Among the meteorites that survive entry to Earth, a subset carries water-bearing minerals. Known as hydrated minerals, they are chock-full of water and hydroxide (OH) molecules: about 10% water by mass. Planetary scientists love hydrated minerals because they provide a look at the physical and chemical conditions of the early solar system. They also demonstrate how asteroid impacts can deliver water to young terrestrial planets.
But curiosity about hydrated minerals extends beyond the planetary science community: Cosmic prospectors are keenly interested in the water-laden crystals found on near-Earth objects still floating in space. The commercial space industry has boomed over the past decade, and many private companies—and even some countries—have begun researching how to mine asteroids. Hydrated minerals can be used to refuel communications satellites and, when harvested from asteroids, can be delivered with significantly less energy than shipments launched from Earth. Before mining can commence, however, we must first understand how many hydrated asteroids exist in near-Earth space.
In a new research study, Rivkin and DeMeo provide an initial estimate of how many hydrated asteroids exist in near-Earth space and how much water they carry. They modeled their approach on the famous Drake equation, used to estimate the expected number of civilizations in the galaxy. In addition to serving as a guide for space exploration, the estimates also provide insight into a niggling scientific question: How did lunar ice arrive in the first place?
The authors focused on carbon-rich (C-type) asteroids in near-Earth space. C-type asteroids are one of several known asteroid types and a variety in which scientists can confidently identify hydrated minerals with remote sensing. In C-type asteroids, the fingerprints of hydrated minerals are seen at wavelengths in the 3- and 0.7-micrometer ranges.
The study found that 53 ± 27 asteroids larger than 1 kilometer in diameter are expected in near-Earth space. Of those objects, 17 ± 9 are expected to be more accessible than a round-trip journey to the Moon (they calculated asteroid accessibility by comparing Δv values, which are a measure of the energy required for a spacecraft to change orbits). Removing size limitations, the authors estimated 700 ± 350 asteroids in near-Earth space. Assuming each asteroid contains 7% water by mass, they found 800 billion kilograms (±50%) of water more accessible on asteroids than the ice on the Moon. For comparison, that is a fraction of what the contiguous United States receives in precipitation every day.
Using these estimates, the authors concluded that there is likely less water on the Moon delivered by asteroids than previously thought. To fill the Moon’s polar cold traps, they calculated there would need to have been roughly 60,000 lunar asteroid impacts, which would take longer to occur than the age of the solar system. Therefore, the water in those cold traps must have a different source.
The authors acknowledged that several of their research assumptions—using observed numbers of asteroids instead of theoretical values, for example—may have resulted in conservative estimates. Their approach and findings, however, represent a significant first look at near-Earth space water availability, which is of interest to both space science and commerce. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2018JE005584, 2019)
—Aaron Sidder, Freelance Writer