The 54-pound meteor in this picture is the largest known piece of Mars on Earth. A PROXY is a substitute for something else. It is tempting for scientists to use Martian meteorites as PROXIES for studying the Red Planet’s geochemical properties. But some researchers caution that a true understanding of Martian geology should not be based upon random meteorites. PROXY has become a favorite answer on recent vocabulary in context questions. For example, one passage pointed out that paleontologists use the respiratory systems of modern birds as PROXIES for studying dinosaurs.