This article is presented in the spirit of science outreach associated with the Purple Mountain Observatory (PMO), Chinese Academy of Sciences (CAS). For educational sharing.

Meteorites are rocky specimens—fragments of extraterrestrial bodies—that survive their passage through Earth’s atmosphere and fall to the ground. Every year, hundreds of meteorites land in different corners of our planet. The vast majority, however, fall into the boundless oceans or sparsely populated deserts. Only a tiny fraction appear over densely inhabited regions as brilliant fireballs, flashing across the sky and vanishing in an instant—sometimes accompanied by rolling, thunder-like detonations.

For most people, such an astronomical spectacle is exceedingly rare; those who witness it with their own eyes are even fewer than lottery jackpot winners. For scientists, these meteorites are exceptionally precious: they are a crucial window into the origin of the Solar System. At present, apart from lunar samples brought back by NASA’s Apollo missions and dust returned from asteroid Itokawa by Japan’s Hayabusa spacecraft, meteorites are the only extraterrestrial rock samples that we can literally hold in our hands and study directly.

Meteorites are like messengers from the cosmos. They carry “codes” related to the formation and origin of the Solar System—and even of the universe itself—arriving in our world and waiting for scientists to decipher them one by one. So, among meteorites, which cosmic messengers are the most famous of all?

No. 1 — The Allende Meteorite

Why it made the list: It overturned humanity’s understanding of the Solar System and rewrote the history of Solar System evolution.

On 8 February 1969, at 07:05 GMT (01:05 local time), the village of Allende in Mexico’s state of Chihuahua experienced what is considered the largest carbonaceous chondrite shower in recorded history. The recovered total mass exceeded 2,000 kg, and over the past 50+ years, additional fragments have continued to be found in the region.

Allende is often regarded as “the most thoroughly studied meteorite in history,” for three main reasons:

1. An enormous amount of material (>2,000 kg).
   This made it possible for scientists worldwide to carry out rich and varied research programs. Thousands of scientific papers on Allende have been published; more than 100 papers on it have appeared in Science and Nature alone.

2. A once-in-a-lifetime timing advantage.
   The United States’ Apollo crewed lunar landing program was set to launch in July 1969. Prior to that, major laboratories around the world upgraded their most advanced research equipment and assembled large teams of planetary scientists—eagerly preparing for the first batch of lunar rock samples to be returned by Apollo 11. The Allende fall early that year was like a “timely rain”: it gave scientists a perfect “pre-game warm-up,” enabling them to demonstrate their capabilities by launching broad and in-depth investigations into this carbonaceous chondrite.

3. Exceptionally rich scientific content.
   The Allende meteorite (type CV3) contains abundant calcium–aluminum-rich inclusions (CAIs), which are thought to be among the earliest high-temperature condensates formed in the Solar System. These inclusions dramatically refreshed humanity’s picture of the early Solar System:

   • They include new minerals not found on Earth, such as panguite.
   • They revealed oxygen isotope anomalies in the Solar System.
   • Because CAIs formed extremely early, their formation age effectively represents the age of the Solar System itself, determined as 4.56732 ± 0.00016 billion years.

In addition, CAIs host multiple isotope anomalies (e.g., in calcium and aluminum) and record the Solar System’s highest abundance of the initial $(^{26}\mathrm{Al}/^{27}\mathrm{Al})$(26Al/27Al) ratio, about $5.25 \times 10^{-5}$5.25×10−5. Isotopic studies indicate that $^{26}\mathrm{Al}$26Al is produced by nucleosynthesis in red giants, AGB stars, and supernovae, and was delivered into the primitive solar molecular cloud via stellar winds. One hypothesis proposes that a supernova shock wave may have triggered the collapse of the solar nebula—or even directly contributed to the birth of the Solar System.

Because $^{26}\mathrm{Al}$26Al is a short-lived radionuclide with a half-life of roughly 0.7 million years, it functions like a “clock installed inside meteorites,” allowing scientists to constrain the time of the supernova explosion to less than about 2 million years before Solar System formation.

No. 2 — The Murchison Meteorite

Why it made the list: A beacon for exploring the origin of life.

Like Allende, the Murchison meteorite was another extraordinary gift from the sky in 1969. On 28 September 1969, at around 10:58 a.m. local time, a bright fireball over the town of Murchison in Victoria, Australia split into three flaming masses, fell, and disappeared—leaving a plume of smoke. About 30 seconds later, a shock wave was heard. Many meteorite fragments were found across an area exceeding 13 km, and the final collected mass exceeded 100 kg. This is the world-famous Murchison meteorite.

Murchison is among the rarest meteorite types known to humanity: a CM2 carbonaceous chondrite. Its primitive composition makes it like a time capsule, sealing away abundant information from the early Solar System. In terms of composition, it contains 22.13% total iron, about 12% water, and is rich in organic matter—making it one of the most extensively and deeply studied meteorites in the world. A large fraction of what we currently know about meteorite organic compounds comes from this sample.

More than 100 amino acids have been identified in Murchison. These include common amino acids such as glycine, alanine, and glutamic acid, as well as rarer amino acids such as isovaline, tert-leucine, and diamino acids. The range of organic compounds is extensive, with approximate abundances as follows:

• Amino acids: 17–60 ppm
• Aliphatic hydrocarbons: >35 ppm
• Aromatic hydrocarbons: 3319 ppm
• Fullerenes: >100 ppm
• Carboxylic acids: >300 ppm
• Hydroxy acids: 15 ppm
• Purines and pyrimidines: 1.3 ppm
• Alcohols: 11 ppm
• Sulfonic acids: 68 ppm
• Phosphonic acids: 2 ppm

In 2001, polyols were discovered in Murchison. In 2008, nucleobases—purines and pyrimidines—were also reported. Isotopic analyses indicate that these compounds are not the result of terrestrial contamination; rather, they came from outer space.

In January 2020, Proceedings of the National Academy of Sciences (PNAS) reported the discovery in Murchison of 40 relatively large presolar silicon carbide (SiC) grains. “Presolar grains” are mineral particles that existed before the Solar System formed. Their cosmic-ray exposure ages suggest these grains formed about 7 billion years ago, more than 2 billion years older than our Solar System. Presolar grains are the oldest solids that can currently be directly dated, providing remarkable physical samples for understanding our Milky Way before the Solar System existed.

Many compounds found in Murchison also exist within Earth’s terrestrial biosphere. These discoveries are profoundly significant for better understanding the birth and evolution of life on Earth. One hypothesis suggests that during the earliest stages of life’s emergence, organic compounds carried by countless small bodies impacting Earth “seeded” the early planet, gradually shaping it into the vibrant world we know today.

Even now, multiple theories about the origin of life remain in circulation. “The road ahead is long and winding; I will search high and low”—the famous line by Qu Yuan captures the spirit with which scientists continue to pursue this ultimate question.

No. 3 — The Martian Meteorite Allan Hills 84001 (ALH 84001)

Why it made the list: It sparked worldwide debate about life on Mars.

ALH 84001 is a Martian meteorite discovered on 27 December 1984 in Antarctica’s Allan Hills by the Antarctic Search for Meteorites (ANSMET) program. It weighs 1.93 kg. Isotopic dating indicates that ALH 84001 is among the oldest Martian meteorites ever obtained by humans. It formed about 4.1 billion years ago in the ancient southern highlands of Mars. Roughly 17 million years ago, it was ejected from the Martian surface by an impact event, overcame Mars’ escape velocity, and then fell onto the Antarctic ice sheet on Earth about 13,000 years ago.

In 1996, ALH 84001 made global headlines. A NASA-led research team published a paper in Science claiming the meteorite contained microfossils of Martian bacteria. Under a scanning electron microscope, tiny rod-like structures appeared clearly, with diameters around 20–100 nm, similar in size to the then-theorized “nanobacteria.” Scientists interpreted these as fossils of bacteria-like life forms. If these structures truly represented petrified life, they would constitute the most direct evidence yet for extraterrestrial life.

For a time, ALH 84001 was like a drop of water splashing into a boiling pan—igniting worldwide debate. U.S. President Bill Clinton even delivered an official televised statement, fueling boundless public imagination about life on Mars.

Today, most scientists hold that morphology alone cannot be used unambiguously as a tool to detect primitive life. Moreover, while the rod-like structures resemble some modern terrestrial bacteria and associated features, they are much smaller than typical Earth bacteria. As a result, many scientists argue these structures do not represent any form of life and may instead arise from non-biological mineral processes.

Subsequently, multiple organic compounds—including polycyclic aromatic hydrocarbons (PAHs)—were also reported in this Martian meteorite. In April 2020, nitrogen-bearing organic compounds were newly reported. However, the presence of organic matter remains far from a definitive indicator of extraterrestrial life.

Based on carbonate records in ALH 84001, Mars during the Noachian period likely had a near-surface aqueous environment. The co-variation of carbon and oxygen isotope ratios suggests these carbonates formed at a near-constant temperature of 18 ± 4 °C, through interactions among water, carbon dioxide in the ancient Martian atmosphere, and gradually evaporating groundwater—depositing carbonates as they formed. The water body may have been a shallow aquifer a few meters to a few tens of meters beneath the surface.

Thus, although whether Mars ever hosted life remains unknown, one point seems clear: when ALH 84001 formed more than 4 billion years ago, Mars was in a warmer and wetter era. The Martian surface very likely once held abundant liquid water—perhaps even ancient oceans.

The search for life on Mars is a long and difficult journey. With China’s first independently developed Mars probe, Tianwen-1, launched in July 2020, we will gradually come to understand the past and present of this red planet.

No. 4 — The Canyon Diablo Iron Meteorite

Why it made the list: It “leaked” the age of Mother Earth.

In the desert of northern Arizona, USA, there is a massive impact crater with a diameter of 1,200 m and a depth of 182 m. It formed about 50,000 years ago when an asteroid roughly 50 m in diameter struck Earth. This exceptionally well-preserved crater was the first meteorite crater on Earth to be scientifically confirmed, and it is named the Barringer Crater in honor of mining engineer Daniel Barringer. In the early 20th century, Barringer was among the first to propose an impact origin for the crater—and, with keen insight, he invested to purchase the crater site, leaving his descendants a substantial fortune.

Panoramic view of Barringer (Meteor) Crater, Arizona, USA.

In the 1950s, Caltech geologist Eugene Shoemaker and USGS Chinese-American geologist Zhao Jingde conducted multiple expeditions there and ultimately found the crucial evidence supporting an extraterrestrial impact origin: coesite and stishovite—two minerals that form when quartz experiences enormous shock pressure in an instant. This evidence finally ruled out the long-debated volcanic-origin hypothesis.

Later, large numbers of iron meteorite fragments were found around the Barringer Crater, with a total mass exceeding 30 tons; the largest single piece weighs 639 kg. It turns out that the visitor from space responsible for the crater was a multi-million-ton iron meteorite composed mainly of iron–nickel metal: the Canyon Diablo iron meteorite. Chemically it belongs to the IAB group and exhibits a coarse octahedrite structure.

Canyon Diablo made two major contributions to the history of science:

1. Dating the Earth.
   In 1953, Caltech professor Clair Cameron Patterson separated Canyon Diablo samples to obtain ultra-clean primordial lead. He measured the lead and uranium contents; because iron meteorites contain essentially no uranium, the lead content within them is not altered by uranium decay. Thus, the lead in iron meteorites can represent the lead composition at Solar System formation—i.e., the primordial lead composition at Earth’s formation. Combined with other meteorite data, Patterson determined Earth’s precise age as 4.55 ± 0.07 billion years, a value still widely recognized as one of the best estimates of Earth’s formation age.

2. A sulfur isotope standard.
   The mineral troilite (FeS) in Canyon Diablo meteorites has been widely used as an international reference standard for sulfur isotopes.

Today, Barringer Crater is a popular tourist destination. Visitors from around the world can experience nature’s grandeur firsthand and even physically touch the largest piece of this extraterrestrial visitor on-site. Because the crater is so similar to lunar craters, the location has also been used as a training base for American astronauts.

No. 5 — The Fukang Meteorite

Why it made the list: The ultimate favorite among meteorite enthusiasts worldwide.

In 2000, a local resident in Fukang, Xinjiang, discovered a meteorite weighing more than one ton in the Gobi Desert. Five years later, this meteorite—named after its fall/find location—suddenly appeared on the U.S. market. It was cut and sold publicly at USD 300 per gram.

The Fukang meteorite is a rock specimen formed 4.5 billion years ago deep inside a differentiated asteroid parent body, representing material from the asteroid’s core–mantle boundary. It is a stony-iron meteorite, also known as a pallasite. Roughly half consists of silvery iron–nickel metal and half consists of golden olivine crystals. It resembles an exquisite inlay artwork—dazzling under sunlight, like a treasure.

No. 6 — The Jilin Meteorite

Why it made the list: The largest stony meteorite—listed in the Guinness World Records.

At 15:00 on 8 March 1976, with an ear-splitting roar, an unprecedented meteorite shower descended, spectacular beyond description. This was the extraordinary stony meteorite shower that fell in Jilin, China—scattering over a plain region of about 500 km² across the outskirts of Jilin City, Yongji County, and near Jiaohe City.

This event is known as the Jilin meteorite fall, classified as an H5 ordinary chondrite, with a total recovered mass exceeding 4,000 kg. The largest single mass weighs 1.77 tons, making it the largest stony meteorite in the world. It is currently collected and exhibited at the Jilin City Museum. Such a spectacular meteorite shower is rare in human history; fortunately, it caused no casualties.

No. 7 — The Xinjiang Altay Iron Meteorites

Why it made the list: The world’s largest-scale meteorite strewn field.

In 2011, an enormous iron meteorite was discovered in the Altay region of Xinjiang: the Wuxilike iron meteorite, weighing 5 tons. Iron meteorites formed deep within differentiated asteroid parent bodies in the earliest Solar System and represent samples from the very core of such bodies.

Research indicates that the Wuxilike iron meteorite is paired with two other iron meteorites found in the same region—the Xinjiang iron meteorite (Armanty) and the Ulasitai iron meteorite. All belong to the III E group, indicating a common parent body. When they passed through Earth’s atmosphere, they exploded and scattered, creating an ejecta/strewn range reaching 430 km—far exceeding the previously recognized “world’s longest” meteorite strewn field, Namibia’s Gibeon strewn field (about 275 km).

The Meteoritical Society has officially approved grouping these iron meteorites under the collective name Aletai iron meteorites. This is, to date, the world’s longest known meteorite strewn field. Establishing this strewn field provides important insights into the evolution of asteroid orbits and the history of Earth impacts.

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