As I recently reported, a Canadian homeowner recently captured what is to believed to be the first audio and video of a meteorite impact. Well, it may seem like a stupid question, but what is a meteorite? In fact, as I learned as a young space nerd, it’s kind of complicated.
For starters, the fireball you see streaking across the sky is not a meteorite. It’s a meteor. It only becomes a meteorite if it survives re-entry and impacts on the ground.
But for millions of years before you chanced to see its burning entry into Earth’s atmosphere, it was hurtling through space as a meteoroid.
So: meteoroid, meteorite, meteoroid. The three stages of life of that little bit of space rock. What sort of rock? It gets more complicated.
They are commonly designated as three types: stony meteorites, iron meteorites, and stony-iron meteorites.
Stony meteorites come in two types.
The most common are the chondrites, which have round objects inside that appear to have formed as melt droplets. These comprise 85 per cent of all meteorites found on Earth.
Scientists aren’t finished.
Most are known as “ordinary chondrites”. They are then divided into three broad classes – H, L and LL – based on the iron content of the meteorites and the distribution of iron and magnesium in the major minerals olivine and pyroxene. These silicate minerals are the mineral building blocks of our Solar System and are common on Earth, being present in basalt.
“Carbonaceous chondrites” are a distinct group. They contain high amounts of water in clay minerals, and organic materials such as amino acids. Chondrites have never been melted and are direct samples of the dust that originally formed the solar system.
The less common of the two types of stony meteorites are the so-called “achondrites”. These do not have the distinctive round particles of chondrites, because they experienced melting on planetary bodies.
Confused, yet? Well, let’s look at where they come from. Surprisingly, at it happens, most from just a few places in the Solar System.
Surprisingly, because there are a lot of meteors and meteorites. Roughly 17,000 impact the Earth every year, an average of nearly 50 a day.
Scientists know that while some of these meteorites come from the Moon and Mars, the majority come from asteroids. But two separate studies published in Nature today have gone a step further. The research was led by Miroslav Brož from Charles University in the Czech Republic, and Michaël Marsset from the European Southern Observatory in Chile.
The papers trace the origin of most meteorites to just a handful of asteroid breakup events – and possibly even individual asteroids.
Asteroids, as you surely know, mostly orbit in the Asteroid Belt, between Mars and Jupiter.
The asteroid belt itself consists of millions of asteroids swept around and marshalled by the gravitational force of Jupiter.
The interactions with Jupiter can perturb asteroid orbits and cause collisions. This results in debris, which can aggregate into rubble pile asteroids.
These ‘gravelly’ asteroids are the type recent robotic probes, such as Hayabusa and OSIRIS-REx, landed on and eventually returned samples to Earth.
These missions established the connection between distinct asteroid types and the meteorites that fall to Earth.
S-class asteroids (akin to stony meteorites) are found on the inner regions of the belt, while C-class carbonaceous asteroids (akin to carbonaceous chondrites) are more commonly found in the outer regions of the belt.
The common, ordinary chondrite meteorites originate from collisions between asteroids larger than 30 kilometres in diameter, less than 30 million years ago.
The Koronis and Massalia asteroid families provide appropriate body sizes and are in a position that leads to material falling to Earth, based on detailed computer modelling. Of these families, asteroids Koronis and Karin are likely the dominant sources of H chondrites. Massalia (L) and Flora (LL) families are by far the main sources of L- and LL-like meteorites.
The L chondrite meteorites originated from the Massalia family of asteroids. Computer modelling suggest that an asteroid collision roughly 470 million years ago formed the Massalia family. As it happens, this time period on Earth, the Ordovician, yields abundant fossil meteorites.
So, a collision deep in the Asteroid Belt resulted, just a few million years later (a tick of the clock in Deep Time) in a shower of meteorites on Earth.
Which is something to ponder, as you watch the next falling star streak across the sky.
