Location and Orbit
Missions to Mercury
References & Links
Mercury: Physical Structure
Facts in Brief
|Max Dayside Temperature
|Max Nightside Temperature
||3.30 x 1024 kg
||10 to 15 Bars
|Dipole Magnetic Field Strength
Mercury was originally believed to be a very simple planet--no rotation, no magnetosphere, an atmosphere-less orb half-baked and half-frozen. The more that astronomers investigate the small planet, however, the less these previous beliefs hold up. The Mariner 10 probe discovered Mercury's magnetic field, for instance, and radio telescopes have detected evidence of water on the surface. To best convey the information that we have on Mercury, we'll work from the inside out.
Discovered, along with most of the scientific findings about Mercury, by the Mariner 10, the discovery of Mercury's magnetic field upset the previous model of the planet's interior composition. Up until this time, scientists believed that Mercury's small size would have caused its core to have solidified long ago. The presence of a magnetic field indicates proves that a planet has an iron core that is at least partially molten. Despite this, however, the presence of nearly undisturbed, billion-year-old craters reveals that there is no tectonic activity on the planet. The crust, however cracked and cratered, has no fault lines. There are no active volcanoes, though in the past there may have been volcanic activity.
Mercury has a high density, 5.44 g/cm3 (compare to Earth's 5.52g/cm3 density). In an uncompressed state, Mercury's density is 5.5 g/cm3 where Earth's is only 4.0 g/cm3. This high density indicates that the planet is 60 to 70 percent by weight metal, and by weight 30 percent silicate. This suggests a core radius of 75% of the planet radius and a core volume of 42% of the planet's volume. Surrounding the out-sized core is an intermediate rocky layer and a thin crust, which is believed be an effective insulator to keep the planet's outer core liquid despite the very cold temperatures on the planet's dark side.
The surface is discussed in greater depth in the Topography section. In short, Mercury's planetscape is host to a large assortment of plains, cliffs, and craters. Despite the extreme heat that the surface is exposed to (it's hot enough to melt lead or tin), it is also quite possible that ice exists, sheltered in craters on the planet's north pole. Because of its density, the force of gravity is about one-third of that on Earth's surface, despite being only slightly larger than the Earth's moon.
Mercury was originally believed to have no atmosphere. Scientists now think one exists, albeit a very scanty one--it exerts a pressure one million billionths that of Earth. Believed atmospheric composition: 42% oxygen, 29% sodium, 22% hydrogen, 6% helium, 0.5% potassium. Possible trace amounts of argon, carbon dioxide, water, nitrogen, xenon, krypton, and neon may exist. These gases are generally diffused from the surface of the planet. One reason for the weakness of Mercury's atmosphere may be the solar winds which the planet is exposed to. When gases are ionized by solar radiation, they are lost to the atmosphere through the interaction of the planet's magnetic field with the solar wind.
Mariner 10 showed that Mercury has a magnetic field that is 1% as strong as Earth's. This magnet field is inclined 7 degrees to Mercury's axis of rotation and produces a magnetosphere around the planet. Magnetic fields are generated from the rotation of a conductive molten core and is known as the "dynamo effect."