Mercury, the Sun's Closest Planetary Neighbor
Mercury's History & Naming
Mercury is the closest planet to the sun. As such, it circles the sun faster than all the other planets, which is why Romans named it after the swift-footed messenger god Mercury.
Mercury was known since at least Sumerian times roughly 5,000 years ago, where it was often associated with Nabu, the god of writing. Mercury was also given separate names for its appearance as both a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus believed that both Mercury and Venus orbited the Sun, not the Earth. [Latest Photos: Mercury Seen by NASA's Messenger Probe]
Mercury's Physical Characteristics
As the planet nearest the sun, the surface of Mercury can reach a scorching 840 degrees F (450 degrees C). However, since this world doesn't have a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275 degrees F (minus 170 degrees C), a more than 1,100 degrees F (600 degree C) temperature swing that is the greatest in the solar system.
Mercury is the smallest planet — it is only slightly larger than Earth's moon. Since it has no significant atmosphere to stop impacts, the planet is pockmarked with craters. For instance, about 4 billion years ago, a roughly 60-mile-wide (100-kilometer-wide) asteroid struck Mercury with an impact equal to 1 trillion 1-megaton bombs, creating a vast impact crater roughly 960 miles (1,550 kilometers) wide. Known as the Caloris Basin, this crater could hold the entire state of Texas.
Amazing, as close to the sun as Mercury is, ice may exist in its craters. In 1991, astronomers using radar observations discovered that water ice may lurk at Mercury's north and south poles inside deep craters that are perpetually shadowed and cold. Comets or meteorites might have delivered ice there, or water vapor might have outgassed from the planet's interior and frozen out at the poles.
Mercury apparently shrank about 0.6 to 1.2 miles (1 to 2 kilometers) as it cooled in the billions of years after its birth. This caused its surface to crumple, creating lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high.
Mercury is the second densest planet after Earth, with a huge metallic core roughly 2,200 to 2,400 miles (3,600 to 3,800 kilometers) wide, or about 75 percent of the planet's diameter. In comparison, Mercury's outer shell is only 300 to 400 miles (500 to 600 kilometers) thick.
A completely unexpected discovery Mariner 10 made was that Mercury possessed a magnetic field. Planets theoretically generate magnetic fields only if they spin quickly and possess a molten core. But Mercury takes 59 days to rotate and is so small — just roughly one-third Earth's size — that its core should have cooled off long ago. The recent discovery from 2007 Earth-based radar observations that Mercury's core may still be molten could help explain its magnetism.
Although Mercury's magnetic field is just 1 percent the strength of Earth's, it is very active. The magnetic field in the solar wind — the charged particles streaming off the sun — periodically touches upon Mercury's field, creating powerful magnetic tornadoes that channel the fast, hot plasma of the solar wind down to the planet's surface.
Instead of a substantial atmosphere, Mercury possesses an ultra-thin "exosphere" made up of atoms blasted off its surface by solar radiation, the solar wind and micrometeoroid impacts. These quickly escape into space, forming a tail of particles.
Mercury's Orbital Characteristics
Mercury speeds around the sun every 88 Earth days, traveling through space at nearly 112,000 miles per hour (180,000 kilometers per hour), faster than any other planet. Its oval-shaped orbit is highly elliptical, taking Mercury 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 Mercury when it is nearest to the sun, it would appear more than three times as large as it does when viewed from Earth.
Oddly, due to Mercury's highly elliptical orbit and the 59 Earth days or so it takes to rotate on its axis, when on the scorching surface of the planet, the sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the sun appears to set, rise again briefly, and then set again.
Composition & Structure
- Atmospheric composition (by volume)
No atmosphere: Mercury possesses an exosphere containing 42 percent oxygen, 29 percent sodium, 22 percent hydrogen, 6 percent helium, 0.5 percent potassium, with possible trace amounts of argon, carbon dioxide, water, nitrogen, xenon, krypton, neon. [Inside Planet Mercury (Infographic)]
Roughly 1 percent the strength of Earth's.
- Internal structure
Orbit & Rotation
Average Distance from the Sun
English: 35,983,095 miles
Metric: 57,909,175 km
By Comparison: 0.38 Earth's distance from the Sun.
English: 28,580,000 miles
Metric: 46,000,000 km
By Comparison: 0.313 times that of Earth
English: 43,380,000 miles
Metric: 69,820,000 km
By Comparison: 0.459 times that of Earth
Length of Day:
58.646 Earth days
Mercury's Moons or Rings
Mercury has no moons or rings.
Research & Exploration
The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface and detected its magnetic field. NASA's MESSENGER orbiter is the second spacecraft to visit Mercury, and has made three flybys as it slowly works its way into orbit. When it arrived in March 2011, MESSENGER became the first spacecraft to orbit Mercury, where studied Mercury for one year, mapping nearly the entire planet in color. NASA has extended the mission beyond the initial one-year science plan to continue to study Mercury. [First Photos of Mercury from Orbit]
Please see this website, http://www.nasa.gov/worldbook/mercury_worldbook.html. There is a section about the phases of Mercury that provides additional information/content about Mercury.
RELATED: See our Solar System Planets overview, or our broader Solar System Facts overview, or learn more about each of the other planets: Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and the demoted dwarf planet Pluto.