Astronomy 1010
Planetary Astronomy
Course Announcements
SW-chapter 9 due: Wed. Nov. 18
SW-chapter 10 due: Mon. Nov. 23
I will collect the L-T books on Monday, Nov. 23
1st Quarter Obs. (last one of semester): Thurs. Nov. 19
Reports Due: Monday Nov. 23 – AT CLASS TIME!
Water Cycle
 Convection along with Earth’s water cycle creates
violent weather patterns on Earth, including
thunderstorms, tornadoes, and hurricanes.
 Dust devils can also be created on Mars.
 Lightning on Venus and the giant planets.
 Parts of Earth
are heated differently.
 Convection distributes
surface heating.
 Hadley circulation
transports heat
between equator and
 Global winds carry
heat from hot to cool
 On Earth, Venus, and
Mars, the circulation
depends on heating
pattern and rotation
 The Coriolis effect
breaks the larger
convection cells into
smaller ones.
 Climate is the average
state of an atmosphere.
 Can be affected by
geological, and
biological mechanisms.
 Astronomical: changes
in the Sun’s energy
output, change in a
planet’s obliquity, and
 Geological: volcanic
and tectonic activity.
 Biological:
photosynthesis by
bacteria and plants,
methane byproducts,
human activity.
 Paleoclimatology has yielded much
information about Earth’s climate change.
 Has found temperature and climate cycles,
including ice ages.
 Earth’s climate is very sensitive to changes
in temperature.
 The temperature of a gas is a measure of the
average kinetic energy of its molecules.
 The motion of the gas molecules in an
atmosphere creates its pressure.
 If the speed of a gas
molecule is less than a
planet’s escape speed, it
will be more easily
 The speed is
represented by the
 If the gas speed is less
than 1/6 of the escape
speed, the planet will
keep its atmosphere.
 When presented with complex science (such
as climate change), it is important to analyze
the issue inclusively from different angles.
Jupiter, Saturn, Uranus,
and Neptune are the
giant planets.
Jupiter and Saturn:
mainly hydrogen and
5.2 AU from Sun,
and 9.6 AU from Sun.
Called gas giants.
 Uranus and Neptune:
smaller, have much
more water, water ice,
and other ices.
 19.2 AU from Sun,
and 30.0 AU from Sun.
 Called ice giants.
 Jupiter and Saturn
were known to the
 Uranus was too faint
to be discerned from
the other stars.
 Discovered in 1781
by Herschel by
accident (at first
thought it was a
 Neptune was found
because Uranus was
straying from its
predicted orbit.
 Gravity of Neptune
was tugging on
 Found in 1846 by
Galle after being
predicted by Le
Verrier and Adams.
 Called giant planets because of their mass—
from 14.5 Earth masses (Uranus) to 318
(Jupiter)—and also, their physical size.
 No solid surfaces: We just see the cloud layers
in the atmospheres.
More Giant Characteristics
 We see atmospheres (some very cloudy,
some not), not surfaces.
 They are less dense than the terrestrial
planets; in fact, Saturn would float in a
large enough tub of water.
 Jupiter’s chemistry is like the Sun: mostly
hydrogen and helium.
 Saturn has some more heavy elements,
but is like Jupiter; Uranus and Neptune
have much more heavy elements.
 Planetary diameters are found by observing
how long it takes for a planet to pass over a
star: stellar occultation.
 Planetary masses are found by observing the
motions of a planet’s moons and effects of
i_Clicker Question
Jovian Planets: Jovian Transit
Jovian Planets: Jupiter Formation
 All giants have rapid
rotation and therefore
different amounts of
 They also have different
• Jupiter: 3°.
• Uranus: 98°, which results in
extreme seasons.
i_Clicker Question
Jovian Planets: Oblateness
Strong dark and light bands.
A long-lasting giant storm (Great Red Spot).
Many smaller storms.
Colors indicate complex chemistry.
 Saturn has a similar band structure
to Jupiter, but less pronounced.
 It has violent storms and a feature similar to
Earth’s jet stream.
 Infrared observations let us see details of
structure on Uranus.
 Weak banding on both Uranus and Neptune.
 Small, scattered bright or dark clouds.
 Transient large storms (Great Dark Spot on
The gas giants have
similar cloud layers.
Temperature, pressure
increase downward.
Different heights
of cloud layers.
 Clouds on Jupiter:
• Ammonia (NH3) at
T = 133 K.
• Ammonium hydrosulfide
(NH4SH) at T = 193 K.
Under the Clouds: Uranus/Neptune
 Unlike Jupiter and
Saturn, the highest
clouds on Uranus
and Neptune are
methane ice.
 Bluish because
of scattering of light
by the methane.
 Clouds on Jupiter
and Saturn are
colored by
Helium Rain
 In Jupiter and Saturn, Temperature and Pressure meet the
correct conditions to condense Helium in the lower
atmosphere. In essence, this cause liquid Helium drops to
form and fall down as “rain”.
 This contributes to the internal heating of both planets.
 Mixed in the Helium drops is Neon, which helps explain
the low levels of neon in the atmospheres of both
i_Clicker Question
Jovian Planets: Helium Rain
 Rapid planetary rotation results in strong
Coriolis forces. This causes storm rotation.
 Most extreme equatorial winds are in Saturn’s
and Neptune’s atmospheres, with maximum
speeds up to 2,000 km/hr.
 Alternating east–west winds make banded
clouds on Jupiter.
 Circulation pattern differs from planet to
planet in ways not understood.
 Wind speeds on gas
giants can be
measured by observing
the movement of clouds
above an assumed
 Using the
circumference of the
planet, you can find
how much a spot
travels in a given time.
 All but Uranus
have significant
internal heat.
 Heat flows from
the hot interior
 Heat has a big
effect on global
 Jupiter, Saturn, and Neptune radiate away
more energy than they get from the Sun.
 A small increase in internal temperature leads
to a large increase in emitted energy.
 Something has to be increasing the
temperature. It is believed that the planets are
still shrinking, with gravitational energy being
converted into heat during that process.
 For Jupiter:
 Jupiter/Saturn: At depths of a few 1,000 km,
gases are compressed so much they liquefy.
 At higher pressure and temperature, this
liquid hydrogen can act like a metal.
 Cores are a liquid mixture of water, rock, and
 Uranus and Neptune are smaller and have
less pressure than the gas giants.
 They have more water and ices (ammonia,
 Deep oceans containing dissolved gases and
salts are present.
 Jupiter and Saturn formed from the
protoplanetary accretion disk while
hydrogen and helium were still present.
 Solar wind later blew out these gases.
 Uranus and Neptune formed later, by the
merger of icy smaller bodies.
 All four possess a dense liquid core
containing rocky materials.
 Many details are still not understood.
 Magnetic fields
are generated
by the motion of
the electrically
 Their orientation
is at an angle to
the rotation axis.
 Like a bar
Magnetospheres are huge (Jupiter’s is 6 AU).
They interact with the solar wind.
Auroras (“Northern lights” on Earth).
Produce strong radio waves/synchrotron
i_Clicker Question
Jovian Planets: Magnetic Fields
 Strong
concentrate charged
particles in
radiation belts,
including the plasma
torus created by
particles from
Jupiter’s moon Io.
 Powerful flux tubes
create bright
 Accelerating charged
particles emit photons.
 When moving, charged
particles are forced in
circles by magnetic
 Occurs often with
magnetic fields and
electrons around planets
and stars.
 Synchrotron radiation.
 “Hot Jupiters” are seen orbiting close to their
stars in extrasolar planetary systems.
 Computer simulations show that the giant
planets may not have formed where they
exist now, but rather could have migrated to
their positions due to gravitational influences.
 Theories make predictions that serve as
opportunities to falsify them or strengthen
confidence in the theory.
 These predictions are essential to scientific
 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
 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
 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
 Eruptions of silicate
 Has no craters and
a very young
 For a moon to be
geologically active,
it must have internal
 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
 Eruptions of silicate
 Has no craters and
a very young
 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
 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
 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
 Methane in Titan’s atmosphere is most likely
renewed by active geology.
Definitions & Terms -1
Photon: A particle of light.
Spectrum: The flux of an object as a function of