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ASTRONOMY 161 Introduction to Solar System Astronomy Class 18 Mercury Friday, February 23 Mercury: Basic characteristics Mass = 3.302×1023 kg (0.055 Earth) Radius = 2,440 km (0.383 Earth) Density = 5,427 kg/m³ Sidereal rotation period = 58.6462 d Albedo = 0.11 (Earth = 0.39) Average distance from Sun = 0.387 A.U. Mercury: Key Concepts (1) Mercury has a 3-to-2 spin-orbit coupling (not synchronous rotation). (2) Mercury has no permanent atmosphere because it is too hot. (3) Like the Moon, Mercury has cratered highlands and smooth plains. (4) Mercury has an extremely large iron-rich core. (1) Mercury has a 3-to-2 spin-orbit coupling (not synchronous rotation). Mercury is hard to observe from the Earth (because it is so close to the Sun). Its rotation speed can be found from Doppler shift of radar signals. Mercury’s unusual orbit Orbital period = 87.969 days Rotation period = 58.646 days = (2/3) x 87.969 days Mercury is NOT in synchronous rotation (1 rotation per orbit). Instead, it has 3-to-2 spin-orbit coupling (3 rotations for 2 orbits). Synchronous rotation (WRONG!) 3-to-2 spin-orbit coupling (RIGHT!) Time between one noon and the next is 176 days. Sun is above the horizon for 88 days at the time. Daytime temperatures reach as high as: 700 Kelvin (800 degrees F). Nighttime temperatures drops as low as: 100 Kelvin (-270 degrees F). (2) Mercury has no permanent atmosphere because it is too hot (and has low escape speed). Temperature is a measure of the 3kT v = random speed of m atoms (or v typical speed of atom molecules). = At a given temperature, T = temperature (Kelvin) massive atoms move m = mass ot atom (molecule) more slowly than k = a constant of nature low-mass atoms. HOT/COLD If an atom’s speed is greater than the escape speed, it will fly away on a hyperbolic orbit. Low-mass atoms escape first, then “sluggish”, massive ones. 2GM v = esc r v = escape speed M = mass of planet r = radius of planet G = universal constant of gravitation Daytime on Earth: T = 300 Kelvin, vesc = 11.2 km/sec Hydrogen and helium escape. Daytime on Mercury: T = 700 Kelvin, vesc = 4.3 km/sec All gases escape. Daytime on the Moon: T = 260 Kelvin (no greenhouse effect) vesc = 2.4 km/sec Because vesc is low, all gases escape, despite low T. Only one spacecraft has visited Mercury: Mariner 10 (a fly-by). It has only imaged 45% of Mercury’s surface. (3) Like the Moon, Mercury has cratered highlands and smooth, lava-covered plains. The hemisphere of Mercury that has been photographed resembles the Moon: (Crater walls are slightly lower.) Highlands are heavily cratered. Low-lying impact basins, such as the Caloris basin, are covered with smoother lava flows. Unique mercurian feature: Opposite the Caloris basin is “jumbled terrain”, filled with closely spaced hills. “Jumbled terrain” was created when seismic waves from the Caloris impact converged on the antipodes. (Planet acts as a giant “lens”). Another strange mercurian feature: Mercury has scarps, or cliffs, as much as 3 km high, 500 km long: They are probably the result of cracking as the crust cooled. (4) Mercury has an extremely large iron-rich core. Mercury is exceptionally dense: 5400 km/m3. The Earth is slightly denser, but only because it is compressed by its own strong gravity. Mercury must have a huge dense core (probably mostly iron, like Earth’s). Radius of Mercury = 2400 km. Radius of iron core = 1800 km. Why does Mercury have a big metal core? • Perhaps the solar nebula was so hot that only metals could condense easily. • Perhaps an intense solar wind blew away most of Mercury’s rocky mantle. • Perhaps an impact with a giant planetesimal stripped the mantle. We still have much to learn about Mercury Head-on impact of Mercury with a giant planetesimal: Most of mantle attains escape speed. Return to Mercury: The MESSENGER spacecraft (MErcury Surface, Space ENviroment, GEochemistry and Ranging) Launched 2004: Mercury orbit in 2011 Scientific mission of MESSENGER 1) Why is Mercury so dense? 2) What is the geologic history of Mercury? 3) What is the structure of Mercury's core? 4) What is the nature of Mercury's magnetic field? 5) What are the unusual materials at Mercury's poles? 6) What volatiles are important at Mercury? Few closing questions: 1) Name the major differences and similarities between Mercury and the Moon. 2) Name the major differences and similarities between Mercury and the Earth. 3) How dense is the atmosphere of Mercury? 4) Is there magnetic field on Mercury? 5) What is the name of Mercury’s moon? 6) Are there volcanoes on Mercury?.
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