The following is an extract from our monthly Launchpad newsletter, which explores the solar system and beyond. You can sign up for Launchpad for free here.
There is no better place to start explaining what so fascinates me about meteorites than Winston Churchill’s fish pond. Churchill lived for many years at a grand old house called Chartwell, which happens to be close to my home. The property and its elegant grounds are now open to the public, so I often take my children there for a run around, and we always pause to throw a coin or two into the pond, a tradition countless visitors have carried on over the years.
What’s that got to do with meteorites? Well, the fish are trapped in the water, with only a dim perception of the wide world that lies beyond. Yet every so often, an artefact from that expanse above is cast into their world, which, if the fish could read it, would provide a clue to human civilisation. Often, when I stand by the pond with my boys, I think that we’re not so different from the fish. Confined to Earth, we can look out at space with telescopes, but we get an imperfect view. Yet occasionally we do get a messenger from space dropped down into our midst.
Advertisement
I first started thinking about this years ago and it led me to write my book The Meteorite Hunters. The first part of the book tells the incredible stories of the adventurers and scientists that hunt meteorites by scouring wild deserts, Antarctic ice sheets and even the rooftops of Oslo, Norway. The second part investigates how the secrets we’ve read from these extraterrestrial treasures are helping to write a new history of the solar system – and that is what I want to share here.
The thing you need to know about the solar system is that it’s a neat and tidy place: four small rocky planets close to the sun and four larger gas ones further away – including Jupiter, the largest. For a long time, astronomers assumed this was the natural, normal way of things and that the planets must have formed where they are now.  After all, the solar system began as a vast ring of dust and gas encircling the sun and there would have been a temperature gradient radiating through the ring that would have determined what kinds of materials could have existed as solids and glommed into planets. Ices of compounds like hydrogen and carbon dioxide, which constitute the gas giants, could only have survived in the colder outer reaches, far from the star.
But, starting in the mid-1990s, we began to observe other planetary systems in which vast gas planets orbited in close proximity to their star. Since no one thought those planets could form in such zones, this led to a radical idea: do the orbits of planets move?
To cut to the chase, most astronomers now think they probably do. There are two main theories about how this may have worked in our own solar system. One is known as the Nice model, after the French city where it was developed. The original version says that the four gas giants all formed close to each other before moving apart into their current orbits. The other, known as the grand tack model, says that very early in the history of the solar system, Jupiter moved inwards towards the sun before swinging back outwards (or “tacking”, to use the sailing term) to where it is now. These ideas were radical at the time, but they have merit because they explain some of the strange features of our cosmic neck of the woods. For instance, Mars is unusually small, with about 10 per cent the mass of Earth. Perhaps that’s because, as Jupiter swung inwards, its gravity sucked in all the dust and gas that would otherwise have fed the growth of the Red Planet.
The trouble is, beyond modelling these ideas in computer simulations, it’s hard to get any hard evidence that supports them. That’s where meteorites come in.
Discovering the great dichotomy
To understand what these extraterrestrial stones have to say about the history of the solar system, we first need to know something about isotopes. All atoms contain a certain number of subatomic particles called protons and neutrons in their nucleus. The number of protons determines which element you have: one for hydrogen, six for carbon and so on. But atoms of the same element can have slightly different numbers of neutrons, and these versions of an element are called isotopes. Various natural processes mean atoms gain or lose neutrons over time, which is why archaeologists use isotopes to date ancient bones or artefacts. When it comes to extremely ancient materials, like meteorites, their balance of isotopes can reveal the environments they have been exposed to through deep time.
In 2011, at the University of California, Los Angeles, looked at a raft of data on the many isotopes in a wide variety of meteorites. He noticed a pattern: it seemed that all the meteorites could be assigned to one of two groups based on the balance of their isotopes, the most diagnostic ones being titanium, chromium and oxygen. This was the first sniff of an important result.
His work flew under the radar for a while, but it did catch the attention of at the Max Planck Institute for Solar System Research in Germany. Kleine and his colleagues investigated further, measuring the abundances of other isotopes in meteorites. Looking specifically at molybdenum, he found the same pattern Warren had identified: meteorites sat in one of two groups based on their molybdenum isotopes. In practice, this means that if you measure the molybdenum isotopes in all known meteorites and plot the values on a graph, you get two lines. Any given meteorite will fit one or the other of these two lines. This time, the split was more obvious – and since molybdenum is present in just about all meteorites, it soon became clear that the whole lot followed the rule. These two groups became known as the non-carbonaceous (NC) meteorites (which are stony and contain little carbon or moisture) and carbonaceous (CC) meteorites (wet and brimming with carbon-based molecules). This division is now called “the great dichotomy”.
So what does it mean? Well, the existence of this dichotomy indicates that all meteorites originally formed from one of two separate reservoirs of dust and gas. And the composition of the meteorites in each group indicates that one reservoir was closer to the sun, the other further away. Think of these reservoirs as being like a solar system-sized jammy dodger (a bit like a Linzer cookie, for those who are unfamiliar): you have one reservoir (the jam) in the middle and a second (the biscuit) around the outer edge. What was it that kept those two regions of gas and dust from merging and mixing? “The only likely explanation we could come up with at the time was a large planet,” says , who previously worked with Kleine and is now based at the Lawrence Livermore National Laboratory in California. “And Jupiter seemed like the best candidate.”
If Jupiter kept those two reservoirs apart, it must have been around from very early on to stop the dust mixing. In 2017, Kruijer wrote a paper explicitly making this argument and using the great dichotomy to date the formation of Jupiter. According to his calculations, the planet must have grown to at least twenty times the mass of Earth – not its full size, but still hefty – within 1 million years of the birth of the solar system. It was the first time the formation of Jupiter had ever been empirically dated and it supported the grand tack model, with its vision of Jupiter forming and moving around in the first flush of the solar system’s youth.
It didn’t end there. The most profound consequence of the dichotomy, says Kruijer, is that it changed how we think about where asteroids – and hence meteorites – ultimately come from. Today, the majority of asteroids orbit in a belt between Mars and Jupiter. The dichotomy showed us that many of them could not have formed there, but instead must have accumulated further out into the colder reaches of the solar system. That means something must have pushed them inwards. And again, that fits perfectly with the theory of planetary migration. The gravitational turbulence of Jupiter and its friends moving around would be just the thing to send these two reservoirs of asteroids pinballing around the solar system. Today, the boulders in the asteroids between Mars and Jupiter contain material from both of the original reservoirs.
Meteorites, then, have provided some crucial evidence to support the idea of planetary migrations. But there is something that has always bothered me. It is hard to believe that asteroids from different reservoirs never smashed into each other in the chaos of the early solar system. But if even a small portion of them had, why did all meteorites fit exclusively into one or other of the two groups of the great dichotomy? If the separate reservoirs were the jam and biscuit of a jammy dodger, why had we never seen any meteorites that contained a mixture of both jam and biscuit?
Well, in fact, we have found one example. In 2021, at the Max Planck Institute for Solar System Research announced he had measured the molybdenum isotopes of a meteorite that fell in a place called Nedagolla in India in 1870 – and it didn’t fit on either the CC or the NC line. Spitzer remembers colleagues saying, “No, this can’t be right.” But it quickly became clear that this was a sensational result. The best explanation for the Nedagolla rock, Spitzer told me, is that it formed in a collision between two asteroids, one from the NC reservoir and the other from the CC reservoir. “Basically, Nedagolla is direct evidence of collision-generated mixing,” says Spitzer. Analysis of the meteorite suggests the impact occurred about 7 million years after the birth of the solar system, which broadly fits with the grand tack model.
Meteorites are not all extremely wonderful to behold. Though there are some specimens of incredible beauty, most of them (let’s face it) just look like old rocks. But one of the things I most enjoyed about writing my book is that I have learned to see past that. These extraterrestrial stones are like time capsules dropped into our pond, and – if we learn to read them – they can tell us some remarkable tales about the solar system.
Josh Howgego is deputy head of features at żěè¶ĚĘÓƵ. His book is out in the UK on 6 February.
