New Class of Comets May Be Source of Earth's Water

by Henry Hsieh and David Jewitt

Main-belt comet orbits are shown in red.

Earth is believed to have formed hot and dry, meaning that its current water must have been delivered after the planet cooled. Possible candidates for supplying this water are comets and asteroids that collided with Earth. Because of their large ice content, comets were the leading candidates for many years, but recent analysis of comet water has shown that it is significantly different from typical ocean water on Earth.

Three icy comets orbiting among the rocky asteroids in the main asteroid belt between Mars and Jupiter may hold clues to the origin of Earth's oceans. This newly discovered group of comets, which we have dubbed "main-belt comets," has asteroid-like orbits and, unlike other comets, appears to have formed in the warm inner solar system inside the orbit of Jupiter rather than in the cold outer solar system beyond Neptune. The existence of these main-belt comets suggests that asteroids and comets are much more closely related than previously thought and supports the idea that icy objects from the main asteroid belt could be a major source of Earth's present-day water.

As conventionally defined, comets and asteroids are very different. Both are a few to a few hundred miles across and orbit in our solar system. Comets, however, are thought to originate in the cold outer solar system and consequently contain much more ice than the asteroids, most of which are thought to have formed in the asteroid belt between Mars and Jupiter.

Comets also have large, elongated orbits and thus experience wide temperature variations. When a comet approaches the Sun, its ice heats up and sublimes, so it vents gas and dust into space. This creates a tail and a distinctive fuzzy appearance. In contrast, asteroids have nearly circular orbits and are expected to be mostly baked dry of ice by their confinement to the inner solar system. Essentially, they should be just rocks. With the discovery of the main-belt comets, we now know this is not the case, and that the conventional definitions of comets and asteroids are in need of refinement, since main-belt comets do not fit neatly into either category.

Asteroidal ice may give a better match to Earth's water. Until now, scientists thought that any ice that the asteroids may have once contained was either long gone or so deeply buried inside large asteroids as to be inaccessible. The discovery of main-belt comets means that this ice is not gone and is still accessible on at least some objects in the main belt. Spacecraft missions to the main-belt comets could provide more detailed information about their ice content and give us new insights into the origin of water and, ultimately, life on Earth.

More about main-belt comets