(☿) The closest planet to the Sun. It has the most elliptical orbit (eccentricity 0.206) of all the major planets, so that at perihelion it is only 46 000 000 km from the Sun's centre, but 69 820 000 km at aphelion. Its mean geometric albedo, 0.11, is similar to the Moon's, and its overall colour is grey. Mercury's mean magnitude at greatest elongation is 0.0, but it keeps close to the Sun in the sky and so is seldom visible to the naked eye. Its period of axial rotation is exactly two-thirds of its orbital period, an example of spin–orbit coupling. As a result, two lines of longitude, spaced by 180°, experience the Sun overhead at perihelion, making these two regions the hottest on Mercury.
Mercury has no permanent atmosphere, although some hydrogen and helium from the solar wind is temporarily captured. The surface temperatures average about 170°C, but Mercury has the most extreme temperature range of any planet in the Solar System, becoming extremely hot during the day, over 450°C at the subsolar point at perihelion, and rapidly dropping below –183°C during the long night. Dark and bright surface markings can be glimpsed through a telescope, but they have much lower contrast than the markings on Mars or the Moon. The Mariner 10 probe photographed half the planet in detail, revealing a lunar-like landscape heavily scarred with impact craters, many with bright rays. The largest known crater is Beethoven, 640 km wide. There are some slight differences in cratering style: on Mercury, secondary craters fall closer to the main crater than they do on the Moon because of the higher gravity, and inner rings are seen in smaller craters than on the Moon. The largest impact structure on Mercury, the Caloris Basin, is 1300 km across, similar in size to the Moon's Imbrium Basin. The very small axial tilt of Mercury means that, like the Moon, it is likely to have craters in its polar regions that are never sunlit. Such craters are cold enough that they can preserve water ice, perhaps from impacting comets. Evidence for this has come from terrestrial radar studies that indicate the presence of water ice in some craters at both the south and north poles.
There are no obvious volcanoes on the planet, nor any sinuous rilles to indicate lava eruption, nor are there any dark maria. However, the widespread smooth plains material, which has obscured part of the rim of the Caloris Basin, for example, and infilled many impact craters to make them flat-floored, is probably lava, although it could be ejecta or impact melt from the large basins. Lobate scarps up to 500 km long are found in many areas on Mercury, and appear to be thrust faults resulting from sideways compression. These scarps and the general lack of tensional features such as the graben found on the Moon are evidence that the planet has contracted, probably as a result of cooling.
Mercury's high density suggests that it is composed of about 70% iron, probably concentrated in a central core, and 30% rock. The iron-rich core probably has a diameter 75% of that of the planet, proportionally the largest of any planetary body known. The planet has a weak magnetic field with a strength of 3 × 10−7 tesla, around 1% that of the Earth. It has no natural satellites.
Subjects: Astronomy and Astrophysics.