[2] The planet Mercury played an important role in the religious life of many ancient civilizations. Although Mercury was probably seen by prehistoric man, the first recorded observation was by Timocharis in 265 B.C. The early Greeks believed that the east and west elongations of Mercury represented two separate objects which they called Hermes (evening star) and Apollo (morning star). When later Greeks recognized that Mercury was one object, they designated it Hermes, the messenger of the gods and god of twilight and dawn who announced the rising of Zeus. The ancient Egyptians, however, first discovered that Mercury (called by them Sabkou) orbited the Sun. To the Teutonic peoples Mercury was known as Woden, and our anglicized version of the midweek day Wednesday is derived from the original Woden's Day. The present name Mercury is derived directly from the Latin name Mercurius, which is the Roman designation for the Greek name Hermes. 1, 2
The Italian astronomer Zupus first observed the phases of Mercury in 1639. They were later observed independently by Hevelius in 1644. The transit of the Sun by Mercury, first predicted by Kepler in 1630. was observed by Gassendi. and the first recorded observations of surface markings were by Schroter and Harding in 1800. In the same year, Schroter incorrectly measured a rotation period of 24 hours with a rotation axis inclined 70° to the orbital plane. Another incorrect rotation period of 88 days determined by Schiaparelli 1, 2 - 80 years later was not corrected until the advent of recent radar observations, which in turn were confirmed by measurements made by Mariner 10. 3, 4
MERCURY
Of all the planets in the solar system. Mercury is closest to the Sun ( Figure 1). Because it is never more than 28 angular degrees from the Sun as viewed from the Earth, telescopic observations must be made during daytime or at twilight through a long path length of the Earth's atmosphere. As a consequence. telescopic observations are poor compared with those of most other planets.
Mercury is the smallest terrestrial planet, with a diameter of 4878 km ( Figure 2). In size it lies between the Moon and Mars. Its orbit has greater eccentricity (0.205) and inclination to the ecliptic plane (7°) than any other planet except Pluto. This pronounced eccentricity causes the apparent solar intensity at Mercury to vary by more than a factor of two throughout a Mercurian year. Table 1 lists the best current values of the more important orbital and physical properties of the planet.
Table 1. Orbital and Physical Data for Mercury.
.
Semimajor axis
0.3871 AU (5.79 x
107 km)
Perihelion distance
0.3075 AU (4.60 x
107 km)
Aphelion distance
0.4667 AU (6.98 x
107 km)
Sidereal period
87.97 days
Synodic period
115.88 days
Orbital eccentricity
0.20563
Inclination of orbit to
ecliptic
7.004 deg
Mean orbital velocity
47.87 km/s
Rotational period
58.646 days
.
.
Radius
2439 km
Surface area
7.475 x
107 km2
Volume
6.077 x
1010 km3
Mass
3.302 x
1026 g
Mean density
5.44 g/cm
Surface gravity
370 cm/s2
Escape velocity
4.25 km/s
Surface temperature extremes
~100 to 700° K (-173
to 427°C)
Normal albedo
0.125
Magnetic dipole moment
4.8 (± 0.5) x
1022 gauss cm3
[3] The best Earth-based and Mariner 10 measurements indicate that the rotation period (58.64 days) is in twothirds resonance with the orbital period (87.97 days), as shown schematically in Figure 3. Therefore, at Mercury's equator, longitudes 0° and 180° are subsolar points near alternate perihelion passages and are called "hot poles," whereas equatorial longitudes 90° and 270° are subsolar points near alternate aphelion passages and are called 'warm poles" because they receive less solar energy per "day" on Mercury (175 terrestrial days) than do the "hot poles:' The equatorial temperatures vary from about 100°K at local midnight to 700°K at local noon at perihelion, or a range of 600°K during a Mercurian "day:' This temperature range is greater than that of any other planet or satellite in the solar system.
In the past, Earth-based observations at visible, infrared, and microwave wavelengths led most observers to conclude that the Mercurian atmosphere was, at best, tenuous, with a total pressure < 0.1 mb. Mariner 10's ultraviolet spectroscopy and radio science experiments confirmed this inference, but extended the upper limit estimates downward by seven orders of magnitude to 10-12 bar. A very thin (10-15 bar) helium atmosphere was detected, and the question of its origin is now under discussion.5 The natural decay of uranium and thorium in crustal rocks may have resulted in the generation of the helium, or it may have accreted from the solar wind. If the observed helium is internally generated, then a crustal thickness can be estimated.
Before the Mariner 10 mission, it was generally believed that, because of Mercury's slow rotation and presumed interaction with the solar wind, its magnetic field would be similar to that of the Moon. One of the most important discoveries made by Mariner 10 on its first encounter with Mercury was the existence of a planet-related magnetic field, as indicated by the detection of a bow shock and magnetosphere together with accelerated protons and electrons in the interaction region. The first encounter data did not give a unique answer on the origin of the magnetic field, i.e., whether it was internally generated or induced by a complex interaction with the solar wind. However, Mariner 10's third Mercury encounter provided strong evidence that the field is of internal origin.6 The magnetic field data obtained during the third encounter duplicated those predicted on the basis of an intrinsic field model. Furthermore, the correlative plasma data showed the Mercurian magnetosphere to be a scaled-down (1/30) replica of the Earth's.7 Therefore, Mercury has an intrinsic dipole magnetic field with a moment 4 x 10-4 that of the Earth's dipole moment. The maximum field intensity is 400 gammas, or 20 times larger than the interplanetary field at Mercury's distance from the Sun.6
[5] The precise mechanism for field generation remains unknown, as fossil magnetization and an active internal dynamo cannot be distinguished from the data. The magnetic field observations provide independent evidence that Mercury possesses a large, metal-rich core.
Probably the most anomalous property of Mercury is its high mean density of 5.44 g/cm3, which is comparable to that of the Earth (5.52 g/cm3). However, Mercury is only about one-third the size of the Earth; its uncompressed average density of 5.3 is considerably greater than that of the Earth (4.04). This indicates that Mercury is composed of 65 to 70 percent by weight of metal phase (probably iron), and only some 30 percent by weight of silicate phase. Therefore, Mercury apparently contains twice as much iron (in terms of percentage composition) as any other planet in the solar system. Measurements of the magnetic field and evidence of volcanism in the Mariner 10 photography suggest that Mercury is chemically differentiated.8 If this is correct and most of the iron is concentrated in a core, then the core volume is about 50 percent of the total volume, and its radius is about 70 to 80 percent of the radius of the planet.
As a consequence of Mercury's high mean density, its surface gravity (370 cm/s2) is virtually the same as that of Mars, although it is considerably smaller. The gravity scaling of surface processes is the same for both bodies.
The photometric, polarimetric, and thermal properties of Mercury derived from Earth-based measurements are very similar to those of the Moon and indicate a surface covered by a dark, porous, fine-grained particulate layer.9 The thermal properties of the Mercurian surface measured by the Mariner 10 infrared radiometer are also consistent with the presence of a lunar-like regolith of insulating silicate particles constituting at least the upper tens of centimeters. However, spatial variations in the thermophysical properties of this layer suggest large-scale regions of enhanced thermal conductivity which could be areas of more compacted soil, or areas in which boulders or outcroppings of rock are exposed. 10
The best Earth-based telescopic photographs of Mercury have a resolution of about 700 km. These photographic and visual observations show that the surface of Mercury consists of dark and light regions somewhat similar to the maria and highlands of the Moon seen at comparable resolution. Although radar altitude profiles and reflectivity maps in the equatorial regions suggested the presence of a cratered surface, it was not known before the Mariner 10 mission that the topography was similar to that of the Moon. 11 Most planetologists believed that Mercury would show a cratered surface, although the amount of cratering was in dispute. Some believed that the crater density would be much less than that on the Moon or Mars because of Mercury's great distance from the asteroid belt, whereas others believed it would show a crater density comparable to that of the Moon. Questions concerning the presence or absence of volcanism, the tectonic framework, and the surface history were unresolved.
Mariner 10 dispelled many mysteries about Mercury and exposed its surface to detailed studies previously possible only for the Moon and Mars. The best pictures of Mercury acquired by Mariner 10 have a resolution of 100 m, an improvement by a factor of about 7000 over Earth-based resolution. As demonstrated by the pictures contained in this Atlas, the tremendous increase in resolution has resulted in a quantum jump in man's knowledge of the planet.
