Their home is our destination.
MATERIALS WITH A LONG JOURNEY
Lunar meteorites are fragments of the Moon that have reached the Earth in a form of a meteorite. They are exceedingly rare. As of today, 17th of October 2019, there are 387 lunar meteorites of the total 61,929 meteorites registered in the NASA´s International Meteoritical database. They are compared to the material brought back from the lunar missions and are on display in the natural history museums around the world.
HOW DID LUNAR METEORITES REACH THE EARTH?
Lunar surface is due to the absence of atmosphere fully exposed to the bombardment of meteorites. Most of them are the size of dust grain. However, in rare cases of large asteroid impacts, the rocks from the surface of the Moon are ejected into the space. Because of the relatively small Moon's mass, the escape velocity of the Moon is only 2.38 km/s. Escape velocity of an astronomical object is the minimum speed a particle needs to escape its gravitational influence. Once material escapes Moon`s gravity, rock fragments are captured by either Earth`s or Sun`s gravitational fields. Thousands to several million years later rocks from the Moon may fall on the Earth as meteorites.
HOW DO WE RECOGNISE THEM?
After their find, meteorites are examined by meteorite experts specialized in analyzing space material for laboratory research. Classified material is then entered into the Meteoritical Bulletin Database from the Lunar and Planetary Institute https://www.lpi.usra.edu/meteor/.
Lunar meteorites are recognised upon following:
There are two main types of lunar meteorites: feldspatic regolith breccias and mare basalts.
Feldspatic breccias originate from the lunar highlands. They were formed when the initial asteroid impact shattered the rock, and the next one glued the rock back together. These rocks are highly fragmented, composed of light-colored clasts in a dark fine-grained matrix. They are composed mostly of the mineral plagioclase. Plagioclase (anorthite) is a greyish mineral, giving the Moon its familiar greyish color in the night sky.
Mare basalts are volcanic rocks that formed 3.9 - 1.5 billion years ago when several large asteroids protruded Moon`s mantle creating circular basins, which are today known as lunar seas or maria. Basins were subsequently filled with basaltic lava. Since there are way fewer lunar seas than lunar highlands, these meteorites are less common types of already rare Lunar meteorites. They are mainly composed of minerals pyroxene, olivine, and plagioclase.
According to the Meteoritical Bulletin*, the lunar meteorite NWA11273 is "breccia composed of mineral clasts of anorthite, olivine, exsolved pigeonite, pigeonite, augite, chromite, Ti-Cr-Fe spinel, kamacite, taenite and troilite in a finer grained matrix containing small vesicles and minor barite. Rare basalt clasts and glass fragments are also present."
The simple mineralogical composition of Lunar rocks means that the chemical composition of their minerals is characteristic and highly predictable. Most of the plagioclase is calcium-rich. Lunar meteorites are mainly recognized upon geochemistry based analysis.
For example, the lunar meteorite on the image 1 (NWA 11273) is classified in the Meteoritical Bulletin as feldspathic breccia. The chemical composition of minerals is following*: (Olivine (Fa8.7-59.7, FeO/MnO = 89-111), pigeonite (Fs28.8Wo11.2, FeO/MnO = 56), clinopyroxene host (Fs15.3Wo40.9, FeO/MnO = 44), orthopyroxene exsolution lamella (Fs34.0Wo2.7, FeO/MnO = 56), augite (Fs16.8Wo41.7, FeO/MnO = 62), plagioclase (An95.9-96.5Or0.2).
Absence of metal as a consequence of the Moon`s formation
Unlike most of the other meteorite types, lunar meteorites don`t contain any significant quantities of metal. Therefore they are not magnetic and can not be detected with metal detectors. According to the giant-impact hypothesis, the Moon was formed when Earth collided with Mars-sized planet 4.6 billion years ago, that is couple hundred million years after the formation of our Solar System. During that time heavy elements such as iron and nickel have already sunk to the Earth`s core. The collision was only powerful enough to eject in space lighter elements from the Earth`s crust and mantle, from which later the Moon was formed. This also explains the lower density of the Moon. Lunar meteorites are among the least dense outer space material.
Stable isotopes are kind of "fingerprints" of rocks. In nature, there are three different stable isotropies of oxygen 16^O, 17^O and 18^O. The ratios between the isotopes are characteristic for each celestial body in the solar system. In Lunar meteorites, the ratios are similar to the Earth´s rocks, suggesting the common origin of both celestial bodies.
Cosmic radiation and solar wind exposure
Lunar meteorites are so distinctive from terrestrial, that they can not be forged. The direct exposure of lunar rocks to the cosmic radiation has caused crystal damage in some minerals. Inside of lunar rocks, there is a small amount of trapped gases (hydrogen and noble gases) that were brought by the solar wind. When lunar rocks enter the Earth`s atmosphere, the gases are released, causing the formation of the bubbles in the fusion crust of the meteorites.
LUNAR METEORITE UNDER THE ELECTRONIC MICROSCOPE
WHERE CAN I SEE LUNAR METEORITES?
Lunar meteorites can be seen in natural history museums all over the world. The Natural History Museum in Vienna has the oldest and largest meteorite display in the world. They are curated next to the material brought back from the lunar missions.
Next to museums, world renowned collectors hold timeless collections of precious outer space material.
Writing: Bojan Ambrozic PhD, Master of Science in Meteoritics