Planetary Science: Meteorites

I recently picked up a fantastic book called Worlds Apart: A Textbook in Planetary Sciences. It’s an older text from 1994, but still contains a wealth of information about the planets and moons of our solar system, as well as some discussion on meteorites, asteroids, the sun, and origins of our solar system.

I thought it would be fun to start a series based on content in the book. I’ll be summarizing what the book presents as well as providing some additional details from other sources (which you can find at the bottom of each post).

What is a meteorite?

meteorite is defined as a chunk of rock and/or metal from space that has entered Earth’s atmosphere and impacted the ground. For the sake of completeness, it is called a meteoroid when it travels through space and a meteor while it is travelling through Earth’s atmosphere.

The History of Meteorites

Believe it or not, the idea the meteorites originated from the sky was laughed at in scientific circles until the early 19th century. The idea that bits of the heavens could fall down to Earth didn’t align with the view of an eternal and unchanging universe that was prolific during that time. The first paper to propose that “fireballs in the night sky” were actually true physical phenomena was published by Ernst Chladni in 1794, a German Physicist. The paper was concisely named Über den Ursprung der von Pallas gefundenen und anderer ihr ähnlicher Eisenmassen und über einige damit in Verbindung stehende Naturerscheinungen (“On the Origin of the Iron Masses Found by Pallas and Others Similar to it, and on Some Associated Natural Phenomena”) [2]. If you can read German, you can read the original version online here, courtesy of Saxon State and University Library Dresden (unfortunately I was unable to find an English translation).

laigle_meteoryt
L’Aigle Meteorite (image courtesy of Wikimedia Commons)

The first accepted evidence was not found until 1803 when the French Academy of Sciences dispatched the young Jean-Baptiste Biot to the town of L’Aigle after the town residents reported a rain of stones falling from the sky [3]. Painstaking field work by Biot confirmed that the area around the town was littered with dark stones that were unlike any native to the area.

Chondrites vs. Achondrites

Meteorites can be classified into two main categories: chondrites and achondrites. Chondrites possess chondrules, while achondrites do not. Chondrules (from the Greek word, chondros or grains) are little beads of rock found inside meteorites. So how and when did these chondrules form?

chondrules
A cross-section of a chondritic meteorite, showing the component chondrules. Sizes of the chondrules range from less than 1 mm to several mm in diameter.

The solar system started out as a cloud of gas and dust. As the cloud collapsed under its own gravity, it formed a flattened disk with a protosun at the center and other fragments orbiting the sun. Much of the material ended up accreting to form the planets. The rest of the material collected together in little clumps, much like dust bunnies collect under our bed [1]. Once the Sun reached a critical mass, it ignited, sending a blast of intense heat across the solar system. This wave of pressure and heat melted the clumps of dust, forming molten beads (chondrules). Some of the beads agglomerated together to form even larger objects. The internal heat of some of these objects was high enough to melt the chondrules together, yielding achondritic meteorites. Smaller objects had a lower internal heat, such that some chondrules survived, resulting in chondritic meteorites [4].

If you would like more information on chondrules, Northern Arizona University’s Meteorite Laboratory has a fantastic page on chondrules with some beautiful examples of different types of chondrules.

Types of Chondrites

Chondrites can be split into three main categories: ordinary, carbonaceous, and enstatite.

Ordinary chondrites are composed primarily of the minerals olivine, pyroxene and bits of metallic iron. They are broken down further by their iron content: H (very high), L (low), and LL (very low). Ordinary chondrites also tend to contain many chondrules.

Carbonaceous chondrites– as the name implies- contain carbon as well as hydroxides and complex hydrocarbons. They also tend to have a high water content. Carbonaceous chondrites are further broken up depending on how well they compare with other well-known carbonaceous chondrites.

Lastly, enstatite chondrites are mainly composed of the mineral enstatite. They also contain metals and iron sulfide in small amounts. An interesting fact about enstatite chondrites is that they contain minerals that have not been found to occur naturally on the Earth, such as osbornite, a form of titanium nitride [1].

Types of Achondrites

Similar to chondrites, achondrites can be subdivided into several different classes as well.

Beginning with basaltic achondrites, these meteorites are composed of basalts. Several specimens bear a striking resemblance to rocks recovered from the moon. Others are theorized to have come from a larger asteroid. One theory I find very thought provoking is that several types of basaltic achondrites were rocks blasted into space from volcanoes on Mars that eventually were ensnared by Earth’s gravitational pull. If this is true, it lends some credence to the idea of panspermia.

One famous example of a Martian meteorite is ALH84001, found in 1996. Some interesting features of this meteorite led researchers to believe it contained evidence of fossilized bacterial life [5]. These claims were subsequently disputed.

alh84001s
A view of ALH84001 under an electron microscope. The “worm-like” features in this image were initially believed to be the fossilized remains of microscopic bacteria.

Ureilites are an exotic type of stony meteorite that contain tiny diamonds. One possible explanation for the formation of the diamonds is that an ordinary meteorite had a high energy impact with a carbon-rich asteroid.

The other main class of achondrites is enstatite achondrites. These meteorites are similar to the chondrites of the same name, in that they formed under conditions of little oxygen and excess sulfur.

Scratching the Surface

This post has meant to serve merely as an introduction into the fascinating science of meteoritics. Study of this heavenly debris is helping to answer some of the most pressing questions in astronomy today as the origins of our solar system. For more information on the topic, please consult some of the links in the references below as a starting point.

References

[1] Consolmagno, Guy, and Martha Schaefer. Worlds apart: A textbook in planetary sciences. Vol. 1. 1994.

[2] Ernst Chladni. Über den Ursprung der von Pallas gefundenen und anderer ihr ähnlicher Eisenmassen und über einige damit in Verbindung stehende Naturerscheinungen. Saxon State and University Library Dresden.

[3] L’Aigle meteorite. Wikipedia.

[4] Chondrules and the Origin of Meteorites. Northern Arizona University – Meteorite Laboratory.

[5] Allan Hills 84001. Wikipedia.

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