New research, reported in the journal Nature, changes scientists’ understanding of how terrestrial planets attain their physical and chemical characteristics.
A young planetary system. Image credit: NASA / FUSE / Lynette Cook.
Planets grow by a process of accretion (a gradual accumulation of additional material) in which they collisionally combine with their neighbors. This is often a chaotic process and material gets lost as well as gained.
Massive planetary bodies impacting at several miles per second generate substantial heat which, in turn, produces magma oceans and temporary atmospheres of vaporized rock.
Before planets get to approximately the size of Mars, gravitational attraction is too weak to hold onto this inclement silicate atmosphere.
Repeated loss of this vapor 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 lead author Dr. Remco Hin, from the University of Bristol’s School of Earth Sciences.
“Magnesium isotope ratios change as a result of silicate vapor loss, which preferentially contains the lighter isotopes. In this way, we estimated that more than 40% of the Earth’s mass was lost during its construction.”
In this research, Dr. Hin and colleagues analyzed samples of the Earth together with meteorites from Mars and the asteroid Vesta.
“The main findings are three-fold,” they said. “Earth, Mars and Vesta have distinct magnesium isotope ratios from any plausible nebula starting materials.”
“The isotopically heavy magnesium isotope compositions of planets identify substantial (40%) mass loss following repeated episodes of vaporization during their accretion.”
“This slipshod construction process results in other chemical changes during growth that generate the unique chemical characteristics of Earth.”
Dr. 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.”
“We now show that vapor loss during the high energy collisions of planetary accretion has a profound effect on a planet’s composition.”
“This process seems common to planet building in general, not just for Earth and Mars, but for all planets in our Solar System and probably beyond, but differences in the collision histories of planets will create a diversity in their compositions.”
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R. Hin et al. Magnesium isotope evidence that accretional vapour loss shapes planetary compositions. Nature 549: 511-515; doi: 10.1038/nature23899
This article is based on text provided by the University of Bristol.
