Jupiter with Io and Europa in foreground (Voyager Mission image)
"And now for something completely
different," as they used to say on Monty Python.
The large "Jovian" planets (Jupiter, Saturn, Uranus, and Neptune) are
entirely unlike the terrestrial planets (see more discussion
in Study Guide 11). They may have rocky
cores, like larger versions of the Earth, at their centers, but these
are enveloped in giant gaseous atmospheres. Only the outermost
skins of these atmospheres can be studied directly. This is
meteorology, instead of the geology/topography we discussed for
the terrestrials. However, it can be just as extreme with respect to
Earth-bound meteorology as are the canyons and mountains of Mars
compared to those of Earth.
Another major distinction of the Jovians is the large number of
satellites they possess. The satellites, observed at close range
by spacecraft, exhibit an astonishing diversity of surface
types and features. Unlike the three terrestrial planet satellites,
the larger Jovian satellites are rich in water ice and exhibit
many different phenomena as a consequence. In many ways, they are
more interesting than their parent planets. They may even
harbor biospheres. The ring systems, which are present
around all 4, Jovians are probably the remnants of distintegrated
Many examples of a third kind of planet have recently been
discovered outside the orbit of Neptune. These are perhaps most aptly
called the "ice dwarfs," of which Pluto is the archetype.
A. History of Discovery/Exploration
Prehistory: Jupiter, Saturn known
1600's: Early telescopic studies reveal satellites of J & S, red spot of J,
rings of S
discovers Uranus (accidentally) using a small telescope---the first
new planet in recorded history
1790+: Deviations in U's orbit, assumed to be caused by the gravity of
an unknown planet, lead to a prediction of its location based on
Newton's law of gravity.
1845: Neptune discovered as predicted = a
triumph of Newtonian mechanics
1930: Tombaugh discovers Pluto. Much more difficult than Neptune.
Although the search for Pluto followed the pattern for Neptune, it is now
clear its discovery was accidental: Pluto is too small to have affected
Neptune's motion significantly, nor were the claimed distortions in
Neptune's orbit accurate.
1950+: Deeper searches: no planet larger than Neptune found within
100's of AU's from the Sun
Flyby orbits of Voyager Missions to outer planets
1979-89: The Voyager 1 & 2 spacecraft fly by all 4 Jovians. Highly
successful missions, but require elaborate planning (over 12,000
person-years of effort). The Voyager spacecraft have continued on
into interstellar space.
1992: First "Kuiper Belt Object" discovered. Over a thousand are now
known. Pluto is now recognized to be a large KBO.
1994: Comet Shoemaker-Levy 9 collides with Jupiter; acts as
an atmospheric probe.
1995: Galileo orbiter/probe mission arrives at Jupiter; sends
probe ~ 50 miles into atmosphere; orbiter continues to study
Jupiter and its satellites
2004: Cassini-Huygens mission enters orbit around Saturn.
2005: The Huygens probe lands safely on Titan, Saturn's largest
satellite. The Cassini orbiter continues its studies of the Saturn system
and discovers a water plume from Enceladus, indicating liquid water below the surface
of this small satellite.
2006: International Astronomical Union debate over the status of Pluto
and other KBO's. Pluto is redefined to be a "dwarf planet."
B. Jovian Planets (J,S,U,N): Properties
These four share gross properties. Pluto is entirely different (see
Distant from Sun: 5-30 AU. (Pluto is at 39 AU.) The outer solar system is
vast (over 10,000 times the volume of the inner solar system out to
Mars) and sparsely populated.
Large: Radii are 4-11 x Earth. Masses are 15(U)-318(J) x Earth.
J. contains twice as much mass as all other planets combined. An
animated timelapse image of Jupiter's rotation and surface features
is shown at the right. Click on the image for a more current, high
resolution HD video.
Jupiter is midway between planets and stars on a power of ten
mass scale. Objects only 13 times more massive are considered to be small
Their low mean densities (~ 1 gr/cc) imply the Jovians
are mainly composed of H and He, with only small rocky cores,
perhaps Earth-size. U,N have larger complements of heavy elements
are entirely different from terrestrial planets because of their
predominant hydrogen composition. That is a product of their
formation out of the cool regions of the solar nebula, dominated by
icy (H-rich) solids. (Click on the cross section drawing at right for
an enlarged version.)
High internal pressures in J,S convert hydrogen to liquid or "metallic"
form in their interiors
No solid surfaces: these are "gas giants"
Visible surfaces = cloud layers, about 150 miles deep.
The clouds consist of 3 main types of ice crystals: ammonia, ammonium
hydrosulfide, and water. Colors are from trace compounds. Thin, white
clouds on Neptune are methane crystals.
"Spots", e.g. Jupiter's Red Spot (large oval in image at right:
22,000 mi long ~ 3x Earth) are long-lived cyclonic storms. There are similar
features on other Jovians (e.g. the transient "Great Dark Spot" on Neptune).
Atmospheric banding is caused by lateral windstreams and
rising/falling convection currents. Winds reach 300-600 mph on J,S
and a maximum of 1300 mph on Neptune. See the enhanced pseudocolor
image of Saturn's atmospheric banding below right.
Shoemaker-Levy-9: was captured by Jupiter and hit
the planet in 1994. Impact acts as a "natural" atmospheric probe;
fragments produce multiple enormous, concentrated energy releases in
atmosphere; explosions bright; scars lasting.
Strong magnetic fields are generated by motions in the liquid metallic
hydrogen interiors of Jupiter & Saturn. These produce
strong radiation belts, up to
100x those of Earth.
Pseudo-color infrared image of
C. Ring Systems
Saturn has the brightest rings, but rings are present around all 4
Rings are not solid: the inner rings revolve faster than the outer
ones, as expected for objects in Keplerian gravitational orbits
They are composed of billions of ice-coated particles (typically about 10 cm in
size). Different particle sizes and coatings produce some of the
structure visible in the rings.
Origin: debris from tidally/collisionally fragmented
Rings lie inside the planet's Roche Limit. Inside
this distance from the planet's core, gravity tides would pull apart a
large body, such as a satellite.
Structure: complex! (at right). The biggest gaps are "resonance"
effects produced by the cyclical gravitational tug of the satellites
outside the ring. The ringlets may be produced by the self-gravity
of the material in the rings.
Spacecraft images of the four Galilean satellites of Jupiter
(Io, Europa, Ganymede, and Callisto). Each is a unique world in its own right.
D. The Jovian Satellites
Click here for a Java animation of orbits of
the satellites of each planet
Diverse(!) characteristics; often violent histories
Larger moons are mixtures of rocky/icy materials
3 as large as Mercury
The large moons formed at the same time as their parent planet
Because of their large ice content, their surfaces are more plastic
than those of the terrestrial planets; some show extensive evidence of
melting and resurfacing.
e.g. Hyperion (Saturn), are
irregular in shape
Most of these are rocky or icy
planetesimals/asteroids, gravitationally captured over
continual volcanic eruptions caused by heating from tidal
flexing in Jupiter's gravitational field. Much more active today
than even the Earth. Io's volcanic plumes at the time of their discovery are
shown above right.
ice-coated and extraordinarily smooth. Few craters, indicating
a young surface. Most scientists believe the ice shell covers an underlying
ocean, kept warm by tidal flexing (less severe than for Io).
Long, dark lines on the surface may be places where the shell has
cracked, allowing filling by younger ice. There is much speculation
about a possible biosphere on Europa
(see Study Guide 23).
(S): Saturn's largest moon has a thick
atmosphere!---mostly nitrogen with a small amount of methane. The
atmosphere can be retained, despite Titan's small mass, because of its
low temperature at Saturn's distance from the Sun.
Titan is a main target of the European Cassini-Huygens
Mission. While the primary spacecraft stays in orbit around
Saturn, the Huygens probe was detached and successfully landed on Titan's surface in
January 2005, relaying data during its descent and for a short period
on the ground.
Solar UV light interacting with methane has produced a rich mixture of
clouds and obscuring haze. There is probably hydrocarbon rain
here for an atmospheric profile.
Recent radar data from the orbiting Cassini spacecraft shows that there
are large lakes
on Titan, probably of methane or ethane.
In company with Europa and Enceladus (see below), Titan is now regarded
as possibly hosting a biosphere --- but with lifeforms based on
utilizing methane rather than carbon dioxide.
Enceladus(S): Although only a
small satellite, Enceladus was unexpectedly discovered by the Cassini
orbiter to possess huge water/ice geysers; the plumes contain both
water vapor and complex hydrocarbons. Interpretation: warm liquid
water reservoirs beneath the surface are heated by tidal flexing; jets
escape through deep vents. See drawing above. The outflow from
Enceladus feeds Saturn's "E ring."
Miranda (U): shattering
collision & reassembly? or surface scars from internal convection?
Artist's Concept of Huygens Probe Landing On Titan
E. Pluto and the Kuiper Belt
Pluto is entirely unlike the other four large outer planets. It
is smaller by a factor of 2 than
any of the other 8 planets. It is a rocky/icy object rather
than a gas giant. Its orbit is the most highly inclined to the
ecliptic plane of any of the classical "9 planets."
When first discovered, Pluto was thought to be isolated at the edge of
the Solar System. However, in the last 20 years, astronomers have
discovered many more such bodies, some with
sizes comparable to Pluto. These are all members of the "Kuiper
The Kuiper Belt is a
huge volume beyond the orbit of Neptune, centered on the ecliptic
plane, but extending many AU's above and below the plane. Over 1000
"Kuiper Belt Objects" (KBOs) have been discovered in this volume to
The plot at the right shows the distribution of the known KBO's
as of 2012. Click for an enlargement, with scales giving distances
The most massive known KBO---yes, it's more massive than
Pluto---is Eris, also the most distant known KBO (97 AU). It was
discovered in 2005. Its size (2300 km diameter) is comparable to
for a page describing Eris by its discoverer, Mike Brown.
discovered in 2003, has an aphelion (greatest orbital distance from
the Sun) of 937 AU, although at present it is at only 90 AU. Its
orbital period is about 11,400 years. It is distant enough that it
may be a member of
the Oort Cloud
rather than the Kuiper Belt.
These discoveries, particularly that of Eris, precipitated the messy
discussion at the International Astronomical Union in the summer of
2006. Astronomers held a debate over the meaning of the term
"planet"---specifically whether or not Pluto and the other large KBO's
should be placed in a separate category. In the end, the IAU voted to
create a new category of "dwarf planet" for these latter
objects but was then forced to add the asteroid Ceres for consistency.
All this was handled very clumsily, and it generated needless
controversy. It turns out many non-astronomers were fond of Planet
Even before the discovery of Pluto, we had already known of many
small, rocky objects in separate orbits around the Sun---the
"asteroids." Now, we know about many similar, but icy, objects.
Sensible designations for these types, above some threshold in size,
are as "rock dwarf planets" and "ice dwarf planets."
New Horizons, the first mission
to Pluto and the Kuiper Belt, was launched in 2006 and, having received
a gravity assist from Jupiter, is now approaching Pluto with a planned
flyby date of 14 July 2015. The spacecraft was originally intended to
fly within 6500 miles of Pluto's surface, but the discovery of a total
of 5 moons in orbit around Pluto has raised concerns about an invisible
ring system or debris field near Pluto, and the trajectory may have
to be adjusted. Following the flyby, New Horizons will be retargeted
to approach other Kuiper Belt Objects, assuming good candidates can
be found within its limited range of maneuver.