The recent announcement that scientists have spotted what they believe to be evidence suggesting ancient life on Mars seemed like the latest in an endless cycle of tantalizing hints followed by pleas for more funding. What will it take for them to declare something definitive?

This time, the SUV-sized Perseverance Rover detected minerals called vivianite and greigite, which typically form in marshes, lake bottoms, or hydrothermal sediments — environments where microbes thrive. Here on Earth, microbes play a role in their formation by altering the chemical forms of iron and sulfur. A NASA team concluded that life is the most likely explanation for these minerals’ presence on Mars.

Scientists already have several lines of evidence suggesting life may have arisen on Mars. Raising the probab

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