In 1976, NASA’s Viking 1 lander touched down in Chryse Planitia. In the process, it became the first successful Mars lander. To Alexander Hayes, it also marked the beginning of the modern hunt for life on Mars. And it started with evidence for ancient water.
“The first piece of evidence came when we had Viking images that first showed precipitation patterns that were carved by what we believe to have been rain in an environment very similar to Earth,” Hayes tells Inverse. “Mars had persistent long-standing water that carved the surface up and filled up its craters into lakes.”
Hayes, director of the Cornell Center for Astrophysics and Planetary Science, says that Viking was the first to prove that Mars’ environment was once conducive to life. Now, 45 years later, evidence of Mars’ watery past has added up.
In the process, it’s built a dossier for promising signs that Mars was once habitable. Searching for life beyond Earth has become an all-hands affair in many ways, and is currently centered around our neighboring Red Planet. But the existence of the mythical Martian goes beyond a cosmic hunch.
Mounting evidence suggests that Mars may have hosted life during its early history when the planet was a wet, warm world. All the right conditions were there for life, but it’s just a matter of actually finding it. As NASA’s Perseverance rover begins its hunt of ancient life on Mars, scientists are betting on that evidence to find the first signs of life on another planet.
“We know that Mars at one point was a place that that was suitable for life,” Hayes says. “What we don't know is what is the probability of having the right building blocks in the right environment means that life actually evolved.”
If Perseverance successfully finds evidence of life on Mars, it may finally settle debates over controversial or incomplete evidence of ancient life on our cosmic neighbor. While there has been no smoking gun, some pieces of evidence have hinted not just at the right conditions for life, but at life itself, with big, big, big maybes.
Here are a few pieces of evidence that, while inconclusive, hint that researchers may have been on the right path.
5. Did the Viking missions find evidence of life on Mars?
On June 19, 1976, the Viking 1 lander was the first spacecraft to successfully land on Mars. Its companion, Viking 2, landed September 3 that same year. Both landers opened the door to our modern exploration of Mars, partly by showing that there was more than meets the eye to the desert world than Mariner 4 had found a decade before that had left planetary scientists ready to write off Mars.
Viking provided the first evidence that Mars had ancient river beds and vast flooding, as well as all of the elements that are essential to life on Earth, such as carbon, nitrogen, hydrogen, oxygen, and phosphorus.
The evidence of precipitation on Mars also proves that the planet once had a thicker atmosphere in the past. But over time, the planet lost its blanket of life-friendly gasses through atmospheric escape to outer space, leaving behind just a whisp of what was once there.
Along with cameras and other instruments to study the Martian environment, the Vikings had small astrobiology “labs” aboard to perform experiments looking for present-day life on Mars.
The Labeled Release (LR) experiment mixed small samples of Martian soil with drops of water that had a nutrient solution and some radioactive carbon to test for gaseous waste that would be produced by living organisms on Mars.
The soil sample tested positive for metabolism, but scientists disagreed on whether or not this result is a sure sign that life existed on Mars since the gas could’ve been produced by something other than life. For instance, ultraviolet light from the Sun could’ve activated the soil, creating off-gassing.
Another experiment performed by the Viking mission tested for traces of organic material and found none, suggesting that there was no life on Mars.
Forty years later, scientists look back at these initial experiments with skepticism.
4. Did Mars meteorites have life stowing away?
On December 27, 1984, a piece of Mars was found in the Allan Hills in Antarctica. The meteorite, dubbed ALH84001, weighed about four pounds and likely originated on Mars before an ancient collision sent it hurtling toward Earth.
In 1996, a group of scientists peered inside a fragment of the meteorite, and in the process, made a startling discovery. They found structures inside the meteorite similar to those that would have been formed by microbes in ALH84001, as well as the presence of organic materials.
The initial claims of life on Mars were not widely accepted as scientists found other ways to interpret the structures within the meteorite, claiming that contamination with materials from Earth may have caused the presence of organic material.
But by that time, the idea of Martian life was rapidly gaining popularity all over again.
3. Curiosity finds organic materials within Mars rocks
Nearly 20 years later, NASA’s Curiosity rover found traces of sulfur, nitrogen, hydrogen, oxygen, phosphorus, and carbon (key ingredients of life) while drilling a Martian rock.
The rover also found minerals like sulfates and sulfides that could have been utilized as food for ancient microbes on Mars billions of years ago. Scientists believe that primitive forms of microbes could have found sufficient energy by feeding on the Martian rocks.
The minerals also showed the chemical composition of the water itself after it evaporated from Mars.
“There are papers that have been published saying that not only was there water on Mars but that water had a chemical makeup or such that we could have drunk it,” Hayes says. “We could have sat there and sipped it and been very happy.”
2. Was Martian cauliflower made by microbes?
In 2008, NASA’s Spirit rover stumbled upon an odd shape poking out of the Martian surface in the Gusev Crater.
The structure was dubbed “cauliflower” because it was shaped like broccoli’s more delicious cousin. Images taken by the rover show a series of mineral deposits made up of opaline silica.
On Earth, these silica formations are associated with microbial activity. When the rover snapped images of these structures on Mars, some were quick to suggest that these had been formed by Martian bacteria. However, it could also be shaped by non-biological processes (like wind erosion) since Mars’ climate is very different than that of Earth’s.
1. Why did Mars emit methane?
In 2018, the Curiosity rover found evidence of methane in the Martian air that may have been produced by a form of life. This seemingly confirmed previous observations of trace amounts of methane on the surface, seen by orbiters and rovers alike. On Earth, methane is considered a biosignature, and an indication of life. Because methane is volatile, it doesn’t last long after its production, instead breaking down into other molecules.
The findings also suggested that the methane goes up and down seasonally and occasionally spikes. That made scientists suggest that the methane is produced by living organisms on Mars, which contributes to the rise and fall of the gas.
And while organic molecules often translate to life, other scientists suggest that the methane could have been produced by unknown inorganic chemical processes on Mars.
Although none of the findings provided a conclusive idea of Martian life, they made up different pieces of the puzzle of Mars’ past and called for more research on the Red Planet.
Perseverance rover’s hunt for life on Mars
On February 18, NASA’s Perseverance rover landed on Mars and will shortly begin its hunt for signs of ancient microbial life on the Red Planet.
Other Mars missions have tried to build a comprehensive picture of Mars’ past, but Perseverance will be the first NASA mission since the Vikings to conduct robust hunts for life on Mars.
Perseverance will examine Mars’ Jezero Crater — a 28-mile wide, 1,600-foot deep crater located in a basin slightly north of the Martian equator — for signs of ancient life.
Jezero Crater once housed a lake estimated to have dried out 3.5 to 3.8 billion years ago, making it the perfect environment to look for signs of ancient microorganisms that may have lived in the lake’s waters.
Perseverance will not only examine Martian rocks, but the rover will also pick up the samples of rock and stow them away for a future return mission to Earth, where they will be examined in a lab. After all the circumstantial or ambiguous evidence, those samples could provide the most concrete answer.
“I think the Perseverance rover is the first step towards answering that question,” Hayes says. “It's the first step towards trying to understand if Mars had life on it.”