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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 • 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 sun day 88 day 59 sun day 117 sun sun day 147 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. Albedo • 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. 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.