The Terrestrial Planets

1. The Terrestrial Planets Astronomy 311 Professor Lee Carkner Lecture 9

2. Where did the Earth come from? • It accreted from rocky planetesimals • It accreted from rocky and icy planetesimals • It rapidly condensed out of the solar nebula • It was captured by the Sun’s gravity • It was ejected from a collision of Jupiter and Saturn

3. Why do we think the planet Jupiter formed quickly? • Samples from Jupiter indicate it is older than the other planets • It is mostly made of hydrogen gas which was only around in the early solar system • We see many Jupiter-type planets around very young stars • Jupiter’s orbit is right at the spot where the solar system was born • Jupiter has a very rapid rotation rate

4. 1962 -- Mariner 2 Venus Fly-by 1964 -- 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 Early Missions to the Inner Planets

5. Planetary Missions • First wave of exploration from 1960-1979 • Very large number of Soviet missions, most were failures • Venus: 15 successes, 31 missions • Smaller number of US missions, but higher success rate • Mercury: 1 success, 1 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

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

7. 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 Inner Planet Facts

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

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

10. 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

11. 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

12. 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): 0.003

13. 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

14. 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

15. Asteroid Facts • Asteroids • Diameter: <0.14 • Mass: <0.02 • Orbital Radius: 2.8 • Most have orbits within the asteroid belt (~2-3.5 AU)

16. 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: <0.01 • All are small compared to the gas giants (Neptune is ~4 times the diameter of the Earth and ~64 times the volume)

17. Atmospheres • Mars • Surface pressure = • Composition = 95 % CO2, 3 % N (also water vapor, oxygen) • Venus: • Surface pressure = • Composition = 96 % CO2, 4 % N (also sulfur compounds such as sulfuric acid, H2SO4)

18. Atmospheres (cont.) • Earth: • Surface pressure = • Composition = 77 % N, 21 % O2 (also water vapor, CO2, 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

19. The Carbonate-Silicate Cycle Atmosphere Water + CO2 (rain) CO2 Volcano Ocean CO2 + silicate (subvective melting) Carbonate + water (stream) Carbonate + silicate (Sea floor rock)

20. CO2 and Greenhouse Effect • Water washes CO2 out of atmosphere where it is eventually deposited as rock • CO2 is a greenhouse gas • More CO2 = higher temperature

21. Hot more CO2 washes out cools off Cool less CO2 washes out heats up Carbonate-Silicate Feedback

22. CO2 and the Inner Planets • Venus: • all the water boiled off and was disassociated • thick CO2 atmosphere and high temperatures • Mars: • no way to get CO2 out of rocks • thin CO2 atmosphere and low temperatures • Earth: • mild temperature and atmosphere

23. Composition • Density of rock (silicates) ~3000 kg/m3 • What makes up the difference? • Iron • “Rocky” planets could also be called the “metal” planets

24. Interior Structure

25. 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

26. Next Time • Read Chapter 8 • but just the Mercury parts

27. 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

28. 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 CO2 is in atmosphere • Mars -- no plate tectonics means CO2 is in rocks • Earth -- carbonate-silicate cycle balances greenhouse effect