1. Max-Planck Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
2. Dipartimento di Astronomia and CISAS, Università di Padova, Vicolo dell'Osservatorio 5, 35100 Padova, Italy
3. Instituto de Astrofisica de Andalucía-CSIC, C/Camino Bajo de Huétor, 50, 18008 Granada, Spain
4. Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, Les Trois Lucs BP 8, 13376 Marseille, France
5. DLR Institute for Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany
6. European Space Agency, ESTEC, SCI-SB, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlands
7. Physikalisches Institut, Abteilung Weltraumforschung und Planetologie, Universität Bern, Sidlerstr. 5, 3012 Bern, Switzerland
8. Uppsala Astronomical Observatory, Box 515, 75120 Uppsala, Sweden
9. University of Maryland, Department of Astronomy, College Park, Maryland 20742-2421, USA
10. Università di Padova, Departimento di Ingegneria Meccanica, Via Venezia, 1, 35131 Padova, Italy
11. Armagh Observatory, College Hill, Armagh BT61 9DG, UK
12. Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
13. Observatoire de Paris, Section de Meudon, 92195 Meudon, France
14. Services d'Aéronomie du CNRS, Route des Gatines, BP 3, 91371 Verrières le Buisson, France
15. Cornell University, 424 Space Sciences Building, Ithaca, New York 14853-6801, USA
16. Università di Padova, Departimento di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
17. Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
18. Università di Padova, Center of Studies and Activities of Space (CISAS) "G.Colombo", Via Venezia 1, 35151 Padova, Italy
19. Università di Padova, Center of Studies and Activities of Space (CISAS) "G.Colombo", Via Venezia 1, 35151 Padova, Italy
20. ISAS, International School for Advanced Studies, Strada Costiera, 11, 34014 Trieste, Italy
21. Institut für Datentechnik und Kommunikationsnetze der TU Braunschweig, Hans-Sommer-Str. 66, 38106 Braunschweig, Germany
22. National Central University, Institute of Astronomy, 32054 Chung-Li, Taiwan. 15DLR Institute for Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany
23. DLR Institute for Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany
24. Laboratoire d'Astrophysique de Marseille (CNRS/LAM), Traverse du Siphon, Les Trois Lucs, BP 8, 13376 Marseille Cedex 12, France
25. Instituto de Astrofísica de Andalucía, PO Box 3004, Camino Bajo de Huétor 24, 18080 Granada, Spain
26. Università di Padova, Departimento di Fisica, Via Gradenigo 6A, 35151 Padova, Italy
27. Università di Padova, Departimento di Elettronica et Informatica, Via Venezia 1, 35151 Padova, Italy
28. Instituto Nacional de Tecnica Aeroespacial, Carretera de Ajalvir, p.k. 4, 28850 Torrejon de Ardoz (Madrid), Spain
29. Universidad Politecnica de Madrid, Instituto Ignacio D'Riva, Plaza Cardenal Cisneros s/n, 28040 Madrid, Spain
30. Laboratoire d'Astrophysique de Marseille (CNRS/LAM), Traverse du Siphon, Les Trois Lucs, BP 8, 13376 Marseille Cedex 12, France
31. Università di Padova, Departimento di Elettronica et Informatica, Via Venezia 1, 35151 Padova, Italy
32. European Space Research and Technology Centre, Space Science Department, Keplerlaan 1, Postbus 299, 2201 AZ Noordwijk ZH, The Netherlands
33. Lists of participants and affiliations appear at the end of the paper

Correspondence to: Michael Küppers¹ Correspondence and requests for materials should be addressed to M.K. (Email: kueppers@mps.mpg.de).

Abstract

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.

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