With another instrument on Curiosity, scientists have discovered that the levels of methane, an organic compound, in the Martian atmosphere change seasonally, peaking near the end of the summer in the planet’s northern hemisphere. The researchers say that seasonal changes in temperature could lead to the release of methane buried beneath the surface, trapped in water-based crystals that could be hiding in large deposits. On Earth, the seafloor is teeming with methane capable of sustaining bustling ecosystems.
These findings are not evidence of past life on Mars. Scientists are still dreaming of that kind of discovery.
But together, the results color in more of the picture of early Mars as we understand it: a warm environment, organic material, and liquid water—all the right conditions for life to arise. They show that Mars may be rich with clues about its history, and the Curiosity rover, along with future generations of rovers, could dig them up from the rock or sniff them out in the air.
The tricky thing about such Mars data is determining its source. The organic material and the methane concentrations could have origins in several different phenomena. They may have emerged from geological processes, like interactions between rock and water. They could have crash-landed on Mars inside a meteor that blew apart into bits, scattering debris and mixing with indigenous material. Or they might have been produced from biological activity, from the breath and digestion and movements of living beings.
What’s more, the Martian surface is a hostile environment. Without a protective atmosphere like ours, the ground is ravaged by radiation. The soil samples the Curiosity rover dug up are so damaged from these conditions that it’s difficult to tell with any certainty where the organic molecules within them originated.
Jennifer Eigenbrode, a biogeochemist and geologist at NASA and the lead author of the study on organic matter, says she’s hopeful spacecraft will someday find spots on Mars where the radiation is a little less harsh, and where carbon-bearing molecules may be better preserved. Perhaps at a fairly young crater, where a meteor exposed bedrock, long hidden from the sun, upon impact.
So, what would a signature of life in some dusty soil look like? “There are various chemical patterns or structures that we might expect to see in organic molecules,” Eigenbrode says. One particularly promising indicator is lipids, the carbon-based molecules that make up membranes in cells. Lipids are hearty things, capable of resisting breakdown by other chemicals and and surviving in terrestrial sediment for several billion years.
“When organisms die, their biomass becomes food for other organisms. If there’s protein, amino acids, maybe some nucleotides or sugar or carbohydrates—all of the organisms around it will go, ‘Look, it’s dessert!’ and go and eat it,” she says. “What they don’t tend to eat are lipids.” Eigenbrode says they’re almost everywhere in ancient Earth rocks.