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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 33:
Mercury

Key Ideas:

Mercury is the innermost planet

Rotation is locked in a 3:2 resonance with the Sun

Surface:

Interior:


Mercury at a Glance

Orbital Data
Semi-Major Axis: a = 0.386 AU
Orbital Period: P = 87.969 days
Eccentricity: e = 0.206
Perihelion: 0.31 AU
Aphelion: 0.47 AU
Inclination: i = 7° (Most elliptical orbit of the planets)

Planetary Data
Radius: R = 2439 km (0.382 REarth)
Mass: M = 0.055 MEarth
Rotation Period: 58.65 days, exactly 2/3 of its orbital Period. Mercury's Day is 176d long as a consequence
Axis Tilt: 2°

Resonant Rotation

Mercury's rotation period is exactly 2/3 its orbital period.

Caused by its elliptical orbit:


Spacecraft Visits to Mercury

Only one spacecraft has ever visited Mercury. The US Mariner 10 spacecraft flew past Mercury 3 times in 1974 and 1975. The timing of the fly-bys was such that only 45% of the surface was in sunlight, so more than half the planet has never been imaged up close.

The NASA Discovery-class spacecraft Messenger was launched in August 2004. Using one Earth flyby in 2005 and two Venus flybys in 2006 and 2007 for gravity assistance, it has executed three Mercury flybys, twice in 2008 and once in 2009, timed so as to be able to map the entire surface of the planet. In March 2011 it will go into orbit around Mercury and begin an extended close-up study mission.

For more info, see the Messenger website.

A future ESA/Japanese spacecraft, BepiColumbo, is currently in the planning stages, approved for a 2008-2013 mission window.


Surface of Mercury

Mercury is heavily cratered, like the Moon.

Terrain Features:


Caloris Basin

Impact Basin: Made by a large asteroid impact.

Antipodes: (opposite side of Mercury from the Caloris Basin)


Mercury's Atmosphere

Mercury has virtually no atmosphere:

Lost its primordial atmosphere:


Surface Temperatures

Since Mercury has virtually no atmosphere, there are extreme Day/Night temperatures:

Poles are in perpetual twilight:


Mercury's Interior

Mercury is intermediate in size between the Moon and Mars. We expect:

See lobate scarps, signs of tectonic disturbance (but NOT plate tectonics):


Deep Interior

Rocky mantle is about 700 km thick.

Unexpectedly large iron core:

Revealed by:


How did Mercury get such a huge core?

One idea is that during formation Mercury collided nearly head-on with a large body.

The weak magnetic field is due to a molten core:


Planetary Impacts

Once again, we see how impacts between planets and asteroid-sized bodies can play an important role in determining a planet's properties.

Impacts are an essential element of the history of the Solar System.


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