As luck would have it, there was once an active volcano within roughly 50 kilometers (31 miles) of Cheyava Falls. This volcano appears to be much younger than the rock, though, and there is no evidence that hydrothermal activity happened at the right time and place to explain what we see in Cheyava Falls. 

Still, it conceivably could have. The bigger issue is that, even if sulfide did get introduced to the rock this way, it probably would not give you the exact patterns we see today. 

Adding sulfide from outside Cheyava Falls, instead of making it within, would be more likely to give you splotches instead of sharply defined spots. It’d also change where you might expect there to be the most greigite in each individual leopard spot. The sulfide would likely be dwindling as it percolates deeper into the rock instead of being most concentrated deep within, at the center of each spot. 

For all these reasons, the spot patterns seem to be more naturally described by microbes consuming one sort of energy source and then switching to another.  

Room for interpretation 

That may sound like pretty good evidence in favor of Martian microbes. But there is still a lot we don’t know.

For one, just because a convincing alternate explanation doesn’t exist right now doesn’t mean one never will. Now that one group of scientists has published their analysis of Cheyava Falls, other researchers around the world will try to come up with their own ideas. They may discover some new process, entirely unknown to us today, that could make something just like Cheyava Falls without any life.

For Bell, one of the biggest reasons to remain hesitant about Cheyava Falls is simple.

“It’s on Mars,” he said, “And we’ve never found any evidence of past or current life on Mars.”

Of course, that is part of why they’re looking. But if you can explain every other thing on Mars without invoking life, then it takes all the more evidence to make the claim for the very first time.

Nuance goes both ways, too. Even if life didn’t form the greigite, for instance, the mineral would still hint at an environment where microbes could have thrived. Cheyava Falls would have hosted all three of life’s main ingredients: liquid water, an energy source, and organic molecules.