Most of the meteorites that land on Earth today can be traced back to just three recent collisions out in the asteroid belt, according to a pair of recent studies.
It’s hard to trace meteorites back to exactly where they came from — and for centuries, that’s been one of the most confusing questions we’ve asked about how our world relates to the sky above us. Only about 6 percent of meteorites ever found have been traced back to where they came from. But by studying the chemical makeup of meteorites on Earth and asteroids in space, and by simulating dramatic collisions between those asteroids, a team of researchers recently found that the most common types of meteorites landing on our planet today are debris from three cosmic collisions in the asteroid belt.
Astronomers Michael Marsset of the European Southern Observatory, Miroslav Brož of the University of Helsinki, and their colleagues published their work in two papers in the journal Nature.
A Cosmic Crime Scene
Marsset, Brož, and their colleagues used infrared telescopes to measure the chemical composition of several groups, or families, of asteroids orbiting between Mars and Jupiter. They compared those distant asteroids’ chemical fingerprints to the makeup of rocky meteorites found here on Earth, and found three likely culprits: families of asteroids that suffered collisions in the last few million years, producing clouds of small rocky debris.
The researchers also measured the ages of rocky meteorites, based on the ratio of different isotopes of the element argon (which can reveal the last time part of the rock was heated to its melting point and then cooled, during something like a volcanic eruption or a collision between two fast-moving behemoth asteroids). Those ages matched astronomers’ estimates of how old three particular asteroid families are, based on simulations of how the orbits probably evolved over time.
In the end, Brož and his colleagues say most of the rocky meteorites, called chondrites, that land on Earth today are debris from three specific collisions between asteroids out in the asteroid belt, between Mars and Jupiter. One happened 40 million years ago, and its remnants are a large family of asteroids called Massalia. The other two are much more recent, just 7.6 million and 5.8 million years ago, and they trace back to the formation of a young asteroid family called Koronis — and an even younger family that’s part of Koronis, called Karin.
When asteroids collide out in the asteroid belt, the resulting chunks tend to end up on roughly the same orbital path around the Sun: They move at around the same speed, tilted at the same angle relative to the Sun’s equator, and they stay at about the same distance from the Sun. We call these groups of survivors asteroid families. And they can be pretty dramatic, just like real families: Often, rocks within those families later have their own mishaps, spawning little families of their own inside the main one, like Karin and Koronis.
But the smaller bits of debris from those collisions can have even more eventful lives; they tend to drift out of their orbit thanks to an effect called Yarkovsky drift, which happens because sunlight heats their surfaces unevenly, releasing little bits of gas that push them onto new paths. They’re also prone to getting nudged onto different orbits by the gravity of passing planets. Over time, it’s easy for these small, wayward bits of collision debris to wind up on orbits that cross Earth’s path — and eventually, they end up being found by meteorite hunters in a field somewhere.
Messy Family Dynamics... in Space
Massalia, for example, is a family of asteroids that orbits about 2.41 times farther than Earth from the Sun. Based on simulations of its orbit, Massalia formed in a catastrophic collision about 470 million years ago, which broke a 125-mile-wide asteroid apart into smaller chunks. And it’s been a source of drama ever since.
The family’s largest member is about 87 miles wide, and it’s rocky but also rich in iron. That makes it very similar to a type of meteorite called a type H chondrite. But according to Marsset and his colleagues’ observations, most of the smaller Massalia asteroids have a slightly different makeup, one with much less iron — more like meteorites known as type L chondrites.
What’s interesting about that is that a flurry of L chondrites rained down on Earth 466 million years ago. The rain of meteorites was so intense that the dust they kicked up actually caused an ice age. Marsset and his colleagues dated the argon isotope ratios in tiny bits of meteorite found in rock layers from the time. Then they compared those ages to the age of the Massalia family and found a match.
As if the Massalia family hadn’t been through enough, two more of its members, both around 19 miles wide, smacked into each other around 40 million years ago, creating a fresh supply of rocky debris, which accounts for a lot of the rocky meteorites that land on Earth today, according to Brož and his colleagues’ work. The Massalia family, clearly, is the gift that keeps on giving (space rocks).
The rest of the rocky meteorites that occasionally bump into our planet today mostly come from two even more recent smashups in space — one 7.6 million years ago and one 5.8 million years ago. Massalia, Koronis, and Karin together account for about 70 percent of the meteorites that hit Earth today.
Within about 100 million years, the supplies of debris from those collisions will have run out. But the asteroid belt is a dynamic, busy place, so by then it’s likely that we’ll be tracing meteorites to new collisions. Over time, the meteorites that land on Earth will be able to tell the story of events happening 200 million miles away in space.
