I would be inclined to send an e-mail back to the person who forwarded it as follows:
Well mate, you better go get your family and prepare to spend your last few days with them, because after Mars has left its orbital path and is on a collision course for earth orbit, the earths crust will crack, earthquakes and volcanoes, tidal waves that Steven Spielberg couldn't make look real, fire, ice brimstone and death will spell the end of all life as we know it.
Have a nice day.
YOU TWIT!
You know the word "Gullible" is not in the dictionary?