‘Asteroid forensics’ could reveal the origin of water

‘Asteroid forensics’ could reveal the origin of water

By studying water-rich meteorites on Earth, Museum scientist Helena Bates is working out where in the solar system the meteorites – and the water they contain – originated from.

From the crushing pressures at the bottom of the ocean to the super-salty lakes beneath Antarctica, or the boiling temperatures of Yellowstone’s hot springs – wherever we find water on Earth, we find life.

Without liquid water, life as we know it simply would not exist. Yet this water didn’t originate on our planet.

Scientists believe that almost all water on Earth was delivered from within asteroids that pelted the young planet after it formed. Now scientists are trying to find out more about these celestial bodies which may be the source of water.

Helena is conducting her PhD at the Museum, while also carrying out experiments at Oxford University, to investigate how these water-rich asteroids formed in space.

‘I’m kind of doing asteroid forensics,’ explains Helena. ‘I’m taking meteorites and then working out their history. I then try to unravel what has happened to them before they landed on Earth, including what they’re made of.’

Later this month, the spacecraft OSIRIS-REx will start its approach of the asteroid Bennu – the first NASA mission to sample an asteroid. It is hoped that the data collected can then be combined with the work Helena is carrying out.

This will not only tell us more about how the celestial bodies formed, but also the evolution of other terrestrial planets in the solar system and even the origins of life itself.

A smash up between two asteroids in the early solar system

As larger asteroids smashed into each other, they send debris flying. Some of this hit Earth, bringing water to the young planet © NASA/JPL-Caltech

The blue marble

With 70% of the surface of the planet covered in water, it is easy to see why we think of Earth – the blue planet – as a water world. But it hasn’t always been this way.

When Earth formed roughly 4.5 billion years ago, the environment was simply too hot for water to condense. This means that the water on our planet, along with other substances known as volatiles – such as carbon dioxide and carbon monoxide – came from elsewhere.

While Earth was taking shape deep within the hot, chaotic swirl of dust and gas as the Sun coalesced at the centre, other bodies were forming further out where the temperatures were cooler.

‘We have this hypothetical boundary called the snow line,’ explains Helena, ‘beyond which, if you go further away from the Sun, water vapour can condense into water ice. Because there is no pressure it’s not going through its liquid phase, but rather going straight to solid.’

This means that water ice was available to be accreted, or incorporated, in these more distant objects such as the gas giants and asteroids as they formed.

‘So we think that in order for water and volatiles to have come to Earth, they must have come from bodies that formed far out in the solar system, and then travelled inwards once the Sun reached a more stable state,’ says Helena.

By looking at the chemical signature of water, scientists have been able to figure out that almost all water on Earth must have been delivered here by early asteroid impacts.

‘It almost all came from asteroids,’ says Helena. ‘There is actually a lot less water on Earth compared to the total volume of the planet itself.’

The volume of water on Earth compared to the Earth itself

All the water on Earth compared to the volume of Earth itself © Howard Perlman/USGS

The right rocks

Broadly speaking there are three main types of meteorites and the asteroids they are thought to have come from. The most water-rich of these are known as carbonaceous chondrites, which can be up to 28% water. It is these meteorites that are of interest to scientists exploring how the wet stuff got to Earth.

The rock isn’t packed full of ice, rather the molecules that make up water (H2O and OH) are tacked on to the mineral’s structure. When the meteorites heat up, such as during an impact with our planet, they release this water as gas. The gas is then trapped by the planet’s atmosphere.

But figuring out which asteroids out in space are responsible for the carbonaceous chondrite meteorites that we have on Earth is not particularly easy.

Helena’s work involves taking meteorites that have survived their passage through Earth’s atmosphere and matching them up to the asteroids they likely came from.

She uses a spectrometer to shine a wide range of wavelengths of light onto the rocks, then measures what is reflected back.

‘When you shine a light on something, bonds in that material absorb some of that light,’ explains Helena. ‘This means that you see gaps when you look at the reflected light where certain bonds have absorbed a particular wavelength.’

By knowing what wavelength of light the bonds in water absorb, scientists can analyse the light reflected from asteroids and see whether there are gaps where those specific wavelengths have been absorbed, suggesting that they contain water.

chondrite-two-column

Carbonaceous chondrites can be as much as 28% water

‘I then look at the different wavelength ranges absorbed by the meteorites, and I compare it to space telescope data of asteroids in space, and hopefully eventually to spacecraft data,’ says Helena.

By tying what we know about the meteorite’s history to the data collected from the spectrometer, Helena can start to look for patterns. ‘I then look for these same patterns in the asteroid data so that I can say something about its mineralogy and what has happened to it,’ she adds.

From asteroid to life

These next few years will be particularly exciting for scientists studying asteroids.

Not only will this month give us our first detailed look at asteroid Bennu as OSIRIS-REx begins its approach, but the Japanese craft Hayabusa2 has already started its observations of Ryugu, the asteroid it is now currently orbiting.

Over the next few years, Helena’s research will feed into the data that these spacecraft collect, allowing us to better understand where and how the water-rich meteorites on Earth may have originated.

‘Bennu and Ryugu have been floating around the Sun ever since they were formed around 4.567 billion years ago,’ says Helena. ‘They contain some of the oldest solids in the solar system.’

Because of this, the objects give us an unprecedented snapshot of what the early solar system was like mere moments after it began to form, including the origin of that all-important water. As such, it lets us peer far back into the beginnings of life.

‘We do this because we are inherently interested in our own origins,’ says Helena.

Source: Natural History Museum

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