Beneath a beam of helium ions in a laboratory, a fragment of the Murchison meteorite has been made to endure eight centuries of solar wind in a few hours, and what it revealed is that the Sun does not erode the carbon of the cosmos evenly. The fragled chains of aliphatic hydrocarbons give way while the stubborn aromatic rings endure, a finding reported in late October 2026 by Futing Yin, Hong Tang and their colleagues. It is a quiet correction to how we read the surfaces of dark asteroids, and by extension the samples that Hayabusa2 and OSIRIS-REx carried home, for the organic record we see at
Solar Wind Preferentially Strips Aliphatic Organics From Asteroid Surfaces, Leaving Aromatics Intact
The Sun does not wear down organic matter evenly.
Mark
Why does a lab experiment on one meteorite matter for whole asteroids?
Mimi
Murchison is the closest lab analog to C-type asteroids like Ryugu and Bennu, so what the solar wind does to it is a model for what happens on their surfaces.
Mark
And the key result is the selective damage?
Mimi
Yes. Aliphatic chains lose the most, while aromatic rings survive because their delocalized electrons share the energy of a broken bond.
Luke
Careful with the scope, though. This is one meteorite slice, one ion type, one dose.
Mimi
True. They used He+ at 1.2 × 10^17 ions per square centimeter, standing in for 800 years.
Mark
Why 800 years?
Mimi
Regolith gets churned by impacts and shaking on timescales of centuries to millennia, so fresh material keeps reaching the surface.
Luke
So the claim that spectra underestimate aliphatics is an inference, not a measurement of Bennu or Ryugu.
Mimi
Right. It follows from the lab result and fits earlier work, including Thompson 2020 and the pristine Ryugu subsurface grains, but it is not a direct check on those samples.
Mark
What do we still not know?
Luke
By how much. The text I have doesn't give a quantified correction for the real asteroid ratios.
Le Pouls
- Solar wind is quietly stripping the flexible aliphatic chains from asteroid surface organics, while the sturdier aromatic rings are left standing.
- Lab irradiation of Murchison has made the surface chemistry look altered, raising doubts about what C-type asteroid spectra actually show.
- Researchers are cross-checking Raman and nano-IR data against earlier irradiation studies to pin down how much aliphatic carbon has already been lost.
- The Ryugu and Bennu sample results are now being reread against this new baseline, and the original aliphatic content may turn out to be higher than the surface suggests.
Beneath a beam of helium ions in a laboratory, a fragment of the Murchison meteorite has been made to endure eight centuries of solar wind in a few hours, and what it revealed is that the Sun does not erode the carbon of the cosmos evenly. The fragled chains of aliphatic hydrocarbons give way while the stubborn aromatic rings endure, a finding reported in late October 2026 by Futing Yin, Hong Tang and their colleagues. It is a quiet correction to how we read the surfaces of dark asteroids, and by extension the samples that Hayabusa2 and OSIRIS-REx carried home, for the organic record we see at the surface may be a survivor's residue rather than the original inheritance.
In a laboratory, a polished slice of the Murchison meteorite sat under a beam of helium ions, absorbing a dose equal to 800 years of solar wind on a carbonaceous asteroid. The results, published October 31, 2026, in the Journal of Geophysical Research: Planets, showed that the Sun does not wear away organic matter evenly. Futing Yin and Hong Tang's team found that the bombardment destroys aliphatic hydrocarbons while the aromatic ring structures largely survive.
The experiment was designed around the way asteroid surfaces actually renew themselves. Micrometeorite impacts and seismic shaking constantly turn over regolith, pushing fresh material into the sunlight where it begins to collect damage immediately. Murchison, which fell in Australia in 1969 and has been studied ever since, was an ideal subject: its organics include a cross-linked aromatic network with aliphatic chains bridging it, alongside amino acids and nucleobases. Using Raman spectroscopy and nano-infrared mapping, the team traced six classes of organic groups clustered around clay minerals before irradiation, then watched the network grow more disordered and lose much of its signal afterward.
The central finding is the difference between the two kinds of carbon. Single-bonded aliphatic chains broke apart, while aromatic rings spread the energy of a bond break across their delocalized electrons and held together. The surface, then, is a residue: rich in aromatics and thin in aliphatics. This matters for the sample-return missions, since Ryugu and Bennu grains came from regolith exposed to the same wind. If aliphatics erode in proportion to dose, the organic content seen in surface spectra and surface samples may understate what the parent body originally held. The next step is to reread those returned materials against this baseline.
A polished slice of the Murchison meteorite sat under a beam of helium ions in a laboratory, taking in a dose equal to 800 years of solar wind on the surface of a carbonaceous asteroid. The result, published October 31, 2026, in the Journal of Geophysical Research: Planets, is that the Sun does not wear down organic matter evenly. Researchers led by Futing Yin and Hong Tang found that the bombardment destroys aliphatic hydrocarbons while leaving aromatic ring structures largely intact.
The team delivered He+ ions at a fluence of 1.2 × 10^17 ions per square centimeter. They chose that dose because micrometeorite impacts and seismic shaking keep turning over asteroid regolith, bringing fresh material to the surface on timescales of centuries to millennia, where it begins collecting solar wind damage at once. At 1 astronomical unit, the solar wind delivers roughly 3 to 4 × 10^8 ions per square centimeter per second at several hundred kilometers per second, with energies near 1 kiloelectronvolt per atomic mass unit. Those ions break bonds, set off cascades of displaced atoms, and sputter carbon-bearing fragments into vacuum.
Murchison is a natural test subject. It is a CM2 carbonaceous chondrite that fell near Murchison, Victoria, Australia, on September 28, 1969, with more than 100 kilograms recovered, and it has been studied for five decades. Its organics fall into two groups. Insoluble organic matter is a cross-linked aromatic network, loosely like terrestrial kerogen, with aliphatic chains bridging the aromatic cores. Soluble organic matter includes amino acids, carboxylic acids, nucleobases and aldehydes. CM chondrites are the closest laboratory match to the dark C-type asteroids, including the parent bodies of Ryugu and Bennu.
The team used Raman spectroscopy alongside nanoscale infrared spectroscopy, or nano-IR. Nano-IR pairs an atomic force microscope with a tunable infrared source and maps absorption at tens of nanometers, about 100 times finer than conventional infrared microspectroscopy. That let the researchers place organic functional groups at the scale of individual mineral grains. Before irradiation they found six classes, all co-located with phyllosilicate clays: carboxylic acids, aliphatic hydrocarbons, aromatic hydrocarbons, amides, amino acids and aldehydes. That clustering fits the picture of water-driven alteration on the parent body.
After irradiation, the Raman D/G band ratio rose, indicating greater structural disorder in the macromolecular carbon network. Total organic content measured by nano-IR fell. The source attributes this mainly to sputtering, plus direct bond damage that weakens the infrared signatures of carbon that stays in place. It is consistent with earlier He+ work on Murchison by Laczniak et al. in 2023.
The differential result is the central finding. Aliphatic groups, the single-bond CH2 and CH3 chains, took the heaviest modification. Aromatic rings, including benzene and polycyclic aromatic frameworks, held up far better because their delocalized pi electrons spread the energy of a bond break across the whole ring. Surface organics are therefore a post-irradiation residue, enriched in aromatics and depleted in aliphatics. Thompson et al. in 2020, using pulsed-laser weathering of Murchison, saw increased aromatic signatures in irradiated regions, which fits.
That bears on the sample-return missions. Hayabusa2 brought 5.4 grams from Ryugu in December 2020, and OSIRIS-REx returned 121.6 grams from Bennu in September 2023. Both came from regolith with solar wind exposure. Ryugu subsurface grains have already been shown to be pristine and free of the space-weathering features seen in surface grains, and the new work offers a mechanism for the organic difference. If aliphatics erode in proportion to dose, aliphatic-to-aromatic ratios in surface material understate what was originally there, and so do reflectance spectra of C-type surfaces. The source text breaks off while discussing amino acids and carboxylic acids, whose signatures also change under irradiation. The next test is how Ryugu and Bennu results are read against this baseline.
Citations marquantes
Aliphatic hydrocarbons suffered the greatest modification from the helium ion dose, while aromatic structures proved considerably more resistant.— Findings of Yin, Tang and colleagues, JGR: Planets (paraphrased)
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