Mercury (pic 1)

Mercury (pic 2)

Hills on Mercury

MERCURY (PLANET)

Mercury, the planet closest to the Sun, is a cratered world.
It is the smallest of the inner planets, probably because the
heat of the nearby Sun as Mercury formed, about 4.6 billion
years ago, prevented most of the gases present in the vicinity
from becoming part of the protoplanet.  Mercury's surface is
very hot, sometimes reaching extremes of more than 470 deg C
(more than 1,380 deg F)--especially at two "hot spots" opposite 
one another on the equator.  The heat and the planet's low
gravity make it impossible for Mercury to retain any
significant atmosphere. Trace amounts of hydrogen, helium, and
oxygen above the surface probably derive from the solar wind,
while similar traces of sodium and potassium atoms may
represent gases diffusing up through the planet's crust.
 
Astronomical Data
 
Mercury orbits the Sun once every 88 days at distances varying
from 70 million to 46 million km (43 million to 29 million mi). 
Because of the great difficulty in observing this small and
distant planet, which never appears more than 28 deg from the
Sun in the sky, it was thought as late as the early 1960s that
Mercury also rotated with an 88-day period, so that one
hemisphere always faced the Sun.  Radar observations have since 
shown, though, that the true rotation period is 58.6 days.
Mercury rotates three times for every two trips around the Sun, 
so that during every alternate perihelion (closest approach to
the Sun) the same face points directly at the Sun.  The
perihelion of Mercury's orbit advances 43 seconds of arc per
century.  This effect is only fully explained by Einstein's
general theory of RELATIVITY.
 
Physical Characteristics
 
Mercury's diameter is about 40% that of the Earth, and its mass 
is about 6% of the Earth's.  Its high density implies that
there is a large iron or nickel-iron core inside the planet.
Mercury is thought to contain a higher percentage of iron than
the Earth does.  Current computer models set Mercury's core
radius at 1,800 km (1,100 mi)--75% of the radius of the planet. 
(The Earth's core has a radius that is only about 55% of the
planetary radius.) This large iron core, part of which is
probably molten, is undoubtedly responsible for Mercury's
intrinsic magnetic field.  Discovered in 1974 by the MARINER 10 
spacecraft, the field is only about 1% as strong as the Earth's 
at the surface.  This is enough, however, to disturb the SOLAR
WIND as it streams past the planet.
 
Mariner 10 photographed about 40% of Mercury's surface in
detail.  Extensive cratered highlands cover much of the
observed surface, making Mercury look like the Moon.  Such
highlands are probably quite ancient.  Dark, smooth plains that 
look like those on the Moon are also seen.  They are probably
younger than the highlands.  The largest plain, 1,300 km (800
mi) wide, is called the Caloris Basin because it is located at
one of Mercury's "hot spots"--either of the two points where
the Sun can be directly overhead when Mercury is nearest the
Sun.  Radar observations made in 1991 of the planet's north
polar area--a region unphotographed by mariner 10--suggest that 
despite Mercury's nearness to the Sun, some water ice may exist  
in this area at protected sites.
 
Mercury is quite different from the Moon, however.  It is 40%
larger, 4.5 times more massive, and much richer in iron.  Even
Mercury's surface is different from the Moon in two important
ways.  First, although heavily cratered, Mercury's highlands
are not saturated with craters.  Extensive, gently rolling
plains predominate.  These plains may represent the original
crust of Mercury showing through the effects of cratering.
Mercury's higher surface gravity prevents meteoric impacts from 
spreading their ejecta as far as they do on the Moon, so that
some of the precratering surface may have remained intact.
Secondly, there are large, long, and winding scarps, or
one-sided ridges, that cross the surface of the cratered
highlands for hundreds of kilometers.  These scarps are thought 
to have formed during contraction of the crust as Mercury's
large core cooled and partially solidified, much as an apple
skin wrinkles as the apple begins to dry out.  Thus, while
externally Mercury looks like the Moon, internally it is more
like the Earth in terms of its large iron core and related
magnetic field.  The formation and development of this core is
what primarily distinguishes the evolution of Mercury from that 
of the Moon.
 

Bibliography:  Strom, R. G., Mercury: The Elusive Planet
(1987) and "Mercury," Sky and Telescope, December, 1990;
Time-Life Books Editors, The Near Planets (1989);  Vilas,
Faith, et al., eds., Mercury (1988);  Yenne, Bill, The Atlas of 
the Solar System (1989).

                  CHARACTERISTICS OF MERCURY



MERCURY (PLANET)

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Mean distance from Sun           57,900,000 km (36,000,000 mi)
 
Length of year                   88 days
 
Length of day                    58.6 days
 
Inclination of axis              7 deg
 
Equatorial diameter              4,880 km (3,030 mi)
 
Mass compared to Earth           .054
 
Specific density (water = 1)     5.4
 
Atmosphere                       trace
 
Mean surface temperature         350 deg C (660 deg F) day
                                 -170 deg C (-270 deg F) night
Satellites                       none
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