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.