c. 4.5 Billion BCE

Mercury

All the planets in our solar system formed around the same time, about 4.5 billion years ago, as the Solar Nebula cooled and tiny grains condensed, collided, stuck together, and eventually grew into a small number of big objects. In the warm zone close to the Sun, the planets were rocky. Farther out, beyond the “snow line,” they were mixtures of rock, ice, and gas.

Mercury is the closest in of the so-called terrestrial planets, with a diameter of 3,032 miles (4,880 kilometers); Earth’s diameter, by comparison, is 7,926 miles (12,756 kilometers). Mercury orbits the Sun at an average distance of only 0.38 astronomical units (or AU; 1 AU = 93 million miles = 150 million kilometers = Earth’s average orbital distance from the Sun). Mercury is the Roman name of the Greek god Hermes, the fleet-footed messenger. The planet was aptly named: even the ancients knew that Mercury takes only 88 days to complete a circuit in the sky, which we now know represents its orbital period around the Sun.

Mercury is a small world of harsh extremes and curious enigmas. There is no atmosphere, and temperatures range from only 90 kelvins in permanently shadowed craters near the poles to more than 700 kelvins (above the melting point of lead) in the harsh midday sunlight. Earth-based radar observations indicate that there may be ice in those polar craters. Mercury has a very high density and a large iron core that spans 75 percent of the planet’s radius. The core might be partially molten, perhaps explaining Mercury’s weak magnetic field (1 percent as strong as Earth’s). Images from the two space missions that have encountered Mercury (Mariner 10 in 1974–1975 and MESSENGER in 2011–2015) reveal a heavily cratered surface and some evidence of ancient volcanic activity similar to the Moon’s. Perhaps most surprising, the planet preserves a network of large tectonic thrust faults (scarps) that seem to indicate that Mercury may have been completely molten early in its history and then shrank by a few percent when it cooled.

NASA’s MESSENGER spacecraft flew past Mercury three times in order to set itself up to go into orbit around the planet in 2011. This third flyby image was taken in January 2008 and revealed many never-before-seen craters and other features.