III.

Surface and Composition

Like the Moon, Mercury preserves a record of a violent early period when asteroids, comets, and other debris bombarded the newly formed planets and satellites of the solar system at much higher rates than currently observed. Although Mercury’s heavily cratered surface appears very similar to the surface of the Moon, there are some significant differences. Laser altimeter data indicates that craters on Mercury are shallower than those on the Moon. Debris ejected from impacts on Mercury also falls closer to craters than on the Moon, an effect of Mercury’s stronger gravitation. Unusual features not seen elsewhere in the solar system include a system of troughs radiating from around a moderate-size impact crater at the center of the giant Caloris Basin. The Caloris Basin itself is the largest geological feature on Mercury and the result of a massive ancient impact. Smooth, lavalike plains inside the Caloris Basin appear lighter in color than surrounding higher terrain, unlike the smooth mare plains of the Moon, which are much darker than surrounding highlands. Also unlike the surface of the Moon, the surface of Mercury is crisscrossed by long escarpments, or cliffs, indicating a period of surface contraction as the planet cooled early in its history.

Mercury is a poor reflector of sunlight because its surface consists of dark, dry soil called regolith created by micrometeorite impacts over billions of years. The planet’s albedo, or the amount of sunlight it reflects, is only about 12 percent, about the same as our Moon. Earth, in contrast, reflects about 39 percent of the sunlight that strikes it, thanks mainly to clouds, water, and ice, while cloud-covered Venus, the most reflective planet in the solar system, reflects about 76 percent.

Surface temperatures on Mercury vary more than those of any other major body in the solar system, with a maximum range of about 650°C (1170°F/ 650°K) between the hottest and coldest extremes. The side facing the Sun gets very hot—up to 450°C (840°F/725°K)—while the side facing away quickly cools to frigid temperatures, -183°C (-297.4°F/90°K). Because its axis is vertical, Mercury does not have seasons. The floors of craters at the north and south poles receive very little sunlight and always remain extremely cold—about -200°C (-328°F/70°K)—while its equatorial region experiences extreme changes, reaching 450°C (840°F/725°K) at perihelion when facing the Sun—hot enough to melt zinc. (The surface of Venus is even hotter because of the greenhouse effect caused by its dense atmosphere, reaching 462°C (864°F/736°K), hot enough to melt lead). The same spot on Mercury faces the Sun at perihelion every second orbit. Scientists named a basin found near one of these so-called “hot poles” the Caloris Basin, from the Latin word calor “heat.” The Caloris Basin is the largest known geographical feature on the planet and is thought to be a huge impact crater filled by lava.

Mercury’s high density indicates that the relatively dense and abundant element iron accounts for a large proportion of the planet’s composition. The surface of Mercury, however, contains little iron, suggesting that most of Mercury’s iron is now concentrated in a large iron core. Collisions with other protoplanets early in the history of the solar system may have stripped away much of Mercury’s low-density crust, leaving behind a dense, iron-rich core. Alternatively, Mercury could have formed from material enriched in iron close to the Sun early in solar system history.

Mercury is the only rocky planet other than Earth to have a global magnetic field, which is about 1 percent as strong as Earth’s. However, scientists are puzzled as to why Mercury’s magnetic field is relatively weak. Theory predicts that it should be about 30 times stronger if it is generated in the same way proposed for Earth’s magnetic field. The presence of the field and its global extent suggest that the core of the planet is largely liquid iron, which produces a magnetic field as it moves. Scientists believe that Mercury’s crust insulates the planet’s outer core, allowing the planet to retain heat from radioactive decay and keeping the core liquid despite the very cold temperatures on the dark side of the planet.

In 1991 powerful radio telescopes on Earth revealed signs of possible deposits of ice in the polar regions of Mercury. These ice deposits occur in areas where sunlight never falls, such as crater bottoms near both of the planet’s poles. Similar ice deposits may have been found during the 1990s near the poles of the Moon by the Clementine and Lunar Prospector spacecrafts. The ice on Mercury likely comes from comets or water-bearing meteorites that have hit Mercury over the planet’s history up through the present.

Scientists use a technique called spectroscopy to conduct studies of the light that Mercury reflects. These studies indicate that planet has only an extremely thin atmosphere, containing sodium and potassium. Apparently these elements slowly escape as gases from the crust of the planet or are blasted off the surface by the solar wind, high energy particles that stream from the Sun.