Comets were formed from gas, ice, and dust about 4.5 x 10⁹ years ago in the outer solar system, and their chemical composition reflects that of the primordial solar system. Each time a periodic comet makes a close approach to the Sun, volatile gases and dust are released from the surface to form the comet’s tail that so delights the casual onlooker. Over time the surface composition changes: the evaporation of volatiles leaves behind a coating of dark silicate dust that reflects hardly any sunlight. The icy interior, however, preserves the original composition. The purpose of NASA’s Deep Impact mission was to blast through the silicate crust and release pristine volatiles for spectroscopic analysis.

Deep Impact consisted of two spacecraft: the impactor itself, and a flyby mothership to observe the fireworks and relay the data. Almost all of the world’s large observatories recorded the event on July 4, 2005, a level of activity unprecedented in the history of astronomy. Much of the science from the space mission was actually conducted on Earth’s surface.

Close-up images of Tempel 1 obtained just before impact show signs of geological activity. Its surface shows many classical impact craters, as well as scarps and geological strata that hint at a layered interior.

Prior to the collision, comet researchers had not been sure what to expect. Perhaps the comet would be smashed to smithereens, or, maybe, the impactor would be swallowed up like a pole hitting quicksand. In the event, the impact surprised the experts: it blew out a huge cloud of microscopically small particles (1-100 m m) that created a dense dust cloud shielding the impact crater from view. The quantity of dust expelled was of on the order of ~10⁷ kg. This fine material is probably tens of meters deep.

After the dust had settled, scientists could clearly see that it was compositionally different from the normal surface. There were large increases in organics during and after the event. Emission features in the ejecta spectrum include H₂O, HCN, CO₂, as well as vibration mode signals from H₂CO (formaldehyde) and CH₃OH (methanol).

By looking at the rate at which the ejecta cone expanded, principal investigator Michael A’Hearn (University of Maryland) and his colleagues conclude that the bulk density of the cometary nucleus is 0.6 g cm^-3, broadly similar to freshly fallen snow. This comet is decidedly fluffy: half of it is empty space rather than rock-hard ice. About two-thirds by mass is pure water ice.

Most comets, when approaching the Sun, undergo outbursts. By looking at these outbursts before and after impact, researchers have concluded that the natural and impact-induced outbursts are very similar, which means that a comet’s tail is composed of material that originates below the surface.