Geology Reference
In-Depth Information
CHAPTER 5
Mercury
5.1 Introduction
known (
Table 1.1
), including its mass, as estimated from
Mercury
'
s interactions with Venus and other bodies. In the
mid 1960s, radar experts Gordon Pettengill of MIT and
Rolf Dyce tracked Mercury using the Arecibo radar
facility in Puerto Rico and determined that the planet
As a planetary
“
wanderer,
”
Mercury probably has been
recognized as long as the heavens have been viewed.
Although many cultural mythologies refer to this planet,
the
first recorded mention of Mercury was in 265 B.C. by
the Greek Timocharis. With the advent of telescopes and
their use in science, careful observations by Giovanni
Zupus in 1639 revealed that Mercury goes through phases
similar to the Moon. In the 1800s, several well-known
planetary observers noted various aspects of Mercury,
including supposed surface markings. For example,
Giovanni Schiaparelli and Percival Lowell, both of Mars
fame, made simple maps of Mercury and named features
that they thought could be seen. While most of these
features turned out not to exist, some of the names are
still used (
Fig. 5.1
).
s
spin rate equaled a rotation period of 59 days, a value later
re
ned to 58.6 days. This is exactly two-thirds of
Mercury
'
s orbital period, which means that its spin and
orbit are locked into a 3:2 resonance. Thus, the planet
spins on its axis three times for every two orbits around the
Sun. If one were to stand on the surface, sunrise would be
repeated every two orbits or 176 days, and the temperature
would vary by 600 °C.
Most of our knowledge of the geologic aspects of
Mercury comes from the Mariner 10 and the
MESSENGER (MErcury Surface Space Environment,
GEochemistry, and Ranging) missions. Mariner 10 was
launched in late 1973 and, after a
flyby of Venus and a
journey of nearly 5 months,
flew past Mercury in March
1974, returning the
first close-up views of this Sun-baked
planet. Although this was not in the original plan, talented
engineers at the Jet Propulsion Laboratory, the NASA
facility responsible for the mission, analyzed the trajectory
of the spacecraft and found that, after looping around the
Sun, Mariner 10 couldmake a second and then a third
yby
of Mercury (
Fig. 5.2
). This resulted in a tremendous
increase in data return over the original plan, all nearly for
the price of one
yby. Mariner 10 returned more than 2,700
useful images, covering about 45% the surface of Mercury,
as well as other data. It should be noted, however, that the
resolution of these images ranges from about 100m to 4 km
per pixel, comparable to that for Earth-based images of our
Moon. Consequently, the analysis of Mercury
'
sgeomor-
phology was similar to that conducted for the Moon using
Earth-based telescopes prior to the Space Age.
MESSENGER is a Discovery-class mission with Sean
Solomon as the Principal
'
5.2 Mercury exploration
In some ways, Mercury has been the forgotten planet. Until
recently, only NASA
s Mariner 10 spacecraft,
flown in the
early 1970s, had returned data from this, the closest planet
to the Sun. Because of its orbit within the inner Solar
System, Mercury is dif
cult to observe from Earth tele-
scopically, never being more than 28° from the Sun. In fact,
many observers are reluctant to train their telescopes in the
direction of Mercury for fear that stray light from the Sun
would damage the instruments. Nonetheless, some cautious
observations were made, which provided key data on the
physical properties and astronomical characteristics of the
planet. In addition, Earth-based radar observations pro-
vided insight into Mercury, including hints of some very
large surface features.
By the dawn of the Space Age and before the
flight of
Mariner 10, the major characteristics of Mercury were
'
Investigator who leads an
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