1_Grand Tour - Grand Tour of the Grand Tour of the Solar...

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Unformatted text preview: Grand Tour of the Grand Tour of the Solar System, Part 1 Discussion Questions Discussion Questions If the Moon was formed by an impact on Earth knocking some of the Earth’s mantle into orbit, why is its density ~40% less than the Earth’s? Why is Mercury’s density about the same as Earth’s What’s wrong with Pink Floyd’s album title “The Dark Side of the Moon?” What’s the chain of logic that is used to infer that there was a period of very frequent impacts on the Earth early in its history? Why is the night side of Mercury one of the coldest places in the solar system? What does the density of a planet tell you about its bulk composition? Inner Solar System Outer Solar System Planetary Sizes The Sun and nine Planets of the Solar System are shown here to scale in terms of their relative sizes, but not their distances. The Solar System at 1:1010 Scale The closest star system to the Sun, Alpha Centauri, is located 4.4 light years away. At the 1:1010 scale of the Smithsonian model, it would be located in San Francisco. Planetary Categories Planetary Categories The 9 8 major planets of the Solar System can be divided into 2 classes: Terrestrial Planets Mercury, Venus, Earth, Mars "Terrestrial" or Earthlike worlds Jovian Planets Jupiter, Saturn, Uranus Neptune "Jovian" (Jupiter­like) or Giant planets Pluto is now known as a “Dwarf Planet”. Large moons, asteroids, and comets are in classes of their own. Fundamental Differences Fundamental Differences Property Terrestrial Planets Jovian Planets Size & Mass Small Large Density High Low Composition Rock + Metal Gas Atmosphere Thin Thick Oxygen­rich Hydrogen­rich Spin Slow Fast Moons Few Many Rings No Yes Surface Temp. Higher Very Cold Internal Heat Little/None Lots Mass and Density Mass and Density Where’s mass in Solar System? Object % Total Mass Sun 99.80 Jupiter 0.10 All Comets 0.05(?) All Other Planets 0.04 Moons and Rings 0.00005 All Asteroids 0.000002 Cosmic Dust 0.0000001 Density ρ = Mass / Volume Volume V = 4/3 (π R3) where R is the planet’s radius. Clues to planet's composition: Examples: Water: ρ = 1 g/cm3 = 1 g/cm Rock: ρ ≈ 2.5 g/cm3 2.5 g/cm Metal: ρ ≈ 8­10 g/cm3 8­10 g/cm Earth: ρ ≈ 5.5 g/cm3: implies rock+metal interior 5.5 g/cm All the inner planets have high densities: 3.9­5.5 g/cm3 The Moon is somewhat less dense: 3.3 g/cm3 Gas Giants much less dense: 0.7­1.6 g/cm3 Terrestrial Planets Terrestrial Planets Mercury Venus Earth Mars D = 0.39 AU D = 0.72 D = 1.0 D = 1.5 ρ = 5.4 g/cm3 ρ = 5.3 ρ = 5.5 ρ = 3.9 T = 700 K P = 0.24 years T = 740 P = 0.62 T = 290 P = 1.0 T = 215 P = 1.88 Jovian Planets Jovian Planets Jupiter Saturn Uranus Neptune D = 5.2 AU D = 9.5 D = 19 D = 30 ρ = 1.3 g/cm3 ρ = 0.7 ρ = 1.2 ρ = 1.6 T = 125 K P = 12 years T = 95 P = 30 T = 60 P = 84 T = 60 P = 165 The Moon The Moon: The View from Earth The Moon: The View from Earth The Moon is in synchronous rotation: it always shows the same side to the Earth: its nearside. Because the Moon’s orbit is elliptical, the Earth-Moon distance varies over time. At apogee, the Moon is farthest from Earth. At perigee, the Moon is closest to Earth. The apparent size of the Moon can change by as much as shown below… Apogee Fig. credit: http://www.fourmilab.ch/earthview/moon_ap_per.html Perigee The Moon: Basic Properties The Moon: Basic Properties Value Comments Radius 1,738 km 0.27 REarth Mass 7.35x1022 kg 1/81 MEarth Density 3.3 g/cm3 < ρEarth ~ 5.5 g/cm3 Composition Rocks, metals Gravity 1.62 m/s2 0.17 gEarth Distance from Earth 384,000 km ~ 357,000 – 407,000 km range Orbit around Earth 27.32 days Sidereal period of revolution Rotation (or Spin) 29.53 days “Synchronous rotation” Surface Temperature 380 K / 120 K Day / Night range The Moon: Highlands vs Maria The Moon: Highlands The Moon presents two main types of surface terrains: Highlands and Maria. Highlands: light-colored, heavily cratered, topographically high, and very rugged. Maria (plural of mare): dark-colored, sparsely cratered, low, and relatively smooth. The maria actually cover only 16% of the lunar surface. Most are found on the nearside. The view of the Moon at left is NOT the one we typically see from the Earth; it was taken from spacecraft. The Moon: Highlands The Moon: Highlands Highlands are areas on the Moon where its impact cratering record has been best preserved. They are the oldest terrain on the Moon. Rock samples from Highlands are 3.8 - 4.4 GY old. Much of the nearside and almost all of the farside of the Moon are made up of Highlands. Highlands are composed mainly of anorthosite (silicate rich in plagioclase feldspar). The Moon: Maria The Moon: The Maria are dark-colored areas on the Moon where the impact cratering record is sparser. They are younger terrain than the Highlands. Samples of Maria are ~ 3.3 - 3.8 GY old. Maria are concentrated on the nearside, although some occur on the farside. Maria are composed mainly of basalt. The Moon: Craters The Moon: Craters • The lunar surface presents many craters. • Originally thought to be of volcanic origin, they became widely accepted to be of impact origin instead only ~ 45 years ago. The debate was finally settled when lunar rock samples were brought back. • Terrain on the Moon with higher crater concentrations (Highlands) yield rock samples ~3.8 - 4.4 GY in age. Terrain with lower crater densities (Maria) yield rock samples ~3.3 - 3.8 GY old. • The big difference in impact crater densities between Highlands and Maria compared to their relatively modest difference in age tells us that impacting rates dropped drastically between 4.4 GY ago and 3.8. GY ago. Conclusion: there was an initial stage of Heavy Bombardment after which meteorite impact rates dropped quickly to ~ present levels. • This Heavy Bombardment must have been experienced by all bodies in the Solar System. The major role of impacts in the formation and subsequent evolution of all planetary bodies is one of the key lessons learned from the study and exploration of the Moon. The Moon: Landed Missions Surveyor (USA) Apollo (USA) Luna (USSR) The Moon: Surface The Moon: Surface Lunar surface is uniformly gray, mantled by thick dust, and has many boulders. Topography is relatively soft and smooth: no jagged peaks; rounded hills instead. Distances are difficult to estimate because of lack of atmosphere and references. The Moon: Lunar Soil The Moon: Lunar Soil Lunar soil includes very fine-grained material and also coarser chunks of rocks. Over time, micrometeorite and meteorite impacts comminute (break down and pulverize) surface materials, producing a surface rubble layer called regolith. The regolith contains impact-welded breccia rocks and glassy agglutinates. As “weathering” is extremely slow, footprints on the Moon may last >>10 6 years. The Moon: Lunar Rocks The Moon: Lunar Rocks Almost all lunar rocks brought back are breccias: rocks formed by the welding together of many other rock fragments, in general under the effect of impacts. A given breccia may contain bits and pieces that originated from different areas on the Moon. Highland rocks are mainly anorthosites, igneous rocks made up of plagioclase feldspar (lightweight aluminum-rich silicates). Highland rocks formed early in the Moon’s history from the crystallization of material that floated up to the top of an initial global magma ocean. Apollo 17 breccia 79135 Maria rocks are mainly basalts, igneous rocks made up of relatively heavierweight ferro-magnesian minerals. Mare basalts were emplaced at the surface of the Moon via very fluid volcanic eruptions that took place hundreds of million years after the anorthosite crustal rocks crystallized. Lunar Ice Lunar Ice Radar, neutron remote sensing, and infrared spectroscopic measurements all consistently point to the existence of water ice in permanently­ shadowed regions near the Moon’s poles. The Moon: Interior The Moon: Interior The Moon’s interior is poorly known but based on gravity data from robotic orbiters, Apollo seismic data, and theoretical models, it is likely differentiated. The lunar crust is asymmetric in thickness: thin on nearside (~20-60 km), thicker on farside (>~100 km). The Moon probably has a huge mantle made of rocks roughly similar to Earth’s mantle rocks. The Moon might have a very small iron core. A large iron core is not possible because the Moon’s average density is low: ~3.3 g/cm3. Also, the absence of any active magnetic field indicates that most of the iron in any core is probably no longer molten. Copyright 1999 Calvin J. Hamilton Possible Origin: Head­On Collision Possible Origin: Head­On Collision Outcome consistent with all data available: The Moon forms by reaccretion of mostly mantle material from both planets. Mercury Mercury Mercury: Closest to the Sun Mercury: Closest to the Sun Mercury: Basic Properties Mercury: Basic Properties Radius Value 2436 km Comments 0.38 REarth Mass 3.3x1023 kg 0.055 MEarth Density 5.4 g/cm3 < ρEarth ~ 5.5 g/cm3 Composition Rocks, metals Gravity 3.7 m/s2 0.38 gEarth (same as Mars) Distance from Sun 0.39 AU Orbital Period 58x106 km (range: 46­ 70) 88 Earth days Obliquity 0o Rotation (or Spin) 58.6 Earth days No seasons! 2/3 of orbital period exactly Atmosphere None (almost) Surface Temperature ­150oC to +425oC Moons None ~ 3 Earth months Less hot than Venus! Mercury: Mercury: Moon­Like Surface Little was known about Mercury’s surface from Earth-based visible telescopic observations (below). A lunar-like appearance was revealed when Mariner 10 imaged the planet in 1974 (right). Mercury’s surface is heavily cratered, much like the Lunar Highlands. Mercury: Mariner 10 Mercury: Mariner 10 Mercury has been visited by only 2 spacecraft: Mariner 10 (US) and Messenger (US). Mariner 10 made three fly-bys of Mercury from 1974 to 1975. ~ 45% of Mercury’s surface was imaged by Mariner 10. Mercury: Messenger Mission Mercury: Messenger Mission Messenger New NASA mission to Mercury currently underway. Launch: 2004 Arrival: 3 years ago for a flyby Orbiter will map Mercury for 1 Earth year starting March 18, 2011: Imaging, spectroscopy, altimetry, magnetic field studies. Spacecraft use solar power (of course!) Spacecraft also has a sunshade… Mission website: http://messenger.jhuapl.edu/ Mercury’s Unseen Side Mercury’s Unseen Side ~80% now imaged Mercury: Impact Craters Mercury: Impact Craters Because Mercury’s surface gravity is ~twice that of the Moon, impact craters tend to be shallower and ejecta blankets less extensive than on the Moon. Mercury also bears the scars of giant impacts, like the Caloris Basin (left). The formation of Caloris led to chaotic terrain on the opposite side of the planet (below)… Mercury: Volcanism Mercury: Volcanism Most of Mercury’s surface is saturated with craters, suggesting most terrains date back to the end of the Heavy Bombardment, i.e., ~3.8 BY old or more. In some areas, Mercury seems to have been resurfaced: Relatively dark, smooth areas with fewer impact craters are believed to be younger lava plains, not unlike the lunar maria although much less extensive. Mercury: Interior Mercury: Interior Mercury’s density is high considering the planet’s small size. Large dense core. Mercury has a (weak) magnetic field, which suggests this core is metallic, It is not known whether the magnetic field is still active or remnant; thus, it is not known whether the core is still molten. Resemblance between Mercury and the Moon is only skin deep. They are very different worlds. Mercury: Tectonics Mercury: Tectonics As Mercury’s mantle and huge core cooled and solidified, they shrank significantly in size. As a result, the planet’s crust shriveled and got crumpled: compressional tectonics. This did not happen as dramatically on the Moon because the amount of shrinking was not as great there: the Moon does not have a huge metallic core like Mercury does. Lobate scarps: Mercury: Mercury: Ice at the Poles? Radar studies from Earth show strong reflectivity at Mercury’s poles. Is it H2O ice trapped in permanently shadowed craters? Alternative: sulfur deposits? Discussion Questions Discussion Questions If the Moon was formed by an impact on Earth knocking some of the Earth’s mantle into orbit, why is its density ~40% less than the Earth’s? Why is Mercury’s density about the same as Earth’s What’s wrong with Pink Floyd’s album title “The Dark Side of the Moon?” What’s the chain of logic that is used to infer that there was a period of very frequent impacts on the Earth early in its history? Why is the night side of Mercury one of the coldest places in the solar system? What does the density of a planet tell you about its bulk composition? ...
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