What Is Mercury Like? The Scorched, Shrunken Planet Nearest the Sun

Mercury orbits the Sun in 88 Earth days, at a distance of about 58 million kilometres, in an orbit more elliptical than any planet's but the dwarf Pluto's. It rotates slowly, once every 59 days, and the two are locked together in a 3-to-2 resonance: the planet turns three times for every two orbits, a state into which the Sun's tides pulled it long ago. The consequence is that a solar day on Mercury, from one sunrise to the next, lasts 176 Earth days, two of its years, and an observer on the surface at some longitudes would see the Sun rise, stop, go backwards for a while and then resume, as the planet's orbital speed near perihelion briefly outruns its spin. There is no atmosphere to speak of, only a thin exosphere of atoms knocked off the surface by the solar wind, so nothing spreads the heat, and the day side bakes while the night side freezes.

The surprise of Mercury is its density. It is only slightly larger than the Moon but nearly as dense as the Earth, and the reason is that its iron core takes up about 85 percent of its radius, against 55 percent for Earth's, so that the planet is a ball of metal with a thin rocky shell. Why is not settled: a giant impact early in its history may have stripped away most of the original mantle, or the young Sun may have vaporised the lighter rock from the innermost planet, or the material that condensed so close to the Sun was simply iron-rich to begin with. The core is partly liquid, since the planet has a magnetic field, weak but real, generated by the same dynamo process as Earth's and unexpected in a body so small and so slowly turning.

Mariner 10 photographed half of Mercury in three flybys in 1974 and 1975, and Messenger orbited it from 2011 to 2015 and mapped the rest. The surface looks like the Moon's, cratered from four billion years of impacts with no weather to erase them, and it has features of its own:

• •The Caloris basin, an impact crater 1,550 kilometres across, one of the largest in the solar system, ringed by mountains and with a patch of chaotic terrain at its antipode where the seismic waves of the impact converged
• •Lobate scarps, cliffs up to three kilometres high and hundreds long, formed as the planet cooled and shrank; Mercury's radius has contracted by about seven kilometres, and the crust buckled along these faults like the skin of a drying apple
• •Hollows, bright shallow pits with no equivalent elsewhere, apparently where volatile material is sublimating out of the surface even now
• •Volcanic plains, from eruptions that ended about 3.5 billion years ago
• •Ice: radar from Earth in 1991 saw bright reflections at the poles, and Messenger confirmed water ice, hundreds of billions of tonnes of it, in the floors of polar craters that lie in permanent shadow at minus 170 degrees, a few hundred kilometres from rock at 400 degrees

Mercury is close, and getting to it is harder than getting to Jupiter, because a spacecraft leaving Earth is already moving at Earth's orbital speed, and to drop inward toward the Sun it must lose a great deal of that speed, which takes as much fuel as gaining it. Falling toward the Sun also accelerates the craft, so it arrives too fast to be captured unless it can brake, and Mercury has no atmosphere to brake in. Messenger took six and a half years and six planetary flybys, one of Earth, two of Venus and three of Mercury itself, to shed enough speed to enter orbit, and the European-Japanese BepiColombo, launched in 2018, is taking seven years and nine flybys to arrive in 2026. Once there, the craft must survive sunlight eleven times as strong as at Earth and the heat radiated back from the surface, which is why both carry sunshades and instruments tucked behind them.

Mercury is the end member of the rocky planets, the one that formed closest to the Sun, and its iron-rich composition, its shrinking, its unexpected magnetic field and its ice are all tests of how the inner solar system was assembled. It is also a laboratory for gravity: the slow rotation of its elliptical orbit, 43 seconds of arc per century more than Newton's laws predicted, was the first evidence for Einstein's general relativity, in 1915, before any experiment had been done to test it. The planet that Copernicus never saw was the one that showed Newton was incomplete.

Mercury is the smallest planet and the nearest to the Sun, orbiting in 88 days and turning three times for every two orbits so that its solar day lasts two of its years, with a surface that swings between 430 degrees and minus 180 for lack of an atmosphere. It is mostly iron core, has shrunk enough to crack its crust into great scarps, keeps water ice in permanently shadowed polar craters, is harder to reach than Jupiter because a spacecraft must shed so much speed, and its orbit gave the first proof of general relativity.