# Why is the Moon without an atmosphere, though the Earth has one?

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Myn Is Meposted 12 years ago

Why is the Moon without an atmosphere, though the Earth has one?

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MisterHposted 12 years ago

I believe it's to do with the mass of the moon relative to the earth.  We have a much greater mass which means the effect of our gravity is greater which in turn can hold an atmosphere in place.

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Physicshelpposted 12 years ago

The Moon does have an atmosphere, just much less dense than that of the earth, so much so that it's pretty much negligible. MisterH is right in saying that this is due to it's low mass. This leads to a small escape velocity - the speed at which objects have to be travelling to escape a body.  So compared to earth, the escape velocity of the moon is low, and hence its atmosphere is negligibly dense.

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William Haloupekposted 12 years ago

The escape velocity at the surface of the moon is only 2.38 km/sec, compared with 11.19 km/sec for Earth. That's a factor of about 4.7.

Earth has trouble holding onto its hydrogen and helium, because in the upper atmosphere, it gets hit my sunlight and cosmic rays, and gets going faster than the escape velocity, and goes flying off into space. So we are slowly losing our hydrogen and helium.

Kinetic energy is proportional to velocity squared, so the Moon can only hold onto molecules with speed less than Earth can, by a factor of 4.7 squared, which is about 22. Now if an oxygen atom, with mass 16 times that of a hydrogen atom, got as much energy from photons as hydrogen atoms do in Earth's atmosphere, they would be even harder for the Moon to hold onto than hydrogen is for Earth to hold onto.

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Astronutposted 12 years ago

Because, owing to its small size, the moon had not sufficient attractive power to retain the atmosphere which once enveloped it. The atoms of this gradually escaped into space. The earth, being much larger than the moon, and having a greater pull, has been able to prevent its atmosphere leaving it.

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Illumindposted 12 years ago

Because that's what NASA tells us, and we trust NASA. :-)

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Mlssilvaposted 12 years ago

The moon has no atmosphere because, due to its small mass, does not have enough gravity to retain gases. Even if gravity were sufficient to retain the atmosphere, due to total absence of magnetic field the solar wind would have direct access to the atmosphere and due to its effects of erosion, the atmosphere would be slowly taken away.

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