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