It would be an extinction-level impact, were there any life on Mars.
Unlike Global Warming, this stuff is actually real and worrying.
Discovered on January 3, C/2013 A1 Siding Spring was typical of comet finds: a dim moving speck on a photograph. However, recent observations suggest that Comet Siding Spring could be extremely interesting indeed – its current orbit brings it only 37,000 km from Mars.
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Should the C/2013 A1 impact Mars next October, scientists will have a front row seat for one of the rarest of all geological events – an impact capable of causing a mass extinction had it been aimed at Earth instead. As the comet makes its closest approach to Mars, it will be racing outward from the sun at approximately 56 km/s relative to Mars. If the comet were to impact Mars, the energy released would be on the order of trillions of megatons, while the resulting crater could be up to 500km across and 2.5km deep depending on the exact size of the comet.
The 37,000 km is just current best-guess so it may pass further away… or it might hit Mars. Even if it doesn’t hit Mars, it will probably catch the planet in its tail, and the planet may be bombarded with particles that interfere with our robotic scouts there.