Fractures on Asteroid Boulders: Understanding Orientation Biases Caused by Lighting

Introduction:

The observed presence and orientations of fractures on boulders on the asteroids  (101955) Bennu and (65803) Didymos-Dimorphos have been instrumental in establishing the role of thermal fatigue and fragmentation on asteroids (Molaro et al., 2020, Lucchetti et al. 2024). With cratering processes, thermal fatigue is hypothesized to be one of the primary mechanisms forming regolith on small asteroids (Delbo et al., 2014).  However, lighting and viewing conditions can introduce ambiguity in the orientation and number of surface lineaments  that are identified on an asteroid’s surface (Buczkowski et al., 2008). Similarly, lighting conditions can affect the identification and orientation of fractures observed on boulders. Therefore, any unidentified bias in determining the orientation of these boulder fractures could influence any conclusions on the importance of thermal fatigue relative to cratering.

To understand what imaging bias might exist, and to determine a way to correct for it, we explore systematically how lighting influences measurement of fracture orientations on asteroid boulders. We render several local digital terrain models (DTMs) of boulders on Bennu generated using data collected by the OSIRIS-REx Laser Altimeter (OLA; Daly et al., 2020, 2017). The modeled boulders were chosen because they possess numerous linear structures on their surfaces that are well captured by the high-resolution, 5 cm per pixel ground sample distance (GSD) of the OLA-derived DTMs. The boulders considered in this study range in size from 6–33 m in diameter.

Figure 1. Setup used for rendering surface boulders observed by OLA on Bennu. Camera is looking straight down on surface with an elevation on 90°

Methodology:

In this analysis, each local boulder DTM had its slope de-trended, such that the boulder was sitting on a flat-horizontal plane.  Images of the boulder were rendered using a ray tracing algorithm and a modified Lommel-Seeliger photometric function identical to what was done in Daly et al., (2022). The camera was always placed at an elevation angle of 90° (0° emission) and a range of 1 km from the surface, looking directly down at the boulder (Figure 1). The modeled camera had 1024x1024 pixels with a focal length of 500 mm and an instantaneous field of view (IFoV) of 4x105 radians, resulting in images with a ground sample distance of 4 cm/px. These camera properties were picked so that the resulting image pixel scales were similar to the GSD of the DTMs. The boulder DTMs were rendered using a range of solar azimuths (α of 0° to 360°) and elevations (β -30° to 30° via 90°) (Figure 2). Images were rendered every 30° in α and 20° in β.

 

Figure 2. Boulder images rendered for differing sun azimuths α and elevations β. The image with the yellow outline indicates the observing conditions that existed during the DART impact (Daly et al., 2023).

Members of our team were then tasked with mapping lineaments on a subset of the synthetic images. Each investigator was only given images for one lighting condition when they undertook their lineament assessment, in order to minimize any bias that might have arisen by having more than one viewing orientation. They were also given a downsampled and smoothed version of the boulder DTM so that they could map lineaments on the rendered images in the Small Body Mapping Tool (SBMT, Ernst et al. 2018) if they so desired. Once all the rendered images were mapped and the result turned in, each investigator was given a full resolution boulder DTM with all of its renderings, so that a complete set of lineaments could be identified. These were then compared to the lineaments obtained from the individual rendered images. The results for any biases were then assessed.

Preliminary results:

The mapping is ongoing, and a full report of our findings will be presented at EPSC 2024. However, some preliminary findings can be mentioned. As is already well-known, the morphology of surface features, including boulder fractures, are not easily recognized when the sun elevation is high (immediately overhead): features get washed out. Images taken with low sun β (<20°) are also problematic because shadows hide any significant morphology. The best viewing geometry for finding boulder fractures seems to be between 20°< β <60° solar elevation. Using two orthogonal solar azimuths further reduce bias.

For lighting at the DART impact site (where the Sun was at about β = 30° and α= 10°), many of the fractures present on boulders can probably be identified. Cracks that are orientated in N-S, NE-SW and NW-SE are easily identifiable. Some E-W fractures could be missed.

Conclusion:

Lighting needs to be considered when evaluating orientation of boulder fractures. Biases related to lighting geometries may have implications for past fracture studies on Bennu and elsewhere. The evidence used to establish the importance of thermal fatigue operating on asteroids may need to be re-evaluated.

Acknowledgemnet: O.S.B and R.-L.B. are funded by NASA New Frontiers Data Analysis Program grant number 80NSSC22K1035.

References:

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