One of the goals of the missions to Mars is to find signs of life. Some of the best signs to date are periodic “burps” of methane – a compound that can be produced by living organisms. But the burps aren’t common – and they’re hard to see.

In June, for example, the Curiosity rover detected the heaviest concentration of methane yet seen – more than 20 parts per billion. That’s dozens of times higher than the average level. But two spacecraft in Mars orbit passed above Curiosity within hours – and didn’t “sniff” a thing.

Here on Earth, most of the methane in the atmosphere is produced by life – from bacteria to people. So methane on Mars could be produced by life as well. But it also could be produced by chemical reactions in the rocks, or by volcanoes.

Several methane outbursts have been seen over the years – by orbiters, telescopes on Earth, and especially by Curiosity.

Some of the rover’s detections have also been seen by Mars orbiters. But the one in June wasn’t. That might be because Curiosity was looking at night, the orbiters during the day. Sunlight destroys methane, so it might have disappeared by the time the orbiters rolled into view.

Scientists are trying to pinpoint where the methane is coming from. That should help them figure out whether it’s produced by life on the Red Planet.

Mars teams up with the Moon at first light tomorrow. It looks like an orange star above the Moon. Saturn and Jupiter line up to their upper right.

Script by Damond Benningfield