1
Inner or Terrestrial Planets
• All the inner planets
formed at the same time.
• Their composition is also
very similar.
• They lack the huge
atmospheres of Jovian
planets.
• Yet all are large enough
for gravity to shape them
into spheres.
• Much of the difference
we see in these planets
has to do with their size
and distance from the
Sun.
2
Density of the Planets
• Density is simply the mass divided by the volume (M/V).
• The density of the Terrestrial planets is much higher than for the
Jovian planets.
• The Jovian planets are more massive than Terrestrial planets but
their volume is much higher so their densities are actually lower
• Earth has a density of 5.52 grams/cm3, Jupiter has a density of
1.33 grams/cm3 .
3
Planetary Interiors
When the planets formed they were very hot. This heat
came from three sources:
– Accretion - impacts from asteroids and comets
– Differentiation - heavy material sinks, light material rises
– Radiation - radioactive materials
4
Planetary Interiors
• Differentiation caused the heavy materials such as iron and nickel
to sink towards the core. The lighter material rose to the surface
and cooled forming the crust.
• The interior heat of planets drives earthquakes, volcanoes, etc.
• All the planets are cooling down. The smaller the planet, the
faster the planet cools. Earth still has a hot interior, Mercury’s
interior may have cooled completely.
5
Impact
Cratering
• All of the inner planets
experienced tremendous
amounts of impact cratering
• The number of craters in an
area can be used to tell the age
of the surface. Fewer craters,
younger area.
• On some planets craters have
been removed by lava flows,
others by erosion.
• Planets with atmospheres
cause small objects to burn up
before they hit the ground.
• Planets without atmospheres
are continually bombarded
with dust-sized
micrometeorites.
A heavily cratered area
of the Moon
Barringer Meteor
Crater, AZ, USA
6
Mercury
Unmanned missions to Mercury:
• Mariner 10 (1974-75),
• MESSENGER (2011-2012),
• BepiColombo (2011-2012).
• Smallest of the inner planets. Large
metallic core. Geologically dead although
magnetic field detected. Why?
• Its rotation rate is slow and is exactly 2/3
of its orbital period. One Mercury year is
87.9 Earth days, one Mercury day is 58.6
Earth days. This is an example of a spinorbit resonance.
• It has a very elliptical and inclined orbit.
• The surface facing the Sun is very hot
because Mercury is so close to the Sun.
• However, since Mercury’s axial tilt is near
0°, craters near the poles receive no
sunlight and are very cold.
• Scientists may have detected ice at the
poles.
• No atmosphere, no satellites.
7
Mercury
Spin-Orbit Resonance
8
Venus
Radar image of Venus
Unmanned missions to Venus:
• Magellan (1989-1994)
• Pioneer Venus (1978-1992)
• Vega 1 & 2 (1985)
• Venera 1 - 16 (1961-1983)
• Mariner 5 (1967)
• Mariner 2 (1962)
• Nearly the same size as Earth.
Probably still geologically active.
• Completely covered in clouds. Only by
radar have we observed the surface and
measured the rotation rate.
• Very thick atmosphere mostly CO2
• Runaway greenhouse effect causes
very high surface temperatures and
pressures.
• Hottest surface temperature of any
inner planet. Hotter than Mercury.
• Surface pressure is 100 times higher
than Earth’s
• Slowest rotation of any planet (243
days) and spins backwards.
• No magnetic field, no satellites
9
Orbit of Venus
10
The Greenhouse Effect
• When the gases in an
atmosphere allow sunlight to
strike the surface the surface
heats up and gives off
infrared radiation.
• If the atmosphere however
prevents the infrared
radiation from radiating back
out to space the temperature
of the planet can increase,
this is the Greenhouse Effect.
• Carbon Dioxide CO2 behaves
this way and is an important
greenhouse gas. Venus’
atmosphere is 95% CO2.
11
Earth
• Large enough to maintain hot
interior (volcanoes, earthquakes,
continental drift).
• Thick atmosphere and mild
greenhouse effect allows liquid
water to remain on the surface.
• Erosion has eliminated nearly all
impact craters.
• Rapid spin and molten interior allow
a magnetic field to exist.
• One satellite, Moon.
Earth and Moon as seen
from Martian orbit
Earth as seen from Lunar orbit
12
Mars
Nearly 40
unmanned
missions to Mars
since 1960
Olympus Mons
• About half the size of Earth. No
geological activity likely now.
No magnetic field.
• Has the largest volcano in the
solar system, Olympus Mons.
• Evidence of massive water
erosion some time in the past.
Scientists are searching for liquid
water now.
• Very thin CO2 atmosphere, polar
caps of mostly frozen CO2 and
water. Since its atmosphere is
thin and cold there is very little
greenhouse effect.
• Two satellites, Phobos and
Deimos (possibly captured
asteroids)
13
The Search for Life
on Mars
• Among all of the planets besides Earth,
Mars appears to have had conditions
that might have been most suitable for
life.
• What appear to be dry lake beds and
water erosion on Mars are visible from
orbit.
• If liquid water once existed on the
surface of Mars did life also? Might it
still be there?
• The current series of Mars Exploration
Rovers are on the surface looking at the
geology of Mars to find chemical and
physical evidence of water.
• Their results are that liquid water had
existed on Mars at some time in the
past.
14
Unmanned Missions to Mars’ Surface
Sojourner/Pathfinder (1997)
Spirit & Opportunity MER
(2004-2005)
Viking 1&2 (1976)
15
Magnetic Fields of the Inner Planets:
Mercury, Earth, Venus
• Magnetic field is caused by dynamo of rotating molten
iron/nickle core
– Also require temperature gradient between core and mantle
• Mercury: has a magnetic field (1% of Earth) but its origin
is unclear
– Mercury’s interior is cool (but perhaps has a temperature
gradient?) and Mercury has slow rotation
– stay tuned for MESSENGER probe results.
• Venus: hot interior, maybe lacks required temperature
gradient; extremely slow rotation
• Earth: has a magnetic field; conditions are just right
16
Magnetic Fields of the Inner Planets
Mars
• Magnetic field is caused by dynamo of rotating molten
iron/nickle core
– Also require temperature gradient between core and mantle
• Mars: no present day dynamo magnetic field; massive
impact theory:
– Massive impact heated mantle; reduced temperature gradient
between core and mantle
– Dynamo Magnetic field was drastically reduced or turned off
– Core + mantle cooled uniformly (no gradient  no dynamo
magnetic field
– Massive impact may have blasted Mars’ atmosphere, leaving thin
atmosphere observed today
– After dyanamo magnetic field was turned off, solar wind may have
started eroding Mars’ atmosphere
– Evidence of past dynamo magnetic field  recent measurements
of Mars’ crustal magnetic field
17
Evidence for Past Magnetic Dynamo
on Mars: Crustal Magnetic Field
18
Atmospheric Composition
CO2~0.037%
Atmospheric Pressure 90 atm
1atm
0.007 atm
19
Interior Composition
20
Planetary Evolution
21
Terrestrial Planet Differences
• Mass and Radius
– Smaller planets cool faster
– Cooler planets are less geologically active
– Geological activity maintains atmosphere for
inner planets, so reduced interior heat and
activity leads to reduced atmosphere
22
Why is Earth’s Atmosphere so
Different from Mars’ and Venus’?
• Water + CO2 makes carbonic acid = soda water
• Rain on Earth removes CO2 from the atmosphere
and locks it into the rocky ground
• Venus’ atmosphere is too hot for water to
condense out  no water rain to remove CO2
• Mars’ atmosphere is too thin and cold for water
rain (may have fog)
– Mars does have CO2 snow at poles
– Mars currently has very little water in its atmosphere
23
Why is Earth’s Atmosphere so
Different from Mars’ and Venus’?
• Role of Biology on Earth
• Plants use carbon-dioxide to make cellulose
• Sea creatures use carbon-dioxide runoff (from
rain) to make shells (calcium carbonate).
• Plants break down water and carbon dioxide by
photosynthesis, releasing oxygen into the
atmosphere
• Geological processes melt rock in the hot mantle
re-releasing carbon-dioxide into the atmosphere
24
Why is Earth’s Atmosphere so
Different from Mars’ and Venus’?
• Role of Geology
• On Earth geological processes melt rock in the hot
mantle re-releasing carbon-dioxide and other
molecules into the atmosphere
• Mars has little interior activity, therefore it cannot
replenish its own atmosphere
– Although Mars’ atmosphere is dominated by carbon
dioxide, that atmosphere is too thin to support a
greenhouse effect, and it is unable to retain heat.
25