6. Mercury: a battered world
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.
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.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University