Is a comet due to hit Mars in October, 2014? A March 4, 2013 article in Space.com poses that question. The short version of the answer is, probably not. The consensus is that comet C/2013 A1, discovered recently, will pass by Mars by a distance of about 63,000 miles. But there is a great deal of uncertainty surrounding the question.
Due to uncertainties about the comet’s size and mass, its exact direction, and a comet’s tendency to alter course due to out-gassing, scientists do not really know what could happen. C/2013 A1 could give Mars a wide berth. Or it could hit the Red Planet dead on.
The effects of a collision would be a planet wide catastrophe. The UK Daily Mail suggests that the resulting explosion would be in the range of a billion megatons, sending debris that would destroy every probe on the surface or in orbit around Mars. RT posits that climate change that would result would be almost unimaginable. Water and frozen carbon dioxide in the comet would be vaporized and spread out around Mars, raising its temperature. On the other hand, dust would be kicked up in the Martian atmosphere and long dormant volcanoes could erupt, providing a cooling effect.
If the comet were to hit Mars, scientists would have a once in an eon opportunity to observe the effects of such an event. MAVEN, the NASA probe that is designed to examine the Martian atmosphere, is due to assume orbit around the Red Planet shortly before the comet would hit. It would be in position to examine the effects until and unless (if the UK Mail is correct) the blast destroys it.