From the Economist (h/t Instapundit):
For a start, [an impact] should be simply spectacular. Given the unusual speed of the comet (which is moving so fast that it may well be coming from outside the solar system—cool upon cool) and the fact that it is travelling the wrong way round the sun, from a planet’s point of view, Mr Plait estimates that its impact should yield a blast equivalent to that of a billion megatons of TNT. It would be an event on the same sort of scale as the impact that drove the dinosaurs extinct 65m years ago. If it really is that big, and if the comet were to hit the side of Mars facing Earth (it seems that it might do, but it might also hit the far side), then the blast could well be visible to the naked eye, even in daylight.
It is worth pointing out that, for C2013 A1 (Siding Spring), this is the nearest thing to a direct effect on Earth that can be imagined. The recent meteor break-up over Russia, and the close pass of asteroid 2012 DA14, have reminded people that Earth, and all other planets, are vulnerable to intervention from outside. But this particular comet is not coming anywhere near Earth.
That does not mean it would not have scientists salivating. Observing such an impact, which might leave a crater hundreds of kilometres across, would be a huge scientific boon. Of the geological processes which shape the surfaces of planets, impacts are the ones that humans have had the least opportunity to observe up close. The faces of the moon, of Mars and of Mercury show the aftereffects of such impacts, but the processes by which they hollow out their craters and raise their rims and central peaks have only been witnessed in miniature. If a fair sized comet smacks into the surface of Mars, telescopes on Earth and in orbit around it will be able to see large-scale cratering in real time and remarkable detail. Telescopes orbiting Mars will do better still—but perhaps not for long. The amount of debris thrown up from the planet’s surface and into space would seem likely to make their orbits quite hazardous.
The cratering process would be interesting to geologists. Its aftermath would be fascinating to astrobiologists. There is a lot of ice frozen into the Martian crust. The heat of an enormous impact would melt a huge amount of it. If, as some believe, there are microbes living deep under the Martian surface, such a burst of warm, wet conditions over a substantial chunk of the planet would give them a brief chance to thrive at and close to the surface before the planet refroze. It’s not obvious how to observe such exciting developments, but there are surely already people at NASA and elsewhere giving thought to the matter. And they will have time. Parts of the surface and subsurface in the impact region, if there is an impact, will stay warm for decades.
In all likelihood there won’t be an impact. But even a near miss could be exciting and, for humankind’s emissaries on Mars, spectacular. The various spacecraft now orbiting Mars will get to see a fresh comet at very close quarters. There is a good chance that they will pass through the comet’s “coma”, the comparatively dense (though still remarkably thin) cloud of dust and gas that surrounds the solid nucleus. This will subject them to some risk, as indispensable planetary science blogger Emily Lakdawalla points out—the coma has some grit in it, and at 55km a second a little grit can do a lot of damage. The risks a close approach would pose to the spacecraft currently on the Martian surface, Opportunity and Curiosity, would probably be negligible, and they, too, should get some spectacular pictures to send back to Earth. As Donald Yeomans of NASA says, "unless this comet completely fizzles, it should be extraordinary as seen with Mars-based assets".
And if the comet passes close enough to the planet it may allow a natural experiment. Over the past decade there has been much discussion of the possibility that there might be methane on Mars, possibly produced by the aforementioned subterranean microbes. Various observers claim to have seen evidence for the gas, but theoretical arguments cast serious doubt on their results. One of the questions in play is how fast the Martian environment can oxidise organic compounds (such as methane) which get pumped or dumped into it. A very close encounter with a comet might result in a measurable pulse of organic matter being introduced into the upper atmosphere; its fate would be interesting to track.