The revealing key to the manufacture of meteorites at the birth of the solar system

April 26, 1803 was an unusual day in the small town of L'Aigle in Normandy, France: there was rain of rocks.

More than 3000 of them fell from the sky. Fortunately, no one was injured. The French Academy of Sciences investigated and proclaimed, from many eyewitness stories and the unusual view of the rocks, that they came from space.

The Earth is constantly bombarded with rocks and gravitates around the Sun, adding about 50 tons to the mass of our planet each day. Meteorites, as these rocks are called, are easy to find in the deserts and on the ice plains of Antarctica, where they emerge as a sore thumb. They can even land in backyards, treasures hidden among ordinary land rocks. Fans and professionals collect meteorites, while the most interesting go to museums and laboratories around the world for exhibition and study. They are also bought and sold on eBay.

Despite decades of intense studies by thousands of scientists, there is no general consensus on the formation of most meteorites. As an astronomer and geologist, we have recently developed a new theory of what happened during the formation of the solar system to create these precious vestiges of our past. As planets form from the collisions of these early rocks, this is an important part of the Earth's history.

The mysterious chondrules

About 10% of meteorites are pure iron. These are formed by a multistep process in which a large fused asteroid has sufficient gravity for the iron to flow at its center. This builds an iron core like that of the Earth. Once this asteroid solidified, it can be broken into meteorites by collisions with other objects. Ferrous meteorites are as old as the solar system itself, proving that large asteroids are formed rapidly and that those that are completely melted are once abundant.

The remaining 90% of meteorites are called "chondrites" because they are filled with tiny spheres of rock known as "chondrules". No terrestrial rock has a chondrule inside. It is clear that chondrules formed in space during a brief period of intense warming when temperatures reached the melting point of the rock, about 3000 degrees Fahrenheit, for less than an hour. What could possibly explain that?

Researchers have made many assumptions over the last 40 years. But no consensus has been reached on how this brief flash of heating occurred.

The complex problem is so difficult and controversial that when we announced to colleagues a few years ago that we were working on it, their reaction was to smile, to shake their heads and to offer their condolences. Now that we have come up with a solution, we are preparing for a more critical response, which is good, because that is how science progresses.

The flyby model

Our idea is quite simple. A radioactive dating of hundreds of chondrules shows that they formed between 1.8 and 4 million years after the beginning of the solar system – 4.6 billion years ago. Meanwhile, the completely melted asteroids, the parent bodies of ferrous meteorites, were abundant. Volcanic eruptions on these asteroids have released enormous amounts of heat into the space around them. Any smaller object that would pass during an eruption would suffer a short and intense heat of heat.

To test our hypothesis, we divided the challenge. The astronomer Herbst analyzed the numbers to determine how much heating was needed and how long to create chondrules. Then geologist Greenwood used an oven in our Wesleyan lab to recreate the conditions and see if we could make our own chondrules.

The experiments proved to be quite successful.

We put in a small capsule fine dust from terrestrial rocks with compositions resembling dust of space. We placed them in our oven and varied the temperature in the planned range. A beautiful synthetic chondrule is out. Case closed? Not so fast.

Two problems appeared with our model. In the first place, we ignored the biggest question of how chondrules became part of the meteorite. What is their relationship to chondrule stuff – called matrix? In addition, our model seemed a little too risky. Only a small part of the primitive matter will be heated as we proposed. Would it be enough to account for all these meteorites filled with chondrules hitting the Earth?

Make whole meteorites

To solve these problems, we expanded our initial model to take into account the overflight heating of a larger object, up to several kilometers long. As this material gets closer to a hot asteroid, some of its parts vaporize like a comet, creating an atmosphere rich in oxygen and other volatile elements. This proves to be the kind of atmosphere in which chondrules are formed, based on previous detailed chemical studies.

We also expect the pressure of heat and gas to harden the boiling object into a complete meteorite through a process called hot isostatic compression, used commercially to make metal alloys. When the chondrules merge into small spheres, they will release gas into the matrix, trapping these elements as the meteorite hardens. If the chondrules and chondrites are formed together in this way, we expect the matrix to be improved exactly with the same elements that the chondrules are exhausted. This phenomenon, called complementarity, has been observed for decades and our model provides a plausible explanation.

Perhaps the most innovative feature of our model is that it directly associates chondrule formation with meteorite hardening. Since only well-hardened space objects can cross the earth's atmosphere, we expect the meteorites in our museums to be full of chondrules, as they are. But hardened meteorites, full of chondrules, would be the exception, not the rule, in space, since they form in a relatively random process – hot flyover. We should know soon enough if this idea has water, because it predicts that chonders will be rare on asteroids. Japan and the United States both have on-going missions on nearby asteroids that will return samples in the next few years.

If these asteroids abound with chondrules, like the hardened meteorites that reach the Earth's surface, our model can be thrown away and the search for a solution to the problem of chondrules can continue. If, on the contrary, chondrules are rare on asteroids, the flyby model will have passed an important test.