The Sun has been used as the chemical yardstick for astronomy since scientists first learned to read the dark lines in its spectrum. That yardstick developed a puzzling flaw when measurements of silver in sunlight failed to match the silver preserved in primitive meteorites. A new analysis led by Dr. Sema Caliskan at Uppsala University, with collaborators at the University of Liege, reports that the apparent deficit was caused by the model used to interpret the solar atmosphere rather than by a real difference in the Sun's composition.
Published in Astronomy & Astrophysics, the study raises the estimated solar silver abundance by 55 percent. The result brings the Sun and ancient CI chondrite meteorites close to agreement, restoring confidence in a comparison that reaches back to the formation of the solar system.
The Meteorite Paradox | Two Records of the Same Solar System
The Sun and primitive CI chondrite meteorites formed from the same collapsing cloud of gas and dust about 4.6 billion years ago. CI chondrites are among the most chemically primitive rocks available to scientists because their bulk composition has been altered less than that of many other meteorites. For elements that did not evaporate or separate during planet formation, their relative abundances should closely reflect the starting material that also became the Sun.
Silver did not appear to follow that rule. Earlier solar spectroscopy placed the Sun's silver abundance at approximately log epsilon Ag = 0.87 on the astronomical scale where hydrogen equals 12.00. The corresponding meteorite value was about 1.21. That 0.25 dex difference made the Sun look roughly 44 percent poorer in silver than the rocks that formed beside it.
Solar Spectroscopy | How the Missing Silver Was Hidden
Astronomers measure elements in the Sun by splitting sunlight into its component wavelengths. Each element absorbs light at characteristic frequencies, producing dark spectral lines that act as chemical fingerprints. The strength of those lines depends on how many atoms occupy the energy levels involved in the transition, so interpreting a line requires a physical model of the atmosphere where it formed.
Earlier calculations used one-dimensional static models. Those models reduced a turbulent, convecting plasma to a uniform vertical stack of layers. They also assumed local thermodynamic equilibrium, or LTE, in which collisions and local temperature determine the distribution of atoms among energy states.
The real solar atmosphere is more complicated. Hot plasma rises, cooler material sinks, and radiation travels across regions with different temperatures and densities. That radiation can push silver atoms out of the energy distribution predicted by local temperature alone. In this non-LTE state, silver atoms spend more time in excited levels, changing the strength and formation depth of their ultraviolet absorption lines.
The earlier models treated the weakened lines as evidence that fewer silver atoms were present. The updated calculation shows that the lines were weakened by the atoms' radiation-driven state. Once that effect is included, the same observations support a substantially larger amount of silver.
The Revised Numbers | Solar Abundance Meets Meteorites
The new analysis reports a solar silver abundance of approximately log epsilon Ag = 1.15 +/- 0.08. That is much closer to the CI chondrite value of 1.21 +/- 0.04 than the earlier solar estimate of 0.87 +/- 0.10. The residual difference is about 0.06 dex, within the combined uncertainty of the measurements and models.
| Measurement | Earlier solar model | Updated 3D non-LTE model | CI chondrites |
|---|---|---|---|
| Silver abundance, log epsilon Ag | 0.87 +/- 0.10 | 1.15 +/- 0.08 | 1.21 +/- 0.04 |
| Difference from meteorites | -0.25 dex, about 44% lower | -0.06 dex, within uncertainty | Baseline |
| Internal line scatter | 0.175 dex | 0.020 dex | Not applicable |
The smaller internal line scatter is an important part of the result. When multiple silver lines are interpreted with the revised model, they produce a more consistent abundance instead of a broad spread. That agreement suggests the model is correcting a physical bias in the analysis rather than simply shifting one selected measurement.
Heavy Elements | What the Solar Baseline Changes
Silver is not made in ordinary stellar fusion. It belongs to a group of elements that require environments rich in free neutrons, where atomic nuclei can rapidly capture neutrons before radioactive decay occurs. Astrophysicists associate much of this rapid neutron-capture process, or r-process, with extreme events including neutron star mergers and some collapsars.
To understand when and where those events enriched the Milky Way, researchers compare the abundances of heavy elements in stars of different ages and compositions. The Sun is the reference point for those comparisons. If the solar silver baseline is too low, models of galactic chemical evolution can misidentify the amount of silver produced by earlier generations of stars or infer the wrong balance between possible r-process sources.
Dr. Caliskan and her collaborators argue that the improved solar composition should be applied beyond silver. Three-dimensional atmospheric dynamics and non-LTE effects can influence other elements, especially when their diagnostic lines form in the ultraviolet or in layers where radiation dominates over collisions. The correction therefore offers a method for improving abundance measurements across stellar populations, not only a solution to one solar-system discrepancy.
The finding also complements OzoneNews reporting on second-generation black holes in LIGO data, which examines how extreme cosmic environments build heavy objects over multiple generations. It connects to our coverage of the wind from the Milky Way's central black hole and the first true sugar detected in interstellar space. For the broader stream of research reporting, visit the OzoneNews Science section.