Inner or Terrestrial Planets

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, Mercurys 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

• 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 spin-orbit 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 Mercurys 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.

• Unmanned missions to Mercury
• Mariner 10 (1974-75),
• MESSENGER (2011-2012),
• BepiColombo (2011-2012).

7
Venus

• 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 Earths
• Slowest rotation of any planet (243 days) and
  spins backwards.
• No magnetic field, no satellites

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)

8
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.

9
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
10
Mars

• 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)

Nearly 40 unmanned missions to Mars since 1960
Olympus Mons
11
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.

12
Unmanned Missions to Mars Surface
Sojourner/Pathfinder (1997)
Spirit Opportunity MER (2004-2005)
Viking 12 (1976)