Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 32: Sunny Mercury

Key Ideas:

Mercury is the innermost planet
Rotation is locked in a 3:2 tidal resonance
Surface:: Heavily cratered; Virtually no atmosphere
Interior: Large iron core and weak magnetic field; Weak tectonic activity (wrinkles as it cools)

3:2 Tidal Resonance

Mercury's rotation period is exactly 2/3 its orbital period
In a 3:2 Tidal Resonance with the Sun: Completes 3 rotations for every 2 orbits
Caused by its highly elliptical orbit: Tides are strongest at perihelion (closest to Sun); Rotation is the same if it were in a circular orbit at its perihelion distance and tidally locked

Surface of Mercury

First imaged by Mariner 10 in 1974 and 1975
Messenger launched in 2004 for 2008, 2009 flybys and 2011 orbit
Mercury is heavily cratered like the Moon: Surface is ~3.8-4 Gyr old like the lunar highlands (last epoch of heavy bombardment); Craters are flatter than lunar craters because Mercury's gravity is 2 times stronger
Terrain features:: Highlands and lava basins like the Moon; Caloris Basin: Huge ringed impact basin; Lobate scarps and jumbled terrain

Caloris Basin

Impact Basin: Large asteroid impact punched through crust; Left a ringed basin 1340 km across
Antipodes: Jumbled terrain where the seismic waves from the impact converged

Mercury's Atmosphere

Mercury has virtually no atmosphere: Pressure is 10^-12 times that of the Earth; H and He captured from the Solar Wind; Atoms of Sodium and Calcium knocked out of rocks by energetic solar wind particles
Lost its primordial atmosphere: Too small for gravity to hold onto it; Too hot because close to Sun

Surface Temperatures

Since Mercury has no atmosphere, there are extreme day/night temperature changes:: Daytime: 500 K (441 F); Nighttime: 100 K (-279 F); Some daytime locations are as hot as 600 K
Poles are in perpetual twilight: Axis has virtually no tilt; Polar soid is cold: 125 K (-234 F)

Mercury's Interior

Mercury is between the Moon and Mars in size: Thinner lithosphere than the Moon, but thicker than Earth or Mars
Lobate Scarps:: Thrust faulting (cliffs); Signs of tectonic disturbance, but NOT plate tectonics; The lithosphere wrinkles as the interior cools and contracts

Deep Interoir

Rocky mantle ~700 km thick
Unexpectedly large iron core: ~75% of the radius of Mercury!; Contains ~60% of the planet's mass
Revealed by:: Weak magnetic field: ~1% as strong as Earth's; High density: 5.43 g/cm³ (compare to Mars, which is more massive yet only has a density of 3.9 g/cm³)

Origin of the Core

Leading idea: head-on collision: Collider was smaller than Mercury; Impact blew off most of Mercury's mantle; Re-formed planet had a huge iron core left behind
The weak magnetic field is a mystery: Is it due to fluid motions in a semi-molten core? or is it remnant permanent magnetism?

Planetary Impacts

Impacts between planets and asteroid-sized bodies have played an important role in determining a planet's properties: In the case of Mercury, a large head-on impact is invoked to explain its unusually large iron core

Impacts are an essential part of the history of the Solar System

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.