Mercury

Mercury is the innermost planet of the solar system. Its diameter is 4800 km and its mean distance from the Sun 0.39 AU. The eccentricity of the orbit is 0.21, which means that the distance varies between 0.31 and 0.47 AU. Because of the high eccentricity, the surface temperature of the subsolar point varies substantially: at the perihelion, the temperature is about 700 K; at the aphelion, it is 100 K lower. Temperature variations on Mercury are the most extreme in the solar system

Table 7.3. Theoretical and observed temperatures of some planets

Albedo

Distance from the Sun [AU]

Theoretical temperature [K] (7.52)

(7.53)

Observed maximum temperature [K]

Mercury

G.G6

G.39

525

44G

7GG

Venus

G.76

G.72

27G

23G

75G

Earth

G.36

1.GG

29G

25G

31G

Mars

G.16

1.52

26G

215

29G

Jupiter

G.73

5.2G

11G

9G

13G

because in the night side the temperature drops below 100 K.

The precession of the perihelion of Mercury is more than 0.15° per century. When the Newtonian perturbations are subtracted, there remains an excess of 43". This is fully explained by the general theory of relativity. The explanation of the perihelion precession was one of the first tests of the general theory of relativity.

Mercury is always found in the vicinity of the Sun; its maximum elongation is only 28°. Observations are difficult because Mercury is always seen in a bright sky and close to the horizon. Moreover, when closest to the Earth in the inferior conjunction, the dark side of the planet is toward us.

The first maps of Mercury were drawn at the end of the 19th century but the reality of the details was not confirmed. As late as in the beginning of the 1960's, it was believed that Mercury always turns the same side toward the Sun. However, measurements of the thermal radio emission showed that the temperature of the night side is too high, about 100 K, instead of almost absolute zero. Finally, the rotation period was established by radar. One revolution around the Sun takes 88 days. The rotation period is two-thirds of this, 59 days. This means that every second time the planet is in, say, perihelion, the same hemisphere faces the Sun (Fig. 7.22). This kind of spin-orbit coupling can result from tidal forces exerted by a central body on an object moving in a fairly eccentric orbit.

Re-examination of old observations revealed why Mercury had been presumed to rotate synchronously. Owing to its geometry, Mercury is easiest to observe in spring and autumn. In six months, Mercury orbits twice around the Sun, rotating exactly three times around its own axis. Consequently, during observations, the same side was always facing the Sun! The details visible on the surface are very obscure and the few excep tional observations were interpreted as observational errors.

The best (and thus far unique) data were received in 1974 and 1975, when the US space craft Mariner 10 passed Mercury three times. The orbital period of Mariner 10 around the Sun was exactly twice the period of Mercury. The two-thirds-factor meant that the same side of the planet was illuminated during every fly-by! The other side is still unknown.

The Mariner 10 data revealed a moon-like landscape (Fig. 7.23). Mercury's surface is marked by craters and larger circular areas, caused by impacts of minor planets. The craters are 3000-4000 million years old, indicating that the surface is old and undisturbed by

Fig. 7.22. Length of day in Mercury. The positions of Mercury during the first revolution are shown outside the ellipse. Upon returning to the aphelion, the planet has turned 540° (1% revolutions). After two full cycles the planet has rotated three times around its axis and the same side points toward the Sun. The length of the day is 176 d, longer than on any other planet

Fig. 7.22. Length of day in Mercury. The positions of Mercury during the first revolution are shown outside the ellipse. Upon returning to the aphelion, the planet has turned 540° (1% revolutions). After two full cycles the planet has rotated three times around its axis and the same side points toward the Sun. The length of the day is 176 d, longer than on any other planet

Fig. 7.23. (Left) A mosaic picture of Mercury. (NASA) (Right) Surface details on Mercury. One of the most prominent scarps photographed by Mariner 10 during it's first encounter with Mercury in 1974. The scarp is about 350 kilometres long

continental drift or volcanic eruptions. Most of Mercury's surface is covered by old and heavily cratered plains but there are some areas that are less saturated and the craters are less than 15 kilometres in diameter. These areas were probably formed as lava flows buried the older terrain.

The largest lava-filled circular area is the 1300 km wide Caloris Basin. The shock wave produced by the Caloris impact was focused to the antipodal point, breaking the crust into complex blocks in a large area, the diameter of which is about 100 km. There are also faults that were possibly produced by compression of the crust. The volume change probably was due to the cooling of the planet.

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  • IIDA NURMINEN
    What is the main sequence of mercury?
    9 months ago

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