SAN FRANCISCO — Life on Mars? Today? The notion may not be so far-fetched after all.

A year after reporting that NASA’s Curiosity rover had found no evidence of methane gas on Mars, all but dashing hopes that organisms might be living there now, scientists reversed themselves on Tuesday.

Curiosity has now recorded a burst of methane that lasted at least two months.

For now, scientists have just two possible explanations for the methane. One is that it is the waste product of certain living microbes.

“It is one of the few hypotheses that we can propose that we must consider as we go forward,” said John P. Grotzinger, the mission’s project scientist.

The scientists also reported that for the first time, they had confirmed the presence of carbon-based organic molecules in a rock sample. The so-called organics are not direct signs of life, past or present, but they lend weight to the possibility that Mars had the ingredients required for life, and may even still have them.