We are all aware of the 50th anniversary of the first Apollo Moon landing. However, we may not be aware of two other events that occurred that year: two significant meteorite falls, Allende and Murchison. Meteorites are our major source of material from other worlds, “rocks from space.”
At 1:05 am on February 8, 1969, a fireball (a very bright meteor) was seen over the Mexican state of Chihuahua. The fireball broke up in the atmosphere and ultimately yielded over 2,000 kilograms (4,400 pounds) of meteoritic material. This was the Allende meteorite, the largest carbonaceous chondritic meteorite ever found. There were probably thousands of pieces spread out over about 150 square kilometers (60 square miles) near the village of Allende.
At about 10:58 am on September 28, 1969, a fireball was seen over the state of Victoria in Australia. The fireball broke up into three pieces and over 100 kilograms (220 pounds) of fragments were eventually found spread out over an area of over 13 square kilometers (five square miles) near the town of Murchison, Victoria. This was the Murchison meteorite. If you did not already know or haven’t figured it out, meteorite falls are named after the location where they fell: a city, river, mountain range, etc.
Stony meteorites make up about 97% of all known meteorites, and chondrites make up about 94% of the stony meteorites. Ordinary chondrites make up more than 94% of the known chondrites (hence the term “ordinary”). Chondrites are primitive meteorites. A primitive meteorite refers to an undifferentiated meteorite that has a composition similar to that of the Sun, minus the “volatile” elements (hydrogen and helium, for example). Chondrites are stony (non-metallic) meteorites that are made up mostly of silicates, traces of iron, and sometimes carbon and water. Many, but not all, chondrites contain chondrules. Chondrules form as molten or partially molten droplets in space before being accreted to their parent asteroids. Chondrites represent one of the oldest solid materials within our Solar System and are believed to be the building blocks of the planetary system. Chondrites have not been modified due to melting or differentiation of the parent body.
Both Allende and Murchison are classified as carbonaceous chondrites. Carbonaceous chondrites represent about 4% of all chondrites (5% by mass). Allende is classified as a CV3 meteorite: C for carbonaceous chondrite; V for Vigarano, the first meteorite of the class, and 3 for the extent of alteration due to thermal metamorphism. Allende is noted for the presence of chondrules as well as abundant white calcium-aluminum-rich inclusions (CAIs), one of the condensates from the Sun’s cooling protoplanetary disk. Some CAIs have been dated as forming 4.568 billion years ago! Allende also contains about 0.3% carbon.
Murchison is classified as a CM2: M for Meghei, the first meteorite of the class, and 2 which implies no thermal metamorphism, but extensive alteration by water (probably less than 20 degrees Centigrade). Murchison contains small chondrules and abundant CAIs. It is rich in carbon, and 15 types of amino acids have been identified in it. Murchison also contains 12% by weight water in the form of clay minerals.
While the source asteroids, called parent bodies, of these two meteorites are not known, 75% of all asteroids are what are called C-group (“carbonaceous") asteroids. By mass, they represent much more than 75%, as three of the four largest asteroids are C-group asteroids.
Included in the C-group asteroids are asteroids 162173 Ryugu and 101955 Bennu, both of which have spacecraft in orbit around them that will be returning samples from their surfaces in the next few years.
Personally, Murchison is near and dear to my heart. When I first came to the University of Arizona many years ago, an associate of mine sent me a spectrum (reflected light as a function of wavelength, “color”) of Murchison and noted an absorption feature that was due to water in clay minerals. Knowing that most meteorites came from the asteroids, I was able to observe the asteroid 1 Ceres and was able to make the first definitive detection of clay minerals on asteroids! Here is a spectrum of Ceres vs. Murchison. Since then, it has been determined that the surface of Ceres is much more complex than previously thought. More recently, the OSIRIS-REx team has observed Bennu. Here is a spectrum of Bennu, comparing it to other CM meteorites and confirming the composition of the surface of Bennu.
To put the masses of Allende and Murchison into context, the Apollo missions brought back 382 kilograms (842 pounds) of lunar rocks. In comparison, the total known mass of Allende meteorites is over 2,000 kilograms (4,400 pounds), 6% of the total mass of all carbonaceous chondrites, and the total known mass of Murchison meteorites is 100 kilograms (220 pounds).