"Space rocks" found in the desert, subverting human understanding of the solar system?

Space rock could rewrite early solar system history

Space rocks (Image credit: Steve Jeanswest/Wikimedia, CC BY-SA)

In May 2020, some unusual rocks containing unique green crystals were discovered in a sand sea in a dune area of ​​the Sahara Desert in southern Algeria. Upon closer inspection, the rocks turned out to be from outer space - they are rubble from billions of years ago, left over from the dawn of the solar system.

"They are all fragments of what are called meteorites, the oldest volcanic rocks ever found, the product of melting long ago in the fires of some now-vanished ancient protoplanet. In new research published in Nature Communications, we analysed the lead and uranium isotopes in it and calculated that it is about 4,556,556 million years old, give or take 120,000 years. This is one of the most precise ages ever calculated from space, and our results call into question some common assumptions about the early solar system."

The Secret Life of Aluminum

About 4.567 billion years ago, our solar system formed from a giant cloud of gas and dust. Among the many elements in this cloud, aluminum existed in two forms. First, in its stable form, aluminum-27, and second, in aluminum-26. This is a radioactive isotope produced primarily by exploding stars, and over time it decays into magnesium-26. Aluminum-26 is extremely useful stuff for scientists who want to understand how the solar system formed and grew. Because space rocks decay over time, we can use it to date events—particularly within the first four or five million years of the solar system's life. The decay of aluminum-26 is important for another reason: We think it was the primary source of heat in the early solar system. This decay influenced the melting of small, primitive rocks that later came together to form the planets.

Uranium, lead and age

However, to use aluminum-26 to learn about the past, we need to know whether it's evenly distributed in some places or more densely clumped together than in others. To figure this out, we need to calculate the absolute age of some ancient space rocks more precisely. Looking at aluminum-26 alone won't let us do this, because it decays relatively quickly (after about 705,000 years, half of a sample of aluminum-26 will have decayed into magnesium-26). It's used to determine the relative ages of different objects, but not their absolute ages (measured in years). But if we combine the data for aluminum-26 with that for uranium and lead, we can make some progress. Uranium has two important isotopes (uranium-235 and uranium-238), which decay into different isotopes of lead (lead-207 and lead-206, respectively). The uranium isotopes have much longer half-lives (710 million years and 4.47 billion years, respectively), which means we can use them to directly calculate how long ago an event happened.

Space rocks like Erg Chech 002 provide clues about the early days of the solar system. (Image credit: Yuri Amelin, CC BY)

Meteorite Group

Erg Chech 002 is known as an "unassembled achondrite." Achondrites are rocks that formed from melted planetesimals, solid chunks of the cloud of gas and debris that we call the solar system. The origins of many of the achondrites found on Earth have been identified. Most belong to the so-called Howardite-Eucrite-Digeneite clan, believed to have originated from Vesta 4, one of the largest asteroids in the solar system. Another group of achondrites are known as the Wrathites, and they share an unknown parent body. Still other achondrites, including Erg Chech 002, are "unclassified": their parent bodies and family relationships are unknown.

Large piece of aluminum

In our study of Erg Chech 002, we found that it contains high abundances of lead-206 and lead-207, as well as relatively large amounts of undecarbonated uranium-238 and uranium-235. Measuring the ratios of all lead and uranium isotopes helped us estimate the age of the rock with unprecedented precision. We also compared our calculated age with previously published data on aluminum-26 for Erg Chech 002, as well as data from various other achondrites.

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- Meteor showers and meteors: formation and history The comparison with a group of achondrites called volcanic furies is particularly interesting. We find that the parent body of Erg Chech 002 must have formed from a material containing three to four times more aluminum-26 than the source of the furies' parent body. This suggests that aluminum-26 was indeed rather heterogeneously distributed in the cloud of dust and gas from which the Solar System formed. Our results contribute to a better understanding of the early stages of the development of the Solar System, and of the geological history of the emerging planets. Further studies of various achondrite groups will undoubtedly continue to refine our understanding and enhance our ability to reconstruct the early history of the Solar System. This article is reproduced with permission from Communications.

By: Evgenii Krestianinov

FY: He Danhuai

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