Ice among the rocks.

Researchers have found a trio of icy comets hidden among the thousands of rocks in the main asteroid belt between the orbits of Mars and Jupiter. Such comets, which don't fit into any known class, could have been primary sources of water that transformed the early, dry Earth.

Each member of the trio, dubbed main-belt comets by codiscoverers Henry Hsieh and David Jewitt of the University of Hawaii in Honolulu, appears to have formed inside Jupiter's orbit. That's in contrast to most other comets, which were born beyond Neptune in the chilliest parts of the solar systems.

The existence of the main-belt comets indicates that asteroids and comets--rocks and ice chunks, respectively--could be more closely related than previously proposed. If icy bodies are relatively common in the asteroid belt, they could have provided some of the water delivered to Earth about 4 billion years ago, Jewitt and his collaborators suggest in an upcoming Science.

Using the Gemini North Telescope atop Hawaii's Manna Kea, the team late last fall found that an object, known as asteroid 118401, was spewing an abundance of dust, just as a comet does during its journey toward the sun. Two other known comets, called 133P/Elst-Pizarro and P/2005 U1, show similar behavior and orbits.

Most comets have elongated, tilted orbits relative to the planets' paths around the sun. But, like asteroids, the main-belt comets have circular orbits that lie in the planets' plane. The dust-spewing main-belt comets thus resemble a hybrid of asteroids and comets, the team suggests.