Meteoroids from the Moon and Mars

Prior to the 1969 Apollo lunar landings, meteorites represented the only extraterrestrial material available for scientific study. In 1979, scientists found an unusual meteorite in Antarctica that resembled a certain type of moon rock. Numerous comparative studies confirmed that this Antarctic meteorite was of lunar origin and the result of an impact event. Subsequent studies on a small group of anomalous meteorites provided evidence to support their Martian origin.

Overview

Throughout human history, people have been aware of rocks falling from the sky. In earlier times, they were thought to be either gifts from the gods or the result of severe weather picking up rocks from one location and then dropping them at another. Later, as human curiosity developed into science, people began to look differently at these strange rocks and associated the appearance of a bright fireball with the fall of a meteorite. They also noted that meteorites were composed of different substances. Some were made of nearly pure metal, while others had the appearance of some volcanic rocks. Scientists later performed chemical and mineralogical analyses that identified elements and minerals in meteorites that were either rare or unknown on Earth. Eventually, all this evidence led to the conclusion that meteorites originated in space and were not from Earth itself.

Meteorites were once the only extraterrestrial material available for scientific study. That changed when the six Apollo lunar landings returned to Earth with more than 383 kilograms of rocks and other lunar samples. Later, Soviet Luna missions returned a small sampling of lunar material. Among the rock specimens collected were examples of breccia, basalt, anorthosite, and gabbro. Along with regolith samples, this sampling of lunar rocks has provided scientists with enough information to gain a good understanding of the Moon’s composition, internal structure, and geological history throughout time. However, Apollo samples were limited to only six locations, and a couple of these sites were very similar to each other in their geological settings. Vast areas of the lunar surface, including the lunar far side, were not represented. Scientists were anxious to get their hands on rock specimens from these unexplored regions. Little did they know that meteorites would once again provide the required research material.

Along with the 1969 Apollo lunar landings, another rather important event took place in planetary science that year. In Antarctica, a team of Japanese scientists working in the Yamato Mountains region came across nine meteorites sitting on the ice in close proximity to one another. Later, after these meteorites had been classified, it was determined that they represented several different falls. This was extraordinary and suggested that Antarctica might be a “treasure house” for meteorite finds. Subsequent search teams have recovered tens of thousands of meteorites, confirming the theory that Antarctica offers a unique situation for meteorite finds. Found among this huge number of specimens is the full range of stone, stony-iron, and iron meteorite types. Also present are a relatively large number of the biologically important carbonaceous chondrites and a variety of achondrites.

During the 1979–80 Antarctic field season, another group of Japanese scientists searching for meteorites in the Yamato Mountains recovered what was at first believed to be an anomalous achondrite. Upon later examination, researchers recognized the appearance of a lunar breccia. After additional analyses and comparison to Apollo lunar rocks, this specimen was determined to be a lunar meteorite. It had been blasted off the Moon’s surface during a large impact event and was later pulled to Earth and entered the atmosphere just like any other meteorite. Confirming that this specimen came from the Moon could not have been possible without the availability of the Apollo lunar specimens for comparison.

Confirmation that lunar rocks have reached the surface of Earth as meteorites excited the scientific community. Not only did these finds provide additional lunar material for study, but they also represented material from areas on the Moon that had not been explored by the Apollo astronauts. Each year, as more lunar meteorites were being found in Antarctica, meteorite dealers and collectors from around the world began to search other locations to find lunar meteorites. Soon, the deserts of Australia, Northwest Africa, Libya, and Oman became prime search areas for meteorites. Within a relatively short period, meteorites found by the local inhabitants began turning up in marketplaces and were quickly purchased by mineral dealers. Gradually these meteorites found their way to the scientists for study. Found among the huge numbers of ordinary meteorites were the occasional gems, the lunar meteorites and the equally exciting SNC meteorites.

Prior to the discovery of lunar meteorites in Antarctica, a small group of achondrite meteorites called SNCs (pronounced “snick” and standing for shergottites, nakhlites, and chassignites) could not be readily explained. They were different from most meteorites because of their relatively young radiometric ages and the fact that they were essentially volcanic rocks. When compared to the isotopic chemistry of Earth or Moon rocks, they were clearly different, and their origin was unknown. At this time, it was generally believed that all meteorites came from the asteroid belt as a result of numerous impact events early in the history of the solar system. This clearly was not the case for the SNCs. As early as 1979, based on the new understanding of lunar meteorites, some scientists suggested that the SNCs might have come from Mars. Initially, most scientists did not readily accept this theory, but that quickly changed. A couple of the suspected Martian meteorites contained glass in which a tiny amount of gas had been trapped long ago while the rock was on Mars. When scientists analyzed this gas, it was found to have a nitrogen and noble gas content that closely matched the gases found in the Martian atmosphere, as sampled by the Viking landers in 1976. Also, many geologists believed that the massive volcanoes on Mars, such as Olympus Mons, could have been active at the time that these meteorites were believed to be on Mars. When scientists connected the relatively young age of the SNC meteorites to the volcanoes, it seemed to make sense that the SNCs might be of Martian origin.

To many scientists, it seemed quite clear that the best place of origin for these intriguing SNC meteorites had to be Mars. For others, the evidence was not as convincing. The main argument against a Martian origin for the SNC meteorites rests on the fact that scientists do not have a documented sample of a Martian rock for comparison. No astronauts or robotic explorers have collected samples from the Martian surface and returned them to Earth. With no definitive Martian rocks for comparison, the SNCs cannot be verified as truly Martian. The robot explorers Pathfinder, Spirit, and Opportunity have traveled across Mars and viewed hundreds of rocks close up, but they have not seen a type of rock that matches any of the SNC meteorites. Although this lack of evidence does not preclude the possibility that SNCs are of Martian origin, it will take a major sampling effort by both robotic and human explorers to make that determination, and therefore, the debate over the origin of the SNC meteorites will continue to spark scientific discussions for years if not decades to come. NASA is hopeful that its Mars Perseverance rover, which has been collecting new samples from the planet, will eventually be able to return those samples to Earth for further study.

Knowledge Gained

The basic study of meteorites paved the way for the development of the scientific technology that was required for the examination of materials brought back from the Moon by the Apollo astronauts. Geologists and geochemists studying meteorites have a more difficult task than those who study Earth rocks. The average meteorite fall usually consists of a relatively small amount of material. Proper classification and additional detailed studies require destructive analyses. With only a small amount of material available, scientists had to develop analytical techniques that required only minimal amounts of the precious material available to them. Instruments such as the electron microscope, electron and ion microprobes, and mass spectrometer gave researchers tools to explore the secrets of the universe from the chemical composition of a meteorite. As is usually the case, technology specifically developed for pure scientific research was immediately integrated into the commercial market and became valuable tools for industry and medical applications.

The rock and regolith samples returned by the six Apollo landings were not representative of the entire Moon and its major geological terrains. Many questions concerning the Moon’s origin and relationship to Earth could not be answered with only the Apollo material. Scientists would have to wait patiently for some future missions to bring back samples from different areas. The 1979 find of lunar meteorites in Antarctica changed all that, giving scientists the opportunity to study additional lunar materials. A lunar meteorite represents material blasted off the Moon’s surface during an impact event; hence, it does not represent a single location but could have originated anywhere on the lunar surface. By examining lunar meteorites that originated from different locations on the Moon, scientists have been able to construct a more comprehensive picture of lunar geology.

The case for Martian meteorites proved to be equally exciting, especially with the meteorite ALH 84001. This unique meteorite was found during the 1984–85 field season in the Allan Hills region of Antarctica. Initially classified as an anomalous achondrite, it was later reclassified as a SNC Martian meteorite. Then, in 1996, after years of intensive study, scientists at the National Aeronautics and Space Administration (NASA) announced the discovery of possible microscopic fossil bacteria within carbonate deposits present in the meteorite. This revelation created much excitement within the scientific community and sparked intense controversy and debate. Additional studies provided evidence that the features in the meteorite resembling bacteria could have been the result of an inorganic process, too. Neither side in the debate could definitely prove the other wrong, so in the end, the “fossil” was deemed inclusive with regard to the evidence of organic material, and scientists concluded that additional evidence was required.

Although the argument for fossil bacteria in ALH 84001 was not fully accepted, it inspired other scientists to look for microscopic evidence of life in more promising meteorites, notably the carbonaceous chondrites. The recognition of lunar and Martian meteorites also suggests that planets can exchange material with each other and possibly distribute the essential chemical compounds of life through giant impacts.

In 2013, a team from the University of New Mexico's Institute of Meteoritics reported on their analysis of a meteorite, Northwest Africa 7034 (NWA-7034), nicknamed "Black Beauty," which had been discovered in the Sahara Desert in 2011. Led by Carl Agee, the team stated that NWA-7034 may be the first known Martian meteorite to come from the planet's surface or crust. It differs from other Martian meteorites in several key ways, including its high water content, older age, chemical composition, and texture, and offers researchers unique insight into planetary conditions circa two billion years ago during what scientists think was an important period in Martian history. The meteorite also provides a frame of reference for the Mars Curiosity rover as it looks for mineral samples in the Gale Crater.

Context

Confirmation of lunar meteorites led to the possibility of meteorites coming from Mars. Once scientists became comfortable with the idea that rocks could be blasted off the Moon’s surface and pulled to Earth, they expanded their options to include other worlds. Because of its relative closeness to Earth and similar geological features, Mars became the logical first choice. Using the same technology developed for lunar studies, scientists began to look at a number of intriguing anomalous meteorites (SNCs) with uncertain places of origin. By comparing gas compositions and isotopic data from these meteorites to data obtained through the Viking Mars Project, it was concluded that they could have come from Mars in a manner similar to the way lunar meteorites arrived. Although this Mars-origin theory for these meteorites is not conclusive, most scientists seem to have accepted it as fact and moved on from there. Now, other anomalous types of meteorites are being examined for evidence that may have them coming from Mercury or possibly Venus. Perhaps one day, a particularly interesting meteorite may produce an ancient piece of Earth rock tossed out by an early giant impact event that has finally returned home.

Bibliography

Agee, Carl B., et al. "Unique Meteorite from Early Amazonian Mars: Water-Rich Basaltic Breccia Northwest Africa 7034." Science 339.6121 (2013): 780–85. .

Bevan, Alex, and John De Laeter. Meteorites: A Journey through Space and Time. Washington, DC: Smithsonian Institution P, 2002.

Greicius, Tony, and Brian Dunbar. "Researchers Identify Water Rich Meteorite Linked to Mars Crust." NASA: Mars. NASA, 3 Jan. 2013.

Hartmann, William K. Moons and Planets. 5th ed. Belmont: Thomson, 2005.

Humayun, Munir. "Planetary Science: A Unique Piece of Mars." Science 339.6121 (2013): 771–72.

Kring, David A. “Unlocking the Solar System’s Past.” Astronomy 34.8 (2006): 32–37.

Lauretta, Dante S., and Harry Y. McSween, eds. Meteorites and the Early Solar System II. Tucson: U of Arizona P, 2006.

“Mars Rock Samples Collected by Perseverance Rover.” NASA Mars Exploration, mars.nasa.gov/mars-rock-samples/#20. Accessed 15 Sept. 2023.

NASA. "Meteors & Meteorites: Read More." NASA: Solar System Exploration. NASA, 2 July 2013.

Norton, O. Richard. The Cambridge Encyclopedia of Meteorites. Cambridge: Cambridge UP, 2002.