Rare meteorites provide a window into the early solar system

Scientists have studied starlight for ages, yet there is more to learn about the photosphere, the outermost layer of the Sun where light is emitted. Getting accurate data on which elements exist in the Sun, and in what amounts, is no simple feat.

According to Dr. Katharina Lodders, a researcher at Washington University in St. Louis, matching the Sun’s element abundances with materials found in CI-chondrites (a rare type of stony, carbonaceous meteorite) has opened fresh avenues for discussing where our planetary building blocks come from. 

Dr. Lodders’ detailed investigation highlights that these peculiar meteorites, which carry water-rich signatures, share an elemental similarity with the Sun’s composition, once the most volatile elements are accounted for.

Elements of the solar system

Elements underpin the chemistry behind planetary bodies, stars, and the vast swaths of interstellar material. When scientists discuss solar “metallicity,” they are referring to every element heavier than hydrogen and helium, the two most abundant gases in the Sun.

Our star’s chemical makeup sets the tone for how planets form, why certain materials accumulate, and what life’s essential ingredients might be.

Understanding these elements in the Sun also sheds light on the processes that occurred billions of years ago, back when the proto-solar nebula took shape.

Connecting the Sun with meteorites

Researchers recognized long ago that meteorites reflect early solar system material. But CI-chondrites stand out for closely resembling the solar photosphere’s elemental profile, especially for less volatile elements.

These meteorites are so rare that only a handful have ever been retrieves. Their compositions hint at how minerals like silicates and oxides came together in the young solar nebula before larger bodies – like asteroids and planets – formed.

Solar data challenges

Studying the Sun’s chemical makeup is tricky. Temperatures and gas dynamics just beneath the photosphere create spectral lines, and scientists must correct for processes like convection and radiative transfer when interpreting the data.

Advances in computing have enabled three-dimensional models that simulate how gases behave in the solar atmosphere. Recent studies also try non-local thermodynamic equilibrium (NLTE) approaches in an attempt to reduce previous errors caused by oversimplified assumptions.

Helium, hydrogen, and metals

Most of the Sun consists of hydrogen. Next comes helium, with a small but vital sliver of heavier elements, such as oxygen and iron.

Over billions of years, heavier particles in the solar envelope can drift inward, making the photosphere slightly poorer in these “metals.” This phenomenon is called element settling, and many analyses now correct for it when estimating the original, or proto-solar, abundance of elements.

Matching the puzzle pieces

Scientists look at CI-chondrites for clues about planetary material that was around in the earliest days. Yet some elements, like sulfur or mercury, move around over time in water-rich conditions, which complicates the data.

For more reliable results, researchers compare stable element ratios, such as zirconium to hafnium, across different meteorite groups. This ratio-based approach has revealed that certain metals remain unaffected by water or heat and thus preserve their original proportions.

Peering into the Sun’s core

Neutrino experiments, like Borexino, peer into the Sun’s core by measuring particles generated through fusion reactions. They offer an indirect check on core abundances of carbon and nitrogen because these elements drive fusion in deeper layers.

Meanwhile, helioseismology, the study of pressure waves in the Sun, uncovers details about its interior density and temperature. The interplay of these findings provides an extra layer of validation for updated solar models.

Refining models of planet formation

The Earth and other planets formed from leftover solar system material. By tracking how elements are partitioned between the photosphere and meteorites, researchers are able to refine models of planet formation and the distribution of key ingredients for life.

Even subtle differences in iron or magnesium abundances can change how scientists interpret early solar nebula processes.

More precision means better insight into how Earth got its water, or why the gas giants differ in composition from the rocky planets that are closer to the Sun.

Comparing meteorites and the Sun

As instrumentation becomes more refined, additional comparisons between meteorites and the Sun’s photosphere will clarify whether certain elements have been under- or overestimated in models.

Researchers also anticipate new data from sample-return missions that will target asteroids akin in composition to known CI-chondrites.

Planned solar probes will improve measurements of solar wind particles. These samples, combined with thorough lab tests, might close gaps in knowledge about noble gases and more volatile elements that are hard to assess through classical spectroscopy.

The study is published in Space Science Reviews.

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