Comet-Like Asteroids

Water-rich asteorids
These images from the University of Hawaii's 2.2-meter telescope show the three known main-belt comets at the center of each panel. The fuzz around each comet is suspected to be dust that is ejected as water ice sublimates.
H. Hsieh and D. Jewitt (University of Hawaii)
Two astronomers have uncovered a new class of objects orbiting the Sun between Mars and Jupiter: comets disguised as asteroids.

Henry Hsieh and David Jewitt (University of Hawaii) report in the March 23rd Science Express that main-belt Asteroid 118401 (1999 RE70) is ejecting a comet-like dust tail. Although the tail is too faint for spectroscopic measurements, Hsieh and Jewitt infer from the dust's velocity and the long duration of activity that the tail forms as water ice sublimates (turns directly from solid to gas) off the surface.

If the dust is indeed water rich, the discovery, along with another tail-spewing asteroid (P/2005 U1), and 133P/Elst-Pizarro, means there are now three main-belt "comets."

Icy bodies normally dwell in the coldest parts of the solar system the Kuiper Belt and Oort Cloud. When they venture inside Jupiter's orbit, they sublimate, which is how comets acquire their tails. Astronomers have long assumed that any ice on main-belt asteroids disappeared eons ago. But Hsieh says the venting asteroids provide compelling evidence for water today. "The activity we see can only be explained by ice sublimating," he says.

The three comet-like asteroids appear to be in stable, roughly circular orbits, just like the rest of the main-belt asteroids. Since computer models suggest that it is highly unlikely that outer-solar-system comets could be deflected into a circular orbit, the asteroids probably formed within the main belt, with a thick regolith protecting their icy interiors. We are seeing sublimation now, perhaps because an impact might have punched a hole in this regolith, allowing ice to escape.

Hsieh and Jewitt speculate that other asteroids between Mars and Jupiter might be similarly ice-rich, and that these ice-rich asteroids might have contributed to Earth's oceans. Geologists have long thought that comet impacts must have delivered most of Earth's water. But when researchers examined ice from Oort Cloud comets, they found isotopic ratios drastically different from those seen in water from Earth's oceans. Dynamically speaking, the ancient orbits of main-belt asteroids could be the answer, and these results help to provide the chemical evidence needed to back up those claims.

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