Post by Dr. O. Ruesch, European Space and Technology Center, European Space Agency (ESA), the Netherlands.
A fragmented block is referred to a cluster of fragments formed by the disruption of a parent block. The identification of such features on planetary surfaces is possible due to the minor spatial dispersion of the fragments away from the parent block. This morphology is to be distinguished from clusters of fragments formed by mass wasting like rockfall or disintegration during block rolldown. Observations of fragmented blocks have been reported on almost every rocky planetary body where images captured by orbital and surface craft resolved features in sufficient spatial detail. Despite the fact that disrupted blocks can reveal important clues on the formation process of soil (regolith) on planetary surfaces, they have started to receive attention only in recent years.
On the airless surface of the Moon, fragmented blocks display a wide range of morphologies (Images 1 and 2). In general, the configurations of the fragments can be described by a continuum from highly catastrophic to sub-catastrophic. Image 1 shows an example of a catastrophic fragmentation where the number and size of the fragments indicate that the parent block was much larger than the largest fragment. The radial pattern formed by small fragments and brighter areas is diagnostic of disruption by a meteoroid impact.
Image 1. Example of a block fragmented catastrophically near crater Copernicus on the Moon, where the largest fragmented in considerable smaller than the original parent block. LROC/NAC image M127063668LE. http://bit.ly/2mAl0CB
Lunar blocks fragmented sub-catastrophically correspond to clusters of fragment where the largest fragment is only slightly smaller than the parent block and/or where the fragments display minimal displacement. Image 2 (left) shows a configuration where the largest fragments are not located at the center but appear to surround it. In Image 2 (right), fractures bifurcate from a point approximately at the center of the block and extend over its entire width. In this case, the sub-catastrophic disruption might indicate an impact of a meteoroid of relatively lower energy (e.g., smaller velocity or size of the projectile). Although most lunar fragmented blocks are expected to be due to meteoroid bombardment, the action of additional processes cannot be completely excluded. For example, thermal stresses by diurnal temperature variations might enhance disruption of blocks and smaller boulders, and could lead to cracking such as in Image 2 (right). Blocks fragmented by a network of cracks observed on Earth and Mars have been associated to the role of thermally induced stresses. These stresses are proportional to temperature variations between day and night and their effects are a function of the number of cycles, i.e., length of day. Thus, fast-rotating objects orbiting relatively close to the Sun (e.g., near Earth asteroids) could provide favorable conditions for thermal cracking and thermal fatigue to play a non-negligible role in landscape evolution.
Image 2. Left: Example of a fragmented block near lunar crater Giordano Bruno where the few largest fragments were not dispersed far away from the parent block. LROC/NAC image M156924032LE. Right: Example of a fragmented block near crater Giordano Bruno displaying cracks without displacement of the fragments. LROC/NAC image M156924032LE.
Further Reading
Nakamura, A.M., et al., 2008. Impact process of boulders on the surface of asteroid 25143 Itokawa—fragments from collisional disruption. Earth Planets Space 60, 7–12.
Dombard, A., et al., (2010), Boulders and ponds on the Asteroid 433 Eros, Icarus 210, 713–721.
Eppes, M.-C. et al. (2015), Cracks in Martian boulders exhibit preferred orientations that point to solar-induced thermal stress, Nature Communications, 6:6712.
Ruesch, O., et al., (2020), In situ fragmentation of lunar blocks and implications for impacts and solar-induced thermal stresses, Icarus 336, 113431.
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