minor planets; asteroids: individual: 7 Iris; methods: observational; methods: numerical
Abstract :
[en] Context. Asteroid (7) Iris is an ideal target for disk-resolved imaging owing to its brightness (V 7-8) and large angular size of 0.33'' during its apparitions. Iris is believed to belong to the category of large unfragmented asteroids that avoided internal differentiation, implying that its current shape and topography may record the first few 100 Myr of the solar system's collisional evolution. <BR /> Aims: We recovered information about the shape and surface topography of Iris from disk-resolved VLT/SPHERE/ZIMPOL images acquired in the frame of our ESO large program. <BR /> Methods: We used the All-Data Asteroid Modeling (ADAM) shape reconstruction algorithm to model the 3D shape of Iris, using optical disk-integrated data and disk-resolved images from SPHERE and earlier AO systems as inputs. We analyzed the SPHERE images and our model to infer the asteroid's global shape and the morphology of its main craters. <BR /> Results: We present the 3D shape, volume-equivalent diameter D[SUB]eq[/SUB] = 214 ± 5 km, and bulk density ρ = 2.7 ± 0.3 g cm[SUP]-3[/SUP] of Iris. Its shape appears to be consistent with that of an oblate spheroid with a large equatorial excavation. We identified eight putative surface features 20-40 km in diameter detected at several epochs, which we interpret as impact craters, and several additional crater candidates. Craters on Iris have depth-to-diameter ratios that are similar to those of analogous 10 km craters on Vesta. <BR /> Conclusions: The bulk density of Iris is consistent with that of its meteoritic analog based on spectroscopic observations, namely LL ordinary chondrites. Considering the absence of a collisional family related to Iris and the number of large craters on its surface, we suggest that its equatorial depression may be the remnant of an ancient (at least 3 Gyr) impact. Iris's shape further opens the possibility that large planetesimals formed as almost perfect oblate spheroids. Finally, we attribute the difference in crater morphology between Iris and Vesta to their different surface gravities, and the absence of a substantial impact-induced regolith on Iris. The reduced images are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A121">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A121</A>Based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 199.C-0074 (PI: P. Vernazza) and 086.C-0785 (PI: B. Carry).
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Hanuš, Josef; Institute of Astronomy, Charles University, Prague, V Holešovičkách 2, 18000, Prague 8, Czech Republic
Marsset, M.; Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA ; Astrophysics Research Centre, Queen's University, Belfast, BT7 1NN, UK
Vernazza, Pierre; Aix Marseille Université, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France
Viikinkoski, Matti; Department of Mathematics, Tampere University of Technology, PO Box 553, 33101, Tampere, Finland
Drouard, Alexis; Aix Marseille Université, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France
Brož, M.; Institute of Astronomy, Charles University, Prague, V Holešovičkách 2, 18000, Prague 8, Czech Republic
Carry, B.; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France
Fetick, R.; Aix Marseille Université, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France ; ONERA, The French Aerospace Lab BP72, 29 avenue de la Division Leclerc, 92322, Chatillon Cedex, France
Marchis, F.; SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Mountain View, CA 94043, USA
Jorda, Laurent; Aix Marseille Université, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France
Fusco, T.; Aix Marseille Université, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France ; ONERA, The French Aerospace Lab BP72, 29 avenue de la Division Leclerc, 92322, Chatillon Cedex, France
Birlan, M.; IMCCE, Observatoire de Paris, 77 Avenue Denfert-Rochereau, 75014, Paris Cedex, France
Santana-Ros, T.; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland
Podlewska-Gaca, Edyta; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland ; Institute of Physics, University of Szczecin, Wielkopolska 15, 70-453, Szczecin, Poland
Jehin, Emmanuel ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Origines Cosmologiques et Astrophysiques (OrCa)
Ferrais, Marin ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Origines Cosmologiques et Astrophysiques (OrCa)
Grice, J.; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France ; Open University, School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
Bartczak, P.; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland
Berthier, J.; IMCCE, Observatoire de Paris, 77 Avenue Denfert-Rochereau, 75014, Paris Cedex, France
Castillo-Rogez, J.; Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
Cipriani, F.; European Space Agency, ESTEC - Scientific Support Office, Keplerlaan 1, Noordwijk, 2200 AG, The Netherlands
Colas, F.; IMCCE, Observatoire de Paris, 77 Avenue Denfert-Rochereau, 75014, Paris Cedex, France
Dudziński, G.; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland
Dumas, C.; TMT Observatory, 100 West Walnut Street, Suite 300, Pasadena, CA 91124, USA
Ďurech, J.; Institute of Astronomy, Charles University, Prague, V Holešovičkách 2, 18000, Prague 8, Czech Republic
Kaasalainen, M.; Department of Mathematics, Tampere University of Technology, PO Box 553, 33101, Tampere, Finland
Kryszczynska, A.; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland
Lamy, P.; Aix Marseille Université, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France
Le Coroller, H.; Aix Marseille Université, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France
Marciniak, A.; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland
Michalowski, T.; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286, Poznań, Poland
Pajuelo, M.; IMCCE, Observatoire de Paris, 77 Avenue Denfert-Rochereau, 75014, Paris Cedex, France ; Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Apartado, Lima 1761, Peru
Tanga, P.; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France
Vachier, F.; IMCCE, Observatoire de Paris, 77 Avenue Denfert-Rochereau, 75014, Paris Cedex, France
Vigan, A.; Aix Marseille Université, CNRS, LAM, Laboratoire d'Astrophysique de Marseille, Marseille, France
Witasse, O.; European Space Agency, ESTEC - Scientific Support Office, Keplerlaan 1, Noordwijk, 2200 AG, The Netherlands
Yang, B.; European Southern Observatory (ESO), Alonso de Cordova 3107, 1900 Casilla Vitacura, Santiago, Chile)
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