The Earth lost over 40 per cent of its mass during a violent and chaotic process that led to its formation, according to a study which sheds new light on the creation of our planet and its neighbour Mars. Planets grow by a process of accretion – a gradual accumulation of additional material – in which combine with their neighbours during collision. This is often a chaotic process and material gets lost as well as gained. Massive planetary bodies impacting at several kilometres per second generate substantial heat which, in turn, produces magma oceans and temporary atmospheres of vaporised rock.
Before planets get to the size of Mars, gravitational attraction is too weak to hold onto this inclement silicate atmosphere. Repeated loss of this vapour envelope during continued collisional growth causes the planet’s composition to change substantially. “We have provided evidence that such a sequence of events occurred in the formation of the Earth and Mars, using high precision measurements of their magnesium isotope compositions,” said Remco Hin from the University of Bristol in the UK. “Magnesium isotope ratios change as a result of silicate vapour loss, which preferentially contains the lighter isotopes. In this way, we estimated that more than 40 per cent of the Earth’s mass was lost during its construction,” said Hin. Dubbed as the “cowboy building job”, this process was also responsible for creating the Earth’s unique composition,” he said.
The research, published in the journal, Nature was carried out in an effort to resolve a decades long debate in Earth and planetary sciences about the origin of distinctive, volatile poor compositions of planets. Researchers analysed samples of the Earth together with meteorites from Mars and the asteroid Vesta, using a new technique to get higher quality measurements of magnesium isotope ratios than previously obtained.
They found that Earth, Mars and asteroid Vesta have distinct magnesium isotope ratios from any plausible nebula starting materials. The isotopically heavy magnesium isotope compositions of planets identify substantial (about 40 per cent) mass loss following repeated episodes of vaporisation during their accretion. This construction process results in other chemical changes during growth that generate the unique chemical characteristics of Earth.
“Our work changes our views on how planets attain their physical and chemical characteristics,” Hin said. “While it was previously known that building planets is a violent process and that the compositions of planets such as Earth are distinct, it was not clear that these features were linked,” he said.
“We now show that vapour loss during the high energy collisions of planetary accretion has a profound effect on a planet’s composition,” he added. The process seems common for all planets in our Solar System and probably beyond, not just for Earth and Mars, but differences in the collision histories of planets will create a diversity in their compositions.