A piece of EC 002. (A. Irving, public domain)

This Is a Piece of a Lost Protoplanet, And It's Officially Older Than Earth

9 MARCH 2021

A chunk of meteorite found in the desert sands of Algeria could be a piece of a baby planet that never made it.

According to an in-depth analysis of the rock's composition and age, not only is the meteorite known as Erg Chech 002 older than Earth, it formed volcanically - suggesting that it could have once been part of the crust of an object known as a protoplanet.

 

As such, it represents a rare opportunity to study the early stages of planet formation, and learn more about the conditions in the earliest days of the Solar System, when the planets we know and love today were still forming.

EC 002 was just found in May of last year, several chunks of rock with a combined weight of 32 kilograms (70 pounds) in the Erg Chech sand sea in southwestern Algeria. It was fairly quickly identified as unusual; rather than the chondritic composition of most recovered meteorites - which form when bits of dust and rock stick together - its texture was igneous, with pyroxene crystal inclusions.

It was therefore classified as an achondrite, a meteorite made of what seems to be volcanic material, originated on a body that has undergone internal melting to differentiate the core from the crust - a protoplanet, one of the middle stages of planet formation.

Of the tens of thousands of meteorites that have been identified, only a few thousand - 3,179, according to the Meteoritical Bulletin Database - are achondrites.

Most of these achondrites come from one of two parent bodies, and are basaltic in composition. This means that they cannot tell us much about the diversity of protoplanets in the early Solar System.

 

EC 002, on the other hand, is not basaltic, but a type of volcanic rock known as andesite, a team of scientists led by geochemist Jean-Alix Barrat of the University of Western Brittany in France has determined.

Of all the meteorites we have found to date, even among achondrites, that makes EC 002 extremely rare - and opens up a new avenue for understanding planet formation.

According to the team's analysis, the rock is ancient. The radioactive decay of isotopes of aluminium and magnesium suggest that these two minerals crystallised around 4.565 billion years ago, in a parent body that accreted 4.566 billion years ago. For context, Earth is 4.54 billion years old.

"This meteorite is the oldest magmatic rock analysed to date and sheds light on the formation of the primordial crusts that covered the oldest protoplanets," the researchers wrote in their paper.

Unlike basalt, which forms from the rapid cooling of lava rich in magnesium and iron, andesite is composed primarily of sodium-rich silicates, and - on Earth, at least - forms in subduction zones, where the edge of one tectonic plate is pushed underneath another.

 

Although it's found rarely in meteorites, the recent discovery of andesite in meteorites found in Antarctica and Mauritania prompted scientists to investigate how it might occur. Experimental evidence suggests that it can form from the melting of chondritic material.

Because chondritic bodies are so common in the Solar System, it's possible that the formation of protoplanets with andesite crusts was also common. However, when the team compared the spectral characteristics of EC 002 - that is, the way it interacts with light - with the spectral characteristics of asteroids, they could find nothing in the Solar System that matched the meteorite.

Andesitic crustal remains are not only rare in the meteorite record; they are also rare in the asteroid belt. Which raises the question: if the formation process was so simple and common, then where the heck did all the differentiated protoplanets get to?

The same place most of the material in the Solar System ended up, probably: they either got pulverised, or incorporated into larger rocky bodies; or, perhaps, a combination of both.

Since EC 002 is a little older than Earth, it's even possible that its protoplanetary siblings went on to help build Earth from a knot of denser material in the dust cloud that orbited the baby Sun.

Although we have a pretty decent grip on how baby planets are born, growing over millions of years as clumps of rocks and dust stick together, the specifics of the process are a little more mysterious.

EC 002 represents a spectacular opportunity to fine-tune our understanding of how our home system emerged from the dust.

The research has been published in PNAS.