I was unclear. The detected oxygen does not come from meteors, but from the surfaces of the moons instead. Remember, surfaces of these moons (except for Io) are icy.

Atmosphere composed of oxygen is not terribly good sign of life. But if we discover an Earthlike planet with ozone in its atmosphere (meaning there is oxygen) and methane we can be certain that something very interesting is happening there. Methane doesn't survive for long in an oxygen atmosphere, so something must replenish it. On Earth, that something is life. Some methane is produced by volcanism, though. On Mars, the methane detected is likely generated by geological processes where certain compounds from rocks react with water. Or from volcanism, meaning the planet is not dead. And, if the least likely and the most profound possibility is true, there's living organisms under the surface of Mars.

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Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.
-- Richard Feynman