Moons of Jovian Planets

Clicker Question: The only Jovian planet without a large moon is: A: Jupiter B: Saturn C: Uranus D: Neptune

The Galilean Moons of Jupiter Closest to JupiterFurthest from Jupiter (sizes to scale) Radii range from 1570 km (Europa, slightly smaller than our Moon), to 2630 km (Ganymede - largest moon in Solar System). Orbital periods range from 1.77 days (Io) to 16.7 days (Callisto). The closer to Jupiter, the higher the moon density: from 3.5 g/cm 3 (Io) to 1.8 g/cm 3 (Callisto). Higher density indicates higher rock/ice fraction. Io EuropaGanymedeCallisto

8.1 The Galilean Moons of Jupiter Their interiors

Voyager 2 (1979) Galileo (1996) Volcanic activity causes Io’s surface to slowly change over the years:

The Galilean Moons of Jupiter: Io Io is the densest of Jupiter ’ s moons, and the most geologically active object in the solar system. It has many active volcanoes, some quite large. Io can change surface features in a few weeks. Io has no craters; they fill in too fast – Io has the youngest surface of any solar system object.

Volcanic activity requires internal heat. Io is a small body. Should be cold and geologically dead by now. What is source of heat? First, Io and Europa are in a "resonance orbit": Day 0 Europa Io Day 1.77 Europa Io Day 3.55 Europa Io The periodic pull on Io by Europa makes Io's orbit elliptical. Jupiter

Io orbital speed slower orbital speed faster - Tidal bulge always points to Jupiter. So the angle of the bulge changes faster when Io is closer to Jupiter. (exaggerated ellipse) - But Io rotates on its axis at a constant rate. - So bulge moves back and forth across surface => stresses => heat => volcanoes

Europa may have Warm Ocean beneath Icy Surface 860 km 42 km Icebergs or "ice rafts" suggest broken and reassembled chunks. Dark deposits along cracks suggest eruptions of water with dust/rock mixed in (Europa’s density => 90% rock, 10% ice). Fissures suggest large moving ice sheets. Hardly any impact craters.

Europa Europa has no craters; surface is water ice, possibly with liquid water below. Tidal forces stress and crack ice; water flows, keeping surface relatively flat.

What is source of heat? Similar to Io: resonant orbits with Ganymede and Io make Europa's orbit elliptical => varying tidal stresses from Jupiter => heat. Warm ocean => life? Jupiter Io Europa Jupiter Ganymede (exaggerated ellipses)

The Galilean Moons of Jupiter: Ganymede Ganymede is the largest moon in the solar system – larger than Pluto and Mercury. It has a history similar to Earth ’ s Moon, but with water ice instead of lunar rock.

The Galilean Moons of Jupiter: Callisto Callisto is similar to Ganymede; no evidence of plate activity.

The Large Moon of Saturn: Titan Titan has been known for many years to have an atmosphere thicker and denser than Earth ’ s; mostly nitrogen and argon. Makes surface impossible to see; the picture at right was taken from only 4000 km away.

Saturn's Titan: A Moon with a Thick Atmosphere Surface pressure is 1.6 atmospheres, T=94 K. Atmosphere 90% Nitrogen. Evidence for methane rain, a few possible slushy lakes of methane/ethane, drainage channels, liquid-eroded rocks, icy volcanoes (replenishing the methane?), complex hydrocarbons in atmosphere (e.g. benzene C 6 H 6 ). Mostly dry now - liquid flow may be episodic. Surface from Huygens probe Origin of atmosphere: probably gases trapped in water ice at formation, released by heat from natural radioactivity and volcanos into atmosphere. Trapped by Titan’s cold temperature and relatively high gravity. Taken during Huygens’ descent From Cassini- Huygens mission

Saturn's Rings (all Jovians have ring systems) - Inner radius 60,000 km, outer radius 300,000 km. Thickness ~100 m! - Composition: icy particles, 10m in diameter. Most a few cm. - A few rings and divisions distinguishable from Earth.

Origin of Saturn's Rings: If a large moon, held together by gravity, gets too close to Saturn, the tidal force breaks it apart into small pieces. The radius where this happens is called the Roche Limit. Total mass of ring particles equivalent to 250 km moon. Perhaps a collision between moons sent one inwards this way, or a captured stray body. Rings expected to survive only million years.

Voyager probes found that rings divide into 10,000's of ringlets. Structure at this level keeps changing. Waves of matter move like ripples on a pond.

Origin of Cassini Division: another resonance orbit Approximate radius of Mimas' orbit Mimas' orbital period is twice that of particles in Cassini division. Makes their orbits elliptical. They collide with other particles and end up in new circular orbits at other radii. Cassini division nearly swept clean. Other gaps have similar origins.

Rings of other Jovian Planets The rings of Uranus. Discovered by "stellar occultation". Jupiter, Uranus, Neptune rings much thinner, much less material. Formed by breakup of smaller bodies? Also maybe "sandblasting" of material off moon surfaces by impacts. Given rings have short lifetime and all Jovian planets have them, their formation must be common. Neptune's moon Triton is spiraling in to the planet and should produce spectacular ring system in 100 million years.

8.4 Planetary Rings Jupiter has been found to have a small, thin ring.

8.4 Planetary Rings Neptune has five rings, three narrow and two wide.

Bizarre Orbits of some of Saturn's Moons Telesto and Calypso share orbit with Tethys, and are always 60 deg. ahead and behind it! They stay there because of combined gravity of Saturn and Tethys. Janus and Epimethius are in close orbits. When the approach each other, they switch orbits! Tethys Janus and Epimethius

Clicker Question: Jupiter’s moon Europa is thought to have a large ocean of liquid water under a frozen surface. What is the heat source that keeps it from freezing? A: Heat trapped inside the moon since formation. B: A strong greenhouse effect from a dense atmosphere. C: Tidal forces exerted by Jupiter, Io and Ganymede. D: Radioactive decay of heavy elements in the mantle.

Clicker Question: Saturn’s rings are not perfectly uniform. What causes the observed gaps? A: The gravitational influence of Saturn. B: The gravitational influence of Saturn’s moons. C: Radiation pressure from Saturn. D: The gravitational influence of the Sun and Jupiter.

Pluto Predicted to exist by remaining irregularities in Uranus' orbit. Discovered in 1930 by Clyde Tombaugh ( ). Irregularities later found to be incorrect!

Pluto Predicted to exist by remaining irregularities in Uranus' orbit. Discovered in 1930 by Clyde Tombaugh ( ). Irregularities later found to be incorrect! Model created from HST images. This is the most detail we have. Discovery image of Pluto's moon Charon (1978). Charon is 1/6 the size of Pluto and tidally locked

Mass M Earth or 0.2 x mass of Moon Radius 1150 km or 0.2 R Earth Density 2.0 g/cm 3 (between Terrestrial and Jovian densities. More like a Jovian moon) Basic Properties of Pluto Icy/rocky composition Eccentric, tilted orbit Moons: Charon: radius about 590 km or 0.1 R Earth. Pluto and Charon tidally locked. S/2005 P1 and S/2005 P2: about km. For over 70 years, the largest object in Solar System past Neptune

But should Pluto be a planet? Pluto the oddball

Pluto is smaller than the larger moons

Orbits of the 8 Planets All orbit in same direction. Most orbit in same plane. Elliptical orbits, but low eccentricity for most, so nearly circular.

Exceptions: Mercury Pluto (no longer a planet) orbital tilt 7 o orbital tilt 17.2 o eccentricity 0.21 eccentricity 0.25 (Earth: orbit eccentricity 0.016: nearly circular orbit) Pluto’s orbit around the sun is different

Density = mass / volume Densities of common materials Water1.0 g / cm 3 Ice0.92 g / cm 3 Air g / cm 3 Gasoline0.70 g / cm 3 Milk1.03 g / cm 3 Gold19 g / cm 3 Lead11 g / cm 3 Osmium22.5 g / cm 3 Planet Densities – Does Pluto fit?

Planet Densities – Which of these is not like the others

Planet Densities – What is Pluto made of ? Terrestrial Planets are mostly rocky Gas Giants are dense gas Moons are ice (~ 1 g / cm 3) and rock (3-5 g / cm 3) What is Pluto made of?

The Kuiper Belt Objects More than 1200 found since Probably 10,000's exist. Icy/rocky. Orbits tend to be more tilted, like Pluto's. Leftover planetesimals from Solar System formation? Also called Transneptunian objects

The New “Dwarf Planet” (2003 UB313 = Eris) Sequence of discovery images of 2003 UB313(aka Eris)

Radius 1200 ± 50 km so bigger than Pluto. Icy/rocky composition, like Pluto. More massive than Pluto. The New “Dwarf Planet” (2003 UB313 = Eris) It too has a moon (Keck telescope) Very eccentric orbit. Aphelion 98 AU, perihelion 38 AU. Period 557 years. Orbit tilt 44°. orbit

2006 International Astronomical Union defines what a planet is A celestial body that (a)is in orbit around the Sun (b)Has sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round – spherical) shape, and (c)Has cleared the neighborhood around its orbit

2006 International Astronomical Union also defines new term => dwarf planet A celestial body that (a)is in orbit around the Sun (b)Has sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round – spherical) shape, (c)Has NOT cleared the neighborhood around its orbit, and (d)Is not a satellite (not a moon of another body)

Dwarf Planets compared to Terrestrial Planets "Terrestrial" Mercury, Venus, Earth, Mars Dwarf Planets Pluto, Eris, Makemake, Haumea, Ceres, and others, expected to be found or classified Close to the Sun Small Far from the Sun Very small Few Moons No Rings Main Elements Fe, Si, C, O, N Mostly Rocky High Density ( g/cm 3 ) Slow Rotation ( days) Rock and Ice Moderate Density (2 - 3 g/cm 3 ) Few Moons No Rings Main Elements Fe, Si, C, O, N And an icy surface Rotation?

Pluto and the Dwarf Planets

March 8, 2007: New Mexico Legislature declares that, “as Pluto passes overhead through New Mexico’s excellent night skies, it is declared a planet and that March 13, 2007 be declared ‘Pluto Planet Day’”

Today: What matters is not how the International Astronomical Union defines what a planet is What matters is that we understand the structure of the solar system and the roles of the major players (Sun & Planets) and the minor players (everything else).

The Solar System Ingredients? ● The Sun ● Planets ● Terretrial ● Gas Giants (Jovians) ● Dwarf Planets ● Moons and Rings ● Comets ● Asteroids (size > 100 m) ● Meteoroids (size < 100 m) ● Kuiper Belt ● Oort cloud ● Zodiacal dust ● A lot of nearly empty space

Oort Cloud The size, shape, and orientation of cometary orbits depend on their location. Oort cloud comets rarely enter the inner solar system.

Shoemaker-Levy Impact

Summary of Chapter 8 Outer solar system has 6 large moons, 12 medium ones, and many smaller ones. Titan has a thick atmosphere and may have flowing rivers of methane. Triton has a fractured surface and a retrograde orbit. Medium-sized moons of Saturn and Uranus are mostly rock and water ice. Saturn ’ s rings are complex, and some are defined by shepherd moons.

Summary of Chapter 8, cont. The Roche limit is the closest a moon can survive near a planet; inside this limit rings form instead. Jupiter, Uranus, and Neptune all have faint ring systems. Pluto has five known moons including Charon, Nix, and Hydra. Dwarf planets beyond Neptune (including Pluto) are now known as plutoids.