Researchers looking at Earth’s growth as a planet in the early solar system have uncovered a new chaotic twist: planetary bodies like ours gained and lost a “vapor envelope” that caused our planet’s composition to change drastically. This envelope resulted after collisions with small bodies that created a temporary atmosphere of vaporized rock.
Prevailing theory holds that planets grow by accretion, which means gradually gathering gas and dust over time. This sometimes also includes collisions with smaller neighbors. A famous example of the latter was Earth’s long-ago collision with a Mars-sized body; the debris from the crash eventually created the Earth’s moon.
The new research, which was recently published in the journal Nature, shows that the larger bodies that struck Earth and Mars — traveling at several miles a second — created a lot of heat that generated magma oceans, and the temporary vaporized-rock atmosphere. However, Earth as a young planet (when it was smaller than Mars) was too small to have enough gravitational attraction to hold on to this atmosphere, so it eventually bled into space.
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Over time, the researchers said, these collisions, and the gain and loss of temporary atmospheres, greatly altered the makeup of Earth and Mars. The evidence is based on looking at samples from Earth rocks, as well as meteorites from Mars and the asteroid Vesta.
“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, a senior research associate with the University of Bristol’s school of Earth sciences in a statement.
Isotopes are different forms of an element. In this case, the element studied was magnesium.
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“Magnesium isotope ratios change as a result of silicate vapour loss, which preferentially contains the lighter isotopes,” Hin went on. “In this way, we estimated that more than 40 per cent of the Earth’s mass was lost during its construction. This cowboy building job, as one of my co-authors described it, was also responsible for creating the Earth’s unique composition.”
Researchers carried out the work to add weight to a lengthy debate about why planets have poor volatile compositions — a “volatile” is a chemical element or chemical compound with a low boiling point, such as helium. The debate has two main points: the volatiles were lost either because of planetary growth, or due to some process related to the gas and dust environment in which planets in the solar system were born.
“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. We now show that vapour loss during the high-energy collisions of planetary accretion has a profound effect on a planet’s composition.”
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