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

Hanuš, J. and Marsset, M. and Vernazza, P. and Viikinkoski, M. and Drouard, A. and Brož, M. and Carry, B. and Fetick, R. and Marchis, F. and Jorda, L. and Fusco, T. and Birlan, M. and Santana-Ros, T. and Podlewska-Gaca, E. and Jehin, E. and Ferrais, M. and Grice, J. and Bartczak, P. and Berthier, J. and Castillo-Rogez, J. and Cipriani, F. and Colas, F. and Dudziński, G. and Dumas, C. and Ďurech, J. and Kaasalainen, M. and Kryszczynska, A. and Lamy, P. and Le Coroller, H. and Marciniak, A. and Michalowski, T. and Michel, P. and Pajuelo, M. and Tanga, P. and Vachier, F. and Vigan, A. and Witasse, O. and Yang, B. (2019) The shape of (7) Iris as evidence of an ancient large impact? Astronomy and Astrophysics, 624 . Art. No. A121. ISSN 0004-6361.

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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.

Item Type:Article
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URLURL TypeDescription Paper
Hanuš, J.0000-0002-2934-3723
Additional Information:© 2019 ESO. Article published by EDP Sciences. Received 30 October 2018; Accepted 13 February 2019; Published online 24 April 2019. 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). We thank Simone Marchi for his pertinent and constructive remarks. This work has been supported by the Czech Science Foundation through grants 18-09470S (J. Hanuš, J. Durech) and 18-04514J (M. Brož) and by the Charles University Research program No. UNCE/SCI/023. P. Vernazza, A. Drouard, J. Grice, and B. Carry were supported by CNRS/INSU/PNP. We thank Chris Magri for kindly providing the radar shape model published in Ostro et al. (2010).
Group:Thirty Meter Telescope
Funding AgencyGrant Number
Grantová Agentura České Republiky18-09470S
Grantová Agentura České Republiky18-04514J
Charles UniversityUNCE/SCI/023
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
Institut National des Sciences de l'Univers (INSU)UNSPECIFIED
Subject Keywords:minor planets, asteroids: individual: 7 Iris – methods: observational – methods: numerical
Record Number:CaltechAUTHORS:20200311-134200192
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Official Citation:The shape of (7) Iris as evidence of an ancient large impact? J. Hanuš, M. Marsset, P. Vernazza, M. Viikinkoski, A. Drouard, M. Brož, B. Carry, R. Fetick, F. Marchis, L. Jorda, T. Fusco, M. Birlan, T. Santana-Ros, E. Podlewska-Gaca, E. Jehin, M. Ferrais, J. Grice, P. Bartczak, J. Berthier, J. Castillo-Rogez, F. Cipriani, F. Colas, G. Dudziński, C. Dumas, J. Ďurech, M. Kaasalainen, A. Kryszczynska, P. Lamy, H. Le Coroller, A. Marciniak, T. Michalowski, P. Michel, M. Pajuelo, P. Tanga, F. Vachier, A. Vigan, O. Witasse and B. Yang. A&A, 624 (2019) A121; DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101858
Deposited By: Tony Diaz
Deposited On:11 Mar 2020 21:08
Last Modified:11 Mar 2020 21:08

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