Moons of the Jovian Planets: Satellites of Ice and Rock
• What kinds of moons orbit the jovian planets? • What makes Jupiter’s Galilean moons unusual? • What makes Saturn's moon Titan different from other moons? • Why are small icy moons more geologically active than small rocky planets? What kind of moons orbit the jovian planets? • Two kinds: Medium and large moons: mostly formed at the same time as their planets (nearly circular orbits, all in the same direction). Small moons: mostly captured asteroids and comets (mildly to extremely elliptical orbits, even retrograde ones). Medium & large moons • L o ts o f ic e • Active resurfacing in the past (some moons even today) • Enough self gravity to be spherical: young, `molten' moon rock and ice ran `downhill' [like water on Earth] until it made a sphere [on which there is no more `downhill']
What makes Jupiter’s Galilean moons unusual?
IO EUROPA Ganymede Callisto Io’s Volcanoes
Io is the solar system's most volcanic world. Tidal stress cracks Europa’s surface ice, which floats to new positions on a subsurface ocean of water or slush. Interiors of Io & Europa are warmed by tidal heating.
What makes Saturn's moon Titan different from other moons? What makes Saturn's moon Titan different from other moons? • Only moon with an atmosphere: 90% nitrogen, plus argon, methane, hydrocarbons (smog!). • Methane & ethane are greenhouse gases. • Still cold: 93 K (180 degrees C) • Chemical reactions on Titan produce organic, chemicals (hydrocarbons, etc.) • Cassini spacecraft images show a young surface (few craters) but with evidence of hydrocarbon lakes only at the poles. • Possibly a captured Neptune’s Moon Triton K u i p e r b e l t o b j e c t : orbiting Neptune opposite Neptune’s direction of rotation • Neptune's tidal bulges (caused by Triton) are pulling backwards on Triton, slowing it down into a future crash into Neptune. • Smaller than Earth’s Moon, yet has recent geological activity (nitrogen geysers). Uranus’ small moon Miranda shows huge cliffs & tectonic activity, and few craters. Why are small icy moons more geologically active than small rocky planets? • Hot interiors needed for geological activity. • Ice deforms more easily than solid rock – Less internal heat is required – Smaller objects can be geologically active. • Tidal heating important for some icy moons, but not for rocky planets (example: the Moon's tides are slowing down the Earth, but not heating it up by any significant amount). What have we learned?
• What kinds of moons orbit the jovian planets? • We can categorize the more than 100 known moons as small, mediumsize, or large. • Most mediumsize and large moons probably formed with their planet in the disks of gas that surrounded the young jovian planets. • Small moons are most often captured asteroids or comets. • What makes Jupiter’s Galilean moons unusual? What have we learned? • Io is the most volcanically active object in the solar system. Europa may have a deep, liquid water ocean under its icy crust. Ganymede also thought to have a subsurface ocean. • What makes Titan different from other moons? • Titan is the only moon in our solar system with a thick atmosphere. It has a young surface with lots of contrasting features, but we don't yet know what they are. What have we learned?
• Why are small icy moons more geologically active than small rocky planets? • Ices deform and melt at much lower temperatures than rock, allowing icy volcanism and tectonics at surprisingly low temperatures.