Mercury

Learn more about the first planet in our solar system.

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This picture, taken from NASA's MESSENGER Mission using a Mercury Dual Imaging System, shows the Caloris basin, which at approximately 1525 km (948 mi) across, is the largest known crater in the solar system.

Mercury's elliptical orbit takes the small planet as close as 29 million miles (47 million kilometers) and as far as 43 million miles (70 million kilometers) from the sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, the sun would appear almost three times as large as it does when viewed from Earth.

Temperatures on Mercury's surface can reach 800 degrees Fahrenheit (430 degrees Celsius). Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -280 degrees Fahrenheit (-170 degrees Celsius).

Because Mercury is so close to the sun, it is hard to directly observe from Earth except during twilight. Mercury makes an appearance indirectly, however, 13 times each century. Earth observers can watch Mercury pass across the face of the sun, an event called a transit. These rare transits fall within several days of May 8 and November 10.

Scientists used to think that the same side of Mercury always faces the sun, but in 1965 astronomers discovered that the planet rotates three times during every two orbits. Mercury speeds around the sun every 88 days, traveling through space at nearly 31 miles (50 kilometers) per second faster than any other planet. The length of one Mercury day (sidereal rotation) is equal to 58.646 Earth days.

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This computer simulation shows Mercury close up with Earth, Mars, and Saturn visible in the background. 

A Planet With No Atmosphere

Rather than an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off its surface by solar wind and striking micrometeoroids. Because of the planet's extreme surface temperature, the atoms quickly escape into space. With the thin exosphere, there has been no wind erosion of the surface and meteorites do not burn up due to friction as they do in other planetary atmospheres.

Mercury's surface resembles that of Earth's moon, scarred by many impact craters resulting from collisions with meteoroids and comets. While there are areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile (1.6 kilometers) high, formed by early contraction of the crust. The Caloris Basin, one of the largest features on Mercury, is about 800 miles (1,300 kilometers) in diameter. It was the result of an asteroid impact on the planet's surface early in the solar system's history and is oen of the largest known impact craters in the solar system.

Over the next half-billion years, Mercury shrank in radius about 0.6 to 1.2 miles (1 to 2 kilometers) as the planet cooled after its formation. The outer crust contracted and grew strong enough to prevent magma from reaching the surface, ending the period of geologic activity.

Mercury is the second smallest planet in the solar system, larger only than Pluto. Mercury is the second densest planet after Earth, with a large iron core having a radius of 1,100 to 1,200 miles (1,800 to 1,900 kilometers), about 75 percent of the planet's radius. Mercury's outer shell, comparable to Earth's outer shell (called the mantle), is only 300 to 400 miles (500 to 600 kilometers) thick. Mercury's magnetic field is thought to be a miniature version of Earth's, but scientists are uncertain of the strength of the field.

Mercury's surface is marked with "ray craters," features usually found only on moons. These hub-and-spoke formations are created when a large o 500Error reading 'logLevel' on type com.nationalgeographic.componentry.sling.model.logging.LoggerModel

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