PSRD: The Importance of When

Unraveling a planet's geologic evolution requires knowing what happened and when it happened. Two recent studies of the ages of Martian meteorites illuminate the early and late stages of Martian crust formation. One, by Munir Humayun (Florida State University) and colleagues in Australia and France studied Martian meteorite NWA 7533, an impact melt breccia composed of alkali-rich basalt similar to those at the Gusev landing site analyzed by instruments onboard the Spirit rover, and impact-modified products of such basalts. Humayun and colleagues determined the age of rock fragments in the meteorite by using secondary-ion mass spectrometry (SIMS) to date the mineral zircon. They report an age of 4.428±0.025 billion years, indicating formation of a substantial amount of the Martian crust by that time, a mere 100 million years after the planet formed.