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Mercury. Mercury’s Orbit. Mercury has a short year. 88 Earth days = 1/4 Earth year 0.4 AU from the Sun This is predictable from Kepler’s third law. The period squared compared to the distance cubed Period (0.25) x (0.25) = 0.063 Distance from Sun (0.4) x (0.4) x (0.4) = 0.064. Tidal Lock.

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## Mercury

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**Mercury’s Orbit**• Mercury has a short year. • 88 Earth days = 1/4 Earth year • 0.4 AU from the Sun • This is predictable from Kepler’s third law. • The period squared compared to the distance cubed • Period (0.25) x (0.25) = 0.063 • Distance from Sun (0.4) x (0.4) x (0.4) = 0.064**Tidal Lock**• With the strong gravity from the nearby sun, Mercury should have tidal friction. • One side would always face the sun • Measurements with radio telescopes showed that Mercury rotates in 58.7 days. • There is a lock, but Mercury takes three rotations every two revolutions of the sun.**Mercury’s Day and Year**• The apparent day on Mercury is twice as long as the year. day 0 day 29 day 59 sun sun sun day 88 day 117 day 147 sun sun sun**Close Up**• In 1974-5 Mariner 10 flew by Mercury. • Measured surface • Still orbiting the Sun • In 2004 Messenger was launched. • Flyby in 2008 • Orbit since 2011**Solar Broil**• Mercury is the closest planet to the sun (0.4 AU). • Surface temperature reaches 700 K (870 ºF) • Poles at 100 K (-280 ºF) • There is water under the surface at the poles. • The atmosphere is mostly burned off. • Gravity too low • Traces of gas coming from the surface**Cratered Surface**• Mercury is a terrestrial planet - it has a rocky surface. • The surface is heavily cratered like the moon, due to impact in the early solar system. • There are signs of lava flows and tectonics.**Planets reflect the light of the sun.**The fraction of the light reflected compared to what falls is called albedo. Mercury and the Moon both have low albedo (0.06-0.07). The dark cratered surface does not reflect well. Albedo**Mini Magnet**• The mass of Mercury is 1/20 of Earth. • Almost equal density • The remaining rock is more iron rich than Earth. • Mercury has an iron core and weak magnetic field.

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