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Space Topics: MercuryMercury Facts and Pictures
Small, fast-moving Mercury is one of the least understood objects in the solar system. Because of its proximity to the Sun, it is very difficult to observe from Earth. It always rises or sets within two hours of the Sun, so its features are washed out by the Sun’s glare. Its proximity to the Sun also makes it a challenge to visit with spacecraft. Only one spacecraft has ever been to Mercury -- Mariner 10, which flew by three times in 1973. Those three flybys saw the same hemisphere of Mercury, so fully half of the planet’s surface has never been seen up close. Mercury’s rotationPrior to Mariner 10, little was known about Mercury. It was once thought that Mercury always kept the same face toward the Sun, as the Moon always keeps the same face toward Earth. This phenomenon -- known as a “1:1 spin-orbit resonance” or “synchronous rotation” is very common for small spherical bodies located close to giant ones in our solar system: in fact, nearly all of the major moons in the solar system rotate synchronously. If Mercury rotated synchronously, it would have an intensely hot side always facing the Sun, and an intensely cold side that never saw the Sun. However, early radio astronomy observations revealed that Mercury actually does rotate once every 58.6441 days -- exactly 2/3 its orbital period. In other words, Mercury is an a spin-orbit resonance, but it’s a 3:2 spin-orbit resonance, rotating 3 times for every 2 times it travels around the Sun. The combination of the two, rotation and revolution, give Mercury solar days that are 176 days -- or 2 Mercury years -- long. Mercury’s densityThe next thing that was learned about Mercury was its density. Its bulk density -- mass divided by volume -- was found to be quite high, comparable to Earth’s. Mercury is sometimes called the densest object in the solar system, though that’s not exactly true. Earth actually has a higher bulk density. But Earth is much larger than Mercury (Mercury is only 5.5% as massive as Earth). The greater mass and gravity of Earth cause its interior to be more highly compressed than Mercury, despite the fact that Earth is, on average, made of lighter stuff. If you could remove the effect of gravity, Mercury would be the densest planet, because it has the largest metal core, relative to its size, of all the planets. Mercury’s iron core occupies 75% of its diameter. Earth’s, by comparison, is only 54% of its diameter. Because Mercury is a small planet, models predict that it should have radiated away enough of its primordial heat for its core to have frozen solid. Its surface is ancient enough that it appears there has been no internally generated geologic activity for billions of years. Yet Mariner 10 discovered a magnetic field at Mercury, which is only possible with a liquid outer core, like Earth has. Why Mercury has such a large core, and why it is not solid, are two great Mercury mysteries. Mercury’s surface
The Mariner 10 flybys revealed that Mercury is almost completely covered with impact craters, indicating that the surface is quite ancient. The craters have broad similarities to lunar craters, though their shapes are somewhat different because of Mercury’s higher gravity. There are some areas on Mercury that, like the Moon, show fewer craters. However, unlike the Moon, the albedo (or brightness) of the surface isn’t related to how cratered and ancient it is. On the Moon, the heavily cratered highlands are relatively bright, while the less heavily cratered maria are dark. Mercury is crisscrossed by long fault scarps. Scarps and thrust faults form when compressional forces act on a body. Scientists theorize that when Mercury’s initially hot core cooled and shrank, the lithosphere (outer rocky layer) was already very stiff and solid. Mercury’s shrinking caused the lithosphere to crack and crumple together, forming the scarps. Like the Moon, Mercury has a few large impact basins. The largest observed on Mercury is the Caloris Basin, only half of which was seen in the Mariner 10 flybys. On the exact opposite side of Mercury from Caloris is a very strange-looking, rippled, warped terrain. Scientists theorize that when the Caloris impact happened, the shock waves from the impact traveled all the way around Mercury and converged on the point exactly opposite to Caloris, causing the formation of this bizarre-looking terrain. It was a lucky fluke that Mariner 10, which only saw half of Mercury, had a field of view that overlapped part of Caloris and part of the terrain on the exact opposite side of the planet from Caloris.
Because Mercury’s rotational axis is almost exactly vertical with respect to its orbit, there are no seasons. Consequently there are areas in the walls and floors of craters at Mercury’s poles that never see the Sun. Earth-based radar observations of Mercury hint at different surface materials lurking in the walls and floors of these polar craters. Could it be ice -- preserved for most of the age of the solar system? Mercury’s “atmosphere”There are gases floating in the space above Mercury, but they are so thin that it is usually referred to by scientists as an “exosphere,” not an atmosphere. Six elements are known to exist in Mercury's exosphere: (1) hydrogen, (2) helium, (3) oxygen, (4) sodium, (5) potassium, and (6) calcium. How abundant these are, and how exactly they got there, are unknown. |
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