The Sulfur Enigma

The Mysterious Case of the Missing Sulfur

In 2010, scientists studying asteroid 433 Eros made a puzzling discovery: its surface showed dramatic sulfur depletion compared to its interior. This finding was later confirmed by analysis of samples from asteroid Itokawa, where Fe-rich whiskers protruded from sulfide grains like microscopic stalagmites ¹ . These discoveries revealed a fundamental gap in our understanding of space weathering—particularly how sulfur behaves on airless bodies bombarded by cosmic forces.

For decades, planetary scientists assumed solar wind radiation was the primary driver of surface alteration. But when laboratory simulations failed to fully replicate the mysterious sulfur depletion and whisker formation, researchers realized another cosmic sculptor must be at work: micrometeoroid bombardment. This article explores the revolutionary discovery of metal-sulfide agglutinates—space-forged composites that rewrite our understanding of asteroid surface evolution.

Cosmic Alchemy: Space Weathering's Secret Recipe

The Birth of Agglutinates

Asteroid surfaces are constantly reshaped by space weathering—a combination of solar wind irradiation, cosmic rays, and micrometeoroid impacts. When microscopic impactors strike at hypersonic speeds (up to 20 km/s), they generate instantaneous temperatures exceeding 1,100°C. This flash-melts surface minerals, creating glassy composites called agglutinates that cement surrounding particles together ¹ .

Unlike lunar agglutinates (iron-dominated), asteroidal varieties contain abundant sulfide minerals like pentlandite ([Fe,Ni]₉S₈) and pyrrhotite (Fe₁₋ₓS). These sulfur-rich compounds behave dramatically differently when heated:

The Whisker Enigma

Hayabusa's return of Itokawa samples revealed S-depleted rims on sulfide grains adorned with filamentous Fe-Ni structures up to 3 µm long. Initially attributed to solar wind, their origin remained unexplained until 2025, when transmission electron microscope (TEM) experiments uncovered a startling formation mechanism ¹ .

The Smoking Gun Experiment: Simulating Cosmic Impacts

Methodology: Inside the Space Weathering Simulator

Researchers designed a groundbreaking experiment to recreate micrometeoroid strikes:

1. Sample Preparation: Natural pentlandite grains were crushed into micron-sized particles mimicking asteroidal regolith ¹ .
2. Vacuum Chamber: Particles were loaded into a TEM equipped with a laser heating system replicating impact conditions (10⁻⁶ mbar pressure).
3. Thermal Pulses: Samples received millisecond-scale laser pulses reaching 1,100°C—matching micrometeoroid impact temperatures.
4. Progressive Bombardment: Each sample endured up to three heating cycles to simulate repeated impacts.
5. In Situ Analysis: High-resolution imaging and spectroscopy tracked real-time changes in chemistry and microstructure ¹ .

Revolutionary Findings

After the first thermal pulse:

• Grains developed rounded morphologies indicating partial melting
• Curved Fe-Ni whiskers sprouted from surfaces (Fig. 1b)
• Whiskers were entirely sulfur-free despite originating from S-rich pentlandite

Cosmic Implications: Rewriting Asteroid History

Solving the Sulfur Depletion Mystery

This experiment demonstrated that micrometeoroid impacts:

Ryugu's Organic-Sulfur Connection

Analysis of Hayabusa2 samples revealed even more complexity:

• Thiol organosulfurs coat sulfate grains in Ryugu particles
• Calcium thiosulfate grains show anomalous δ³⁴S values (+12‰)
• Isotopic signatures point to presolar photochemistry in molecular clouds ⁹

This suggests sulfur participates in both mineralogical and organic space weathering cycles—a revelation for understanding prebiotic chemistry.

The Psyche Mission Connection

NASA's upcoming mission to metal-rich asteroid Psyche will directly observe these processes. As Dr. Bose explains: "Sulfides record both the early solar system environment and active processes on airless bodies today" ^{1 9} . Instruments will map surface sulfides to test models of impact-driven sulfur loss.

The Scientist's Toolkit: Decoding Sulfur Signatures

Conclusion: Sulfur's Double Life in Cosmic Evolution

Sulfur's role in solar system evolution is now revealed as a tale of two environments:

This dual narrative transforms sulfur from a mere mineral component to a central player in both surface weathering and deep planetary evolution. As sample return missions from Bennu and Ryugu continue to yield surprises, and Psyche launches toward its metal world, we stand poised to decode sulfur's cosmic diary—one whisker at a time.