Mercury

Here is a web page with general information and data on Mercury:
http://nssdc.gsfc.nasa.gov/planetary/planets/mercurypage.html

People have been aware of Mercury for thousands of years. The planet is referred to in some of the oldest known texts, including The Epic of Gilgamish, but was likely recognized as a "wandering star" since prehistoric times. The Greeks were among the first to associate the planets with Gods. Because of its rapid motion, the planet Mercury was associated with the Greek messenger god Hermes.

Early Exploration and Discoveries

Mercury is a hard place to explore, but even before the space age important discoveries were being made. In 1639 Italian astronomer Giovanni Zupus discovered that Mercury has phases (the phases of Venus were discovered by Galileo in 1610). These observations strengthened the Copernican view of the Solar System.
 
About 300 years later Mercury provided a test of Einstein's theory of General Relativity. Since the 18th century astronomers had known that Mercury's orbit was not quite in agreement with Newton's laws (Mercury precesses faster than it should). However, Einstein's prediction of Mercury's motion matched observations exactly.

Despite this, because Mercury is so hard to observe, very little was known about Mercury's surface itself.

A. Schiaparelli (1889), B. Lowell (1896), C. Jarry-Desloges (1920)

Mercury in the Space Age

On November 3rd, 1973, NASA launched the Mariner 10 spacecraft.

Mariner 10

Mariner 10 was the first spacecraft to use a gravity-assist trajectory and thereby visit two planets in a single mission. It was also the first spacecraft to return to a planet it had visited. All previous missions consisted of a single flyby.

The Mariner 10 spacecraft was set up to encounter Mercury several times.  However, the Mariner 10 orbital period was set at 2 Mercury periods.  This meant that Mariner 10 could only see the same side of Mercury at each encounter.

Some of the early generalizations about Mercury's geology were based on incomplete evidence.  We now have full coverage from the Messenger orbiter.

Mercury's Orbit

Mercury rotates in a spin-orbit resonance of the type 3:2, which is unusual in the solar system.  This appears to be related to Mercury's high orbital eccentricity.  The resonance causes Mercury to "track" the Sun at periapse.  The "hot poles" of Mercury are the two antipodal sides of Mercury that alternately face the Sun at periapse.

Here is a movie clip demonstrating how Mercury would rotate if it were in a 1:1 resonance like the Moon:1:1 movie clip.
Here is a movie clip demonstrating how Mercury actually rotates: 3:2 movie clip.

Mercury's 3:2 resonance and its high eccentricity create a strange "day" on Mercury. At perihelion Mercury's orbital speed is faster than its rotational speed. An observer standing at 90 degrees longitude would see the Sun rise, hover in the sky, set over the same horizon, and then rise again.

Mercury's Interior Structure

How do we know?

(1) The mean density of Mercury (5.4 g/cm³) is about the same as the Earth's, although Mercury is much smaller (only 0.055 of the Earth's mass).  Because of this high mean density, researchers believe that Mercury has a large iron core:

(2) There is also geological evidence (scarps) on Mercury that suggests the presence of a core. Mercury shows evidence of crustal shrinkage, not seen on the Moon, which may be related to the cooling of its iron core.

The scarps are consistent with thrust faults, indicating that the entire planet has shrunk. The diameter of the planet today may be 1-2 km less than when the core was hot.

(3) Mercury has an intrinsic magnetic field, which may be generated by electrical currents in the core. The field is very similar to the Earth's magnetic field but only 1% of the strength.

Mercury's magnetic field is something of a mystery. Production of a magnetic field requires convection (fluid motion) within a liquid core. A planet as small as Mercury should have cooled to the point where such convection does not occur, and no magnetic field is present.  But other evidence also points to a liquid core.

It has been theorized that Mercury's magnetic field is produced by "remanent magnetization." When certain minerals (especially magnetite Fe₃O₄) are placed in a magentic field they become magnetized. If there are a lot of these minerals in the crust of Mercury they can produce a weak magnetic field. This type of field has been detected on Mars and the Moon. However, the production of a remanent magnetic field on Mercury would require a layer of highly magnetic minerals 30 km thick!  Very unlikely.

Mercury's Surface

For recent Messenger images of Mercury's surface check out:

http://photojournal.jpl.nasa.gov/index.html

and for maps, see

http://planetarynames.wr.usgs.gov/Page/mercuryQuadMap

Because of its position at 0.4 AU, Mercury's surface temperature can be extremely hot (though not the hottest: that honor goes to Venus). On the day-side of the planet, average temperatures can reach 427^o C. Surprisingly, the night-side temperatures drop to -183^o C. The reason for the large temperature swing is Mercury's lack of atmosphere.

The planet's surface superficially resembles the Moon's: ancient and heavily cratered.  However, no precise analogs to lunar maria are seen. The surface of Mercury is also less cratered than the Moon. These regions are termed intercrater plains, and are the dominant terrain type on Mercury

These plains units were most likely formed by volcanism that filled in many small craters. However, no volcanoes are seen on Mercury, suggesting that the magma must have been erupted from widely distributed fissures. The period of volcanism probably occurred 4.0 - 3.7 Ga ago.

We also see very large impact structures on Mercury. The largest is the Caloris basin, located near one of the hot poles. The ring structure of the Caloris basin was formed during one of the largest impacts to strike the surface. It is likely that there is a big mascon there, which could help to stabilize the 3:2 spin:orbit resonance.

Antipodal to the Caloris Basin, we see fractured terrain that may be related to the Caloris impact event:

 A big iron core could have focused the impact energy to the other side of Mercury. Computer simulations suggest that an impact of the size that formed Caloris could have caused vertical ground motions greater than 1 km, and tension fractures down to depths of 10 km. Such disruption would break the surface into a jumble of blocks and depressions and provide magma a chance to reach the surface.

Remote-sensing evidence for Mercury shows that much of its surface resembles the lunar highlands: refractory and volatile depleted.  But curiously, radar reflactions from its poles indicate that the bottoms of craters which never see the Sun may be filled with water ice.

http://www.nrao.edu/pr/2000/vla20/background/mercuryice/

In this image, red indicates strong reflection of the radar signal and yellow, green, and blue, progressively weaker reflection. The bright red dot at the top of the image indicates strong radar reflection at Mercury's north pole. In fact, it resembles the strong radar echo seen from the ice-rich polar caps of Mars.

Similar attempts to find ice at the poles of the Moon have not been totally successful, although a result from Lunar Prospector suggests that a small amount of dispersed ice may be present there.

Current Mercury Exploration

The MESSENGER mission, launched on 8/03/04, first encountered Mercury three times, and then in March 2011 it was inserted into a Mercury orbit.

http://messenger.jhuapl.edu/index.php

Questions addressed by the Messenger mission:
1. Why is Mercury so dense?
2. What is the geologic history of Mercury?
3. What is the structure of Mercury's core?
4. What is the nature of Mercury's magnetic field?
5. What are the unusual material's at Mercury's poles?
6. What volatile are important at Mercury?

The spacecraft is equipped with 2 cameras, 3 spectrometers, a magnetometer, a plasma detector, and a radio science experiment

Messenger's Instrument Package

Animation