Ryugu, which meanders between Earth and Mars, is now confirmed to be akin to the Ivuna-type carbonaceous chondrites (CIs), meteorites that scientists prize for their ancient and unaltered nature. These CIs are like time capsules, believed to have conserved the primordial soup from which the planets emerged.
Anomalies in isotopic ratios, particularly chromium (54Cr/52Cr) and titanium (50Ti/47Ti), denoted as e54Cr and e50Ti, serve as cosmic fingerprints, leading scientists to the nucleosynthetic origins of these space-traveling materials. While Ryugu's overall isotopic signature aligns with its meteorite cousins, the Cr isotopic variations hinted at a narrative more complex than a shared birthplace.
To decipher this cosmic puzzle, an international team spearheaded by Professor Tetsuya Yokoyama from the Tokyo Institute of Technology, undertook a meticulous study of these isotopic signatures in five Ryugu samples, findings of which were published in Scientific Advances.
"Previous research suggested a common cradle for Ryugu and Ivuna-type carbonaceous chondrites in a distant part of the solar system," Prof. Yokoyama elucidated. "The slight isotopic discrepancy we found in Cr between Ryugu and CIs raises questions about whether this heterogeneity is a tale of different origins or a sequel to subsequent events post-accretion."
Employing inductively coupled plasma mass spectrometry (ICP-MS) and thermal ionization mass spectrometry (TIMS), the team dissected samples as light as a feather - weighing less than 24 milligrams - to those with more heft. Their findings? The e50Ti was in harmony with CI data, but the e54Cr variances strayed from the expected values.
Intriguingly, when samples tipped the scales at over 90 milligrams, a uniform isotopic landscape emerged, suggesting a macroscopic resemblance to CIs. This pointed to an uneven spread of Cr isotopes within Ryugu's body at a microscopic scale.
Further scrutiny painted a picture of Ryugu's infancy, where water within the asteroid played a cosmic game of hide and seek with chromium. As water dissolved soluble Cr, the 54Cr-rich presolar nanoparticles remained steadfast, leading to a dance of minerals within the asteroid's body, some depleted in 54Cr. This was supported by the radiogenic 53Cr isotope analysis, which clocked the secondary mineral precipitation at about 5.2 million years post-solar system's creation.
The study sheds light on the role of water in redistributing elements within asteroids, a factor that might explain the observed 54Cr anomaly. This suggests that the apparent isotopic variations could be due to parent body processes rather than nebular inheritance.
Prof. Yokoyama reflects, "The Ryugu samples, spared from Earth's altering touch, are invaluable for demystifying our solar system's chemical lineage. They retain a chemical innocence, making this research a significant stride towards grasping our chemical origins."
As the Hayabusa2 mission continues to inspire and propel our understanding of the universe's early chemistry, these findings not only add a piece to the puzzle of our solar system's past but also enhance our understanding of the role of water in shaping the asteroidal bodies floating in our celestial neighborhood.
Research Report:Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample
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Tokyo Institute of Technology
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