Callisto: a dead moon of Jupiter, or not so dead after all… ? As seen from Galileo satellite Callisto exhibits a dirty ice surface with many craters. While there are many large craters, very few are found with a diameter less than 1km.

- It is not known how small craters have been eroded away - It is not known where does the dark dusty material blanketing the surface came from.

The same size as the planet Mercury, 0.378 Earth’s diameter or 4820km. Mass of 0.018 Earth mass. Density of 1.83 that of water, so it is mainly made of silicate rocks and water ice. Like Io, Europa and Ganymded, Callisto was discovered by Galileo Galile in 1610.

Additional high resolution (close up) view of the surface showing “material” covering craters and small features being eroded: The erosion is due to sublimation (transition from solid to gas) of water ice from within the surface, this erodes the surface, as the non-water material collapses.

In addition callisto has a magnetic field like Ganymede which indicates a liquid conducting layer - salty water ? Callisto has a liquid conductive layer (of salty water?) at least 10km thick and up to 250-300km thick. Ammonia and other substance (5% by weight) act like antifreeze. Callisto is only partially differentiated in its interior.

Some Galileo research scientists even suggested the layer might be some organic material (a kind of vegetation, in such a frigid temperature?) and even indicate the slightly “greenish” color observed here

Callisto has a tenuous atmosphere of CO2 due also to sublimation of CO2 from the icy crust/surface.

The multi ring impact structure 3800km in diameter, the largest feature on Callisto’s surface.

Saturn’s Moon Titan – with a methane cycle - Titan is larger than Mercury with a diameter of 5150km. - It has a thick atmosphere (30 percent more than Earth’s).

The atmosphere is made mainly of - Nitrogen (from ammonia NH3, where H has escaped due to photo dissociation) 95% - Methane CH4 (methane must be renewed constantly to be present) - Ethane C2H6 - Complex hydrocarbons (“organic compounds”) such as C2H2 (acetylene), C2H4 (ethylene), C2H6

(ethane), C3H8 (propane) …

- It is the largest moon of Saturn and the only moon known to have a dense atmosphere. - Composed of water ice and rocky materials.

Titan was extensively studied with the Cassini-Huygens mission in 2004. - Discovery of liquid hydrocarbon lakes on its surface young surface - Active cryovolcanism (volcanoes of ice ejecting water, ammonia or methane)

Titan has a climate with winds and rains, creating surface features such as sand dunes, rivers, lakes and seas (of liquid methane or ethane) and shorelines. Titan is viewed as an “early Earth”, but at a much lower temperature. Titan is differentiated with several layers:

- A 3,400km rocky core - Several layers composed of different crystal forms of ice - A possible liquid layer composed of water and ammonia between the ice and crust (the

ammonia allows the water to remain liquid at these temperatures). - The crust is decoupled from the interior, evidence for a liquid layer beneath it.

Huygens Probe landed on Titan (2004) The findings indicate in the upper atmosphere:

- Winds of up to 120m/s - T=88Kelvin - Methane clouds

On the surface: - No wind (the atmosphere is very thick behaving like a liquid at the surface) - T=112Kelvin - Methane rivers and methane lakes, methane rain

The atmosphere of Titan :

- Denser than earth’s - The atmosphere is blocking most of the sunlight to reach the surface which is obscure.

The composition of the atmosphere is mainly nitrogen (see beginning). The material blocking the surface is made of large molecules (10,000 times the mass of a hydrogen atom) charged negatively. The material is believed to be “tholins”

Clouds in the atmosphere if Titan:

The cloud is possibly made of Ethane rather than methane.

The surface of Titan is complex, and geologically young. There are volcanoes releasing water and ammonia (cryo-volcanoes). Some impact craters have been filled with raining hydrocarbons. No surface feature elevated above 1000m.

No ocean, but rather many lakes:

Sand Dunes on Earth (top) and on Titan (bottom)

Pictures from the Huygens probe:

There are signs of erosion at the bottom of some of the rocks, indicating fluvial activity (running liquid on the surface).

Rivers and other features on Titan:

Additional moons of Saturn…

Enceladus shows some evidence of tectonic (ice) processes, cryovolcanism …

Mimas: the death star

Enceladus

Mimas: the Death Star…

Uranus & Neptune: the Ice Giants

- The planets we have studied, Mercury, Venus, Mars, Jupiter and Saturn are all visible to the

naked eye and have been known for thousands of years. Uranus can barely seen with the naked

eye in the best observing conditions, Neptune is seen only with a telescope.

- Uranus is tilted basically 90deg as if it had been hit by a large body some 4 billion yrs ago or

around the time of its formation, so its axis of rotation nearly lies in its orbital planet draw

diagram

- The planet Uranus was discovered and recognized as a planet by William Herschel (German

astronomer) in 1781.

Herschell’s 40 foot telescope

- However Uranus’s observed orbit and computed orbit appeared to not agree over time. - Le Verrier (French Astronomer) and Adams (English Mathematician) showed that the motion of

Uranus was perturbed by a more distant planet. The planet was discovered in 1846 based on the predictions of Le Verrier and Adams, by the German Astronomer Galle. The planet was named Neptune.

Family Portraits …

John C. Adams (English) Urbain J. J. Le Verrier (French) Johann G. Galle (German) Nepture is so far, that traveling by plane at 500miles/hr would take 500 years to get there. With ground

telescopes not too much was known about these two distant planets.

We had to wait for space age with the following missions:

- Pioneer 10 & 11 (1974-1975, 1979) - Voyager 2, reaching Uranus in 1986 and Neptune in 1989. - Galileo and Cassini missions only visited Jupiter and Saturn respectively. Pioneer (below)

Comparison of Ice Giants with Gas Giants:

Ammonia and water are found deeper in the layer of the planets, they have more methan than Jupiter &

Saturn giving them their blue-green color.

Atmosphere Dynamics: winds, internal heat, features.

Uranus has no internal heat and its atmosphere only exhibits winds flowing in the same direction as the

planet rotation with maximum speed in the equator reaching 700km/hr (440mile/hr).

Image from HST in ultraviolet, visible and infrared light (enhanced colors to show cloud patterns).

Neptune is almost 20 percent more massive than Uranus. Neptune, unlike Uranus but like Jupiter and

Saturn, is still “cooling down” since its formation and generates internal heat. Because of that Neptune

atmosphere is more dynamic and show zones and belts as well as a Great Dark Spot.

Clouds made of methane crystals.

Clouds made of crystals of methane ice.

The interior of the Ice Giants

Magnetic Fields They are tilted to the rotation axis and are off-centered.

The tilt and off-center are either due to magnetic field reversal as on Earth, or due to a catastrophic

collision.

Uranus and Nepture also have Rings:

-Uranus has 13 rings, very narrow (~km), widely spaced (100skm), and material from the last (inner) ring maybe spiraling into the atmosphere of the planet. - The rings are Carbon-rich, also rich in organic compounds, they are dark and possibly originated from the core of comets?

-Neptune has 6 rings, 3 of them narrow. - the material from a ring clumps together to form “arc segments” (15km wide and up to 10,000km long) - made of ice silicates from comets. - also rich in Carbon and organic compounds.

The Moons of Uranus Uranus has 27 known moons.

- In the picture below: Puck, Miranda, Ariel, Umbriel, Titania and Oberon.

Some of the moons show sign of past geologic activity, possibly due to tidal forces.

Miranda (470km in diameter) show signs of “resurfaced” terrains, possibly due to past tidal heating.

It is made primarily of ice, while the other moons have equal amounts of rock and ice.

Miranda:

It has 9 irregular moons, orbiting far away from the planet.

The Moon of Neptune. Neptune has 13 moons, 6 regulars (Naiad, Thalassa, Depina, Galatea, Larissa, Proteus) and 7 irregulars

(Triton, Nereid, Halmede, Sao, Laomedeia, Psamathe, Neso).

THe most important moon is the irregular Triton with a diameter of 2,700km and RETROGRADE motion.

TRITON: is a captured moon, with a thin atmosphere, ices and frosts of methane and nitrogen on the its

surface and few craters young surface.

Triton is characterized by its “cantaloupe” terrain with less craters (younger)

It has : - vein-like features - basins similar in appearance to lunar maria due to water, methane, nitrogen ice that melted

inside and erupted like magma (geyserlike cryovolcanoes), carrying away silicate ice grains

This is a basin similar to a lunar maria… but in frozen water rather than dried lava