Mercury

Mercury (planet)

I

INTRODUCTION

Mercury (planet), first planet in distance from the Sun in the solar system. The smallest of the rocky or terrestrial planets that include Venus, Earth, and Mars, Mercury has a global magnetic field, but only a trace of an atmosphere and no moons of its own. It is the second hottest planet after Venus. Mercury circles the Sun every 88 Earth days at an average distance of 58 million km (36 million mi) and takes 59 days to turns on its axis. It retains an ancient cratered surface that has changed little since the formation of the solar system, making the planet of special interest to planetary scientists. Mercury was named for the fleet-footed messenger of the gods in Roman mythology.

Mercury’s diameter is 4,879 km (3,032 mi), about 40 percent the diameter of Earth or about 40 percent wider than the Moon. Mercury’s volume and mass are about one-eighteenth that of Earth. Mercury’s mean density, 5.4 g/cm³, is nearly as great as that of Earth and is higher than that of any of the other planets. The force of gravity on the planet’s surface is about one-third of that on Earth’s surface or about twice the surface gravity on the Moon and about the same as the surface gravity on Mars, which is larger than Mercury but less dense. Two moons in the solar system—Jupiter’s Ganymede and Saturn’s Titan—are also larger than Mercury but are much less dense and hence have lower gravity (about the same as the Moon).

II

ORBIT AND ROTATION

Mercury orbits the Sun every 87.97 Earth days at an average distance of approximately 58 million km (about 36 million mi), or 0.3871 astronomical unit (AU). An AU is equal to the average distance between the Earth and the Sun, or about 150 million km (93 million mi). However, Mercury’s orbit is highly elliptical and ranges from 46 million km (28,580,000 mi/0.3075 AU) at its nearest point to the Sun (perihelion) to 69.8 million km (43,380,000 mi/0.4667 AU) at its farthest point (aphelion). As a result, sunlight is over 2.3 times stronger at perihelion than at aphelion—during a single orbit Mercury receives as much as 11 times the intensity of sunlight that Earth does to a minimum of about 4.5 times. Mercury’s orbital velocity is also about 46 percent faster at perihelion than at aphelion. The planet’s orbit is tilted 7 degrees to the plane in which Earth orbits around the Sun.

The point in Mercury’s orbit at which the planet is closest to the Sun (perihelion) moves a tiny amount every orbit, too much to be accounted for solely by the gravitational influence of other planets. The observation of these changes in Mercury’s perihelion was one of the first confirmations of Einstein’s general theory of relativity, which predicted such variation due to the curvature of space caused by the enormous mass of the Sun.

Like Earth and most other planets, Mercury turns counterclockwise (west to east) when seen from its north pole. Mercury’s axis is almost perfectly vertical, unlike Earth’s axis, which is tilted 23.5 degrees. Radar observations of Mercury show that it rotates only once every 58.65 days, two-thirds of its period of revolution around the Sun. As a result, only three rotations of the planet occur during every two of its years. This relationship is called a 3:2 spin-orbit resonance. It is thought to be the result of differences in the pull of the Sun’s gravity on Mercury as the planet moves nearer and farther away in its orbit, an effect called solid body tidal forces.

The 3:2 spin-orbit resonance means that a solar day on Mercury (the time when the Sun next passes the noon point in the sky) is very different from the planet’s actual period of rotation (called a sidereal day). In fact, a complete solar day on Mercury lasts 175.84 Earth days, or two of Mercury’s years, and a night and a day at the equator each last one Mercurian year (87.97 Earth days). The Sun’s movement across the daytime sky from east to west would look very strange to a human observer, however. The planet’s eccentric orbit, changing orbital velocity, and slow rotation combine to make the Sun appear to stop and reverse direction before returning to a westward path. This effect occurs when Mercury is closest to the Sun and the planet’s orbital velocity becomes faster than its rotational speed around its axis.The Sun’s apparent size would also change during an orbit, from over 3 times to about 2 times its average size (about 0.5 degree of arc) when seen from Earth.

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. The smooth, lava-like plains on Mercury, for example, are not as dark as the smooth mare plains of the Moon. 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.

IV

OBSERVATION AND EXPLORATION

Because Mercury orbits so near the Sun, it can be difficult to observe from Earth. The planet is only a few degrees above the horizon for short periods in the early evening or just before dawn, often visible only during twilight and seen through hazy air. Like the Moon and Venus, it goes through phases and varies noticeably in brightness. Optical telescopes on Earth have revealed little detail of its surface and space telescopes such as the Hubble Space Telescope cannot be safely pointed at an object so close to the Sun. Radar, however, can observe Mercury in the sky during the daytime. Radar studies in the 1960s discovered its 3:2 rotation and orbital period relationship—scientists had previously assumed that Mercury always kept the same face to the Sun the way the Moon does with the Earth. Radar was also able to estimate the planet’s size. More recent studies using microwaves and radar have made other discoveries, including mapping Mercury’s surface and detecting possible ice at the poles. (The 7 percent inclination of Mercury’s orbit relative to Earth’s orbit allows the planet’s polar regions to be studied by Earth-based radar more easily than the polar regions of the Moon.)

The first up-close study of Mercury came with NASA’s Mariner 10 spacecraft, which passed Mercury twice in 1974 and once in 1975. It sent back pictures of a moonlike, crater-pocked surface. The spacecraft also detected a magnetic field and provided data about the planet’s density and some of its surface chemistry. However, Mariner 10 could not orbit the planet and was only able to photograph about 45 percent of its surface, often in sunlight conditions that did not bring out features in maximum detail.

A much more ambitious mission to Mercury called MESSENGER was launched by the National Aeronautics and Space Administration (NASA) in 2004. MESSENGER will orbit the planet to conduct an in-depth study of its entire surface, in contrast to Mariner 10’s quick flybys. MESSENGER is an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging—instruments on board include detectors to help analyze the planet’s mineral composition, topography, geological processes, possible ice, internal structure, and the origin of its magnetic field. It will take MESSENGER nearly seven years to adjust its path and lose enough energy to be captured into orbit around Mercury. This process involves a complicated series of flybys of Venus and Earth, and includes three flybys of Mercury in 2008 and 2009, during which the mapping of the planet will begin. MESSENGER’s orbital phase will start in 2011 when it begins orbiting the planet for at least one Earth year, equivalent to four Mercurian years.

The European Space Agency (ESA) has announced a Mercury mission of its own called BepiColombo, set for launch in 2013, to be begin orbiting Mercury in 2019. The mission is a collaboration with Japan and will include two separate orbiters: Mercury Planetary Orbiter (MPO), built by the ESA; and Mercury Magnetospheric Orbiter (MMO), built by the Japanese space agency ISAS/JAXA. In addition to studying the planet’s surface, interior structure, and magnetic field, the BepiColombo mission will refine measurements of the relativistic effects of the Sun on Mercury’s orbit.

Renewed interest in Mercury stems from recent progress in understanding the evolution of the solar system. Because Mercury represents a kind of extreme among the terrestrial planets, it offers special insights into the formation and early history of planets in the inner solar system, especially when compared to other rocky planetary bodies.

Reviewed By:
Paul D. Spudis

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