For the first time, researchers have detected water on an asteroid. Two research teams independently determined that the 24 Themis asteroid, which orbits between Mars and Jupiter, is completely covered in a thin layer of frost. The discovery indicates that comets and asteroids may be more similar than previously thought. Comets are traditionally defined as small lumps of rock and ice that form vapor tails when they venture close to the Sun. Asteroids are usually characterized as large dry rocks. Now the distinction between these two types of space rock is not so clear.

The ice on 24 Themis appears to have been present for billions of years. Researchers suggest that an icy reservoir within the asteroid may continually replenish the surface frost. This find is prompting astronomers to hypothesize that an asteroid collision with early Earth might explain how water was delivered to our planet.