Evidence That Overturns Theories Of The Origin Of Water On Earth
University of Oxford researchers have helped overturn the popular theory that water on Earth originated from asteroids bombarding its surface. Instead, the material which built our planet was far richer in hydrogen than previously thought. The findings have been published today in the journal Icarus.
A team of researchers at the University of Oxford have uncovered crucial evidence for the origin of water on Earth. Using a rare type of meteorite, known as an enstatite chondrite, which has a composition analogous to that of the early Earth (4.55 billion years ago), they have found a source of hydrogen which would have been critical for the formation of water molecules.
Crucially, they demonstrated that the hydrogen present in this material was intrinsic, and not from contamination. This suggests that the material which our planet was built from was far richer in hydrogen than previously thought.
Without hydrogen, a fundamental elemental building-block of water, it would have been impossible for our planet to develop the conditions to support life. The origin of hydrogen, and by extension water, on Earth has been highly debated, with many believing that the necessary hydrogen was delivered by asteroids from outer space during Earth’s first approximately 100 million years.
But these new findings contradict this, suggesting instead that Earth had the hydrogen it needed to create water from when it first formed. The research team analysed the elemental composition of a meteorite known as LAR 12252, originally collected from Antarctica. They used an elemental analysis technique called X-Ray Absorption Near Edge Structure (XANES) spectroscopy* at the Diamond Light Source synchrotron at Harwell, Oxfordshire.
A previous study led by a French team had originally identified traces of hydrogen within the meteorite inside organic materials and non-crystalline parts of the chondrules (millimetre-sized spherical objects within the meteorite). However, the remainder was unaccounted for – meaning it was unclear whether the hydrogen was native or due to terrestrial contamination.
The Oxford team suspected that significant amounts of the hydrogen may be attached to the meteorite’s abundant sulphur. Using the synchrotron, they shone a powerful beam of X-rays onto the meteorite’s structure to search for sulphur-bearing compounds.
Close up of a section of the meteorite LAR 12252. Particles can be seen both scattered and arranged in a roughly spherical structure. Thin section photo of sample LAR 12252 in plane-polarised light with 5x magnification. Credit: NASA.
When initially scanning the sample, the team focussed their efforts on the non-crystalline parts of the chondrules, where hydrogen had been found before. But when serendipitously analysing the material just outside of one of these chondrules, composed of a matrix of extremely fine (sub-micrometre) material, the team discovered that the matrix itself was incredibly rich in hydrogen sulphide.
In fact, their analysis found that the amount of hydrogen in the matrix was five times higher than that of the non-crystalline sections. In contrast, in other parts of the meteorite that had cracks and signs of obvious terrestrial contamination (such as rust), very little or no hydrogen was present. This makes it highly unlikely that the hydrogen sulphide compounds detected by the team originated from an Earthly source.
Since the proto-Earth was made of material similar to enstatite chondrites, this suggests that by the time the forming planet had become large enough to be struck by asteroids, it would have amassed enough reserves of hydrogen to explain Earth’s present-day water abundance.
Tom Barrett, DPhil student in the Department of Earth Sciences at the University of Oxford, who led the study, said: “We were incredibly excited when the analysis told us the sample contained hydrogen sulphide – just not where we expected! Because the likelihood of this hydrogen sulphide originating from terrestrial contamination is very low, this research provides vital evidence to support the theory that water on Earth is native – that it is a natural outcome of what our planet is made of.”
The meteorite LAR 12252 on a lab bench. Next to it is a smaller fragment and small fragments contained in a crucible. Metal tags with ‘LAR 12252’ are next to each and a ruler is at the front. Lab photo of sample LAR 12252. Credit: NASA.
Co-author Associate Professor James Bryson (Department of Earth Sciences, University of Oxford) added: “A fundamental question for planetary scientists is how Earth came to look like it does today. We now think that the material that built our planet – which we can study using these rare meteorites – was far richer in hydrogen than we thought previously. This finding supports the idea that the formation of water on Earth was a natural process, rather than a fluke of hydrated asteroids bombarding our planet after it formed.” The study ‘The source of hydrogen in earth’s building blocks’ has been published in the journal Icarus.
X-ray Absorption Near Edge Structure (XANES) spectroscopy is a technique that is used to identify what elements are in a material and what their chemical state is. It works by shining X-rays onto a sample, causing the atoms to absorb energy in a way that depends on what the element is, the chemical form it is in (e.g., an oxide, a sulphide, etc), and how the atoms are bonded with others.
Astrobiology, Astrogeology, Astrochemistry,
