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Mercury: Inside and Out

Looking in on Mercury:

Planet in Cross-Section

On the outside:




Mass and Density

In 1841, the German astronomer Johann Franz Encke determined Mercury’s mass by measuring its gravitational effect on a comet that now bears his name. This measurement was within 20% of the best modern measurement of 3.3 x 1023 kilograms, or about 5.5% of Earth’s mass.

Knowing the mass of Mercury doesn't tell us its exact composition, but since we know the planet's volume as well, we can estimate what the interior is made of. We start by dividing the planet's mass by its volume to find its average density. Each element or compound has its own characteristic density, and a planet is made up of many materials of different densities. From the average density, scientists can guess what materials, and how much of each, the planet contains.

For example, Earth has an average density of 5.5 g/cm3, which is between the density of iron (8 g/cm3) and that of the silicate compounds found in rocks (3 g/cm3). We believe that Earth has an interior core of iron making up about 16% of the Earth’s volume. Mercury has a density similar to Earth, 5.4 g/cm3. Because of Mercury’s small size and high density, scientists believe that about 70% of Mercury’s mass is composed of iron, mostly contained in its core.

Odd but true: Mercury is the second smallest planet, but it is also the second densest body in our solar system. Only Earth is denser than Mercury. Jupiter, the gas giant, is the most massive of our planets, but at 1.3 g/cm3 it is denser only than Saturn.



The Mariner 10 mission sent back more than just pictures of Mercury's surface. On-board instruments detected a weak magnetic field - about 100 times weaker than Earth's. This came as a surprise, since the photographs from this mission showed a Moon-like surface, and the Moon's magnetic field is about 100 times weaker yet. A planet's magnetic field shields it from constant bombardment of charged particles streaming outward from the Sun - the solar wind.

Mercury’s magnetic field and high density both indicate that the planet has a large, iron-rich core, probably about 3600 kilometers in diameter. This core takes up a large fraction of the planet's volume, since Mercury's total diameter is only 4880 kilometers. Mercury’s ratio of core size to planet size is the largest of any planet in the solar system. Mercury probably has only a thin lunar-like silicate mantle.

Odd but true: Mercury’s iron core is thought to extend from the planet’s center to nearly three quarters of the way to the surface, making it about the size of our Moon.


Before the Mariner 10 mission in 1974, Mercury’s surface features were little more than a blur to Earth-bound observers. Mariner 10 made the first flyby of Mercury on March 29, 1974 at a distance of about 700 kilometers. Its high-resolution photographs of about 50% of Mercury’s surface allowed scientists to view the planet close-up. On its surface, Mercury closely resembles our Moon. Impact craters cover the majority of the planet but unlike the Moon, Mercury’s cratered upland regions are covered with large areas of smooth plains. The most distinguishing features on Mercury’s surface are scarps, or long cliffs. These wind across Mercury’s surface for tens to hundreds of kilometers and range from 100 meters to over 1.5 kilometers in height. What makes these cliffs so unique is that no other planet or moon features such a vast number of them. They are thought to be thrust faults created when the planet, as it cooled, shrunk by up to 4 km in diameter.

Scarp on Mercury
Discovery Scarp
This 300 km scarp (above) runs from upper right corner of the picture to the lower left. (Click on the image to see an enlarged version.)
Discovery Scarp: This scarp is about 350 kilometers long and transects two craters. The maximum height is about 3 kilometers. (Click on the image to see an enlarged version.)
The largest surface feature photographed by the Mariner 10 mission is the Caloris basin. This is a multi-ringed (resembling a bull's-eye) impact basin 1,340 km across - almost of the full diameter of the planet. The basin includes a series of circular mountain ranges up to 3 km in height - the tallest mountains on Mercury. Caloris is thought to have been produced when a very large asteroid collided with the planet about 4 billion years ago. The massive impact sent seismic waves echoing through the planet. Coming to a focus on the opposite side of Mercury, these intense waves created there a region of hilly and broken terrain.

Odd but true: The Caloris Basin is so large that it would engulf the entire state of Texas. The name Caloris is derived from the Latin word "calor" or heat.

Caloris Basin, from Mariner 10
Floor of Caloris Basin
Hilly terrain on the surface of Mercury opposite the Caloris basin
The multi-ringed Caloris Basin, taken from Mariner 10. Only half of the basin appears in this photomosaic. (Click on the image to see an enlarged version.)
Ridges and fractures on the floor of the Caloris Basin
Hilly and jumbled terrain on the side of Mercury opposite the Caloris Basin. (Click on the image to see an enlarged version.)


A planet’s ability to keep an atmosphere depends largely on its mass and surface temperature, as well as on the composition of the atmosphere itself. Large planets with strong gravitational fields, such as Earth, are more likely to retain their atmosphere. Since Mercury is a small planet compared to Earth it was once thought that any gases on Mercury would have escaped into space long ago. Yet the onboard instruments of Mariner 10 showed that Mercury does have a trace atmosphere. The atmospheric pressure is quite small - about one trillion times less than that at sea level on Earth. Elements known to be present in the Mercurian atmosphere include helium, sodium, and oxygen.

Odd but true: Atmospheric size and composition determine how the sky will appear to someone on a planet's surface. For instance, the sky on Earth appears blue while the sky on Mars would appear to be a pinkish-red. Due to the absence of a substantial atmosphere on Mercury, the sky would appear black even during daytime - except for the Sun, which would appear two to three times larger than we see on the Earth. With no atmosphere, a visitor to Mercury would hear no sounds.


Mercury experiences the most extreme temperature range of any of the planets. When Mercury is farthest from the Sun (at aphelion) the predawn temperature reaches a frigid -180°C (-300°F). As Mercury’s orbit reaches perihelion the mid-afternoon temperatures can skyrocket to 430°C (800°F). Mercury’s slow rotational period, its nearness to the Sun, and its lack of a substantial atmosphere are together responsible for this enormous variation in temperature. The period from sunrise to sunset lasts for 88 Earth days allowing the intense solar radiation to heat the surface. But during the nights (also 88 days long) the temperature falls dramatically since heat is not trapped within a thick atmosphere, as it is on Earth or Venus.

The seasons on Earth are caused by the tilt of its axis of rotation. Mercury’s axis is nearly perpendicular to the plane of it’s orbit, so it has no significant tilt and the planet has no seasons. The poles of Mercury never receive sunlight, and the temperature there remains below -160°C. It is here that scientists believe that frozen water may be found.

Odd but true: Although Mercury is the closest planet to the Sun, it is not the hottest. That distinction goes to Venus, which has a surface temperature of about 460°C. This is due to the dense atmosphere of carbon dioxide, a very effective greenhouse gas. Solar radiation penetrates Venus' clouds and its heat becomes trapped.

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