What do you think of when you hear "meteor"?
Perhaps a romantic evening watching shooting stars flash across the sky? More likely, a threat to all life on Earth and the death knell of the dinosaurs.
But while massive space rocks hurtling toward Earth do have frightening destructive potential, they’re also a force for creation.
Rocks floating around in space are what formed the Earth in the first place — and they may even hold the keys to how life on Earth began.
How did space rocks form?
Asteroids, meteors, and meteorites are three different names for the same thing.
A rock floating in space is an asteroid. While it is zooming through our atmosphere it is called a meteor. Once it hits the ground, we call it a meteorite.
They’re the debris left over from the formation of solar systems.
About 5 billion years ago, our solar system was just a huge cloud of dust and gas.
As it collapsed under its own weight the the sun ignited in the centre, and a disc formed around it — like the rings of Saturn, just way, way bigger.
That disc is where the planets formed. Dust particles stuck together, crashed into each other to form bigger clumps, which collided some more… until the clumps became so large that their gravity started pulling even more stuff in, which eventually formed the planets we know and love.
The little tiny bit of stuff left over makes up the asteroids, most of the comets, and all the other shrapnel left over from the violent beginnings of our solar system.
While there aren’t as many asteroids as there used to be, some are still entering our atmosphere as meteors.
If they make it to the ground without disintegrating, we call them meteorites — and studying them closely can tell us lots about the history of our planet.
How do you recognise a space rock?
Hold on, hold on, I hear you say… if asteroids are made from the same stuff as the Earth, how do we know which rocks on the ground have come from space?
It’s a great question. Meteorite hunters identify meteorites by the fact that they look different to the rocks on the surface of the Earth.
But if meteorites and the Earth were made from the same stuff, how can they be so different? The answer is actually really cool — or hot, as the case may be.
When the Earth first formed it was a ball of molten rock, so hot that all the rock was in liquid form. And just like in liquid water, in liquid rock, heavy stuff sinks.
That means that the heavy minerals, things like iron and nickel, sank to the core of the Earth, while lighter stuff like silicon and magnesium floated to the top.
We think that over 30 per cent of the Earth’s mass is in iron, but only 5 per cent is near the surface.
When asteroids land on Earth they still have the primordial composition – the stuff the Earth started out with before the heavy stuff sunk – so they are very different to normal Earth rocks.
Since meteorites often have way more iron, one way to identify them is to just pick them up with a magnet.
What does all this have to do with the origin of life on Earth?
Some space rock is a bit different, including one type of asteroid called a “carbonaceous chondrite”.
As the name implies, it’s made up of lots of carbon – the same stuff that makes up most organic compounds, including the backbone of our DNA.
Turns out that in the process of making the solar system, not all asteroids were created equal.
These carbonaceous chondrites are some of the most pristine asteroids, which formed early on, haven’t been heated much, and are closest in chemical composition to the Sun itself.
One of the best examples is the Murchison meteorite, a rock that landed in country Victoria in the late 1960s.
The Murchison meteorite is chock-a-block full of amino acids, the building blocks of life.
Intriguingly, this means that meteorites quite possibly had an even more important role in our existence than just building the planet we stand on.
They may have also delivered the molecules that would go on to form life.
Life on Earth is made up of a few basic elements. Critical to our DNA are the elements collectively known as “CHNOPS”: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur. Together they can make strong bonds and complex molecules needed for life.
Once the Earth had cooled, and was developing a solid crust, more asteroids and comets would have continued to rain down on the fledgling planet.
Some of these would have brought water. Others, similar to the Murchison meteorite, brought pre-built organic molecules.
Experiments have shown that the molecules delivered in carbonaceous chondrites, when heated in hot springs, can form primitive cells.
Like dropping a stock cube in your boiling soup, dropping a meteorite in a hot spring may have delivered an intense burst of “flavour” … the ingredients needed to evolve into life, and eventually into us.
So did life actually come from space?
Does this mean we are all in some sense “aliens”?
That remains an open question.
There’s a hypothesis called “panspermia” that suggests that life is spread all over the universe, floating throughout space in dust and asteroids, ready to land anywhere a suitable environment forms.
Meteorite evidence shows that certainly the building blocks of life — amino acids — form in space. Whether more complex systems we’d recognise as living organisms also form in space is still a point of speculation.
The fact that meteorites likely did bring organic molecules to Earth may explain why life formed so rapidly here.
Fossil and chemical evidence shows that basically as soon as Earth had a solid surface, life emerged.
It makes one wonder anew how much life there may be on other planets. Have we not seen life on other planets because they didn’t have environments in which life could thrive, or have we just not looked hard enough yet?