Long thought to be bone dry, the moon has recently been confirmed as relatively water rich. But a big question remains: Where did the wet—or more accurately, icy—stuff come from?
A new study might have the answer: The moon's water may have, in a sense, sailed in on the solar wind. The discovery hints at a previously unknown method of delivering water to the inner solar system—and a new way to produce water and rocket fuel for future space missions.
So where does the water come from? Two main theories have been suggested.
Water could be delivered to the moon by the impacts of meteorites and comets, which can contain large amounts of ice.
Another theory is based on the fact hydrogen atoms reach the moon as a result of the solar wind – the continuous stream of particles ejected from the sun. The theory goes that hydrogen atoms then react with oxygen in the surface minerals to form water and hydroxyl
The new study, published by Yang Liu from the University of Tennessee and colleagues, seeks to distinguish between these two theories by looking at the isotopic composition of the hydrogen.
Analysis of the Apollo moon samples, which began in the 1970s, previously had uncovered the presence of hydrogen inside volcanically produced glass beads in the soil. In 2008, scientists found hydrogen in a phosphate mineral in lunar rocks, and last year found it again inside another mineral, olivine.
Three robotic probes, including NASA's LCROSS experiment, also have found evidence for water ice on the moon. But where the water came from has been a mystery.
Using two new techniques to dig down into chemistry of hydrogen inside lunar soil grains, Liu and colleagues determined that most of it came from the solar wind, a steady stream of charged particles from the sun that permeates and defines the boundaries of the solar system.
The solar wind streams off of the Sun in all directions at speeds of about 400 km/s (about 1 million miles per hour). The source of the solar wind is the Sun's hot corona. The temperature of the corona is so high that the Sun's gravity cannot hold on to it. Although we understand why this happens we do not understand the details about how and where the coronal gases are accelerated to these high velocities. This question is related to the question of coronal heating.