The following is an extract from our monthly Launchpad newsletter, in which resident space expert Leah Crane journeys through the solar system and beyond. You can sign up for Launchpad for free here.
It seems like once every year or two, I have a couple weeks where it seems like all I’m writing about is rocks in space. Asteroids, comets (which are just wet asteroids), dwarf planets (big asteroids), moons (trapped asteroids), you get the gist. And with the return of samples from the asteroid Bennu last week, plus the launch of a mission to the asteroid Psyche mission this week, it looks like it’s that time of year. Even though asteroids might seem like just a bunch of rocks, it turns out that they’re amazingly diverse – and they’re really important to understanding our place in the cosmos.
In the beginning of the solar system, there was a cloud of dust and rocks. That cloud was clumpy, so gravity did its work and drew more and more material to the clumps, creating a star with a disc of material around it. A similar process turned clumpier areas into the planets we know now, but not every clump became a planet. What was left over mostly got shepherded by the planets into belts around the sun – the asteroid belt, the Kuiper belt and the less-structured Oort cloud.
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And then, for billions of years, much of what went into those belts just… hung around. The asteroid belt is nothing like what you might picture from sci-fi films – each rock is separated from its neighbours by hundreds of thousands of kilometres. So the rocks left over from planet formation rarely got hit by anything bigger than a proton. They’re the crumbs left over from the birth of the solar system, and they’ve been in deep freeze since then.
At this point I’m going to steal a metaphor that I heard from Lindy Elkins-Tanton, the principal investigator of the Psyche mission. She told me to imagine the solar system as a cake. It’s been painstakingly put together over a long time with a very specific mix of ingredients. Now that the cake is baked, it’s nearly impossible to reverse-engineer those ingredients from the completed treat – you can tell from taking a bite of cake that it’s got sugar and flour and eggs in it, but not the exact proportions, or the exact type of sugar, or the colour of the eggs or the order in which they were all combined. In the same way, we pretty much know what the planets are made of and what they’re like now, but the exact details of their formation remain hazy.
Studying asteroids is like having a look at the area where you baked before you wipe it down. Is there flour scattered everywhere? (There always is.) Is there a half-used carton of eggs? A measuring spoon with vanilla left on it? Each asteroid can represent one of these things.
Ingredients for life
Rocky asteroids such as Bennu are important to study because they may have provided Earth with some of the most crucial ingredients for life: water and organic compounds. We don’t know exactly how or when water came to Earth. The planet may have been born with some, but the violent collision that created the moon and the subsequent molten state of Earth’s surface would have probably caused any such water to evaporate. If that is the case, our now-ubiquitous water would have been delivered later on by asteroids and comets.
There’s already some evidence that rocky asteroids carry water, but whether it’s enough to account for the abundance on Earth is still up in the air. We also know that these space rocks carry the types of organic compounds that life on Earth needs, but whether the spark for life came from an asteroid is still an open question – one that could potentially be solved by a combination of laboratory experiments and sample return missions like OSIRIS-REx, which recently brought us those samples from Bennu that I mentioned.
Rocky asteroids aren’t the only type out there, though – there are also metallic asteroids such as Psyche. These are much rarer, so we’ve never seen one up close before. But that could change soon with the Psyche mission. Psyche may not be a leftover from the planetary formation process in the same way that other asteroids are – instead, it might be a failed planet, which formed its iron core and then had its outer layers somehow stripped away. If that’s the case, asteroids like this might be the only way for us to see a planetary core up close, which is somewhere between difficult and impossible to do when that core is still in the middle of its planet.
“We are so attached as a species to rocky planets, but the one missing ingredient that we have never been able to inspect up close is the metal core,” Elkins-Tanton told me. “It’s like we’re making cakes and we’ve never met an egg before. Now we’re going to go meet an egg.” And if we meet enough asteroids, maybe we’ll be able to piece together how Earth formed and where it got each of its ingredients.
