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Union University Department of Physics

The Science Guys

Science Guys > January 2002

January 2002

Is there air on our Moon? Does the Moon have gravity?

A full Moon is a lovely sight. From our Earth we can look up and see some surface details- details that are never obscured by the scattering of light from an atmosphere. Planets with an atmosphere sometimes have their surface features obscured - Venus’ atmosphere is so thick that none of its surface features is visible from the Earth. Mars’ surface has been obscured recently because of a planet-wide dust storm. So visually we can infer that the Moon has no atmosphere because there are never any visual disturbances. If you view the moon with a telescope of sufficient magnification, pay particular attention to the edge on the Moon. You see profiles of crater edges and mountains- details that an atmosphere would make quite difficult to see.

Why the moon has no atmosphere is actually related to the gravity question. While the fundamental nature of gravity is still a mystery, the successful gravitation theories of Isaac Newton (1642-1727) and Albert Einstein (1879-1955) rely on the fact that the mere presence of matter is enough to produce a gravitational field. So, simply by virtue of having a mass, the Moon has a gravitational field. Many of you are old enough to recall astronauts walking on the Moon during the NASA Apollo program. The Moon’s gravitational field exerting a pull on those astronauts and held the astronauts to the surface.

The gravitational field is an entity that we consider "attached" to a planet or moon. The gravitational field moves with the Moon and exerts an attractive force on anything that wanders into the field. However, the field on the Moon is much weaker than the field on Earth. A 180-pound person on Earth would weigh only 30 pounds on the Moon because of the Moon’s weaker gravity. The Moon’s smaller mass and radius combine to produce a gravitational field at its surface only one-sixth that of our Earth.

Now consider the atmosphere question, keeping in mind the Moon’s weaker gravity. It is the gravitational field that holds an atmosphere to a planet. Because of the Earth’s gravitational field, an object leaving the Earth would have to be thrown upward with a speed of 25,000 mph in order to leave and not fall back to the Earth. This speed is called the escape speed. There are very few air molecules in the upper atmosphere with sufficient speed to escape Earth’s gravity. (In a previous article we indicated that air molecules can move thousands of mph but not tens of thousands of mph)

The Moon’s much weaker gravity corresponds to an escape speed of only 5400 mph, a speed gas molecules can attain. Even if man created an atmosphere on the Moon, this significantly lower escape speed means that, over time, the molecules that constitute any atmosphere on the Moon would eventually escape into space.