Question

Why do Mercury and the Moon have (almost) no atmosphere?

a. The gravity at their surfaces is low so most gas molecules travel fast enough to escape.
b. The temperature at their surfaces is high so most gas molecules travel fast enough to escape.
c. The only gas molecules they originally had were very low in mass, and immediately escaped.
d. They are both highly reflective.

Answer 1

Mercury and the Moon lack substantial atmospheres mainly due to their low gravity and inability to retain gas molecules, with the Moon having weak gravity and being depleted in volatile materials, and Mercury being too hot to hold gases.

Step-by-step explanation:

The primary reasons why Mercury and the Moon have almost no atmosphere are their low gravity and the properties of the gases that could have formed their atmospheres. Both celestial bodies have insufficient mass and gravitational pull to retain an atmosphere. For the Moon, it has only one-eightieth the mass of Earth, resulting in about one-sixth of Earth's surface gravity.

This low gravity makes it too weak to hold onto gas molecules which escape into space in a process similar to the way a rocket leaves a planet. Moreover, any temporary atmosphere gained from comet impacts on the Moon is quickly lost by freezing onto the surface or escaping to space.

The Moon's material is also depleted in volatile materials, which are substances that evaporate at relatively low temperatures. Mercury, on the other hand, is not only small but also too hot, which makes the retention of an atmosphere impossible. In both cases, any atmosphere they might have had would be lost rapidly on geological timescales.