Mercury (planet)

Mercury Astronomical symbol of mercury
How to say it how to say: /ˈmɜrkjəri/
Reference date J2000
Longest distance from the Sun 69,816,900 km
0.466 697 AU
Shortest distance from the Sun 46,001,200 km
0.307 499 AU
Longest distance from the center of its orbital path
("semi-major axis")
57,909,100 km
0.387 098 AU
How long it takes to complete an orbit 87.969 1 d
(0.240 846 a)
How long an orbit seems to take
(from the central body)
115.88 d
Average speed 47.87 km/s
Mean anomaly 174.796°
Angle above the reference plane
7.005° to Ecliptic
3.38° to Sun’s equator
6.34° to Invariable plane
Natural things which orbit it None
Size and other qualities
Average radius 2,439.7 ± 1.0 km
0.3829 Earths
Surface area 7.48×107 km²
0.147 Earths
Volume 6.083×1010 km³
0.054 Earths
Mass 3.3022×1023 kg
0.055 Earths
Average density 5.427 g/cm³
Surface gravity 3.7 m/s²
0.38 g
Escape velocity 4.25 km/s
Turning speed 10.892 km/h (3.026 m/s)
Angle at which it turns
(in relation to its orbit)
2.11′ ± 0.1′
Angle above the celestial equator
How much light it reflects

0.119 (bond)

0.106 (geom.)
Surface temp. Min. Avg. Max.
0°N, 0°W 100 K 340 K 700 K
85°N, 0°W 80 K 200 K 380 K
Seeming brightness
("apparent magnitude")
up to −1.9
Make-up 42% Molecular oxygen
29.0% sodium
22.0% hydrogen
6.0% helium
0.5% potassium
Trace amounts of argon, nitrogen, carbon dioxide, water vapor, xenon, krypton, & neon

Mercury is the smallest planet in the Solar System. It is the closest planet to the sun. It makes one trip around the Sun once every 87.969 days. Mercury is bright when it is visible from Earth, ranging from −2.0 to 5.5 in apparent magnitude. It cannot be easily seen as it is usually too close to the Sun. Because Mercury is normally lost in the glare of the Sun (except during a solar eclipse), Mercury can only be seen in the morning or evening twilight.

Compared to what is known about the other planets in the Solar System, little is known about Mercury. Telescopes on the Earth show only a small, bright crescent. The first of two spacecraft to visit the planet was Mariner 10, which mapped only about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which finished mapping the planet in March 2013.

Mercury looks a lot like Earth's Moon. It has many craters with areas of smooth plains, no moons around it and no atmosphere as we know it. However, Mercury does have an extremely thin atmosphere, known as an exosphere. Unlike Earth's moon, Mercury has a large iron core, which gives off a magnetic field about 1% as strong as that of the Earth. It is a very dense planet due to the large size of its core. Surface temperatures can be anywhere from about 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Known sightings of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers thought that Mercury was two different objects: one able to be seen only at sunrise, which they called Apollo; the other that was only able to be seen at sunset, which they called Hermes. The English name for the planet is from the Romans, who named it after the Roman god Mercury, which they thought to be the same as the Greek god Hermes. The symbol for Mercury is based on Hermes' staff.

Even though Mercury is the closest planet to the Sun, it is not the warmest. This is because it has no greenhouse effect, so any heat that the Sun gives to it quickly escapes into space.

Inside Mercury

Mercury is one of four inner planets in the Solar System, and has a rocky body like the Earth. It is the smallest planet in the Solar System, with a radius of 2,439.7 km (1,516.0 mi) Mercury is even smaller than some of the largest moons in the solar system, such as Ganymede and Titan. However, it has a greater mass than the largest moons in the solar system. Mercury is made of about 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only a little bit less than Earth’s.

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