Sometimes life can be very frustrating!
Just when you thought you had it all figured out, along comes a new sample and everything changes.
It’s so annoying, isn’t it?
Well no, actually that’s the fun thing about studying meteorites. You just never know what’s around the corner.
So, now we have a new meteorite sample from Mars that doesn’t want to play by the rules. It’s called Northwest Africa 7034 (NWA 7034) and has already got a lot of media coverage.
What’s all the fuss about? Here’s the deal:
Unlike the Moon, samples from Mars have yet to be returned to Earth by spacecraft. Instead, our understanding of Martian geology is based on remote sensing observations by rovers and satellites, and through laboratory studies of meteorites from Mars. Our current suite of Martian meteorites consists of 114 officially named specimens, of which 5 are witnessed falls. Martian meteorites are sometimes also referred to as SNCs; an acronym based on the three most famous falls in the group: Shergotty, Nakhla and Chassigny. The shergottite suite are the most important Martian sub-group (96 specimens), followed by the clinopyroxene-rich Nakhlites (13 specimens), the olivine-rich chassignites (2 specimen), and three other samples including the famous orthopyroxenite ALH 84001, the ungrouped sample NWA 2646 and now NWA 7034.
Professor Carl Agee, Director of the Institute of Meteoritics, University of New Mexico, who led the study of the Martian meteorite NWA 7034. The sample he is holding is the iron meteorite Sikhote-Alin. (Inage: Carl Agee)
In their recent Science paper on NWA 7034, Carl Agee and co-workers point out that the SNC meteorites show a relatively poor match to the geochemical data for the exposed Martian crust obtained by recent NASA orbital and lander missions. Thus, the basalts analysed by the MER Spirit rover at Gusev crater are significantly different to the SNC meteorites, as is the average Martian crust based on Odyssey Orbiter measurements. In contrast, Agee and co-workers suggest that the average bulk composition of NWA 7034 gives a much closer match to the Martian crust as defined by this remote sensing data.
Samples from Mars have yet to be returned by astronauts from the red planet. What we know (or think we know) about Mars comes from remote observations by rovers or orbiting satellites, and laboratory study of Martian meteorites. A new study suggests that our present collection of Martian meteorites may not be representative of the surface of the red planet. Instead a newly discovered meteorite NWA 7034 may be a better match. But NWA 7034 is like no Martian meteorite seen before. (left image: Curiosity Rover; upper right: Mars Odyssey; lower left: Nakhla meteorite) (images: NASA).
So what sort of meteorite is NWA 7034?
NWA 7034 is a basaltic breccia consisting of a complex mixture of up to millimetre-sized crystals and rock fragments set in a fine-grained matrix. The rock fragments show quite diverse textures, ranging from very fine to quite coarse-grained types. The dominant minerals in NWA 7034, plagioclase feldspar and pyroxene, display very wide compositional ranges. It is suggested that the meteorite is a “monomict” breccia. This is important because the term “monomict” means that all the material in the meteorite is from a single source and so does not contain a foreign component introduced during an impact event.
Left-hand image: Main mass of NWA 7034 with saw-cut surface revealing heterogeneous, brecciated texture. White crystals are plagioclase phenocrysts, dark crystals are pyroxene phenocrysts. Clasts are plagioclase-pyroxene aggregates, melt pockets, and other domains resembling volcanic spherulites. Right-hand image:BSE image showing NWA 7034 pyroxene phenocrysts (light gray), plagioclase phenocrysts (dark gray), oxides and sulfides (white), and fine-grained matrix. Plagioclase-pyroxene clast is partially visible above the scale bar. (Caption text and images: Agee et al. 2012)
And why is this important?
Well, NWA 7034 has one particularly unusual property. It has an oxygen isotope composition unlike any other Martian meteorite. You see Mars is a planet and like the other terrestrial planets it grew at the expense of other smaller bodies. Significant amounts of energy would have been released during such collisional growth, resulting in extensive melting and leading to the development of a magma ocean. It is widely believed that this would have resulted in homogenization of the oxygen isotope composition of Mars, as appears to have also been the case for the other planetary bodies for which we have samples: Earth, Moon and the asteroid 4 Vesta. In fact all of the Martian meteorites that have been measured so far show only limited oxygen isotope variation and scatter about a single unique line, often referred to as the Mars Fractionation Line. Rocks from the Earth and Moon fall on a distinct line called the Terrestrial Fractionation Line and samples from the asteroid 4 Vesta define a third line. As can be clearly seen in the diagram below, analyses of NWA 7034 do not plot on the Mars Fractionation line. (A group of meteorites known as the angrites are also shown in the diagram below, however, it remains unclear what sort of parent body they are derived from).
Oxygen isotope composition of NWA 7034 in relation to other Martian meteorites (SNCs), meteorites from Vesta (HEDs) and angrites. Rocks from the Earth and Moon plot on the terrestrial fractionation line (TFL).
So if NWA 7034 is from Mars how can we explain this?
Agee and co-workers suggest that unlike the Earth and Moon, Mars never experienced a global magma ocean event. Instead, they suggest that the differentiation of Mars may have been dominated by basin-forming impacts, or even hemisphere-wide igneous complexes, each of which had a unique oxygen isotope and geochemical composition. One potential objection to this model is the fact that apart from NWA 7034 all other Martian meteorites define a single distinct line. If Mars really was heterogeneous with respect to oxygen isotopes then a blurring of the distinction between the two groups might be expected. A more radical explanation might be that NWA 7034 samples a different planet to the other SNCs. The 2,100 million year crystallization age of NWA 7034 strongly suggests that it is from a planet with a long-lived geological history. So perhaps NWA 7034 is from Mars and the other SNCs are not? This is unlikely, because one of the most important pieces of evidence linking the SNCs to Mars are the similarities in isotopic composition between the Martian atmosphere, as measured by the NASA Viking landers, and trapped gases in SNCs such as EETA 79001 and Tissint.
A further possibility to explain the oxygen isotope data for NWA 7034 is that the meteorite does in fact contain a foreign component introduced during impact processes. This would make the breccia polymict rather than monomict. Polymict breccias are common on the asteroid 4 Vesta and it is now known that these can have oxygen isotope compositions that were modified by impact-related events. However, for the moment there is no evidence to support this possibility in the case of NWA 7034.
There remain a lot of unanswered questions about the origin of NWA 7034. However, it is already clear that it is an important and unique sample that will provide significant new information about the geology of Mars. As long as it really does come from Mars of course!
Follow the link here to watch the facinating video put together by Carl Agee’s team at the University of New Mexico about their work on NWA 7034.
Blog image credit: Erfoud, Morocco is a major centre for the international trade in meteorites. NWA 7034 was purchased in 2011 by Jay Piatek from a Moroccan meteorite dealer Aziz Habibi. The sample is now on deposit with the Institute of Meteoritics, University of New Mexico. (image: Wikipedia)
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