Transcript Astro-Lecture-Ch09 - Physics and Astronomy

Chapter 9
Lecture Outline
Worlds of Gas and Liquid—
The Giant Planets
21st CENTURY ASTRONOMY
THIRD EDITION
Hester | Smith | Blumenthal | Kay | Voss
The Giant Planets
• Jupiter and Saturn:
mainly hydrogen and
helium.
• 5.2 AU from Sun,
and 9.6 AU from Sun
• Called gas giants
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The Giant Planets
• Uranus and Neptune: smaller,
have much more water.
• 19.2 AU from Sun,
and 30.0 AU from Sun
• Called ice giants
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Finding the Giant Planets
• Jupiter and Saturn were
known to the ancients.
• Uranus was too faint to
be discerned from the
other stars.
• Discovered in 1781 by
Herschel.
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Finding the Giant Planets
• Neptune was found
because Uranus was
straying from its
predicted orbit.
• Gravity of Neptune was
tugging on Uranus.
• Found in 1846 by Galle
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Characteristics of the Giant Planets
• Called giant planets because of their mass—from 15
Earth masses (Uranus/Neptune) to 300 (Jupiter)—
and also, their physical size.
• No solid surfaces: we just see the cloud layers in
the atmospheres.
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Measuring the Giants
• Find diameters by observing how long it takes for a
planet to pass over a star: stellar occultation.
• Find masses by observing the motions of a planet’s
moons and effects of gravity.
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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 vat
of water.
• Jupiter’s chemistry is like the Sun: mostly hydrogen
and helium.
• Saturn has some more massive elements; Uranus
and Neptune have much more.
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More Giant Characteristics
• All giants have rapid rotation.
• They have different obliquities.
• Jupiter: 3°.
• Uranus: 98°, which results in
extreme seasons.
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Cloud Patterns – Jupiter
•
•
•
•
Strong dark and light bands.
A long-lasting giant storm (Great Red Spot).
Many smaller storms.
Colors indicate complex chemistry.
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Cloud Patterns – Saturn
• Similar band structure
to Jupiter, but less
pronounced.
• Has violent storms, as well
as something similar to
Earth’s jet stream.
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Cloud Patterns – Uranus and Neptune
•
•
•
•
Infrared observations let us see structure.
Weak banding.
Small, scattered bright or dark clouds.
Transient large storms (Great Dark Spot on
Neptune).
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Under the Cloud Tops: Jupiter/Saturn
• 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.
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Under the Clouds: Uranus/Neptune
• Unlike Jupiter and
Saturn, the highest
clouds are methane
ice.
• Bluish because
of scattering of light
by the methane
• Jupiter and Saturn’s
clouds are colored
by impurities
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Winds and Storms
• Rapid planetary rotation results in strong Coriolis
forces. This imparts a rotation to storms.
• Most extreme winds are in Saturn’s atmosphere
(1,650 km/hr).
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Winds and Storms
• Alternating east/west winds make banded clouds
on Jupiter.
• Circulation pattern differs from planet to planet
in ways not understood.
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Internal Heat
• All but Uranus have
significant internal
heat.
• Heat flows from the
hot interior outward.
• Heat has a big effect
on the global
circulation patterns.
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Internal Heat
• Jupiter is hotter than
it would be just from
sunlight.
• Jupiter radiates about
65% more energy
than it receives from
the Sun.
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Models of the Interior
• 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 liquid water and rock.
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Models of the Interior
• Uranus/Neptune:
• Have more water and ices (ammonia, methane).
• Have less pressure than the gas giants.
• Deep oceans containing dissolved gases and salts.
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Origins
• Jupiter/Saturn formed from accretion disk while
hydrogen/helium still present.
• Solar wind later blew out these gases.
• Uranus/Neptune formed later, by merger of icy
smaller bodies.
• All four possess a dense core containing rocky
materials.
• Many details are still not understood.
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Magnetic Fields
• Magnetic fields are
generated by the
motion of the
electrically
conducting liquids.
• Their orientation is
at an angle to the
rotation axis.
• Like a bar magnet.
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Magnetospheres
•
•
•
•
Magnetospheres are huge (Jupiter’s is 6 AU).
They interact with the solar wind.
Auroras (“Northern lights” on the Earth).
Produce strong radio waves.
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Concept Quiz – Heat
How do we know Jupiter has a significant amount of internal
heat?
A. It emits strong radio waves.
B. Its atmosphere is mainly hydrogen and helium.
C. It is hotter than expected at its distance from the Sun.
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Concept Quiz – Composition
Which two giant planets have a larger fraction of water than the
other two?
A.
B.
C.
D.
E.
Jupiter and Saturn
Uranus and Neptune
Jupiter and Neptune
Saturn and Neptune
Uranus and Jupiter
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Concept Quiz – Density
You discover a giant planet around another star. It is as big as
Jupiter, but much more dense. What does the density tell you?
A. It has less hydrogen and helium than Jupiter.
B. It has a lower mass than Jupiter.
C. Like Jupiter, it is probably hot inside.
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This concludes the Lecture PowerPoint presentation
for Chapter 9
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21st CENTURY ASTRONOMY
THIRD EDITION
Hester | Smith | Blumenthal | Kay | Voss
©2010 W.W. Norton & Company, Inc.
©2010 W.W. Norton & Company, Inc.