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Deep In A Comet: Scientists See Signs Of Evaporating Ice Grains

University of Wisconsin-Madison

Peering deep into the heart of comet Hyakutake, scientists have found evidence that small, evaporating ice particles in the tail and surrounding the nucleus of the comet are producing most of the water and other gases seen from Earth.

The discovery, reported in today's (Aug. 1) issue of the journal Science by a team led by University of Wisconsin-Madison astronomer Walter M. Harris, adds new and difficult-to-detect features to the anatomy of a comet, and helps portray how the nucleus of a comet survives many orbits about the sun without being wasted away by the effects of solar radiation.

Using state-of-the-art optics to look deep within comet Hyakutake, astronomers have found that a halo of marble-sized ice particles is responsible for most of the gaseous features of the comet as seen from Earth. The new work, reported in the Aug. 1 issue of the journal Science, adds new, previously unseen features to the anatomy of a comet, including two gaseous arcs seen in the tail. The arcs are apparently unique to comet Hyakutake. This false-color reconstruction of the comet is based on observations made by University of Wisconsin-Madison astronomers using the 3.5 meter WIYN Telescope atop Kitt Peak, Ariz. The new WIYN Telescope was used to make some of the most precise optical measurements ever of a comet by a ground-based telescope.

"For the first time, we are getting a detailed look at the inner workings of a comet from a ground-based observatory," said Harris of the comet that orbited within 10 million miles of Earth early last year. It was the closest encounter of a major comet with the planet since the advent of modern telescopes capable of probing the hearts of the icy balls of dust and water that spend most of their lives on the frigid margins of the solar system.

Using the new 3.5 meter WIYN Telescope atop Kitt Peak, Ariz., Harris and his colleagues captured some of the most detailed optical readings ever performed on a comet.

Their findings infer that a previously detected swarm of several hundred billion marble-sized particles surrounding the relatively small nucleus of the comet is composed of icy material evaporating at a rate much greater than that of the nucleus. The observations suggest the halo of ice particles occupies a large volume of space around the nucleus, just 2.5 kilometers in diameter, and is constantly replenished by more particles blown off the nucleus.

The ice particles, said Harris, are literally blown off the nucleus of the comet by the wind created as ice boils off the surface. As it sweeps through the inner solar system, the comet evaporates more rapidly, producing more and larger grains. It is these grains, Harris reported, that apparently produce most of the gaseous features characteristic of comets.

Although the particles are only a fraction of the total mass of the comet, they could be responsible for as much as 90 percent of the gas observed in the coma, a cloudlike mass that with the nucleus form the comet's head.

"What Hyakutake showed us is that the nucleus need not be the only thing producing gas in a comet. And that tells us a lot about comets," said Harris.

"Although the ice grains must be replaced as they evaporate, the total mass of the halo is small enough that a comet can still survive many passages through the inner solar system without losing a significant fraction of its nucleus," Harris noted.

According to the Wisconsin astronomer, the WIYN observations of an ice-particle halo around Hyakutake confirm an idea first suggested by theorists in the 1970s: "It's something we long thought was there, and now we know it almost certainly is there" and that it is a feature of all comets.

The discovery of the ice-particle halo was possible because the orbit of Hyakutake, although a much smaller comet than the more recent Hale-Bopp, carried the comet 120 million miles closer to Earth. It's closer proximity to Earth enabled astronomers using WIYN and its state-of-the-art optics to capture the comet in more detail than any previous comet.

The WIYN observations of Hyakutake also revealed a previously unobserved and possibly unique feature of the comet: bright arcs of neutral gas emission in the tail. Such a feature has never been observed before and Harris speculates that the two arcs seen in Hyakutake may have been the result of two interacting gas flows, one from the head of the comet, and one from the tail.

In addition to Harris, the scientists contributing to the report in Science include: Michael R. Combi of the University of Michigan, R. Kent Honeycutt of Indiana University, Béatrice E. A. Mueller of the National Optical Astronomical Observatories (NOAO) and Frank Scherb of the University of Wisconsin-Madison.

The WIYN Telescope is operated by a consortium that includes the University of Wisconsin-Madison, Indiana and Yale Universities and NOAO. Completed in 1994, the WIYN Telescope has at its heart a 3.5-meter spincast mirror, one of the first of a new generation of telescope mirrors that promises greater resolution and performance than traditional mirrors.

CONTACT: Walter M. Harris, 608-265-3436,

(Editor's note: Astronomer and lead author Walt Harris will be traveling this week, but will be accessible via email. Images of the comet that are suitable for print publications are posted at on a Web page.)


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