2.9 Where does water on the Moon’s surface come from?
There are two main candidates for the origins of water on the surface of the Moon: water carried to the Moon from elsewhere and water formed in situ on the Moon’s surface.
First, with no atmosphere to protect it, the Moon is regularly bombarded by comets and asteroids, both of which contain water from elsewhere in the Solar System. As you saw earlier in the course, many of the outer Solar System moons contain large amounts of water and ice.
During a comet impact, its water molecules evaporate, and some are blasted away from the surface back out into space, but some are trapped back down onto the lunar surface and are adsorbed onto mineral surfaces or into cold ‘traps’, in permanently shadowed craters. Some craters on the Moon hold the record for having the coldest recorded temperatures in the Solar System, down to 20 kelvin (-253 °C). Because the Moon experiences very little reworking of surface material (unlike the Earth), water delivered to the Moon in this way can build up slowly via thousands of impacts, over millions of years, to form the icy deposits observed in polar craters.
The other possibility involves the production of water or hydroxyl (OH) on the lunar surface due to the interaction of charged particles from the Sun (the solar wind, which you met in Week 4) with material on the Moon’s surface. The solar wind bathes illuminated parts of the Moon’s surface with charged particles, mostly protons, which are hydrogen ions (H+) for the purposes of chemistry. When these protons come into contact with oxygen atoms inside the structures of minerals that make up the rocks and soil on the lunar surface, they form a chemical bond together and make hydroxide ions (OH−). These OH− ions are adsorbed onto the surface of lunar rock or soil minerals, and so are trapped there. If enough protons are delivered to the immediate area, or if another hydroxide group forms nearby, an extra proton may be added to the OH−, forming H2O (water).
This newly formed water molecule then experiences a series of successive episodes of evaporation and condensation. Heat from the Sun causes the water to evaporate (remember the Moon’s surface heats and cools far more than the surface of the Earth because it has no atmosphere) and when it is free from being trapped to a mineral surface, it will ‘bounce’ around. If it hits a warm surface it will rebound, but if it hits a cold surface it will tend to stick. It will evaporate again when the surface warms up, but if the surface is permanently cold, it will stay there. Over time, water molecules can ‘hop’ across the lunar surface, from the warmer sunlit regions where they form, to the cold permanently shadowed polar craters where they accumulate as water-ice and are measured in greater abundance.
However, both of these processes – the delivery of water from elsewhere in the Solar System and its generation in situ – may be important for making lunar surface water, and they form the focus of a lot of on-going research; so keep your eyes open for new revelations as our understanding of the Moon’s water evolves.