Survey
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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. 1 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 . 2 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 3 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. 4 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 5 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. 6 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 7 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. 8 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 9 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) 10 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. 11 Unmanned Missions to Mars’ Surface Sojourner/Pathfinder (1997) Spirit & Opportunity MER (2004-2005) Viking 1&2 (1976) 12