Fundamental Information about Mercury

M ercury stands out from the rest of the terrestrial planets in several ways. It is the smallest of the four (the others are Venus, Earth, and Mars; see the table below), but it has the largest core. Mercury's core takes up three-quarters of the planet's radius and contains twice the metal than do Earth's, Venus's, or Mars's cores. Mercury's huge core makes the bulk planet exceptionally dense and therefore also gives it a strong gravity field for its size. The cause of its large core is unknown, as is the cause of its magnetic field. These and other mysteries about Mercury may be partially solved from data sent back by two new missions to the planet, the first to visit since 1975.

Fundamental Facts about Mercury

equatorial radius 1,515 miles (2,440 km), or 0.38 of the Earth's radius

ellipticity ([equatorial radius-polar

0, meaning the planet is almost a perfect sphere,

radius]/polar radius)

within the accuracy of the current radius measurements


1.30 X 1010 cubic miles (5.4 X 1010 km3), 0.05 times Earth's volume


7.3 X 1023 pounds (3.3 X 1023 kg), or 0.055 times the Earth's mass

average density

341 pounds per cubic feet (5,430 kg/m3), the second densest after Earth, and if gravitational compression is not considered, Mercury is the densest planet

acceleration of gravity on the

12.1 feet per second squared (3.70 m/s2), 0.37 times

surface at the equator

the Earth's gravity

magnetic field strength at

2 X 10-7

the surface





The astronomical symbol for Mercury has upstanding lines that resemble the wings on the helmet of the god Mercury, as shown in the figure on page 66. Each planet, the Sun, and a number of asteroids

Many solar system objects have simple symbols; this is the symbol for Mercury.

have their own symbols, which are often used in scientific figures and notes and other places where shorthand is helpful or space is at a premium.

Because of their spin, most planets are not perfect spheres. Spinning around an axis creates forces that cause the planet to swell at the equator and flatten slightly at the poles. Planets are thus usually shapes called oblate spheroids, meaning that they have different equatorial radii and polar radii. Mercury's radii, to the extent that they have been measured, are almost equal. Mercury is probably not truly spherical (Venus is the planet closest to a sphere, probably because it rotates exceptionally slowly), and better measurements will clarify Mercury's shape.

Because most planets' equatorial radii are longer than their polar radii, the surface of the planet at its equator is farther from the planet's center than the surface of the planet at the poles. To a lesser extent, the distance from the surface to the center of the planet changes according to topography, such as mountains or valleys. Being at a different distance from the center of the planet means there is a different amount of mass between the surface and the center of the planet. Mass pulls with its gravity (for more information on gravity, see the sidebar "What Makes Gravity?" on page 67). Gravity is not a perfect constant on any planet: Variations in radius, topography, and the density of the material underneath make the gravity vary slightly over the surface.This is why planetary gravitational accelerations are

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  • katharina
    Where is natula mercury?
    7 months ago

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