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The Moon's atmosphere is drastically different from that of Earth, and understanding this difference can help explain why liquid water cannot exist on its surface. The Moon is often described as having no atmosphere. However, it does have a very thin exosphere comprised mostly of hydrogen, helium, neon, and trace amounts of other gases. This exosphere isn't thick enough to be considered as a traditional atmosphere like Earth's.

Due to the Moon's lower gravity, which is about one-sixth that of Earth's, it can't hold onto a thick atmosphere. Any gases that might accumulate on the Moon are quickly lost into space. As a result, what remains is not significant enough to offer any protection or support any stable conditions we associate with an atmosphere, such as temperature regulation or air pressure. This is one of the key reasons the Moon cannot sustain bodies of liquid water.

Atmospheric pressure plays a crucial role in determining the state of water. On Earth, atmospheric pressure is approximately 101.3 kPa at sea level, which is sufficient for maintaining water in its liquid form at a range of temperatures. This pressure supports a stable climate and allows water to exist in various states depending on temperature.

On the Moon, the absence of a thick atmosphere means that the atmospheric pressure is nearly non-existent. In fact, the pressure is so low that it can be considered almost a perfect vacuum. Without enough atmospheric pressure, there is no force to hold water molecules together in a liquid state. Therefore, water on the Moon cannot exist as a liquid; it would instantly boil into vapor or freeze depending on the environmental conditions.

Water can exist in three primary states: solid, liquid, and gas. The transition between these states depends significantly on temperature and pressure. On Earth, liquid water is possible because the atmospheric pressure is sufficient to sustain it within a specific temperature range—generally from 0°C to 100°C at sea level.

On the Moon, the extreme temperature fluctuations and lack of atmospheric pressure prevent water from existing in a stable liquid form. During the lunar day, surface temperatures can soar to around 127°C, causing any water to quickly vaporize. During the night, temperatures can plummet to -173°C, which would freeze any water.

• Solid: Water is ice; occurs below 0°C during the lunar night.
• Liquid: Cannot naturally occur on the Moon's surface due to low pressure.
• Gas: Water vaporizes under the Moon's high daytime temperatures.

These conditions make it clear why liquid water is simply not feasible on the lunar surface in the absence of adequate pressure and stable temperatures.