Planetary achondrites

Rocks knocked off the Moon and Mars by impacts.

Classification

Stony meteorites › Achondrites › Planetary achondrites

In our catalogue

286 meteorites, 5 seen to fall

Heaviest

Northwest Africa 16788, 24.67 kg

Earliest recorded fall

Chassigny, 1815

Most recent fall

Tissint, 2011

A few achondrites come not from asteroids but from much larger worlds. A large impact can throw rock off the Moon or Mars faster than escape velocity, and some of it eventually reaches Earth. Lunar and Martian meteorites are rare and highly prized.

Escaping a planet

To leave the Moon, a rock must travel faster than about 2.4 km/s; to leave Mars, about 5 km/s. For many years scientists doubted that any impact could launch rock that fast without melting or vaporising it. The answer lies in spallation: near the surface, the outgoing shock wave from an impact meets its own reflection, which cancels much of the pressure, so surface rocks can be flung off at high speed while being only moderately shocked. Martian meteorites are typically shocked to pressures of 20 to 45 gigapascals, enough to turn their feldspar to glass, but not to melt them.

Once in orbit around the Sun, the rocks travel for thousands to tens of millions of years before some cross Earth's path. Cosmic rays striking them during the journey create new isotopes, which record how long they were in space.

How their home is proved

Lunar meteorites match Apollo and Luna samples in their minerals, their chemistry and their ratio of iron to manganese, about 60 in pyroxene, and their oxygen isotopes lie on the terrestrial line. Martian meteorites carry trapped gas that matches the Martian atmosphere measured by spacecraft, distinct oxygen isotopes, and young crystallisation ages that only a large, geologically active world could produce.

Abundance and value

Several hundred lunar and several hundred Martian meteorites have been named, but many are pieces of the same falls, so the true number of separate meteorites is much smaller. Most are finds from the hot deserts of north-west Africa and Oman, and from Antarctica. No confirmed meteorite from Mercury or Venus has been found; Venus's thick atmosphere and high escape speed make it very hard for rock to leave, and Mercury lies deep in the Sun's gravity.

Planetary achondrites are the only rocks we have from Mars, and sample the Moon far more widely than the missions that brought rock back. They are highly prized by collectors, so they are also the most often faked; buy only with proper classification records.

They also act as ground truth for spacecraft. Comparing their minerals with spectra measured from orbit helps scientists read the make-up of distant surfaces, and their ages help calibrate crater counting, the method used to date regions of the Moon and Mars that have never been sampled.

Groups

Lunar meteorites

Rocks from the Moon, found mainly in deserts and Antarctica.

165 in the catalogue

Martian meteorites

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

121 in the catalogue

Related groups

• Lunar meteorites Rocks from the Moon, found mainly in deserts and Antarctica.
• Martian meteorites Rocks from Mars, recognised by gas from the Martian atmosphere trapped inside them.
• Achondrites Igneous stones from bodies that melted: asteroids, the Moon and Mars.

Photos shared under Creative Commons licences (Wikimedia Commons).