Mars was not always the cold, dry world we see today. Billions of years ago, scientists believe it may have been warm, wet, and wrapped in a much thicker atmosphere, creating conditions that could have supported simple microbial life. Even so, proving that life ever existed there remains one of the biggest challenges in planetary science.
NASA's rovers have already detected organic molecules in Martian rocks, but those compounds alone cannot confirm that life was once present. Beginning in 2030, the European Space Agency's Rosalind Franklin rover is expected to join the search with specialized instruments designed to look for stronger chemical evidence. Now, researchers from the Max Planck Institute for Solar System Research (MPS), the University of Göttingen, and Côte d'Azur University in Nice (France) have put one of the rover's key detection methods through a demanding test.
Searching for Ancient Martian Biosignatures
Finding convincing evidence of ancient Martian life means telling apart organic molecules created by living organisms from those produced through ordinary chemistry. Researchers believe two hydrocarbons, pristane (C19H40) and phytane (C20H42), could help answer that question.
These molecules originate from living organisms and are also found in petroleum on Earth. Because they are unusually stable, scientists think they could survive for billions of years under the right conditions.
"If life once existed on Mars, then molecules like pristane and phytane represent important molecular biosignatures that could have survived to this day," said MPS scientist Guillaume Leseigneur, lead author of the new study.
Why Mirror Image Molecules Matter
Pristane and phytane have another important characteristic that makes them attractive targets in the search for life. Like many organic compounds, they are chiral, meaning they exist in two mirror image forms called enantiomers. The two versions contain the same atoms but arranged as mirror images of one another, much like a person's left and right hands.
"Chirality is a valuable tool in the search for past extraterrestrial life," said co-author Uwe Meierhenrich of Côte d'Azur University.
Living organisms typically produce almost exclusively one mirror image of a chiral molecule. Scientists expect the same pattern would apply to any life elsewhere in the universe because living systems reproduce themselves. In contrast, molecules formed without biology should contain both mirror image forms in roughly equal amounts.
Testing the Rosalind Franklin Rover
The Rosalind Franklin rover will search for these subtle differences using the Mars Organic Molecule Analyzer (MOMA), an instrument developed and built under the leadership of the MPS. MOMA combines a gas chromatograph, a mass spectrometer, small furnaces, and an excitation laser.
Rock samples are first heated in the furnaces to release volatile compounds. Those gases are then analyzed and passed through specially coated capillary tubes. Because the mirror image versions of the same molecule interact differently with the coatings, they move through the tubes at different speeds, allowing the instrument to separate them.
For the new study, researchers used identical replicas of MOMA's capillary tubes. For the first time, they successfully separated the chiral forms of both pristane and phytane, despite the molecules being extremely unreactive.
"Chiral separation of pristane and phytane requires high instrument sensitivity and measurement accuracy, both of which we show MOMA can achieve," explained co-author and MOMA team member Fatma Yesil Sahan from the MPS.
Meteorite Reveals an Unexpected Twist
Instead of Martian rocks, the team tested samples from the famous Murchison meteorite, which fell in Australia in 1969. Like many meteorites, it contains a mixture of organic compounds. Some were present when the meteorite formed, while others likely came from biological contamination after it landed. The researchers initially suspected pristane and phytane belonged to this second category.
The results, however, told a different story.
The meteorite contained equal amounts of every mirror image version of pristane and phytane. That pattern does not match biological material that could have contaminated the meteorite where it was found.
Instead, the researchers concluded the contamination was probably picked up while the meteorite passed through Earth's atmosphere, where it encountered aerosols produced by fossil fuel combustion.
Comparisons with pristane and phytane found in oil shales supported that explanation. These sedimentary rocks contain petroleum precursors that have spent millions of years deep underground.
"Petroleum forms in these rocks over millions of years at great depths under the influence of heat and pressure," said co-author Manuel Reinhardt from the University of Göttingen.
Over time, those conditions erase the natural imbalance between the mirror image forms of the molecules, leaving them in equal proportions. That closely matches what the team observed in the Murchison meteorite.
Preparing for the Search for Life on Mars
The researchers say the work does more than validate MOMA ahead of its mission to Mars. It also raises new questions about how organic molecules found in meteorites acquire contamination and what increasing levels of petroleum related pollution in Earth's atmosphere may mean for future studies.
MOMA is part of ESA's ExoMars Mission to Mars and was developed and built under a program of, and funded by, the European Space Agency.
