FACTS ABOUT MERCURY
Mercury has been known to humanity since ancient times and
although its discovery date is unknown, the first mentions of the planet are
believed to be around 3000 BC by the Sumerians.
A year in Mercury is 88 days, yet a Mercury day is 176 Earth
days. Mercury is nearly tidally locked to the Sun – also known as a gravitational
lock – and over time this has slowed the rotation of the planet to almost match
its orbit around the Sun.
Mercury orbits so quickly around the Sun that early
civilizations believed it was actually two different stars – one which appeared
in the morning and another which appeared in the evening.
Mercury is the smallest planet in the solar system with a
diameter of 4,879 km and is one of five planets that is visible to the naked
eye.
After the Earth, Mercury is the second densest planet. Despite
its small size, Mercury is very dense because it is composed mainly of heavy
metals and rock – the main characteristic of terrestrial planets.
Mercury is named after the messenger of the Roman gods, who is
also known as Hermes in Greek mythology. This is because of the speed in which
Mercury orbits the Sun and the speed with which Mercury the Roman deity was
able to deliver messages.
Astronomers didn’t realize that Mercury was a planet until
1543 when Copernicus published his Sun-centered model of the Solar System –
putting the Sun as the centre of the solar system rather than the previously
believed centre, the Earth.
The planet has just 38% of the gravity on Earth. This means
that Mercury isn’t able to hold the atmosphere it has and it instead gets blown
away by solar winds. However those same solar winds are also bringing in new
gases, radioactive decay and dust from micrometeorites – replenishing the
atmosphere.
Mercury has no moons or rings because of its low gravity and
lack of atmosphere.
It was once believed that a planet called Vulcan existed
between the orbit of Mercury and the Sun – however the existence of such a
planet was never found.
The orbit of Mercury is an ellipse rather than circular. It
has the most eccentric orbit in the solar system and the least circular of all
of the planets, according to scientists and astronomers.
Mercury is only the second hottest planet. Venus, though
farther from the Sun than Mercury, actually experiences higher temperatures.
This is because Mercury has no atmosphere to regulate temperature and results
in the most extreme temperature change of all the planets – ranging from -170°C
(-280°F) during the night to 430°C (800°F) during the day.
Mercury does not experience any seasons. The axis of Mercury
has the smallest tilt of all other planets, and this results in a lack of
seasons on its surface.
Mercury is the only planet which doesn’t rotate exactly once
every year – instead rotating three times for every two orbits of the Sun. This
is because it is nearly tidally locked to the Sun.
The orbit of Mercury was important in proving Albert
Einstein’s theory of General Relativity.
Mercury has a large iron core that is around 40% of its volume
(compared to a core volume of 17% for Earth) in its centre whose radius is 1800
to 1900 kilometers (1100 to 1180 miles). Scientists believe the core of Mercury
is probably molten.
The outer shell of Mercury is only 500 to 600 kilometers (310
to 375 miles) thick. Earth’s outer shell (the mantle and crust) is 2930
kilometers (1819 miles) thick.
Mercury has a very thin atmosphere, which is made up of atoms
from the surface of the planet that have been blown away by solar winds. As
Mercury is so hot, these atoms quickly escape into space and so its atmosphere
is constantly being replenished.
Mercury has a weak magnetic field whose strength is about 1%
of the magnetic field on Earth.
Only two spacecraft have ever visited Mercury. It is difficult
to reach the planet due to its proximity to the Sun and any spacecraft visiting
would need to travel 91 million kilometers into the Sun’s gravitational
potential well. The Mariner 10 visited during 1974-75, flying by Mercury three
times and mapping half its surface. On March 24, 1975 it ran out of fuel and is
still believed to be orbiting the Sun.
The MESSENGER probe was launched in 2004
to explore Mercury’s high density, its geological history, the nature of its
magnetic field and more. Another mission, BepiColombo, is to be launched in
2015 by the European Space Agency and Japan is expected to reach Mercury in
2019.
Mercury has more craters and impact marks that any other
planet. The surface is similar to that of the Moon, as unlike most planets,
Mercury isn’t geologically active and cannot “self heal” from impacts with
asteroids and comets. Most of the Mercurian craters are named after famous
writers and artists. If a crater is larger than 250 km in diameter, it is known
as a Basin. The largest Basin on Mercury, the Caloris Basin, is around 1,550 km
in diameter and was discovered by the Mariner 10.