The core of the comet will be moving 56 kilometers per sec when it smashes into Mars. The explosion it creates will be equal to 40 BILLION 500kt nuclear bombs(avg US and Russ arsenal weapon) going off at once, or 20 billion megatons of TNT, or 1.33 trillion Hiroshima bombs, or 1.3 billion Castle Bravo explosions. Its enough to kill all life on the planet if any were living there.

Astronomers predict a large-scale disaster on the Red Planet in October 2014, when a comet may ram into Mars, creating a 500-kilometer crater, the website of the Russian observatory ISON-NM said.
[link to english.pravda.ru]


comet core 30 miles across
collision speed 35 miles per second (126,000 miles per hour)

It may make Mars temporarily as bright as the sun. The crater left behind will be 500 kilometers across. A lot of rock will be ejected into space and become stray asteroids. This will be an epic event.