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Physicists working on the Borexino experiment in Italy have measured the carbon and nitrogen abundances in the Sun’s core for the first time using solar neutrinos. Their review of nearly five years of data intensifies a decades-long controversy among scientists regarding the Sun’s true composition. The disagreement rests on different methods of detecting what’s in the Sun, and the Borexino experiment has the ability to weigh in based on the neutrino, a subatomic particle of negligible size. Approximately 99 percent of the Sun’s energy is generated when hydrogen fuses into helium via the proton-proton (pp) chain. The remainder is produced through the CNO cycle, which involves the fusion and decay of various carbon, nitrogen, and oxygen isotopes. Both processes generate neutrinos as byproducts.

Physicists reported in 2020 that they had for the first time discovered neutrinos from the Sun’s CNO cycle using Borexino, an underground detector beneath Italy’s Gran Sasso mountain range. Between January 2017 and October 2021, they collected this new information by observing an average of 4.8 CNO neutrinos every day. Now, physicists have reduced their measurement errors by half compared to 2020, allowing them to detect the carbon and nitrogen abundances in the Sun’s core. The findings have been published in Physical Review Letters. Before these findings, astronomers had two methods for determining the composition of the Sun: spectroscopy and helioseismology.

Spectroscopists separate the Sun’s light into a spectrum, searching for chemical fingerprints known as absorption lines, which indicate which components have absorbed specific frequencies of light. On the other hand, helioseismologists measure sound waves that have reached the solar surface after bouncing about within the Sun. The heavier the Sun’s elements, the more they inhibit sound waves from reaching its surface. (Metals are elements heavier than hydrogen and helium; the higher the metallicity of the Sun, the more metals it contains.)

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Once, the two separate methods were in agreement. In the 2000s, however, new and improved measurements caused them to diverge dramatically. In what has become known as the solar abundance dilemma, helioseismologists now argue for a greater metallicity than what spectroscopists find. In May of 2022, a team lead by spectroscopist Ekaterina Magg found 26% more metals on the surface of the sun than earlier spectroscopic examinations. The new carbon-nitrogen abundances derived from Borexino neutrino data appear to be consistent with Magg’s conclusion, but with a wide margin of error. The Borexino statistics indicate that the interior contains between 9 to 58% more metals than prior investigations.

“Our measurement accords quite well with the so-called high-metallicity compilations,” the team writes in its article. “However, there is moderate… disagreement with the low-metallicity compilations.” The Yale University helioseismologist Sarbani Basu, who was not engaged in the neutrino investigation, considers this finding to be noteworthy. “I believe the Borexino results support the helioseismic restrictions on metallicities, given that Magg and colleagues’ spectroscopic results also indicate a greater metallicity,” she says.

Yet a recent research accepted for publication in Astronomy & Astrophysics casts doubt on this alleged consensus. “Taking into account the impact of the Sun’s spin and the reported depletion of lithium at the solar surface, even models with large CNO abundances from Magg and colleagues contradict with the Borexino results,” explains the study’s lead author, Gal Buldgen (University of Geneva). If Buldgen is true, then CNO neutrinos and helioseismology would argue for a high metallicity in the solar interior, whereas spectroscopy would continue to suggest a lower metallicity near the solar surface.

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Both camps may be right. If the Sun acquired material from the protosolar disk unevenly, a November 2022 study, also published in Astronomy & Astrophysics, suggests that the core metallicity might be 5% higher than at the surface. More observations of CNO neutrinos would help lower the uncertainty surrounding these preliminary data, but they won’t come from Borexino. The experiment was terminated in October 2021, and no other facilities can observe these low-energy neutrinos at this time. It appears, at least for the time being, that this lengthy argument will continue.

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