Mercury Astronomy 311 Professor Lee Carkner Lecture 10

Which of the following things was Mercury the Roman god of? a)War b)Love c)Agriculture d)Commerce e)Death

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

Why does Mercury have 3-2 spin orbit coupling instead of 1-1? a)Because of the Caloris impact b)Due to resonance with Venus c)Because of its large iron core d)Because it is not quite close enough to the sun e)Because of the relatively high eccentricity of its orbit

Why do we think there might be water ice at Mercury’s poles? a)Radar reflects strongly off of polar regions b)We see the spectroscopic signature of hydrogen c)The Caloris impactor should have lowered temperatures at the poles enough for ice to form d)There is evidence of comet impacts at the poles e)The slow rotation of Mercury is ideal for ice formation

Mercury -- Messenger of the Gods  Closest planet to the Sun, shortest orbital period  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

Mercury From Earth

Solar Transit of Mercury

Viewing Mercury From Earth  View Mercury with the naked eye in the morning or evening  Mercury is always near the Sun  Daytime often best for telescope views  Very hard to see any surface features  Mercury shows phases like Venus  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  smallest planet  Orbit: ~0.4 AU  smallest orbit, smallest period  Description: small, airless, cratered

Mariner 10  The first spacecraft to explore Mercury  Launched by NASA in 1973  Visited both Venus and Mercury  Did not image all of Mercury (only about 45%)

Mariner 10

Mercury from Mariner 10

Rayed Craters

Scarp

Surface of Mercury  Mercury has very little atmosphere (trace amounts of material blasted off the surface by the solar wind)  Heavily cratered  Newer craters on top of older  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

VLA Image of Mercury

Caloris Impact Basin

Opposite Caloris

Impacts  Solar system objects interact by direct contact as well as gravitationally  Impacts have been very common during the history of the solar system  Look at the Moon  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?  Kinetic energy (KE) is released  KE = 1/2 mv 2  m is the mass of the impactor (in kg)  v is the velocity (in m/s)  KE is in joules (j)  a Watt is a joule per second

Caloris Basin  Large (1300 km) crater  Caused by very large impactor  The impact sent a shock wave through the planet upthrusting hills on the other side  Not many craters on top of it, so it must be fairly young

Geological Map of Caloris Region

Spin-Orbit Coupling  The Sun’s gravity produces a tidal bulge, elongating Mercury  The Sun pulls on this bulge slowing down the rotation of the planet (the spin)  The spin wants to slow down to match the period of the orbit  Mercury has 3 to 2 spin orbit coupling  due to eccentric orbit, rotation and revolution periods are not quite equal  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 takes 59 days to complete one rotation  Mercury has long days and long nights  Sun is above or below horizon for about 3 earth months  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  It also is small, so the high density is not due to gravitational compression  Why is density so high?  Answer: Large iron core  Possibly caused by a large impact stripping off much of the lighter outer layers leaving a disproportionately large core  Mercury has a magnetic field, so core must be molten to generate it via the dynamo effect  Why is core molten?  Mercury is small, so core should have cooled

Inside Mercury

MESSENGER

MESSENGER Testing

Mercury from MESSENGER

Caloris from MESSENGER

Volcano on Mercury?

Future Mercury Exploration  The MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) mission  Launched August 3, 2004  Will only start orbiting Mercury in 2011  Flew by in January 2008 and took images  Orbits Mercury for 1 year and will make detailed maps of the surface  Will also explore atmosphere and magnetic field and look for ice at poles

Why does Mercury have 3-2 spin orbit coupling instead of 1-1? a)Because of the Caloris impact b)Due to resonance with Venus c)Because of its large iron core d)Because it is not quite close enough to the sun e)Because of the relatively high eccentricity of its orbit

Why do we think there might be water ice at Mercury’s poles? a)Radar reflects strongly off of polar regions b)We see the spectroscopic signature of hydrogen c)The Caloris impactor should have lowered temperatures at the poles enough for ice to form d)There is evidence of comet impacts at the poles e)The slow rotation of Mercury is ideal for ice formation

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