034, which had not yet received its official, certified name from the Nomenclature Committee of the Meteoritical Society [ MetSoc link ], they realized what had been sitting on Carl Agee's shelf: a piece of Mars. Dr. Piatek generously donated a slice of the valuable (scientifically and financially) sample to the UNM Institute of Meteoritics.
|Three pieces of NWA 7034 (cube is 1 centimeter on a side). Note how dark and smooth the surfaces are, typical of the fusion crust of a fresh, unweathered meteorite. The striking dark color led Carl Agee and his colleagues to nickname the rock "Black Beauty."|
Certain chemical, mineralogical, and textural features about space rocks are so distinctive that cosmochemists can make startlingly quick decisions that a rock is asteroidal, lunar, Martian, or just some kind of Earth rock. Sometimes we can tell just looking at the sample. Other times we need to look at a thin slice of it in an optical microscope. Still other times, and to be sure, we need to determine some key chemical parameters, such as the ratio of iron (Fe) to manganese (Mn), or oxygen isotopic composition. Identifying the original home of Black Beauty required some chemistry. The ratio of Fe/Mn is useful because these two elements behave quite similarly during melting and crystallization (within some geochemical limits), so their concentration ratio reflects their ratio in the planetary body in which they formed. The diagram below shows the Mn and Fe concentrations in the mineral pyroxene in NWA 7034, compared to Martian meteorites, Earth rocks, and lunar rocks.
|Concentrations of manganese (Mn) and iron (Fe) in pyroxene crystals in NWA 7034. The units are atomic formula units, often used for minerals, which show the relative abundances of atoms of ions making up a mineral. The blue dots are data from NWA 7034; the lines show the trends for Martian meteorites (red line labeled "Mars"), Earth rocks (purple), and lunar samples (green). The dots fall along the Mars line.|
One of the most definitive measures of planetary heritage is the oxygen isotopic composition. (Oxygen has three isotopes with masses of 16, 17, and 18.) Although some types of meteorites overlap in oxygen isotopic composition, most do not. The mix of oxygen isotopes in NWA 7034 is different from all other meteorite types and the Earth (and Moon, which is indistinguishable from Earth). NWA 7034 is slightly different from the other Martian meteorites (see diagram below), but not so different to lead Carl Agee and his colleagues to conclude that the rock did not come from Mars. Instead, they suspect that the slight difference reflects some inherent variation in the oxygen isotopic composition of the Martian interior. Another possibility is that oxygen isotopic abundances were modified by interaction with the Martian atmosphere, which is thought to be higher than the meteorites in Δ 17O (the vertical displacement of any point from the terrestrial fractionation line on the plot shown below). A third, more speculative, possibility is that an exotic cometary or carbonaceous chondrite component was added to the rock through impact processes on Mars.
|Oxygen isotopic abundances in NWA 7034 compared to Martian meteorites and the line representing Earth rocks (TF, for Terrestrial Fractionation line). The plot shows the 17O/16O and 18O/16O ratios in the meteorite samples in parts per thousand, also called per mil (‰). Data have been standardized to standard mean ocean water (SMOW) and plotted as deviations from that value. The important points are that the NWA 7034 data are similar to, yet slightly different from, those of Martian meteorites and they are distinctly different from the terrestrial line and (though not shown) from any other meteorite type. Note that the oxygen isotope measurements were made in two different laboratories, one at the University of California, San Diego (UCSD) and one at the University of New Mexico (UNM).|
Any expert on Martian meteorites would not immediately shout, "Hey, this is a Martian meteorite!" NWA 7034 does not look like any of the