Orthopyroxenites represent one of the rarest and most scientifically significant groups of Martian meteorites. This category is uniquely defined by a single specimen: Allan Hills 84001 (ALH 84001), discovered in Antarctica in 1984.
ALH 84001 is composed almost entirely of orthopyroxene, indicating crystallization deep within the Martian crust more than 4 billion years ago, making it one of the oldest known rocks from Mars. Its mineralogy and textures differ fundamentally from volcanic Martian meteorites such as shergottites, nakhlites, and chassignites, recording early crustal processes rather than surface lava flows.
In the mid-1990s, ALH 84001 attracted worldwide attention following the discovery of carbonate globules and microscopic features initially interpreted as potential biosignatures. Subsequent studies demonstrated that these carbonate formations resulted from low-temperature alteration processes and that the debated features were best explained by abiotic mechanisms, with several lines of evidence pointing to terrestrial contamination after the meteorite’s arrival on Earth. While the biological interpretation has been rejected, the meteorite remains of exceptional importance for understanding aqueous alteration, shock events, and early geochemical processes on Mars.
