Mercury's Rotation
Mercury's rotation is quite slow, about 59 days, and its
axis is roughly perpendicular to the plane of its orbit.
A given region of the planet is exposed to sunlight and
then remains in the shade for a very long time.
Mercury practically does not have an atmosphere--given
its small mass and gravitational attraction, it was
never able to sustain one. This phenomenon is similar to
what happened to the Moon and to most of satellites in
the solar system. Because of the lack of the moderating
influence of an atmosphere and its slow period of
rotation around its axis, Mercury's surface temperatures
are the most extreme in the solar system. In areas
illuminated by the sunlight, the temperature can soar to
750 K, while on the dark side of the planet it plummets
to 150 K.
Figure 3.1: A global view of Mercury, from images of
Mariner 10
The reason for the slow rotation of Mercury is the
action of 'tidal forces' between the planet
and the Sun. This is an important effect when two
celestial bodies are relatively close to
each other. Tides are important, for instance, between
the Earth and the Moon, deforming
both from a perfectly spherical shape. It is quite
possible that the period of rotation of
Mercury was much shorter long ago, at the time of the
planet's formation. The Sun attracts
the closest side of Mercury slightly more than the
farthest side. Such difference causes the
planet to assume a slightly
prolate shape in the
direction of the Sun.
For the sake of the argument let us assume that Mercury
did have a fast period of rotation, with an axis roughly
perpendicular to that of the planet's orbit around the
Sun. Concentrate your attention on one particular point
of the planet surface, say a point at the equator. As
the rotation of the planet proceeds, the distance
between that point of the surface and the center of the
planet periodically changes. Layers of rock in any given
region of the planet's interior get stretched and
contracted on a massive scale. Since the forces of
friction resist such a motion, the energy of the
planet's rotation gradually gets transformed into
internal heat, while the rotation slows down.
If Mercury's orbit had been circular, the process we
have just described would have slowed down the rotation
to a period equal to the period of revolution of the
planet around the Sun. Our Moon has the two periods in a
1 : 1 ratio, and that it why we always see the same half
of our satellite's surface. But Mercury has an eccentric
orbit, and the period of rotation of Mercury stabilized
when it reached a value 2 : 3 of the period of
revolution, instead of being in the simple ratio 1 : 1.
