One possibility is that they formed deep underground and were later pushed to the surface by internal geological processes. Just as on Earth, those internal forces are mostly powered by heat from the decay of radioactive elements. The heat is carried away by the motion of the material, just as heating water at the bottom of a pot causes it to rise and then make complex convection patterns. The strength of the forces depends on the rate at which the heat leaks from the deep interior to the ground. That is, heat is a form of energy, and a faster flow of heat energy (and thus of material) would provide a more powerful internal engine to drive minerals to the surface.

Heat flows from hot (far underground) to cold (the surface, which is exposed to space). It is at least 80 degrees Fahrenheit (50 degrees Celsius) colder near Ceres’ north and south poles than near the equator. That means the strength of the geological pressure pushing minerals to the surface should depend on the latitude, which would translate into different compositions at different latitudes. But that is not what Dawn sees. The minerals show up everywhere we look. Their prevalence is a fact that is inconsistent with a deep underground origin followed by a heat-driven movement to the surface. Science tells us we need to formulate a different explanation for why minerals produced in water under high pressure now can be found on the ground.

Scientists recognize a more likely explanation. The minerals may have formed in an ocean early in Ceres’ history, when radioactive elements were so abundant that it would have been warm enough to keep a large volume of water as a liquid. But as Ceres aged, it would have cooled (perhaps some readers have experienced this as well), because the supply of radioactive elements would have gradually been depleted as they decayed. Almost the entire ocean would have frozen, encasing Ceres in a shell of ice. But that wouldn’t be the end of the story.

Ice cannot last long on Ceres (except in special places). Cold though it is on that world, there is enough warmth from the distant sun that ice sublimates, turning from a solid into a gas as the water molecules escape into space. Even as that gradual phenomenon occurred at the microscopic level, ice was lost through a much more dramatic and abrupt process. It was blasted away by asteroids that slammed into it. The rain of rocks that fall onto Ceres over millions of years is a familiar hazard to anyone who has lived in the main asteroid belt for millions of years. In fact, scientists estimate that a frozen ocean three miles (five kilometers) thick could have been lost in only a few tens of millions of years, a blink in geological time. (And even if that ice shell had been much thicker, it would still have been lost on a geologically short timescale.)