Allan Hills 84001 (ALH84001) is a piece of a meteorite from Mars that was found in the Allan Hills of Antarctica on December 27, 1984, by a group of American scientists working on the ANSMET project. Like other meteorites in the shergottite–nakhlite–chassignite (SNC) group, ALH84001 is believed to have come from Mars. However, it does not belong to any of the known SNC groups. When it was discovered, its weight was 1.93 kilograms (4.3 pounds).

In 1996, scientists studied the meteorite and found tiny structures that looked like microscopic fossils of bacteria, which suggested that these structures might have formed on Mars. This discovery received worldwide attention, including a speech by U.S. President Bill Clinton about the possible finding. However, the claims were debated from the start. After further study, most scientists concluded that the unusual features in the meteorite could be explained without needing to involve life. Although no clear evidence of life on Mars was found, the original research and the widespread interest it generated are seen as important moments in the study of astrobiology.

History and description

ALH 84001 was discovered by Roberta Score, who was the Laboratory Manager of the Antarctic Meteorite Laboratory at the Johnson Space Center, on the Allan Hills Far Western Icefield during the 1984–85 season.

ALH84001 is believed to be one of the oldest Martian meteorites. Scientists think it formed from melted rock about 4.091 billion years ago. Chemical tests suggest it came from Mars when liquid water was present on the planet’s surface.

In September 2005, Vicky Hamilton, from the University of Hawaii at Manoa, used data from the Mars Global Surveyor and 2001 Mars Odyssey spacecraft to study where ALH84001 might have originated. Her analysis suggested that Eos Chasma in the Valles Marineris canyon could be the meteorite’s source. However, the results were not certain because the study focused only on areas of Mars not covered by dust.

Scientists believe ALH84001 was sent into space when a meteorite hit Mars about 17 million years ago. It landed on Earth roughly 13,000 years ago. These dates were determined using several radiometric dating methods, such as samarium–neodymium (Sm–Nd), rubidium–strontium (Rb–Sr), and potassium–argon (K–Ar) dating. Other meteorites with possible signs of life have not been studied as much because they do not contain rock from a time when Mars had liquid water on its surface. ALH84001 is the only meteorite that may have come from a period when Mars had surface water.

In October 2011, scientists reported that isotopic analysis showed the carbonates in ALH84001 formed at a temperature of 18 °C (64 °F) using water and carbon dioxide from Mars’ atmosphere. The ratios of carbon and oxygen isotopes in the carbonates suggest they formed from water that was slowly drying up underground, likely in a shallow aquifer located meters or tens of meters below the surface.

In April 2020, researchers announced they found nitrogen-containing organic materials in Allan Hills 84001.

Hypothetical biogenic features

On August 6, 1996, a group of scientists, including NASA researcher David S. McKay, announced that a meteorite might contain small signs of life from Mars. This finding was published in the science journal Science a few days later. Using a scanning electron microscope, scientists saw tiny structures that some believed could be fossils of bacteria-like lifeforms. These structures, found in the meteorite ALH84001, were 20–100 nanometers wide—similar in size to theoretical nanobacteria but smaller than any known living cells at the time. If the structures were indeed fossils, as some scientists suggested, they would have been the first solid evidence of life beyond Earth, unless the structures came from Earth-based contamination.

The claim of possible extraterrestrial life caused much debate. When the discovery was shared, many people believed it was the first clear proof of life from another planet, leading to worldwide media coverage. President Bill Clinton even made a televised statement to highlight the event.

McKay argued that microbial contamination from Earth, found in other Martian meteorites, did not look like the structures in ALH84001. The shapes in ALH84001 appeared to be embedded in the meteorite’s natural material, unlike contamination from Earth. Although scientists have recreated similar structures in labs without using biological materials, McKay noted that these experiments used unusually pure materials and could not explain all features in ALH84001. He stated that a valid non-biological explanation must account for all the properties scientists have linked to possible life in the meteorite. Many scientists disagreed with McKay’s views.

In January 2010, a team at Johnson Space Center, including McKay, reported that new evidence from 2009 supported the idea that the structures might be fossils. They found three times as much fossil-like data in two other Martian meteorites and more evidence in parts of the Allan Hills meteorite. However, many scientists warned that shape alone is not enough to prove life, as interpreting shapes can be unclear and has led to past mistakes.

Features in ALH84001 that scientists thought might be microfossils include:

A study in January 2022 concluded that ALH84001 did not contain life from Mars. The organic molecules found were linked to non-living processes, such as chemical reactions that occurred on early Mars 4 billion years ago, like serpentinization and carbonation reactions caused by hydrothermal fluids altering basalt rock.

In popular culture

The 1996 X-Files episode "Tunguska" was influenced by the 1984 discovery of the Allan Hills 84001 meteorite.

The 1997 movie Contact included edited footage of President Bill Clinton speaking about the Allan Hills 84001 meteorite. The footage was altered to make it appear as though Clinton was talking about searching for signals from space. This use of the edited video led to a formal letter from White House Counsel Charles Ruff, who objected to the use of Clinton’s digitally altered image.

The 2001 mystery novel Deception Point by Dan Brown, which tells the story of a meteorite that appears to show evidence of alien life, was inspired by the Allan Hills 84001 meteorite.