Australian National University scientists are a step closer to pinpointing the source of methane on Mars, which they believe could be a sign of life or other biological activity under the planet's surface.
The origin of methane puffing from a large crater on the red planet has been a topic of speculation among scientists for more than a decade.
Gale Crater, which is about 3.8 billion years old and 154 kilometres wide, is thought by some to contain an ancient lakebed.
The mystery of where the methane is coming from deepened when the ExoMars Trace Gas Orbiter satellite and the NASA rover Curiosity collected data that showed wildly different detections of methane and seemingly contradicted each other.
A new study led by Dr John Moores, an Australian National University visiting fellow based in Canada, has shed light on the reasons behind the conflicting data, revealing that methane concentrations on Mars change over the course of each day.
"We were able to resolve these differences by showing how concentrations of methane were much lower in the atmosphere during the day and significantly higher near the planet's surface at night, as heat transfer lessens," Dr Moores said.
"This new study redefines our understanding of how the concentration of methane in the atmosphere of Mars changes over time, and this helps us to solve the bigger mystery of what the source might be."
Dr Moores said with its improved estimates of the amount of methane on Mars, the research team had calculated the rate of seepage from Gale Crater for the first time.
He said the figure was equivalent to an average of 2.8 kilograms per Martian day.
Professor Penny King, a co-researcher from the ANU Research School of Earth Sciences, said there were several equally plausible explanations for the methane detected on Mars.
"Some microbes on Earth can survive without oxygen, deep underground, and release methane as part of their waste," she said.
"The methane on Mars has other possible sources, such as water-rock reactions or decomposing materials containing methane."
The exploration of Mars is heating up, with six spacecraft scheduled to launch for Mars missions next year alone. They include NASA's Mars 2020 rover, the European Space Agency's ExoMars rover, the United Arab Emirates' Hope orbiter, a Japanese orbiter and a Chinese rover. SpaceX, owned by entrepreneur Elon Musk, also plans to send one of its Dragon capsules.