The dwarf planet Ceres reoriented itself early in its history due to changes in the density of its crust, suggests a study published online this week in Nature Geoscience. Such a reorientation may explain some fractures and a planet-circling ridge seen on Ceres’ surface.

Some planets and moons in our solar system are thought to have reoriented, shifting the location of their poles in response to changes in the distribution of their masses. Reorientations can result from processes such as mountain building. Mars, for example, is thought to have shifted as to move the massive Tharsis volcanic region to a more stable position nearer the equator. Plate tectonics, which can also drive reorientations, is believed to have gradually reoriented Earth in the past. Orbiting Ceres, NASA’s Dawn spacecraft has found evidence that the dwarf planet has a variably ice-rich crust, which could indicate large differences in density across the body.

Pasquale Tricarico analyses gravity and shape data taken by Dawn, finding an area of dense crust near Ceres’ present equator that might have driven a reorientation. He also finds evidence of an ancient ridge circling the dwarf planet at an angle of 36° from the present equator. This ridge may represent Ceres’ original equator, which could have bulged outwards during reorientation because of its relatively thick crust. The distribution of the large fractures seen on the surface of the crust is also consistent with the proposed reorientation event - suggesting a complex interior history for Ceres, and perhaps other similarly sized icy bodies.