The Astronomical Planet
Mercury is the second smallest planet in the solar system, larger only than Pluto, the most distant planet in our solar system. If Earth were the size of a baseball, Mercury would be the size of a golf ball. Viewed from Mercury, the Sun would look almost three times as large as it does from Earth.
The small and rocky planet Mercury is the closest planet to the Sun. Because Mercury is so close to the Sun, it is hard to see from Earth except during twilight.
Mercury speeds around the Sun in a wildly elliptical (non-circular) orbit that takes it as close as 47 million km and as far as 70 million km from the Sun.
Mercury completes an orbit every 88 days, speeding through space at nearly 50 km per second. Faster than any other planet!
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Planetary Statistics of Mercury:
Mercury: First Planet from the Sun
Diameter: 3,031 miles
Distance from the Sun: 36 million miles
Planetary Year: 87.96 Earth days to make one complete revolution around the Sun.
Planetary Day: 58.7 Earth days to make one complete revolution on its axis.
Temperature: Mean; 452 K (179 C, 354 F), surface temperature extremes 100 K (-173 C, -280 F) to 700 K (427 C, 800 F)
Chronology of Mercury
Until 1965, scientists thought that the same side of Mercury always faced the Sun. Then, astronomers discovered that Mercury completes three rotations for every two orbits around the Sun.
Only one spacecraft has ever visited Mercury. That was Mariner 10 in 1974-75. Mariner 10 discovered that Mercury has a very weak magnetic field. The fact that it is similar to, but weaker than Earths, was a major surprise.
In 1991, astronomers using radar observations showed that Mercury might have water ice at its north and south poles. The ice exists inside deep craters. The floors of these craters remain in a perpetual shadow, so the Sun cannot melt the ice.
Environment of Mercury
Mercury is the second densest body in the solar system after the Earth. Its interior is made of a large iron core with a radius of 1,800 to 1,900 km. Nearly 75 percent of the planet's diameter and nearly the size of Earth's Moon! Mercury's outer shell, comparable to Earth's outer shell (called the mantle) is only 500 to 600 km thick.
The planet has less than a million-billionths the pressure of Earth's atmosphere at sea level.
Surface temperatures on Mercury can reach a scorching 427 degrees Celsius. But because the planet has hardly any atmosphere to keep it warm, night time temperatures can drop to a frigid -173 degrees Celsius.
Like our Moon, Mercury has almost no atmosphere. What little atmosphere exists is made up of atoms blasted off its surface by the solar wind. With no atmosphere to protect the surface, there has been no erosion from wind or water, and meteorites do not burn up due to friction as they do in other planetary atmospheres.
Mercury is composed chiefly of oxygen, sodium, and helium. Because of its extreme surface temperature, these atoms quickly escape into space and are constantly replenished.
Mercury's surface very much resembles Earth's Moon. It is scarred by thousands of impact craters resulting from collisions with meteors. While there are areas of smooth terrain, there are also cliffs. Some soar up to a mile high being formed by ancient impacts.
The Caloris Basin, one of the largest features on Mercury, is about 1,300 km in diameter. It was the result of an asteroid impact on the planet's surface early in the solar system's history. This impact was the probable cause of the strange surfaces on the opposite side of the planet.
Over the next half-billion years, Mercury actually shrank in radius from 2 to 4 km as the planet cooled from its formation. The outer crust, called the lithosphere, was compressed and grew strong enough to prevent the planet's magma from reaching the surface. This effectively ended the planet's period of geologic activity. Evidence of Mercury's active past is seen in the smooth plains in the Caloris basin.
Information Courtesy of NASA Jet Propulsion Laboratory at Caltech
Images and Information
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