So, seeing as I put myself firmly on the soap box a few posts ago, I should practise what I preach and tell you about my latest astrobiological research. You might be familiar with the idea of biomarkers – the concept that inhabited planets look fundamentally different from uninhabited planets. For example, significant amounts of oxygen in a planet’s atmosphere is a pretty good (if not entirely conclusive) sign that something is continually producing it; otherwise, the oxygen would disappear pretty quickly. Anyway, this is an indication that something interesting is going on there, and we should continue exploring. Biomarkers tell us that there’s a good chance the planet is inhabited by life – noomarkers tell us that a planet or planetary system has intelligent life. What my collaborator Martin Elvis and I were interested in was whether asteroid mining could provide us with a noomarker.
Let’s take a step back for a moment, and think about the asteroids themselves. We know for a fact that many rocky bodies exist in our own Solar System. We classify them in families – the Kuiper-Edgeworth Belt, the asteroid belt between Mars and Jupiter, the Oort Cloud, and so on – and we have a reasonable idea of what they’re made of. We also know that we are not the only planetary system to have such rocky bodies – we have detected debris discs around other stars, which are generally rings of rocky bodies which (we are reasonably certain) have similar properties to our own rings of rubble. These debris discs are the leftovers of planet formation. After the star has been born with a circumstellar disc, the gas and dust within the disc coalesces into planets (we’re still figuring out some of the kinks as to exactly how this happens, but we’ve got a pretty good idea), and some dust and gas is lost from the system by stellar winds and ionisation fronts. The end result is a star, some planets, and “the little planets that couldn’t” – these are the asteroids and comets we see today.
Because they’re made from the same feedstock as the planets, they have quite similar chemical compositions (roughly). We can argue that in fact there will come a time when this similarity is incredibly useful. If we run out of raw materials for the next generation of technologies (like hydrogen fuel cells and C02 scrubbers, which require precious metals such as platinum) or just for some very big construction projects (like a space elevator), we’re going to find the Earth’s resources pretty stretched (just as we would on any other planet). The vast amounts of material in the asteroids might be an inevitable step in our evolution towards a spacefaring species – if we can’t solve the problems associated with targeted asteroid mining (TAM), then how else are we to explore the stars?
Let’s argue that this evolutionary step is a common one in the development of extraterrestrial civilisations. If that’s the case, then can we see them asteroid mining? After all, mining’s a pretty destructive process – you have to drill, collect the good stuff and take it home, and find somewhere to dispose the waste. If you look at the Earth’s surface, you know we’ve been busy in this regard! The key word here is disequilibrium. Just as life pushes the Earth’s atmosphere out of physico-chemical equilibrium, creating biomarkers, asteroid mining is going to push the debris disc system out of equilibrium, creating noomarkers.
We looked at three types of disequilibrium – chemical disequilibrium (or, “Where’s the Platinum gone?”), mechanical disequilibrium (or, “What’s with all the mess?”), and thermal disequilibrium (or, “Did you see that glowing blob?”).
We would expect that certain chemicals will be preferred for mining over others – silicates are very common on terrestrial planets, so that wouldn’t be particularly desirable, but differentiated elements (i.e. elements that sink to the centre of rocks when they’re melted, like iron and nickel) and rare elements (like platinum) are more likely to be mined.
But, because they’re differentiated, they’ll be even harder to spot. The only we can see them is by taking a spectrum of the system, and looking for spectral lines. We’ve managed to (just) see absorption lines of iron and nickel in recent observations, but these signatures are degenerate (i.e. there’s more than one way to make these signatures, and we don’t know which is the real one). We’ve got to model these systems very carefully to make sure we understand them, and more often than not, any single TAM signature can be explained by natural causes, like comets falling towards their parent star and evaporating.
This sad trend continues with the other two types of disequilibrium. Mechanical disequilibrium from TAM basically says that we will find that the size distribution of objects (i.e. how many objects are big versus how many are small) won’t make sense. We might find more small objects than we expect, for example. Again, what we found was that while unusual size distributions are just that, they can be explained by natural causes, like recent high velocity impacts between large bodies in the debris disc system.
Thermal disequilibrium is a similar problem. We might see dust production in an unusual part of the disc (with an associated increase in temperature). This temperature increase might also produce tektites (which we find on Earth as a result of meteorite impacts), which are minerals that are strongly heated until they become glassy. Again, you can make tektites by smashing two bodies together very hard, so there is another natural explanation. The more subtle thermal signatures (mining starting and stopping producing weird fluctuations) are far outside our current ability to detect them (for one thing, we simply don’t look at debris discs often enough or long enough to see these fluctuations).
This all sounds depressing, I know, but there is a silver lining. Just like biomarkers, noomarkers usually have a natural explanation. But get enough of them together in one system, and you’ll raise eyebrows among SETI researchers. The more of these signatures occurring together in one system, the less likely that a natural explanation can cover them all. Again, this isn’t conclusive proof that ET is mucking around in the asteroids, but it is a way to flag the system for further investigation. We can then look at the planets in the system for signs of life, and search for radio signals or other communication coming from that part of space.
And the best part is, we can get this flag for free. Astronomers study debris discs for their own sake, and they don’t plan to stop. The current stream of data about these systems can only increase in size and quality. SETI researchers can then look for the signatures that we have described in our paper, and who knows? We might see ET prospecting in the vacuum, trying to spark off the next interplanetary Gold Rush.