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# 6.4 Mercury

Mercury is the planet closest to the Sun. Not surprisingly, it is very hot, with surface soil temperatures well above 500 K (441°F) on the sunlit side. (Science fiction writers used to describe pools of molten metal on Mercury, but this is unlikely; the temperature isn't quite high enough to melt common metals, especially in the sub-surface soil.) There is essentially no atmosphere, and without a blanket of air to hold in the heat, the surface temperature drops to an extreme low of about 100 K (-279°F) at night.

Mercury is about a third the size of Earth but about a third bigger than the Moon. It is so small and far away that its angular size is too small for even the Hubble Space Telescope to show many surface features. However, it was photographed at close range in 1974 and 1975 by the American robotic space probe Mariner 10. The photos revealed that Mercury closely resembles the Moon: it is heavily cratered and has areas that appear to be lava plains. Further analysis of the Mariner 10 data in 1997, as well as Earth-based observations, strengthen the view that the dark plains are basaltic plains superimposed on a more feldspar-rich crust — very similar to the features of the Moon. Only one hemisphere was photographed by Mariner 10. The other still remains to be explored. One region of Mercury contains not only craters, but also a large, multi-ring impact basin, similar to examples found on the Moon. The similarity of craters on Mercury and our Moon shows that impact cratering of planets has happened widely throughout the solar system.

Mercury takes 88 Earth-days to go around the Sun. In 59 days, it makes one complete turn on its axis with respect to the distant stars (in other words the time it takes for any star to return to the same place in the Mercury night sky). Do you notice a relationship between the two numbers? The 59-day rotation (a Mercury "day") is exactly two thirds of Mercury's 88-day "year." Is it only chance that the ratio is exactly 2/3? No. Due to tidal forces and complex gravitational effects called resonances, Mercury's 59 day rotation period has stabilized at two thirds of the 88-day "year." This phenomenon is also observed in patterns in the orbits of moons and rings elsewhere in the solar system.

There is an odd noontime-to-noontime "day" on Mercury. The combination of the 59-day spin and the 88-day "year" means that the time from one noontime to the next at any spot on Mercury's surface averages 176 days! The number 176 is not mysterious; it is the lowest number that is an even multiple of the spin rate and the rotation rate around the Sun. In other words, Mercury experiences about 88 days of burning daylight followed by 88 days of frigid night. In addition, the Sun's movement across Mercury's sky is not as regular and steady as across Earth's sky because Mercury's orbit is not as circular as the Earth's. This creates a slow "wobble" in the Sun’s slow movement across the sky, relative to the horizon.

Mercury's orbit is not circular, so it has a certain point in its orbit where it is closest to the Sun — called the perihelion (from the Greek roots peri = around, and helios = the Sun). In the 1800s, scientists were surprised to find that this point shifts in position slowly around the Sun, by a tiny angle, from year to year. The shift could not be explained by Newton’s laws of gravity. In the late 1800s, scientists thought the shift must come from the gravitational force of an unknown planet between Mercury and the Sun. (This hypothetical planet was called "Vulcan." This is the origin of the name for Mr. Spock's home planet on the TV show Star Trek.) But later observations showed that no such planet exists.

The solution to the mystery of Mercury’s orbit came in 1915, when Albert Einstein modified and improved Newton's laws with his new theory of relativity. Einstein's modifications describe the relationships between gravitation, space, and time differently than Newton did. Einstein's laws predicted exactly the rate of shift that was observed in Mercury's orbital orientation. Thus, the solution to the mystery of Mercury's orbit played a major role in worldwide acceptance of Einstein's theory of relativity early in this century. This is dramatic example of how science works. It is rare that a single observation leads to a new theory. Yet this example of a small shift in the orbit of a single planet led to a new conception of gravity.

Mercury is generally known as a very hot planet, since the daytime side gets broiled by the nearby Sun. It was all the more amazing, then, what was discovered in 1991 by bouncing radar signals off Mercury. Radar images revealed strange deposits located in the shadows of craters. These deposits suggested the existence of ice caps at the poles, which were reminiscent of the ice deposits on polar craters on the Moon. Like the Moon, Mercury has polar craters whose floors are in permanent shadow. These cold craters may trap water molecules released when comets hit the surface of the planet. Familiar terrestrial substances like water exist throughout the solar system — even on barren, lifeless worlds like Mercury and the Moon.

Imagine what it would be like to visit the north pole of Mercury. Unlike the Earth, with its 23.5° axial tilt, Mercury has a 0° axial tilt. This means that if you stand at the north pole of Mercury, the Sun is exactly on the horizon all year long. (As seen from Earth's north pole, in contrast, the Sun stays above the horizon throughout spring and summer, rising as high as 23.5° above the horizon in June, and then drops below the horizon throughout autumn and winter.) If you stood on the depressed floor of a crater at Mercury's pole, the Sun would always be hidden by the crater walls, and the floor would be in perpetual shadow.

Mercury by comets, which sometimes hit the planet. Ice and sulfur contained in the comet might have vaporized and created a cloud of gas molecules near Mercury's surface; the water vapor or sulfur gas molecules might then have condensed into solid frost on the cold rocks in the dark craters at the poles. If the deposits really contain H2O ice, they could be a welcome resource if human explorers ever attempt to land on the otherwise forbidding planet.

We got a close-up look at Mercury recently when NASA sent the Messenger spacecraft there a few years ago. Messenger was launched in 2004 and in 2011 became the first spacecraft ever to orbit Mercury. Its mission lasted four years and on April 30, 2015, it used the last of its propellant and executed a planned crash landing into the surface. Data from Messenger yielded a 3D map of the surface in exquisite detail. There was an unexpected discovery of large amount of water vapor high in Mercury's thin atmosphere. Spectroscopy of the surface showed water ice and organic compounds in permanently shadowed craters near the north pole. There was evidence of past volcanic activity, surprising for such a small planet, barely larger than Earth"s Moon. As you can tell, any time we get more detailed information about a Solar System object, there are surprises.

Mariner 10 probe that visited Mercury in the 1970s. Click here for original source URL.

Mercury as seen by the Messenger spacecraft in late 2008. Click here for original source URL.