Most of the water inside the Moon was delivered via asteroids during the Moon’s early history, about 4.5 to 4.3 billion years ago, according to a new study published in Nature Communications this week.
The Moon is believed to have formed from the debris generated by the collision of a Mars-sized planet with the Earth about 4.5 billion years ago. Soon after its formation, an ocean of magma was present on the Moon. Water is also known to have been present in the interior of the Moon. However, until now, the timing and nature of the delivery of water to the Moon, and the relative contributions of asteroidal and cometary sources, have not been clear.
Jessica Barnes and colleagues constrained the rate, source and timing of water delivery using a combination of numerical models and measured isotopic compositions of lunar samples from a range of previous studies. They found that water was delivered to the interior of the Moon over a period of ten to 200 million years, while the lunar magma ocean was also present. Based on hydrogen and nitrogen data from the samples, the authors show that a water-rich class of asteroids known as carbonaceous chondrites were responsible for most of the water in the lunar interior, with comets accounting for less than 20 % of the total water budget of the Moon. In this model, comets and asteroids collided with the magma ocean on the Moon, and a thermal lid - which formed at the surface of the magma ocean - prevented loss of volatiles such as water through degassing to space, allowing water to be retained in the Moon’s interior.
Although these results suggest that most of the water may have come from asteroids, some of it is also likely to have been derived from the early Earth during the Moon-forming impact event, note the authors.