A new 3D non-LTE model of the Sun's atmosphere finds far more silver than the standard reference value assumed, closing most of a long-standing mismatch with primitive meteorites — though a small gap remains.
Astronomers measure the Sun's chemical make-up by matching sunlight's dark absorption lines — one distinctive pattern per element — against physical models of the solar atmosphere.1 For silver, the standard value has rested since 2015 on a model that assumed the gas was in local thermodynamic equilibrium, meaning every atom's excitation state was set by the local temperature alone.1
Researchers at Uppsala University built a new atomic model of silver and combined it with a 3D simulation of the Sun's turbulent outer layers, dropping that equilibrium assumption. Applied to the two ultraviolet silver lines used to measure the element, the revised model raises the recommended solar abundance from log ε = 0.96 to log ε = 1.15 — an increase of 0.19 on the logarithmic scale astronomers use, equivalent to 55% more silver.13 The calculations ran on Tetralith, Sweden's national supercomputer at Linköping.2
The Sun and the oldest, most chemically primitive meteorites condensed from the same cloud of gas and dust 4.6 billion years ago, so their heavy-element make-up should broadly agree.1 Silver was the outlier: the 2015 solar value sat 0.25 dex — about 78% — below the meteoritic one, a discrepancy the study describes as more than two standard deviations out of line among elements with comparably precise measurements.1
Silver matters beyond bookkeeping, too. Around 80% of the solar system's silver is thought to come from the r-process, the rapid neutron-capture reactions inside stellar explosions, and silver is considered a useful tracer of a distinctive "weak" component of that process that remains poorly understood.1 Lead author Sema Caliskan, who carried out the work during her PhD studies at Uppsala, said the new model let the team interpret the diagnostic lines more precisely than before; the same approach will next be applied to other stars.2
A residual 0.06 dex gap to the meteorite value remains, and the paper does not resolve whether this reflects real depletion of silver from the Sun's outer layers or leftover measurement error on one side or the other — the authors call it an open, debated question.1 The largest modelling uncertainty comes from how silver atoms collide with hydrogen in the solar atmosphere, estimated to contribute around 0.05 dex to the total error bar.1 One of the two diagnostic spectral lines is also blended with a predicted, not directly measured, iron line — a complication the authors flag for confirmation by future solar observations.1