Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 31: Sunny Mercury

Key Ideas:

Mercury is the innermost planet and the smallest planet in the Solar System
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)
Importance of Giant Impacts

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 as 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; Smooth in the center, similar to lunar maria (indicates magma); Albedo of center is similar to rest of surface (unlike lunar maria)
Antipodes: Disrupted terrain where the seismic waves from the impact converged; May also be where ejecta converged

Mercury's Atmosphere

Mercury has virtually no atmosphere: Pressure is 10^-12 times that of Earth's atmosphere; H and He captured from the Solar Wind, held only temporarily; Atoms of Sodium and Calcium knocked out of rocks by energetic solar wind particles
Lost its primordial atmosphere: Too low mass for gravity to hold it; Too hot because close to the 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 soil is cold: 125 K (-234 F); High reflectivity of some craters suggest ice

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 (3.9 cm³), which is a larger planet; Almost as dense as the Earth, yet Mercury is not as compressed

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
Alternative idea is that the proto-Sun vaporized the original surface
The weak magnetic field is likely due to the dynamo effect: Circulation in the liquid, iron-rich core

Attraction of Satellites

The Hill Sphere is the region around a body where its gravity dominates over all others
The radius is
For Mercury, this is only 220,000 km

Planetary Impacts

Impacts between planets and asteroid-sized bodies have played an important role in determining the properties of the planets
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.