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September 7, 2005 02:30 PM (EDT)

News Release Number: STScI-2005-27

Largest Asteroid May Be 'Mini Planet' with Water Ice

September 7, 2005: Observations of Ceres, the largest known asteroid, have revealed that the object may be a "mini planet," sharing many characteristics of the rocky, terrestrial planets like Earth. Ceres' mantle, which wraps around the asteroid's core, may even be composed of water ice. The observations by NASA's Hubble Space Telescope also show that the asteroid has a nearly round shape like Earth's and may have a rocky inner core and a thin, dusty outer crust. Astronomers enhanced the sharpness in these images to bring out features on Ceres' surface, including brighter amd darker regions that could be asteroid impact features. The observations were made in visible and ultraviolet light between December 2003 and January 2004.

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Q & A: Understanding the Discovery

  1. 1. Why is Ceres different from other asteroids?

  2. Ceres has a nearly round body and a "differentiated interior," with denser material at the core and lighter minerals near the surface. All terrestrial planets have differentiated interiors. Pictures of asteroids much smaller than Ceres show those objects to be irregularly shaped and to have a uniform composition.

  3. 2. Does the possibility of water ice on Ceres mean that life could exist?

  4. Ceres does not have an atmosphere or liquid water, which are necessary for life as we know it. The water ice that may be present on Ceres is beneath the surface. It may be, however, a natural resource for future space explorers.

  5. 3. How are asteroids different from comets?

  6. Asteroids are composed mostly of rock while comets are made up mostly of dust and ice. The asteroid Ceres, however, may be a layered mixture of rock and ice. The vast majority of asteroids is found in the asteroid belt, a region between the orbits of Mars and Jupiter. Comets, on the other hand, orbit the Sun in paths that either allow them to pass by the Sun only once or that repeatedly bring them through the solar system (as in the 76-year orbit of Halley's Comet). A comet's "signature" long, glowing tail is formed when the Sun's heat warms the coma or nucleus, which releases vapors into space. Asteroids do not have tails.

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Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), L. McFadden (University of Maryland, College Park), and M. Mutchler and Z. Levay (STScI)