Aupctre was formed when

Did the earth come into being faster than expected?

Dust instead of lumps? Most of our planet could have formed in just five million years - far faster than previously thought, as researchers report. Instead of growing gradually from collisions of ever larger planetary components, the proto-earth was formed rapidly from an agglomeration of dust in the primordial cloud. The scientists found evidence of this process in comparative analyzes of meteorite and earth rock.

According to popular theory, the earth and the other rocky planets of the solar system formed slowly and in several phases. Turbulence in the primeval cloud initially created objects several hundred kilometers in size, which then gradually attracted more and more particles the size of dust grains due to their gravity. When the primordial cloud then disintegrated around four to five million years after its formation, the planetary embryos grew into protoplanets through collisions with one another over the course of millions of years - as far as the usual scenario.

From solar dust instead of lumps?

But there is an alternative scenario: “The other possibility is that the earth was balled up from dust - millimeter-sized grains that rained on the growing celestial body and formed it in one go,” explains lead author Martin Schiller from the University of Copenhagen. That would mean that the main accretion phase of the earth took place entirely in the protoplanetary disk - in the first five million years of the solar system.

Some time ago, observations of asteroids provided the first indications of this accelerated growth, but also newer models of planet formation, as Schiller and his team report. If this scenario is correct, however, it should be possible to determine this by comparing the iron isotope distribution of the earth's mantle and meteorites. Because if a large part of our planet was created through late collisions with larger chunks, then the remaining chunks - asteroids - would have to reflect its composition.

Iron isotopes as clues

Schiller and his team have now investigated whether this is the case. To do this, they analyzed samples from various meteorites and earthly rock samples for their content of various iron isotopes. They paid particular attention to the ratio of iron-54 to iron-56. It turned out that only one type of meteorite material contains the same distribution of iron isotopes as the earth today: the so-called CI chondrites.

"If the young earth only became a full-grown planet through accidental collisions of different chunks, then its iron composition should not only match one type of meteorite," says Schiller. “You would have to find a mixture.” Because towards the end of the accretion phase assumed according to current theory, chunks from different zones of the solar system should have collided with the proto-earth. "But the only group of chondrites that matches the terrestrial iron composition are the CI chondrites," the researchers say.

Proto-planet after just five million years

The interesting thing about it: CI chondrites are considered to be the meteorites whose composition comes closest to that of the primordial cloud and the sun. "The only time when CI-like material was abundantly available in the formation zone of the Earth-like planets was during the lifetime of the protoplanetary disk," explain Schiller and his colleagues. In their opinion, the earth must therefore have grown much earlier than previously assumed.

According to your scenario, the proto-earth agglomerated relatively quickly from the cosmic dust of the protoplanetary disk. Within just five million years - and thus within the lifespan of the primordial cloud - our planet had already reached 80 percent of its current mass. "This period represents the time in which the growth of the proto-sun attracted incident material through the disk," explain the researchers. This process, in their view, also enabled the earth to grow rapidly.

Water from the primeval cloud

If this scenario is confirmed, it could also explain where early earth got its water from. Because the early dust of the protoplanetary disk had not yet dried up due to the intense solar radiation. It therefore contained even more volatile elements than the material from the later planet formation time. "Water would then only be a by-product in the formation of a planet like Earth," says Schiller.

For a long time, planetary researchers suspected that the primordial earth got a large part of its water from impacts by comets and water-rich asteroids. But measurement data from comets are now calling this assumption into question. Because the isotope signature of the cometary water does not match that of the terrestrial water. As early as 2018, scientists therefore put forward the thesis that the young earth could have already noticed its water from the primeval cloud. The results of Schiller and his team now support this. (Science Advances, 2020; doi: 10.1126 / sciadv.aay7604)

Source: University of Copenhagen

February 21, 2020

- Nadja Podbregar