Solar wind and space dust combine to create water; process works on the Moon, too.
Water ice is the most abundant solid material in the Universe. Much of it was created as the byproduct of star formation, but not all of it. John Bradley of Lawrence Livermore National Laboratory and his team may have discovered a new source of water in our solar system. His lab experiments reveal that the solar wind may be creating water on interplanetary dust.
The sun ejects high-speed charged particles in all directions. Bodies in the inner solar system get bombarded by this wind of particles, which continuously varies in intensity. Small bodies, such as dust particles or tiny asteroids, can be eroded by these harsh winds. Larger bodies that do not have an atmosphere, such as the Moon, are bombarded by both the solar wind and tiny meteorites. This form of bombardment causes a phenomenon called space weathering.
Bradley’s team attempted to locate water using a highly sensitive method of analysis called valence electron energy-loss spectroscopy. The method involves exposing a sample to a beam of electrons that, upon hitting the material, will get deflected at different speeds. The deflection and the speeds can reveal how much energy was lost by the electrons in the process, which is based on the type of atom it hits. The instrument can identify the composition of a material at very small scales, just enough for Bradley to analyze silicate rims.
The best way to determine whether water forms on silicate rims is to do these experiments on the types of silicate material that exist in space. Bradley did this by using three types of these minerals: olivine, clinopyroxene, and anorthine. These were exposed to charged hydrogen and helium particles, which were a proxy for the solar wind.
If water is formed by the solar wind, it would only be found in the samples that were exposed to hydrogen—not in those exposed to helium. And that is what happened. As reported in PNAS, Bradley’s sensitive tests repeatedly found water, but only in the samples that were bombarded by hydrogen.
Martin McCoustra at Heriot-Watt University in Edinburgh finds the work convincing. He said, “I am not very surprised that water could be formed on silicates. However, now that they have shown that it can, it could be an important source of water.”
Bradley’s work implies that water molecules must have been forming for billions of years on interplanetary dust particles, on the Moon, and possibly on asteroids. However, McCoustra warns that “[t]his source of water, albeit new, won’t be able to account for a large proportion of water in the solar system. Most of that water was formed during the process of star formation that our sun went through.”
Some have argued that water-rich comets planted water on our planet, but McCoustra believes that a single source is unlikely. This study provides another potential source for the material that helps make our planet habitable.