ASTR 330: The Solar System
Announcements
• Homework #4 due back today: October 31st.
• Mid-term exam #2 is Tuesday, November 7th.
• Homework #5 also distributed then.
• Extra credit term paper: choose subjects and
register with me by November 14th.
• Submit extra credit papers by December 5th.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 18:
Jupiter and Saturn
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Outer Solar System
• We now have ‘left’ the inner solar system in the lecture course, having
looked at the Sun, the terrestrial planets, meteorites, comets and
asteroids.
• We now turn our attention to the outer solar system, beyond the
asteroid belt, in the realm of the giant planets.
• The four giant planets of the outer solar system of course have no solid
surface, unlike the inner planets, so no record of aging through cratering
or tectonics, and no volcanology to study.
• However, the moons of the outer planets in many respects resemble
the terrestrial planets: we will discuss these beginning next week.
• Let us begin our discussion of the outer planets by looking at the
spacecraft missions which gave us most of our information.
Picture credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pioneers 10 & 11
• The Pioneer missions of the early
1970s dramatically increased our
knowledge of Jupiter and Saturn.
• Pioneer 10 (launched 3/2/72)
carrying 15 scientific experiments on
board was the first spacecraft to pass
through the asteroid belt, and the first
to study Jupiter close-up (12/3/73). Its
last communication was on 4/27/03.
• Sister ship Pioneer 11 (launched
4/5/73) also reached Jupiter (12/2/74),
and then sling-shotted on to visit
Saturn (9/1/79). Last communication
was on 11/95.
Picture credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Pioneer Mission Highlights
• Pioneer 10:
• First spacecraft to pass through asteroid belt.
• First close-up pictures of Jupiter.
• First in-situ maps of Jovian radiation belts and magnetic field.
• Discovery that Jupiter is mainly liquid.
• Pioneer 11:
• Close-up images of jovian Great Red Spot, and first good view of poles.
• Determined the mass of Callisto.
• Discovered 2 moons of Saturn, and one new ring.
• Charted magnetic field, measured Titan’s temperature.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Voyagers 1 & 2
• Where Pioneers 10 & 11 blazed the trial, Voyagers 1 & 2 followed.
• Voyager 1 (launched 9/5/77) reached Jupiter first, in March 1979. Using
a ‘sling-shot’, it reached Saturn in November 1980. Making a close fly-by
of Titan, it then headed on a course to leave the solar system. Currently
at 90 AU and still functioning, V1 is the furthest human-made object.
• Voyager 2 (launched 8/20/77)
encountered Jupiter in July 1979
and Saturn in August 1981. Further
gravity-assist maneuvers allowed it
to continue to Uranus (1/86) and
Neptune (8/89), the only spacecraft
to ever reach these worlds. Voyager
2 is also still functioning as it leaves
the solar system, currently at 70 AU.
Picture credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Voyager Mission Highlights
• The Voyager missions rank as perhaps the most ground-breaking and
successful interplanetary missions ever.
• Highlights include:
• Discovery of the Uranian and Neptunian magnetic fields.
• 22 new satellites discovered (3 Jupiter, 3 Saturn, 10 Uranus, 6 Neptune).
• Discovery of volcanoes on Io and geysers on Triton.
• Aurorae of Jupiter, Saturn, Neptune.
• Many new ring features, including Jovian rings, Saturnian spokes etc.
• Storms on Neptune (Great Dark Spot).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Galileo
• Previously, the Pioneer and Voyager missions had made fast fly-bys of
the outer worlds. The next logical step was to send an orbiter.
• Galileo was a dual orbiter and atmospheric entry probe sent to Jupiter.
• Plagued by many delays, Galileo was finally launched on 10/18/89 by
the Shuttle Atlantis. Technical problems began right away, when the
folded main antenna refused to unfurl.
• Galileo went into orbit in December 1995.
• The probe was released in July, and entered the atmosphere just as
the orbiter was firing its rockets to brake into orbit.
• Galileo was finally directed into impact with Jupiter in September 2003,
to prevent it from ever crashing into and contaminating a moon
(especially Europa).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Galileo Spacecraft
Picture credit: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Probe Mission Highlights
• The probe (mass 320 kg, diameter 1.3 meters) entered the atmosphere
at 172,200 km/h using a heat shield to protect itself, and later a parachute
to brake and descend.
• The probe entered a
hotspot near the equator,
which was largely cloud
free. It survived for about
58 minutes as it dropped
through the atmosphere,
eventually being crushed
or vaporized at 22 bars and
450 K. The probe mass
spectrometer gave us our
first in situ measurements
of the elemental
composition.
Picture credit: NASA/NSSDC
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Galileo Highlights
• Many, many highlights include:
• First multispectral images of Earth’s Moon, and confirmation of far-side
impact basin.
• First close flyby of an asteroid (Gaspra) and discovery of first asteroidal
moon: Ida’s Dactyl.
• Only direct observations made of S-L 9 impact with Jupiter.
• Downward revision of Jovian water abundance and lightening activity.
• Re-surfacing of Io since the Voyager visits.
• Evidence for oceans under Europa crust.
• High speed winds seen on Jupiter; new radiation belt discovered.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cassini-Huygens
• Cassini is the current mission to Saturn, which carries a European-built
probe called Huygens. Look carefully at the picture below: where is the
probe being dropped?
• Launched in October 1997, Cassini made 4 gravity assists: Venus x2,
Earth, and finally Jupiter in December 2000, en route to Saturn.
• Cassini arrived on July 1st 2004,
went into orbit as the first artificial
satellite of Saturn, and began a 4year prime mission.
• Cassini carries 12 scientific
experiments on board: cameras,
spectrometers, and detectors for
radio waves, charged particles and
magnetic fields, and more.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comparison of Giants
• Jupiter and Saturn are both large, but have substantial differences,
apart from the prominent rings of Saturn.
• Jupiter is more
massive, larger and 2x
closer than Saturn,
and orbits the Sun in
just over 1/3 the time.
• Jupiter also rotates
faster than any other
planet: quite
remarkable given its
size.
• Jupiter has the most
moons (at present).
Jupiter
Saturn
Diameter (km)
142984
120536
Mass (Earth=1)
317.8
95.2
Density (g/cm 3)
1.3
0.7
Semi-major axis (AU)
5.2
9.54
11.86
29.42
Spin Period (hrs)
9.8
10.6
Number of Moons
62
31
Orbital Period (yrs)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Internal Structure
• The densities of Jupiter and Saturn are slightly more than, and slightly
less than water, respectively.
• Note that these are less than a third the density of the terrestrial
planets.
• Densities tend to increase with size, therefore, if Jupiter was mostly
rock and metal, it should be much denser than the Earth.
• The fact that it is not, tells us that the planet must be made of lighter
stuff which also is harder to condense.
• In fact, we know already that these giant planets are largely made of
hydrogen and helium, the two lightest elements.
• There must also be a small dense core of yet unknown composition.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
States of Hydrogen
• At the very outer layers, Jupiter has a
hydrogen gaseous atmosphere of H2
molecules – very thin: not shown here.
• Below this, the outer 25% by radius of
Jupiter is liquid molecular hydrogen (H2).
• Most of the inner 75% is hydrogen
metal, a liquid state where the single
electron and proton are separated, and is
hence electrically conducting.
• In the very core resides some denser
compounds, made of silicon, oxygen,
heavy volatiles and metals.
• Saturn is similar to Jupiter, but the
metallic part is a smaller fraction of the
planet.
Picture credit: Kaufmann and Comins (book)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cores and Fusion*
• The core of Saturn is a similar size to the core of Jupiter (contrary to
the previous diagram): about 10-15 Earth masses.
• Hence Saturn may be considered to be a ‘Jupiter’ which is depleted in
hydrogen and helium.
• How much more massive would Jupiter need to have been, to fuse
hydrogen and become a star?
• Answer: about 80 times heavier; however, if it was just 13 times bigger,
it would be able to fuse deuterium and generate some nuclear energy.
• Interestingly, if Jupiter was 10 to 50 times bigger, it would actually be
smaller! The increased mass would cause further gravitational
compression, which would more than compensate for the extra mass.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Internal Energy
• Both Jupiter and Saturn radiate more energy than they receive from the
Sun: Jupiter 2X as much (4x1017 watts) and Saturn 3X (2x1017 watts).
• This energy is not in the visible part of the spectrum, but in the infrared
(i.e. heat).
• If Jupiter was in heat balance with incoming solar energy (i.e. radiating
the same as it receives) then it should be about 107 K. In fact, it is closer
to 130 K!
• This sounds like a greenhouse effect, but in fact it is not. These planets
are warmer because they really are generating their own energy.
• The answer lies in gravitational contraction, the same process once
used to explain the power source of the Sun, before fusion was
understood.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Helium Rain
• In the simplest form of the model, material ‘falling’ inwards in a gravitational
field releases energy, which becomes heat (imagine a large asteroid impacting
the Earth).
• However, this simple theory, where the whole planet gradually contracts, does
not account for the present-day energy source: this phase should have been
finished by now, and the planet much cooler.
• In fact, the present-day energy release comes from a sort of differentiation in
the interior.
• In the past, helium would have been ‘dissolved’ in the hydrogen, and the two
well-mixed together. But as the planets have cooled, helium has began to
‘condense’ out of the hydrogen solvent, like sugar crystals in cold water.
• Nowadays, we have helium droplets ‘falling’ or raining down inside the interior,
moving towards the center under gravity.
•This explains why Saturn releases comparatively more energy for its size than
Jupiter: it is colder, and so comparatively less helium is now dissolved.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
He/H Abundances
• We can test this theory by measuring the He/H fraction in these
planets.
• If the differentiation is going on, then we expect the outer layers of the
planet, the atmosphere, to appear depleted in He compared to the
starting value (assumed to be like the Sun).
• So, we expect He/H (Saturn) to be less than He/H (Jupiter), which in
turn should be less than He/H (Sun).
• This is exactly what we find for the mass fractions:
He/H (Sun)
= 0.28
He/H (Jupiter)
= 0.24 (Galileo probe)
He/H (Saturn)
= 0.21 (Voyager)
• The theory and the observations agree well so far.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Atmospheric Composition
• What do we expect the atmospheres of Jupiter and Saturn to be
composed of?
• Firstly, hydrogen and helium of course, the light elements which must
make up most of the mass and give us the low densities.
• Other common reactive elements (C, O, N, P, S) are likely to be in
hydrogenated form: combined into molecules with as many hydrogen
molecules as possible:
CH4 (methane)
NH3 (ammonia)
PH3 (phosphine)
H2S (hydrogen sulphide)
H2O (di-hydrogen monoxide?)
• Also, we expect to see the other ‘noble’ gases: Ne, Ar, Xe, Kr.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Measuring the Composition
• How do we normally measure atmospheric composition?
1. Remote sensing, i.e. spectroscopy.
2. In situ measurements, i.e. a probe and mass spectrometer.
• We expect difficulties in making spectroscopic identifications from the
Earth however: the monatomic gases (e.g. He) absorb only in the
ultraviolet: blocked by the Earth’s ozone layer.
• Hydrogen absorbs in the infrared: but very weakly. Also, many
infrared wavelengths are blocked by H2O, CO2 etc.
• Remote sensing measurements progressed through the 20th century.
• Methane was identified first on Jupiter and Saturn, along with
ammonia on Jupiter, around 1940. In 1960s H and He were finally
detected, along with Saturn’s ammonia.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Galileo Probe Mass Spectrometer
• The best
measurements
we have of
atmospheric
composition
come from the
Galileo Probe
Mass
Spectrometer:
• Note that this
table gives the
gas abundances
in terms of
molecules, not
mass.
Figure credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Trace Species and Chemistry
• Jupiter’s atmosphere also contains some other trace species, other
than the hydrides and noble gases.
• We see some hydrocarbons, such as acetylene (C2H2) and ethane
(C2H6), which are produced by the action of sunlight on methane (CH4) in
the upper stratosphere.
• This process, of breaking molecules apart by sunlight and then
chemically re-combining them, is known as photochemistry.
• Photochemistry is the cause of the Earth’s ozone, and the complex
chemicals we find in cometary heads.
• We also see the gas carbon monoxide, produced by a hot reaction at
1200 K:
CH4 + H2O  CO + 3 H2
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Volatile Species
• A full table of gaseous abundances is given in Morrison and Owen,
Table 13.1. These abundances refer to the upper atmospheres of the
planets: once we get down into the cloud layers, then several species
are able to condense, like water in the Earth and CO2 on Mars.
• Of course, the name we give to such species which can change
physical state is volatiles.
• The question of the water abundance on Jupiter remains contentious.
When the Galileo probe dropped into the atmosphere, it found no water
at all, even though water should be the third most abundant species
(after H2 and He).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Icy Planetesimals*
• The gas abundances of minor species on Jupiter lead us to a surprising
conclusion about Jupiter’s formation.
• We expect that the core of Jupiter was formed from the same
planetesimals which became comets, so Jupiter should have the same
relative abundances of elements as comets.
• In fact, Jupiter has the same heavy-element abundances as the Sun,
and is not depleted in argon and nitrogen like comets are. (Jupiter’s
atmosphere is depleted in helium, which rains out in the core, and neon,
which dissolves in hydrogen).
• Hence, the type of planetesimals which formed Jupiter seem to be
different from the cometary ones. These planetesimals must have
formed cold (<37 K) to retain the highly volatile nitrogen and argon, at
about the current position of Neptune. These objects were also the major
repositories of mass in the solar system outside the Sun.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Temperature-Pressure Profile
• The graph (right) shows the
variation of temperature and
pressure in Jupiter’s
atmosphere.
• The initial determination was
carried out by the Voyagers and
Pioneers, using the technique of
radio occultation (observing a
radio emitter go behind the
planet).
• The profile was then confirmed
by the Galileo entry probe.
Saturn is similar to Jupiter, but
colder.
Figure credit: Arizona Press
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Appearance of Jupiter
• Jupiter seen through a telescope in visible light shows a pattern of
alternating bright and dark bands.
• The brighter, white-colored
bands are known as ‘zones’ while
the dark, reddish-brown bands
are known as ‘belts’.
• At the equator we have the
white ‘equatorial zone’, and
further north or south the bands
are named after the Tropical,
Temperate and Polar latitude
ranges of the Earth. However,
these regions are ‘tropical’ and
‘temperate’ in name only!
Figure credit: Charles Cowley, U Michigan
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Red spots, White and Brown Ovals
• To either side of the equatorial zone, in the tropical belts, we see
prominent white and brown oval features. Further south, lies the Great
Red Spot, which has been observed we believe for over 300 years now!
Figure credit: James Schombert, U. Oregon; NASA Voyager 1
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
What are the white ovals?
• Three white oval features
were observed to persist from
1939 until they suddenly
merged in 2000,
• The visible image of the ovals
(upper right) was taken by the
HST, while the infrared image
of the same region (bottom)
was taken by Galileo.
• This shows that the white
ovals are dark in the IR, i.e.
they are topped by high cold
clouds.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Brown Ovals
• The brown ovals or barges, which we see near 18° north, are thought to
be holes in the upper clouds, through which we glimpse the deeper levels
below.
• These ovals, which are dark in the
visible, are much brighter than the
surroundings viewed in infrared.
• Hence, we are seeing hotter, deeper
levels of the atmosphere.
• The Galileo probe unfortunately
entered one of these regions:
unfortunately, because many of the
instruments which were designed to
analyze clouds found very little clouds
to measure!
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Great Red Spot
• The Great Red Spot at 22° south is the most remarkable feature of
Jupiter. Measuring 18,000 by 12,000 km, it could easy swallow up the
Earth! Its size may be one reason for its longevity.
• The Voyager image (right) shows
the GRS in false color to emphasize
cloud structure and turbulence.
• But what is it? It is a huge
anticyclone, with an anti-clockwise
rotation period of about 6 days. The
top protrudes well above the
surrounding cloud layers.
• Clearly, it is a high-pressure storm
center, but we still do not really know
the mechanism, or even what causes
the red color!
Picture credit: NASA Voyager
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Clouds Of Saturn
• The false-color image below has been made by combining information
at several infrared wavelengths.
• The blue color is the main cloud layer of ammonia ice crystals, while
the green and yellow regions show overlying haze layers.
Picture credit: HST/Arizona
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
‘Dragon Storm’
• Amongst the many new
insights to come from
Cassini, are dramatic
pictures of a band known as
‘storm alley’ in Saturn’s
southern hemisphere.
• The so-called ‘Dragon
Storm’ of mid-2004 appears
to be a flare-up of a longlived storm deep in the
atmosphere.
• The D.S. was a source of
lightening and radio
emission bursts, and so is
probably a thunderstorm.
Picture credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Winds on Jupiter
• Jupiter has many prominent features which we can track.
•The equatorial and polar clouds
rotate in different times, so we
must use a different concept
when we wish to talk about
Jupiter’s rotational period.
• The method used is the rotation
period of the radio emissions
(System III), giving a time of 9
hrs, 55 mins for the deep interior.
• Relative to System III, Jupiter
exhibits alternating eastward and
westward flowing jets of material
at different longitudes.
Figure credit: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Winds On Saturn
• In general, the appearance of Saturn is much less interesting than
Jupiter: the cloud features are harder to detect and track.
• Saturn also has a measured radio
rotation period (System III), which is 10
hrs 39 minutes.
• Saturn also has a strong eastward
jet, but whereas Jupiter’s has a
velocity of 360 km/hr, similar to jet
streams on the Earth, Saturn has an
amazing 1600 km/hr jet extending to
40° either side of the equator.
• Again, beyond the equator we see
the alternating pattern of eastwards
and westward jets.
Picture credit: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Types of Clouds
•
Until now we have concerned ourselves with the visible appearance
of Jupiter, in the light and dark bands. Now let us examine the vertical
cloud structure.
• There are three main ways in which clouds can arise:
1. Condensation clouds: liquid droplets or ice crystals of condensed
volatile. For the Earth, we mean water. On Jupiter, we can have
ammonia or more exotic condensates occurring as well.
2. Dust clouds: very prominent on Mars. We can also have dust
clouds on Earth, or, dust particles may serve as seeds for liquid
droplets.
3. Photochemical haze, or smog. Produced by chemical reactions
which bind molecules into long chains, usually powered by
sunlight.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Photochemical Smog*
• On the Earth, the raw materials of smog are anthropogenic
emissions. But on the outer planets, the raw materials are the
hydrogen-rich gases in the atmosphere.
• The formation of haze begins with the absorption of solar UV light by
molecules such as CH4, which breaks apart into CH2 and other
fragments. The pieces then chemically recombine to form C2H2, C2H6
etc.
• The absorption of UV by molecules and hazes also causes the
thermal inversion in the atmospheres: i.e. the temperature minimum
at the tropopause and subsequent temperature rise in the
stratosphere. We see a stratosphere on the Earth for similar reasons.
• Note that the haze layer is thin: it is above the visible cloud tops
which we see in visible images, which are lower condensate clouds.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Jupiter Condensate Clouds
•
Below the haze layers, there are expected to be at least three cloud decks:
1. A high, cold cloud layer of ammonia
cirrus, at about 0.6 bar pressure. This
is thought to cause the whitest cloud
regions on visible images.
2. A cloud of ammonium hydrosulfide
(NH4SH) – formed from NH3 and H2S
– at 1.6 bar. We believe that this level
gives rise to the orange-brown
appearance of much of Jupiter.
However, NH4SH is white, so we
believe that some impurities are
dissolved.
3. Water clouds beginning at 4-5 bar.
Figure: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Probe Results
• The Galileo probe
found much less
cloud than
expected.
• Observations
seem to indicate
that the probe went
into a ‘hole’ in the
clouds (one of the
brown hot-spot
regions), and so
only faint evidence
for the upper two
cloud levels was
found, and none
for the deep water
cloud.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Vertical Picture
• Let us now try to explain the full picture of Jupiter’s bright and dark
bands in a vertical context.
• The white zones are thought to be
regions of upwelling, warm air from the
deep interior, carrying volatile species.
As the air rises, it cools, and the
various cloud layers form. These
appear cold in the IR because we see
the high cloud tops.
• The brown belts are the regions of
sinking cool air which is largely
depleted of volatiles. Paradoxically,
these cooler belts appear warmer in
the infrared, because we see through
to the deeper warmer levels.
Picture credit: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Saturn Cloud Layers
•
Saturn is thought to have the same three cloud levels at Jupiter.
• Saturn is colder however, and all
three of the cold layers are thicker.
• This results in a more uniform
appearance than Jupiter: we are
less able to see through the very
first and upper cloud layer.
• Saturn’s cloud layers are about
200 km deep, compared to 80 km
on Jupiter. This is due to the
weaker gravity of Saturn, 2.5
times less at the 1-bar level, which
cause less compression.
Figure: Nanjing University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Jovian Magnetic Field
• Jupiter and Saturn both have magnetic fields.
• Jupiter’s field is many times stronger than that of the Earth, with an an
equatorial value of 4.3 gauss compared to the Earth’s 0.3 gauss.
• The magnetic dynamo on the Earth is driven by convection within the
electrically conducting layers of molten iron.
• On Jupiter, the conducting material
is the liquid metallic hydrogen of the
interior.
• The orientation of Jupiter’s field is
inclined at 10° to the rotation axis
(like the Earth) but in the opposite
polarity: the magnetic north is near
the rotational south pole.
Picture credit: JPL/NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Saturnian Magnetic Field
• Saturn has a much weaker field: only 0.2 gauss.
• Saturn’s field is also in the opposite sense to the Earth’s field, but there
is no inclination to the spin axis: the north magnetic pole is exactly on the
south rotational pole.
• Our understanding of magnetic
field generation is still incomplete,
although we expect field reversals
from time to time.
• We not have an adequate theory
for explaining the alignment of the
spin and magnetic axes: we
expect there to be some offset.
Picture credit: NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Saturn’s Aurorae
• Saturn’s magnetic field
leads to aurorae similar to
those observed on the Earth.
• The mechanism is similar:
charged particles from the
solar wind flow along field
lines and crash into the
poles, heating the molecules
which then re-radiate energy
at specific frequencies.
• This HST picture shows UV
emission from hydrogen
molecules, overlaid on visible
light pictures.
Picture credit: NASA/JPL/University of Colorado
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Jovian Radio Emissions
• It is also interesting to note that both Jupiter and Saturn
emit radio waves.
• Jupiter has intense radio emissions of two types:
1. Decametric (10s of meters) wavelength radio emissions: which
are highly erratic, and linked to the inter-action with the moon
Io.
2. Decimetric (10s of cm): a more continuous emission.
• The radio emissions are non-thermal: i.e. they are not caused by
heat. The cause is the interaction of charged particles – electrons and
ions – spiraling in the planet’s magnetic field. Io volcanoes are one
source of charged particles.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dr Conor Nixon Fall 2006
Listening To Saturn
• Saturn also generates radio emissions at much longer wavelengths: typically
several km, like AM radio. These waves are reflected from the Earth’s ionosphere,
so were not detected until we sent the Voyager spacecraft. The cause is electrons
from the solar wind being deflected in the magnetic field of Saturn.
November 22nd sounds
July 25th sounds
• Listen to some sounds of radio emission from Saturn’s aurorae. The sounds
have been pitch-shifted lower into the human hearing range, and sped up in time
by factors of 2 and 25 respectively.
Credits: NASA/JPL/University of Iowa
ASTR 330: The Solar System
Quiz-Summary
1. Which were the first spacecraft to reach Jupiter and Saturn?
2. Which spacecraft have told us the most about Jupiter and Saturn?
3. What does the density of the outer planets tell us about composition?
4. Describe the main liquid and solid layers of Jupiter? Is Saturn the same
or different?
5. Both Jupiter and Saturn are wamer than we would expect if they were
purely in the thermal balance with incoming sunlight. How do we
explain this phenomenon? Is there additional proof of this hypothesis?
6. In what chemical form are many of the reactive atoms, such as C, N, O
and S?
7. What do the elemental ratios tell us about planetesimals?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-Summary
8. Describe the banded appearance of Jupiter. What are the zones and
belts? Which are brighter (a) in the visible (b) in the infrared?
9. What is meant by System III longitude, and why is it used?
10. Relative to System III, what sort of cloud movements do we see at the
equator of both planets?
11. Is the equatorial jet of Saturn stronger, weaker, or the same as
Jupiter’s?
12. Describe the main layers of condensate clouds in Jupiter’s
atmosphere, and relate these to belts and zones.
13. Compare and contrast the Jovian and Saturn magnetic fields to that of
the Earth?
14. What causes the non-thermal radio emissions?
Dr Conor Nixon Fall 2006