A Martian meteorite is a rock that formed on Mars, was sent away from the planet by a powerful impact, and traveled through space before landing on Earth as a meteorite. As of September 2020, 277 meteorites had been identified as Martian, which is less than 0.5% of the 72,000 meteorites that have been classified. The second-largest complete, uncut Martian meteorite, Taoudenni 002, was found in Mali in early 2021. It weighs 14.5 kilograms (32 pounds) and is displayed at the Maine Mineral and Gem Museum.

There are three main groups of Martian meteorites: shergottites, nakhlites, and chassignites, which are together called SNC meteorites. Some other Martian meteorites do not belong to these groups. These meteorites are believed to be from Mars because their chemical and isotopic makeup matches rocks and gases found on Mars, as measured by spacecraft, landers, and rovers. The term "Martian meteorite" does not include rocks found on Mars, such as Heat Shield Rock.

History

By the early 1980s, scientists noticed that the SNC group of meteorites (Shergottites, Nakhlites, and Chassignites) were very different from most other types of meteorites. These differences included younger ages, different oxygen isotope patterns, signs of water interacting with rocks, and some chemical similarities to rocks analyzed on Mars by the Viking landers in 1976. Some scientists believed these traits suggested the SNC meteorites came from a large parent body, possibly Mars.

In 1983, scientists discovered trapped gases inside the EET79001 shergottite, a type of SNC meteorite. These gases matched those found in the Martian atmosphere by the Viking mission. This discovery provided strong evidence that the SNC meteorites originated from Mars. In 2000, a study by Treiman, Gleason, and Bogard reviewed all the evidence supporting the idea that the SNC meteorites (14 had been found by then) came from Mars. They wrote, "It seems unlikely that the SNC meteorites are not from Mars. If they came from another planet, that planet would have to be very similar to Mars as we now understand it."

Subdivision

As of April 25, 2018, 192 of the 207 Martian meteorites are divided into three rare groups of stony meteorites: shergottites (169), nakhlites (20), chassignites (3), and others (15). These others include the orthopyroxenite (OPX) Allan Hills 84001 and 10 basaltic breccia meteorites. Because of this, Martian meteorites as a whole are sometimes called the SNC group. They have chemical clues that match each other but do not match Earth. The names come from the locations where the first meteorite of each type was found.

About three-quarters of all Martian meteorites are shergottites. They are named after the Shergotty meteorite, which fell in Sherghati, India, in 1865. Shergottites are igneous rocks made of mafic to ultramafic materials. They are grouped into three main types: basaltic, olivine-phyric (like the Tissint group found in Morocco in 2011), and lherzolitic shergottites. These groups are based on the size of their crystals and the minerals they contain. Shergottites can also be divided into three or four groups based on their rare-earth element content. These two classification systems do not match, suggesting complex relationships between the rocks and magmas that formed them.

Shergottites may have formed as recently as 180 million years ago. This is surprising because most of Mars’s surface appears very old, and Mars is a small planet. Some scientists think the shergottites might be older than this, but this idea, called the "Shergottite Age Paradox," remains unsolved and is still being studied.

A 3-million-year-old crater called Mojave, 58.5 km in diameter, was once thought to be the source of these meteorites. However, a 2021 study suggested instead that the 28 km crater Tooting or the crater 09-000015 might be the source of some meteorites ejected 1.1 million years ago.

Nakhlites are named after the Nakhla meteorite, which fell in El-Nakhla, Egypt, in 1911. It weighed about 10 kg. Nakhlites are igneous rocks rich in augite and formed from basaltic magma during at least four eruptions over 90 million years, from 1416 ± 7 to 1322 ± 10 million years ago. They contain augite and olivine crystals. Their ages suggest they formed on large volcanic areas like Tharsis, Elysium, or Syrtis Major Planum on Mars.

Nakhlites were exposed to liquid water about 620 million years ago and were ejected from Mars by an asteroid impact around 10.75 million years ago. They fell to Earth within the last 10,000 years.

The first chassignite, the Chassigny meteorite, fell in France in 1815. Only one other chassignite, Northwest Africa (NWA) 2737, has been found. It was discovered in Morocco or Western Sahara in 2000. Scientists confirmed it came from Mars and is similar to Chassigny.

The meteorite Allan Hills 84001 is different from other Martian meteorites because it is an orthopyroxenite, a type of igneous rock mostly made of orthopyroxene. It was studied closely after scientists found structures that looked like fossilized bacteria. However, by 2005, scientists concluded these structures were likely from Earth, not Mars. Allan Hills 84001 is as old as other groups of Martian meteorites—about 4.1 billion years.

In 2004, scientists suggested the Kaidun meteorite, which fell in Yemen in 1980, might have come from Phobos, one of Mars’s moons. However, Kaidun is not a Martian meteorite in the strict sense because it is a carbonaceous chondrite, a type of meteorite that contains carbon. It may still have small pieces of Martian material.

The Martian meteorite NWA 7034, nicknamed "Black Beauty," was found in the Sahara in 2011. It has ten times more water than other Martian meteorites found on Earth. It contains parts as old as 4.42 ± 0.07 billion years and was heated during the Amazonian period on Mars.

A meteorite that fell in Dayanpo, China, in 1986 contains a mineral called "Elgoresyte," which is not found on Earth.

Origin

Most SNC meteorites are relatively young compared to other meteorites, suggesting that volcanic activity on Mars occurred only a few hundred million years ago. The young ages of Martian meteorites helped scientists recognize that they likely came from a planet like Mars. Among Martian meteorites, only ALH 84001 and NWA 7034 are older than about 1.4 billion years. Nakhlites, as well as Chassigny and NWA 2737, have similar formation ages of about 1.3 billion years, as determined by radiometric dating methods. Many shergottites have varied formation ages, mostly between 150 and 575 million years.

Scientists are still learning about the history of shergottites. Some researchers suggest that a few may have formed earlier than their radiometric ages indicate, but most scientists disagree. Formation ages of SNC meteorites are often connected to their cosmic-ray exposure (CRE) ages, which are measured by studying how cosmic rays interact with the meteorites in space. Nakhlites have nearly identical CRE ages of about 11 million years, which, combined with their similar formation ages, suggests they were ejected from a single location on Mars by one impact event. Some shergottites also form distinct groups based on their CRE and formation ages, indicating they may have been ejected by a single impact. However, shergottites have widely varying CRE ages (about 0.5 to 19 million years), meaning multiple impact events were likely needed to send all known shergottites into space. Earlier claims that no large young craters on Mars could be sources for these meteorites were later challenged, as some studies identified possible sources for ALH 84001 and other shergottites.

In a 2014 study, researchers suggested that all Shergottite meteorites may have originated from the Mojave Crater on Mars. The time meteorites spent traveling from Mars to Earth can be estimated by analyzing how cosmic radiation has affected noble gas isotope ratios in the meteorites. Meteorites often form groups that match distinct impact events on Mars. Scientists believe these meteorites likely came from a small number of impacts on Mars that occur every few million years. The objects causing these impacts would be several kilometers in size, creating craters tens of kilometers wide on Mars. Models of impacts on Mars support these findings.

Ages since impact determined so far include…

Possible evidence of life

Several Martian meteorites have been found that some scientists believe may contain signs of ancient life on Mars. The most important of these is a meteorite discovered in the Allan Hills of Antarctica, called ALH 84001. This meteorite was likely sent into space from Mars about 16 million years ago and reached Earth around 13,000 years ago. Cracks inside the rock show signs of carbonate materials, which suggest that water may have once been present on Mars between 4 and 3.6 billion years ago. Scientists have found certain types of organic chemicals called polycyclic aromatic hydrocarbons (PAHs) in the meteorite, with higher levels found farther from the surface. Other meteorites from Antarctica do not contain these chemicals. Scientists believe that Earth-based contamination would most likely be found at the surface of the meteorite. Some minerals inside the cracks, such as magnetite (a type of iron), are said to form in ways similar to how they do on Earth when living things are involved. Tiny, oval-shaped and tube-like structures in the carbonate material may be similar to fossils of extremely small bacteria, according to some researchers. However, a scientist named Schopf, who studies ancient life on Earth, examined the meteorite and said the structures are too small to be Earth-based bacteria and do not clearly look like life. The size of these structures matches what is known as "nanobacteria," but the existence of nanobacteria itself is not widely accepted by scientists.

Many studies have questioned whether the structures in the meteorite are evidence of life. For example, scientists found that much of the organic material in the meteorite likely came from Earth. However, a 2009 study suggested that the magnetite in the meteorite could have been created by microbes on Mars. This study, published in the Journal of the Geochemical and Meteoritic Society, used more advanced tools than those used in 1996. A major challenge in linking the magnetite to life is that most of the magnetite crystals are arranged in specific ways with the surrounding carbonate minerals, which suggests they formed through non-living chemical processes rather than biological ones.

While the presence of water is not proof of life, many Martian meteorites found on Earth show signs of water, including NWA 7034, which formed during the Amazonian period of Mars' history. Other signs of liquid water on Mars, such as recurring slope lineae, are still debated by scientists but are generally consistent with earlier findings from Martian meteorites. Any liquid water present on Mars is likely in very small amounts and may not be enough to support life.