Why Does Mercury Have a Big Iron Core? (IMAGE) Tohoku University Caption The Cores of the terrestrial planets. The four terrestrial planets each have iron-rich cores surrounded by rocky mantles. Core size scales with distance from the sun, Mercury has the biggest and Mars the smallest. Credit NASA Usage Restrictions Credit: NASA License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.