Tuesday, August 12, 2014

Complimentary craters, south of Maclear

South of Maclear in northwest Mare Tranquillitatis, two complimentary craters of very similar location, size and origin, but additionally of widely different ages. The relentless bombardment of small debris "gardens" the lunar surface at an average rate of 3 mm every 2 million years. In addition to the nearly billion year long cycle of cosmic ray dark-reddening, "space weathering" ages, or "optically matures" the lunar surface at a predictable rate, adding to crater counts and super-positioning another useful tool to the craft of dating lunar features from a distance. LROC NAC observation M131515002R, LRO orbit 4515, June 18, 2010; 79.75° sunrise incidence angle, resolution 85 cm from 40.68 km over 9.09°N, 20.14°E [NASA/GSFC/Arizona State University].
Raquel Nuno
LROC News System

There are several distinguishing properties of craters that help lunar scientists determine their ages. As craters get older their appearance changes through exposure to solar wind bombardment and other impacts (collectively called space weathering), and even gravity has an effect.

Effects of the solar wind lower the reflectance of the surface; so regolith (soil) that was excavated by recent impacts has higher reflectance than the background surface, this is why small young craters have visible crater rays. New impacts pulverize rocks that were ejected during the formation of an older crater and disturb the shape by causing moonquakes. Also, gravity works to alter the shape of a crater by pulling material down its walls in a process called slumping, this causes craters to have a smoother appearance.

1.69 km field of view from LROC NAC Commissioning observation M106748283R, LRO orbit 873, September 5, 2009; 29.84 low-angle incidence, resolution 1.17 meters from 133.64 km over 9.82°N, 20.15°E [NASA/GSFC/Arizona State University].
Today's Featured Image showcases two similarly sized adjacent craters (each ~500 m in diameter) located in Mare Tranquillitatis (see WAC context image below) with very different appearances. The area surrounding the top crater is littered with boulders in all directions. Wheras the more southerly crater has only a few rocks near its rim. Where did the boulders come from in the first place? And did the lower crater originally have boulders?

Locating two co-located 500 meter "complimentary craters" (arrow) good for comparing rates of general space weathering, in west-northwest Mare Tranquillitatis. LROC Wide Angle Camera (WAC) monochrome (566 nm) observation M131514941C, captured simultaneous with the NAC observation opportunity shown in the Featured Image at the top of this post. LRO orbit 4515, June 18, 2010; 79.75° incidence, resolution 57.7 meters from 40.72 km over 10.17°N, 20.14°E [NASA/GSFC/Arizona State University].
Since the mare basalt formed from layers of lava that hardened into solid rock, it is likely the boulders are coherent fragments of those thick layers (a few to tens of meters thick) that were broken up and ejected during the impact event. Since these two craters are so close and both formed in the mare it is very likely that the lower crater also had a large grouping of boulders in its ejecta field. The dissimilarity between these two craters is most likely due to age difference. Over time (perhaps a couple of billion years) the original boulders around the lower crater were slowly ground down by micro-meteorite bombardment - think of this process as cosmic sand-blasting! The boulders around the younger crater (top) have not had time to be pulverized by other impacts, but stick around for a billion years and you can watch these boulders slowly disappear!

Explore the full resolution NAC HERE.

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