BRIEF HISTORY OF MARTIAN METEORITE ALH84001
On 27 December 1984, during a snowmobile ride on the far western Allan Hills icefield, located at the eastern terminus of the Transantarctic mountain range (76°54’S, 157°01’E), a National Science Foundation team discovered the ALH84001meteorite (http://curator.jsc.nasa.gov/antmet/mmc/index.cfm). The field notes documenting the find describe it as a “highly-shocked, grayish-green, achondrite, 90% covered with fusion crust (with the additional comment ‘Yowza-Yowza’)” (http://curator.jsc.nasa.gov/antmet/mmc/index.cfm). Since this meteorite was visually the most unusual rock collected during the 1984-1985 field season, it was the first to be processed and was classified (49, 54) incorrectly as a rare achondritic meteorite known as a diogenite. It took another 9 years before it was correctly identified as a Martian meteorite (47, 53) based on oxygen isotope analysis (18).
ALH84001 is the oldest of the 18 age-dated samples (49) in our current inventory of 26 identified Martian meteorites (http://curator.jsc.nasa.gov/antmet/mmc/index.cfm). It has a radiogenic Rb-Sr crystallization age of 4.5 billion years (49; Nyquist et al., Abstr. Pap. Submit. Lunar Planet. Sci. Conf. 26:1065-1066, 1995), indicating that it formed shortly after the planet Mars itself was formed. Note that the crust of Earth, unlike Mars, is continually being modified, either being created or destroyed. As a result, there are no rocks on Earth that have an age comparable to the age of ALH84001. The oldest known terrestrial samples are zirconium grains with U-Pb ages of 4.4 billion years (62). The oldest known terrestrial rocks are in the Acasta Gneiss Complex in North America, dated at 4.03 billion years old (56), and the Isua Supercrustal rocks in west Greenland, dated at 3.7 to 3.8 billion years old (1). ALH84001 was subsequently ejected into interplanetary space from the Martian surface 16 million years ago (30), presumably as a consequence of a collision of an asteroid or comet with Mars (46). About 13,000 years ago it was captured by the Earth’s gravity field and fell as a meteorite in Antarctica (35).
With a mass of 1.94 kg, ALH84001 is primarily a volcanic, silicate-rich rock composed mainly of orthopyroxene [(Mg, Fe)SiO3] with minor phases of chromite (FeCr2O4), olivine [(Mg, Fe)SiO4], pyrite (FeS2), apatite [Ca3(PO4)2], and Si-rich glass (47). Secondary carbonate mineral assemblages, formed at temperatures compatible with terrestrial microbes (51, 60, 61), range in size from 1 to 500 µm and are found within small cracks and fissures in the meteorite (45, 47). They comprise 1% (vol/vol) of the rock (47) and have radiogenic Rb-Sr ages of 3.90 ± 0.04 billion years (11), indicating that they formed while the rock was still on Mars. The 13C isotopic composition (40 relative to Pee Dee Belemnite) for the carbonates further substantiates the Martian origin of these secondary minerals (51). The carbonate globules consist of an optically golden core concentrically zoned in Ca, Mn, Fe, and Mg carbonate (45, 51, 60, 61) in which nanometer-size magnetites (Fe3O4) are evenly distributed as a minor phase (45, 57, 58). Surrounding the core is an inner rim and an outer rim composed mainly of tens-of-nanometer-size magnetite crystals embedded in a Mg-rich Fe carbonate matrix (45, 57, 58), which are separated by a band of nearly pure Mg carbonate (45). Optically, this appears as a layered black-white-black rim (Fig. 1). Since the carbonate globules are Martian, then by association the magnetites are also of Martian origin. In support of this we note that even though the Antarctic environment is oxidizing, little or no conversion of the magnetites to maghemite and hematite has occurred. Magnetite is a mixture of Fe and O, with one-third of the Fe atoms occurring as Fe2+ and two-thirds of the atoms occurring as Fe3+. The near-surface environment on the ice fields of Antarctica is oxidizing, and nearly all Antarctic meteorites show evidence of rust on their outer surfaces (29). Nanometer-size magnetite crystals exposed to atmospheric levels of oxygen rapidly oxidize to maghemite (-Fe2O3;Fe3+) or hematite (-Fe2O3;Fe3+) (48). The fact that little or no conversion of the magnetites to maghemite and hematite has occurred in ALH84001 suggests that during the meteorite’s residence time in Antarctica, there was little terrestrial alteration of the carbonate assemblages.
Thomas-Keprta, K. L., Clemett, S. J., Bazylinski, D. A., Kirschvink, J. L., McKay, D. S., Wentworth, S. J., … Romanek, C. S. (2002). Magnetofossils from Ancient Mars: a Robust Biosignature in the Martian Meteorite ALH84001. Applied and Environmental Microbiology, 68(8), 3663–3672. https://doi.org/10.1128/aem.68.8.3663-3672.2002. (Full article)