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THE INNER
PLANETS
The Inner Planets
are Mercury, Venus, Earth and Mars. All four planets are very different from
each other, but also have their similarities. Their main similarities are that
they all have solid surfaces, are fairly similar in size (in comparison with the
sizes of the Outer Planets) and are all quite close to the Sun. The picture below shows the orbits of the Inner Planets
around the Sun, and their distances from the Sun and from each other, plus the orbit of Jupiter (the first of the Outer Planets), just about fitting onto the picture.
This shows how much further away Jupiter is from the Inner Planets.
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Each
planet has its own characteristics, although none of them, apart
from Earth, are known to be able to support life. Below is a
short description of each planet. For a more detailed look at the
planet, select on the planet's name (underlined in blue) or
picture.
MERCURY
Mercury is the closest
planet to the Sun, and the second
smallest in the Solar System. It has a surface covered in
craters, just like Earth's Moon, and has temperatures which can reach up to 350°c on
its sunlit side and -170°c on its dark side. A year on Mercury (the length of time it takes for
the planet to orbit the Sun) lasts for only 88
days, although a day (the length of time it takes for the
planet to turn completely on its axis) takes 59 days. Mercury has no atmosphere, so the sky
appears dark all the time.
VENUS
Venus is the only planet
in the Solar System to turn in a clockwise direction on
its axis, and it does this very slowly, taking 243 Earth
days to do so (so a day on Venus is 243 days long).
This is even longer than a year on Venus, which is 225 Earth days long. Venus is covered in poisonous clouds containing
sulphuric acid, and an atmosphere containing
dangerous carbon monoxide. Venus' surface remained a mystery
because of its thick cloud cover until 1990-1994 when
special imaging equipment on the Magellan space craft
managed to look through the clouds to reveal a volcanic surface. Temperatures on Venus reach up to 480°c because heat
from the Sun cannot escape through the thick atmosphere, which acts like a
greenhouse covering the entire planet.
EARTH
Earth is the only planet in the Solar System
known to contain life. It takes 23 hours and 56 minutes
for a day to take place on Earth (the amount of time it
takes the planet to turn completely on its axis) and 365
and a quarter days for it to complete one orbit around
the Sun. Every four years, an extra day is added to the
year to make up for the extra quarter of a day it takes
for the planet to orbit the Sun. Earth is the only planet
to contain water in its three forms, as a solid (ice), as
a liquid (sea, rain, etc.) and as a gas (steam, clouds,
atmosphere). It is still geologically living, meaning
that volcanoes, earthquakes and weather still form and
reform its surface. Its thin atmosphere is made up mainly
of Nitrogen, giving the sky a blue colour, and Oxygen,
placed into the atmosphere by the presence of life.
Hundreds of years ago, people believed Earth to be the
centre of the universe, although scientists later
discovered the truth. Earth is the biggest of the Inner
Planets, and has its own moon orbiting it.
MARS
Mars is a small red
planet with a very thin atmosphere of Carbon Dioxide. It
is believed that it once had flowing water, and would
have appeared like Earth from the distance.
Life has not been
discovered on the planet, despite many missions aiming to find evidence of
primitive forms of life. The planet has a
rusty surface and a pink sky. It is covered in rocks and
impact craters, but has seasons like Earth and water droplets can form frost,
fog and clouds. The Solar System's biggest volcano,
Olympus Mons, took place on Mars, millions of years
ago when dinosaurs lived on Earth. Mars also has huge valleys, which
people believe were created when volcanic eruptions tore
up the ground. Mars takes 687 days to
orbit the Sun (so a year is 687 days long on Mars), but a day on Mars is 24 hours and 37 minutes long,
about forty minutes longer than a day on Earth. The planet has two small moons orbiting it.
After
the Inner Planets is the Asteroid Belt, an area of rocks, the
size of some of the smaller moons orbiting the planets. This belt
is millions of kilometres wide, and separates the Inner Planets
from the Outer Planets.
STATISTICS
ABOUT THE INNER PLANETS:
AVERAGE
DISTANCES FROM THE SUN (in kilometres):
DIAMETER
AT EQUATOR (WIDTH) (in kilometres):
VISIT THE OUTER PLANETS
COMPARE
THE INNER AND OUTER PLANETS
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