456 PART 4^ |^ THE SOLAR SYSTEM
In 1962, radio astronomers detected blackbody emissions
from the planet and concluded that the dark side was not as cold
as it should have been if the planet kept one side in perpetual dark-
ness. In 1965, radio astronomers made radar contact with Mercury
by using the 305-m Arecibo dish (see Figure 6-21) to transmit a
pulse of radio energy at Mercury and then waiting for the refl ected
signal to return. Doppler shifts in the refl ected radio pulse showed
that the planet was rotating with a period of only about 59 days,
noticeably shorter than the orbital period of 88 days.
Mercury is tidally coupled to the sun but in a diff erent way
than the moon is coupled to Earth. Mercury rotates not once per
orbit but 1.5 times per orbit. Th at is, its period of rotation is
two-thirds its orbital period. Th is means that a mountain on
Mercury directly facing the sun at one place in its orbit will point
away from the sun one orbit later and toward the sun after the
next orbit (■ Figure 21-12).is only a fourth of Earth’s diameter, and Mercury is just over
a third of Earth’s diameter. Th eir rotation has been altered
by tides, their surfaces are heavily cratered, their lowlands
are fl ooded in places by ancient lava fl ows, and both are air-
less and have ancient, inactive surfaces. Th e impressive dif-
ferences between them also will help you understand the
nature of these airless worlds.
Mercury is the innermost planet in the solar system, and
thus its orbit keeps it near the sun in the sky, viewed from
Earth. It is sometimes visible near the horizon in the evening
sky after sunset or in the dawn sky just before sunrise. Earth-
based telescopes show the small disk of Mercury passing
through phases like the moon and Venus. Th e Mariner
10 spacecraft looped through the inner solar system in 1974
and 1975, taking photographs and other measurements dur-
ing three fl ybys of Mercury. Astronomers managed in 2007
to make impressively high-resolution images from Earth
of the parts of Mercury not covered by Mariner 10’s images
(■ Figure 21-10). A new spacecraft called MESSENGER
(MErcury Surface, Space ENvironment, GEochemistry, and
Ranging mission) has made two fl ybys of Mercury at the time of
this writing and will make one more before it goes into orbit
around the planet in 2011 and begins a yearlong close-up study
(■ Figure 21-11).
Rotation and Revolution
During the 1880s, the Italian astronomer Giovanni Schiaparelli
sketched the faint features he thought he saw on the disk of
Mercury and concluded that the planet was tidally locked to the
sun and kept the same side facing the sun throughout its orbit.
Th is was actually a very good guess because, as you will see, tidal
coupling between rotation and revolution is common in the solar
system. You have already seen that the moon is tidally locked to
Earth. But the rotation of Mercury is more complex than
Schiaparelli thought.
■ Figure 21-11
The MESSENGER spacecraft will make three fl ybys and then spend a year
orbiting Mercury, observing from behind a fabric sunscreen. (NASA/Johns
Hopkins University Applied Physics Lab/Carnegie Institute of Washington)■ Figure 21-10
Using the 4.1-meter SOAR telescope in Chile by remote control from their
offi ces in North Carolina, astronomer Gerald Cecil and undergraduate stu-
dent Dmitry Rashkeev resolved details on Mercury as small as 150 km during
March and April 2007, the highest-quality images of that planet ever made
from Earth. The observations were timed to view mostly the side of Mercury
that had not been previously imaged by the Mariner 10 probe. The telescope
aperture was masked down to 1.35 meters in diameter to reduce the effect
of Earth atmosphere “seeing.” A series of very rapid exposures were taken
with a digital camera, with most images being discarded, saving only the
highest-resolution data. (G. Cecil/University of North Carolina)