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The Planet
Mercury
The planet Mercury
was first photographed in detail on March 29, 1974, by the U.S. probe Mariner
10. The probe was about 130,000 miles (210,000 kilometers) from Mercury. Image
credit: NASA
Mercury is the closest planet to
our Sun. It is named for the ancient Roman god of trade and profit. Legend says
Mercury's winged sandals gave him super speed. Mercury the planet is super fast,
too. It zips around the Sun every 88 days - faster than any other planet.
Mercury's elliptical orbit takes
the small planet as close as 47 million kilometers (29 million miles) and as far
as 70 million kilometers (43 million miles) from the Sun. If one could stand on
the scorching surface of Mercury when it is at its closest point to the Sun, the
Sun would appear almost three times as large as it does when viewed from Earth.
Temperatures on Mercury's surface can reach 430 degrees Celsius (800 degrees
Fahrenheit). Because the planet has no atmosphere to retain that heat, nighttime
temperatures on the surface can drop to -170 degrees Celsius (-280 degrees
Fahrenheit).
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.
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 second smallest
planet in the Solar System, larger only than previously measured 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.
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Average Distance from the Sun
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Metric: 57,909,175 km
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English: 35,983,095 miles
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Scientific Notation:
5.7909175 x 107 km (0.38709893 A.U.)
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By Comparison: Earth is 1
A.U. (Astronomical Unit) from the Sun.
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Metric: 46,000,000 km
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English: 28,580,000 miles
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Scientific Notation:
4.600 x 107 km (0.3075 A.U.)
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By Comparison: 0.313 x
Earth
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Metric: 69,820,000 km
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English: 43,380,000 miles
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Scientific Notation:
6.982 x 107 km (0.4667 A.U.)
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By Comparison: 0.459 x
Earth
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Metric: 2,439.7 km
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English: 1,516.0 miles
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Scientific Notation:
2.4397 x 103 km
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By Comparison: 0.3825 x
Earth
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Metric: 15,329.1 km
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English: 9,525.1 miles
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Scientific Notation:
1.53291 x 104 km
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Metric: 60,827,200,000 km3
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English: 14,593,200,000
mi3
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Scientific Notation:
6.08272 x 1010 km3
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By Comparison: 0.054 x
Earth's
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Metric:
330,220,000,000,000,000,000,000 kg
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Scientific Notation:
3.3022 x 1023 kg
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By Comparison: 0.055 x
Earth's
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Metric: 5.427 g/cm3
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By Comparison: 0.984 x
Earth
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Metric: 74,800,000 km2
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English: 28,900,000
square miles
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Scientific Notation: 7.48
x 107 km2
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By Comparison: 0.108 x
Earth
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Equatorial Surface Gravity
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Metric: 3.7 m/s2
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English: 12.1 ft/s2
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By Comparison: If you
weigh 100 pounds on Earth, you would weigh 38 pounds
on Mercury.
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Metric: 15,300 km/h
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English: 9,500 mph
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Scientific Notation: 4.25
x 103 m/s
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By Comparison: Escape
Velocity of Earth is 25,022 mph
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Sidereal Rotation Period (Length
of Day)
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58.646 Earth days
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1407.5 hours
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By Comparison: 58.64 x
Earth
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Sidereal Orbit Period (Length of
Year)
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0.241 Earth years
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87.97 Earth days
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By Comparison: 0.241 x
Earth
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Metric: 172,341 km/h
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English: 107,088 mph
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Scientific Notation:
47,872.5 m/s
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By Comparison: 1.61 x
Earth
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0.20563069
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By Comparison: 12.3 x
Earth
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Orbital Inclination to Ecliptic
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Equatorial Inclination to Orbit
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0 degrees
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By Comparison: Earth's
equatorial inclination to orbit is 23.45 degrees.
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Metric: 356,000,000 km
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English: 221,000,000
miles
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Scientific Notation: 3.56
x 108 km
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By Comparison: 0.385 x
Earth
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Minimum/Maximum Surface
Temperature
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Metric: -173/427 °C
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English: -279/801 °F
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Scientific Notation:
100/700 K
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By Comparison: Earth's
temperature range is ~ 185/331 K.
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By Comparison: Earth's
atmosphere consists mostly of N2, O2
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Additional
Information:
Namesake:
Messenger of the Roman Gods
Successive Sunrises: 175.97 days
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Mercury/Earth
Comparison
Bulk parameters
Mercury Earth Ratio (Mercury/Earth)
Mass (1024 kg) 0.3302 5.9736 0.0553
Volume (1010 km3) 6.083 108.321 0.0562
Equatorial radius (km) 2439.7 6378.1 0.383
Polar radius (km) 2439.7 6356.8 0.384
Volumetric mean radius (km) 2439.7 6371.0 0.383
Ellipticity (Flattening) 0.0000 0.00335 0.000
Mean density (kg/m3) 5427 5515 0.984
Surface gravity (eq.) (m/s2) 3.70 9.80 0.378
Surface acceleration (eq.) (m/s2) 3.70 9.78 0.378
Escape velocity (km/s) 4.3 11.2 0.384
GM (x 106 km3/s2) 0.02203 0.3986 0.0553
Bond albedo 0.119 0.306 0.389
Visual geometric albedo 0.106 0.367 0.289
Visual magnitude V(1,0) -0.42 -3.86 -
Solar irradiance (W/m2) 9126.6 1367.6 6.673
Black-body temperature (K) 442.5 254.3 1.740
Moment of inertia (I/MR2) 0.33 0.3308 0.998
J2 (x 10-6) 60. 1082.63 0.055
Number of natural satellites 0 1
Planetary ring system No No
Orbital parameters
Mercury Earth Ratio (Mercury/Earth)
Semimajor axis (106 km) 57.91 149.60 0.387
Sidereal orbit period (days) 87.969 365.256 0.241
Tropical orbit period (days) 87.968 365.242 0.241
Perihelion (106 km) 46.00 147.09 0.313
Aphelion (106 km) 69.82 152.10 0.459
Synodic period (days) 115.88 - -
Mean orbital velocity (km/s) 47.87 29.78 1.607
Max. orbital velocity (km/s) 58.98 30.29 1.947
Min. orbital velocity (km/s) 38.86 29.29 1.327
Orbit inclination (deg) 7.00 0.00 -
Orbit eccentricity 0.2056 0.0167 12.311
Sidereal rotation period (hrs) 1407.6 23.9345 58.785
Length of day (hrs) 4222.6 24.0000 175.942
Obliquity to orbit (deg) ~0 23.45 0.
Mercury Observational Parameters
Discoverer: Unknown
Discovery Date: Prehistoric
Distance from Earth
Minimum (106 km) 77.3
Maximum (106 km) 221.9
Apparent diameter from Earth
Maximum (seconds of arc) 13.
Minimum (seconds of arc) 4.5
Maximum visual magnitude -1.9
Mean values at inferior conjunction with Earth
Distance from Earth (106 km) 91.70
Apparent diameter (seconds of arc) 11.0
Mercury Mean Orbital Elements
(J2000)
Semimajor axis (AU) 0.38709893
Orbital eccentricity 0.20563069
Orbital inclination (deg) 7.00487
Longitude of ascending node (deg) 48.33167
Longitude of perihelion (deg) 77.45645
Mean Longitude (deg) 252.25084
North Pole of Rotation
Right Ascension: 281.01 - 0.003T
Declination : 61.45 - 0.005T
Reference Date : 12:00 UT 1 Jan 2000 (JD 2451545.0)
T = Julian centuries from reference date
Mercurian Magnetosphere
Dipole field strength: 0.0033 gauss-Rh3
Dipole tilt to rotational axis: 169 degrees
Longitude of tilt: 285 degrees (from Mercury I flyby)
115 degrees (from Mercury III flyby)
Note: Rh denotes Mercurian
radii, 2,439 km
Mercury Atmosphere
Surface pressure: ~10-15 bar (0.001 picobar)
Average temperature: 440 K (167 C) (590-725 K, sunward side)
Total mass of atmosphere: <~1000 kg
Atmospheric composition: 42% Oxygen (O2), 29% Sodium (Na),
22% Hydrogen (H2), 6% Helium (He), 0.5% Potassium (K),
possible trace amounts of Argon (Ar), Carbon Dioxide (CO2),
Water (H2O), Nitrogen (N2), Xenon (Xe), Krypton (Kr), Neon (Ne)
(The atmosphere of Mercury is essentially a vacuum.
Compositional values are not well constrained, values from "Mercury",
Vilas, Chapman, and Matthews, eds., University of Arizona Press, 1988)
Credit:NASA
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