Astronomy 1010-H
Planetary Astronomy
Fall_2015
Day-38
Course Announcements
•
SW-chapter 11, 12 due: Wed. Dec. 9
•
Presentations are tomorrow at lab time. 10 minutes,
+ 5 for questions…
•
1st Thursday Art Walk – 5-8pm on Study Day
 Rings are kept stable by shepherd moons.
 Shepherd moons can also distort rings.
 Gravity can cause distortions, including what
look like twists and waves.
 Other
distortions
include
scalloped
shapes and
appearance of
transient
spokes in
Saturn’s rings.
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Rings do not last forever.
Collisions and sunlight destroy rings.
Shepherd moons can help stabilize rings.
Orbital resonances can create gaps.
Earth does not have a ring because it
lacks shepherd moons to contain the
material.
MATH TOOLS 11.2
 The tidal force between a planet and its
moon depends on their masses, the size of
the moon, and the distance between them:
 Can use this to find the relative tidal forces
for different moons of the same planet.
MATH TOOLS 11.3
 The moons of the giant planets have a much
lower escape velocity than that of Earth,
which is 11.2 km/s or >40,000 km/h.
 Cannot easily hold on to particles ejected
during volcanic activity.
 Enceladus:
 Its cryovolcanic plumes are nearly 2,200
km/h.
Concept Quiz—Moons and
Rings and Rings
You discover a moon of Saturn in a gap of
its rings. The moon most likely
A. recently accreted from ring material.
B. maintains the gap due to its gravitational
influence.
C. is not the reason for the “twists” in nearby
rings.
D. is a figment of your imagination; the gaps
are completely empty.
 The identification of extremophile bacteria
on Earth has led to consideration of the
possibility of life in the extreme
environments of the Solar System’s
moons.
 The combination of liquid water, heat, and
organic compounds could be present.
 Enceladus, Europa, Titan, and Callisto are
possibilities for life.
CONNECTIONS 11.1
 Small particles are best viewed when they
are between the observer and light source.
 Backlighting allows for the rings of the planets
to be viewed most easily.
 Most light that hits the particles still comes to
the observer instead of being scattered away.
PROCESS OF SCIENCE
 Apparent violations
of well-supported
theories are exciting
for scientists
because they must
be reconciled.
 This often means
that something new
is about to be
discovered.
 Planetesimals left over from the formation
of the solar system include asteroids and
comets, as well as meteorites and
meteroids.
 Five large planetesimals deserve their own
classification: dwarf planets.
 Four reside in the Kuiper Belt beyond
Neptune’s orbit: Pluto, Haumea,
Makemake, and Eris.
 Ceres is in the main asteroid belt.
 Pluto is about
1/400 the mass
of Earth.
 “Double planet”:
Pluto/Charon.
 Eccentric orbit.
 Rock and ice.
 Thin methane
atmosphere.
 Eris, larger than Pluto, is
the most distant.
 Has moon, Dysnomia.
 Orbit has greater
inclination than Pluto’s.
 Ceres used to be known
as the largest asteroid.
 Spherical, about 4% the
mass of the Moon.
 Asteroids are rocky
planetesimals.
 Most are in the
asteroid belt
between Mars
and Jupiter.
 Near-Earth
asteroids have
orbits that come
close to the orbit of
Earth.
 Amor, Apollo, and
Aten asteroids have
orbits near Earth.
 Apollo and Aten
asteroids have orbits
that could cross
Earth’s—could
collide with Earth.
 Called near-Earth
objects.
 Asteroids are fragments of rock.
 Generally not large enough to be spherical.
 Erratic rotation periods.
 S- and M-type asteroids differentiated.
 C-type asteroids did not.
 S-type are similar to igneous rock; M-type to
iron and nickel.
 Spacecraft have visited seven asteroids.
 It is possible for them to have moons.
Comets
 Comets are icy
planetesimals found
beyond the planets.
 Far from the Sun,
these objects remain
small, icy bodies that
are very hard to see
from Earth.
 Located either in the
Kuiper Belt or the Oort
Cloud surrounding the
Solar System.
 Short-period
comets:
 Periods < few
centuries.
 Near ecliptic plane.
 Prograde orbits,
circular or
somewhat
elongated.
 Kuiper Belt.
 Long-period comets:
 Periods of almost
1000 to perhaps 1
million years.
 Prograde or retrograde
orbits, from the Oort
Cloud.
 Large tilts from the
ecliptic, very elongated
orbits.
 Nucleus not “worn
out.”