Future moon settlers could produce all the water they need -- by capturing solar winds.
Streams of charged particles propelled from the surface of the sun (known as 'solar wind') slam into the Moon's surface every day. It's not a gentle process -- these particles reach speeds in excess of 450 kilometers per second (nearly 1 million miles per hour) -- but it does enrich the lunar surface with the building blocks of water, a new study reports.
Water from a stone
"We think of water as this special, magical compound," said William M. Farrell, a plasma physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, one of the study's co-authors. "But here's what's amazing: every rock has the potential to make water, especially after being irradiated by the solar wind."
The team ran a computer simulation to see what chemical changes take place in lunar rocks under the effect of solar winds.
Solar wind is basically a flow of protons. It continually blasts the Moon's surface, breaking the bonds among molecules in regolith -- lunar soil -- pulling apart the atoms of silica (SiO2, basically sand) and iron oxides found within the majority of the Moon's soil. Some of these protons also grab onto electrons in the lunar surface, producing hydrogen atoms. These atoms then work their way up through the regolith leeching the released oxygen. Together, hydrogen and oxygen make the molecule hydroxyl (OH), which is two-thirds of the water (H2O) molecule.
The findings should help further our goal of sending humans up to the Moon to establish a permanent presence there, says Orenthal James Tucker, a physicist at Goddard who led the research.
"We're trying to learn about the dynamics of transport of valuable resources like hydrogen around the lunar surface and throughout its exosphere, or very thin atmosphere, so we can know where to go to harvest those resources," he explains.
The research drew on infrared measurements performed on the lunar surface by several spacecraft -- including NASA's Deep Impact spacecraft NASA's Cassini spacecraft, and India's Chandrayaan-1. These readings offered us insight into the chemistry of the lunar surface, all of them finding evidence that water or its components -- hydrogen and hydroxyl -- were present in the regolith.
Exactly how such compounds wound up on the moon, however, was still a matter of debate. It was possible that they arrived on the back of meteorites impacting its surface, or that these impacts initiated the chemical reactions that created hydrogen, hydroxyl, and water. Tucker's simulation, which traces the life cycle of hydrogen atoms on the Moon, supports the solar wind hypothesis.
"From previous research, we know how much hydrogen is coming in from the solar wind, we also know how much is in the Moon's very thin atmosphere, and we have measurements of hydroxyl in the surface," he says. "What we've done now is figure out how these three inventories of hydrogen are physically intertwined."
The findings also helped us understand why spacecraft have found fluctuations in the amount of hydrogen in different regions of the Moon. All the hydrogen atoms created by solar wind bombardment eventually escape into space (since it's much less dense than all other compounds). However, hydrogen tends to accumulate predominantly in the Moon's colder areas since it gets energized by sunlight, making it escape much faster.
"The whole process is like a chemical factory," Farrell said.
A key implication of the findings, Farrell said, is that every exposed body of silica in space -- from the Moon down to a small dust grain -- has the potential to create hydroxyl and thus become a chemical factory for water.
The paper " Solar Wind Implantation Into the Lunar Regolith: Monte Carlo Simulations of H Retention in a Surface With Defects and the H 2 Exosphere" has been published in the Journal of Geophysical Research.