Miércoles 14 de Septiembre de 2005, Ip nº 126

Composition of a Comet Poses a Puzzle for Scientists
Por KENNETH CHANG

Although comets form at the frigid edges of the solar system, they appear somehow to contain minerals that form only in the presence of liquid water, and at much warmer temperatures, scientists are reporting today.

On July 4, as planned, part of the Deep Impact spacecraft - essentially an 820-pound, washing machine-size bullet - slammed into the comet Tempel 1 at 23,000 miles an hour. The collision tossed up thousands of tons of ice and dust from the comet that were observed by telescopes on Earth as well as small flotilla of spacecraft.

One of the observers was the Spitzer Space Telescope, a NASA mission that takes pictures in the infrared part of the spectrum. In the burst of light after the collision, Spitzer detected specific colors of infrared light that indicated that Tempel 1 contained clays and carbonates, the minerals of limestone and seashells.

Clays and carbonates both require liquid water to form.

"How do clays and carbonates form in frozen comets where there isn't liquid water?" said Carey M. Lisse, a research scientist at the Applied Physics Laboratory at Johns Hopkins University who is presenting the Spitzer data today at a meeting of the Division for Planetary Sciences in Cambridge, England. "Nobody expected this."

Spitzer also detected minerals known as crystalline silicates. Astronomers had already known that comets contain silicates, but silicates line up in neat crystal structures only when they are warmed to 1,300 degrees Fahrenheit - temperatures reached at around the orbit of Mercury - and then cooled.

"How do you do that and then how do you put that stuff into a comet that forms out by Pluto?" Dr. Lisse said.

Dr. Lisse said that the presence of the clays, carbonates and crystalline silicates indicated that material in the solar system's primordial cloud had somehow become well-mixed with dust from the inner solar system migrating outward, or that the minerals had formed through unexpected chemical reactions.

"Both of these are speculation," Dr. Lisse said. "Hopefully in a few months I can tell you."

Another possibility is that the changes are more recent. Within the last 10,000 years, Tempel 1 migrated to the warmer inner solar system, between Mars and Jupiter, but Dr. Lisse said he doubted that was long enough for the makeup of the comet to change much.

Observations of the Deep Impact collision confirmed that the comet is mostly empty space. The outer layers of Tempel 1 are "unbelievably fragile, less strong than a snow bank," said Michael A'Hearn, the mission's principal investigator, during a telephone news conference yesterday.

"There is no indication we got down to any solid ice."

The comet, about five miles long and three miles wide, is fluffy and porous, with about 75 percent of it just empty space, Dr. A'Hearn said.

"The ice is all in the form of tiny grains" from 0.00004 to 0.004 inches in diameter, he said.

Dr. Lisse described Tempel 1 as an object the size of a city "that has the strength of lemon meringue."

Earth-based telescopes also spotted a jump in the number of organic molecules flying out from the comet after the impact.

Astronomers are interested in comets because they are remnants of the early system and are thought to have supplied Earth with many of the chemical building blocks that led to life.

It is still not clear how large a crater Deep Impact carved out, but Dr. A'Hearn said it was probably tens of yards deep and about 100 yards wide.

Three scientific papers describing initial scientific results from Deep Impact will be published this week on the Web site of the journal Science.


  05/09/2005. The New York Times.