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No ‘nanofossils’ in martian meteorite

Abstract

Elongated, segmented forms found on fracture surfaces within the martian meteorite ALH84001 have been proposed to be martian ‘nanofossils’1, even though they appear too small to be fossilized bacteria2,3,4. We have examined similar forms and find that the majority are (non-biological) lamellar growth steps on pyroxene and carbonate crystals. Their segmented surface microstructures are laboratory artefacts resulting from the deposition of conductive heavy-metal coatings.

Main

Using transmission electron microscope (TEM) imaging, the definitive method for identifying nanofossils5, we have previously observed magnetite whiskers but no nano-fossils on or near the surfaces of the carbonates6. We suggested that some of the purported nanofossils1 might be similar high-temperature, vapour-deposited magnetite whiskers, but the dissimilarity between specimen preparation and electron-beam analysis techniques used in the two studies makes direct comparisons difficult.

We have now examined fracture surfaces from carbonate-rich regions using the techniques described by McKay et al.1. Chips of ALH84001 with exposed carbonate and pyroxene crystals were mounted, coated with 2-20 nm of gold or gold/palladium, and examined in a field emission scanning electron microscope (FE-SEM) using secondary electron imaging and incident electron beam energies of 2-20 keV. We measured mineral compositions using energy dispersive X-ray spectroscopy (EDS).

The surface topography is highly irregular on a nanometre scale, with emergent lamellae following the major cleavage direction of the substrate (Fig. 1a ). Some regions exhibit typical pyroxene cleavage, whereas others display lamellar features (Fig. 1b ). Close examination of these features over a range of tilt angles reveals that they are angled directly into the pyroxene substrate and are the emergent edges of substrate lamellae. Many of these lamellae have segmented, curvilinear surface microstructures resembling the elongated forms proposed to be nanofossils by McKay et al.1.

Figure 1: ‘Elongated forms’ in the meteorite ALH84001.
figure 1

a, Secondary electron image of pyroxene surface. Parallel elongated forms (from upper left to lower right) are emergent lamellae related to substrate cleavage direction. Inset, pyroxene surface oriented such that elongated forms resemble nanofossils. Both surfaces have a 10-nm-thick coating of gold, measured by TEM imaging of ultramicrotomed cross-sections. b, Secondary electron images of highly fractured pyroxene surface. Inset, high-magnification image of lamellae. Coating: 20 nm Au, resulting in more pronounced segmentation. c, Secondary electron image of carbonate crystals (coated with 9 nm of Au/Pd). The average grain size is 9. d, Secondary electron image of carbonate surface within fracture zone. Inset, high-magnification image of elongated forms on carbonate, some of which may be magnetite whiskers6. Coating: 10 nm Au.

The surfaces of carbonates within the fracture zones of ALH84001 are also decorated with emergent lamellae (Fig. 1c, d ). Although we suspect that some are magnetite whiskers (composition and crystal structures of magnetite cannot be established by SEM-EDS), their abundance far exceeds that of the whiskers that we described previously6. As with the pyroxenes, the lamellae are angled directly into the carbonate substrate and their orientations are controlled by local substrate cleavage directions.

The segmented surface microstructures of the elongated forms in ALH84001 (Fig. 1a, b ) are largely an artefact of the conductive metal coating. Similar coating artefacts have previously been misidentified as (terrestrial) nanofossils7. Au and Au/Pd coatings produce segmentation on elongated forms, although Au/Pd imparts a finer-scale segmentation (as seen in the images in refs 1 and 8). In addition, the segmentation becomes more pronounced as the coating thickness is increased7 (compare Fig. 1a and b ). As noted by McKay et al.1, specimen charging is a problem with some chips of ALH84001, because the minerals are insulators and because some chips are shattered (shock-fractured) on a submicrometre scale. A conductive coating is desirable for secondary electron imaging, and the optimum coating (composition and thickness) is a trade-off between charging effects, surface artefacts (enhanced segmentation) and required image resolution.

The complexity of the pyroxene and carbonate fracture surfaces in ALH84001 probably reflects the superposition of severe shock and/or thermal events on indigenous surface microstructures of the minerals9,10. Many of the lamellae relate to internal cleavage planes within the pyroxene and carbonate substrates, and their orientations may reflect localized shock deformation, partial melting, or even the onset of secondary alteration (for example, clay formation11). Where lamellae are exposed on fracture surfaces, their enhanced secondary electron yield (a characteristic of lamellar surfaces12,13) and segmented surface structures (an artefact of the metal coating) combine to produce the nanofossil-like appearance.

Although some of the elongated forms of ALH84001 could conceivably be martian nanofossils, the majority appear to be either emergent substrate lamellae or magnetite whiskers6. Some lamellae may exhibit indigenous segmentation14, but conductive metal coatings both produce and accentuate segmentation which is particularly prominent in high-resolution FE-SEM images. TEM imaging is therefore more appropriate for distinguishing coated lamellae from ‘nanofossils’, as it is possible to observe internal microstructures characteristic of non-biogenic6 and biogenic5 ‘elongated forms’.

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Bradley, J., Harvey, R., McSween, H. et al. No ‘nanofossils’ in martian meteorite. Nature 390, 454–455 (1997). https://doi.org/10.1038/37257

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