Several billion years ago, when it was a young planet, Mars was different. Liquid water pooled in deep craters carved out of the landscape by violent collisions with space rocks. Rivers snaked through the jagged terrain. The planet was wrapped in a warm, thick atmosphere, and mountaintops pierced a sky not unlike our own.
Today, Mars is a cold, barren desert world, with a wispy atmosphere and no hope for liquid water. But thanks to the work of a small, diligent rover, scientists are finding intriguing relics of the planet’s ancient past.
In a pair of papers published Thursday in Science, NASA researchers report new findings, gleaned from the thin Martian atmosphere and Martian soil, that further enrich our understanding of the planet and its bygone days of potential habitability.
Using the Curiosity rover’s drilling and soil-analysis equipment, scientists have identified organic matter in deposits beneath the surface that are not unlike the organic-rich sedimentary rock on Earth. The material—several different types of carbon-bearing molecules—came from two sites in the crater that Curiosity has roamed for nearly six years. Such organic carbon molecules are the building blocks of life as we know it on Earth. The rover has found similar organic matter on Mars before, in samples excavated from other spots along the crater in 2013.
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.
But lipids can have abiotic sources, Eigenbrode says. Meteors can carry lipid-like molecules. If scientists find traces of these structures in ancient deposits on Mars, it won’t be immediately clear whether they hitched a ride there on a space rock or emerged from living things.
For now, we can rely on our imaginations (and cool computer-generated animations) for glimpses of early Mars. The samples in Eigenbrode’s study came from what was once a large lake.“It was probably much like the types of lakes that we have on Earth,” she says. “The size could have been something between the Dead Sea and the Great Salt Lake [in Utah],” Eigenbrode says. The lake was surrounded by miles and miles of rocky terrain. There were no grass or trees, she says, just rusty-red rock and whatever beings might have sustained themselves on the landscape. But the view skyward would have beautiful.
“There would have been clouds,” Eigenbrode says. “If we have water on the ground, there’s clouds.”