NASA's Curiosity rover has detected methane on Mars several times since landing in 2012, but last week, it measured the highest level yet: 21 parts per billion. This is an "unusually high" level of the odorless, colorless gas, NASA says, but the agency cautioned that while methane is produced by living organisms, this is not absolute proof of life on Mars. "While increased methane levels measured by @MarsCuriosity are exciting, as possible indicators for life, it's important to remember this is an early science result," NASA's Thomas Zurbuchen tweeted. On Earth, major sources of methane include cattle and the production of fossil fuels. Scientists will analyze the information and plan on conducting more observations.

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