Allan Hills A81005 or ALH A81005 (sometimes also named without the "A" in front of the number) was the first meteorite to be recognised as a lunar meteorite. The meteorite Yamato 791197 was discovered in 1979 but its lunar origin was not recognised until 1984. ALH A81005 was found in 1982 in the

Allan Hills The Allan Hills () are a group of hills, mainly ice free and about long, lying just north-west of the Coombs Hills near the heads of Mawson Glacier and Mackay Glacier in the Oates Land and Victoria Land regions of Antarctica. Exploration and n ...

at the end of the

Transantarctic Mountains The Transantarctic Mountains (abbreviated TAM) comprise a mountain range of uplifted rock (primarily sedimentary) in Antarctica which extends, with some interruptions, across the continent from Cape Adare in northern Victoria Land to Coats L ...

, during a meteorite gathering expedition ( ANSMET).

Discovery and naming

ALH A81005 was found on 17 January 1982 by John Schutt and Ian Whillans. It is named after the

, a mountain chain in

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

where many

meteorites A meteorite is a rock that originated in outer space and has fallen to the surface of a planet or moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical interactions with the atmospheri ...

are gathered by expeditions. The large number of meteorites collected in Antarctica and the lack of geographic terms that could be used for names have led to the adaption of the "Antarctic rules" for meteorite naming. Every meteorite found in Antarctica receives the names of the collection area (Allan Hills) and a number. The number consists of the year the expedition started "81" and a three digit number that is given out consecutively (005). The "A" in front of the number stands for meteorites collected by ANSMET expeditions and can be considered optional. The definition of the year is used because the year changes during the Austral summer season (December to March) and this avoids samples from one expedition having different years. This is the reason ALH A81005 has the year "81" in its name despite being found on 17 January 1982.

Description

ALH A81005 measures . It has a dark fusion crust on the outside. The interior is made up of a black to dark grey groundmass (

matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...

) with larger grey and white angular crystals ( clasts). This appearance is typical for

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

s, including those originating on Earth. The size of the larger crystals ranges from sub-millimeter to in diameter. Thin section analysis revealed that the crystals are mostly

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

, with some

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

and

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

. It was also discovered that the meteorite had similarities to terrestrial

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

. Microprobe analysis showed that the

was very calcium-rich. The crystals are a solid solution of 97%

anorthite Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

albite Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicat ...

. The

s have a variable composition lying between enstatite, ferrosilite and

wollastonite Wollastonite is a calcium Silicate minerals, inosilicate mineral (calcium, Casilicon, Sioxygen, O3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or D ...

. The

is a solid solution of 11 to 40%

fayalite Fayalite (, commonly abbreviated to Fa) is the iron-rich endmember, end-member of the olivine solid solution, solid-solution series. In common with all minerals in the olivine, olivine group, fayalite crystallizes in the orthorhombic system (spac ...

with the rest being

forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich Endmember, end-member of the olivine solid solution series. It is Isomorphism (crystallography), isomorphous with the iron-rich end-member, fayalit ...

. ALH A81005 is classified as a "lunar anorthosite breccia" and belongs to the group "lunar anorthosite" (abbreviated Lun-A).

History

The determination that ALH A81005 was of lunar origin was made by Robert Clayton and Toshiko Mayeda, researchers at the

University of Chicago The University of Chicago (UChicago, Chicago, or UChi) is a Private university, private research university in Chicago, Illinois, United States. Its main campus is in the Hyde Park, Chicago, Hyde Park neighborhood on Chicago's South Side, Chic ...

, following the determination by

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...

scientist Brian Harold Mason that the meteorite was similar in chemical and isotopic composition to rocks returned by the

Apollo program The Apollo program, also known as Project Apollo, was the United States human spaceflight program led by NASA, which Moon landing, landed the first humans on the Moon in 1969. Apollo followed Project Mercury that put the first Americans in sp ...

astronauts from lunar highland areas. Evidence that ALHA 81005 is a lunar sample, was presented at the 18 March 1983 meeting of the

Lunar and Planetary Institute The Lunar and Planetary Institute (LPI) is a scientific research institute dedicated to study of the Solar System, its formation, evolution, and current state. The Institute is part of the Universities Space Research Association (USRA) and is ...

. The evidence included fabric data, mineralogical data, compositional data, oxygen isotope data,

noble gas The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...

data,

cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...

exposure history,

magnetic properties Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...

, nuclear particle tracks, and thermoluminescence data.

References

{{Meteorites by name , state=collapsed Lunar meteorites Meteorites found in Antarctica Transantarctic Mountains