118 Encyclopedia of the Solar System
FIGURE 1 Shaded relief map of Mercury showing the quadrangle names and major features. About 55% of the planet is unknown.prominent feature viewed byMariner 10is named the
Caloris Basin(Basin of Heat) because it nearly coincides
with one of the “hot poles” of Mercury.
1. General Characteristics
Mercury’s diameter is only 4878 km, but it has a relatively
large mass of 3. 301 × 1023 kg. Because of its large mass in
relation to its volume, Mercury has an exceptionally high
mean density of 5440 kg/m^3 , second only to the density of
the Earth (5520 kg/m^3 ). The manner in which it reflects light
(its photometric properties) is very similar to the way light
is reflected by the Moon. The brightness (albedo) of certain
terrains is greater than comparable terrains on the Moon.
Mercury is covered with aregolithconsisting of fragmen-
tal material derived from the impact of meteoroids over
billions of years. Mercury’s surface is heavily cratered with
smooth plainsthat fill and surround large impact basins.
Longlobate scarpstraverse the surface for hundreds of
kilometers, and large expanses ofintercrater plains(the
most extensive terrain type) fill regions between clusters of
craters in the highlands. Also a peculiar terrain consisting of
a jumble of large blocks and linear troughs occurs antipodal
to the Caloris Basin.2. Motion and Temperature
Mercury has the most eccentric (0.205) and inclined (7◦)
orbit of any planet. However, over periods of a few million
years, its eccentricity may vary from about 0.1 to 0.28 and its
inclination from about 0◦to 11◦. Its average distance from
the Sun is 0.3871 AU (5.79× 107 km). Because of its large
eccentricity, however, the distance varies from 0.3075 AU
(4.6× 107 km) at perihelion to 0.4667 AU (6.98× 107 km)