Mercury is the hot, nearly airless planet that orbits closest to the Sun. It is about 1.5 times the size of the Moon and about 4.5 times the Moon's mass. The smallest of the terrestrial planets, it has an old (about four billion years), desolate, heavily cratered surface very much like the Moon, though its craters are flatter and thinner rimmed because of the planet's higher surface gravity. Also present on Mercury's surface are large multi‐ringed lava‐filled craters, called basins, and long, high cliffs, or scarps, produced either during the early cooling and contraction of the planet or by the solar tidal effect. The impact that produced the largest basin, Caloris, also sent shock waves through the interior of the planet; these converged on the planet's opposite side to produce an exceedingly rugged surface. Evidence also shows extensive volcanic events, in the form of younger intercrater plains and smooth plains, with fewer and smaller craters than adjacent areas.

The planet has a surprisingly high mean density, 5.4 g/cm 3, suggesting that its iron‐nickel core is disproportionately large, possibly due to partial loss of its mantle in a collision with another large object (see Figure ). This iron core is responsible for its weak magnetic field (1 percent that of Earth), likely a residual field left over from an early time when more rapid rotation generated electrical currents in the core, which in turn generated the magnetic field. Its proximity to the sun produces an extreme temperature range between 700 K (760°F) during its daytime and about 100 K (–280°F) during its night. Ice appears to exist in the permanently shadowed interiors of deep craters near the poles. Little atmosphere exists—about one‐trillionth that of Earth, and is composed of solar wind gases temporarily held by the planet's gravity and some sodium and potassium likely released from surface rocks by the impact of the solar wind particles.

Figure 1

The interior of Mercury.

Of the inner eight planets, Mercury has the largest orbital eccentricity (e = 0.21). Its large variation of distance from the Sun has produced, via tidal friction, an unequal resonance, or relation between its rotation and its orbit. With a 59‐day rotation and 88‐day orbit, Mercury's resonance has a 3:2 ratio, or three rotations in every two orbits about the Sun. The combination of its rotation and revolution about the Sun produces a Mercury day equal to two orbital periods, or 176 Earth days. Careful consideration of the dynamics of the planet shows that this is the result of tidal locking at perihelion, when the planet is closest to the sun and the tidal forces are strongest.

Mercury has also been the object of study because its orbit cannot be adequately explained by only using Newton's Law of Gravity. According to Newton, the gravitational force between a spherical Sun and Mercury produces an elliptical orbit that perfectly repeats itself each time the planet moves around the Sun (Kepler's Laws). Because the other planets also gravitationally tug on Mercury and because the Sun is not perfectly spherical, each successive elliptical path is actually rotated slightly with respect to the previous orbit, like a spirograph pattern. The position of the closest approach of Mercury (its perihelion) thus slowly drifts around the Sun (an effect termed precession of the perihelion) at the rate of 5600”/century = 1.56°/century = 13.5”/orbit. When the effect of the non‐spherical Sun and the gravitational perturbations by the other planets are subtracted, there is left over a precession (rotation) of 43”/century = 0.1”/orbit, a tiny change that can be accounted for only by Einstein's theory of general relativity, but not by Newton's theory of gravitation. Mercury's precession is thus one of the fundamental tests showing that the theory of general relativity is a far more accurate description of gravitational phenomena that is the Newtonian theory, although the difference in predictions between the two is negligible under most circumstances. See Table 1 for Mercury's physical and orbital data.