Deciphering the cratering record of (7) Iris

Description

As part of our ESO large program (ID 199.C-0074), we observed asteroid (7) Iris with the VLT/SPHERE/ZIMPOL instrument throughout its rotation during two consecutive nights in October 2017 (five different epochs). Iris, which is one of the four D>200 km S-type main belt asteroids along with (3) Juno, (15) Eunomia and (29) Amphitrite, is an exceptional target for an adaptive optics (AO) campaign due to its large angular size as seen from the Earth (0.35'') during opposition. Considering the large size of Iris, one pixel represents 2.3 km at distance of Iris on our AO images. We identified several topographic features in the ZIMPOL AO images that we interpreted as impact craters. Crater identification was performed manually on the images, by looking for circular features with a clear brightness contrast. Craters were first extracted on each of the five epochs. We compared all the images within a given epoch to confirm the genuineness of the identified features, removing the possibility that they are deconvolution artifacts. In the end, we checked the pairing of craters between the different epochs, that lead to a total number of six individual craters with diameters larger than 20 km. We compared the number and size of the identified craters to the cratering records on Ceres and Vesta. We used the well established ADAM (All-Data Asteroid Modeling) inversion technique for the reconstruction of a highly-detailed 3D shape model (craters included), volume and the spin of Iris using its disk-integrated data (optical lightcurves) and disk-resolved images as inputs. We also estimated the bulk density to 2.4±0.3 g cm^-3. No moons were identified in our images. This work was supported by the grant 18-09470S of the Czech Science Foundation.

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