Methane readings on Mars are once again sparking scientific controversy back on Earth. Recently, sensors on NASA's Mars rover, Curiosity, registered a sizable spike in methane gas, leading researchers to believe microbial life might not be far away.

On Earth, most methane gas is produced by living organisms, so large concentrations of the gas have long been considered a sign of life. But not long after the Curiosity readings set the scientific community abuzz, one scientist began second-guessing the source.

"I am convinced that they really are seeing methane," Kevin Zahnle, a researcher at NASA Ames Research Center who wasn't involved in the recent Curiosity research, told Astrobiology Magazine. "But I'm thinking that it has to be coming from the rover."

Zahnle recently voiced his disagreement at a seminar in April at the NASA Astrobiology Institute's Virtual Planetary Lab.

The scientists who analyzed the methane readings -- readings that were made over the course of two months in 2014 -- deliberately took their time to double-check their work, waiting more than a year to finally publish the findings.

Methane has previously been measured in Mars' atmosphere by orbital probes. But each time, the measurements have been undermined or disregarded, deemed unverifiable or a mathematical mistake. But this time, the scientists working with Curiosity are standing by their research.

"We are continuously monitoring that methane amount and there hasn't been evidence of any leakage during the entire mission," explained Chris Webster, a senior research scientist at NASA's Jet Propulsion Laboratory and lead author of the study on the 2014 methane readings -- published in January in the journal Science.

"And while it's true that the concentration of methane in that chamber is 1,000 times higher than in Mars's atmosphere, the comparison is actually misleading," Webster added. "You have to look at the amount of methane, not the concentration."

"The concentration of methane on the rover may seem high, but the actual amount is very small because the chamber is very small," Webster continued. "To produce the amount we detected in Mars's atmosphere, you'd need a gas bottle of pure methane leaking from the rover. And we simply don't have it."

Still, Webster acknowledged that Zahnle's concerns are valid, and that more research must be done to completely rule out the rover as a source of methane. Webster says he and his team will continue to watch for methane spikes and consider new explanations for the 2014 readings.