Scientists propose a new approach for the detection of alien life
Two rovers have had big breaks over the past few years in the case of whether Mars ever hosted life.
Curiosity, which is exploring Gale Crater, conducted a chemistry experiment on a rock sample that revealed complex carbon compounds. Perseverance, about 2,300 miles away at Jezero Crater, found fossilized material that an ancient alien microorganism could have excreted.
Despite the compelling leads, NASA doesn't know whether ancient living things produced these organic molecules or if some other process, such as chemical reactions between rock and water, did. Because of instrument limitations, it's doubtful the U.S. space agency will ever be able to rule that possibility out, unless the samples come back to Earth.
"This finding by our incredible Perseverance rover is the closest we've actually come to discovering ancient life on Mars," said Nicky Fox, NASA's associate administrator for science, last year.
But a team of scientists has suggested a new way to look at samples that could help close the gap of ambiguity. The idea is surprisingly simple: Rather than try to look for specific molecules in a sample, study the overall pattern of chemicals within it.
The researchers compared samples from living organisms, fossils, ocean sediments, meteorites, asteroid material, and lab experiments that simulated early-Earth or space chemistry. They specifically homed in on amino acids, which build proteins, and fatty acids, which contribute to cell membranes. They found that life organizes chemicals differently.
It turns out a strong statistical divide exists between biological and nonbiological samples, according to a new study. The results are published in Nature Astronomy. Though looking for molecular diversity in a sample is not a silver bullet for detecting aliens, it could offer one more strong piece of evidence to weigh in the balance.
"Astrobiology is fundamentally a forensic science," said Gideon Yoffe, lead author of the paper and a researcher at the Weizmann Institute of Science in Israel, in a statement. "We’re trying to infer processes from incomplete clues, often with very limited data collected by missions that are extraordinarily expensive and infrequent."
In the study, amino acids from biological samples usually contained a wider and more balanced and organized mix of compounds because cells actively make many compounds for specific jobs. Abiotic samples — specimens formed without life — tended to look sparse, with a few simple amino acids dominating the mixture. Some contaminated meteorites shifted closer to the biological group, indicating that biology changes chemical patterns in recognizable ways.
The researchers also found samples that leaned in the other direction. Biological samples that had suffered heavy damage from heat, radiation, or age started to resemble nonliving chemistry because they lost molecular diversity over time. Ancient rocks, fluids from hydrothermal vents, and some fossils all showed signs of this deterioration.
Scientists wondered whether radiation could erase the biological signal. They simulated conditions in the icy surface layers of Europa, one of Jupiter's moons, and found that the diversity pattern often survived, even after substantial chemical damage.
Fatty acids, on the other hand, showed the opposite trend but still clearly distinguished life from non-life. Because living cells rely on a narrower set of fatty acids for membranes, the biological samples appeared less evenly distributed. Nonliving chemistry produced broader, more uniform mixtures, according to the study.
The statistical strategy could improve current space missions designed to perform chemical analyses and some life-detection tests. Scientists often search for unusual isotope ratios or molecular "handedness," but those signals can fade over time and require sensitive instruments.
Curiosity or Perseverance could potentially do this kind of statistical test if ever the rovers were to detect a broad suite of related organic compounds and measure the reliable relative amounts of those different molecules, said Fabian Klenner, a UC Riverside assistant professor of planetary sciences and coauthor on the paper. The current limitation isn't that the rovers are incapable of analyzing molecular diversity, but that they need a sample rich and varied enough in organic data.
The technique might be especially useful for NASA's eight-rotor Dragonfly aircraft, which is expected to explore Titan, an icy moon of Saturn, in the mid-2030s. The aircraft will have a mass spectrometer device designed to analyze and characterize organic molecules.
"Dragonfly is another interesting case," Klenner told Mashable. "If it can resolve organic molecules and their relative abundances, then I would love to see our diversity approach applied to the data."
