Scientists have made great strides in predicting what will happen to Earth's climate. Unfortunately, here is a fundamental problem: We only have one climate to test our hypotheses in. We can't irreversibly hack Earth's climate to test whether our assumptions are right or wrong. We would test by pumping it full of toxic gases, for example, but that, obviously, would be disastrous for Earth's inhabitants. The car-sized mobile chemistry lab that dropped spectacularly onto the surface of Mars yesterday, will give scientists a rare chance to test their assumptions about how climate change works on Earth.