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origin, internal structure, and composition of 21 : Low , Lutetia—is its mean (bulk) density, derived from the mass and the volume. Observations of the High Density Optical, Spectroscopic, and Infrared Remote Im- aging System (OSIRIS) camera and ground ob- M. Pätzold,1* T. P. Andert,2 S. W. Asmar,3 J. D. Anderson,3 J.-P. Barriot,4 M. K. Bird,1,5 servations using adaptive optics were combined B. Häusler,2 M. Hahn,1 S. Tellmann,1 H. Sierks,6 P. Lamy,7 B. P. Weiss8 to model the global shape. The derived volume is (5.0 T 0.4) × 1014 m3 (4). The volume leads to Asteroid 21 Lutetia was approached by the Rosetta spacecraft on 10 July 2010. The additional a bulk density of (3.4 T 0.3) × 103 kg/m3.This Doppler shift of the spacecraft radio signals imposed by 21 Lutetia’s gravitational high bulk density is unexpected in view of the on the flyby trajectory were used to determine the mass of the asteroid. Calibrating and correcting low value of the measured mass. It is one of the for all Doppler contributions not associated with Lutetia, a least-squares fit to the residual highest bulk densities known for (5). frequency observations from 4 hours before to 6 hours after closest approach yields a mass of Assuming that Lutetia has a modest macropo- (1.700 T 0.017) × 1018 kilograms. Using the volume model of Lutetia determined by the rosity of 12%, it would imply that the bulk den- Rosetta Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) camera, the bulk sity of its material constituents would exceed density, an important parameter for clues to its composition and interior, is (3.4 T 0.3) × 103 that of stony meteorites. Unless Lutetia has anom- kilograms per cubic meter. alously low porosity compared with other as- teroids in its size range, its high density likely steroid 21 Lutetia, discovered in 1852, corresponding to a mass of (1.700 T 0.017) × indicates a nonchondritic bulk composition en- is one of the larger main-belt asteroids. 1018 kg (error, 1.0%). The uncertainty in GM (2) riched in high atomic number like . It may AIn 2004, it became the flyby target as- considers the error from the least-squares fit also be evidence for a partial differentiation of teroid for the Rosetta spacecraft mission. An mainly driven by the frequency noise (0.55%), the asteroid body (6). important characteristic of an asteroid is its bulk the uncertainty in the 21 Lutetia ephemeris in- References and Notes density, derived from its mass and its volume. Al- troduced by the uncertainty in the flyby distance et al Geophys. Res. Lett. T 1. T. P. Andert ., 37, L09202 (2010). though there are a number of asteroid mass de- of 7.5 km (0.24%), and the considered uncer- 2. Materials and methods are available as supporting termination techniques, by far the most accurate tainty in the tropospheric correction introduced material on Science Online. on May 26, 2015 is spacecraft tracking during a close flyby. by the zenith delay model and the mapping func- 3. As shown in (7), the expected final postencounter Doppler shift of a two-way radio carrier signal is The velocity of a spacecraft flying by a body tion of the ground station elevation (0.8%). These f D f(t → ∞)=4 X GM ⋅ sin a′ cos b , where a′ = c d ⋅ v0 of sufficient size and at a sufficiently close dis- contributions yield a total uncertainty of 1.0%. 172.18° is the direction to Earth projected into the tance is perturbed by the attracting force of that The values for GM and Df agree within the error flyby plane and b = 3° is the direction angle to Earth body. The perturbed velocity is estimated from with the analytical solution (3). The derived mass above the flyby plane. Using the fit solution for Lutetia −2 3 2 the additional Doppler shift of the transmitted is lower than other mass determinations of Lutetia of GM = (11.34 T 0.11) × 10 km /s , the analytical result of the relation above is 36.4 T 0.4 mHz. radio signal in comparison with the expected from astrometry (fig. S7). 4. H. Sierks et al., Science 334, 487 (2011). Doppler shift of an unperturbed trajectory (1). One of the most important global geophys- 5. Similar high bulk densities are known for the asteroids The Rosetta spacecraft was tracked during ical parameters—which provides clues to the 4 , , 20 Massalia, and 22 Kalliope, all the flyby of asteroid 21 Lutetia on 10 July 2010 www.sciencemag.org with NASA’s Deep Space Network (DSN) 70-m antenna (DSS-63) near Madrid, Spain (2). The flyby distance was d = 3168 T 7.5 km, the high 0.04 relative flyby velocity was v0 = 14.99 km/s, and the projection angle between the relative veloc- ity and the direction to Earth was a = 171.2°, all

of which define the postencounter amplitude of Downloaded from the Doppler shift (3). 0.00 After correcting for contributions not asso- ciated with 21 Lutetia (2), the final Doppler frequency shift 6 hours after Rosetta’s closest Df T approachtotheasteroidwas =36.2 0.2 mHz -0.04 (Fig. 1). The value of GM (gravitational constant G × body mass M ) from a least-squares fitting procedure is GM =(11.34T 0.11) × 10−2 km3s−2,

-0.08 1Rheinisches Institut für Umweltforschung, Abteilung Planeten- frequency residuals at X-band (Hz) forschung, an der Universität zu Köln, 50931 Cologne, Ger- many. 2Institut fürRaumfahrttechnik, Universitätder Bundeswehr München, 85579 Neubiberg, Germany. 3Jet Propulsion Labora- tory, California Institute of Technology, Pasadena, CA 91125, 4 USA. Géosciences du Pacifique Sud, Université de la Polynésie -0.12 Française, BP 6570, 98702 Faa’a, Tahiti, Polynésie Francaise. -4 -3 -2 -1 0123456 5Argelander Institut für Astronomie, Universität Bonn, 53121 Bonn, Germany. 6Max-Planck-Institut für Sonnensystemforschung, time relative to closest approach (hours) 37191 Katlenburg-Lindau, Germany. 7Laboratoire d’Astrophysique de Marseille, Marseille, France. 8Department of Earth, Atmospheric Fig. 1. Filtered and adjusted frequency residuals at X-band frequency from 4 hours before closest and Planetary Sciences, Massachusetts Institute of Technology, approach to 6 hours after closest approach. Two tracking gaps (light red shaded zones) are indicated Cambridge, MA 02139, USA. from 5 min before closest approach to 45 min after closest approach as planned (7), and from 192 min *To whom correspondence should be addressed. E-mail: to 218 min after closest approach when DSS 63 accidentally dropped the uplink. The red solid line is a [email protected] least-squares fit to the data from which GM is determined.

www.sciencemag.org SCIENCE VOL 334 28 OCTOBER 2011 491 REPORTS

of which are larger than Lutetia (8). Bulk densities Acknowledgments: The Rosetta Radio Science Investigation European Space Tracking Network and Deep Space of more primitive C-type asteroids are in the range experiment is funded by the Deutsches Zentrum für Network ground stations for their continuous support. 1200 kg/m3 to 2700 kg/m3. Luft- und Raumfahrt (DLR), Bonn under grants 6. B. P. Weiss et al., Evidence for thermal metamorphism or 50QM1002 (T.P.A. and B.H.) and 50QM1004 (M.P., M.H., Supporting Online Material partial differentiation of asteroid 21 Lutetia from Rosetta, S.T., and M.K.B.), and under a contract with NASA www.sciencemag.org/cgi/content/full/334/6055/491/DC1 paper no. 2077, 42nd Lunar and Planetary Science (S.W.A. and J.D.A.). We thank T. Morley for valuable Materials and Methods Figs. S1 to S7 Conference, The Woodlands, TX, 7 to 11 March 2011. comments and all persons involved in Rosetta at the 9–26 7. M. Pätzold et al., Astron. Astrophys. 518, L156 (2010). European Space Research and Technology Centre, References ( ) 8. J. Baer, S. R. Chesley, R. D. Matson, Astron. J. 141, 143 European Space Operations Centre, European Space 16 February 2011; accepted 4 October 2011 (2011). Astronomy Centre, Jet Propulsion Laboratory, and the 10.1126/science.1209389

Analysis of the normalized spectral variation The Surface Composition and across the observed surface at highest resolution and over a wide range of phase angles shows a Temperature of Asteroid 21 Lutetia remarkable uniformity of the surface spectral prop- erties, showing a maximum fluctuation of 3%. This implies that any albedo variation would be As Observed by Rosetta/VIRTIS related to regolith transport and/or sorting processes rather than compositional variation (Fig. 3A). m A. Coradini,1 F. Capaccioni,2* S. Erard,3 G. Arnold,4 M. C. De Sanctis,2 G. Filacchione,2 The region between 3.5 and 5.1 m is dom- F. Tosi,1 M. A. Barucci,3 M. T. Capria,2 E. Ammannito,1 D. Grassi,1 G. Piccioni,2 S. Giuppi,1 inated by the presence of the thermal emission of ’ G. Bellucci,1 J. Benkhoff,5 J. P. Bibring,6 A. Blanco,13 M. Blecka,7 D. Bockelee-Morvan,3 the asteroid s surface, from which we evaluated F. Carraro,1 R. Carlson,8 U. Carsenty,9 P. Cerroni,2 L. Colangeli,5 M. Combes,3 M. Combi,10 the surface temperature and the spectral emissiv- J. Crovisier,3 P. Drossart,3 E. T. Encrenaz,3 C. Federico,11 U. Fink,12 S. Fonti,13 L. Giacomini,1 ity. The temperature observed varied between W. H. Ip,14 R. Jaumann,9 E. Kuehrt,9 Y. Langevin,6 G. Magni,2 T. McCord,15 V. Mennella,19 170 and 245 K, with a direct correlation of the S. Mottola,9 G. Neukum,16 V. Orofino,13 P. Palumbo,21 U. Schade,22 B. Schmitt,17 F. Taylor,20 temperature with topographic features and with D. Tiphene,3 G. Tozzi18 maximum temperatures obtained for the smaller incidence angles on the left-hand side of the im- age (Fig. 3B). The Visible, InfraRed, and Thermal Imaging Spectrometer (VIRTIS) on Rosetta obtained We have applied a thermophysical model of hyperspectral images, spectral reflectance maps, and temperature maps of the asteroid 21 Lutetia. the heat conduction into the asteroid surface de- No absorption features, of either silicates or hydrated minerals, have been detected across the rived from cometary nucleus evolution models observed area in the spectral range from 0.4 to 3.5 micrometers. The surface temperature reaches a (8). The model was used to fit the measured maximum value of 245 kelvin and correlates well with topographic features. The thermal inertia is surface temperatures as a function of the den- in the range from 20 to 30 joules meter−2 kelvin−1 second−0.5, comparable to a lunarlike powdery sity, specific heat, and thermal conductivity, as- regolith. Spectral signatures of surface alteration, resulting from space weathering, seem to be missing. sumed similar to those of a lunar regolith (9–11). Lutetia is likely a remnant of the primordial planetesimal population, unaltered by differentiation A self-heating parameter (12) accounts for the processes and composed of chondritic materials of enstatitic or carbonaceous origin, dominated contribution of unresolved topography and micro- by iron-poor minerals that have not suffered aqueous alteration. roughness. Iteratively, we optimized the result-

1Istituto di Fisica dello Spazio Interplanetario, Istituto Na- he visible, InfraRed, and Thermal Imag- 550 nm, consistent with the value measured by zionale di Astrofisica (INAF), 00133 Rome, Italy. 2Istituto di As- ing Spectrometer (VIRTIS) on board the OSIRIS (5). trofisica Spaziale e Fisica Cosmica, INAF, 00133 Rome, Italy. TRosetta spacecraft is an imaging and The full-disk spectra (Fig. 1) show a varia- 3Laboratoire d’etudes spatiales et d’instrumentation en astro- high-resolution spectrometer with two indepen- bility limited to within 3% shortward of 3.5 mm. physique (LESIA), Observatoire de Paris, CNRS, UniversitéPierre m et Marie Curie, UniversitéParis-Diderot, 92195 Meudon, France. dent channels: VIRTIS-M, which acquires hyper- Beyond 3.5 m, the surface thermal emission 4Institute for Planetology, 48149 Münster, Germany. 5European spectral images in the spectral range from 0.25 to becomes more and more relevant, and the spec- Space Research and Technology Centre (ESTEC), European 5.1 mm with a spatial resolution of 250 mrad, and tral variations reflect the local time changes, from Space Agency, 2200 AG Noordwijk, Netherlands. 6Institut d’Astro- 7 VIRTIS-H, a high-resolution infrared spectrom- sunset to sunrise, and the corresponding surface physique Spatial, CNRS, 91405 Orsay, France. Space Research m Center, Polish Academy of Sciences, 00-716 Warsaw, Poland. eter in the 2- to 5- m range with spectral res- temperature changes. 8Jet Propulsion Laboratory, National Aeronautics and Space Ad- olution as high as 3000 (1–3). The VIRTIS-M spectra show no absorption ministration, Pasadena, CA 91109, USA. 9Institute of Planetary VIRTIS-M obtained hyperspectral images of features in the range from 0.5 to 2.5 mmtoanrms Research, Deutsches Zentrum für Luft- und Raumfahrt (DLR), 10 asteroid 21 Lutetia with spatial resolution varying level of less than 5% (Figs. 1 and 2), thus con- 12489 Berlin, Germany. Department of Atmospheric, Oceanic and Space Sciences, The University of Michigan, Ann Arbor, MI between 12 km and less than 1 km, at closest firming previous disk-integrated ground-based 48109, USA. 11Università di Perugia, 06100 Perugia, Italy. approach. VIRTIS-H was operated throughout observations (6, 7). Asteroid 21 Lutetia has a mod- 12Lunar Planetary Laboratory, University of Arizona, Tucson, the flyby [supporting online material (SOM)]. erately red slope in the range of 0.5 to 0.8 mm, AZ 85721, USA. 13Dipartimento di Fisica, Università di Lecce, m 73100 Lecce, Italy. 14National Central University, Taipei, Taiwan. Because of the orientation of the Lutetia spin whereas in the region of 0.8 to 2.5 m the spec- 15 16 axis, only about 50% of the surface has been trum is essentially flat. Both VIRTIS-M and Bear Fight Institute, Winthrop, WA 98862, USA. Freie Univer- sität, 14195 Berlin, Germany. 17UniversitéJoseph Fourier–Grenoble imaged, ranging in latitude from the north pole VIRTIS-H consistently did not identify any spec- 1 /CNRS-INSU, Institut de Planétologie et d’Astrophysique de down to the equator according to the OSIRIS tral feature associated with OH absorptions. In Grenoble, 38041 Grenoble, France. 18Osservatorio Astrofisico shape model (4). particular, VIRTIS-H, within its calibration accu- di Arcetri, 50125 Firenze, Italy. 19Osservatorio di Capodimonte, 80131 Napoli, Italy. 20Department of Physics, Oxford Uni- VIRTIS observed Lutetia at zero phase angle racy of 2%, did not identify hydrated minerals 21 m versity, Oxford OX1 2JD, UK. Universitàdegli studi di Napoli (0.007°), located at northern latitude 16.0° and bands at 1.9, 2.7, and 3 m, thus ruling out their “Parthenope,” 80133 Napoli, Italy. 22Helmholtz-Zentrum east longitude 358.3°. The measured radiance presence on the explored surface (SOM). VIRTIS Berlin für Materialien und Energie, 14109 Berlin, Germany. factor (I/F,whereI is the calibrated radiance and did not identify any spectral features associated *To whom correspondence should be addressed. E-mail: F is the solar flux) at zero phase is 0.21 T 0.01 at with organic materials in the 3.3- to 3.6-mm range. [email protected]

492 28 OCTOBER 2011 VOL 334 SCIENCE www.sciencemag.org Asteroid 21 Lutetia: Low Mass, High Density M. Pätzold et al. Science 334, 491 (2011); DOI: 10.1126/science.1209389

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