Rethinking The C1 Carbonaceous Chondrite Standard. New analysis of data from the Mars Pathfinder Mission has revived a nagging question that was first posed nearly 50 years ago: why do the inner planets exhibit different mean densities when presumably they formed from the same material? The new analysis, performed at the Carnegie Institution of Washington, suggests that one current theory explaining density variations is wrong, and that future modelers of inner solar system accretion must account for a set of inner planets with differing elemental compositions.