The Alan Hills meteorite is a part of history to Mars aficionados. It came from Mars and meteorite hunters discovered in Antarctica in 1984. Scientists think it’s one of the oldest chunks of rock to come from Mars and make it to Earth.
The meteorite made headlines in 1996 when a team of researchers said they found evidence of life in it.
The Alan Hills meteorite (ALH84001) is a precious scientific object and part of Mars lore now. Twelve years after its discovery, a team of scientists claimed to find evidence of microscopic bacterial fossils in the meteorite.
From that paper’s abstract: “The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, the formation of the globules by biogenic processes could explain many of the observed features, including the PAHs (polycyclic aromatic hydrocarbons.) The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota.”
That was controversial, and it was huge news. So huge that then-President Bill Clinton saw fit to make a speech about it. Clinton was suitably circumspect when he said, “Like all discoveries, this one will and should continue to be reviewed, examined and scrutinized. It must be confirmed by other scientists.”
Other scientists have studied it many times, and they’ve concluded that the strange, organic-seeming morphologies inside the meteorite were not biological in origin.
So what created them?
There are a lot of false positives when it comes to fossilized evidence of microscopic life. Purely geological structures can mimic organic structures. So it’s easy to see how scientists interpreted the Alan Hills meteorite as evidence of life in 1994. But now we know that a host of geological processes can produce organic-looking structures. Scientists call these microscopic structures “chemical gardens.”
Anybody who saw these images could easily assume that they were organic. And the idea that they could be biological had to be examined. But now we know they’re not biological; they’re geological.
In a new study, a team of researchers took a deeper look into the microscopic organic-looking structures in ALH84001. Their goal was to understand the geological processes that created them and learn something new about Earth and Mars.
The paper’s title is “Organic synthesis associated with serpentinization and carbonation on early Mars.” The team published their paper in the journal Science, and the lead author is Andrew Steele. Steele is a Senior Staff Scientist at Carnegie University’s Geophysical Laboratory.
Interactions between water and rock don’t produce life directly. But those interactions are likely a precursor to habitability on both Earth and Mars. They produce organic molecules and create the mineralogical diversity necessary for life. The scientists who found evidence of life in the Alan Hills meteorite were incorrect, but they were onto something.
Now that the Perseverance Rover is on Mars, a deeper understanding of geological interactions is gaining importance. Perseverance is searching for fossilized evidence of life in the Jezero Crater. By understanding the geological interactions and the false positives they can create, scientists are better positioned to understand the evidence Perseverance gathers. The Alan Hills meteorite can play a role in this.
“Analyzing the origin of the meteorite’s minerals can serve as a window to reveal both the geochemical processes occurring early in Earth’s history and Mars’ potential for habitability,” Steele said in a press release. Steele has extensively researched organic material in Martian meteorites and is a member of both the Perseverance and Curiosity rovers’ science teams.
Mars rovers have found organic compounds in ancient rocks on Mars. And both rovers and orbiters have detected methane, which can have a biological source. Organic compounds containing carbon, oxygen, hydrogen, nitrogen, sulphur, and other elements are associated with living processes. But non-biological processes can also produce them. Those processes are called abiotic organic chemistry. The Alan Hills meteorite contains organic carbon, and its presence poses a question: What process produced the organic carbon?
There are different hypothetical answers to that question, including volcanic activity, hydrological exposure, and impact events on Mars. Living processes are also a hypothetical answer. Ancient Martian life could’ve created them, or Earthly life post-impact.
Scientists have new investigative techniques at their disposal now. One of them is nanoscale imaging. Nanoscale imaging wasn’t available to researchers studying the Alan Hills meteorite in 1994, and recent breakthroughs have increased its power. In this new study, the researchers used nanoscale imaging along with spectroscopy and isotopic analysis to deepen their understanding of ALH84001.
The team’s evidence showed that the meteorite underwent two types of hydrological interactions.
One is called serpentinization, a name that brings organic activity to mind. But serpentinization is purely geochemical. It occurs when igneous rocks rich in iron or magnesium interact with low-temperature circulating water. Serpentinization produces hydrogen, and it also changes the mineralogy of the rocks. The rocks absorb large amounts of water, lowering their density and destroying their initial structure.
The second process is called carbonation. From Wikipedia: “Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid.” Carbonation is the reaction between rocks and slightly acidic water containing dissolved CO2.
The team’s results show that these processes occurred rapidly in ALH84001. But they weren’t able to determine if they occurred simultaneously or sequentially. From the paper: “We find complex refractory organic material associated with mineral assemblages that formed by mineral carbonation and serpentinization reactions. The organic molecules are colocated with nanophase magnetite; both formed in situ during water-rock interactions on Mars.”
These reactions occurred about 3.9 to 4.1 billion years ago on Mars, during the Late Noachian period. The Late Noachian was a period of intense impacts on Mars, and the planet also likely had extensive surface water. It roughly coincides with the rise of Life on Earth. Surface geology from this time is prime hunting ground in the search for fossilized evidence of life. The Jezero Crater, where the Perseverance Rover is searching, dates from the Noachian period.
These new results are in line with other recent developments. A 2011 study showed that the carbonates in ALH84001 were formed during a period of surface water evaporation.
The mineralogical features caused by serpentinization and carbonation are rare in Martian meteorites. Orbital surveys of Mars found evidence of both processes on the planet’s surface, and scientists have found carbonation in other Martian meteorites, all younger than ALH84001. But the Alan Hills meteorite is the first evidence of both processes occurring on ancient Mars.
Steele has found organic molecules in other Martian meteorites and on Mars with the SAM (Sample Acquisition at Mars) instrument on the Curiosity Rover. So scientists are reasonably sure that these abiotic processes have been at work on Mars for much of the planet’s history.
Their presence indicates that Martian geology supplied some necessary materials for life to exist.
“These kinds of non-biological, geological reactions are responsible for a pool of organic carbon compounds from which life could have evolved and represent a background signal that must be taken into consideration when searching for evidence of past life on Mars,” Steele concluded.
But the results extend beyond Mars. They tell us something about Earth, and maybe about Saturn’s moon Enceladus.
“Furthermore, if these reactions happened on ancient Mars, they must have happened on ancient Earth and could possibly explain the results we’ve seen from Saturn’s moon Enceladus as well. All that is required for this type of organic synthesis is for a brine that contains dissolved carbon dioxide to percolate through igneous rocks. The search for life on Mars is not just an attempt to answer the question ‘are we alone?’ It also relates to early Earth environments and addresses the question of ‘where did we come from?’”