9. Moon, Mercury, Venus

Transcription

1 9. Moon, Mercury, Venus

2 All the heavier elements were manufactured by stars later, either by thermonuclear fusion reactions deep in their interiors or by the violent explosions that mark the end of massive stars.

3 The planets formed by the accretion of planetesimals and the accumulation of gases in the solar nebula

4 Concept Question As the solar nebula contracts, it A) flattens out into the ecliptic plane around the Sun's poles. B) spins faster due to conservation of angular momentum. C) cools due to condensation. D) loses angular momentum.

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6 Concept Question In terms of composition, A) all planets are condensed from the same nebula and have similar compositions. B) the jovian planets are more like the Sun than are the terrestrials. C) the terrestrials are more like the Sun, since they formed close to it. D) the Sun is unique, made of nothing but hydrogen and helium.

7 The Moon Mass = 7.4 x kg = M earth Radius = 1738 km = 0.27 R earth Density: ρ= 3.3 g/cm 3 (Earth 5.5 g/cm 3 ) Gravity = 1/6 that of Earth

8 Tides A feature of oceans (but solid materials have small tides too). Two high and two low tides per day. Tides are due to the gravitational pull being stronger on side of Earth closest to it (Sun causes smaller tides) Earth-Moon gravity keeps them orbiting each other. But side of Earth closest to Moon has slightly stronger pull to Moon => bulges towards it. Other side has weaker pull => bulges away compared to rest of Earth. The Earth spins once a day while the bulge always points towards and away from the Moon => high and low tides.

9 Tides

10 Tides Spring Tide Neap Tide

11 Tidal Bulge

12 Tidal locking We always see the same face of the Moon. This means: period of orbit = period of spin Top view of Moon orbiting Earth Earth Why? The tidal bulge in the solid Moon elongates it slightly (2-3 km) along an axis pointing to Earth. If orbit period is faster than the spin period, tidal bulge would have to move around surface of Moon, creating friction, which slows the Moon s spin down until tidal bulge no longer migrates around.

13 Far versus Near side of the moon

14 The Lunar Surface Large, dark featureless areas: marias or seas. Lighter areas at higher elevation: highlands. Loads of craters (due mostly to meteorite impacts). No winds to erode them away. Highlands have 10 times the crater density of maria. marias highlands

15 Lunar Volcanism (long ago) Remember: volcanism is a way of losing internal heat Evidence: Maria: result of old, widespread lava flows (filled in largest, early impact craters). ditches indicating old lava flows. Linear chains of craters (not formed by impacts), probably marks ancient fault, collapsed lava domes.

16 Cratering Impact speeds several km/s Ejecta blanket of pulverized rock surrounds crated Impacts => regolith: ~20m thick layer of pulverized rock covering the Moon.

17 Cratering Rates Small meteoroids common, large ones rare. So same is true for craters: Crater Size Occurrence 10 km Every 10 million years 1 m Every month If no other processes (erosion, lava flows) change the surface, the number of craters in an area tells you the age of the surface.

18 Moon s History Age: 4.5 billion years 3.9 billion years ago: Heaviest meteoritic bombardment ended billion years ago: Volcanism created maria. Maria are just the largest crates, filled in. 3.2 billion years present: No volcanism, cratering continued at lower rate. Geologically dead!

19 Lunar Structure Inner Core and Outer Core take up small fraction of volume compared to Earth case the Moon is more rigid And no atmosphere, so no wind or erosion. Surface reflects geologic history well.

20 Concept Question When do the largest high tides occur? A. When the Moon is at first quarter B. When the Moon is full C. When the Earth is at aphelion (furthest from sun) D. When the Moon is at 3 rd quarter

21 Concept Question The surface gravity of the moon is 1/6 that of Earth. If Matt weights 120 lbs on Earth, how much does he weigh standing on the moon? A. 60 lbs B. 30 lbs C. 20 lbs D. 10 lbs

22 Concept Question Suppose the Moon was half as dense, but the same size. How much would Matt (120 lbs on Earth) weigh? A. 60 lbs B. 30 lbs C. 20 lbs D. 10 lbs

23 How did the Moon form? We re not quite sure! Three older theories: 1. Fission: The material that would be the Moon was thrown off the Earth and coalesced into a singled body. Problem: Earth not spinning fast enough to eject large amount of material. 2. Co-formation: The Moon and the Earth formed out of the same material at the beginning of the Solar System. Problem: Moon has different density and composition. 3. Capture: The Moon was a stray body captured into orbit around Earth. Problem: An extremely unlikely event, given the Moon s size is a substantial fraction of Earth s.

24 Now: Impact theory preferred Early in Solar System, when many large planetesimals were around, a Mars-sized object hit the forming Earth, ejecting material from the upper mantle which went into orbit around the Earth and coalesced to form the Moon. Computer simulations suggest this is plausible. Similar spin orientations. Moon samples indicate that the Moon's surface was once molten. The Moon has a relatively small iron core. Evidence exists of similar collisions in other star systems Giant collisions are consistent with the leading theories of the formation of the Solar System.

25 Mercury Mass = 3.43 x kg = M earth Radius = 2439 km = 0.38 R earth Density: ρ= 5.4 g/cm 3 (Earth 5.5 g/cm 3 ) Gravity = 0.38 that of Earth Semi-major axis = 0.39 AU Orbital Period = 88 days

26 Solar Transit Transits occur about twelve times a century when the sun, Earth and Mercury are aligned There was a transit on November 8, 2006

27 Best Earth-based Views of Mercury Difficulties observing Mercury from Earth led early astronomers to incorrectly decide that Mercury always kept the same face towards the sun in synchronous orbit Note phases like the moon

28 Radio telescope observations from sites such as Arecibo gave evidence of a non-synchronous orbit

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30 Discovery of Water Ice on Mercury Goldstone 70 m radar received by the VLA. Polar regions could be 125 K and never warmed by the Sun.

31 Mercury rotates slowly and has an unusual spin-orbiting coupling

32 Strong tidal effects, Mercury s slightly elongated shape and its very eccentric orbit cause this strange 3- to-2 orbit A day of solar light on Mercury would be 88 earth days

33 Mercury: Heavily cratered surface Less dense cratering than moon Gently rolling plains Scarps No evidence of tectonics

34 The Moon: Note how much more densely the craters occur on the moon s surface.

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36 The Caloris Basin is evidence of a large impact

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40 The magnetosphere blocks the solar wind from reaching the surface of the planet

41 Venus Mass = 0.82 M earth Radius = 0.95 R earth Density: ρ= 5.2 g/cm 3 (Earth 5.5 g/cm 3 ) Gravity = 0.90 that of Earth Average distance from Sun = 0.72 AU Orbital period = 225 days Rotation period = 243 days (longer than orbital period, and retrograde!)

42 Venus has a hot, dense atmosphere and corrosive cloud layers Spacecraft measurements reveal that 96.5% of the Venusian atmosphere is carbon dioxide The remaining balance of the atmosphere is nitrogen. Venus s clouds consist of droplets of concentrated sulfuric acid. The surface pressure on Venus is 90 atm, and the surface temperature is 460 C (860 o F) Both temperature and pressure decrease as altitude increases

43 The Greenhouse Effect Main greenhouse gases are H 2 O and CO 2.

44 Runaway Greenhouse Effect 1. Water and CO 2 evaporate from oceans and ejected from volcanos into atmosphere 2. Greenhouse effect more efficient 3. Temperature rises 4. More evaporation, less liquid water to absorb CO 2 (back to #1) => Complete evaporation of oceans. Thick atmosphere.

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47 Radar data (Pioneer Venus mission) reveal altitude variations on surface. Flatter than Earth, no evidence for plate boundaries => no large scale plate tectonics. But plenty of evidence of stresses and fractures on smaller scales => much small-scale shifting of crust.

48 The density of craters suggests that the entire surface of Venus is no more than a few hundred million years old. According to the equilibrium resurfacing hypothesis, this happens because old craters are erased by ongoing volcanic eruptions

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50 Concept Question The Moon was most likely formed: A. By fission as a large chunk of the Earth was thrown off B. By the same stuff that formed the Earth, coformation C. By an impact of a proto-planet with the proto- Earth D. Out of the proto-planetary nebula and later captured by the Earth

51 Concept Question We think Mercury could have ice at the poles because: A. Mercury is so far from the Sun B. Optical images show white polar caps C. Radar images show high reflectivity at the poles D. Mercury is in a 3:2 spin:resonance orbit around the Sun

52 Concept Question Why is Venus the hottest planet in the Solar System? A. It is the closest planet to the Sun B. There is a lot of radioactive material in the crust. C. There is a large concentration of CO 2 in the atmosphere D. There is no liquid water

53 9/25: reading assignment Chapter : Mars