Climate
Mercury's equatorial plane is inclined less than 2° to its orbital plane (Klaasen, 1976; Murray and others, 1981, p. 28); its rotation period of 58.64 terrestrial days is in two- thirds resonance with its orbital period of 87.97 terrestrial days (Colombo, 1965; Colombo and Shapiro, 1966). The resulting lag and orbital eccentricity create a variation of mean temperature not only with latitude, as on the Earth, but also with longitude. However, because of Mercury's relatively slow rotational period, diurnal variations in temperature probably greatly exceed mean-temperature variations along latitude and longitude, even in the high latitudes. Its pronounced orbital eccentricity (0.2563) causes the apparent solar intensity at Mercury to vary by more than a factor of 2 throughout a Mercurian year, corresponding to about a 20 percent change in equilibrium temperature. Further, conservation of orbital angular momentum and spin-orbit coupling cause considerable variation in the length of daylight. Dawns and sunsets are prolonged by the long transit time of the Mercurian horizon across the solar disk, so that daylight is lengthened and nighttime reduced by several terrestrial days at sunset and vice versa at sunrise (Robert Wildey, U.S. Geological Survey, oral commun., 1982). Despite these considerations and despite the daily range in surface temperatures of several hundred kelvins, the subsurface temperature in the polar regions always remains well below freezing (Murray, 1975).
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Famous quotes containing the word climate:
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—Elizabeth Bowen (1899–1973)