标题： (7) 轨道倾角作为古老大型撞击的证据？ 显示英文标题

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

摘要： 灶神星(7) Iris 是一个理想的解析成像目标，由于其亮度（V$\sim$7-8）和出现期间的较大角直径0.33弧秒。 据信，Iris 属于大型未碎裂小行星类别，避免了内部分化，这意味着其当前的形状和地形可能记录了太阳系碰撞演化的最初几亿年。 我们从欧洲南方天文台大计划框架下获取的VLT/SPHERE/ZIMPOL解析图像中恢复了关于Iris形状和表面地形的信息。 我们使用全数据小行星建模（ADAM）形状重建算法来模拟Iris的三维形状，使用光学积分数据和来自SPHERE的解析图像作为输入。 我们分析了SPHERE图像以推断小行星的整体形状及其主要陨石坑的形态。 我们给出了体积等效直径 D$_{{\rm eq}}$=214$\pm$5 km，以及密度$\rho$=2.7$\pm$0.3 g cm$^{-3}$。 其形状似乎与一个具有大型赤道凹陷的扁球体一致。 我们识别出八个直径为20至40公里的疑似表面特征，在多个时期被检测到，我们认为这些特征是撞击坑。 Iris上的陨石坑深度与直径比接近于类似10公里大小的Vesta陨石坑。 Iris的密度与它的陨石对应物LL普通球粒陨石的密度一致。 考虑到与Iris相关的碰撞家族的缺失及其表面上大量陨石坑的数量，我们认为其赤道凹陷可能是古老（至少30亿年）撞击的遗迹。 Iris的形状进一步表明，大型星子可能形成时几乎完美地呈扁球体。 最后，我们将Iris和Vesta之间陨石坑形态的差异归因于它们不同的表面重力。 显示英文摘要

摘要： Asteroid (7) Iris is an ideal target for disk-resolved imaging owing to its brightness (V$\sim$7-8) and large angular size of 0.33 arcsec during its apparitions. Iris is believed to belong to the category of large unfragmented asteroids that avoided internal differentiation, implying that its current shape and topography may record the first few 100 Myr of the solar system's collisional evolution. We recovered information about the shape and surface topography of Iris from disk-resolved VLT/SPHERE/ZIMPOL images acquired in the frame of our ESO large program. We used the All-Data Asteroid Modeling (ADAM) shape reconstruction algorithm to model the 3D shape of Iris, using optical disk-integrated data and disk-resolved images from SPHERE as inputs. We analyzed the SPHERE images to infer the asteroid's global shape and the morphology of its main craters. We present the volume-equivalent diameter D$_{{\rm eq}}$=214$\pm$5 km, and bulk density $\rho$=2.7$\pm$0.3 g cm$^{-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. Craters on Iris have depth-to-diameter ratios that are similar to those of analogous 10 km craters on Vesta. The bulk density of Iris is consistent with that of its meteoritic analog, 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.