Precious metals like gold and platinum would be even rarer had it not been for a single collision between Earth and a large body 4.45 billion years ago, a new paper claims. The theory would have major implications for Earth’s early history, the formation of other rocky planets within the Solar System, and even for the composition of Earth-like planets around other stars.
Models of the way planets form have a hard time capturing what we actually see when we look at Mercury, Venus, Earth, the Moon, and Mars.
A team led by Dr Ramon Brasser of the Tokyo Institute of Technology have produced a radically different model, one in which Jupiter collected most of the inner Solar System's rubble, leaving less to rain down on the rocky planets.
The idea stands in contrast to the visions of steady asteroid bombardment over hundreds of millions of years that represent the usual explanation for the minerals near the surface of the planet, and which shape our perceptions of the conditions under which life formed.
Early Earth was so hot that heavy metals with an affinity for iron, among them gold, platinum, and palladium, melted and sunk to the core.
The presence of these elements in the planet's crust can be explained by subsequent asteroid impacts, just as the layer of iridium in 66 million-year-old rocks provided key evidence for the cause of the dinosaurs' extinction.
Brasser agrees precious metals arrived after Earth's crust solidified. However, in Earth and Planetary Science Letters he presents a model that incorporates the controversial Grand Tack theory, under which Jupiter formed closer to the Sun than its current location, and migrated even further in, before shifting outward.
Unlikely as the Grand Tack theory sounds, it provides an explanation for some anomalies we see today. For example, such a journey by a huge gas giant would have cleared the area inwards of Jupiter's orbit of most small and medium sized objects, preventing them from building Mars to a mass similar to that of Earth, as most other models suggest.
Brasser's modeling of the Grand Tack also leaves far fewer objects to hit Earth than if the Tack did not occur. Instead he concludes one enormous impact, probably the one that caused the formation of the Moon, delivered most of what we can find today.
Brasser's picture of a relatively placid environment after the Moon's formation fits with lunar observations. If Earth was being bombarded by large asteroids around 4.4 billion years ago the Moon should have taken similar hits, but the number of basins and the absence of precious metals in the Moon's crust fits better with Brasser's theory than previous models.
If Brasser is right, the remarkable event that led to the Moon's formation was even more unlikely, and important, than we have previously recognized. If so, exoplanets that appear Earth-like in other ways may seldom have similar metal abundances.
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