Martian meteorites

Rocks from Mars, recognised by gas from the Martian atmosphere trapped inside them.

Classification

Stony meteorites › Achondrites › Planetary achondrites › Martian meteorites

Share of witnessed falls

about 0.5%

In our catalogue

121 meteorites, 5 seen to fall

Heaviest

Northwest Africa 16788, 24.67 kg

Earliest recorded fall

Chassigny, 1815

Most recent fall

Tissint, 2011

Martian meteorites were once called SNC meteorites, after their three original groups: shergottites, nakhlites and chassignites. Several hundred are now known. They are igneous rocks, mostly far younger than other meteorites, and some contain water-formed minerals.

The proof came in 1983, when gas trapped in shock-melted glass inside the Antarctic shergottite EETA 79001 was found to match the Martian atmosphere measured by the Viking landers in 1976. Their oxygen isotopes also form a distinct group of their own.

Only five have been seen to fall: Chassigny (France, 1815), Shergotty (India, 1865), Nakhla (Egypt, 1911), Zagami (Nigeria, 1962) and Tissint (Morocco, 2011). Their ejection ages, measured from cosmic-ray exposure, show they were thrown off Mars in a handful of impacts between about 0.7 and 20 million years ago.

Mineralogy and chemistry

Martian meteorites are igneous rocks: basalts, olivine-rich basalts, pyroxene and olivine cumulates, and breccias. Their minerals include pyroxene, olivine, plagioclase (often turned to glass, called maskelynite, by shock), magnetite, ilmenite, sulphides and phosphates. Compared with asteroidal achondrites they are more oxidised and richer in volatile elements, and several contain minerals formed by water: clays, carbonates, sulphates and salts.

Their oxygen isotopes lie slightly above the terrestrial line, with a Δ17O of about +0.3 per mil, and all Martian meteorites share it. Gases trapped in shock-melted glass match the composition of the Martian atmosphere measured by the Viking landers and the Curiosity rover, including its distinctive ratios of nitrogen, argon and xenon isotopes.

Origin and ages

Most shergottites crystallised between about 150 and 600 million years ago, with some about 2.4 billion; the nakhlites and chassignites about 1.3 billion years ago; Allan Hills 84001 about 4.1 billion years ago; and zircons in Northwest Africa 7034 about 4.4 billion. Such young volcanic ages could only come from a large planet that stayed geologically active. Cosmic-ray exposure ages cluster into several groups, from about 1 million to about 20 million years, showing they were launched by a small number of impacts, each sending off many fragments. Researchers are trying to match each group to a young crater on Mars.

Abundance and famous examples

Several hundred Martian meteorites have been named, many of them paired pieces of the same fall, with a total mass of a few hundred kilograms. Only five have been seen to fall: Chassigny (France, 1815), Shergotty (India, 1865), Nakhla (Egypt, 1911), Zagami (Nigeria, 1962) and Tissint (Morocco, 2011). Elephant Moraine A79001 from Antarctica is the stone in which trapped Martian air was first identified.

How to recognise one

Martian meteorites look like terrestrial basalts or peridotites. Fresh ones have a black fusion crust, sometimes glossy, as on Tissint. Many contain shock-melt glass pockets and veins. They are weakly to moderately magnetic, because of magnetite and pyrrhotite. Only laboratory analysis of oxygen isotopes and mineral chemistry can confirm a Martian origin, and claimed 'Mars rocks' offered for sale should come with a proper classification.

Groups

Shergottites

The most common Martian meteorites: young basaltic and olivine-rich rocks.

99 in the catalogue

Nakhlites

Augite-rich volcanic rocks from Mars, about 1.3 billion years old.

15 in the catalogue

Chassignites

Olivine-rich Martian rocks; the first known fell in France in 1815.

2 in the catalogue

ALH 84001 and other Martian rocks

The ancient Martian rock once claimed to hold fossils, and 'Black Beauty'.

130 in the catalogue

Famous examples

• Nakhla Martian (nakhlite), Egypt, 1911 · 10 kg
• Shergotty Martian (shergottite), India, 1865 · 5 kg
• Chassigny Martian (chassignite), France, 1815 · 4 kg
• Allan Hills 84001 Martian (OPX), Antarctica, 1984 · 1.93 kg
• Tissint Martian (shergottite), Morocco, 2011 · 7 kg
• Zagami Martian (shergottite), Nigeria, 1962 · 18 kg
• Northwest Africa 7034 Martian (basaltic breccia), 2011 · 320 g

Related groups

• Shergottites The most common Martian meteorites: young basaltic and olivine-rich rocks.
• Nakhlites Augite-rich volcanic rocks from Mars, about 1.3 billion years old.
• Chassignites Olivine-rich Martian rocks; the first known fell in France in 1815.
• ALH 84001 and other Martian rocks The ancient Martian rock once claimed to hold fossils, and 'Black Beauty'.
• Lunar meteorites Rocks from the Moon, found mainly in deserts and Antarctica.

See all 121 Martian meteorites

Photos shared under Creative Commons licences (Wikimedia Commons).