DEFINITION OF MERCURY

The closest planet to the Sun, Mercury, is a

midget. It has a mass of only 5.5 percent of the Earth, with a

diameter of 3,031 miles (4,878 kilometers). It has an average

density of 5.4 grams per cubic centimeter, and it rotates once

on its axis every 59 Earth days. It orbits the Sun once every

88 days at a distance of 36 million miles (58 million km).

Since it is so close to the Sun, it is only visible to the naked

eye when the sun is blotted out, such as just before dawn,

after sunset, or during total solar eclipses.

Mercury has such a weak gravitational field that it lacks

an atmosphere, although bombardment by the solar wind

releases some sodium and potassium atoms from surface

rocks, and these may rest temporarily near the planet’s surface.

It has a very weak magnetic field, approximately

1/100th as strong as Earth’s. The surface of Mercury is heavily

cratered and looks much like the Earth’s moon. It shows

no evidence for ever having sustained water, dust storms, ice,

plate tectonics, or life. The surface of Mercury is less densely

cratered than the Moon, however, and some planetary geologists

suggest that the oldest craters may be filled in by volcanic

deposits. It also has some surface scarps, estimated to

be more than 4 billion years old. These are thought to represent

contraction of the surface associated with the core formation

and shrinking of the planet in the first half-billion

years of its history.

The density of Mercury and the presence of a weak magnetic

field suggest that the planet has a differentiated iron-rich

core with a radius of approximately 1,118 miles (1,800 km),

but it is not known whether this is solid or liquid. A mantle

probably exists between the crust and core, extending to

311–373 miles (500–600 km) depth. The small size of Mercury

means it did not have enough internal energy to sustain

plate tectonics or volcanism for long in its history, so the planet

has been essentially dead for the past 4 billion years.

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