Deep Impact

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 soufflé, 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° 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.

The high-velocity spray, which lasted for less than a second, was most likely created as the copper bullet vaporized comet material some 20 to 30 m beneath Tempel 1’s surface. The bullet melted or boiled about 10 times its weight in ice and rocky particles.

Like a hammer that has hit a sand pile, the bullet left behind a slow-moving sound wave or shock wave. This shock wave, spreading gradually through the comet’s interior for as long as 5 minutes, appears to have carved out a deep crater. That’s an indication that the comet is fluffy, Lisse notes. In a more-solid object, the shock would have bored a shallower hole, he says.

The wave kicked up a plume of cool, extremely fine dust that lingered for more than 40 hours. Although the shock ejected an estimated 10 million kg of material—roughly the weight of 10,000 cars—that’s still only about one-ten-millionth of the comet’s mass.

Most of the dust lofted into space by the shock wave had an average velocity of about 1 meter per second. That low speed was still enough to overcome the comet’s weak gravity, which is about one-millionth that of Earth, notes Jim Richardson of Cornell University. He likens the dust particles in the plume to baseballs lofted into the air in slow motion and taking as long as 2 days to fall back to the surface.

Richardson estimates that in 2 days, about 95 percent of the dust particles had fallen back onto the comet. Because the particles were so small—most no more than 100 micrometers in diameter—they scattered nearly all the sunlight falling on them, rendering the funnel-shaped plume opaque. “It was almost a solid fountain of dust,” says Lisse.

The dust shroud hid the crater gouged by the bullet during a critical period, the first 800 seconds after impact, when the craft’s high-resolution camera would have had a close-up view of the comet. A’Hearn and his colleagues had intended to image the bottom of the crater to measure its depth and determine its composition.

Team members are now debating whether they can see signs of the crater in close-up images taken by Deep Impact’s high-resolution camera. A flaw discovered after Deep Impact’s launch had left that camera with a less-than-perfect focus. The researchers have developed software to sharpen the images, using a technique similar to the one used to deblur images taken by the Hubble Space Telescope before shuttle astronauts inserted new optical elements to correct for its flawed mirror.

During the last minute or so of the close-up images taken by Deep Impact, some of the dust had begun to clear. While the crater isn’t apparent, the scientists got a blurry glimpse of the bottom part of the plume, Richardson says. From those observations, the researchers estimate that the crater is about 100 m wide and 30 m deep, in good agreement with Spitzer Space Telescope estimates of the total amount of dust ejected.

Those dimensions imply that the impact excavated cometary material that’s pristine and primitive, from the earliest days of the solar system, notes Melosh. A shallower crater would probably contain material altered by the sun. Warming during Tempel 1’s repeated passages near the sun over thousands of years might have caused sudden eruptions of gas and dust and altered the composition of material several meters beneath the comet’s surface, but not 30 m deep.

The extremely fine texture of the cool dust ejected from the comet in the minutes following the initial violent impact, adds Richardson, also is likely to reflect conditions in the early solar system. The dust could have been trapped in an icy matrix when the comet formed 4.5 billion years ago.

The lighter side

Despite scientists’ difficulties in imaging the crater, other observations reveal that Tempel 1 is extraordinary fragile, composed of small particles bound together only weakly.

The depth to which the bullet penetrated, for example, attests to the fragility of the comet, Melosh adds. Had the comet contained denser, more strongly bound material, the bullet wouldn’t have penetrated to a depth of 30 m.

By analyzing images of the dust plume taken by the Deep Impact craft 45 and 75 minutes after the collision, Richardson and his colleagues measured the expansion rate of the plume. That rate is controlled by the comet’s gravitational field, so once the gravity of the comet is known, researchers can estimate its density.

The expansion measured indicates that the comet is highly porous, with an estimated density just 60 percent that of solid ice, and less than one-quarter that of the lowest-density rocks on Earth.

As weak as the comet’s gravity is, it still managed to keep the expanding plume anchored to the surface during the entire 40 hours that it remained visible. That’s another hint that the gravity is sufficient to hold together the comet as a loose agglomeration of particles.

“You’re talking about something the size of a mountain held together with the strength of the meringue in a lemon meringue pie,” says Lisse.

That may be just what’s expected, Lisse says, if the comet formed gently and gradually, built up over millions of years from the chance collision of tiny particles within the infant solar system’s protoplanetary disk. Such a gradual accumulation of material might also account for the comet’s layering, says Lisse.

Melosh’s calculations suggest that when the comet coalesced, it did so at pressures and temperatures too low for water to be liquid. Liquid water glues together dust and ice particles in many other materials. For the same reason it’s hard to make a snowball on a day too cold to melt ice, the grains of ice and dust that make up Tempel 1 just barely stuck together. As the object grew in size, however, its gravity, although weak, held the pieces in place, conjectures Melosh.

The spectra obtained from both the Deep Impact spacecraft and the Spitzer Space Telescope reveal that the composition and temperature of the plume remained the same from a few minutes to hours after the bullet hit. That’s an indication, says A’Hearn, that the composition of the comet material remains the same from near the surface to 30 m down.

It may also suggest that if Tempel 1 has any crust at all, it’s very thin, says Melosh.

Deep images

Even if Deep Impact’s bullet had missed the comet, the extreme close-ups taken by the spacecraft might have been well worth the visit. A camera aboard the projectile took pictures up to a few seconds before the collision, revealing features only 1 m across.

“We have a resolution better than any previous mission to a comet,” says Veverka. The pictures, which portray about one-third of the comet, show a puzzling variety of smooth and rugged terrains, he notes.

Craterlike outlines on some parts of the surface indicate that, unlike other comets seen close-up, it’s been repeatedly beat up by solar system debris for eons. It’s as if “the whole cratering history [of the solar system] is preserved on its surface,” says Veverka.

Yet some other regions show a smooth, flat, 20-m-high line of cliffs that defies any ready explanation, Veverka notes. Some of the smoothing, he notes, may result from frozen material sublimating from the surface, exposing terrain that lies beneath but then falling back to create a fresh deposit.

The comet also shows layering of different structures, with each segment 20 to 30 m deep. “We went into this mission talking about the so-called icy conglomerate model—a chunk of ice and dirt and organic gunk, all mixed together,” Veverka says. “But instead, we have this frozen onion.”

One possible explanation is that sunlight triggered an icy volcanic eruption meters below the surface. The erupting material, trapped underground, might have spread out horizontally in the porous interior, like the frosting between the layers of a cake.

Several years before Deep Impact was launched last January, Veverka had already proposed a follow-up mission to bring back a sample of a comet to Earth. NASA didn’t fund that proposal, and now the agency is concentrating most of its exploration plans on the moon and Mars.

But only when scientists have a sample, says Veverka, will they know for certain what comets are made of and what role they might have played in spawning life on our planet.