Mercury

Mercury is the innermost and second smallest planet (4,878 kilometers [3,024 miles] in diameter) in the solar system (Pluto is the smallest). It has no known moons. As of the beginning of the twenty-first century, Mariner 10 had been the only spacecraft to explore the planet. It flew past Mercury on March 29 and September 21, 1974, and on March 16, 1975. Mariner 10 imaged only about 45 percent of the surface and only in moderate detail. As a consequence, there are still many questions concerning the history and evolution of Mercury. Two new missions to Mercury will be launched this decade. An American mission called MESSENGER will be launched in March 2004. It will make two flybys of Venus and two of Mercury before going into Mercury orbit in April 2009. A European mission called Bepi Colombo, after a famous Italian celestial dynamicist, is scheduled for launch in 2009.

Motion and Temperature

Mercury has the most elliptical and inclined (7 degrees) orbit of any planet except Pluto. Its average distance from the Sun is only 0.38 astronomical unit (AU). Because of its elliptical orbit, however, the distance varies from 0.3 AU when it is closest to the Sun to 0.46 AU when it is farthest away. Mercury's orbital velocity is the greatest in the solar system and averages 47.6 kilometers per second (29.5 miles per second). When it is closest to the Sun, however, it travels 56.6 kilometers per second (35.1 miles per second), and when it is farthest away it travels 38.7 kilometers per second (24 miles per second).

Mercury's rotational period is 58.6 Earth days and its orbital period is 87.9 Earth days. It has a unique relationship between its rotational and orbital periods: It rotates exactly three times on its axis for every two orbits around the Sun. Because of this relationship, a solar day (sunrise to sunrise) lasts two Mercurian years, or 176 Earth days.

Because Mercury is so close to the Sun, has no insulating atmosphere, and has such a long solar day, it experiences the greatest daily range in surface temperatures (633°C [1,171°F]) of any planet or moon in the solar system. Mercury's maximum surface temperature is about 450°C (842°F) at the equator when it is closest to the Sun, but drops to about -183°C (-297°F) at night.

Interior and Magnetic Field

Mercury's internal structure is unique in the solar system. Mercury's small size and relatively large mass (3.3 × 10²³ kilograms [7.3 × 10²³ pounds]) means that it has a very large density of 5.44 grams per cubic centimeters (340 pounds per cubic foot), which is only slightly less than Earth's (5.52 grams per cubic centimeter [345 pounds per cubic foot]) and larger than Venus's (5.25 grams per cubic centimeter [328 pounds per cubic foot]). Because of Earth's large internal pressures, however, its uncompressed density is only 4.4 grams per cubic centimeter (275 pounds per cubic foot), compared to Mercury's uncompressed density of 5.3 grams per cubic centimeter (331 pounds per cubic foot). This means that Mercury contains a much larger fraction of iron than any other planet or moon in the solar system. The iron core must be about 75 percent of the planet diameter, or some 42 percent of its volume. Thus, its rocky outer region is only about 600 kilometers (370 miles) thick.

Mercury is the only terrestrial planet , aside from Earth, with a significant magnetic field. The maintenance of terrestrial planet magnetic fields is thought to require an electrically conducting fluid outer core surrounding a solid inner core. Therefore, Mercury's magnetic field suggests that Mercury currently has a fluid outer core of unknown thickness.

Exosphere

Mercury has an extremely tenuous atmosphere with a surface pressure a trillion times less than Earth's. This type of tenuous atmosphere is called an exosphere because atoms in it rarely collide. Mariner 10 identified the presence of hydrogen, helium, and oxygen in the atmosphere and set upper limits on the abundance of argon. These elements are probably derived from the solar wind . Later Earth-based telescopic observations detected sodium and potassium in quantities greater than the elements previously known. Sodium and potassium could be released from surface rocks by their interaction with solar radiation or by impact vaporization of micrometeoroid material. Both sodium and potassium show day-to-day changes in their global distribution.

Polar Deposits

High-resolution radar observations show highly reflective material concentrated in permanently shadowed portions of craters at the polar regions. These deposits have the same radar characteristics as water ice. Mercury's rotation axis is almost perpendicular to its orbit, and therefore Mercury does not experience seasons. Thus, temperatures in permanently shaded polar areas should be less than -161°C (-258°F). At this temperature, water ice is stable, that is, it is not subject to evaporation for billions of years. If the deposits are water ice, they could originate from comet or water-rich asteroid impacts that released the water, which was then cold-trapped in the permanently shadowed craters. Sulfur has also been suggested as a possible material for these deposits.

Geology and Composition

In general, the surface of Mercury can be divided into four major terrains: heavily cratered regions, intercrater plains , smooth plains , and hilly and lineated terrain. The heavily cratered uplands record the period of heavy meteoroid bombardment that ended about 3.8 billion years ago.

The largest relatively fresh impact feature seen by Mariner 10 is the Caloris basin, which has a diameter of 1,300 kilometers (806 miles). The floor structure consists of closely spaced ridges and troughs.

Directly opposite the Caloris basin (the antipodal point) is the unusual hilly and lineated terrain that disrupts preexisting landforms, particularly crater rims (see top image on following page). The hilly and lineated terrain is thought to be the result of seismic waves generated by the Caloris impact and focused at the antipodal region.

Mercury's two plains units have been interpreted to be old lava flows. The older intercrater plains are the most extensive terrain on Mercury (see bottom image on this page). The intercrater plains were created during the period of late heavy meteoroid bombardment. They are thought to be volcanic plains erupted through a fractured crust. They are probably about 4 to 4.2 billion years old.

The younger smooth plains are primarily associated with large impact basins. The largest occurrence of smooth plains fill and surround the Caloris basin, and occupy a large circular area in the north polar region that is probably an old impact basin about 1,500 kilometers (930 miles) in diameter. They are similar to the lunar maria and therefore are believed to be lava flows that erupted relatively late in Mercurian history. They may have an average age of about 3.8 billion years. If so, they are, in general, older than the lunar maria.

Three large radar-bright anomalies have been identified on the unimaged side of Mercury. High-resolution radar observations indicate that two of these are similar to the radar signature of a fresh impact crater, and another has a radar signature unlike any other in the solar system. One or both of these craters could account for the polar deposits if they were the result of comets or water-rich asteroid impacts.

Mercury displays a system of compressive faults (or thrust faults ) called lobate scarps . They are more-or-less uniformly distributed over the part of Mercury viewed by Mariner 10. Presumably they occur on a global scale. This suggests that Mercury has shrunk. Stratigraphic evidence indicates that the faults formed after the intercrater plains relatively late in Mercurian history. The faults were probably caused by a decrease in Mercury's size due to cooling of the planet. The amount of radius decrease is estimated to have been about 2 kilometers (1.2 miles).

Very little is known about the surface composition of Mercury. A new color study of Mariner 10 images has been used to derive some compositional information of the surface over some of the regions viewed by Mariner 10. The smooth plains have an iron content of less than 6 percent by weight, which is similar to the rest of the regions imaged by Mariner 10. The surface of Mercury, therefore, may have a more homogeneous distribution of elements that affect color than does the Moon. At the least, the smooth plains may be low-iron basalts . The MESSENGER mission is designed to accurately determine the composition of the surface.

Geologic History

Knowledge about Mercury's earliest history is very uncertain. The earliest known events are the formation of the intercrater plains (more than 4 billion years ago) during the period of heavy meteoroid bombardment. These plains may have been erupted through fractures caused by large impacts in a thin crust. Near the end of heavy bombardment the Caloris basin was formed by a large impact that caused the hilly and lineated terrain from seismic waves focused at the antipodal region. Eruption of lava within and surrounding the large basins formed the smooth plains about 3.8 billion years ago. The system of lobate scarps formed after the intercrater plains, and resulted in a planetary radius decrease of about 2 kilometers (1.2 miles). Scientists will have to await the results of the MESSENGER and Colombo missions to fully evaluate the geologic history of Mercury.

Origins

How Mercury acquired such a large fraction of iron compared to the other terrestrial planets is not well determined. Three hypotheses have been put forward to explain the enormous iron core. One involves an enrichment of iron due to dynamical processes in the innermost part of the solar system. Another proposes that intense bombardment by solar radiation in the earliest phases of the Sun's evolution vaporized and drove off much of the rocky fraction of Mercury, leaving the core intact. A third proposes that a planetsized object impacted Mercury and blasted away much of the planet's rocky mantle, again leaving the iron core largely intact. Discriminating among these hypotheses may be possible from the chemical makeup of the surface because each one predicts a different composition. MESSENGER is designed to measure the composition of Mercury's surface, so it may be possible to answer this vital question in the near future.

see also Exploration Programs (volume 2); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2).

Robert G. Strom

Bibliography

"The Planet Mercury: Mariner 10 Mission." (various papers and authors) Journal of Geophysical Research 80, no. 17 (1975): 2342-2514.

Strom, Robert G. Mercury: The Elusive Planet. Washington, DC: Smithsonian Institution Press, 1987.

——. "Mercury: An Overview." Advances in Space Research 19, no. 10 (1997):1,471-1,485.

——. "Mercury." In Encyclopedia of the Solar System, eds. Weissman, P. R., L. Mc-Fadden, and T. V. Johnson. San Diego: Academic Press, 1999.

Villas, Faith, Clark R. Chapman, and Mildred S. Matthews, eds. Mercury. Tucson:University of Arizona Press, 1988.

Mercury (symbol ♀̆) The closest planet to the Sun. It has the most elliptical orbit (eccentricity 0.206) of all the major planets, so that at perihelion it is only 46 000 000 km from the Sun's centre, but 69 820 000 km at aphelion. Its mean geometric albedo, 0.11, is similar to the Moon's, and its overall colour is grey. Mercury's mean magnitude at greatest elongation is 0.0, but it keeps close to the Sun in the sky and so is seldom visible to the naked eye. Its period of axial rotation is exactly two-thirds of its orbital period, an example of spin–orbit coupling. As a result, two lines of longitude, spaced by 180°, experience the Sun overhead at perihelion, making these two regions the hottest on Mercury.

Mercury in astronomy, nearest planet to the sun, at a mean distance of 36 million mi (58 million km); its period of revolution is 88 days. Mercury passes through phases similar to those of the moon as it completes each revolution about the sun, although the visible disk varies in size with respect to its distance from the earth. Because its greatest elongation is 28°, it is seen only for a short time after sunset or before sunrise. Since observation of Mercury is particularly unfavorable when it is near the horizon, the planet has often been studied in full daylight, with the sun's light blocked off. Mercury has the most elliptic orbit of the planets in the solar system. Its great eccentricity of orbit and its great orbital speed provided one of the important tests of Einstein's general theory of relativity . Mercury's perihelion (its closest point to the sun) is observed to advance by 43″ each century more than can be explained from planetary perturbations using Newton's theory of gravitation , yet in nearly exact agreement with the prediction of the general theory. Mercury is the smallest planet in the solar system, having a diameter of 3,031 mi (4,878 km); both Jupiter's moon Ganymede and Saturn's moon Titan are larger. Its mean density is a little less than that of the earth; its iron core represents about 60% of its mass. Its small mass and proximity to the sun prevent it from having an appreciable atmosphere, although a slight amount of carbon dioxide has been detected. The surface of Mercury is much like that of the moon, as was shown Mariner 10 in flybys in 1974–75 and Messenger in flybys in 2008–9 and in orbit beginning in 2011. Most of its craters were formed during a period of heavy bombardment by small asteroids early in the solar system's history. Messenger, which became the first space probe to orbit Mercury, also has found solid evidence of volcanism. It was long thought that Mercury's period of rotation on its axis was identical to its period of revolution, so that the same side of the planet always faced the sun. However, radar studies in 1965 showed a period of rotation of 58.6 days. This results in periods of daylight and night of 88 earth days each, with the daylight temperatures reaching as high as 800°F (450°C). Night temperatures are believed to drop as low as -300°F (-184°C).

Mercury The innermost planet of the solar system, 0.387 AU from the Sun, its distance from Earth varying from 77.3 × 10⁶ km to 221.9 × 10⁶ km. It has a tenuous atmosphere, with a surface pressure of about 10⁻¹⁵ bar, composed of oxygen (42%), sodium (29%), hydrogen (22%), helium (6%), potassium (0.5%), and possibly trace amounts of argon, carbon dioxide, water, nitrogen, xenon, krypton, and neon (the proportions are approximate). Its diameter is 4880 km; volume 6.085 × 10¹⁰ km³; mass 0.3302 × 10²⁴ kg; mean density 5427 kg/m³ (making it the densest of the planets); surface gravity 3.7 (Earth = 1); visual albedo 0.11; average surface temperature 440 K (on the sunward side ranging from 590 K to 725 K).