A large dust/ice ratio in the nucleus of comet 9P/Tempel 1

NASA's Deep Impact spacecraft met its intended fate on 4 July this year when it collided with Comet Tempel 1 at a relative velocity of over 36,000 km per hour. Telescopes all around the world and beyond it were trained on the event in order to glean information about the internal structure of a comet. Results from OSIRIS cameras on ESA's Rosetta spacecraft provided continuous coverage from 3 days before to 10 days after the impact. The impact created a crater 60 metres in diameter and the nature of the debris points to the comet as an ‘icy dustball’ rather than the more familiar concept of a ‘dirty snowball’. Comets spend most of their life in a low-temperature environment far from the Sun. They are therefore relatively unprocessed and maintain information about the formation conditions of the planetary system, but the structure and composition of their nuclei are poorly understood. Although in situ1 and remote2 measurements have derived the global properties of some cometary nuclei, little is known about their interiors. The Deep Impact mission3 shot a projectile into comet 9P/Tempel 1 in order to investigate its interior. Here we report the water vapour content (1.5 × 1032 water molecules or 4.5 × 106 kg) and the cross-section of the dust (330 km2 assuming an albedo of 0.1) created by the impact. The corresponding dust/ice mass ratio is probably larger than one, suggesting that comets are ‘icy dirtballs’ rather than ‘dirty snowballs’ as commonly believed4. High dust velocities (between 110 m s-1 and 300 m s-1) imply acceleration in the comet's coma, probably by water molecules sublimated by solar radiation. We did not find evidence of enhanced activity of 9P/Tempel 1 in the days after the impact, suggesting that in general impacts of meteoroids are not the cause of cometary outbursts.