REBECCA JONES, VIA EMAIL
HOW DO METEORITES FROM MARS END UP ON EARTH?
There are several strands of evidence implying that some meteorites found on Earth actually originate from Mars. Scientists have used a technique called ‘radiometric dating’ to determine the ages of these chunks of rock. The method, which is analogous to radiocarbon dating of organic material, looks at the ratios of certain radioactive isotopes in the meteorites. Since isotopes decay at well-determined rates, a comparison of their ratios reveals the time since the meteorite was formed, or at least the time since it was last heated. Most meteorites turn out to be about 4.56 billion years old, because they come from asteroids that date from the creation of the Solar System, 4.56 billion years ago. Anything younger than this must be from a planet or moon, the only places in the Solar System that could have formed rocks younger than 4.65 billion years.