Deep impact: a spacecraft breaks open a comet's secrets

Science News, Sept 10, 2005 by Ron Cowen

Moving serenely through space some 130 million kilometers from Earth, Comet Tempel 1 appears no different than it did before July 4, the day a NASA spacecraft called Deep Impact fired a 372-kilogram copper projectile into the comet's icy surface. But if the impact left barely a dent in this 9-kilometer-long, fist-shaped body, the data collected from the collision have made an indelible mark on studies of comets and the formation of the solar system.

Observations of the crash suggest that scientists are for the first time "directly measuring pristine material from deep inside a comet, material that has been locked away since the beginnings of the solar system," says Deep Impact researcher Carey Lisse of the University of Maryland in College Park and the Johns Hopkins Applied Physics Laboratory in Laurel, Md.

The fireworks generated by the impact, along with close-up portraits of Tempel 1 taken just before the collision, have also revealed several surprises. The data, says Lisse, are at odds with a leading model for the structure of comets called the dirty-snowball model. The model assumes that comets, born during the era of planet formation 4.5 billion years ago, consist primarily of an agglomeration of frozen carbon dioxide, water, and other ices, mixed with a smattering of hydrocarbon gunk and grains of dust.

But the data from the Deep Impact mission indicate that although Tempel 1 contains some ices, its primary constituent may be dust particles finer than talcum power. The comet--and perhaps many others--may resemble an icy dirt ball more than it does a dirty snowball, says Lisse.

Held together only by gravity, the comet is much weaker and far more porous than a solid chunk of ice. Its structure is more fragile than that of a souffle, says Jay Melosh of the University of Arizona in Tucson.

What's more, the comet isn't a mere hodgepodge of different materials and structures. "The damn thing is layered like a frozen onion," says Deep Impact scientist Joseph Veverka of Cornell University.

The Deep Impact team, led by Michael A'Hearn of the University of Maryland at College Park, presented its early findings this week at the annual meeting of the American Astronomical Society's Division for Planetary Science in Cambridge, England. Researchers also describe their analyses in a trio of papers posted online this week for publication in an upcoming Science.

Deep Impact's revelations "are going to change lot of our ideas about comets," predicts Melosh.

CHRONICLING THE FIREWORKS Planetary scientists study comets to learn about the early solar system. These icy relics formed from a swirling cloud of gas, dust, and ice that circled the young sun 4.5 billion years ago. Now, comets serve as time capsules from that long-ago era. Researchers also suspect that comets ferried the organic compounds, water, and other ingredients to Earth that set up the chemistry that made life possible on our planet.

Although planets and asteroids coalesced from that same cloud of matter, those big bodies underwent episodes of violent heating and melting that obscured or erased signs of their early history. Comets, on the other hand, spend most of their time in the deep freeze of the outer solar system. There, they remain quiescent and their materials stay relatively unaltered. Only when comets come near the sun do they come alive, vaporizing gas and dust and flaunting their classic tails.

But despite the importance of these icy outposts, astronomers have only the barest storyline of how comets form. "We simply don't have any idea how you go from ... tiny pieces of dust and ice, one-tenth to one-hundredth the width of a human hair, to building a comet," notes Lisse.

From the impact mission, which was monitored by some 80 telescopes in space and on the ground, "we're learning about the initial recipe for making comets--how much carbon, how much rock, how much water," says Lisse. "If we give theorists the recipe, they can tell us how planet formation happens, and that's a giant step."

Much of the information comes from images and spectra of the dust and vapor that belched from Tempel 1 for some 2 days following the Independence Day blast.

Just milliseconds after the impact, the spacecraft recorded a faint flash that faded away in less than a second. Melosh and his collaborators propose that the flash denotes the instant when the 1-meter-wide bullet, coming in at an angle of about 60[degrees] from the vertical, hit the surface.

A fraction of a second later, as the bullet began boring into the comet, an incandescent, hot spray erupted, traveling about 10 km per second. "[That] explosion is so violent that everything in its path is boiled off and swept out," says Lisse.

Consisting of searing vapor and droplets of melted silicate at a temperature of 3,800 kelvins, the spray was so bright that it completely overwhelmed the solid-state detectors on the flyby spacecraft, stationed about 800 km away. Infrared spectra indicate the droplets were 10 to 100 nanometers in diameter.