Why go to the asteroids when the asteroids can come to us? (Javier Zayas Photography/Moment/Getty Images)
The next frontier for precious minerals could be much closer to home than the asteroid belt out past the orbit of Mars.
A new paper proposes that the minerals borne by these asteroids may have come to us, and are now scattered across thousands of impact craters on the Moon. There, up to 6,500 impact craters may contain precious platinum-group metals, such as platinum, palladium, and rhodium.
Furthermore, some 3,400 impact craters may contain water in the form of hydrated minerals – a crucial molecule for any human exploration of our only long-term natural satellite.
"These values are one to two orders of magnitude larger than the number of ore-bearing near-Earth asteroids estimated by Elvis (2014)," writes a team led by independent astronomer Jayanth Chennamangalam, "implying that it may be more advantageous, and hence more profitable, to mine asteroids that have impacted the Moon rather than the ones that are in orbit."
The 226-kilometer (140-mile) wide asteroid Psyche, which lies in the main asteroid belt between Mars and Jupiter. It's thought to be hugely rich in metal. (Peter Rubin/NASA/JPL-Caltech/ASU)
There are numerous potential benefits to mining in space. Earth's resources are finite, for one: There's only so much accessible metal in our planet's crust. Mining is also often quite destructive, damaging land and habitats, and generating tremendous amounts of pollution.
In addition, mining in space has benefits for space exploration; having access to water on the Moon, for example, would help mitigate the challenges of either transporting water from Earth, or recycling it in situ.
Many asteroids are believed to be rich in precious metals, specifically the platinum group metals, which possess properties that make them highly prized for industrial and medical applications, but are rare. But asteroid mining presents its own challenges, even aside from the astronomical (cough) cost.
Near-Earth asteroids are pretty scarce. They're also very complicated to reach, and tumble in space in ways that make them difficult to navigate. The Moon, by contrast, just hangs out in Earth orbit at a predictable distance, with the same side facing us always. That doesn't mean the Moon is easy; but it's a darn sight easier than an asteroid.
The big question that presents itself is, then: Does the Moon have the platinum group metals and water that we need? Chennamangalam and his colleague embarked on a survey of asteroid impact craters to find out.
Certain metallic-type asteroids are thought to be rich in metals, including the platinum group. Meanwhile, carbonaceous asteroids are thought to be rich in hydrated minerals.
When an asteroid smacks into the Moon, some of it vaporizes, but recent evidence suggests that significant portions of the asteroid body can survive under certain conditions. For particularly large, complex craters, the debris is swept into the center of the crater, where it forms a central peak.
By counting the large craters that could be the result of an impact by these asteroids, the researchers estimate that there could be quite a lot of valuable metal lurking on the Moon.
Even factoring in that much of an asteroid may not survive an impact, especially for hydrated bodies that lose water to heat, the statistical estimates still ran into the thousands of craters.
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Up to 6,500 craters larger than 1 kilometer (0.62 miles) across could contain platinum group metals, although the ore might be finely dispersed through the lunar regolith and difficult to access. If the search is narrowed down to craters larger than 19 kilometers across with a well-defined central peak where the ore is likely concentrated, the number drops to 38.
Meanwhile, up to 3,350 craters larger than 1 kilometer could have water, narrowed down to 20 larger than 19 kilometers with concentrated deposits. This means the Moon represents a statistically richer hunting ground for platinum-group metals than near-Earth asteroids.
It may still be that asteroid mining is in humanity's future. However, the Moon appears to represent a more achievable and practical stepping stone towards that goal. The next step is figuring out exactly which craters hide the treasure – and how to mine them.
The researchers suggest that remote sensing from lunar orbit, rather than costly and not-guaranteed-to-succeed landers, is the best option for identifying which targets are most worth pursuing.
The research has been published in Planetary and Space Science.