A common question asked by the public after Wednesday's announcement of evidence of life on ancient Mars based on the analysis of a Martian meteorite was simply, "How do we know the meteorite came from Mars?"
       The answer comes from a Sherlock Holmes-like detective story, where scientists eliminated all impossible sources for a class of meteorites leaving Mars, as improbable as seemed, as the source.
       There are about a dozen meteorites that fall under a classification known as "SNC", pronounced "snick." SNC is an acronym for Shergottite, Nakhlite, and Chassigny, three similar classes of meteorites that are quite different from all other known classes of meteorites..
       In the mid-1970s, studies of the nakhlite class of meteorites showed they were much younger than typical meteorites, with an average age of only about 1.3 billion years, billions of years younger than other small meteorites. The chemical composition of these objects was also unusual, with abundances of rare-earth and other elements more typical of the Earth than of other meteorites.
       The first proposals that the SNC meteorites were from Mars came in the late 1970s. These proposals were formed by process of elimination: all other parent bodies for these meteorites were rejected to their chemical composition or orbital dynamics.
       New experimental techniques in the mid-1980s provided the first solid proof that the SNC meteorites came from Mars. Scientists were able to study tiny pockets of Martian air trapped inside of these meteorites. The ratios of isotopes of argon and xenon, two noble gases, found in the meteorites were very similar to ratios measured by the Viking spacecraft on Mars.
       More substantial proof came soon after, when researchers found an enrichment of the isotope nitrogen-15. This enrichment was very similar to what is found in Mars's atmosphere, and is not found anywhere else in the solar system.
       Although Mars was pinned down as the source of the SNC's, the mechanism for removing the rocks from the Martian surface and bringing them to Earth was still unknown. By the late 1980s, Ann Vickery and Jay Melosh at the University of Arizona found that ejection by a large impact was the most likely method for removing the rocks from Mars.
       Vickery and Melosh proposed that a single impact event around 200 million years ago ejected all the known SNC meteorites, which traveled through space for millions of years before reaching the Earth.