Gold is rare and valuable, but there’s more of it than there should be. Maybe it came from outer space?

Posted on September 20, 2013 by Madsen Pirie

One of the fun things about science is that in many, if not most, fields, there are competing theories, each with its school of adherents, each anxious to validate its view via experiments. Were the dinosaurs killed by volcanic gases, or was it a comet from outer space? Was Tyrannosaurus a predator or a carrion eater? The two schools of thought test each other and science advances. Now let’s look at where gold comes from. There’s about 1.3gm of it per 1,000 tonnes of other stuff, and that’s too much. When most of the iron at Earth’s formation collected at the molten core, most of the gold should have gone with it, but there’s more of it in the crust than there should be. The prevailing theory, the late veneer hypothesis, has it that bombardment from space by meteorites and asteroids gave us most of the gold available to us. Several experiments have backed this up, including examination of lunar rocks following the Apollo visits. Those same meteorites, or similar ones, might also have delivered the carbon, nitrogen and water that were essential to the development of life. Incidentally, isotope measurements suggest that 0.5% of the Earth’s crust is meteoric in origin – that’s about 20 billion billion tones.

Theory number two tries to deal with anomalies in the late veneer hypothesis. It suggests that the Earth’s gold was here all along. Most of it went to the core with the iron, “but a significant proportion – perhaps 0.2% – dissolved into a 700km deep magma “ocean” within the Earth’s outer mantle. Later, the gold was brought back up to the crust by volcanic action.” Under this theory gold has to be more soluble than previously thought in order to match the numbers. The numbers seem to match for the late veneer hypothesis, so it currently finds more favour among scientists.

A common figure for the total quantity of gold ever mined suggests that it would fill about one-third of the Washington monument. And it seems likely that all of it might have come from outer space. I’ll never look at a gold medal in quite the same light…