The impact crater at the origin of the Julia family detected with VLT/SPHERE?

Abstract

Context. The vast majority of the geophysical and geological constraints (e.g., internal structure, cratering history) for main-belt asteroids have so far been obtained via dedicated interplanetary missions (e.g., ESA Rosetta, NASA Dawn). The high angular resolution of SPHERE/ZIMPOL, the new-generation visible adaptive-optics camera at ESO VLT, implies that these science objectives can now be investigated from the ground for a large fraction of D ≥ 100 km main-belt asteroids. The sharp images acquired by this instrument can be used to accurately constrain the shape and thus volume of these bodies (hence density when combined with mass estimates) and to characterize the distribution and topography of D ≥ 30 km craters across their surfaces. Aims. Here, via several complementary approaches, we evaluated the recently proposed hypothesis that the S-type asteroid (89) Julia is the parent body of a small compact asteroid family that formed via a cratering collisional event. Methods. We observed (89) Julia with VLT/SPHERE/ZIMPOL throughout its rotation, derived its 3D shape, and performed a reconnaissance and characterization of the largest craters. We also performed numerical simulations to first confirm the existence of the Julia family and to determine its age and the size of the impact crater at its origin. Finally, we utilized the images/3D shape in an attempt to identify the origin location of the small collisional family. Results. On the one hand, our VLT/SPHERE observations reveal the presence of a large crater (D ~ 75 km) in Julia’s southern hemisphere. On the other hand, our numerical simulations suggest that (89) Julia was impacted 30–120 Myrs ago by a D ~ 8 km asteroid, thereby creating a D ≥ 60 km impact crater at the surface of Julia. Given the small size of the impactor, the obliquity of Julia and the particular orientation of the family in the (a,i) space, the imaged impact crater is likely to be the origin of the family. Conclusions. New doors into ground-based asteroid exploration, namely, geophysics and geology, are being opened thanks to the unique capabilities of VLT/SPHERE. Also, the present work may represent the beginning of a new era of asteroid-family studies. In the fields of geophysics, geology, and asteroid family studies, the future will only get brighter with the forthcoming arrival of 30–40 m class telescopes like ELT, TMT, and GMT.

Closing the gap between Earth-based and interplanetary mission observations: Vesta seen by VLT/SPHERE

“…The data reduction protocol is the same for all the targets in our large programme. We refer the reader to Vernazza et al (2018) for a description of the procedure. A subset of the Vesta images after pipeline reduction and before deconvolution are shown in Fig.…”

Homogeneous internal structure of CM-like asteroid (41) Daphne

“…We process the ZIMPOL data with the ESO pipeline (see details in Vernazza et al, 2018). We then reduce all other imaging epochs with the same suite of IDL routines for consistency.…”

“…The high-quality correction delivered by the AO system of SPHERE (Fusco et al, 2006(Fusco et al, , 2014 compared to previous generations of AO cameras, and the use of shorter wavelength (visible R band compared to the near-infrared J/H/K bands typically used with previous cameras), provides a twofold to threefold improvement in angular resolution. This sharper resolution allows the detailed modeling of asteroid shapes and enhanced satellite detection capability, as recently illustrated on the main-belt asteroids (3) Juno, (6) Hebe, (16) Psyche, (89) Julia, (107) Camilla, and (130) Elektra (Viikinkoski et al, 2015(Viikinkoski et al, , 2018Marsset et al, 2016Marsset et al, , 2017aYang et al, 2016;Hanuš et al, 2017a;Pajuelo et al, 2018;Vernazza et al, 2018).…”

“…The present article focuses on asteroid (41) Daphne as representative of the Ch spectral class, around which a satellite was discovered in 2008 (S/2008 (41) 1, which we call Peneius 1 , Conrad et al, 2008). Daphne was observed within a survey we are currently conducting (ID 199.C-0074, PI P. Vernazza) to image a substantial fraction of main-belt asteroids larger than 100 km in diameter, sampling the main compositional classes (see Vernazza et al, 2018, for a description of the survey). We image these asteroids throughout their rotation at high angular-resolution with the SPHERE/ZIMPOL extreme adaptive-optics (AO) camera (Beuzit et al, 2008;Thalmann et al, 2008) mounted on the European Southern Observa-1 pronounced "peh-NEH-oss" tory (ESO) Very Large Telescope (VLT).…”

SPHERE: the exoplanet imager for the Very Large Telescope

“…Moreover, the stellar candidate companion previously detected by the HST has been confirmed and characterized by these unique observations. The IRDIS classical imaging mode has also been used for several asteroid studies, leading to shape reconstruction (Viikinkoski et al 2015;Marsset et al 2017;Vernazza et al 2018). These studies largely benefit from the high resolution and high Strehl providing much more (Khorrami et al 2017).…”