Human habitats need water. The prospect of human colonies outside of Earth drives the search for extraterrestrial water, especially in liquid form.

The presence of water on planets and other celestial bodies is gleaned initially from data gathered by space missions of the National Aeronautic and Space Administration (NASA). For example, evidence laid out by Georgia Tech College of Sciences researchers in 2015 to establish that liquid water flows on Mars originated from images and spectral data obtained by the Mars Reconnaissance Orbiter. 

Other NASA missions have returned tantalizing suggestions of water on Earth’s satellite, the Moon, according to Thomas M. Orlando, a professor in the School of Chemistry and Biochemistry and an adjunct professor in the School of Physics. “Results from the Moon Mineralogy Mapper on the Chandrayaan-1 spacecraft, the Visual and Infrared Mapping Spectrometer on the Cassini spacecraft, and the extended mission for the Deep Impact spacecraft have implicated the existence of hydroxyl and water on the Moon,” Orlando says

Similarly, Orlando adds, data from the MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) mission suggests the presence of water on or in the polar regions of Mercury.

For both the Moon and Mercury, planetary scientists have posited that the water could come from surface reactions of solar winds bombarding the heavenly bodies. At the ACS National Meeting in Philadelphia, Orlando presents evidence suggesting that solar winds could form water on specific regions of Mercury but not the Moon.

Solar winds are streams of ionized particles—mostly protons and electrons—so energetic that they escape the sun’s gravitational field. They hurtle at speeds of 1 million miles per hour, with temperatures of up to 1 million degrees Celsius. Continuous blasts of these energetic particles would obliterate any life that might exist on a surface. Thanks to its strong magnetic shield, Earth experiences only a fraction of the radiation’s full fury.

The Moon, however, is not protected from solar winds. And Mercury’s shield is not as strong as Earth’s. Researchers have speculated that water could form on the Moon and Mercury as high-energy protons in the solar winds hit surface minerals. Could this be the water hinted by infrared absorptions detected by mission instruments -- around 3 micrometers, characteristic of H2O, but also of hydroxyls (OH)?

NASA doesn’t have assets on the ground that could help explain mission data, which are gathered from tens of kilometers above the surface of the Moon, says William M. Farrell of the Sciences and Exploration Directorate at NASA’s Goddard Space Flight Center. Experiments in the laboratory that replicate extraterrestrial surface processes, he adds, “are the next best approach.”

In the lab, Orlando and others make well-controlled measurements of the physical and chemical changes happening on a mineral sample from the Moon under conditions similar to an onslaught by solar winds. “We make measurements using techniques developed by the surface chemistry and physics communities, and we then collaborate with planetary scientists to help them understand their mission data,” Orlando says.

The measurements suggest that any water molecule formed in the sunlit areas of the Moon would disappear. “Where there is solar flux,” Orlando explains, “the lunar surface warms up too much and the water doesn’t stick.” Water molecules could also desorb and leave the surface when they are bombarded by energetic photons.

So what are mission instruments seeing at 3 micrometers on the sunlit areas of the Moon? “That absorption is due to O-H bending and stretching,” Orlando explains. “Some planetary scientists think it’s water. We’re saying it’s not water. It can’t be water. Water won’t stick.”

The experiments in Orlando’s lab suggest that when the solar wind bombards the Moon, protons embed into the oxide-rich rocky materials on the lunar surface, forming hydroxyls. “I’m in the chemistry department,” Orlando quips. “Water is H2O. If I say water is OH, I’ll get fired.”

Orlando and coworkers have modeled the proton implantation that they believe happens on the Moon. When they apply the model to Mercury, Orlando says, “the story changes for an important reason: Mercury gets really warm, and at those temperatures, the hydroxyls find each other and can produce water by a process known as recombinative desorption.”

Furthermore, the water molecules formed on Mercury would not disappear, because calculations show that they would not have enough velocity to escape Mercury’s gravitational field.

A plausible scenario is that the water molecules hop around Mercury’s surface until they get to the poles. And because temperatures are much lower there, the water molecules stay for a very long time.

“We believe that under specific conditions the solar wind can make water on Mercury and some of this water can eventually deposit on the poles,” Orlando says. For now, this hypothesis is based on extrapolation from modeling. Orlando and his team will be designing experiments to test it.

“What the terrific research at Georgia Tech is finding is that any silica-rich rock exposed to solar wind can generate OH, and if warmed, like at Mercury, may even thermally generate water,” Farrell says.

The hypothesis leads to other questions: If water is formed under specific conditions in Mercury, could it diffuse to the subsurface instead of desorbing? Could Mercury harbor pools of underground water?

“We can’t bring water jugs to the Moon or anywhere else outside Earth,” Orlando says. “That’s why NASA is very interested in the prevalence and availability of water.” At the same time, he adds, “there’s real understanding now in the planetary science and astrophysics community that fundamental chemistry and surface science has to be part of the story.” 

Funding for Orlando’s work is from the NASA Solar System Exploratory Research Virtual Institute (SSERVI) Volatiles, Regolith, and Thermal Investigations Consortium for Exploration and Science (VORTICES) team in the Applied Physics Laboratory at John Hopkins University.