The Robinson Library >> Science >> Astronomy >> Solar System

the closest planet to the Sun and the eighth largest planet in the Solar System (smallest if you consider Pluto to be a planetoid as opposed to a planet)

Mercury as seen by Mariner 10
Mercury as seen by Mariner 10

popular symbol for Mercury
popular symbol for Mercury

In Roman mythology Mercury is the god of commerce, travel and thievery, and the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky. Mercury has been known since at least the time of the Sumerians (3rd millennium B.C.).

Facts and Figures
Distance from Sun  
  Perihelion 28,600,000 miles
  Mean 36,000,000 miles
  Aphelion 43,500,000 miles
Distance from Earth  
  Least 48,500,000 miles
Equatorial diameter 3,010 miles
Period of rotation (day) 58.7 earth days
Period of revolution (year) 88 earth days
Orbital speed 29.8 miles per second
Mass (Earth=1) 0.06
Mean density (water=1) 5.4
Surface gravity (Earth=1) 0.40
Atmospheric pressure at surface (Earth=1 bar)  
Mean surface temperature  
  Dayside 644 F
  Nightside -450 F
Natural satellites 0

View from Earth

Although Mercury is a fairly bright "star," it is never seen in the darkness of true night, but only in the pre-dawn glow or at twilight, and never more than two hours before sunrise or two hours after sunset. This is because Mercury is in a very close position to the Sun when viewed from the distant vantage point of Earth, and is, therefore, usually lost to sight in the brilliance of the solar glare.

Mercury (at the red arrow) passing in front of the Sun, as seen from Earth
Mercury (at the red arrow) passes in front of the Sun only 12 or 13 times each century


Mercury has been visited by only one spacecraft, Mariner 10, which flew by three times in 1974 and 1975. Only 45% of the surface was mapped, and it is too close to the Sun to be safely imaged by the Hubble Space Telescope. A new discovery-class mission to Mercury, Messenger, was launched by NASA in 2004 and began sending data back from Mercury in 2011.

craters on the surface of Mercury, as seen by Mariner 10
craters on the surface of Mercury as seen by Mariner 10


Mercury has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space, meaning that Mercury's atmosphere is constantly being replenished.

Surface Features

Mercury has a dense iron core which is relatively larger than Earth's, probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.

The surface of Mercury is heavily cratered and very old. It is criss-crossed by long escarpments, some up to hundreds of miles in length and up to a mile or more high. Some cut through the rings of craters and other features in such a way as to indicate that they were formed by compression.

One of the largest features on Mercury's surface is the Caloris Basin, which is about 800 miles long. It was probably caused by a very large impact early in the history of the Solar System.

mosaic image of the Caloris Basin
mosaic image of the Caloris Basin

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Some may be the result of ancient volcanic activity, others may be the result of the deposition of ejecta from cratering impacts. Some of the Mariner data suggests the possibility of relatively recent volcanic activity on Mercury, but more data is needed for confirmation.

Despite its proximity to the Sun, radar observations of Mercury's north pole show evidence of water ice in the protected shadows of some craters.

Other Information

Until 1962 it was thought that Mercury's "day" was the same length as its "year," meaning that it kept one face to the Sun much as the Moon does to the Earth. This was proven false by Doppler radar observations in 1965, and it is now known that Mercury rotates three times in two of its years.


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The Robinson Library >> Science >> Astronomy >> Solar System

This page was last updated on 02/24/2017.