees light reflecting off the water ice crystals coming out of the south­ pole geysers. Saturn's moon Titan ❑ Larger (5150 km diameter) than Mercury ❑ Has a thick atmosphere (only moon in Solar System) ❑ Methane absorption seen in reflectance spectrum ❑ But most of the mass is in molecular Nitrogen ❑ Voyager 1's cameras were unable to pierce 200 km atmosphere ❑ N comes from : Titan's atmosphere 2

❑ NH3 + solar UV → N + H(escapes) ❑ Possibility of 95 K liquid hydrocarbon seas ❑ Cassini spacecraft is orbiting Saturn and exploring rings, moons, and the planet ❑ The Huygens probe entered Titan's atmosphere ❑ Cassini orbiter's image of Titan's surface (2 microns). ❑ But the Huygens probe did better! (Artist impression) ❑ But the Huygens probe did better! Images taken from tens of km altitude while the probe was descending on its parachute

Note the `dendritic' pattern. A flowing liquid? ❑ But the landing site turned out not to be liquid

Image from landed Huygen's probe.

Foreground 'pebbles' are about 3­5 cm across

Note that they are rounded?!? ­ Erosion? Uranus has smaller icy moons ● Darkened icy surfaces, with craters showing brighter floors ­ Implies fresh ice exposed via impacts? ­ Largest is Umbriel (1600 km (Photo collage, not to scale) diameter) Orbit in equatorial plane, so formed AFTER Uranus tilted on its side... Miranda a bizarre mix

A broken and re­ assembled moon?

Perhaps sections of the surface heated (tides?) and sunk... Neptune: small inner moons + Triton

❑ Triton is a large (from geysers) retrograde captured moon ❑ Voyager observed geysers ❑ Very thin nitrogen atmosphere ❑ Surface T is only 38K ! Pluto and Charon ❑ Charon half the radius and same density ❑ So mass ration about 1/8, largest in S.S.

❑ Both Pluto and Charon spin with same period, and = to orbital period

❑ How did this arrive?

❑ TIDES despun both ❑ Earth/Moon will eventually do same. Saturn and ring systems

❑ Saturn: the other gas giant ❑ Rings: the dance of Kepler ❑ Sheperds: pastoral harmony in the heavens Saturn's rings

❑ First seen by Galileo but not understood ❑ Was unclear if they were 'attached' to planet ❑ Over decades their appearance changes, getting bigger and smaller ❑ They sometimes disappear! Ring plane is fixed in space and we view it from different angles over a 30­year Saturn period. We see them edge­on during 'ring­plane crossings'

❑ Rings almost disappear when viewed edge on ❑ Implies they must be very thin ❑ Amazingly: locally only tens of meters thick...! ❑ With bends (sheet of paper analogy) about 1 km thick ❑ HST image of 1995 ❑ ring­plane crossing, But nearly 300,000 km note Titan and its across!!! shadow Four main rings+ Cassini division The nature of the rings? ❑ Are the rings a solid ring, or made of particles? ❑ Telescopes showed structure to the rings ❑ A,B,C and Cassini division

❑ Physicist Maxwell proved rings could not be solid ❑ Astronomer Keeler obtained reflection spectrum and Doppler effect showed that speed at each radius same as Kepler speed. Rings composed of a large number of particles

❑ Typical particles are cm or meter sized ❑ Each particle in orbit around Saturn ❑ VERY CIRCULAR ❑ Collisions between particles produce a very flat ring ❑ Shadow of the rings is falling on Saturn. ❑ Dione (left) and Enceladus (right) ❑ Dione in front of the rings. ❑ Shadow of the rings is falling on Saturn. Different sections of the rings have different concentration of particles ❑ The C ring has a low particle concentration (reflect little light) ❑ B ring reflects a lot of light ❑ Cassini division and Encke gap almost empty ❑ F ring thin Rings sculpted by satellite interactions

❑ The satellites just exterior to the rings influence its structure ❑ Example: Cassini division is at the 2:1 resonance with Mimas; recall resonances Mimas Enceladus

Epimetheus

Cassini spacecraft image. Here the rings are backlit, so Cassini gap shows dust Encke gap is cleared by Pan

❑ The 20­km diameter satellite Pan maintains the Encke gap ❑ Found in search of Voyager images (left) ❑ Pan in the Encke gap Cassini image ThThe e F F rriinngg iiss ''sshehepeperrddeed'd' bbyy 22 mmooonsons

☞ Gravitational tugs of the two moons confine the narrow F ring ☞ Conversely, a moon inside the ring can clear a gap ☞ Another example of 'repulsive gravity' in a rotating frame Epimetheus

Pandora

Prometheus AA rreececentnt CCaassssiinini iimmaagge e sshhowowiingng tthhe e twtwoo FF­­rriinng g sshehepphheerrdsds How'd the rings form?

❑ If you added all the mass together, Saturn's rings could form a moon 100 km in diameter. ❑ Why don't the rings accrete together to form a moon? ❑ Answer: tides Recall tidal acceleration ❑ The two bodies m1 and m2 do not feel the same force from the planet ❑ If this 'tidal' force is greater than their force of attraction they will move apart

❑ Objects closer to the planet than a certain distance will not accrete! ❑ This distance (slightly greater than 2 planetary radii) is called the Roche limit. Saturn's inner satellites shape ring ❑ Jupiter's ring ❑ small particles ejected off of small moons Rings of Uranus and Neptune

❑ Uranus and Neptune both have a set of thin rings. ❑ Neptune has some 'arcs' that remain confined... Extra slides

• Not part of course material in 2006 Jupiter has many irregular moons ❑ Irregular moons originate from objects captured into orbit around the planet ❑ Originally formed elsewhere in Solar System Saturn has 14 known irregulars

These 14 are fragments of several moons captured long ago as Saturn formed

Thirteen only discovered in the last 2 years (better CCDs) Uranian and Neptunian Irregular satellites

❑ 9 uranian and 5 neptunian irregular satellites discovered in the last 6 years (all by Canadians) ❑ Appear to be large captured comets ❑ The capture mechanism for irregular satellites is still unclear. Reflected (left) vs scattered (right) light

Voyager image above taken from 'behind' the rings. (Fig 15­6) ❑ B ring transmits very little light ❑ Cassini division not empty