Complex sentence: Since then, methane has been discovered in Mars' atmosphere at 10ppb, thus reopening this debate, although in 2013 the Curiosity rover failed to detect methane at its location in levels exceeding 1.3ppb.
Split into simpler sentences: Since then, methane has been discovered in Mars' atmosphere at 10ppb, thus reopening this debate. Although in 2013 the Curiosity rover failed to detect methane at its location in levels exceeding 1.3ppb.