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CHAPTER 6

Mercury

Robert G. Strom

Department of Planetary Sciences

University of Arizona

Tucson, Arizona

1. General Characteristics 6. Geology and Planet Evolution


2. Motion and Temperature 7. Origin


3. Exosphere 8. The MESSENGER Mission


4. Polar Deposits Bibliography


5. Interior and Magnetic Field

M

ercury is the innermost and smallest planet in
the solar system.^1 It has no known satellites. The
exploration of Mercury has posed questions concerning fun-
damental issues of its origin and, therefore, the origin and
evolution of all the terrestrial planets. The data obtained by
Mariner 10on its three flybys of Mercury on March 29 and
September 21, 1974, and on March 16, 1975, remain our
best source of detailed information on this planet. However,
recent ground-based observations have provided important
new information on the topography, radar, and microwave
characteristics of its surface; discovered new constituents in
its atmosphere; and helped constrain its surface composi-
tion. TheMESSENGERspacecraft is currently on its way to
Mercury and will begin orbiting the planet in March 2011.
The name MESSENGERis an acronym for Mercury,
Surface, Space ENvironment, GEochemistry, and Ranging.
To the ancients, Mercury was the messenger of the gods.
Mercury is often compared with the Moon because it super-
ficially resembles that satellite. However, major differences
set Mercury apart from the Moon and, for that matter,
all other planets and satellites in the solar system.

(^1) Assuming Pluto, a Kuiper Belt object smaller than Mercury, is not a
planet, which is controversial.
Mariner 10imaged only about 45% of the surface at
an average resolution of about 1 km, and less than 1% at
resolutions between about 100 to 500 m (Fig. 1). This cover-
age and resolution is comparable to telescopic Earth-based
coverage and resolution of the Moon before the advent of
space flight. However, unlike the Moon in the early 1960s,
only about 25% of the surface was imaged at Sun angles
low enough to allow adequate terrain analyses. As a con-
sequence, there are still many uncertainties and questions
concerning the history and evolution of Mercury.Mariner
10 also discovered a magnetic field, measured the temper-
ature, and derived the physical properties of its surface.
On Mercury, the prime meridian (0◦) was chosen to
coincide with the subsolar point during the first perihe-
lion passage after January 1, 1950. Longitudes are mea-
sured from 0◦to 360◦, increasing to the west. Craters are
mostly named after famous authors, artists, and musicians
such as Dickens, Michelangelo, and Beethoven, whereas
valleys are named for prominent radio observatories such
as Arecibo and Goldstone. Scarps are named for ships as-
sociated with exploration and scientific research such as
Discovery and Victoria. Plains are named for the planet
Mercury in various languages such as Odin (Scandinavian)
and Tir (Germanic). Borealis Planitia (Northern Plains) and
Caloris Planitia (Plains of Heat) are exceptions. The most

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