Astronomy 1010 Planetary Astronomy Fall_2015 Day-37

Course Announcements SW-chapter 11, 12 due: Mon. Dec. 7 Lab this week is Comparative Planetology … this one you can do on your own and turn in. Next week is the make-up week for labs … We will have the equipment for Lenses & Telescopes and the Spectrometer set up. These are the only two that will be setup in lab. Any other labs (computer based) must be completed PRIOR to Tuesday. I will have a substitute for the cratering lab for those that need it.

 Dozens of “worlds” of rock and ice exist in our Solar System; some large, some small.  Liquid water under some surfaces is possible.

 The moons are made of rock, ice, or both.  Some were formed by accretion and differentiation.  They have many diverse properties, only partially understood.

 Most of the larger moons formed with their planets through the processes of accretion and differentiation.  These are called regular moons.  They revolve around their planets in the same direction that they rotate.  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 were later gravitationally captured by one.  These are called irregular moons.  They are usually on retrograde (“backward”) orbits.  Largest: Triton, moon of Neptune.  Many are only a few kilometers across.

 The giant planets have several large moons, and many are as large as Earth’s Moon.  Some are geologically active, while others used to be.

 Surface markings, craters, and bright/dark areas reveal geological activity.  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.  Tidal stretching by a planet heats the moon’s interior.  Analogy: flexing a paper clip.  Example: Jupiter’s moon Io.

 Io is the most volcanically active object in the Solar System.  Eruptions of silicate magmas.  Has no craters and a very young surface.

 For a moon to be geologically active, it must have internal heat.  Tidal stretching by a planet heats the moon’s interior.  Analogy: flexing a paper clip.  Example: Jupiter’s moon Io.

 Io is the most volcanically active object in the Solar System.  Eruptions of silicate magmas.  Has no craters and a very young surface.

 Enceladus (Saturn): 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.

 Triton 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.

 Europa (Jupiter) is possibly active.  Jupiter’s tidal heating should be too low for volcanism, but should allow for subsurface liquid, perhaps as underground lakes.  Broken slabs of ice that appear to have floated and collided suggest geologic activity.

 Titan is Saturn’s largest moon.  It has a thick, dense, nitrogen-rich atmosphere.  Huygens lander revealed icy “rocks” and a soil rich with organic compounds.  Possibly active.

Possibly Active: Titan  Methane appears to experience a cycle like rain on Earth, involving methane lakes and clouds.  Methane in Titan’s atmosphere is most likely renewed by active geology.

 Formerly active Ganymede (Jupiter) is the largest moon in the Solar System.  Signs of gradually filled-in craters.  Bright terrain from some unknown past tectonic processes.  Some moons of Saturn and Uranus also appear this way.

 Other moons show signs on the surface of being formerly active, including bright and dark areas and tectonic fracturing.  Examples: Mimas and Iapetus of Saturn; Miranda of Uranus.

 Callisto (Jupiter) shows no sign of early geologic activity.  Dark, heavily cratered surfaces.  Bright regions where subsurface ice has been exposed after impacts.  Others include Umbriel (Uranus).

i_Clicker Question Jovian Planets: Europa’s Oceans

Ring Systems  All four gas giants have ring systems.  Saturn’s rings are the largest and brightest.  The fainter rings were discovered by stellar occultation methods.