Mercury (planet)

Mercury is the innermost and now the smallest of the eight planets in the solar system. Of the four terrestrial planets, it has proved the most difficult planet to observe and explore. Historical observations of its orbit around the sun have contributed to a new understanding of physics, and recent observations of its magnetic field might do the same.

Ancient knowledge and naming
The earliest records of Mercury are Sumerian records. The name Mercury is the name of the messenger of the classical gods, and also the swiftest among them. The traditional symbol for Mercury is a combination of the god's winged helmet and the caduceus, which was the traditional symbol of medicine.

Like Venus, Mercury appears in the early evening and the late morning. Also like Venus, ancient astronomers thought that Mercury was two separate objects, and the Greeks were the first to realize that Mercury was one object.

Orbital and rotational characteristics
Mercury is in a highly eccentric orbit around the sun at a distance that varies from 46,000,000 to 69,820,000 kilometers. Its sidereal period is roughly 88 days. Until 1962, astronomers thought that the solar day was also 88 days long. In 1965, however, Doppler radar observations clearly showed that Mercury rotates three times for every two years. This apparent 3:2 spin-orbital resonance, unique in the solar system, continues to intrigue astronomers.

Mercury's orbit is inclined 7.00° to the ecliptic and 3.38° to the equator of the Sun. Its axis of rotation is inclined 0.27°, more slightly than the axis of any other planet.

Orbital oddity
The perihelion of Mercury precesses by 5600 arc-seconds per century, which is 43 arc-seconds per century more than Newtonian physics alone would predict from the respective masses of Mercury, the Sun, and the other planets. This precession was well-known even in the nineteenth century. Astronomers of the period hypothesized either an asteroid belt or another planet (which they named Vulcan) inside Mercury's orbit. Albert Einstein proposed a second-order correction to Mercury's orbit, based on his general theory of relativity. The correction accounted exactly for the 43 arc-seconds of precession and obviated the need to look for any unseen asteroids or innermost planet.

Physical characteristics
Mercury is smaller than any other planet, and smaller even than Ganymede, moon of Jupiter, and Titan, moon of Saturn. Mercury has a density of 5427 kg/m³, second only to that of Earth. Mercury has a very dense core of iron with a radius of at least 1800 km, very close to the planet's overall radius of 2439.7 km.

Mercury has many impact craters, and thus by uniformitarian theories it is an "old" body.

Magnetosphere
Mariner 10 established that Mercury has a significant magnetic field, with about 1% of the flux density seen on earth. The magnetic dipole moment of Mercury as of 1975 was 4.8 * 1019 N-m/T. According to Russell Humphreys's model for the creation of planetary magnetic fields, Mercury should have had a magnetic dipole moment at creation of 7.5 * 1022 N-m/T. Thus Mercury's magnetic field has been decaying rapidly, and Humphreys fully expected the Messenger probe to measure a much lower magnetic dipole moment, on the order of 4.6 * 1019 N-m/T by 2011. In fact, Messenger surveyed the field on January 14, 2008, and NASA's published estimates indicate a magnetic dipole moment of 3.8 +/- 0.7 * 1019 N-m/T. The significance of this measurement for Humphreys' model, and the implications for uniformitarianism, are discussed further below.

Topography
Mercury has multiple craters, multi-ring basins, and areas of lava flow. The largest single feature on Mercury is the Caloris Basin, which measures 1300 km across and is surrounded by concentric rings of mountains.

Satellites
Mercury has no known moons.

Density
Mercury's density demands an iron core having a radius 75% of that of the planet itself. The nebula hypothesis of solar system formation would not predict this. To explain this discrepancy, planetary scientists now theorize that Mercury did form normally, but then a large object impacted Mercury and removed most of its mantle from it, leaving behind the core and the relatively thin mantle. Thus far, however, neither Mariner 10 nor Messenger has returned any image suggesting which face of Mercury suffered the impact.

The magnetic field
Scientists were perplexed to discover that Mercury had a magnetic field, and speculated that the planet's outer core consisted of liquid iron. However, because it is "geologically old," relatively small and hot, Mercury's outer core would long ago have expired. Thus, according to current scientific understandings, the existence of Mercury's magnetic field supports a young-earth position, not evolution. The problem of Mercury's magnetic field is more acute when one considers that Mars, which is slightly larger than Mercury and spins much faster, has a much weaker magnetic field.

Creationist Russell Humphreys has an alternative theory: that planetary magnetic fields do not form by dynamo action, and Mercury's relatively wide and conductive core has preserved its magnetic field. The cores of Mars and Venus are smaller, and thus the magnetic fields have decayed more rapidly. Nevertheless, the decay of Mercury's magnetic field, according to Humphreys, is quite rapid.

Accordingly, in 1984 Humphreys boldly declared:Mercury's decay rate is so rapid that some future probe could detect it fairly soon. In 1990 the planet's magnetic moment should be 1.8 percent smaller than its 1975 value.

In fact, according to Humphreys' extrapolations of the decay time and half-life of the magnetic field of Mercury, the magnetic dipole moment in 2008 should be four percent smaller than that measured in 1975 by Mariner 10. Specifically, this should be (4.5 +/- 0.25) * 1019 N-m/T.

On January 14, 2008, Messenger made rendezvous with Mercury and measured its magnetic field. This rocket probe's measurements are consistent with a magnetic dipole moment of (3.8 +/- 0.7) * 1019 N-m/T. This represents a significant decline from the Mariner 10 measurement, because the two measurements differ by more than the sum of their respective tolerances. More to the point, the higher tolerance range of the current measurement lies well within the predicted magnetic dipole moment that Mercury should have at present. If, on the other hand, the true measurement is less than predicted, that would pose an even greater problem for uniformitarian astrophysics, because such a rapid decay would be utterly inconsistent with an age for Mercury of 4.6 billion years. It would present no problem with the Humphreys model, but would suggest that the core conductivity and/or conductive mass of Mercury is less than Humphreys initially supposed.

Observation and exploration
Mercury is the most difficult terrestrial planet to observe directly. The planet’s greatest angular separation from the sun is a mere 28.3°. Thus one can observe it only in the morning or evening twilight. Galileo Galilei was unable to resolve Mercury sufficiently to detect its phases.

Only two rocket probes have visited Mercury thus far. The first probe to make rendezvous with Mercury was Visiting mission::Mariner 10, and it was able to map only 40%-45% of the surface. Mariner 10 also returned the first measurements of Mercury's magnetic field.

NASA recently launched a second probe, called Visiting mission::Messenger (Mercury Surface, Space Environment, Geochemistry, and Ranging), which has already made one close rendezvous with Mercury (January 14, 2008) and is scheduled to insert itself into orbit around Mercury in on March 18, 2011 after several more such rendezvous.

Related Links

 * NASA "Mercury"
 * Mercury by Wikipedia

Mercure (planète)