The Terrestrial Planets Astronomy 311 Professor Lee Carkner Lecture 9

Early Missions to the Inner Planets  Mariner 2 Venus Fly-by   Mariner 4 Mars Fly-by   1970 Venera 7 Venus lander  first successful landing on another planet  1973 Mariner 10 Venus/Mercury Fly-by   1975 Viking 1 and 2 Mars lander  first successful landing on Mars

Planetary Missions  First wave of exploration from   Very large number of Soviet missions, most were failures  Venus: 15 successes, 31 missions    Smaller number of US missions, but higher success rate  Mercury: 2 successes, 2 mission  Venus: 6 successes, 7 missions   We are now starting to see other countries get more involved with space exploration  Most notably Japan and The European Union

Sources of Information for the Inner Planets  Mercury:  Mariner 10 and MESSENGER --  Venus:  Soviet Venera landers -- surface conditions  Magellan --  Mars:  Viking, Pathfinder, Spirit, Opportunity --  Viking, Global Surveyor, Odyssey, Recon Orbiter -- maps of the surface

Inner Planet Facts  Mercury  Diameter: 0.38  Mass: 0.06  Orbital Radius: 0.4  Venus  Diameter: 0.95  Mass: 0.82  Orbital Radius: 0.7  Earth  Diameter: 1  Mass: 1  Orbital Radius: 1  Mars  Diameter: 0.53  Mass: 0.11  Orbital Radius: 1.5

Determining Planetary Properties  Mass    Distance  Can find directly with radar  Diameter  Can get from the angular diameter and the distance

Determining Planetary Properties (cont.)  Average Density   Atmospheric composition  take a spectrum of the atmosphere, look for the spectral signature of elements

Scale Models  We want to make a scale model to try to understand astronomical distances  Need to find the scale scale = (real size) / (model size)  example: miles per inch or light years per cm  Once you have the scale you can find the model size for any real object (model size) = (real size) / scale

The Planets That Weren’t  There should have been 2 other inner planets   A planet about the size of Mars may have hit the Earth a few billion years ago, the debris formed into the Moon   Jupiter’s gravity disrupted the planetesimals between Mars and Jupiter so they never formed a planet

The Moon   Most of our information comes from the 6 Apollo landings (11-17, excluding 13)  Moon facts  Diameter: 0.27  Mass: 0.01  Orbital Radius (from Earth): 

Moons of the Inner Planets  Venus and Mercury have no moons  Earth has one large moon   Mars has two moons, Phobos and Deimos   Inner planets may be too small to capture moons easily  It is difficult to gravitationally capture something

Asteroids  Millions of small bodies orbit the Sun, most between Mars and Jupiter (the asteroid belt)  Our information comes from 2 sources:   Pieces of asteroids that have fallen to Earth   For example:  NEAR orbiting Eros  Hayabusa landing on Itokawa

Asteroid Facts  Asteroids  Diameter: <0.08  Mass: <  Orbital Radius: 2.8   Most have orbits within the asteroid belt (~2-3.5 AU)

Sizes of the Inner Planets  Sizes relative to Earth  Earth: 1 (diameter = 13,000 km)  Venus: 0.95  Mars: 0.53  Mercury: 0.38  Moon: 0.27  Asteroid: <0.08  All are small compared to the gas giants (Neptune is ~4 times the diameter of the Earth and ~64 times the volume)

Atmospheres   Mars  Surface pressure =  Composition = 95 % CO 2, 3 % N (also water vapor, oxygen)  Venus:  Surface pressure =  Composition = 96 % CO 2, 4 % N (also sulfur compounds such as sulfuric acid, H 2 SO 4 )

Atmospheres (cont.)  Earth:  Surface pressure =  Composition = 77 % N, 21 % O 2 (also water vapor, CO 2, trace elements)  Why are the atmospheres of Venus, Mars and the Earth so different?   The Earth can regulate its atmosphere through the carbonate-silicate cycle, the other planets cannot

The Carbonate-Silicate Cycle Water + CO 2 (rain) Ocean Carbonate + silicate (Sea floor rock) CO 2 Volcano Atmosphere Carbonate + water (stream) CO 2 + silicate (subvective melting)

CO 2 and Greenhouse Effect  Water washes CO 2 out of atmosphere where it is eventually deposited as rock   CO 2 is a greenhouse gas   More CO 2 = higher temperature

Carbonate-Silicate Feedback  Hot   more CO 2 washes out   cools off  Cool   less CO 2 washes out   heats up

CO 2 and the Inner Planets  Venus:   all the water boiled off and was disassociated   thick CO 2 atmosphere and high temperatures  Mars:   no way to get CO 2 out of rocks  thin CO 2 atmosphere and low temperatures  Earth:   mild temperature and atmosphere

Composition   Density of rock (silicates) ~3000 kg/m 3  What makes up the difference?  Iron    “Rocky” planets could also be called the “metal” planets

Interior Structure

Composition (cont.)  Earthquake studies indicate that the Earth has a iron core   Earth has a density gradient, heavier materials near the center, lighter near the surface  We believe that the other inner planets have a similar structure 

Next Time  Read Chapter 8  but just the Mercury parts

Summary  Inner or Terrestrial region  4 planets (Mercury, Venus, Earth, Mars)  1 large moon (The Moon)  thousands of asteroids  Information from 30 years of space missions  Size  Earth and Venus about the same  Mars, Mercury, the Moon, 1/2 -1/4 size of the Earth  Asteroids few km

Summary (cont.)  Composition  silicate rock crust  iron-silicate mantle  iron core  each planet has different proportions of each  Atmosphere  Mercury, Moon, asteroids -- none  Venus -- no water means CO 2 is in atmosphere  Mars -- no plate tectonics means CO 2 is in rocks  Earth -- carbonate-silicate cycle balances greenhouse effect