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Origins of Meteorites

Although Chladni convinced most scientists in the early 19th century that meteorites actually represent genuine rocks from space, nobody really knew where they came from. Did they have their origin on the Moon, on other planets, or did they actually represent chunks of matter wandering homelessly through our solar system? Were they parts of comets or stars, or did they even stem from outer space, some place beyond the confines of our known world? It took more than a century to unlock some of their secrets, and it required an interdisciplinary research, engaging methods of modern physics, astro-physics, astronomy, chemistry, and mineralogy, to reveal the origins of at least some specific groups of meteorites. Historic pamphlet, presuming a lunar origin
Were Meteorites Lunar Rocks?

The Asteroid Belt

Since the late 1950s, scientists have been able to determine the exact orbits of some meteorites, mostly ordinary chondrites, that were photographed by several cameras during their fall - e.g., Pribram, Czech Republic, Innisfree, Canada, and Lost City, USA. The orbits of two other ordinary chondrites, Farmington, USA, and Dhajala, India, have been calculated from independent observations of their falls by eyewitnesses. Each of these orbits has its aphelion, i.e., its most distant point from the Sun, in the asteroid belt, located between the orbits of Mars and Jupiter, which suggests that at least the ordinary chondrites are pieces of asteroids.

This view is consistent with what we know about asteroids. Most of them seem to represent undifferentiated bodies that never accreted into a larger planet because of the perturbing effects of Jupiter's massive gravitational field. Hence, they should be composed of primordial matter that remained more or less unchanged for the last 4.5 billion years - facts that hold true for ordinary chondrites and most other meteorites. Today, it is widely accepted that most meteorites have their origin in the asteroid belt, with the members of each group representing a common asteroidal parent body. >> top...

 

Most meteorites have their origin in the Asteroid Belt
Map of the Orbits of Certain Meteorites

Asteroidal Meteorites

Actually, the asteroid belt is populated by thousands of large asteroids, and yet more small to medium sized bodies. They vary widely from each other, so that looking for the actual parent body of a particular meteorite group is like looking for a needle in a haystack. However, the albedo and the reflectance spectra of asteroids provide the basis for a classification system, and they give us an idea of their compositions. The M-Type asteroids seem to be metallic in composition, exhibiting spectral signatures similar to those of iron meteorites. The C-Type asteroids are dark bodies with nearly featureless spectra, suggesting a carbonaceous composition. The S-Type asteroids represent a spectrally diverse class that probably includes both chondrite and achondrite parent bodies.

Researchers have thoroughly compared hundreds of individual asteroid spectra to meteorite spectra since the late 1980s. In some cases, they found strong similarities, suggesting that a certain asteroid might be the parent body of a certain meteorite class. In other cases, however, they found a close match, as for the H chondrites and the S-type asteroid 6 Hebe, named for the Greek goddess of eternal youth. Another prominent example is 4 Vesta, one of the largest asteroids in our solar system, which proved to be the parent body of the HED group achondrites - the eucrites, howardites, and diogenites. The spectral signatures of these highly differentiated rocks are as unique as the reflectance spectra of their parent body, Vesta.

With a diameter of about 530 km, Vesta is a spherical, differentiated body that shares many features with the terrestrial planets, e.g. Venus, Mars, and Earth. We can resolve a crater near its south pole that is 460 km wide and 30 km deep - the remnant of a huge impact that excavated Vesta's basaltic crust down to its mantle. Due to the immense force of this impact event, large chunks of matter were ejected from Vesta, forming smaller asteroids of similar composition - the so-called Vestoids. Some of these "children of Vesta" subsequently entered a near-Earth orbit, and they are thought to be the actual sources of the HED members that have made their passage to Earth in the form of meteorites. To access more detailed information on this subject, have a look at our HED page. >> top...

 

Asteroid 253 Mathilde, NEAR 1997, Copyright NASA
C-Type Asteroid 253 Mathilde

Asteroid 243 Ida, Galileo 1993, Copyright NASA
The S-Type Asteroid 243 Ida


A model of 4 Vesta, Hubble Heritage, Copyright NASA
Vesta - Home of the Eucrites

Planetary Meteorites

A few meteorites have been proven to have a more prominent origin. In the early 1980s, Japanese and American researchers discovered the first lunar meteorites in the ice fields of Antarctica - actual pieces of the Moon! Subsequently, several other lunaites have been recovered from the hot deserts of Australia, Africa, and Oman, bringing the total number of lunar meteorites to 25, excluding all probable pairings. All of these meteorites were found to exhibit mineral compositions similar to the samples returned by the Apollo and Luna missions, proving their lunar origin. However, they are of major scientific importance because they originate from areas of the Moon that were not sampled before. Most lunaites in our collections have been blasted from the lunar highlands that cover the far side of the Moon. Only a few lunar meteorites have their origin from the smooth lowlands, the maria of the near side, which served as the preferred landing sites for the Apollo missions. Have a look at our Lunar Meteorites page.

The meteorites of the SNC group are even more fascinating since they are genuine pieces of the planet Mars. With crystallization ages between 1.35 and 0.15 billion years, most SNC members are extremely young compared to other achondrites. Obviously, they formed on a parent body that has retained its igneous activity until very recent times, suggesting that the source of these strange rocks is a planet. In the 1980s, the discovery of trapped gas inclusions inside several SNC members provided a final answer. Based on the data obtained by the Viking probes, which landed on Mars in 1976, the composition of this trapped gas is more or less identical to the Martian atmosphere, convincing most scientists that the members of this group actually are genuine samples of our red neighbor, the planet Mars. Consequently, the SNC members are also known as Martian meteorites. Have a look at our Martian Meteorites page to learn more about these most unusual rocks from space. >> top...

 

Just about 25 meteorites are of lunar origin
The Home of Lunar Meteorites


Mars, captured by Hubble, Copyright NASA
The Home of the SNC Group

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