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14.7: Mercury

Difficulty Level: At Grade Created by: CK-12
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How did tiny Mercury get its name?

Mercury was named for the Roman messenger god who traveled rapidly on his winged sandals. From the vantage point of Earth, the planet Mercury travels swiftly across the face of the Sun.


The smallest planet, Mercury, is the planet closest to the Sun. Because Mercury is so close to the Sun, it is difficult to observe from Earth, even with a telescope. However, the Mariner 10 spacecraft, shown in Figure below, visited Mercury from 1974 to 1975.

The MESSENGER spacecraft has been studying Mercury in detail since 2005. The craft is currently in orbit around the planet, where it is creating detailed maps. MESSENGER stands for Mercury Surface, Space Environment, Geochemistry and Ranging.

The Mariner 10 spacecraft, which made flybys of Mercury

(a) Mariner 10 made three flybys of Mercury in 1974 and 1975. (b) A 2008 image of compiled from a flyby by MESSENGER.

As Figure below shows, the surface of Mercury is covered with craters, like Earth’s Moon. Ancient impact craters means that for billions of years Mercury hasn’t changed much geologically. Also, with very little atmosphere, the processes of weathering and erosion do not wear down structures on the planet.

Mercury is covered with craters, like Earth’s Moon

Mercury is covered with craters, like Earth’s Moon. MESSENGER has taken extremely detailed pictures of the planet’s surface.

Short Year, Long Days

Mercury is named for the Roman messenger god, who could run extremely quickly, just as the planet moves very quickly in its orbit around the Sun. A year on Mercury — the length of time it takes to orbit the Sun — is just 88 Earth days.

Despite its very short years, Mercury has very long days. A day is defined as the time it takes a planet to turn on its axis. Mercury rotates slowly on its axis, turning exactly three times for every two times it orbits the Sun. Therefore, each day on Mercury is 57 Earth days long. In other words, on Mercury, a year is only a Mercury day and a half long!

Extreme Temperatures

Mercury is close to the Sun, so it can get very hot. However, Mercury has virtually no atmosphere, no water to insulate the surface, and it rotates very slowly. For these reasons, temperatures on the surface of Mercury vary widely. In direct sunlight, the surface can be as hot as 427°C (801°F). On the dark side, or in the shadows inside craters, the surface can be as cold as -183°C (-297°F)! Although most of Mercury is extremely dry, scientists think there may be a small amount of water in the form of ice at the poles of Mercury, in areas that never receive direct sunlight.

A Liquid Metal Core

Figure below shows a diagram of Mercury’s interior. Mercury is one of the densest planets. It’s relatively large, liquid core, made mostly of melted iron, takes up about 42% of the planet's volume.

Mercury contains a thin crust, a mantle, and a large, liquid core that is rich in iron

Mercury contains a thin crust, a mantle, and a large, liquid core that is rich in iron.


  • Mercury appears to be moving rapidly because it's so close to the Sun.
  • Mercury has short years, just 88 Earth days, but long days, about 57 Earth days.
  • Mercury is extremely hot and has a liquid metal core.


  1. Why is a year on Mercury only 88 days long?
  2. Why is Mercury mostly really hot, but very cold in spots?
  3. Think about the formation of the solar system. Why is Mercury the densest planet?

Explore More

Use this resource to answer the questions that follow.

  1. What is the location and size of Mercury?
  2. Why do the temperatures on Mercury vary so much?
  3. Why was Mercury named for the messenger god Mercury?
  4. Why is Mercury so dense?
  5. What are the features of its core?
  6. Why is the landscape similar to our Moon's?
  7. Why hasn't the surface of Mercury changed over it's history, except for the addition of more impact craters?
  8. How can ice be found on such a hot planet?
  9. Why does it take so much rocket fuel to send a spacecraft to Mercury?
  10. What do probes to Mercury reveal?

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Difficulty Level:
At Grade

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Date Created:
Feb 24, 2012
Last Modified:
Aug 07, 2016
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