Top 10 Things You Should Know About Mercury
Named after the Roman god of commerce, travel and
thievery, Mercury is the second smallest planet in the solar
system, larger only than Pluto, the most distant planet in our
solar system. If Earth were the size of a baseball, Mercury
would be the size of a golf ball. Viewed from Mercury, the Sun
would look almost three times as large as it does from Earth.
1) Mercury is the closest planet to the
Mercury 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. The planet completes a trip around the Sun every 88
days, speeding through space at nearly 50 km per second,
faster than any other planet.
2) Mercury's existence has been known of
since before the third century BC
The Greeks gave it two names, Apollo for
when it appeared as a morning star and Hermes when it
came as an evening star.
3) Mercury's surface very much resembles
Mercury is scarred by thousands of impact
craters resulting from collisions with meteors. While
there are areas of smooth terrain, there are also
cliffs, some soaring up to a mile high, formed by
4) Mercury has a large iron core which is
most likely at least partially molten
Mercury's interior is made of a large
iron core with a radius of 1,800 to 1,900 km, nearly 75
percent of the planet's diameter and nearly the size of
Earth's Moon. Mercury's outer shell, comparable to
Earth's outer shell (called the mantle) is only 500 to
600 km thick.
5) Mercury actually has a very thin
Merucry's atmosphere is made up of atoms
blasted off its surface. Due to the heat of the planet,
these atoms quickly escape into space. Thus unlike the
Earth and Venus which have stable atmospheres, Mercury's
atmosphere is constantly being replenished.
6) Mercury is a planet of extremes
Temperature variations swing from 90 K to
700 K. It's hotter on Venus, but with less fluctuations.
Mercuryalso has a very eccentric orbit; at perihelion it
is only 46 million km from the Sun but at aphelion it is
7) Mercury has no known moons or
While Mercury may resemble our own moon
in many ways, it has no moon of its own.
8) Until 1965, scientists thought that
the same side always faced the Sun
In 1965, astronomers discovered that
Mercury completes three rotations for every two orbits
around the Sun. If you wanted to stay up for a Mercury
day, you'd have to stay up for 176 Earth days.
9) The Caloris Basin is about 1,300 km in
One of the largest features on Mercury,
it was the result of an asteroid impact on the planet's
surface early in the solar system's history. Over the
next 1/2-billion years, Mercury actually shrank in
radius from 2 to 4 km as the planet cooled from its
formation. The outer crust, called the lithosphere, was
compressed and grew strong enough to prevent the
planet's magma from reaching the surface, effectively
ending the planet's period of geologic activity.
10) Mercury is the least explored of our
solar system's inner planets.
Only one spacecraft has ever visited
Mercury: Mariner 10 in 1974-75. Mariner 10's discovery
that Mercury has a very weak magnetic field, similar to
but weaker than Earth's, was a major surprise. NASA is
planning a new mission to Mercury called Mercury
Surface, Space Environment, Geochemistry, and Ranging
(MESSENGER), which will orbit Mercury toward the end of
this decade. MESSENGER will investigate key science
questions using a set of miniaturized instruments.