2023-08-30 15:13:33
4.5 billion-year-old Saharan space rock overturns assumptions about the early solar system. In May 2020, some unusual rocks containing distinctive greenish crystals were found in the Erg Chech Sand Sea, a dune-filled region of the Sahara Desert in southern Algeria. Upon closer inspection, it turned out that the rock came from space: a billion-year-old piece of debris left over from the dawn of the solar system. It was all a piece of a meteorite known as Erg Chech 002, the oldest volcanic rock ever found, melted long ago in the fire of an ancient protoplanet that is now gone. In new research published in the journal Nature Communications, they found lead and uranium isotopes in the rock and calculated that the rock was about 4.56556 billion years old, plus or minus 120,000 years. This is one of the most accurate ages scientists have ever determined for an object from space. This, in turn, casts doubt on some common assumptions about the early Solar System. About 4.567 billion years ago, our Solar System formed from a huge cloud of gas and dust. Among the many elements in the cloud was aluminum, which appeared in two forms. The first is the stable form, aluminum-27. The second is aluminum-26, a radioactive isotope created mainly by exploding stars that decays to magnesium-26 over time. Aluminum-26 is a very useful material for scientists who want to understand how the Solar System formed and evolved. Because it decays over time, we can use it to date events—especially during the first four to five million years of the Solar System’s life. The decay of aluminum-26 is important for another reason: scientists believe it was the main source of heat in the early solar system. This decay affected the melting of tiny, primordial rocks that later coalesced to form the planets. However, in order to use aluminum-26 to understand the past, we need to know whether it was spread evenly or accumulated more densely in some places. To find out, we need to more precisely calculate the age of some ancient space rocks. If we only look at aluminum-26, this will not work because it decays relatively quickly (after about 705,000 years, half of a sample of aluminum-26 decays to magnesium-26). This is useful for determining the relative age of various objects, but not for determining their absolute age in years. However, if we combine the aluminum-26 data with the uranium and lead data, 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). Uranium isotopes have a much longer half-life (710 million years and 4.47 billion years, respectively), which means that with their help we can directly determine how long ago an event took place. Erg Chech 002 is a so-called “ungrouped achondrite”. Achondrites are rocks that form from solid clumps in the cloud of gas and debris that make up the molten Solar System. The source of many achondrites found on Earth has been identified. Most of them belong to the so-called Howardite-Eukrite-Diogenite group, which are believed to originate from Vesta 4, one of the largest asteroids in the Solar System. Another group of achondrites are called angrites, each of which has an unidentified parent body. Other achondrites, including Erg Chech 002, are “ungrouped”, meaning their place of origin is unknown. During the examination of Erg Chech 002, it was found to contain large amounts of lead-206 and lead-207, as well as relatively large amounts of undecayed uranium-238 and uranium-235. It was measuring the ratio of all the lead and uranium isotopes that helped the researchers to estimate the age of the rock with such unprecedented accuracy. Their results contribute to a better understanding of the earliest stages of development of the Solar System and the geological history of the formation of the planets. Further studies of different groups of achondrites will undoubtedly further refine our knowledge and improve our ability to reconstruct the early history of our Solar System.
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