Martian meteorites in depth

Martian meteorites are our only samples of Mars. Their petrology, ages and isotopes reveal the planet's volcanic history, water and atmosphere.

Shergottites

Shergottites, named after Shergotty, are the most abundant Martian meteorites. They are mafic to ultramafic igneous rocks, subdivided by texture into basaltic shergottites, olivine-phyric shergottites with large olivine crystals in a finer groundmass, and poikilitic shergottites with olivine enclosed in large pyroxene crystals.

Their plagioclase is typically converted to maskelynite by shock. Geochemically they range from depleted to enriched in incompatible elements, reflecting different mantle sources.

Most shergottites have crystallisation ages between about 150 and 600 million years, showing that Mars was volcanically active geologically recently. A few, such as NWA 7635, are about 2.4 billion years old, and some researchers argue that they came from the same volcanic region as younger shergottites. Shock-melt glass in shergottites such as Elephant Moraine 79001 trapped the Martian atmospheric gases that first proved their origin.

Nakhlites and chassignites

Nakhlites are clinopyroxenites, cumulate rocks made mainly of augite with some olivine, formed in lava flows or shallow intrusions. Chassignites are dunites, made largely of olivine. Both crystallised about 1.3 to 1.4 billion years ago, and their cosmic-ray exposure ages, about 11 million years, suggest they were ejected together in a single impact.

Many nakhlites contain iddingsite, a mixture of clay minerals, carbonates and iron oxides formed by water. This alteration happened on Mars, as shown by its isotopic composition and its truncation by fusion crust in some samples. It records liquid water in the Martian crust long after the rocks crystallised.

The nakhlites differ in cooling rates and textures, and some researchers have proposed that they represent several flows or layers within one igneous body.

ALH 84001 and NWA 7034

Allan Hills 84001 is an orthopyroxenite that crystallised about 4.09 billion years ago, making it one of the oldest known Martian rocks. It contains carbonate globules that formed around 3.9 to 4.0 billion years ago from fluids. In 1996, a team proposed that features in these carbonates were biogenic. Subsequent work demonstrated plausible non-biological explanations for each feature, and the scientific consensus does not accept the claim, but the debate stimulated the field of astrobiology.

Northwest Africa 7034, nicknamed Black Beauty, and its paired stones are regolith breccias containing clasts of many rock types. They are rich in water compared with other Martian meteorites. Zircons within them have been dated to around 4.4 to 4.48 billion years, showing that Mars had a crust very early. Their composition resembles the average Martian surface measured by orbiters and rovers better than most other Martian meteorites.

Launch sites and limitations

Martian meteorites come from a limited number of ejection events. Cosmic-ray exposure ages cluster in groups, implying perhaps around ten or so separate impacts, each launching rocks of related types. Researchers have proposed specific source craters by combining crater ages, compositions and modelling, though most identifications remain debated.

The collection is biased. Strong, young igneous rocks survive ejection and entry better than weak sedimentary rocks, so Martian meteorites under-represent the sedimentary and altered materials seen by rovers.

That is why returned samples, collected with documented geological context, would be so valuable. Meteorites provide many samples but no context; a sample-return mission would provide a few samples with full context. Both kinds of evidence are needed for a complete picture of Martian history.

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