|Why is air pressure in outer space less than on earth?|
Pressure is nothing more than the weight of
something on you. So, standing on the surface of
Earth, a column of air that goes up to 50 km
weighs upon you. However, in space away from any
planet there is no air upon you, and gravity is
weak. This makes the air pressure in outer space
less than on Earth.
Air pressure is the result of many collisions
between air molecules and some surface.
Essentially, if there are more collisions within a
given area and time between air molecules, there
is higher air pressure. In outer space, there are
very few atoms or molecules per given area, so the
likelihood that there are collisions between atoms
or molecules is very low. To help you visualize
this in a very conceptual way, consider a
billiards table. If we were to put 2 balls on the
billiard table, the balls will probably only
collide once in a while. Now, put 20 balls on the
table. The likelihood that you'll have any two
balls collide is a lot higher now. So you can
imagine the same is true when comparing Earth's
lower atmosphere, which has about
2.7x1019 molecules per
cm3(that's 2 with 19 zeroes after it--
more than a billion billions!), vs. interstellar
space, where there are only between 0.2 and
molecules per cm3.
The Earth is so massive that it attracts things
towards it through gravity. The Earth's gravity
prevents us from flying off the face of the Earth,
and in the same way it prevents most gases from
leaving the atmosphere. (Helium is the only
exception. It is so light that it escapes the
Earth and floats off into space.) Because the
Earth's gravity attracts gas molecules, there is a
large amount of gas near the Earth, which leads to
our atmospheric air pressure. In space, unless
there is a strong gravitational force, gas
molecules are not concentrated in any one area.
The low concentration of gas molecules leads to a
very low pressure. This is the combination of a
very large volume (all of space) with a relatively
small amount of matter (gas).
Earth's gravity holds the Earth's atmosphere onto
the Earth. In space, without a planet to hold gas
down with its gravity, it disperses into the void.
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