Reconciling Earth's origin story

Like other planets, Earth originally formed through the gradual buildup of materials due to gravity. The source of those materials, however, is a subject of much debate. Now, researchers in France believe they may have an explanation for one of the lingering questions about where our planet really came from.

A primitive class of meteorites, called chondrites, are believed to have formed from the primordial cloud of gas and dust known as the solar nebula that gave rise to the sun and planets of our solar system. Scientists think that particular subset of these -- enstatite chondrites -- are made from the same raw material from which Earth formed, because they contain isotopes of chemical elements in very similar proportions to those found on our planet.

But there are exceptions. Among other elemental differences, the planet seems to be much higher in magnesium and lower in silicon than enstatite chondrites.

In a study published Wednesday in Nature Communications, researchers from the French National Center for Scientific Research and Universite Blaise Pascal posit that the discrepancy could be due to the fact that proto-Earth was being pummeled and battered even as it was first coming together as a planet, so that as it grew, it also lost mass, leading to changes in overall chemical composition.

Meteorite impacts that caused Earth to grow in size simultaneously pulverized its surface. The planet maybe have taken 100 million years of abuse before a massive chunk broke off and eventually became the moon. During that time, its crust was eroded, resulting in the loss of a quantity of material the researchers estimate to be equivalent to at least 15 percent of the planet's current mass. A lot of that, the they believe, was silicon, leaving a higher proportion of magnesium behind.

They contend that this accounts for the discrepancy between the ratio of those elements on Earth versus in estatite chondrites.

The repeated destruction may have also caused volatile elements such as sodium and potassium to float off as gas, while elements such as aluminum and calcium were knocked loose from the surface but re-condensed and fell back to Earth as it formed.