Blocks fragmented in place on the Moon

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

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

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Weathering profiles on Earth and Mars

Post by Anne Gaudin, University Nantes, CNRS, Laboratoire LPGN, France

On Earth, weathering profiles that have developed in ultramafic rocks under tropical climate show a mineralogical transition between a Fe, Mg-rich smectite zone and an Al-rich kaolinite-bearing zone (e.g. Colin et al., 1990; Gaudin et al., 2005; Yongue-Fouateu et al., 2009). This evolution is due to an intense leaching of Mg2+ cations during the weathering process. The Murrin Murrin (MM) site is an example of such a profile located in the Archean Eastern Yilgarn Craton, in Western Australia. The MM profile is developed in serpentinized peridotite massifs over a 40 m thick sequence (Image 1) and shows three zones: serpentinized peridotites at the bottom, immediately overlain by Fe/Mg-bearing smectites and then Al-bearing phyllosilicates (kaolinite) mixed with iron hydroxides.

Image 1

Image 1: Weathering profile at the Murrin Murrin site which is currently mined for nickel, located in Western Australia (121º53’41’’E, 28º44’51’’S) (Gaudin et al., 2011).

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