Download Astronomy 110 Announcements: Chapter 8 Jovian Planet Systems

Astronomy 110 Announcements:
• Midterm on Monday – mostly conceptual short
answer questions, some MC, no calculators allowed
Chapter 8
Jovian Planet Systems
• Observing tonight if weather is OK. Meeting at
Watanabe at 7:30pm, then at Kapiolani Park by 8pm.
Check your e-mail around 7pm to check for possible
cancellation. Rain date is Friday—same place and
time. Call me if you’re lost.
Jovian Planets A Different Kind of Planet
•
•
•
•
Bigger & more massive
Lower density, different composition
Rings
Numerous moons
What are jovian planets made of?
• Jupiter & Saturn: almost all H & He, very
little metal & rock (less dense)
• Uranus & Neptune: <50% H & He, the rest
hydrogen compounds (water, methane,
ammonia), with some metal & rock (more
dense)
Why are they different?
Jovian planet formation
• Beyond the frost line, planetesimals could
accumulate ICE.
• Hydrogen compounds are more abundant than
rock/metal so jovian planets got bigger and
acquired H/He atmospheres.
• The jovian differences are in the amount of
H/He gas accumulated.
Differences in Jovian planet
formation
• TIMING: the planet that forms earliest
captures the most hydrogen & helium gas.
Capture ceases after the first solar wind blew
the leftover gas away.
• LOCATION: the planet that forms in a denser
part of the nebula forms its core first.
Why did that amount differ?
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Density (g/cc)
Density Differences
Uranus and Neptune are
denser than Saturn
because they have
less H/He,
proportionately
But that explanation
doesn’t work for
Jupiter….
Jupiter’s “Metallic” Hydrogen
• Jupiter and Saturn are nearly the same size
• But Jupiter is 3x more massive than Saturn
– Mass yields large pressure, which compresses Jupiter
to the point where H atoms are touching (electron
clouds are overlapping! degenerate)
– This results in metallic H – electrons are free to move
from atom to atom
– Increasing the mass of Jupiter further would not
increase its size because the atoms just continue to get
pressed together.
What are jovian planets like on the
inside?
Layers Differ in Phase
Notes: [Density of liquid
water is ~ 1g/cm3.]
• No solid surface.
• Layers under high pressure and temperatures.
• Cores (~10 Earth masses) made of hydrogen
compounds, metals & rock
• The layers are different for the different
planets. WHY?
Metallic hydrogen conducts
electricity; it is not solid.
Core is hydrogen
compounds, metals,
rocks. But not in a form
you’d recognize… 10 x
the mass of Earth inside a
volume the size of Earth.
Jupiter
Why different?
Magnetic Fields
• Jupiter has a powerful magnetic field
generated by its rotating, convecting layer
of metallic hydrogen.
Less mass less gravity less compression.
Boundaries of the layers are deeper in less massive
jovian planets.
The physical states of the cores of the less massive
jovians are less extreme (could be liquid).
What is the weather like on jovian
planets?
Colorful surface features reveal:
– Clouds of different compositions
– Wind speeds
– Storms, some long-lived (Great Red Spot on
Jupiter)
Planet colors
Jupiter’s
colors
• Ammonium sulfide clouds reflect red/brown.
• Ammonia, the highest coldest layer, reflects white.
Planet colors
Saturn’s
Colors
Saturn’s layers are the same, but deeper in and
farther from the Sun --- more subdued.
Uranus and Neptune’s upper layers are colder still,
allowing methane to condense.
Methane gas absorbs red light and transmits blue light
reflected by clouds
Jupiter winds and storms
• Earth’s rotation makes storms ‘spin’.
• Jupiter’s fast rotation stretches storms in to bands
that surround the planet.
• High east/west winds (up to 400 km/hr)
The
Great
Red
Spot
• twice as wide as the Earth
• Has existed for at least 3 centuries
What have we learned?
• What are jovian planets made of?
• Jupiter and Saturn – mostly H, He
• Uranus and Neptune – H compounds mixed w/
metal and rock.
• Originated from ice-rich planetesimals of about
the same size, but captured different amounts of
hydrogen and helium gas from the solar nebula.
What have we learned?
• What are jovian planets like
on the inside?
• All have a core about 10
times as massive as Earth,
consisting of hydrogen
compounds, metals, and
rock. They differ mainly in
their surrounding layers of
hydrogen and helium.
What have we learned?
• What is the weather like
on jovian planets?
• The jovian planets all
have multiple cloud
layers that help
determine the colors of
the planets, fast winds,
and large storms. Some
storms, such as the Great
Red Spot, can apparently
rage for centuries or
longer.
Many moons…
• More than 100 jovian moons and counting…
• 60+ moons of Jupiter alone…
What kind of moons orbit the
jovian planets?
• Medium and large moons mostly formed at
the same time as their planets.
• Small moons are mostly captured asteroids
and comets.
Medium &
large moons
• Enough self-gravity to
be spherical
• Are or were
geologically active.
• Have substantial
amounts of ice.
Medium &
large moons
• Formed in orbit around
jovian planets.
• Circular, equatorial
orbits in same direction
as planet rotation.
Small moons
• Captured asteroids, so
orbits do not follow
patterns.
• Orbits can be tilted,
elliptical, and even
backwards!
Small moons
• Far more numerous than the medium and large
moons.
• Not enough gravity to be spherical: “potatoshaped”
What makes Jupiter’s Galilean
moons unusual?
Io, Europa, Ganymede, Callisto
What makes Jupiter’s Galilean
moons unusual?
Io’s Volcanoes
• Io has volcanoes.
• Europa may have an ocean under its ice.
• Ganymede & Callisto may also have subsurface oceans.
…but they’re so small???
Shouldn’t they be cold & dead?
Orbital
Resonances
Tidal Heating
Io is squished and
stretched as it orbits
Jupiter
Io is the most volcanically active world in the solar
system. HOW?
But why is its
orbit so
elliptical?
Every 7 days,
these 3 moons
line up.
The tugs add up
over time, making
all 3 orbits elliptical.
Europa’s Ocean: Waterworld?
Tidal stresses crack Europa’s
surface ice.
Europa’s interior also warmed by tidal heating
What makes Titan different from
other moons?
• Titan is the only moon
with a thick atmosphere
(90% N2).
• Titan may also have
surface lakes of
methane and ethane.
• Methane and ethane are
greenhouse gases, but
it’s still cold (93 K).
• possible life?
Neptune’s Moon Triton
• Probably a captured
Kuiper belt object:
orbiting Neptune
opposite Neptune’s
direction of rotation.
• Smaller than Earth’s
Moon, yet has recent
geological activity.
8.3 Jovian Planet Rings
Our Goals for Learning
• What are Saturn’s rings like?
• Why do the jovian planets have rings?
Why are small icy moons more geologically
active than small rocky planets?
• Hot interiors are necessary for geological activity.
• Ice deforms more easily than solid rock, so less internal heat
is required, and smaller objects can be geologically active
• Tidal heating is not important for rocky planets.
What are Saturn’s rings like?
• They are made up of numerous, tiny
individual particles
• They orbit over Saturn’s equator
• They are very thin
Earth-based view
Spacecraft view
Artist’s interpretation of a close-up
view
Saturn’s rings
• Many particles of ice and dust orbit Saturn.
• Many particle collisions cause the rings to be
very thin (tens of meters!)
• Gap moons and orbital resonances create the
effect of rings and gaps.
Why do the jovian planets have
rings?
• They formed from dust created in impacts on
moons orbiting those planets
How do we know that?
Implications
• Jovian planets all have rings because they
possess many small moons close-in
• Impacts on these moons are random
• Saturn’s incredible rings may be an ‘accident’
of our time
How do we know?
• Rings aren’t leftover from planet formation-the particles are too small to have survived
this long.
• There must be a continuous replacement of
tiny particles.
• The most likely source is impacts with the
jovian moons.