by Fraser Cain on March 12, 2012

How many moons does Mercury have?

Mercury is the closest planet to orbit the Sun in the Solar System; orbiting at a distance of only 58 million km (just for comparison, Earth orbits at 150 million km). Like all of the planets, Mercury is named after a Roman god; in this case, Mercury was the Roman god of commerce – the same as the ancient Greek god Hermes.

Characteristics of Mercury

Mercury has a diameter of only 4,879 km, making it the smallest planet in the Solar System (now that Pluto is no longer a planet). In fact, Mercury is actually smaller than some of the largest moons in the Solar System: Ganymede and Titan are larger. But Mercury is the second densest planet in the Solar System (second only to Earth), with a metal core that accounts for almost half the volume of the planet. This gives it a higher mass and gravity than you would expect. You would weigh 38% of your regular weight on Mercury.

Mercury orbits the Sun in a highly elliptical orbit, varying its distance to the Sun depending on where it is orbit. At its closest point, called perihelion, Mercury gets within 46 million km, and then it gets to 70 million km at aphelion. At its closest point, Mercury gets within 77 million km from Earth. Mercury takes only 88 days to complete an orbit around the Sun.

At a glance, Mercury looks similar to the Earth’s moon. It has a dry landscape pockmarked by asteroid impact craters and ancient lava flows – the largest crater on Mercury is the Caloris Basin, which measures 1,300 km across. And like our moon, Mercury has no appreciable atmosphere. But underneath the surface, Mercury is much different from the Moon. It has a large core of iron, surrounded by a thick rock mantle and a thin crust. The large amount of metal that makes up Mercury gives the planet a high density. In fact, it’s the second densest planet in the Solar System, after the Earth. If you were to stand on the surface of planet Mercury, you would experience about 1/3rd the gravity you feel standing on Earth.

Mercury rotates slowly on its axis, completing a day once every 59 Earth days. But it’s also traveling quickly around the Sun. So the amount of time it takes for the Sun to rise in the morning, and then set in the evening actually takes a total of 176 Earth days. Astronomers used to think that Mercury’s day was exactly as long as its year (88 days), making it tidally locked to the Sun, but Russian radio astronomers calculated its true day length in the 1960s.

Mercury does have a tenuous atmosphere, made up of captured particles from the Sun, volcanic outgassing and micrometeorite debris kicked into orbit. But it’s almost undetectable. Without an atmosphere, Mercury has no way to retain the heat from the Sun. The side that faces the Sun rises to a temperature of 450° C, while the side in shadow dips down to -170° C. Mercury is actually the second hottest planet in the Solar System after Venus.

The first spacecraft to reach Mercury was NASA’s Mariner 10, which flew past the planet in 1974. It was only able to photograph about half of the planet over the course of a few flybys. And then 2004 NASA launched its latest MESSENGER spacecraft mission to Mercury. At the time of writing, the spacecraft had completed several flybys of the planet, is scheduled to go into orbit in 2011, where it will be able to study the planet in great detail.

Even though you can see Mercury without a telescope, it’s hard to find it because it’s lost within the glare of the Sun most of the time. When Mercury is visible, you’ll be able to see it to the west just after sunset, or to the east just before sunrise. When you do see Mercury through a telescope, it appears to go through phases like the Moon, depending on its point in orbit.

Mercury doesn’t have rings or any moons.

Location and Movement of Mercury

Structure of Mercury

Surface of Mercury

History of Mercury

Other Mercury Articles


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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