Mercury Astronomy 311 Professor Lee Carkner Lecture 10

Mercury -- Messenger of the Gods  Closest planet to the Sun, shortest orbital period   Mercury is hard to see since it always appears near the Sun

Mercury From Earth

Solar Transit of Mercury

Viewing Mercury From Earth   Mercury is always near the Sun  Daytime often best for telescope views   Mercury shows phases like Venus   When viewing Mercury, always be careful of the Sun

Mercury Facts  Size: 1/3 size of Earth   Orbit: ~0.4 AU   Description: small, airless, cratered

Mariner 10   Launched by NASA in 1973   Did not image all of Mercury (only about 45%)

Mariner 10

Surface of Mercury   Heavily cratered   Scarps or cliffs indicate that Mercury shrank as it cooled  Radar studies indicate that the poles are more reflective than the rest of the planet  Possibly due to water ice in crater shadows

Impacts   Impacts have been very common during the history of the solar system   Earth’s weather tends to erase craters  Most occurred during Heavy Bombardment Period, about 3.8 billion years ago

Kinetic Energy   Kinetic energy (KE) is released  KE = 1/2 mv 2   v is the velocity (in m/s)   a Watt is a joule per second

Caloris Basin   Caused by very large impactor   Not many craters on top of it, so it must be fairly young

Geological Map of Caloris Region

Spin-Orbit Coupling   The Sun pulls on this bulge slowing down the rotation of the planet (the spin)   Mercury has 3 to 2 spin orbit coupling  due to eccentric orbit, rotation and revolution periods are not quite equal   or 1.5 rotations per orbit

Mercury’s Spin-Orbit Coupling

Consequences of Spin Orbit Coupling   Mercury has long days and long nights   Day side gets hot, night side gets cold  Day ~700 K, night ~100 K

Day and Night on Mercury Sun 700 K (800 F) 100 K (-270 F) Noon Midnight (3 months later) Brrr Ouch!

Exosphere  Mercury has a very thin layer of gas called an e  Mercury is too small and too hot to hold a normal atmosphere   Sources:   Solar wind blasting molecules off surface   Water from surface ice?

Ions from Mercury’s Exosphere

Interior of Mercury  Mercury has a very high density   Why is density so high?   Possibly caused by a large impact stripping off much of the lighter outer layers leaving a disproportionately large core   Why is core molten?  Mercury is small, so core should have cooled

Inside Mercury

MESSENGER

MESSENGER Testing

Future Mercury Exploration  The MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission   Entered Mercury orbit March 2011   Goals  Detailed surface maps   Study magnetic field 

Next Time  Read Chapter 9

Summary  Mariner 10 and MESSENGER data reveals Mercury as a small, airless, heavily cratered world  Mercury exhibits 3 to 2 spin orbit coupling due to the Sun’s tidal force  1 1/2 rotations on axis per revolution around the Sun  Mercury has a high density, possibly due to large iron core