Lunar Reconnaissance Orbiter Camera

Byrgius A ejecta

A mix of boulders and impact melt lie just beyond the rim of Byrgius A crater. Image width is 1000 m, LROC NAC M1101631740LE [NASA/GSFC/Arizona State University].

We have featured the impact melt flows of Byrgius A before, but today we are focusing on the ejecta. Above we can see that the ejecta blocks have a mixture of two reflectance levels. Maybe the impact is excavating two separate geologic units: one with low reflectance, and the other with high reflectance. Or, perhaps the bright, high reflectance ejecta blocks were covered by darker, lower reflectance impact melt. Can we figure out which hypothesis is correct?

Context image of Byrgius A crater, located at -24.577°, 296.198° E. Today's Featured Image is located within the red box. Discontinuous bright streaks radiating from Byrgius A are boulder fields similar to today's Featured Image. [NASA/GSFC/Arizona State University].

If the crater is excavating two different units, then we should expect to see nearby craters exposing the same. We don't see this in the context image. Several lines of evidence argue instead that impact melt covers some boulders: [1] the darker boulders have a similar reflectance to the impact melt, and [2] the dark boulders have pools of material.

A more detailed view of today's Featured Image. The boxes show areas where reflectance was measured for: [1] impact melt, [2] a bright boulder, and [3] a dark boulder. The red box zooms in on a boulder covered by ponded dark material [NASA/GSFC/Arizona State University].

Nearby melt on the rim has formed a veneer over the original surface, and we can see the melt fracturing parallel to the crater rim. This veneer unit has a reflectance of 0.14, close to the dark boulders with a reflectance of 0.155. The bright boulders have a reflectance of 0.25. The dark boulders also have small ponds of material. Larger melt ponds develop in depressions exterior to their parent crater, and this is likely the same process operating on a small scale.

Can you test the hypotheses further with the full LROC NAC?

Related Posts: Ejecta Starburst

Dark Secondary Crater Cluster

Boulder Mound

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