Properties of craters on the Achaia region of Asteroid (21) Lutetia

We report on the physical properties of the craters of Achaia region of the main-belt Asteroid (21) Lutetia, based on images obtained with the OSIRIS instrument during the Rosetta flyby that took place on 10 July 2010. Images of the surface were acquired with its Narrow Angle Camera, from which Digital Terrain Models (DTM) of the surface were constructed. These DTMs give access to the geometrical properties of the craters of the asteroid. On a complex asteroid shape, slopes and depth-to-diameter ratios (d/D) of craters should be carefully measured taking into account the local topography to obtain a value that is physically related to the work of forces resisting to mass displacement (associated with gravity and/or material strength) occurring in either excavation or degradation processes. We present new measurements of d/D and internal slopes of impact craters of the Achaia region, which offers optimal conditions of observations and a large population of craters. We find that d/D values for Achaia craters differ from previous works on Lutetia and are consistent with the values found on other asteroids such as (243) Ida or (951) Gaspra. The Achaia region may be divided into three units based on geomorphological analysis. The mean d/D values of the three units are different, revealing differences in resurfacing history by impact-related ejecta blanketing and seismic shaking. Some of these geological events may be recent compared to the age of the region since several lineaments intersect most craters of one of the three units. Independent evidence for ejecta blanket have been given for the unit associated with low d/D values confirming the contribution of this process to crater modification. Moreover, we suggest that displacements along faults identified as surface lineaments may have been responsible for the erasure of small craters. Our results are finally integrated into a chronology sequence of events explaining the present characteristics of the Achaia region.