Observational Astronomy - Lecture 6 Solar System I - The Planets

Transcription

1 Observational Astronomy - Lecture 6 Solar System I - The Planets Craig Lage New York University - Department of Physics craig.lage@nyu.edu March 23, / 39

2 The Sun and the Earth The Sun is 23,000 times larger in radius and 330 million times more massive than the Earth. 2 / 39

3 Hydrostatic Equilibrium Gravity Pressure Planets and stars are in Hydrostatic Equilibrium, with the outward pressure balancing the inward pull of gravity. 3 / 39

4 Solar System Formation Gravity pulls a cloud of gas into a spherical proto-star. Most of the matter collapses into the central star. Some of the remaining matter forms a proto-planetary disk. Matter in the disk clumps up into planetesimals, then into larger planets. Radiation pressure from the star blows away most of the remaining gas that has not coalesced into planets. 4 / 39

5 The Composition of the Solar Nebula Element Mass fraction H 70.54% He 27.56% C 0.31% N 0.11% O 0.96% Ne 0.17% Mg 0.06% Si 0.07% S 0.04% Fe 0.18% H Abundances(2) He 5 / 39

6 Escape Velocity and Molecular Speeds c 2GMPlanet d V Escape R V Gas 9 T m Gas Hotter gas molecules move faster Heavier gas molecules move slower. 6 / 39

7 Gravity and Escape Velocity Planet Mass (Me) Radius (Re) Gravity (g) Vescape Sheet1 Temp (K) VH2 VHe VO2 VN2 VCO2 Mercury Venus Earth Moon Mars Ceres Jupiter Saturn Titan Surface Gravity g GM r 2 Escape Velocity c 2GM v r 7 / 39

8 Example 1 - Mercury Planet Mass (Me) Radius (Re) Gravity (g) Vescape Sheet1 Temp (K) VH2 VHe VO2 VN2 VCO2 Mercury Venus Earth Moon Mars Ceres Jupiter Saturn Titan Mercury s gravity is not enough to hold any gases, so it is an airless body. 8 / 39

9 Example 2 - Earth Planet Mass (Me) Radius (Re) Gravity (g) Vescape Sheet1 Temp (K) VH2 VHe VO2 VN2 VCO2 Mercury Venus Earth Moon Mars Ceres Jupiter Saturn Titan The Earth can hold heavier gases, but not Hydrogen and Helium. 9 / 39

10 Example 3 - Moon Planet Mass (Me) Radius (Re) Gravity (g) Vescape Sheet1 Temp (K) VH2 VHe VO2 VN2 VCO2 Mercury Venus Earth Moon Mars Ceres Jupiter Saturn Titan The moon s gravity is not enough to hold any gases, so it is an airless body. 10 / 39

11 Example 4 - Mars Planet Mass (Me) Radius (Re) Gravity (g) Vescape Sheet1 Temp (K) VH2 VHe VO2 VN2 VCO2 Mercury Venus Earth Moon Mars Ceres Jupiter Saturn Titan Mars just manages to hold on to some heavier gases. 11 / 39

12 Example 5 - Jupiter Planet Mass (Me) Radius (Re) Gravity (g) Vescape Sheet1 Temp (K) VH2 VHe VO2 VN2 VCO2 Mercury Venus Earth Moon Mars Ceres Jupiter Saturn Titan Jupiter s gravity is enough to hold all gases. 12 / 39

13 Example 6 - Titan Planet Mass (Me) Radius (Re) Gravity (g) Vescape Sheet1 Temp (K) VH2 VHe VO2 VN2 VCO2 Mercury Venus Earth Moon Mars Ceres Jupiter Saturn Titan Even though Titan s gravity is weak, it has an atmosphere because it is so cold. 13 / 39

14 Mass vs Radius Relation In 1995, only the Solar System planets were known. 14 / 39

15 Mass vs Radius Relation By 2010, we had added many planets around other stars. 15 / 39

16 Mass vs Radius Relation - Composition Smaller planets (like Earth) are rocky. Larger planets (like Jupiter) are gas giants. Intermediate size (like Neptune) are often called ice giants. 16 / 39

17 Mercury Mass Mass Radius Radius Gravity SM axis Orbital Period Rotation Period Temperature kg MC 2440 km RC 0.38 g 0.39 AU 88.0 days 58.6 days -190C to +430 C 17 / 39

18 Venus Mass Mass Radius Radius Gravity SM axis Orbital Period Rotation Period Temperature kg 0.81 MC 6052 km 0.95 RC 0.90 g 0.72 AU days -243 days +460 C 18 / 39

19 The Surface of Venus Taken from the Soviet Venera 13 spacecraft in The spacecraft survived on the surface for 2 hours. 19 / 39

20 Colonizing Venus While Venus seems inhospitable at first glance, Geoffrey Landis of NASA has pointed out that at an altitude of 50 km, the temperature, pressure and gravity are all near Earth normal. Floating cities in the atmosphere of Venus have been proposed. 20 / 39

21 Earth Mass Mass Radius Radius Gravity SM axis Orbital Period Rotation Period Temperature kg 1.00 MC 6378 km 1.00 RC 1.00 g 1.00 AU days 23 h 56 m 15 C 21 / 39

22 Mars Mass kg Mass 0.11 MC Radius 3396 km Radius 0.53 RC Gravity 0.38 g SM axis 1.52 AU Orbital Period 1.88 years Rotation Period 24 h 37 m Temperature -60C 22 / 39

23 The Surface of Mars Taken from the Curiosity rover. 23 / 39

24 Mars Atmosphere The atmospheric pressure on Mars is less than 1% of that of the Earth. 24 / 39

25 Colonizing Mars Artist s conception of proposed Mars One colony. 25 / 39

26 Rocky Planets Compared Mercury, Venus, Earth, Moon, Mars, Ceres 26 / 39

27 Rocky Planet Interiors Possible structure of the Terrestrial Planets. 27 / 39

28 Terrestrial Planet Atmospheres Life has dramatically changed the composition of Earth s atmosphere. 28 / 39

29 The Inner Solar System 29 / 39

30 Jupiter Mass kg Mass MC Radius 71,492 km Radius 11.2 RC Gravity 2.53 g SM axis 5.2 AU Orbital Period 11.9 years Rotation Period 9 h 55 m Temperature -100C 30 / 39

31 Jupiter s Great Red Spot This is an immense storm similar to terrestrial hurricanes. 31 / 39

32 Saturn Mass kg Mass 95.1 MC Radius 60,268 km Radius 9.45 RC Gravity 1.06 g SM axis 9.58 AU Orbital Period 29.5 years Rotation Period 10 h 34 m Temperature -140C 32 / 39

33 Saturn from Behind 33 / 39

34 Uranus Mass kg Mass 14.5 MC Radius 25,559 km Radius 4.0 RC Gravity 0.89 g SM axis AU Orbital Period years Rotation Period 17 h 14 m Temperature -200C 34 / 39

35 Neptune Mass kg Mass 17.1 MC Radius 24,764 km Radius 3.88 RC Gravity 1.14 g SM axis 30.1 AU Orbital Period years Rotation Period 16 h 6 m Temperature -200C 35 / 39

36 Gas Giants Compared Jupiter, Saturn, Uranus, Neptune 36 / 39

37 Gas Giant Interiors Possible structure of the Gas Giants 37 / 39

38 The Outer Solar System 38 / 39

39 Summary 1 Planets (and stars) are in hydrostatic equilibrium, with outward pressure balancing the inward force of gravity. 2 The planets were formed from left-over material from a gas cloud which collapsed to form the sun. 3 Most of the matter in the universe is hydrogen and helium. 4 There are two types (alternatively three types) of planets in the solar system. Rocky planets, also called terrestrial planets. Gas giants. The gas giants, like Jupiter and Saturn, have enough gravity to hold hydrogen and helium, so they are much larger. Some astronomers distinguish Uranus and Neptune as a third type known as Ice Giants. 39 / 39