Liquid water cannot exist on the surface of the Moon as it will rapidly evaporate on its airless surface or get broken down into hydrogen and oxygen by sunlight. However, water can be present in the form of ice within permanently shadowed craters at the Moon’s poles. In such places, the Sun never gets high enough over the horizon to cast its rays over the rims of these craters and illuminate the floors of the craters.
Without ever being warmed by the Sun, these permanently shadowed regions can maintain incredibly low temperatures which make them ideal for water in its frozen form to exist over billion-year timescales. In fact, an instrument onboard NASA’s Lunar Reconnaissance Orbiter recorded temperatures as low as 25 degrees Kelvin or -248 degrees Centigrade in areas within these permanently shadowed regions, making them amongst the coldest known places in the Solar System!
 From whose womb comes the ice? Who gives birth to the frost from the heavens  when the waters become hard as stone, when the surface of the deep is frozen?
- Job 38:29-30 (New international Version)
The permanently shadowed craters at the Moon’s poles can serve as cold traps where water brought to the Moon by impacting comets can accumulate in these places. This week, I researched on the retention of water from the impacts of comets onto the surface of the Moon. Comets are small icy objects which orbit the Sun and they range in sizes from a few hundred meters to tens of kilometers across. Comets are known to contain a large amount of volatiles, especially water in the form of ice. When a comet impacts the Moon, a fraction of the water from the comet can eventually end up in these permanently shadowed craters and accumulate there in the form of ice.
In my research, I derived a method which estimates the fraction of the comet’s mass that remains gravitationally bound to the Moon after the impact. I carried out the computations and analysis for various impact velocities and various impact incident angles. From my results, a significant fraction of the comet’s mass remains gravitationally bound to the Moon after the impact as long as the impact velocity of the comet is less than 30 to 40 kilometers per second. In my analysis, the comet’s mass is assumed to be entirely made up of water.
Of the fraction of the comet’s mass in water which remains gravitationally bound to the Moon after the impact, a portion can survive long enough in its migration across the surface of the Moon to eventually accumulate in the permanently shadowed craters at the Moon’s poles. The presence of water on the Moon is an important factor in determining lunar habitability since a large and easily accessible source of water on the Moon will render needless the prohibitively expensive feat of transporting water from the Earth. Water can be separated into hydrogen and oxygen to provide breathable oxygen and to serve as a form of rocket fuel.
I have also extended my method of analysis in estimating the retention of water from the impacts of comets onto the surface of the Moon to other worlds such as the planet Mercury. With a stronger gravitational field, Mercury is able to retain a larger fraction of a comet’s watery mass and like the Moon; Mercury also has permanently shadowed craters at its poles where frozen water can accumulate.