J/A+A/624/A121 Asteroid (7) Iris Deconvolved disk-resolved images (Hanus+, 2019)
The shape of (7) Iris as evidence of an ancient large impact?
Hanus J., Marsset, M., Vernazza, P., Viikinkoski, M., Drouard, A., Broz M.,
Carry B., Fetick R., Marchis F., Jorda L., Fusco T., Birlan M.,
Santana-Ros T., Podlewska-Gaca E., Jehin E., Ferrais M., Grice J.,
Bartczak P., Berthier J., Castillo-Rogez J., Cipriani F., Colas F.,
Dudzinski G., Dumas C., Durech J., Kaasalainen M., Kryszczynska A.,
Lamy P., Le Coroller H., Marciniak A., Michalowski T., Michel P.,
Pajuelo M., Tanga P., Vachier F., Vigan A., Witasse O., Yang B.
<Astron. Astrophys. 624, A121 (2019)>
=2019A&A...624A.121H 2019A&A...624A.121H (SIMBAD/NED BibCode)
ADC_Keywords: Minor planets
Keywords: minor planets, asteroids: individual: (7) Iris -
methods: observational - methods: numerical
Abstract:
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 100Myr
of the solar system's collisional evolution.
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.
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
Deq=214±5km, and bulk density ρ=2.7±0.3g/cm3 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 20-40km 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 10km craters on Vesta.
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
3Gyr) 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.
Description:
Deconvolved disk-resolved images of asteroid (7) Iris from Keck/Nirc2,
VLT/NaCo and VLT/SPHERE/Zimpol instruments listed in Table C.2. The
final shape model is based on the VLT/SPHERE/Zimpol data and a subset
of the Keck/Nirc2 and VLT.NaCo data. The remaining data of a poor
quality, often affected by deconvolution artifact, were used for
consistency check only. The ordering of the data corresponds to the
ordering in Table C2.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 140 47 List of fits images
fits/* . 47 Individual fits images
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg ? Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg ? Declination of center (J2000)
20- 26 F7.5 arcsec/pix scale ? Scale of the image
28- 30 I3 --- Nx Number of pixels along X-axis
32- 34 I3 --- Ny Number of pixels along Y-axis
36- 58 A23 "datime" Obs.date Observation date
60- 63 I4 Kibyte size Size of FITS file
65- 77 A13 --- FileName Name of FITS file, in subdirectory fits
79-140 A62 --- Title Title of the FITS file
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Acknowledgements:
Josef Hanus, hanus.home(at)gmail.com
(End) Patricia Vannier [CDS] 28-Mar-2019