Mercury Astronomy 311 Professor Lee Carkner Lecture 10

Why do we think the Earth’s core is made of iron? a)Iron meteorites are the most common b)Deep drilling rigs have brought up iron from the core c)The Earth’s bulk density is much higher than the density of rock d)The Moon is mostly made of iron and so the Earth should be too e)Spectroscopic analysis of volcanoes reveal iron

How does the Earth cool off if it gets too hot? a)Some of the hot upper atmosphere is lost into space b)Greenhouse gases react with CO 2 and are neutralized c)The ice caps melt, cooling the oceans d)The greenhouse gases will dissolve in water and rain out of the atmosphere e)It won’t, once the atmosphere gets hot it stays that way forever

Mercury -- Messenger of the Gods   Romans named it Mercury because it changes position in the sky so fast  Mercury is hard to see since it always appears near the Sun

Viewing Mercury From Earth  View Mercury with the naked eye in the morning or evening  Mercury is always near the Sun   Very hard to see any surface features   Mercury can sometimes be seen passing in front of the Sun, called a solar transit  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  The first spacecraft to explore Mercury   Visited both Venus and Mercury 

Mariner 10

Surface of Mercury  Mercury has very little atmosphere (trace amounts of material blasted off the surface by the solar wind)   Newer craters on top of older   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  What happens when two bodies collide?   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   3 rotations on axis for every 2 orbits  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!

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

Future Mercury Exploration  The MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission   Will only start orbiting Mercury in 2011   Orbits Mercury for 1 year and will make detailed maps of the surface 

Next Time  Read Chapter 9

Summary  Mariner 10 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