Chapter 8
Satellites (moons), Rings,
and Plutoids
Reading assignment: Chapter 8
Jovian planets satellites
Four largest Jupiter satellites - Galilean moons
•Jupiter – 79 satellites
•Saturn – 62 (Now 82) satellites
•Uranus – 27 satellites
•Neptune – 13 satellites
Jupiter’s Satellites (Galilean satellites or moons)
•Io - Jupiter’s satellite with active
volcanoes!
•Europa - Jupiter’s satellite covered
with layer of frozen water – strong
indications of an ocean of liquid water
beneath
•Ganymede and Callisto - similar in
size to our moon, just a bit larger.
•Ganymede is the largest satellite in the
solar system. It may have an ocean of
liquid water under the frozen surface
•There are:
•Six large satellites, similar in size to our Moon
•12 medium-sized - 400 to 1500km
•Many small satellites
Medium and large
satellites in the
Jovian planets
All these satellites have
sufficient mass: Self-
gravity force them to be
spherical
Some are now or were in
the past geologically active.
Most of them have
substantial amounts of ice.
Jupiter
Saturn
Uranus
Neptune
The largest satellite
in the Solar system Second largest
Jupiter’s “Galilean Moons”
An Unusual Family
Io Europa Ganymede Callisto
Moon
Io has several active volcanoes.
Europa may have an ocean of liquid water under
its icy crust.
Ganymede (And perhaps Callisto) may also have
sub-surface oceans?
How can we account for the unusual
features?
At that distance from the Sun ( ~5 AU),
shouldn’t they be completely frozen and
showing no activity?
What makes Jupiter’s Galilean satellites unusual?
Io’s Volcanoes Volcanoes were discovered by
Linda Morabito at JPL in images from Voyager’s spacecraft
So far about 80 active volcanoes have been identified using data mainly from Voyager and Galileo spacecrafts
Volcanic eruptions mainly composed of sulfur and sulfur dioxide
Volcanic plumes about 150 km high and 300 km wide (large because of low gravity)
Variety of volcanic hot spots
Large lava lakes made of liquid sulfur
It is the most volcanic active body in the Solar System
Io has a rocky mantle and an iron/iron sulfide core
Io: Two images separated by 15 years (The left one taken by Voyager and the last one on the right by the Galileo
spacecraft)
The plumes of Io’s volcanoes, the material ejected from the satellite, the Io torus and the
low frequency radio emission The gasses ejected from the volcanic activity are ionized by the UV radiation from the Sun and form a torus of ionized
material around the orbits of Io. This ionized material (electrons and ions) are accelerated by Io travel along the magnetic
field. The electrons interact with the magnetic field of the planet and triggers the low frequency radio emission
Io orbital period is about 42 hours. Jupiter rotational period is about 10 hours. Because of the fast rotation, the magnetic
field of Jupiter sweep pass Io and induce an electric field in the satellite. This electric field accelerate the electrons. The
electrons go down toward the planet in spirals along the magnetic field. Some are reflected before the hit the atmosphere
and start spiraling back away from the planet and emit the low radio frequency.
What causes the volcanic activities in Io?
Io torus Electrons spiraling down in Io flux tube
Tidal Heating Io is compressed and stretched as it orbits
Jupiter . This process releases heat, rises the
internal temperature and produce the
volcanic activity
Why is its orbit so
elliptical?
The elliptical orbit of Io and the production of energy
The distance from Io to Jupiter is about 422,000
km
Reference: The distance Moon–Earth is about
384,000 km . Jupiter has 320 times the mass of
Earth
The Jovian Moons Elliptical orbit: Orbital resonance between the orbital periods of Io,
Europa and Ganymede
The 3 closest satellites line up every 7 Earth days (resonance)
Tugging in the same direction distorts the orbit from a circle to an ellipse
1 orbit of Ganymede = 2 orbits of Europa = 4 orbits of Io
(Orbital period of Io ~ 42.5 days; orbital period of Europa ~ 85 days; orbital period of Ganymede ~171 days)
Smooth surface of Europa
Icy surface covering a
large rocky core:
Surface is very smooth &
young.
Just a few impact craters:
Young surface
Fractured into ice rafts &
floes a few kilometers
across,
Repaved by water or
geysers through the cracks
in the ice.
Europa surface Surface is ice covered
Extensive and complex network of
cracks in the icy crust. The darker
color of the cracks is due to the salts
dissolved in the water coming from
below the crust
And indications of internal
geologic activity
Europa surface: a 200 km square area
Europa
“Salty” water oceans below a thick layer of
ice? (twice as much water as Earth!!)
Mostly salty water, some magnesium
sulfate, sulfurs (red color)
Tidal stresses crack Europa’s surface ice. Similar to icebergs,
large chunks of ice that have been broken and reassembled
Does Europa have liquid water?
What lies beneath Europa’s frozen
crust surface?
One possibility:
100-200 km of convective ice above a rocky
core
The most probably scenario based on
measurement of Europa’s magnetic field:
Thin ice crust a few km thick over a 100 km
deep water ocean. (Diameter of Europa=3138
km)
The liquid conductive layer distort the
magnetic field of Jupiter around Europa. That
distortion was detected by the Galileo
spacecraft.
Europa How Europa can maintain liquid water?
•Tidal heating due to the elliptical orbit of
Europa around the large mass of Jupiter
•Thermal vents may bring the heat from
the core
•Heat may keep the interior temperature
above freezing point
Possibility of life?
• The existence of liquid water does not
imply the emergence of life. The salty
water is a hostile environment. But we
have seen on Earth that life can be present
in environment that were considered
hostile
Titan Saturn’s largest satellite
Properties:
Second largest satellite in Solar System
Mass: ~0.02 Earth-mass
Radius: 0.4 REarth
Density: ~1.9 g/cm³ (1900 kg/m³)
Icy mantle over a rocky core.
This is the only satellite (moon) in the solar system that has a permanent and heavy atmosphere
Titan Titan permanent atmosphere mainly compositions: Nitrogen and methane-ammonia
atmosphere
The last spacecraft to explore Titan was Cassini (It was commanded to enter and burn in
the atmosphere of Saturn in Sept. 2017) . The Huygens probe was release from Cassini
and landed on it surface. Rocky surface and evidence of erosion by liquid/slush.
The atmosphere is
thick, the surface
of Titan cannot be
seen in the visual
part of the
spectrum.
On the right, an IR
image taken by
Cassini’s
spacecraft
Titan’s Thick Atmosphere Surface temperature: 94 K (-290 F)
Pressure: 1.5 bar (1.5 times sea level Earth pressure)
Composition:
~98% N2 (nitrogen)
~2% CH4 (methane)
Argon
Hydrocarbons:
Ethane = C2H6
Acetylene = C2H2
Ethylene = C2H4
Propane = C3H8
Clouds of methane & N2 ices
More recent results, using Cassini instruments:
Detection of hydrogen cyanide (CHN) and
Benzene (C6H6) in clouds in the atmosphere
Also using ALMA radio telescope data: Detection of acrylonitrite (vinyl cyanide, CH2CHCN) (possible compound in cell membranes?) in the atmosphere
Titan’s Atmosphere
(Deduced from Voyager 1 spacecraft observation)
The Huygens probe The Huygens lander was carried by
the Cassini spacecraft mission .
The image to the right is an actual
image from the surface of Titan
returned by the Huygens probe.
Colors are close to true colors.
Rocks are probably icy “rocks”
The methane/ethane lakes in Titan.
Radar images taken by Cassini
Lakes on Titan (radar maps)
Titan’s Liquid Lakes
Cassini radar have been
able to image several
smooth regions that have
been identified as lakes of
liquid methane/ethane
Titan, a reflection of sunlight in a methane lake.
(Image taken by Cassini spacecraft)
Titan interior
Model of the interior deduced from gravitational field measurements
during the numerous Cassini flybys.
A layer of liquid water under the ice combined with very complex organic
molecules make it interesting. Possibility of life?
Enceladus
(Saturn’s satellite)
Jets of water gas triggered by hydrothermal
vents: Possibility of life?
Saturn’s satellite Mimas and Star Wars’ Death Star
Image returned by Voyager
spacecraft in 1980 (Fly-by) Star Wars movie released in
1977
Triton
Triton - Neptune’s largest
moon
•It has a retrograde orbit . It orbits in
direction opposite to Neptune
rotation
•Probably a captured object from the
Kuiper belt. Pluto is a Kuiper belt
object
•Voyager 2 detected geysers of
nitrogen gas rising several km high
• The jets of nitrogen gas may come
from liquid nitrogen heated by some
internal source of heat
•A very thin atmosphere of nitrogen
•Temperature about 37 K
Rings
• All of the Jovian planets
have rings
• The most spectacular are
Saturn’s rings
•They are very thin, just a
few km
• Rings are not solid
objects
• They are comprised of
many small solid particles
• All the particles are in
orbit around the planet
• Water ice is the primary
constituent
Why do rings form? The tidal forces of the large planet break
apart a close enough moon or satellite
Rings
Rings consist of billions of
small particles or moonlets
orbiting close to their planet
The size of particle ranges
from the size of grain of
sand to house-sized
boulders
Rings
The orbits of the particles that make
up the rings follow Kepler’s laws inner particles revolve faster than those
farther out
ring are not rotating as a solid body,
rather individual moonlets are revolving
around the planet
If ring particles are widely spaced,
they move independently
If the particles are close to each
other, there is a gravitationally
interaction between them
The gravitational attraction
(resonance) of the satellites (or
moons) and the ring’s particles clear
gaps in the rings
The Cassini
division is
caused by a
resonance with
the orbital
period of
Mimas, one of
Saturn’s
satellites
The Cassini
division is easily
visible with a
moderate size
telescope. It can
be seen with any
of the 8-inch or
bigger
telescopes at the
UF Teaching
observatory
Saturn and its main rings (False colors)
High resolution image of Saturn’s ring in false colors
to enhance the composition
A unusual image of Saturn and its rings taken by the
Cassini spacecraft (October 2013)
An image of Saturn’s rings taken by Cassini spacecraft when
Saturn eclipsed the Sun in 2006
It shows a set of faint rings outside the main rings
(The pale blue dot at 9-10 o’clock is the Earth)
Origin of Rings
Breakup of shattered satellite
Remains of particles that were unable to come together and form satellite
Gravity plays important role
differential force of gravity -- tidal forces
tear bodies apart
inhibit loose particles from coming together
DFg
DFg
The differential force of gravity: Tidal effect
The Roche Limit, a simplfied version
Roche Limit - the closest distance an object can come to another large mass object without being pulled apart by tidal forces
Roche limit
Roche Limit: A simplified version
If the density of the planet is similar to the density of the satellite (moon)
Then, the Roche limit = 2.446 Rplanet
A large moon orbiting inside the Roche limit will be destroyed.
Roche Limits for some planets:
Earth - 18,470 km (Distance to Moon =385,000 km )
Jupiter - 175,000 km
Saturn - 147,000 km Uranus - 62,000 km
For comparison, Saturn outer
diameter of A ring is 137,000.
It is inside the Roche limit
D is Density
Comparing Jovian Ring Systems
Compared to Saturn, the ring
system of other Jovian planets:
• have fewer particles
• are smaller in extent
• have darker particles
The rings of Uranus were
discovered in 1977 when the planet
passed in front of a star and the
rings dimmed the light from the star
The rings of Jupiter and Neptune
were discovered by the Voyager
spacecrafts
Other unsolved mysteries regarding
rings:
• Uranus’ rings are eccentric and
slightly tilted from its
equatorial plane.
• Neptune has partial rings.
The Roche limit of the Jovian planets (Distance of the rings from the planets, in planets radius)
Pluto, a dwarf planet •Discovered in 1930 by Clyde Tombaugh (at Lowell
observatory)
• Pluto is located about 40 times the Earth’s distance from
the Sun (40AU)
•Charon, the first satellite discovered was found in 1978.
Image taken with ground-based telescope
•Pluto has a total of 5 satellites
•The New Horizons mission launched in January 2006
arrived in July 14, 2015 (Fly-by).
•Similar in mass and size to Neptune’s large moon Triton
•Probably formed in the Kuiper belt (comet birth place)
•The Kuiper belt is located outside the orbit of Neptune, at
distances between 30-50 AU
• Pluto has a highly inclined orbital plane
•Orbital period 248 years
•Average density 2000 kg/m³
•Pluto has only 20% the mass of our Moon
•Mass about 0.0022 mass of Earth
•Diameter 1160 km, 0.18 diameter of Earth
•In 2003 it was renamed as a dwarf planet by the
International Astronomical Union (IAU)
Hubble ST image of Pluto
The IAU definition of a planet:
A “planet” is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape
(c) has cleared the neighbourhood around its orbit.
A recent Hubble telescope image of Pluto and its 5 satellites
The name of the satellite P4 is Kerberos. Satellite P5 was discovered in 2012and was
named Styx. The International Astronomical Union (IUA) is the organization that names
celestial objects such as satellites, asteroids, exoplanets. stars, etc
The IAU definition of a “dwarf planet”. Is a celestial body that :
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic
equilibrium (nearly round) shape
(c) has not cleared the neighbourhood around its orbit
(d) is not a satellite.
Modeled “images” of Pluto obtained by processing 24 images
taken by the Hubble Space Telescope. The process produced a
mathematical description of its surface
Recent images of Pluto from the New Horizon spacecraft
The New Horizons mission arrived at
Pluto on July 14, 2015 (Fly by). It
took about 9.5 years to make the trip.
It is the first spacecraft to reach Pluto.
It has been returning the first detailed
images of Pluto.
Tombaugh Regio (“The Heart”)
(Named after Clyde Tombaugh,
Pluto discover).
The surface of Pluto shows few
impact craters. Almost no craters
in the Tombaugh Regio
New Horizon image of Sputnik Planum
Sputnik Planum
shows no impact
craters.
Its surface must
be younger than
10 millions years
old
Region showing
some impact
craters
Details of Pluto surface.
Images from New Horizon
This structures cause by
convection of nitrogen
ice
Pluto haze layers and foggy hazes
Blue skies. The sunlight
scattered in the atmosphere
of Pluto produces a halo of
bluish color
The image was taken when
the spacecraft passed in front
of Pluto and the Sun was
being occulted by Pluto
About a dozen
haze layers in
the atmosphere
above Pluto
surface.
The atmosphere
extend for about
40 km.
Composition of
atmosphere:
nitrogen, carbon
monoxide,
methane
An image of Charon from the New Horizon spacecraft
Darker,
reddish color
in North pole
Probably the
South pole
may be
similar
High resolution images of Ultima Thule from the New
Horizon spacecraft
The New Horizons mission arrived
at Pluto on July 14, 2015 (Fly by).
After the fly by, it continue its trip to
another object. It arrived in December
2018. The targeted object was Ultima
Thule. This is the farthest object
explored by a spacecraft
The object name is (486958) 2014
MU69
It is contact binary planetesimal,
trans Neptunian object in the Kuiper
belt
Size about 31 km. The smaller size
object name is Ultima (10 km) and the
larger size object name is Thule about
14 km
Its has an orbital period of 298 years
and a semi major axis of 44 AU
The image was taken at a distance of
6,000 km and has a resolution of 33 m
Ultima Thule
The Kuiper belt
Kuiper belt • Located outside the orbit
of Neptune
•A region of the solar
system located between 30-
50 AU from the Sun
• Bodies in the Kuiper belt
are composed of “ices”,
mainly methane, ammonia
and water ices.
• Pluto, Eris, Makemake
and Haumea are examples
of Kuiper belt objects
•Some short period comet
(Periods<200 years) are
also object that belong to
the Kuiper belt.
Kuiper Belt Objects (plutoids) compared
The discovery of Eris in 2005 showed that Pluto
was not unique. These objects, along with Pluto
and others, seem to be the largest of the Kuiper Belt
objects, also known as trans-Neptunian objects or
plutoids. Several hundred have been found. It is
estimated that several thousands may form the
Kuiper belt
Eris also has a satellite.
The composition of these objects is mainly ices.
Their composition is different from the composition
of the terrestrial and Jovian planets
A possible ninth planet The object that was nicknamed
“Ninth Planet” is about 10 times
the mass of the Earth.
The distance from the Sun is
about 20 times farther than
Neptune. It is estimated than it
will take 10,000 to 20,000 years
to orbit the Sun.
Several searches have not
being able to located the object.
It could be too faint. May need
larger telescopes than those
available now
Caltech astronomers Konstantin Batygin (right) and Mike Brown
found evidence of the ninth planet by modeling the orbits of several
objects located beyond the Kuiper belt. In order to account for the
distorted and high eccentric orbits of these 6 objects, it is required
the existence of an object with 10 times the mass of the Earth and
located in the modeled orbit.
The results were published in the January 20, 2016 issue of the
Astronomical Journal