New evidence from Curiosity reveals that Mars actually lacks methane, contradicting earlier reports. Methane can be a key sign of life because it is a byproduct of microbial metabolism. But scientists say life on Mars isn’t completely out of the question. There are even some terrestrial microbes that don’t produce methane as a byproduct of metabolism. The rover is still going to keep searching for methane but in much lower concentrations, well under 1 part per billion—less than 1/6 of what earlier studies predicted.