Introduction to Comparative Planetology: Solar System Trivia and Planetary Characteristics

1. Introduction to Comparative PlanetologyChapter 7

2. Solar System Trivia • How many (real) planets are there in the solar system? • Which is the largest planet? • Which is the smallest planet? • Which planet is closest to the Sun? • Which planet would appear faintest from outside the solar system? • Which planet is most dense? • Which planet is less dense than water? • Which planet has the most moons? • Which planet has the highest surface temperature? • Which planet has the most carbon dioxide in its atmosphere?

3. Our Solar System to an Outsider • Orbits of planets • Roughly circular • Roughly in same plane • Clue to origin of Solar System • Keplerian orbits • Inner planets move fastest

4. Which Planets Would Appear Brightest? • Depends on: • How large the planet is • How much light it receives from the Sun • How much light it reflects (albedo) 2. Venus 0.37 4. Earth 0.15 1. Jupiter 1.00 Mars 0.01 3. Saturn 0.19 Uranus 0.01 Mercury 0.05 Neptune 0.003

5. Two Groups of Planets • Inner planets • Mercury, Venus, Earth, Mars • Outer planets • Jupiter, Saturn, Uranus, Neptune

6. Sizes of the Planets • Brightest does not always mean biggest • Uranus & Neptune are large but dim (far from Sun) • Earth and Venus are small but bright (close to Sun) • Terrestrial planets (inner) • Mercury, Venus, Earth, Mars • Small, close to Sun • Jovian planets (outer) • Jupiter, Saturn, Uranus, Neptune • Large, far from Sun

7. How Are Terrestrial Planets Different to Jovian? • Terrestrial planets • Rocky, mountains, craters, valleys, volcanos… • No atmosphere or thin layer • Jovian planets • Thick, deep atmosphere • Is planet mostly gas or liquid? • Prediction: • Jovian planets have lower densities • Need mass and radius (or diameter) to test

8. Masses of Planets • Measure orbits of natural satellites • All but Mercury and Venus have natural satellites • Kepler’s 3rd Law tells you the mass • Measure motion of space-probes

9. The Densities of Planets • Air = 1.2 kg/m3 • Water = 1000 kg/m3 • Concrete = 2000 kg/m3

10. The Densities of Planets • Air = 1.2 kg/m3 • Water = 1000 kg/m3 • Concrete = 2000 kg/m3

11. Terrestrial vs. Jovian Planets • The terrestrial planets are made of rocky materials and have dense iron cores, which gives these planets high average densities. • The Jovian planets are composed primarily of light elements such as hydrogen and helium, which gives these planets low average densities.

12. The Almost-Planets • More than 140 moons in the Solar system • Earth - 1 • Mars - 2 • Jupiter - 62+ • Saturn - 43+ • Uranus - 24+ • Neptune - 13+ • Seven are comparable to Mercury • Similar to terrestrial planets • Either rock or mixed rock & ice

13. The Ex-Parrots Planets • So there are eight planets, right…? • Right. • Pluto is now an ex-planet, a ‘Dwarf Planet’ • One of many Trans-Neptunian Objects • Smaller than our Moon • More than 900 objects in Kuiper Belt beyond Neptune

14. Beyond Average Densities • Why are Jovian planets less dense than Terrestrial planets? • Why is Saturn less dense than Jupiter? • Why is Callisto less dense than Mercury? • We need to know chemical compositions of planets - what are they made of?

15. Spectroscopy in Action! • Kirchhoff’s 3rd Law A cool, transparent gas in front of a source of a continuous spectrum produces and absorption line spectrum

16. Spectroscopy of Titan’s Atmosphere • Methane! • Absorption line of methane does not come from Sun or Earth • UV+IR spectra tell us about more compounds in Titan’s atmosphere

17. Spectroscopy of Europa’s Surface

18. Direct Exploration of Other Planets and Moons Venera 13 on Venus, 1982 Mars Rovers, 2004-present Huygens Probe descending on Titan, 2005

19. Chemical Composition of the Planets • Terrestrial planets (e.g. Mars) • Heavy elements - iron, oxygen, silicon, magnesium, nickel, sulfur • Jovian planets (e.g. Jupiter) • Lightest elements - hydrogen, helium