6. Mercury: a battered world

Basic data

Physical properties of Mercury*

Mass 3.302 x 1023 kilograms = 0.0553 ME
Mean radius 2.439 x 106 meters = 0.382 RE
Mean mass density 5,430 kilograms per cubic meter
Rotation period 58.646 Earth days
Orbital period 87.969 Earth days
Mean distance from Sun 5.79 x 1010 meters = 0.387 AU
Age 4.6 x 109 years
Atmosphere Very tenuous (helium, sodium, potassium)
Surface pressure 2 x 10-13 bars
Surface temperature 90 to 740 degrees kelvin
Magnetic field strength 0.0033 x 10-4 tesla at the equator = 0.01 BE
Magnetic dipole moment 5.54 x 1013 tesla meters cubed
* The symbols ME, RE and BE respectively denote the mass, radius and magnetic field strength of the Earth.

A tiny world in the glare of sunlight

A small, elusive planet

Mercury revolves closer to the Sun than any other known planet, with a mean distance from the Sun of just 0.387 AU. It therefore has the shortest year - 88 Earth days - and the highest orbital speed of any planet.

As planets go, Mercury is a tiny world, with the smallest size of any terrestrial planet and slightly smaller than Jupiter's satellite Ganymede and Saturn's Titan. Mercury's linear radius is easy to measure from its angular radius and distance. Its radius is 2,439 meters or just 1.4 times the radius of our Moon.

Mercury is surprisingly massive for its size. Its volume is only slightly larger than the Moon's and yet it has four times the Moon's mass. This implies a mean mass density of 5,430 kilograms per cubic meter, which is nearly as high as that of the Earth, 5,515 in the same units and a little more than Venus at 5,250.

Fig. .. 

Mercury's small apparent size and its proximity to the Sun make it difficult to see from Earth. The innermost planet never wanders more than 27.7 degrees in angular separation from the Sun. This angle is less than that made by the hands of a watch at one o'clock. From Earth's perspective, Mercury's tight orbit never reaches into the dark night sky, and it can thus be observed only during the day.

Wild temperature swings in an airless world

Solar radiation is ferocious on the surface of Mercury, the closest planet to the Sun. It is subject to the most intense sunlight, and experiences the greatest diurnal temperature variations, of any planet in the solar system. When Mercury is at the closest point in its orbit to the Sun, the noon-time ground temperature on the side facing the Sun soars to 740 degrees kelvin. This is hot enough to melt tin, lead, and even zinc. Because there is almost no atmosphere to hold in the heat, the ground temperature on Mercury plunges to 90 degrees kelvin, or 183 degrees below zero, on the night side.

There is an exceedingly tenuous atmosphere on Mercury, discovered with instruments on the Mariner 10 spacecraft - hydrogen and helium atoms - and subsequently using ground-based telescopes - sodium and potassium atoms. But it is constantly being evaporated away by the Sun's intense heat and replenished from below. The rarefied gas is so thinly distributed that its particles almost never touch each other, and they only hit the surface. This thin atmosphere is a far better vacuum than can easily be produced in a laboratory on Earth.

Radar probes of Mercury

The halting spin of old age

Fig. ..a 

In 1965, Mercury's true rotational period was determined with radio signals that rebounded from the planet. The world's largest radio telescope, located in Arecibo, Puerto Rico, was used to transmit mega-watts of pulsed radio power at Mercury, and to receive the faint echo. This technique is known as radio detection and ranging, abbreviated radar, and it is used to locate and guide airplanes near airports.

Fig. .. 

Each pulse was finely tuned, with a narrow range of wavelengths. Upon hitting the planet, its rotation de-tuned the pulse, slightly spreading the range of wavelengths. One side of the globe was rotating away from the Earth, while the other side was rotating toward our planet. These motions produced slight changes in the wavelength of the echo and from these changes, the speed of the surface and the rotational period were calculated, using the well-known expression for the Doppler effect.

Long, hot afternoons on Mercury

Fig. .. 

The spin-orbit coupling has a curious effect that may have misled astronomers who reported the wrong rotation period. Because three times the true rotation period is equal to twice the orbital period, any surface markings on Mercury would have returned to the same side after two orbital revolutions. Thus, astronomers could have been fooled, because looking at Mercury after two of its orbital periods they would see the same markings on the sunlit side and would find no disagreement with the 88-day period that they expected. Many of the conflicting observations were apparently ignored or missed. This is a striking example of curious observational circumstances and theoretical expectations that misled nearly everyone.

The days are certainly long on Mercury, longer than the planet's year. At any given point on Mercury, the daylight interval between sunrise to sunset lasts 88 Earth days, and the night lasts 88 Earth days more. This means that at one location on the surface, successive sunrises occur every 176 Earth days. So, the full day on Mercury is twice the length of its year.

Possible water ice at the poles of Mercury

Despite the heat on the sunlit side of Mercury, radar astronomers have found evidence for ice at both the north and south poles. Both the intensity and orientation, or polarization, of the bright radar echoes suggest the presence of water ice. Their radar characteristics are similar to those seen on icy surfaces elsewhere in the solar system, such as the ice deposits on the Galilean satellites and the south polar ice cap on Mars.

Fig. .. 

Moreover, the radar-bright features have been matched with specific polar craters, located on Mariner 10 images, and these craters should contain permanently shadowed interiors. On the other hand, the shaded polar craters could contain other volatile substances, such as sulfur, which could produce strong radar echoes but have a higher melting point than water ice. So, we may not definitely know if there is water ice at the top and bottom of Mercury until inquisitive robot spacecraft land there and make the appropriate tests. In the meantime, we turn to the startling results of Mariner 10.

A modified Moon-like surface

Craters like the Moon

Fig. ..  Fig. .. 

At first glance, Mercury closely resembles the Moon, for both worlds are small, heavily cratered, and without a significant atmosphere to cause erosion. Like the Moon, the planet Mercury has highlands that are pockmarked with impact craters, ranging in diameter from impact basins a million meters across to craters only 100 meters in diameter, the limit of Mariner 10's photographic resolution. The ubiquitous craters on Mercury strongly resemble their lunar counterparts, indicating that they were formed by meteoritic impact. As on the Moon, there are small bowl-shaped craters, larger craters with terraces and central peaks, relatively young craters with bright rays, and huge impact basins on Mercury.

Intercrater highland plains

Fig. .. 

In many important respects, Mercury's resemblance to the Moon is superficial. The planet's most densely cratered surfaces are not as heavily cratered as the lunar highlands, and Mercury does not contain regions of overlapping large craters and basins. Also unlike the Moon, the heavily cratered terrain on Mercury is interspersed with large regions of gently rolling, intercrater plains.

Smooth lowland plains

Fig. .. 

Widespread areas of Mercury are covered by relatively flat, sparsely cratered terrain called the smooth lowland plains. They are younger than the intercrater highland plains, and they are about 2 thousand meters lower. Unlike the dark maria on the Moon, the smooth lowland plains on Mercury are about the same brightness or color as the heavily cratered terrain and intercrater plains in the highlands of Mercury.

Fig. .. 

The smooth plains occur within and around the Caloris basin and on the floors of other basins, but a careful study of crater densities suggests that the smooth plains are younger than the Caloris impact. This age difference suggests that the lowland plains on Mercury are volcanic eruptions from the interior, rather than surface material melted and thrown out during basin-forming impacts. An investigation of the colors of sunlight reflected from the surface, and by implication the minerals it contains, support the view that some of the smooth plains originated by volcanic outflow.

Rupes - cliffs or scarps

Fig. .. 

The most remarkable geological features on Mercury are its winding cliffs or scarps, that are widely distributed over the planet. These unique features have been named rupes, Latin for "rock or cliff", each preceded by the name of a ship of discovery or a scientific expedition. An example is Dicovery Rupes, named after the Captain James Cook's (1728-1779) ship on his third and last voyage to the Pacific from 1776-80. The long cliff snakes its way across pre-existing craters and plains, attaining a length of 350 thousand meters and rising to 4 thousand meters, as high as the Pyrenees.

An iron planet

Fig. .. 

The terrestrial bodies - the Moon, Mars, Venus and Earth - exhibit a fairly linear relationship between size and mean mass density, in which bigger objects have greater density. But Mercury does not conform to this relationship. Although it is less than half the size of the Earth and not much bigger than our Moon, the bulk mass density of Mercury, at 5,430 kilograms per cubic meter, is typical of a far larger planet. The most natural explanation of Mercury's high mass density is that it contains an unusual amount of iron, which is cosmically the most abundant heavy element.

In order to account for the planet's large mass density, the apparent dearth of iron on the surface has to be balanced by a large iron core. Relative to its size, Mercury would have the largest metallic core of all the terrestrial planets. The dense iron core takes up 75 percent of the planet's radius, so Mercury is mostly iron core surrounded by a relatively thin silicate mantle. In this respect, it is the opposite of the Moon that has a thick rocky mantle and a relatively small iron core.

A mysterious magnetic field

Fig. .. 

Mariner 10 carried a sensitive device for detecting magnetic fields, a magnetometer. As the spacecraft moved toward Mercury, the magnetometer plotted the fluctuating magnetic field of the solar wind. However, when it reached the neighborhood of Mercury, it abruptly entered a new environment - a magnetic field that emanated from the planet. The increasing strength of this field as the spacecraft approached the planet indicated that the magnetic field of Mercury's surface would be 0.0033 x 10-4 tesla, or 0.01 times the Earth's surface magnetic field. Such a field was strong enough to carve out of the solar wind an elongated magnetic cavity with a tail; pointing away from the Sun.

Einstein and Mercury's anomalous orbital motion

Fig. .. 

Instead of returning to its starting point to form a closed ellipse in one orbital period, Mercury moves slightly ahead in a winding path that can be described as a rotating ellipse. As a result, the point of Mercury's closest approach to the Sun, the perihelion, advances by a small amount, 43 seconds of arc per century, beyond that which can be accounted for by planetary perturbations using Newton's law of gravitation. This unexpected effect is known as the anomalous precession of Mercury's perihelion.

Fig. .. 

According to Einstein's theory, known as the General Theory of Relativity, space is distorted and curved in the neighborhood of matter, and the distortion is the cause of gravity. In the absence of matter, space is not distorted and is described by the geometry developed by the ancient mathematician, Euclid (around 300 BC). In the presence of matter, space becomes curved and it must be described by non-Euclidean geometry.

The result is a gravitational effect that departs slightly from Newton's expression near a very massive object, and the planetary orbits are not exactly elliptical. This produces an advance of perihelion, and the amount predicted by Einstein for Mercury was 43 seconds of arc per century - exactly the observed amount. Because the amount of space curvature produced by the Sun falls off with increasing distance, the perihelion advances for the other planets are much smaller than Mercury's.

Summary Diagram

Fig. ..summary