Researchers studying two of Earth's biggest craters recently discovered that one was caused by an asteroid, not, as was previously thought, by a comet. The difference may seem arbitrary at first, but it may have important implications for scientists attempting to find evidence of primitive life on Mars.

Susan Kieffer of the University of Illinois at Urbana-Champaign, Kevin Pope of Geo Eco Arc Research and Doreen Ames of Natural Resources Canada analysed the structure and composition of the 65-million-year-old Chicxulub crater in Mexico and the 1.8 billion-year-old Sudbury crater in Canada.