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 BulletinDatabase - 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.
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