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The Planet Mercury

The small and rocky planet Mercury is the closest planet to the Sun; it speeds around the Sun in a wildly elliptical (non-circular) orbit that takes it as close as 47 million km and as far as 70 million km from the Sun. Mercury completes a trip around the Sun every 88 days, speeding through space at nearly 50 km per second, faster than any other planet. Because it is so close to the Sun, temperatures on its surface can reach a scorching 467 degrees Celsius. But because the planet has hardly any atmosphere to keep it warm, nighttime temperatures can drop to a frigid -170 degrees Celsius.

Because Mercury is so close to the Sun, it is hard to see from Earth except during twilight. Until 1965, scientists thought that the same side of Mercury always faced the Sun. Then, astronomers discovered that Mercury completes three rotations for every two orbits around the Sun. The length of one Mercury day (sidereal rotation) is equal to 58.646 Earth days.

The Planet Mercury
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. The first two transits of Mercury in the 21st century occur May 7, 2003, and November 8, 2006.

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

Rather than an atmosphere, Mercury possesses a thin exo-sphere made up of atoms blasted off its surface by the solar wind and striking micrometeoroids. Because of the planet's ex-treme 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.

This black and white image shows about half of the planet Mercury.
Mariner 10's first image of Mercury.
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 high, formed by early contraction of the crust. The Caloris Basin, one of the largest features on Mercury, is about 1,300 kilometers (800 miles) in diameter. It was the result of an asteroid impact on the planet's surface early in the Solar System's history. Over the next half-billion years, Mercury shrank in radius about 1 to 2 kilometers (0.6 to 1.2 miles) 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 smallest planet in the Solar System.  larger only than previously measured dwarf planets, such as Pluto. Mercury is the second densest planet after Earth, with a large iron core having a radius of 1,800 to 1,900 kilometers (1,100 to 1,200 miles), about 75 percent of the planet's radius. Mercury's outer shell, comparable to Earth's outer shell (called the mantle), is only 500 to 600 kilometers (300 to 400 miles) 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.

Only one spacecraft has ever visited Mercury: Mariner 10, which imaged about 45 percent of the surface. In 1991, astronomers using radar observations showed that Mercury may have water ice at its north and south poles inside deep craters that are per-petually cold (below -212 degrees Celsius or -350 degrees Fahrenheit). Falling comets or meteorites might have brought ice to these regions of Mercury, or water vapor might have outgassed from the interior and frozen out at the poles.

A new NASA mission to Mercury called MErcury Surface, Space ENvironment, Geochemistry, and Ranging (MESSENGER) will begin orbiting Mercury in March 2011 to investigate key scien-tific areas such as the planet's composition, the structure of the core, the magnetic field, and the materials at the poles.
Mercury: Facts & Figures
Discovered By
Known by the Ancients
Date of Discovery
Average Distance from the Sun
Metric: 57,909,175 km
English: 35,983,095 miles
Scientific Notation: 5.7909175 x 107 km (0.38709893 A.U.)
By Comparison: Earth is 1 A.U. (Astronomical Unit) from the Sun.
Perihelion (closest)
Metric: 46,000,000 km
English: 28,580,000 miles
Scientific Notation: 4.600 x 107 km (0.3075 A.U.)
By Comparison: 0.313 x Earth
Aphelion (farthest)
Metric: 69,820,000 km
English: 43,380,000 miles
Scientific Notation: 6.982 x 107 km (0.4667 A.U.)
By Comparison: 0.459 x Earth
Equatorial Radius
Metric: 2,439.7 km
English: 1,516.0 miles
Scientific Notation: 2.4397 x 103 km
By Comparison: 0.3825 x Earth
Equatorial Circumference
Metric: 15,329.1 km
English: 9,525.1 miles
Scientific Notation: 1.53291 x 104 km
Metric: 60,827,200,000 km3
English: 14,593,200,000 mi3
Scientific Notation: 6.08272 x 1010 km3
By Comparison: 0.054 x Earth's
Metric: 330,220,000,000,000,000,000,000 kg
Scientific Notation: 3.3022 x 1023 kg
By Comparison: 0.055 x Earth's
Metric: 5.427 g/cm3
By Comparison: 0.984 x Earth
Surface Area
Metric: 74,800,000 km2
English: 28,900,000 square miles
Scientific Notation: 7.48 x 107 km2
By Comparison: 0.108 x Earth
Equatorial Surface Gravity
Metric: 3.7 m/s2
English: 12.1 ft/s2
By Comparison: If you weigh 100 pounds on Earth, you would weigh 38 pounds on Mercury.
Escape Velocity
Metric: 15,300 km/h
English: 9,500 mph
Scientific Notation: 4.25 x 103 m/s
By Comparison: Escape Velocity of Earth is 25,022 mph
Sidereal Rotation Period (Length of Day)
58.646 Earth days
1407.5 hours
By Comparison: 58.64 x Earth
Sidereal Orbit Period (Length of Year)
0.241 Earth years
87.97 Earth days
By Comparison: 0.241 x Earth
Mean Orbit Velocity
Metric: 172,341 km/h
English: 107,088 mph
Scientific Notation: 47,872.5 m/s
By Comparison: 1.61 x Earth
Orbital Eccentricity
By Comparison: 12.3 x Earth
Orbital Inclination to Ecliptic
7 degrees
Equatorial Inclination to Orbit
0 degrees
By Comparison: Earth's equatorial inclination to orbit is 23.45 degrees.
Orbital Circumference
Metric: 356,000,000 km
English: 221,000,000 miles
Scientific Notation: 3.56 x 108 km
By Comparison: 0.385 x Earth
Minimum/Maximum Surface Temperature
Metric: -173/427 °C
English: -279/801 °F
Scientific Notation: 100/700 K
By Comparison: Earth's temperature range is ~ 185/331 K.
Atmospheric Constituents
By Comparison: Earth's atmosphere consists mostly of N2, O2

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