The shape of (7) Iris as evidence of an ancient large impact?^★,★★

¹ Institute of Astronomy, Charles University, Prague, V Holešovičkách 2, 18000 Prague 8, Czech Republic
e-mail: pepa@sirrah.troja.mff.cuni.cz; hanus.home@gmail.com
² Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³ Astrophysics Research Centre, Queen’s University, Belfast BT7 1NN, UK
⁴ Aix Marseille Université, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
⁵ Department of Mathematics, Tampere University of Technology, PO Box 553, 33101 Tampere, Finland
⁶ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
⁷ ONERA, The French Aerospace Lab BP72, 29 avenue de la Division Leclerc, 92322 Chatillon Cedex, France
⁸ SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Mountain View, CA 94043, USA
⁹ IMCCE, Observatoire de Paris, 77 Avenue Denfert-Rochereau, 75014 Paris Cedex, France
¹⁰ Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland
¹¹ Institute of Physics, University of Szczecin, Wielkopolska 15, 70-453 Szczecin, Poland
¹² Space Sciences, Technologies and Astrophysics Research Institute, Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium
¹³ Open University, School of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
¹⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁵ European Space Agency, ESTEC – Scientific Support Office, Keplerlaan 1, Noordwijk 2200 AG, The Netherlands
¹⁶ TMT Observatory, 100 West Walnut Street, Suite 300, Pasadena, CA 91124, USA
¹⁷ Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Apartado, Lima 1761, Peru
¹⁸ European Southern Observatory (ESO), Alonso de Cordova 3107, 1900 Casilla Vitacura, Santiago, Chile

Received: 30 October 2018
Accepted: 13 February 2019

Abstract

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.

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.

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.

Results. We present the 3D shape, volume-equivalent diameter D_eq = 214 ± 5 km, and bulk density ρ = 2.7 ± 0.3 g cm⁻³ 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.

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.