Comets played only subordinate role as a supplier of volatile substances as opposed to asteroids.
UK – The surface of the Moon is dry though numerous missions provided evidence of large amounts of water on the Moon pole and in the interior of the moon. So far however, the exact origin of the water is unclear. British and French scientists believe asteroids, according to a study published in the journal Nature Communications, were the most important water source.
According to Jessica Barnes, leader of the study, most of the water on the moon that came over a period of 10 to 200 million years ago was an ocean of magma on the surface. Their assumption is based on isotope analyses of Moon rock samples, which they compared to the composition of asteroids and even comets. The focus of the research is mainly on the elements hydrogen and nitrogen, their isotopic distributions pointed to chondrites from the inner asteroid belt. Comets, had enriched deuterium in water with an increased concentration of the heavy isotope of hydrogen, however, they are responsible for less than 20 per cent of the Moon water.
The Moon was formed in the early history of the solar system after the collision of the proto-Earth with Theia, a Mars-large Protoplanet. Debris of this clash began together gradually. This process dominated by gravity, much energy was released to melt rock and make an ocean of magma on the surface of the moon. After the impact of asteroids and comets in that ocean thermal caps should have formed which kept volatile substances such as water vapour before losing it to the outer space. The water in the Interior of the Moon remained during the following cooling of the magma ocean for millions of years.
The assumption that the asteroids make up most important water source on the Moon is supported by a study presented four years ago by American researcher Conel Alexander and his colleagues at the Carnegie Institution in Washington, D.C, when they researched in the origin of water on Earth. They also came to the conclusion that the majority of the volatile substances and hence the water on our planet came from asteroids.