Number 262 (Story #4), March 14, 1996 by Phillip F. Schewe and Ben Stein IN THE LATE HEAVY BOMBARDMENT (LHB) EPOCH , a span of about 200 million years some 4 billion years ago, the Moon sustained many large impacts. Some astronomers believe that the projectiles responsible may have pestered Mercury, Venus, Earth, and Mars as well. Others assert that the LHB phenomenon was unique to the Earth-Moon system or that it did not happen at all, at least not so suddenly. Now, a group of scientists at the University of Manchester (UK) has dated a rock found here on Earth but which is believed to have been a meteorite originating at Mars. The 4-billion-year age of the object, determined by isotope dating, is much older than previously studied Martian meteorites. The antiquity of the rock, say the researchers, provides evidence for a widespread LHB effect. (R.D. Ash et al, Nature, 7 March 1996.)