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Planets
Planets

Solar System Formation

Terrestrial Planets

Terrestrial Planet Atmospheres

Terrestrial Planet Characteristics
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Jovian Planets
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Trans-Neptunian Objects
Solar System Formation
Solar System
Solar system – consists of six basic categories:
1. Sun (typical small star)
 99.9% of the solar system’s mass
2. Planets and their moons (~ 0.1% of the Sun’s mass)
3. Planetesimals (remainder of the planet-forming debris)
 Asteroids
 Comets
4. Small debris (includes meteoroids and micrometeoroids)
5. Dust (small particles)
6. Gas
 Dominated by hydrogen and helium
Solar System
Solar system composition
Solar system composition is the contents of gas
cloud that collapsed to form the Sun and other
nearby stars



70% H
25% He
5% other stuff
 Ices
 Dust grains
 Gases
Solar System
Early universe composition consisted of only
gases



75% H
25% He
<0.001% other stuff
 3He
 2D (deuterium, or heavy hydrogen)
 4Li
Solar System
Planet formation sequence is actually the sequence f the
formation of a star and its disk of gas and dust
1. Gas cloud collapse and rotation
2. Condensation and crystallization
3. Pre-solar heating
4. Planetary core formation
5. Gas dispersion and transparent disk
6. Planet stabilization
7. Outer solar system remnants
Solar System
Solar system (star) formation

Star (Sun) forms early but requires time to stabilize

Rotating disk around star shapes the position of the planets

Planets form from debris in disk

Planet formation halted by several mechanisms

Some planets change position and energy by dynamical
interactions

Remnants (leftovers) still orbiting the Sun
Solar System
Solar system segregation (as star forms and stabilizes)

Star isolated from planets that begin forming due to intense
energy, gravitational inflow, and particle wind and radiation
outflow

Small and large clumps that will make up larger
planetesimals are created in roughly three zones


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Interior – hot, dense materials remain
Exterior – cold ices and gas remain
Distant – bound by gravity but relatively unaffected by solar heat
Planets build (accrete) from smaller planetesimal material



Interior – rock and metal planets (terrestrial)
Exterior – giant gas and ice planets (Jovian)
Distant – ice bodies and comets
Solar System
Final solar system structure

Star dominates entire solar system in mass and energy

Planets are created in violent collisions between small and
large bodies
 Segregated due to star/Sun heating
 Natural accretion of disk material in rotating star
formation
 Planets are common

Chaos produces varying size and composition
 High temperature materials near star
 Ices and gases beyond “ice line” located between Mars
and Jupiter
Solar System
What was responsible for creating the three planetary zones?

The Sun’s T-Tauri stage created a hot zone nearest the star
 High radiation and high-velocity outward winds swept most
of the inner solar system clear of material
 The remainder fell inward, or was left behind as dense, hightemperature (refractory) debris
 T-Tauri phase named after the 19th brightest star (Tau) in the
constellation Taurus (Tauri) now going through a similar
phase as our Sun passed through 4.6 billion years ago

This created solid planets in a temperate region of the stable
Sun
 Venus
 Earth
 Mars
Solar System
Outside this region was the “ice line” sometimes
referred to as the frost line or freeze line

Temperatures are well below water ice temperature
and ices accumulate on the solid bodies

Located in asteroid belt between Mars and Jupiter

Asteroids inside the ice line have a number of
different surface characteristics than those beyond

Outside the Jovian planets are even colder objects
that have had only minor heating from the Sun over
the 4.6 billion year history of the solar system
Planets

Jovian or Giant planets formed with large cores and
huge gas atmospheres
 Surroundings were not depleted by the Sun’s
strong T-Tauri winds

Composed mostly of hydrogen and helium
 Inner two (Jupiter, Saturn) are closest to the Sun
and composed of 80-95% gas
 Outer two (Uranus, Neptune) are rich in ices
Planets
Body
Rock/metal %
Ices %
Gas %
Terrestrial planets (approximate)
70
30
0
Asteroids (approximate)
70
30
0
Comets, Pluto
15
85
0
Jupiter (gas planet)
2
5
93
Saturn (gas planet)
6
14
80
Uranus (ice planet)
25
58
17
Neptune (ice planet)
27
62
11
Terrestrial Planets
Terrestrial Planets
Inner solar system

Terrestrial means “Earth-like”

Composed of rock and metals

These are refractory (hightemperature) materials

Similar orbit planes with slight
orbital eccentricity

Smaller rock-metal bodies
include many of the asteroids
and our Moon
Terrestrial Planets

Formation of terrestrials was by violent collisional
heating from impacting planetesimals
 Planetesimals are asteroids and comets
 Collisional accretion and impact heating
produced:
 Liquid metal cores
 Liquid rock mantle

After major formation events that included the
heavy bombardment phase, cooling created a thin,
solid crust
Terrestrial Planets

In the early formation stages, the crust was
still very hot, but allowed gas and liquids
(atmosphere and oceans) to begin
accumulating

Cooling increased from accumulating water
primarily from cometary impacts on the
surface

Liquid mantle remained molten because of its
high temperature, and the insulating
properties of the light, thin, solid crust
Terrestrial Planets
Materials that remained closest to the hot Sun were the most dense
and lowest in abundance → a simple hypothesis
Hypothesis test:
 Terrestrial planets farther from the Sun should be larger (closest
should be smallest)
 Terrestrial planets farther from the Sun should be lower in density
(closest should be the most dense)
 Terrestrial planets farther from the Sun should have a smaller
core/mantle ratio (closest should be largest)
Terrestrial Planets
Mercury


Highest density (except for Earth which is gravitationally
compressed) → supports hypothesis
Smallest terrestrial planet → supports hypothesis
Venus
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Lower density than Mercury → supports hypothesis
Larger than Mercury → supports hypothesis
Earth

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Highest density, but because of gravitational compression
Larger than Venus → supports hypothesis
Mars
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Lowest density → supports hypothesis
Smaller than Earth x does not support hypothesis
Terrestrial Planets
Liquid rock and metal interior allowed segregation of
the materials by density (differentiation)
 Heat from impacts and gravitational compression
melts rocky planets and larger moons
 Molten planets differentiate in density
 Gravity produces buoyancy that floats lighter
materials in the liquid state
Liquid metal core is hot and dense, but circulates in a
large rotating planet because of low viscosity (low
resistance to flow)
Terrestrial Planets
Geodynamo magnetic field theory:
Circulating, conductive liquid metal core generates
magnetic fields

First initiated by surrounding magnetic field of Sun

Induced electric current in conductive metal core
generated by a weak external magnetic field
strengthens planet’s internal magnetic field

Increased magnetic field further increases current

Strong geomagnetic field generated in what is known
as the geodynamo process
Terrestrial Planets
Magnetic fields

Liquid metal core must be significant in size (mass
larger than the Moon)

Planet must rotate rapidly enough to circulate
conductive liquid metal core

The exception to this simple geodynamo theory is
Mercury which has a very small mass (6% of the
Earth's mass) and a slow rotation rate of 59 days

Mercury’s small residual magnetic field is most
likely induced by the nearby strong magnetic field
of the Sun, with a core kept liquid by the Sun's
tidal flexing
Terrestrial Planets
Crust

Thicker crust-to-radius ratio for smaller planets
because of their faster cooling
 Earth's crust about 0.5% of radius
 Mars' crust 1% of radius
 Moon's crust about 5% of radius

Thin crust can fracture on larger terrestrial
planets if the rotation is fast enough to circulate
the liquid mantle that supports the thin crust
Terrestrial Planets
Crust

Hypothesis: Fast rotation and the exchange of
heat/mass create a continuous and active surface
geology, including:
 Quakes
 Volcanoes
 Mountain and elevated plateaus
 Crustal motion in broad fragments, or plates
(plate tectonics)
Terrestrial Planets
Geological activity from rapid rotation and thin crust

Earth – yes, fast 24 hr rotation period → supports
hypothesis

Venus – no, too slow (240 day rotation period)
 Large mountains and plateaus formed by different
mechanisms → almost supports hypothesis

Mars - no (rapid rotation, but mass is too small)
 Large mountains and plateaus formed by different
mechanisms → almost supports hypothesis

Mercury - no (slow rotation and too small) → supports
hypothesis
Terrestrial Planets
Mantle

Between the light, thin crust and the dense metal core
of the terrestrial planets lies the molten rock mantle

Composed of various types of rock under tremendous
pressure from the overlying mass

Tremendous pressure can force the liquid mantle
(called lava when it reaches the surface) through
cracks or fissures in the crust
Terrestrial Planets
Mantle

For rapidly rotating planets, the movement of the
dense mantle can stretch and/or fracture the thin
overlying crust

Any break or weakness in the crust allows upward
flow of the liquid rock mantle to the surface =
volcanoes
Terrestrial Planets
Mantle
Semi liquid (plastic) or liquid molten rock between core
and crust

Transfers heat from core to the surface

Thin crust on larger planets allows volcanic activity

Thicker crust and/or slow rotation allows broad, longterm volcanic buildup because the crust has little or
no motion
Terrestrial Planets

Volcanic shields and plateaus found on both Mars
and Venus, but for different reasons
Mars - rapid rotation
 Relatively rapid crustal motion and active
fracturing
 Produced volcanoes on Mars in its early history
(1st billion years)
 Volcanoes are still active on the Earth
Venus - slow rotation
 Large volcanic buildup possible such as the large
plateaus on Venus (e.g., Maxwell Montes) and
volcanic shields
Terrestrial Planets
Lithosphere

Part of the Earth's solid crust is connected to the upper
portion of the mantle, forming the major surface plates
that move in an irregular fashion known as plate tectonics

Defining the crust, lithosphere, and mantle is not by the
structure but by the type of flow

Upper mantle which includes the lithosphere deforms until
fracturing

Layer below the lithosphere known as the asthenosphere
has a semi-liquid (plastic) deformation mode
Terrestrial Planets
Earth’s internal structure
Terrestrial Planet
Atmospheres
Terrestrial Planets
Atmospheres

Primary
 Created during the planets’ formation
 H, He, plus some other atomic gases
 Dispersed by heat since H and He have low escape
velocity

Secondary added by:
 Volcanism, impact deposition from comets, asteroids
 Comets – N2, O2, CH2
 Volcanoes - CO2, SO2, H2S, H2O

Long-term atmospheric gas buildup and retention
 Lighter gases retained according to planetary mass
(escape velocity) and planet temperature
Terrestrial Planets
Atmospheres
Secondary atmospheres remained permanently for
Venus and Mars because of their surface inactivity
(Mars is small, Venus has slow rotation)

H2O lost in Venus' hot atmosphere and from its
lower mass (gravity)

H2O lost from Mars because of its small mass
(gravity)
Terrestrial Planets
Atmospheres

CO2 absorbed in Earth's rock crust, mantle, and oceans

O2 was generated by some H2O breakdown, and early
biological life
Results of the evolution of the three terrestrial planet
atmospheres:

Venus 96% CO2, 4% N2

Mars 95% CO2, 3% N2

Earth 78% N2, 21% O2, 0.04% CO2
Terrestrial Planets – Retained Gases

While the four Giant
planets can retain
any gas in their
atmospheres
including hydrogen
and helium, the
Moon and Mercury
cannot retain even
the heaviest
common gas CO2

Gases retained for a
specific planet or
moon are
represented by the
diagonal lines that
appear below the
planet/moon
Terrestrial Planet
Characteristics
Mercury
Mercury
Mercury which is the closest planet to the Sun
is also the smallest terrestrial planet
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Mass 3.302x1023 kg (0.0553 MEarth)
Radius 2,439 km
Mean density 5.43 g/cm3
Orbital eccentricity 0.2056
Orbit inclination 7.0o
Semimajor axis 5.791 107 km (0.387 AU)
Mercury
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Orbit period - 88.0 days
Rotation period - 58.6 days (3:2 orbit to
rotation resonance period due to nearby Sun)
Magnetic field - 0.0033 gauss (1% of Earth)
Albedo - 10.6% (Earth = 37%, Moon = 12%)
Atmosphere - trace (approx. 1,000 kg total,
composed of K, Na, Ar, O, O2, He and other
trace gases)
Venus
Venus
Venus is the next planet from the Sun and often
called the sister planet of the Earth since it is only
5% smaller
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Mass - 4.87x1024 kg (81% of Earth)
Radius - 6,052 km (95% of Earth)
Mean density - 5.24 g/cm3
Semimajor axis - 0.723 AU
Orbit period - 224.7 days
Rotation period - 243.0 days
Magnetic field - Not significant (~10-5 x Earth)
Albedo - 75% (Earth = 37%)
Atmosphere - 92 bar (92 Earth atmospheres)
96.5% CO2, 3.5% N2
Venus
The surface geology of Venus includes
volcanic features that include large
elevated plateaus, large and small
volcanic cones and shields, and
pancake-shaped formations of lava

Features on the surface of Venus
indicate an age of only 300-500 million
years

This relatively new surface is thought
to be from the periodic remelting of the
lower crust

Magma would then flow through the
cracks, fissures, and craters to cover
much of the lower elevations
Earth
Earth
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Mass - 5.974x1024 kg (1/333,000 MSun)
Radius - 6,378 km (equatorial)
Mean density - 5.52 g/cm3
Semimajor axis - 1.00 AU
Orbit period - 365.24 days
Rotation period - 23 hr 56 min (sidereal)
Magnetic field - 0.308 Gauss
Albedo - 37%
Atmosphere - 1 bar (1 atmosphere)
78.1% N2, 20.9% O2, 0.9% Ar
Mars
Mars
Mars has a small CO2 atmosphere, but a
distant past that had similar
characteristics to Earth including a
magnetic field, liquid surface water, and
active volcanoes
A thick crust and solidification of the
interior of the planet halted most of its
geological activity after the first billion
years as a planet
Mars does contain permanent polar ice
caps of water and CO2 ice and
subsurface ice discovered on recent
exploration missions
Mars
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Mass - 6.421x1023 kg (0.11 MEarth)
Radius - 3,397km (equatorial) (0.53
REarth)
Mean density - 3.93 g/cm3
Semimajor axis - 1.52 AU
Orbit period - 1.88 yr
Rotation period - 24.62 hr
Magnetic field - No dipolar field, but
weak, localized magnetization of the
iron-rich crust
Albedo - 15%
Atmosphere - 0.007 bar 95.2% CO2,
2.7% N2, 1.6% Ar
Jovian Planets
Jovian Planets
Jovian planets are the giant gas and ice planets formed
beyond the terrestrial region

A number of common features exist in the Jovian
planets, including:

Large mass

Innermost rock and metal core

Outer envelope of hydrogen and helium gas

Rapid rotation

Strong magnetic fields
Jovian Planets

Jovian planets began as large ice and rock
(planetesimal) cores

Large, dense gas regions accelerated the process

These cores were large planets in themselves, and
could attract ice and gas unavailable to the terrestrial
planets
 This allowed them to rapidly gain mass because
there was more gas and ices in their neighborhood
Jovian Planets
Jovian Planets
A comparison of the interiors of the Jovian planets shows a distinct
difference between the two largest, Jupiter and Saturn, and the
two smallest, Uranus and Neptune

The largest two, Jupiter and Saturn, are called giant gas planets
because of their principal gas composition, although little is
actually in the gas state

The two smallest are the giant ice planets Uranus and Neptune
 Both Uranus and Neptune do contain a significant hydrogen
and helium atmosphere, but they are dominated by ices in
liquid form

The term "ice planet” should not be confused with the dwarf ice
bodies and planets similar to Pluto, its moon Charon, and a host
of other comet-like planets much smaller
 These have a distinctly different composition than Uranus and
Neptune
Jovian Planets
Jupiter
Jovian Planets
Jupiter

One of the brightest planets in the
night sky because of its tremendous
size (11 x REarth) and light-colored
upper cloud layers (high albedo)

First to observe and document
Jupiter's features and four nearby
moons was Galileo, for which the
four Galilean moons are named

Rapid rotation (9.93 hr) and high
mass (318 x MEarth) generates a huge
magnetic field
Jovian Planets
Jupiter

Largest magnetic field of the planets (11-14 Gauss)

Atmosphere – 90% H, 10% He

Interior composed of liquid H, He, and H
compounds

Deep interior included hydrogen under
tremendous pressure and temperature that forces
the atoms into a degenerate state – metallic
hydrogen
Jovian Planets
Jupiter

Metallic hydrogen mantle is superconducting
which is thought to circulate magnetodynamo
currents to flow without resistance and generate
Jupiter’s enormous magnetic field

Stretches beyond the orbit of Saturn 5 AU away

Orbit
 11.86 yr
 5.2 AU

Rotation period – 9.925 hr (fastest of all planets)
Jovian Planets
Jupiter

Mass – 318 x MEarth

Albedo (reflectance) – 52%

Moons – 63 (as of 2007)
Exploration spacecraft
 Pioneer 10
 Pioneer 11
 Voyager I
 Voyager II
 Ulysses (gravity assist to Sun’s
poles)
 New Horizons (gravity assist to Pluto)
 Galileo (dedicated orbiter)
Jovian Planets
Jupiter

Four largest moons called the
Galilean moons (named after
Galileo Galilei) who
discovered the moons

Io


Closest, and undergoes
highest tidal forces
Tidal flexing heats the
interior to a molten state
and generates the most
geologically active
celestial body in the solar
system
Jovian Planets

Europa

Next moon out is covered
with an icy crust that
overlays a liquid water
interior
 Conditions may be
sufficient to support
primitive life

Weak magnetic field
measured by the Galileo
spacecraft
 Indicative of conductive
water ocean beneath icy
surface

Two possible structures
shown on the right
Jovian Planets
Ganymede
 Largest moon in the solar system
 Weak magnetic field may be made possible by a conductive
water layer beneath the icy crust
Callisto
 3rd largest moon in the solar system
 Interior 40% ice and 60% rock/metal
 Also has a conductive water ocean below the icy surface that
influences Jupiter’s magnetic field
Jovian Planets
Saturn
Jovian Planets
Saturn

Second largest planet in the solar system

Lowest density planet in the solar system (0.69
g/cm3, water is 1.00 g/cm3)

Rapid rotation (10.78 hr) and high mass (95x MEarth)
generates a large magnetic field, but significantly
smaller than Jupiter’s

Orbit


24.86 yr
10.8 AU
Jovian Planets
Saturn

Allbedo – 47%

Also contains a metallic hydrogen mantle

Moons – 60 (as of 2007)

Largest ring system of the planets
Exploration spacecraft

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Pioneer 11
Voyager I
Voyager II
Cassini (dedicated orbiter)
Jovian Planets
Saturn

Hydrogen and helium atmosphere
with trace amounts of methane,
water, ammonia, and hydrogen
sulfide

Most prominent feature is its ring
system
 Extends approximately 67,000
km to 480,000 km
 Saturn's rings composed
primarily of small ice particles
that have been charged
 Allows the orientation and
position of the particles to
change slightly when the
Saturnian magnetic field passes
through
Saturn and its rings in UV
Saturn’s rings
Saturn’s surroundings
Saturn
Titan

Saturn’s largest moon is Titan
 Larger than both Pluto and Mercury

Only moon with a significant atmosphere
found in the solar system

Similar to several of Jupiter's moons
 Rocky core overlayed with a mantle
composed of water and ammonia ice
layers

Roughly 2/3 of the moon is
rock/silicate/metal that remains warm, or
perhaps hot
 Likely creates conditions for a liquid
water layer in the upper ice mantle
Saturn
Titan

High atmospheric pressure (1.5
x Earth’s) and cold temperature
(94 K, -290oF) creates a triplepoint environment for methane
and ethane on Titan’s surface
 Solid, liquid, and gas, like
water on Earth

Ammonia’s low abundance
and variable concentration is
also like water in Earth’s
atmosphere
Saturn
Titan

Rain composed of ethane and
methane erode Titan’s surface
which form large lakes
 Located near poles because
of seasons
 Identified by imaging radar
on Cassini (see image on
right)

Lakes are small in comparison
to Earth’s
Saturn
Titan

Alcohol and ammonia rains create river
channels and broad, flat basins that
appear to be ocean beds

Methane is not stable and not retained on
Titan
 Must be replenished, probably from
porous sands in “ocean” beds

Small, cold volcanoes (cryovolcanoes)
possible source of methane & ethane

Thick haze that prevents direct
observation is due to solar energy
producing hydrocarbons from ethane
and methane
Saturn
Titan

Images of Titan’s surface show
rocks littering flat plateau at the
landing site

Origin of rocks likely from
cryovolcanoes releasing liquid
water that freeze into hard clumps
that persist for a very long time

Water ice is too cold to sublimate
at 94 K

Liquid water interior also
responsible for a different rotation
of its crust compared to its interior
(measured accurately during
Cassini flybys)

Close similarity between Titan’s
and Earth’s geology and
atmosphere
Saturn
Enceladus

Smaller and more distant than
Titan is Enceladus which has
a tortured, icy outer surface

Located in the dense region
of the Saturn's diffuse outer
E-ring

Two different surface
characteristics in northern
and southern hemispheres
 Sparsely-cratered northern
region
 Young, uncratered
southerly zone
Saturn
Enceladus

Tidal heating of the interior by
Saturn generates liquid
cryovolcanoes near the south
polar region which spew frozen
particles into Saturn’s E-ring

Light-colored surface indicative
of active ice formation (celestial
snow)

Water ice from Enceladus
contributes significantly to
Saturn’s E-ring
Saturn
Enceladus

Tidal heating by Saturn
generates continual
cryovolcanism

Discovered during Cassini
flybys with backlighting from
the Sun
Source of tidal heating?

Tidal flexing requires slightly
elliptical orbit, but tides
gradually reduce eccentricity
(Enceladus’ orbit should be
circular by now)

Enceladus’ eccentric orbit due
to perturbation from 2:1 orbital
resonance with Saturn’s moon
Dione
Jovian Planets
Uranus
Jovian Planets
Uranus

One of two Jovian giant ice
planets

Discovered in 1781 by
William Herschel
 First planet discovered by
a telescope
 Previous discoveries were
of large asteroids (minor
planets Ceres and Vesta
were the two largest)

Rotational axis is offset 98o
from the normal
Jovian Planets
Uranus
Structural model consists of three layers

Rocky core
 Small in comparison to giant gas planets

Ice mantle
 Contains the majority of the mass
 Consists of water, ammonia and other
volatiles in liquid form

Outer gaseous hydrogen/helium envelope is a
dull green-blue due to methane
Jovian Planets
Uranus

Mass – 14.5 MEarth

Orbit
 84.32 yr
 19.23 AU

Moons – 27 (as of 2007)
 Aligned with equator (nearly
vertical)
Exploration spacecraft

Voyager II
Jovian Planets
Uranus

Small ring system
discovered in Voyager II
data

False-color image from
Voyager on the right
shows circulation bands
and the storm regions
near the polar band
Jovian Planets
Miranda

One of the most unusual moons in
the solar system is Uranus’ moon
Miranda

Chaotic features include deep,
long canyons 10 times deeper than
the Grand Canyon

Violent collision between moon
during Uranus catastrophic
formation likely broke up the
original moon and reformed it with
jumbled surface features
Jovian Planets
Neptune
Jovian Planets
Neptune

The largest Jovian ice planet

Discovered 1846 by French
astronomer Urbain Le Verrier

Similar to Uranus, the
structural model consists of
three layers
 Rocky core
 Ice mantle
 Outer gaseous
hydrogen/helium envelope
Jovian Planets
Neptune

Mass – 17.2 MEarth

Orbit


164.79 yr
30.10 AU

Moons – 13 (as of 2007)

Small ring system discovered
in Voyager II data
Exploration spacecraft

Voyager II
Jovian Planets
Neptune

Lowest temperatures in upper
atmosphere of all the planets
 Highest wind speeds
measured at approx. 1,500
mph

Circulation bands are visible,
along with giant dark spot
similar to Jupiter’s and highaltitude clouds travelling at
extremely high speeds
Neptune
Triton

Neptune’s largest moon is
Triton

Triton’s diameter is 78% of the
Earth's moon, but only 28% of
its mass
 Density 2.05 g/cm3 (lunar
density is 2.05 g/cm3

Surface shows complex
features with few craters which
indicates a relatively young
surface
Neptune
Triton

Cryovolcanoes of
liquid nitrogen from
beneath the ice crust
indicate tidal heating
and subsurface liquid
nitrogen and possibly
water oceans

Spectral imaging data
shows Triton's
surface covered with
nitrogen ice, water
ice, carbon dioxide
ice, methane ice, and
ammonia ice
Trans-Neptunian
Objects
Beyond Neptune
Trans-Neptunian objects are small icy bodies scattered
by the massive planet Neptune which itself was
scattered outward by Jupiter in its early history
Beyond Neptune
Trans-Neptunian objects are small icy bodies scattered
by the massive planet Neptune which itself was
scattered outward by Jupiter in its early history
Pluto
Pluto
Pluto, the most distant of the
original nine planets has the
largest moon-planet mass
ratio in the solar system

Discovered in 1930 by Clyde
Tombaugh

Pluto’s largest moon
Charon was discovered in
1978
 The large mass of
Charon compared to
Pluto makes them more
of a binary pair than a
planet-moon system
(both orbit around a
center of mass outside
Pluto’s surface)
Pluto
Pluto’s two other moons
Nix and Hydra are 2
and 3 times the
distance of Charon
from Pluto, but much
smaller (also orbit
around the PlutoCharon barycenter)
Exploration spacecraft
 New Horizons (2006
launch for a July 2015
flyby)
Pluto












Mass 1.305×1022 kg (0.2% MEarth)
Radius
1,195 km
Mean density
2.03 g/cm3
Orbital eccentricity
0.24881
Orbit inclination
17.142o
Semimajor axis 39.48 AU
(5.91x109 km)
Orbit period
248.09 yr
Rotation period 6d 9h 17m 36s
Rotational axis tilt
119.59o
Magnetic field Unknown
Albedo
0.49–0.66
Atmosphere
nitrogen,
methane, and carbon monoxide
(surface ice sublimation)
Questions?