An orthopyroxenite is a type of Mars meteorite that is currently represented by only one known specimen, ALH 84001, which was found in the Allan Hills region of Antarctica in 1984. It is a cumulate rock consisting of 97% coarse-grained, magnesium-rich orthopyroxene, with small amounts of plagioclase, chromite, and carbonate. It was initially classified as a member of the HED group, specifically a diogenite, because these achondrites are also made mostly of orthopyroxene. However, the presence of oxidized iron in the chromite of ALH 84001 led to its reclassification as a Martian meteorite; a fact that has been subsequently confirmed by its oxygen isotope composition. With a crystallization age of about 4.4 billion yr, it is by far the oldest meteorite from the fourth planet yet discovered. It probably represents a sample of the early Martian crust, providing evidence for the earliest geologic history of Mars.


However, public and much scientific attention has been focused on a minor aspect of this unique rock during the past years – the presence of small orange-colored carbonate spherules that probably formed 3.9 billion yr ago. These "orangettes" are barely visible to the naked eye, ranging up to 200 microns in size, and they seem to have formed in the presence of liquid water within fractures inside ALH 84001. In 1996, a team led by NASA's David McKay published a paper announcing the discovery of traces of fossil Martian life within the orangettes, consisting of organic molecules, several biominerals, and microfossils that resemble terrestrial nanobacteria. Ever since then, his discovery this been vigorously debated, splitting the scientific community into advocates and prosecutors of the existence of (former) primitive life on Mars. Future missions to Mars will confirm whether we are alone in a hostile universe, or if there are unsuspected neighbors in our own Solar System, thus implying a biophile nature for our universe where life is not the exception but the rule.