Water and the ingredients of life: did space dust deliver them?
Every year, micrometeorites drop tonnes of carbon and water-bearing minerals onto the Earth. On the young Earth the rain of dust was far heavier, and it may have been one of the ways the raw materials for life arrived.
Many micrometeorites are like carbon-rich meteorites: they contain organic molecules and minerals that hold water. Because the dust falls gently, some of that carbon survives. Four billion years ago, when much more dust was falling, it could have added a steady supply of carbon compounds to the surface of the early Earth. Whether it mattered for the start of life is still an open question.
Key facts
- Carbon in dust
- Often a few per cent by mass in unmelted particles; up to about 80% in ultracarbonaceous ones
- Carbon delivered today
- Tens to hundreds of tonnes a year, depending on the estimate
- Early Earth
- Dust supply probably many times higher 4 billion years ago
- Open question
- How much of Earth's water and organics came this way
Organic matter that survives the fall
A large meteoroid hits the air so hard that its outside burns away, though its inside stays cold. A small grain is heated right through, but only briefly and, if it arrives slowly, not very hot. Unmelted micrometeorites therefore keep much of their carbon. Many contain a few per cent of carbon-rich material, and the rare ultracarbonaceous ones are mostly made of it.
Most of that carbon is in the form of large, complex, insoluble molecules, similar to the organic matter in carbonaceous chondrites. Small molecules have been found too: amino acids, the building blocks of proteins, have been reported from Antarctic micrometeorites, including at least one that is rare on Earth but common in carbon-rich meteorites.
Water locked in minerals
Many fine-grained micrometeorites come from bodies where water once flowed through rock, turning it into clays, carbonates and other water-bearing minerals. Some of those minerals lose part of their water during entry, but much survives. Spacecraft samples from the asteroids Ryugu and Bennu show the same kind of water-rich material.
The hydrogen isotopes in some water-bearing micrometeorites and in many carbonaceous chondrites are close to those of Earth's oceans. That is one line of evidence that water-rich asteroids, delivering their material as meteorites and dust, contributed to Earth's water. Comets on average show more deuterium than the oceans, though a few are close.
A heavier rain long ago
Four billion years ago, the Solar System still held far more leftover debris, and the dust supply was probably many times higher than today. Researchers who have modelled it suggest that micrometeorites may then have delivered more unaltered organic carbon to the Earth's surface each year than large impacts, because big impacts destroy most of their organic matter in the blast while dust drifts down gently.
Spread over hundreds of millions of years, that could have added a substantial amount of carbon compounds to the early oceans and land. Some scientists have also proposed that dust gathered by wind, water or melting ice into shallow pools could have concentrated both carbon and phosphorus, another element life needs, in the places where early chemistry might have happened.
What we still do not know
Delivery is only half of the story. Life on Earth could also have begun from carbon compounds made on the Earth itself, in hydrothermal vents, in lightning-charged air or in pools of water. Space dust may have added to that supply, but no one yet knows whether it was essential, helpful or irrelevant. What micrometeorites do show is that organic molecules and water-bearing minerals are common throughout the Solar System and arrive at every planet.
The same dust falls on Mars
Mars receives micrometeorites too, and its thin air lets more of them survive unmelted. Over billions of years, they must have added organic matter to the Martian soil. That matters for the search for life: rovers looking for organic molecules on Mars need to tell any traces of past Martian life apart from carbon delivered by space dust.
Did you know?
Most of the organic matter in carbon-rich meteorites and micrometeorites is a tangled, insoluble solid rather than individual molecules; it is often compared to the material in coal or tar.
The amino acid called AIB, rare in Earth's living things, has been detected in Antarctic micrometeorites. It is a sign that the molecules came from space rather than contamination.
Dust grains are so small that, in proportion to their weight, they have a vast surface on which chemistry can happen in the cold of space.