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/cm 3, Jupiter has a density of 1.33 grams/cm 3.

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 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 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. Unmanned missions to Mercury: Mariner 10 ( ), MESSENGER ( ), BepiColombo ( ).

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 CO 2 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 Radar image of Venus Unmanned missions to Venus: Magellan ( ) Pioneer Venus ( ) Vega 1 & 2 (1985) Venera ( ) 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 CO 2 behaves this way and is an important greenhouse gas. Venus’ atmosphere is 95% CO 2.

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 as seen from Lunar orbit Earth and Moon as seen from Martian 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 CO 2 atmosphere, polar caps of mostly frozen CO 2 and water. Since its atmosphere is thin and cold there is very little greenhouse effect. Two satellites, Phobos and Deimos (possibly captured asteroids) Olympus Mons Nearly 40 unmanned missions to Mars since 1960

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 Spirit & Opportunity MER ( ) Viking 1&2 (1976) Sojourner/Pathfinder (1997)