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


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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Figure 1: The Comet 9P/Tempel 1 as seen by the Osiris NAC.
Figure 2: Number of OH molecules created by the impact, as measured with the OSIRIS/WAC.
Figure 3: Light curve of the cometary dust in the orange filter.


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OSIRIS was built by a European consortium and is part of the payload of the Rosetta spacecraft, provided by the European Space Agency. We acknowledge the work of the Rosetta Science Operations Centre and the Rosetta Mission Operations Centre in coordinating the observation timelines and operating the spacecraft, in particular K. Wirth, V. Dhiri, P. Ferri, E. Montagnon, A. Hubault, J. Morales, V. Companys and M. Lauer. We acknowledge the funding of the national space agencies ASI, CNES, DLR, the Spanish Space Program (Ministerio de Educacion y Ciencia), SNSB and ESA.

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Correspondence to Michael Küppers.

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Küppers, M., Bertini, I., Fornasier, S. et al. A large dust/ice ratio in the nucleus of comet 9P/Tempel 1. Nature 437, 987–990 (2005).

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