1. My understanding is, on Earth, all the heavy metals were drawn to the core while the planet was still molten. What we mine on the surface was deposited by asteroids and comets after the crust hardened. I assume the same thing happened on the Moon. Might as well get the good stuff from the source.
A. It's not necessarily the case that all of the Moon's heavy, siderophile elements sank to its core: the recent "Big Splat" theory
hypothesizes that a second moon formed contemporaneously at one of the Moon's Trojan points. There it would have been dynamically stable for up to 10's of millions of years:
Companion moons are a common outcome of simulations of Moon formation from a protolunar disk resulting from a giant impact, and although most coplanar configurations are unstable, a ~1,200-km-diameter moon located at one of the Trojan points could be dynamically stable for tens of millions of years after the giant impact. Most of the Moons magma ocean would solidify on this timescale, whereas the companion moon would evolve more quickly into a crust and a solid mantle derived from similar disk material, and would presumably have little or no core. Its likely fate would be to collide with the Moon at ~23 km s−1, well below the speed of sound in silicates. According to our simulations, a large moon/Moon size ratio (~0.3) and a subsonic impact velocity lead to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness consistent with the dimensions of the farside highlands.
So it is likely that the farside is enriched with heavy metals compared to the nearside sampled by the Apollo missions.
B. The Moon has concentration mechanisms that don't exist on asteroids. The phenomenon whereby volatiles are concentrated in the polar cold traps is well known. Thus, it was extremely surprising that the LCROSS results reported high concentrations of metals such as zinc, manganese, vanadium--and gold. (Silver peaks were also prominent.) Gladstone et al. reported an upper limit of Au of 0.2% by weight (2,000 g/t); this is an extremely--I repeat--extremely
high concentration of gold, and is very hard to believe, as a quick BOTE shows:
Apollo 12 (IIRC) samples of pristine rocks had average Au concentrations of around 5 ppb, whereas core samples of regolith showed an average of around 2 ppb. That leaves 3 ppb unaccounted for.
Now, when daylight/nighttime boundary (the "terminator") moves slowly around the moon, it causes a big electrical field with some high voltages, which causes the well known phenomenon of electrostatic dust movement. What is less appreciated is that this electrical field will then separate out any tiny particles of native metals. This process of electrostatic separation is well-known technique in the mining industry for separating out metals. Presumably, such particles would be accelerated to high speeds and travel ballistically until they landed somewhere, where they would be relaunched again and again, until they landed in one of the polar cold traps, where, since the Sun doesn't shine there, they would get trapped and concentrated.
So, the Moon's area is 3.8 x 1013
. If the average depth of regolith is 10 m, and if regolith density is 2,000 kg/m3
, then mass of gold liberated is 3.8 x 1014
* 2,000 kg/m3
* 3 x 10-9
= 2.3 x 109
kg Au. If the area of the cold traps is 3.7 x 109
, then 0.6 kg Au/m2
should have been deposited in the polar cold traps. If this amount of Au happened to be concentrated in the top two meters of regolith (about the depth that LCROSS sampled), that would be 0.6 kg Au / 4,000 kg regolith = 150 g/t (ppm).
150 g/t is huge
by Earthly standards, but given the number reported by Gladstone et al., one is tempted to wonder if some other mechanism is at work that could produce higher concentrations than 150 g/t. Thus, lately I've been thinking that the Big Splat could provide a source rock that would jack up the concentration. According to this geologist (Bernard Wood),
there is enough gold in the Earth's core to cover the surface of the Earth to a depth of 1.5 feet (about half a meter). If you crunch the numbers, this works out to an average concentration of Au over the entire Earth of 0.75 g/t. Since the hypothesized Trojan moon would have formed during the giant impact, and since it was presumably relatively undifferentiated, and since it formed a carapace on the Lunar farside, then in theory, pristine rocks found on the Lunar farside could have a similar Au concentration of 0.75 g/t.
Assuming a similar 60% of this in the farside regolith were liberated due to electrostatic separation, then 10 m * 1.9 x 1013
* 2,000 kg/m3
* 0.75 x 10-6
* 60% = 1.7 * 1011
kg Au. If the cold trap area is 3.7 x 109
, then 46 kg/m2
should have been deposited. Assuming the regolith in the cold traps is 10 m, and the Au uniformly distributed, then the predicted Au concentration would be (1.7 * 1011
kg Au) / (3.7 x 1010
* 2,000 kg/m) = ~2,300 g/t (ppm) which actually matches the reported LCROSS results of ~2,000 g/t (ppm) quite nicely. Compare this to the 80 g/t Pt found in a tiny minority of meteorites.
Thus, in other words, both theory and empirical data suggest that a single cubic yard of regolith from polar cold traps is worth on the order of $200,000 USD. Moreover, this is the gold
market we are talking about, which is worth more than PGM's on a per kilgram basis, and is about an order of magnitude larger in terms of annual production and sales, so that a potential market on the order of $100B/year for a Lunar gold mining op would be possible without depressing the price of gold too much.
Thus, in terms of concentration of ore and absolute market size, when it comes to precious metals, the Moon beats asteroids hands down! (IMO YMMV)
2. Much less gravity well to soak up profit.
As the LCROSS results showed, there is also water in the polar cold traps. I've ran the numbers, and enough water would be produced in the process of mining gold to send the gold all the way back to Earth fully propulsively.
Rooting for both to succeed however!