Allan Hills 84001 (ALH84001) is a piece of a meteorite from Mars that was discovered on December 27, 1984, in the Allan Hills of Antarctica by a team of American scientists from 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 match any of the previously identified SNC groups. When it was found, its mass was 1.93 kilograms (4.3 pounds).

In 1996, scientists examined the meteorite and identified tiny structures that resembled microscopic fossils of bacteria, which led to suggestions that these features might have formed on Mars. This discovery received widespread media attention and was discussed in a speech by U.S. President Bill Clinton. However, the findings were debated by scientists, as many later found explanations for the unusual features that did not require the presence of life. Although no clear evidence of life on Mars was confirmed, the original study and the attention it received are regarded as important milestones in the field 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 molten rock about 4.091 billion years ago. Chemical tests show it likely came from Mars when liquid water existed on the planet’s surface.

In September 2005, Vicky Hamilton, from the University of Hawaii at Manoa, shared findings about ALH84001’s origin using data from the Mars Global Surveyor and 2001 Mars Odyssey spacecraft. Her analysis suggested that Eos Chasma, a part of the Valles Marineris canyon, might be the meteorite’s source. However, the results were not certain because the study focused only on areas of Mars not covered by dust.

The theory states that ALH84001 was sent into space when a meteor struck Mars about 17 million years ago. It fell to Earth approximately 13,000 years ago. Scientists used methods like samarium–neodymium (Sm–Nd), rubidium–strontium (Rb–Sr), and potassium–argon (K–Ar) dating to determine these ages. Other meteorites with signs of possible biological activity have not generated as much interest because they do not contain rock from a time when Mars had liquid surface water. ALH84001 is the only meteorite known to have come from such a period.

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’s atmosphere. The ratios of carbon and oxygen isotopes suggest the carbonates formed from a slowly drying underground water source, likely a shallow aquifer located meters or tens of meters below Mars’s surface.

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

Hypothetical biogenic features

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

The claim of possible extraterrestrial life caused strong disagreement among scientists. When the discovery was announced, many people believed it was the first clear proof of life from another planet, leading to worldwide news coverage. Even President Bill Clinton made a televised statement about the event.

McKay argued that microbial contamination from Earth, found in other Martian meteorites, does not look like the shapes seen in ALH84001. The shapes in ALH84001 appear to be grown together or embedded in the meteorite’s natural material, unlike contamination from Earth. Although scientists have recreated similar structures in the lab without using biological materials, McKay said these experiments used materials that were too pure to explain the features in ALH84001. He stated that a believable non-living explanation must account for all the properties scientists have linked to possible life in the meteorite. Many scientists disagreed with McKay’s conclusions.

In January 2010, a team at Johnson Space Center, including McKay, claimed that new evidence supported the biogenic hypothesis. They found three times as much fossil-like data in two other Martian meteorites and additional evidence in other parts of ALH84001 itself. However, many scientists noted that shapes alone cannot reliably prove the presence of life. Interpreting shapes is often unclear and has led to incorrect conclusions in the past.

Features in ALH84001 that some scientists thought might be microfossils include:

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

In popular culture

The 1996 X-Files episode titled "Tunguska" was influenced by the scientific announcement about the Allan Hills 84001 meteorite.

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

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.