The Allan Hills 84001 (ALH 84001) martian meteorite is an approximately four billion-year-old igneous rock composed primarily of the mineral orthopyroxene. It was discovered in Antarctica in 1984 and subsequently studied for its record of the early geological history of Mars. It also garnered attention due to claims from some researchers that it contained microscopic evidence for preserved bacterial fossil remains. Although those claims have since been questioned by other members of the scientific community, there is ongoing debate regarding the origins of its organic material. ALH 84001 provides an opportunity to identify mechanisms of synthesis of organic materials to determine whether the preserved carbon was generated by abiotic or biotic pathways, the latter of which could indicate the presence of life on Mars.
Andrew Steele of the Carnegie Institution for Science and colleagues have recently analyzed ALH 84001 using transmission electron microscopy, nanoscale secondary ion mass spectrometry, and scanning transmission X-ray microscopy to explore the mineralogical, textural, organic, and isotopic composition of this rock. Their investigations focused on previously identified alteration assemblages including a magnetite-rich zone and a carbonate globule. They found that these areas showed textures of intergrown, sawtooth-patterned orthopyroxene, amorphous silicates, and talc-like phases. Refractory organic carbon in these areas contained aromatic compounds. This organic carbon is associated with high deuterium (heavy H) values, consistent with an indigenous source of water on Mars. Their observations indicated that the spatial distribution of minerals and organics formed through abiotic hydrothermal reactions of serpentinization and mineral carbonation, rather than previously suggested mechanisms such as biogenic production and thermal decomposition of siderite. These findings indicate that organic carbon was produced abiotically early in the geologic history of Mars and could be used to explain the presence of species such as methane that are considered potentially relevant to habitability. READ MORE