Planets and moons
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"Mercury." Space, Planets and moons, Q-files Encyclopedia, 10 Feb. 2022.
https://www.q-files.com/space/planets-and-moons/mercury.
Accessed 19 Mar. 2024.
Mercury 2022. Space, Planets and moons. Retrieved 19 March 2024, from
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Space, Planets and moons, s.v. "Mercury," accessed March 19, 2024.
https://www.q-files.com/space/planets-and-moons/mercury
Mercury
Mercury is the smallest planet in the Solar System, and the nearest planet to the Sun. Mercury can be seen from Earth only with difficulty: when visible at all, it is always low in the dawn or twilight sky close to the Sun. For many years, much of what we knew about Mercury was gained by space probe Mariner 10, which flew by the planet in 1974 and 1975. On the third of its three flybys, it came within 327 kilometres (203 miles) of Mercury, but was able to map only between 40 and 45% of the planet's surface. A survey of the planet using radar signals beamed from Earth in 1991 revealed further details of Mercury’s landscape. Between 2011 and 2015, NASA’s MESSENGER space probe orbited Mercury and sent back more detailed images than ever before.
Appearance
On first appearance Mercury looks quite similar to our Moon—bare and rocky and covered with craters and lava plains. Originally molten, Mercury’s surface shrank as it cooled after the bombardment that produced all the craters began to ease about 3000 million years ago. As with a dried fruit, the shrinkage resulted in “wrinkles”. On Mercury’s surface, these wrinkles appear as high cliffs. With practically no atmosphere to cause winds or water to erode the rocks, Mercury’s landscape has remained the same ever since.
Landscape
The landscape of Mercury is dominated by many thousands of craters. There are also “wrinkle ridges”, cliffs sometimes rising more than 4 kilometres (2.5 miles) high. The Sun, huge in the sky, burns with a fierce heat. But temperatures drop dramatically when this face of the planet is turned away.
Asteroids falling on Mercury’s surface billions of years ago have punched craters measuring many kilometres across, some with smaller craters inside. Debris was blasted out in all directions, creating smaller craters and long streaks in the ground. Because Mercury has no weather, the features remain undisturbed.
Orbit
Appropriately named by the Romans after the winged-foot messenger of the gods, Mercury orbits the Sun quicker than any other planet: just 88 days. It spins very slowly, however. A day on Mercury, measured as one complete turn on its axis, is much longer than on Earth: nearly 59 days.
The shape of Mercury’s orbit is also quite unusual. All the other planets (except Pluto) have roughly circular orbits, but Mercury’s is much more elliptical (oval shaped). At its closest, Mercury is 46 million kilometres (29 million miles) from the Sun, but at its most distant it is 70 million kilometres (43 million miles) away. Its orbital path is tilted at about 7° relative to that of Earth.
Because it is so close to the Sun and because the length of its day is so long, Mercury has great extremes of temperature. Where it faces the Sun, it can reach 350°C (660°F), but during the long nights and with almost no atmosphere to keep the heat in, temperatures can plummet to -170°C (-274°F).
Close to the poles there are permanently dark craters that never feel the Sun’s ferocious heat. From photographs taken by the MESSENGER space probe, scientists have discovered that on Mercury, as on our own Moon, there are small areas of water ice.
Composition
Mercury, the densest of all the planets apart from Earth, has a huge metal core making up about 75% of the planet's radius (compared to 50% on Earth). The core, at least partially liquid and possibly with a solid centre, is made of iron and nickel, The relatively thin shell surrounding the core is made up of about 70% metallic and 30% silicate material. Mercury's magnetic field is stronger than Earth's. The planet is almost completely without an atmosphere. This means that its skies are always pitch black, even during the day.
Mercury's story
The MESSENGER space probe discovered that Mercury's crust contains high levels of potassium and sulphur. Both are volatile elements: they are easily vaporized (turned to gas). When the planet formed, elements like these should have been stripped away by the Sun. One possible way to explain the existence of these elements today is that Mercury must have formed much further away from the Sun than its current position—perhaps even beyond Mars. Some astronomers think Mercury was, billions of years ago, during the Solar System's early years, "kicked" by the gravitational force of Jupiter. This sent it careering towards the Sun. The young Mercury then collided with another planet and shattered into pieces. Much of its mantle and crust was lost to space, leaving behind the over-sized core it has today. The collision also knocked Mercury into the lopsided orbit it currently takes around the Sun.
Factfile
Average diameter: 4880 km (3031 miles)
Mass: 0.055 (Earth = 1)
Average density: 5.4 (water = 1)
Surface gravity: 0.38 (Earth = 1)
Day: 176 days
Year: 88 days
Average speed in orbit: 47.9 km/sec (30 miles/sec)
Average distance from the Sun: 58 million km (36 million miles)
Surface temperature: -170°C (-274°F) to +350°C (660°F)
Atmosphere: traces of helium
Moons: none
Name: the Roman messenger of the gods
Consultant: Mike Goldsmith