Why does moon have no air or water?

Why doesn’t the Moon have air and water?

Long before the first astronauts landed there in 1969, astronomers had calculated that the Moon had no air. They suspected that it had no water either.

They were right on both counts. The Moon has no atmosphere, and lunar rocks brought back from that first landing contain no water. (Most Earth rocks hold 1 or 2 per cent.) To preserve the lunar rocks in their original condition, the National Aeronautics and Space Administration (NASA) stores them in its Planetary Materials Laboratory in Houston, Texas, in an atmosphere of dry nitrogen which is absolutely waterless.

The Moon has no air or water because its gravity is too weak to hold on to an atmosphere or to any water vapour. It may seem strange that there isn’t at least some thin air or some wispy clouds of vapour clinging to the Moon. The reason is linked to what is called escape velocity.

When astronauts take off into space, their vehicles need sufficient speed escape velocity to enable them to shake off the pull of Earth’s gravity. The Earth’s escape velocity is 11.2 km (7 miles) a second. That of the Moon is only 2.4 km (1.5 miles) a second. Anything moving faster than that and away from the Moon will head off into space. A speed of more than 8640 km/h (5368 mph), the Moon’s escape velocity, sounds very fast. But molecules of gas regularly go as fast as that and sometimes much faster. At times, when the Sun is overhead, the temperature of rocks on the Moon rises to 110°C (230°F). High temperatures mean higher molecular speeds. Given time, any gas or water molecule on the Moon would reach a speed higher than that of the escape velocity high enough for it to leave the Moon and never return. Any atmosphere and water on the Moon in ancient times would eventually have zoomed away into space.