Most Moon Water Comes From the Sun, Not Comets

The moon’s water came from comet and asteroid impacts, right? As it turns out, the question as to where lunar H2O came from is not so straightforward — it was actually ‘baked’ via reactions with the solar wind.

When Apollo astronauts returned lunar surface samples of rock and regolith (fine, pulverized rocky grains), it was assumed they would be bone dry. After all, the moon has no atmosphere — any water deposited on the surface from impacts would have been long lost to space.

But in recent analysis of the regolith, scientists were surprised to find quantities of water locked in the lunar material.

There are quantities of ice deposits all over the moon’s surface; some large deposits exist in polar craters where sunlight never shines on the crater bottoms. And now we know that even the sun-baked surface and sub-surface has a frozen supply.

According to new research by astrochemists Alice Stephant and François Robert, of Muséum National d’Histoire Naturelle, Paris, the source of the vast majority of the surface water trapped in regolith samples is not from aeons of asteroid and comet impacts — two well known sources of cosmic water ice — it’s the sun. Rather, the water found in lunar regolith is created via chemical reactions with the solar wind.

Their findings have been published in the Proceedings of the National Academy of Sciences.

Through isotopic analysis of deuterium/hydrogen and lithium isotope ratios, the researchers were able to determine that the source of the water did not originate from ancient impacts, but from reactions in and on silicate regolith grains. High-energy protons from the solar wind impact these grains, unlocking oxygen atoms, allowing them to bond with abundant hydrogen atoms, kickstarting the formation of hydroxyl, a tracer for water. Hydroxyl is a simple molecule of one oxygen atom and a hydrogen atom, whereas water has two hydrogen atoms bonded to an oxygen atom.

Read more at Discovery News