Sulfide minerals bear witness to impacts across the solar system

Abstract

Sulfide minerals are ubiquitous in extraterrestrial sample collections and serve as a unique record of the microstructural and chemical characteristics resulting from exposure to interplanetary space on airless planetary surfaces. Among these features are Fe-rich whiskers, identified in samples returned from asteroids and the Moon. While whisker production has previously been attributed to solar wind irradiation, the origin of these enigmatic features is poorly constrained. Here we perform in situ heating experiments to simulate micrometeoroid bombardment of sulfide minerals in the transmission electron microscope. Our results demonstrate that whiskers can form through impact events on airless surfaces and show that sulfides are recording evidence for micrometeoroid bombardment across the solar system. This work contributes to a better understanding of sulfur-depletion previously detected on asteroid surfaces and could be particularly important for the interpretation of data from upcoming missions to sulfide or metal-rich worlds, including asteroid Psyche.

Fig. 1. Results of in situ heating for small pentlandite grains.

a A small (<1 µm) pentlandite grain prior to heating in high-angle annular dark-field (HAADF) STEM imaging showing an angular morphology. b The same pentlandite grain as in (a) after heating once to 1100 °C, showing the rounded morphology and protrusions (whiskers) emanating from the surface; EDS maps show the whiskers, outlined in orange, are enriched in c Fe, and d Ni, but are depleted in e S. Source data are provided as a Source Data file.

Fig. 2. Results of in situ heating for large pentlandite grains, where colored insets correspond to the location of features identified in subsequent panels.

a A large (>1 µm) pentlandite grain after one rapid heating event to 1100 °C, and after b two heating events. Secondary electron (SE) imaging shows the production of whiskers on the grain surface after the first heating event and their degradation with subsequent heating; c, d Various whisker morphologies from regions identified in (a) including truncated and pointed whiskers. e Another pentlandite grain after one heating round, with a filamentous whisker shown in (f). After two heating events, the grain shows whisker degradation in (g) and EDS maps show that even after multiple heating protocols, the whisker in (h) is enriched in i Fe, and k Ni, but depleted in j S. Source data are provided as a Source Data file.

Fig. 3. TEM data of the whisker.

a The FIB section showing the cross section of the grain, including a whisker in the upper left region of the sample surface; b SAED patterns collected from the area outlined in the green circle identify the sample as crystalline pentlandite; EDS maps show the whisker is enriched in c Fe and d Ni and depleted in e S in cross-section. Source data is provided as a Source Data file.

Fig. 4. TEM data of the whisker, where colored insets correspond to the location of features identified in subsequent panels.

a HAADF image and b the TEM image. EDS data shows the grain is enriched in c Fe and d Ni, and depleted in e S. f A line profile extracted from the grain shows the quantitative enrichments and depletions from the grain interior to the whisker; g HRTEM image showing the microstructure of the whiskers which includes crystalline strands interspersed throughout an otherwise amorphous structure. The underlying pentlandite grain is crystalline but enriched in plane defects (e.g., stacking faults). Source data are provided as a Source Data file.

Fig. 5. Schematic demonstrating the process through which whiskers formed during simulated micrometeoroid bombardment.

As temperatures rise, S begins to evaporate from the surface, leaving Fe-Ni-rich material, which is no longer stable as a liquid and quenches. This rapid solidification leads to compressive stresses in the exterior shell, which, when combined with thermal expansion of still-molten sulfide interior, results in whisker formation as a mechanism to relieve compressive stresses. The surface area to volume ratio of the starting particles determines the concentration of whiskers on the surface. Source data is provided as a Source Data file.