Astronomy 1010-H
Planetary Astronomy
Fall_2015
Day-37
Course Announcements
•
SW-chapter 11, 12 due: Wed. Dec. 9
•
Lab this week is Comparative Planetology … this one
you can do on your own and turn in.
•
Presentations are next week, during the normal lab
time. 10 minutes, + 5 for questions…
 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.
 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).
Concept Quiz—Internal Heat
You discover a moon of Jupiter. It orbits very
far from the planet, but it has many
volcanoes. Is this a surprise? Why?
A. No, any moon can have volcanoes.
B. Yes, Jupiter is very far from Earth’s Moon.
C. Yes, tidal forces are less for distant
moons.
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.
 A very complicated system, composed of
thousands of ringlets, each made up of tiny
orbiting particles that obey Kepler’s laws.
 There are bright and dark rings, “gaps,” and
divisions.
MATH TOOLS 11.1
 The moons obey Kepler’s laws as they orbit
the planet:
 For a particular planet, the left-hand side will
be a constant for most of its moons.
 For Jupiter, Ganymede, Europa, and Io are in
an orbital resonance of 1:2:4.
 Can estimate relative orbital distance.
Concept Quiz—Ring Velocities
If you could measure the velocities of ring
particles at each distance from Saturn,
you would find that
A. inner particles orbit at slower speeds.
B. inner particles orbit at faster speeds.
C. orbital speed is the same at all distances.
 Gaps are not empty.
 Brightness/darkness
reflects the amount
of material in each
ring.
 Though wide, the
ring system is
extremely thin.
 If Saturn were a
basketball, a piece
of paper is >1,000
times too thick.
 Diffuse rings are fainter and have no
defined boundaries.
 Saturn’s largest ring is a diffuse dust ring,
discovered in 2009.
 E Ring and G Ring are also diffuse.
 The rings of the other giant
planets are mostly narrow
and diffuse.
 Backlighting brings them
into view.
 Neptune has denser
sections known as ring
arcs.
 Ring particles are from disrupted moons or
from volcanic activity on moons.
 Saturn: bright rings because they are
made of water ice.
 The total mass of Saturn’s bright rings is
about the same as a small icy moon.
 Uranus and Neptune: dark rings from
organic material (darker than coal).
 Jupiter: not as dark as the ice giants, nor
as bright as Saturn’s; most likely
composed of dark silicates.