Asteroid (21) Lutetia: Disk-resolved Photometric Analysis of Baetica Region

Abstract: (21) Lutetia has been visited by Rosetta mission on July 2010 and observed with a phase angle ranging from 0.15 to 156.8 degrees. The Baetica region, located at the north pole has been extensively observed by OSIRIS cameras system. Baetica encompass a region called North Pole Crater Cluster (NPCC), shows a cluster of superposed craters which presents signs of variegation at the small phase angle images. For studying the location, we used 187 images distributed throughout 14 filter recorded by the NAC (Narrow Angle Camera) and WAC (Wide Angle Camera) taken during the fly-by. We photometrically modeled the region using Minnaert disk-function and Akimov phase function to finally reconstruct a resolved spectral slope map at 5 and 20 degrees of phase angle. We observed a dichotomy between Gallicum and Danuvius-Sarnus Labes in the NPCC, but no significant phase reddening. In the next step, we applied the Hapke (2008, 2012) model for the NAC F82+F22 (649.2 nm), WAC F13 (375 nm) and WAC F17 (631.6 nm), enabling us to compose the normal albedo and Hapke parameter maps for NAC F82+F22. On Baetica, the 649 nm global properties are: geometric albedo of 0.205+-0.005, the average single-scattering albedo of 0.181+-0.005, the average asymmetric factor of -0.342+-0.003, the average shadow-hiding opposition effect amplitude and width respectivelly of 0.824+-0.002 and 0.040+-0.0007, the average roughness slope of 11.45+-3 deg. and the average porosity is 85+-0.2%. In the NPCC, the normal albedo variegation among the craters walls reach 8% brighter for Gallicum Labes and 2% fainter for Danuvius Labes. The Hapke parameter maps also show a dichotomy at the opposition effect coefficients, single-scattering albedo and asymmetric factor, that may be attributed to the maturation degree of the regolith or to compositonal variation.

• The paper presents detailed photometric corrections of Lutetia’s NPCC, highlighting minimal phase reddening with specific spectral shift values.
• It employs modeling techniques including Minnaert, Akimov, and Hapke to quantify surface albedo and scattering properties using 187 multi-filter images.
• Findings indicate distinct differences between Gallicum and Danuvius-Sarnus Labes, suggesting varied regolith maturity and geological evolution.

Analyzing the Surface Characteristics of Asteroid (21) Lutetia: A Disk-Resolved Photometric Investigation of the Baetica Region

The article "Asteroid (21) Lutetia: Disk-resolved Photometric Analysis of Baetica Region" undertakes an in-depth exploration of the terrain of asteroid (21) Lutetia, specifically focusing on the Baetica region. Data from the ESA's Rosetta mission's OSIRIS cameras provide the foundation for this comprehensive photometric study. Utilizing 187 images collected via various filters, the authors conduct a meticulous examination of surface features, relying on modeling techniques such as the Minnaert disk-function and Akimov phase function, with further analysis conducted using the Hapke model.

The research primarily concentrates on the North Pole Crater Cluster (NPCC) within the Baetica region. The NPCC exhibits a remarkable variety of crater formations and demonstrates distinguishable photometric characteristics at varying observational angles. The study's objective was to determine the spatial distribution of spectral and albedo variations through photometric corrections and modeling.

Core Findings and Numerical Results:

The study revealed a notable dichotomy between the Gallicum and Danuvius-Sarnus Labes regions of NPCC. One of the significant outcomes is the minimal phase reddening, expressed as a spectral shift metric of $$\bar{\gamma}(5^{\circ})=3.5\pm0.55\%\cdot\mu m^{-1}$$ and $$\bar{\gamma}(20^{\circ})=2.9\pm0.38\%\cdot\mu m^{-1}$$. Such values suggest that the spectral changes or variations in albedo with phase angle are subtle and warrant further investigation.

Hapke model parameters offered detailed insight into Lutetia's surface composition and illumination properties. For instance, the geometric albedo measure for the Baetica region was determined to be $0.205\pm0.005$, with other parameters such as the single-scattering albedo, asymmetric factor, and porosity also quantitatively cataloged. These findings imply a surface dominated by rough, likely porous regolith, with substantial backscatter effects due to large grain size or significant inter-grain porosity.

Theoretical and Practical Implications:

From a theoretical viewpoint, this research augments our understanding of asteroid surface regoliths, particularly in terms of phase curve behavior and optical scattering. The quantitative discrepancies between Gallicum and Danuvius-Sarnus Labes highlight potential differences in regolith maturity or compositional heterogeneity that could be attributed to distinct impact events or varying surface evolution processes.

Practically, these detailed photometric profiles of Lutetia assist in refining models of surface scattering and regolith dynamics on asteroid bodies, which are crucial for interpreting observational data from current and future missions targeting similar bodies. The study’s methodology exemplifies rigorous procedures for extracting meaningful insights from high-resolution space imagery.

Concluding Remarks and Future Directions:

While the study effectively characterizes specific surface regions on Lutetia, the paper emphasizes the need for ongoing research to discern the fine-scale geological processes shaping asteroids. The work also poses challenges, such as confirming the presence of coherent-backscattering, which was not definitively detected in this analysis.

Future studies might focus on integrating these photometric findings with compositional data obtained from spectral analysis, or indeed from sample return missions, to achieve a more comprehensive understanding of the asteroid's history and evolution. Moreover, the methods and results could be extended to other asteroids, providing comparative insights across different asteroid types and aiding our broader comprehension of the early Solar System.