Rocks heat up and cool off relatively quickly. By contrast, soils can’t conduct heat as well and so they stay warmer, longer. The measured night-to-day temperatures for asteroid Bennu, the target of the recent Osiris-Rex sample mission, suggested that its surface was very powdery; that’s what they designed the spacecraft’s sampling gizmo to expect. But close-up images of Bennu revealed a surface covered with boulders, not soil. What went wrong?

Our lab at the Vatican Observatory measures the physical properties of meteorites, including the kind thought to make up Bennu, and recently we discovered that in just the temperature range experienced between day and night on Bennu (-50C to -75C), these particular meteorites flex in an unexpected way that can fill them with many tiny cracks. Yet they can maintain their shapes as boulders, even as they turn into soft clumps of soil, because there’s little gravity or other forces on the asteroid to pull these clumps apart.