Originally published in:
Encyclopedia of Planetary Sciences, edited by J. H. Shirley and R. W. Fainbridge,
476-478, Chapman and Hall, New York, 1997.
Mercury is the smallest of the terrestrial planets. Its radius of 2440 km places it between the Earth's Moon and Mars in size. It is of great importance to those studying planetary magnetic dynamos and to those studying planetary magnetospheres. Its importance to the magnetic dynamo problem stems from its being the smallest and most slowly rotating planet with a presently active magnetic dynamo. Its importance to the physics of planetary magnetospheres stems from its lack of a dynamically important atmosphere or ionosphere. Currents generated by the solar wind interaction, which usually close in the ionosphere, cannot close in the same way at Mercury as they do in other planetary magnetospheres. It is thought therefore that the Mercury magnetosphere may be more strongly coupled to the solar wind than is the case for other planetary magnetospheres.
Because of its small size and outward appearance, due largely to the absence of a significant atmosphere, it is most appropriate to compare Mercury with the Earth's Moon. Mercury rotates more slowly than the Moon, rotating with a period of 59 days compared with the Moon's 28-day period. Mercury also differs from the Moon in that its rotation is not synchronous with its orbital period. Mercury orbits the Sun every 88 days so that every 2 Mercurian years, the same side of the planet again faces the Sun. The slow rotation, close proximity to the Sun and lack of atmosphere causes a very high surface temperature ( 630 K) on the dayside of the planet and very cold temperatures on the nightside. The role of the atmosphere in controlling surface temperature can he appreciated by noting that Venus, with the most massive atmosphere of the terrestrial planets, has day and night temperatures that differ at most by a few degrees.
A unique characteristic of the rotation of Mercury is that its rotation axis is aligned along its orbital pole. Every other planet in the solar system has a rotation axis that is tilted with respect to its orbital pole, affecting, and in most cases dominating, seasonal changes. This oddity may influence both atmospheric and internal processes.
Mercury, with a density of 5.4 cm-3, is much denser than the Moon (whose density is 3.3 g cm-3). In fact, it is much more dense than the Earth, when compared at constant internal pressure (5.3 g cm-3 versus 4.1 g cm-3 at 10 kbar). The high average density implies a metal-rich interior, perhaps 70% iron-nickel and 30% silicate. In the absence of measurements from the surface or from orbit, the interior properties of Mercury are constrained mainly by the total mass and size of the planet. Because of the relatively large density of Mercury, the core must occupy a larger fraction of the planet than is the case for the Earth. Moreover, since Mercury is smaller than the Earth, it should have cooled more rapidly and its solid inner core should be an even larger fraction of the radius of the liquid core than is the case for the Earth. Thus the remaining liquid core may be confined to a rather thin shell. As a result of these differences, it is possible that the dynamo that supports the magnetic field of Mercury differs substantially from the terrestrial dynamo. Only rudimentary constraints are presently available on the nature of the Mercury field.
As discussed elsewhere in this volume, a tenuous sodium and potassium atmosphere has been detected at Mercury. While various mechanisms have been proposed to explain this tenuous atmosphere, one possible source is outgassing of the planetary interior, suggesting in turn an internally active planet.
Mercury has been visited by only one spacecraft, Mariner 10, which made three passes by the planet between March 1974 and March 1975. The first and third passes were suitable for studying the planetary field. On the first pass the spacecraft crossed the darkside of the planet within 723 km of the surface, at which point the field strength reached a maximum of close to 100 nT. The characteristics of the field resembled those of a mini- magnetosphere, in which the solar wind is deflected above the surface of the planet around a distorted dipole field. In contrast, the lunar magnetic field is so weak that the solar wind impinges on the surface and is absorbed. The third Mercury pass also traversed the darkside of the planet, approaching within 327 km of the surface and observing a maximum field of 400 nT. Again the characteristics of the observed field resembled those expected in a miniature version of the Earth's magnetosphere.
These two passes provided weak constraints on the magnitude of the intrinsic magnetic field, its orientation and its harmonic structure, in part because the coverage of the planetary field was poor and in part because of the lack of concurrent observations of the solar wind number density and velocity. The strength of a planetary magnetic field is measured in terms of its magnetic moment, the product of the equatorial surface field and the cube of the planetary radius. Estimates of the dipole moment of Mercury range from about 2 to 6 X 1012 T m3, and the strength of the quadrupole moment and the tilt of the dipole moment are completely unconstrained. The dipole moment is known, however, to be pointed southward like the Earth's. Alternative sources to dynamo generation of the field have been proposed, such as remanent magnetization of an iron-rich crust, but it is difficult to obtain a strong enough field through these alternate mechanisms.
|Fig. 1. Cross-section of the magnetosphere of Mercury in the noon-midnight meridian. Solid lines anchored in the planet represent the direction of the magnetic field. The Mercury magnetic field lines point into the planet in the northern hemisphere as they do on Earth.|
The weak magnetic moment of Mercury, about 4 x 10-4 of that of the Earth, combined with a solar wind pressure about seven times larger than the pressure at Earth, results in a very small planetary magnetosphere (in both absolute dimensions, and relative to the size of the planet). This magnetosphere is sketched in Figure 1 which shows the magnetic field lines in the plane containing the Sun and the magnetic dipole axis. The magnetic cavity deflects the solar wind at a distance of only 1.5 Mercury radii from the center of the planet. Since the solar wind moves faster (relative to Mercury) than the pressure wave needed to deflect the solar wind can propagate in the solar wind, a shock wave is formed in front of the magnetic cavity. This shock wave heats, slows and deflects the solar wind to allow it to flow around the magnetic cavity or magnetosphere. Similar bow shocks are found in front of all planetary magnetospheres. The energy dissipation that is required to heat the flow occurs through collisionless processes in which the electric and magnetic fields scatter the particles and the particles do not make direct collisions with each other. Thus these shock waves are often called 'collisionless' shocks. An important question for all planetary magnetospheres is the coupling of the energy flux in the solar wind to the planetary magnetosphere. In the Earth's magnetosphere, stresses are communicated from the solar wind to the ionosphere and atmosphere and hence the solid body of the planet by electrical current systems which flow along magnetic field lines and then close across the magnetic field in the lower ionosphere. Mercury has no dynamically significant ionosphere or atmosphere, so the coupling must be quite different than in the terrestrial case. Based on dynamic events observed on the first Mariner 10 flyby, the Mercury magnetosphere is thought to be dynamic, varying markedly in the course of minutes. Clearly the energy transfer from the solar wind is much greater than on the Earth. As in the terrestrial magnetosphere, it is thought that the magnetotail is an important site for energizing the plasma of the magnetosphere. However, very little is known about the Mercury magnetosphere.
This work was supported in part by the National Aeronautics and Space Administration under research grant NAGW-2573.
Connerney, J. E. P. and Ness, N. F. (1988) Mercury's magnetic field and interior, in Mercury (eds F. Vilas, C. R. Chapman and M. S. Matthews). Tucson: University of Arizona Press, pp. 494-513.
Russell, C. T. (1987) Planetary magnetism, in Geomagnetism, Vol. 2 (ed. J. A. Jacobs). London: Academic Press, London pp. 457-523
Russell, C. T., Baker, D. N. and Slavin, J. A. (1988) The magnetosphere of Mercury, in Mercury (eds by F. Vilas, C. R. Chapman and M. S. Matthews) Tucson: University of Arizona Press, pp. 514-61.