To date, we've found over a hundred precious Mars rocks that have travelled from the red planet and landed on Earth at some point. Among those, specimen ALH84001 might well be one of the most enigmatic.
This meteorite fragment was picked up during a snowmobile ride in the ice field of Alan Hills in Antarctica in 1984, and is thought to have formed on Mars around 4 billion years ago. Straight away, it was "recognized as the most unusual rock collected" on the trip.
What has especially intrigued scientists since its discovery are the minuscule traces of organic carbon detected as part of the rock's composition. Could this point to early alien life back on Mars all those billions of years ago?
Well, probably not, according to the latest study of the fragment. Instead, the organic molecules found in the meteorite are most likely the result of particular fluid and rock interactions similar to those that happen on Earth.
"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," says astrobiologist Andrew Steele, from the Carnegie Institution for Science in Washington DC.
"As one of the oldest known rocks from Mars, ALH84001 serves as a window into early planetary processes that may also have occurred on early Earth," the team notes in their paper.
"Hypotheses as to the provenance and formation mechanisms of these organics include abiotic production by impact-related, igneous, and/or hydrothermal processes; biological production by putative ancient martian organisms; and terrestrial contamination."
To study the tiny carbon globules found within ALH84001, the team gained access to a thin section and a chip of the meteorite, obtained from NASA Johnson Space Center.
They subjected these fragments to a variety of techniques, including nanoscale-level imaging, an analysis of the isotopes present in the rock, and spectroscopy (using light to study the chemical composition of matter).
Their results showed that the characteristics of the rock indicate it could have easily formed in the presence of non-biological or abiotic processes that are known to produce organic carbon molecules here on Earth. The first one is serpentinization, which happens when igneous rocks (solidified from lava or magma) that are rich in iron or magnesium interact with circulating water, producing hydrogen.
The second process is carbonation, where rocks react with slightly acidic water that has dissolved carbon dioxide in it, resulting in carbonate materials. It's not clear if both processes happened simultaneously, but the study suggests they weren't happening over an extended period of time.
"All that is required for this type of organic synthesis is for a brine that contains dissolved carbon dioxide to percolate through igneous rocks," says Steele.
"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."
Several other abiotic processes have been suggested for the presence of organic material in the meteorite, besides the hypotheses that it is traces of alien life or contamination from Earth: volcanic activity, impact events on Mars, and hydrological exposure have all been put forward as hypotheses in the past.
Serpentinization and carbonation are rarely found in Martian meteorites, especially ancient ones, although these processes have been detected through orbital surveys of the red planet. It seems that the abiotic synthesis of organic molecules has been happening on Mars for almost as long as the planet has been around.
"On Earth, these reactions are responsible for abiotic organic synthesis, methane production, and mineralogical diversity. On Mars, such reactions are relevant to habitability and have been invoked to explain the presence of methane in the atmosphere," the team concludes in their paper.
As with much of the research on Mars, there are implications for Earth as well, which had similar beginnings. One of the ways in which these new findings will be useful in the future is in informing research into the ancient history of our own planet, and how organic molecules first appeared.
"If these reactions happened on ancient Mars, they must have happened on ancient Earth, and could possibly explain the results from Saturn's moon Enceladus as well," says Steele.
"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?'"
The research has been published in Science.