Download Week 5 Lecture

PHYS178
Other Worlds
A/Prof. Orsola De Marco
9850 4241
[email protected]
http:/web.science.mq.edu.au/~orsola/teaching/PHYS178/
Course Outline
• Exploration of the outer solar system.
• Jupiter and Saturn.
• Atmospheres
• Inner composition
• Magnetic fields
• The rings
Jupiter and Saturn
The compositions of
Jupiter and Saturn
• They are mostly made of hydrogen (H) and helium (He).
• Hydrogen’s lack of spectral lines – hard to detect H spectroscopically.
• Low densities of J. and S. indirect evidence that they are not made of rock.
• Presence of CH4 and NH3 indirect evidence for presence of abundant H.
• UV spectra have H lines – 1960s confirmation of H composition.
• Helium was confirmed in 70s and 80s when spacecrafts measured spectra in
detail detecting signature of collisions between H and He.
• J. has “cosmic” (also called solar) abundances: 86% H, 14% He
• S. has much less helium: 96% H, 3% He.
• S. is less massive than J. – it cooled more rapidly. Helium on S. froze and
precipitated to the centre, where it cannot be detected.
Missions to the outer planets
• Pioneer 10: launch 1972, first through asteroid belt; at Jupiter 1973;
lost contact early 2002.
• Pioneer 11: launch 1973, Jupiter 1974, Saturn 1979, lost contact 1995.
• The Pioneer anomaly …
• Voyager 1: launch 1977, Jupiter 1978, Saturn 1980, still going.
• Voyager 2: launch 1977, Jupiter 1989, Saturn 1981, Uranus 1986, Neptune 1989,
interstellar mission still active.
• Galileo: launch 1989, arrived at Jupiter 1995, deployed probe, orbits Jupiter
• Cassini: launch 1997, Jupiter 2000, Saturn 2004, deployed probe on Titan.
• New Horizons: launch 2006, will arrive at Pluto 2015!
The Voyagers
Launched in 1977
Arrived at Jupiter 1979
Arrived at Saturn 1980/81
Arrived at Uranus and Neptune in 1986/89
Still operational – interstellar missions
Gas giants spin with different
periods at different latitudes
The Great Red Spot, the white ovals
and the brown ovals.
Why is the GRS long-lived?
Zonal winds up to 500 km/s circulate
at zone/belt boundaries
The three white ovals merged in
2000 to become a new storm.
Saturn’s rare storms
• Rarely, spots are noticed on S.
• They are thought to be
equivalent to thunderstorms –
rising gas which solidifies into
crystals (ammonia in the case of S.).
• Lightning also detected by
Cassini.
The atmospheres of Jupiter and Saturn
• Heavily influenced by their internal heat.
• E-W and W-E “zonal winds” between 500 km/s
(Jupiter) and 1800 km/s (Saturn). Below Jupiter’s
surface they are faster (650 km/s), showing that the
source of the energy is the planet’s heat.
• Clouds are made of ammonia (highest), ammonium
hydrosulfide (medium) and water crystals (lowest).
They are more compressed on Jupiter than on Saturn
(higher gravity).
• Clouds on Saturn are deeper and details harder to see
hence the overall banding of S. is duller than for J.
•The brown clouds are the lowers, the white the medium
and the red the highest, but why the colours is not
known.
The zones are cool high clouds,
the belts are warm loser strata
We are not sure whether belts or zones are rising
• Galileo’s probe did not find the expected water
Layer – possibly it dropped in a dry area, equivalent
to a desert.
• Jupiter has the same composition as the Sun, but has
more heavy elements – accumulated through impacts.
• Ar, Xe, Kr are also too abundant: clues on origin?
• Jupiter and Saturn are oblate because they
rotate.
• The degree of oblateness depends on the
distribution of mass.
• Models of the mass distribution on J. and
S. indicate that they have 2.6% and 10% of
their mass locked in a small rocky core.
For Jupiter this is 11,000 km in diameter,
8 times Earth’s mass with pressures of
70 million bars.
• Saturn’s larger core can be inferred from its
higher degree of oblateness (due to lesser
gravity). However it would be even more oblate
were it not for its heavier core.
• Both planets have a layer of liquid ices which
have precipitated after collisions. They are
lighter and float on the rocky core.
• Above the ices is a thick layer of helium and
liquid metallic hydrogen – high pressure!
• It is likely that the inner rocky core was
formed first and onto it H and He accreted.
Jupiter’s and Saturn’s magnetic fields
Jupiter’s magnetosphere: 16 times the area of the Moon
• Generated by the spinning liquid metallic hydrogen. 20,000 times Earth’s!
• For Saturn’s only 600 times Earth’s: less liquid metallic hydrogen because
less mass, less pressure.
Jupiter’s magnetosphere: 16 times the area of the Moon
Radio emission from Jupiter
Radio emission due to synchrotron radiation: charged
particles from Io’s volcanos are accelerated in the
magnetic field and emit radiation. Periodic changes reveal
inner period. Saturn has a lesser emission – why?
Saturn’s rings
Noticed by Galileo ~1610
Disappeared by 1612
Reappeared in 1613
Huygens in 1655 predicted
that. This is due to the
equinox – how so?
1675 Cassini noticed
the gap between A
and B rings.
1800s the fainter C
ring was found.
Saturn‘s Equinox
• Saturn’s Equinox is only every 15 years, why?
• During the Equinox objects embedded in the
rings cast the longest shadow.
• We can therefore spot objects that protrude
from the flatness of the rings….
Saturn‘s Equinox: August 2009
Can anybody spot anything???
The composition of Saturn’s rings
• Could the rings be a solid sheet?
• 1857 Maxwell says no.
• 1895 James Keeler sees the inner parts of the
rings rotate faster – it is not a rigid sheet.
• They reflect 80% of the sunlight that falls
on them – might be ice.
• Confirmed in the 70s (Gerarg Kuiper) using IR
spectroscopy.
• Voyager and Cassini spacecrafts found
temperatures to range between -180o and
-200o C – ice would be….icy!
• Radio absorption determined size of particles
to be 1 cm – 5 m. Most particles about 10 cm.
The composition of Jupiter’s rings
• Reflect only 5% of sunlight that falls on them
so they are very faint.
• Have only 1/100,000th of the mass of Saturn’s
rings, so they are faint.
• Mof rocky particles only 1 mm in diameter.
• Cannot be seen from Earth.
• Discovered by Voyager 1 in 1979.
Ring at about 2 Jupiter
radii from the centre of
Jupiter.
The formation of planetary rings
Voyager 1 image of the Saturn’s rings
Cassini image
of the Saturn’s
rings’ drapes
Backlit Saturn’s rings (Voyager 1)
Why is the Cassini division bright?
Compare the Cassini division reflecting light and scattering light
The F ring is made of 1 mm particles – measured by light scattering
Colour variations depend on small amounts of trace elements.
These trace elements have not yet been identified.
Colour variations suggest that there is little mixing from ring to ring.
Rings might have been added at different times and contained traces
of different original materials.
Cassini image
Voyager discovered the D, G and E rings.
E and G likely formed from particles spewed out by Enceladus.
Small moons can clear gaps and make rings:
Pandora and Prometheus shepherd particles into the F ring.
Daphnis clears the Keeler gap
These spokes maintain the same shape throughout the orbital
periods of the rings. Do you find this strange?
Key Ideas
• Composition and Structure: Jupiter and Saturn are both much
larger than Earth. Each is composed of 71% hydrogen, 24%
helium, and 5% all other elements by mass. Both planets have a
higher percentage of heavy elements than does the Sun.
• Jupiter probably has a rocky core several times more massive
than the Earth. The core is surrounded by a layer of liquid “ices”
(water, ammonia, methane, and associated compounds). On top
of this is a layer of helium and liquid metallic hydrogen and an
outermost layer composed primarily of ordinary hydrogen and
helium. All of Jupiter’s visible features are near the top of this
outermost layer.
Key Ideas
• Saturn’s internal structure is similar to that of Jupiter, but its
core makes up a larger fraction of its volume and its liquid
metallic hydrogen mantle is shallower than that of Jupiter.
• Jupiter and Saturn both rotate so rapidly that the planets are
noticeably flattened. The rotation of both planets’ interiors is
revealed by variations in the radio emissions from the planets.
• Both Jupiter and Saturn emit more energy than they receive
from the Sun. Presumably both planets are still cooling.
Key Ideas
• Atmospheres: The visible “surfaces” of Jupiter and Saturn are
actually the tops of their clouds. The rapid rotation of the
planets twists the clouds into dark belts and light zones that run
parallel to the equator. Strong zonal winds run along the belts
and zones.
• The outer layers of both planets’ atmospheres show differential
rotation: The equatorial regions rotate slightly faster than the
polar regions. For both Jupiter and Saturn, the polar rotation
rate is nearly the same as the internal rotation rate.
• The colored ovals visible in the Jovian atmosphere represent
gigantic storms. Some, such as the Great Red Spot, are quite
stable and persist for many years. Storms in Saturn’s atmosphere
seem to be shorter-lived.
Key Ideas
• There are presumed to be three cloud layers in the
atmospheres of Jupiter and Saturn. The reasons for the
distinctive colors of these different layers are not yet known.
The cloud layers in Saturn’s atmosphere are spread out over a
greater range of altitude than those of Jupiter, giving Saturn a
more washed-out appearance.
• Saturn’s atmosphere contains less helium than Jupiter’s
atmosphere. This lower abundance may be the result of
helium raining downward into the planet. Helium “rainfall”
may also account for Saturn’s surprisingly strong heat output.
Key Ideas
• Magnetic Fields and Magnetospheres: Jupiter has a strong
magnetic field created by currents in the metallic hydrogen layer.
Its huge magnetosphere contains a vast current sheet of
electrically charged particles. Charged particles in the densest
portions of Jupiter’s magnetosphere emit synchrotron radiation
at radio wavelengths.
• The Jovian magnetosphere encloses a low-density plasma of
charged particles. The magnetosphere exists in a delicate
balance between pressures from the plasma and from the solar
wind. When this balance is disturbed, the size of the
magnetosphere fluctuates drastically.
• Saturn’s magnetic field and magnetosphere are much less
extensive than Jupiter’s.
Key Ideas
• Rings: Saturn is circled by a system of thin, broad rings lying in
the plane of the planet’s equator. This system is tilted away
from the plane of Saturn’s orbit, which causes the rings to be
seen at various angles by an Earth-based observer over the
course of a Saturnian year.
• Structure of the Rings: Saturn has three major, broad rings (A,
B, and C) that can be seen from Earth. Other, fainter rings
were found by the Voyager spacecraft.
Key Ideas
• The principal rings of Saturn are composed of numerous
particles of ice and ice-coated rock ranging in size from a few
micrometers to about 10 m. Most of the rings exist inside the
Roche limit of Saturn, where disruptive tidal forces are stronger
than the gravitational forces attracting the ring particles to each
other.
• Each of Saturn’s major rings is composed of a great many narrow
ringlets. The faint F ring, which is just outside the A ring, is kept
narrow by the gravitational pull of shepherd satellites.
• Jupiter’s faint rings are composed of a relatively small amount of
small, dark, rocky particles that reflect very little light.