PHY 1060 Lecture Notes - Lecture 6: Escape Velocity, Irregular Moon, The Moons
Module 6: Smaller Solar System Bodies
Chapter 11: Planetary Adornments: Moons & Rings
• Dozens of “worlds” of rock and ice exist in our Solar System
o Liquid water under some surfaces is possible
• The moons are made of rock, ice, or both
o Some were formed by accretion and differentiation
o They have many diverse properties, only partially understood
• Most of the lager moons formed with their planets through the processes of accretion and
differentiation
o These are called regular moons
o They revolve around their planets in the same direction that they rotate
o Almost all are tidally locked, meaning one hemisphere always faces the planet the
moon is orbiting
• Some moons are objects that formed apart from a planet, but later gravitationally
captured by one
o Irregular moons
o Small and on retrograde orbits
o Largest: Triton, moon of Neptune
o Many are only a few kilometers across
• The giant planets have several large moons, and many are as large as Earth’s moon
o Some are geologically active, while others used to be
o Surface markings, craters, bright/dark areas reveal geological activity
o Categorized as active now, possibly active, active in the past, and never active
• For a moon to be geologically active, it must have internal heat
o Tidal stretching by a planet heats the moon’s interior
o Like flexing a paper clip
o Ex: Jupiter’s moon Io
▪ Most volcanically active body in the solar system
▪ Leads to a young surface, with now craters
o Saturn’s moon Enceladus:
▪ Partially young surface
▪ Experiences cryovolcanism, in which the magma is water
▪ Thermal energy melts ice and drives it up to the surface
▪ Enceladus’s low gravity cannot hold onto the icy particles once they are
ejected
▪ This is the source of material for Saturn’s faint E Ring
o Tritan is an irregular moon of Neptune with a retrograde orbit
▪ Cantaloupe-like surface is a clue to its activity
▪ Cryovolcanic activity: geysers of nitrogen
▪ Thin atmosphere
• The moons of the giant planets have a much lower escape velocity than that of Earth,
which is 11.2 km/s or >40,000km/h
o Cannot easily hold on to particles ejected during volcanic activity
o Enceladus has cryovolcanic plumes are 2,200 km/h
o Doesn’t hold onto its material
• You discover a moon of Saturn in a gap of its rings. The moon most likely?
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