Sunlight causes methane to decay, and Mars is close enough to the sun for it to decay quickly. IIRC, the estimate is that, without a source to replenish it, any

Sunlight causes methane to decay, and Mars is close enough to the sun for it to decay quickly. IIRC, the estimate is that, without a source to replenish it, any methane in Mars's atmosphere should be gone after only a few centuries.