Wednesday, September 11, 2013

Bright and Dark Ejecta

LROC Featured Image, September 10, 2013 (M139782204LE)
A relatively recent impact event distributed bright, reflective ejecta across the lunar surface in southeast Mare Tranquillitatis. Smaller craters punch through the ejecta to reveal darker substrate, a contrast easier to see under a high Sun, and thus a lower illumination angle of incidence. A 500 meter-wide field of view from LROC Narrow Angle Camera (NAC) observation M139782204LE, spacecraft orbit 5733, September 22, 2010; a 10.11° angle of incidence, resolution 49 cm per pixel from 44.54 km over 4.37°N, 19.29°E [NASA/GSFC/Arizona State University].
Drew Enns
LROC New System

Fresh (young) impacts on the Moon often display magnificent ejecta blankets (so called because they "blanket" the surrounding terrain). Ejecta is unevenly distributed, which gives rise to its interfingered appearance.

Since space weathering tends to lower the albedo of material on an airless planet, the relative brightness of this ejecta blanket speaks to the young age of the parent crater.

In this case, the parent crater is just to the south of the opening image, and can be seen in the context image.

The same small, relatively fresh crater at local sunrise, when shadows under a higher illumination angle of incidence exaggerate variations in topography over albedo. Even so, the brighter surface rays are as distinct as striations channeled into the terrain by the blast. An 1875 meter-wide field of view from LROC NAC frame M162181924L, LRO orbit 9035, June 8, 2011; 73.88° angle of incidence, resolution 0.83 meters per pixel from 39.7 km [NASA/GSFC/Arizona State University].
But what is providing the small circular patches of dark material? Were the patches formed as part of the impact that formed the ejecta blanket, or later? Was the material excavated from below the bright ejecta? Most likely secondary craters (late stage ejecta) from the initial impact, hit and dug up dark mare material (original surface) from below the thin ejecta blanket. Can we test this idea? How dark is dark? In a more precise sense - do the albedos of the small low reflectance spots match that of the surrounding mare?

LROC Wide Angle Camera (WAC) context for the LROC Featured Image released September 10, 2013, showing the field of view located at located at 4.408 N, 19.230 E (marked by the cross). Nearby linear depressions (one smaller, closer depression is visible in the preceding image) may have provided the darker substrate discussed [NASA/GSFC/Arizona State University].
Your eye could be fooled by all the changes in reflectance. The small dark patches have a higher albedo than the mare (0.07 vs 0.06), which would be consistent with mare material mixing with the brighter (0.09 to 0.11) ejecta blanket. This observation is consistent with the secondary crater interpretation (the underlying mare is mixed with a small amount of the bright immature ejecta). If the reflectance of the dark patches was lower than that of the mare, then something else would have to be at work.

Can you think of other explanations while browsing the full LROC NAC frame, HERE?

Related Posts:
Beautiful Ejecta Patterns
DMD Excavations
Symmetric Ejecta

1 comment:

Anonymous said...

Hey, what's going on!?
We've had no new posts for at least 3 weeks now!