Mercury is the planet closest in to the Sun. Because of its small size it retains barely a trace of atmosphere—a high vacuum, by Earth standards—composed of trapped solar wind, radioactive decay products and vapors from cometary impacts. Because of this lack of significant atmosphere, the temperature varies greatly between day and night and between equator and pole. Polar nights can get down to -193° C, whereas equatorial day temperatures reach nearly 430° C. The crater-pocked surface is drenched in high energy solar radiation.
At one time, Mercury was believed to be phase-locked to the Sun; that is, that it rotated once each orbit, always keeping the same face to the Sun, like the Moon always keeps its same face toward Earth. However, in 1965, radar observations showed that it really spun in just under 59 days, a period that turned out to be exactly 2/3 of its year. This was mathematically shown to be a stable resonance; Mercury completes 3 full days for every 2 of its years.
Mercury's orbit is more eccentric (elliptical) than most planets. One interesting result of this, combined with its slow rotation rate, is that for 4 Earth days, the sun appears to stop in the sky and actually move backwards, before resuming its normal motion. On the parts of the planet experiencing sunrise and sunset, this can have interesting effects.
In the mid 1800s, scientists noted that the perhelion point (point in its orbit when it is closest to the sun) was advancing faster than could be explained by perturbations from other planets. To try to account for this, astronomers proposed the existence of another planet, even closer to the sun (they even gave it the name Vulcan), but no such planet was ever found. In the early 1900s, this perhelion advance was finally explained by Einstein's General Theory of Relativity, thus providing evidence in support of what was then a radical new explanation of space, time and gravity.
The surface of the planet is largely covered by craters, and also bears strong evidence of past volcanic eruptions. It is currently not believed to be geologically active, although models of Mercury suggest that it has a relatively large molten core. Surprisingly, for a planet so close to the Sun, there is evidence from radar imaging of ice in some craters near the north pole, whose rims are high enough that sunlight never reaches their floors.
Another interesting feature of Mercury's surface are so-called rupes. These “wrinkle ridges” crisscross the Mercurian plains all over the planet. The most likely explanation is that the planet has shrunk at some time in its history. Since they cross lava plains and craters, they are newer, indicating that the shrinkage occurred (relatively) recently.
Mercury does not have any natural satellites.
Mercury is one of the 5 planets (aside from Earth, of course!) that was known to the ancients. Because of its proximity to the Sun, it never gets more than 27.8° away from it in the sky. Before the 4th century BC, the Greeks thought that Mercury was actually two objects, and they called it Apollo when it was visible at sunrise, and Hermes (the Greek god equivalent to the Roman god Mercury) when it was visible at sunset. It was the Romans who gave it its present name.
The ancient Babylonians called the planet Nabu, which interestingly was their “messenger to the gods”. The Chinese called it Ch'en-Hsing. To the Hindus it was Budha, and the Germanic pagan god Odin (or Woden) was associated with the planet Mercury, and with the day of the week Wednesday (which comes from “Woden's Day”).
“Preserved on Mercury”
Taken by MESSENGER
In spite of Mercury being one of the closer planets to Earth, it has only been visited by two spacecraft. The first, Mariner 10, made three close flybys before running out of fuel and being shut down. It is believed to still be orbiting the Sun near Mercury and passing close to the planet on a regular basis, but without communication, no one knows for sure. Mariner 10 took a number of close up photos and performed a variety of measurements, including Mercury's magnetic field, which turned out to be similar to Earth's, although only about 1.1% as strong. But because of the combination of Mercury's rotational rate and the spacing of the flybys, nearly half of the planet was still unimaged.
Only recently as Mercury been revisited. The MESSENGER probe was launched in 2004 and sent on a complicated orbit with multiple flybys of Venus and Mercury, with the end goal of putting the probe into orbit around Mercury. The biggest difficulty with putting a probe in orbit around Mercury is the large difference in orbital speed; the complex series of flybys was designed to end up approaching the planet with a small enough relative velocity that its engines could brake it into orbit. It all paid off on March 18, 2011, when MESSENGER successfully fired its braking rockets and entered orbit around Mercury.
Even before entering orbit, MESSENGER had sent back a lot of new science data, including imaging most of the portions of the planet that had been missed by Mariner 10. After entering orbit, it continued to make new discoveries about the closest planet to the sun, as well as send back thousands of high-resolution photos of the surface. After running out of propellant in late 2014, it eventually crashed to the surface on April 30, 2015.
On October 20, 2018, the European Space Agency, in partnership with the Japan Aerospace Exploration Agency, launched the BepiColombo probe. Consisting of two components, the Mercury Planetary Orbiter and the Mercury Magnetospheric Orbiter, it is currently on its way, and is expected to arrive at Mercury late in 2025.
Copyright © 2005-2021 William R. Penning. All rights reserved.