Tuesday, December 3, 2013

Smattering of Self-Secondaries

Self-secondary craters in ejecta
Self-secondaries within the ejecta blanket of a fresh crater. A roughly 2500 meter-wide field of view from LROC Narrow Angle Camera (NAC) observation M1120683999L, LRO orbit 17351, angle of incidence 34.85° at 0.88 meters per pixel resolution from 97 km over 2.21°S, 84.85°E, central Mare Smythii [NASA/GSFC/Arizona State University].
Sarah Braden
LROC News System

Today's Featured Image highlights a small (830 meter) fresh impact crater (located at 2.017°S, 84.948°E) with an ejecta blanket peppered with smaller impact craters. Two of the largest superposed impact craters excavated lower reflectance material (darker) from beneath the main ejecta blanket. This low reflectance material is more mature than the primary crater ejecta. The ejecta from the primary impact is higher reflectance since it is immature material that has not been darkened by space weathering processes. Some of the small impacts have higher reflectance ejecta, which means they did not puncture through the fresh primary crater ejecta blanket.

The craters seen in the Featured Image are a mix of self-secondary and small primary impacts. The term secondary crater describes impact craters formed by ejecta materials from larger impacts. Self-secondary craters form as late stage ejecta is superposed on early ejecta material from the same impact event and are for the most part found near the primary impact crater. The velocity of ejected material increases with distance from the primary crater; therefore, most self-secondary craters are created by low-velocity impacts. Secondary craters formed furthest from the primary crater result from higher velocity ejecta, which makes those secondary craters harder to discern from small primary craters. Secondary craters on the Moon are estimated to have a maximum size of ~4% of the primary crater diameter.

Planetary scientists use the density of craters on impact ejecta as an indicator of age of the impact, based on our understanding of the flux of primary impacts over time. Self-secondaries on crater ejecta are not primary impact craters and inclusion of self-secondary impacts in a measurement will give a higher crater density, and thus result in an older age estimate for that particular surface. The age estimate for the impact crater Giordano Bruno is an example where self-secondaries complicate our understanding of the relationship between surface age and crater density.  

LROC WAC Haldane and Talbot (Context)
LROC WAC 643 nm reflectance mosaic of the area surrounding the fresh crater in the Featured Image (white arrow) [NASA/GSFC/Arizona State University].
The fresh impact crater in today's Featured Image is between two relatively older craters: Haldane (40 km diameter) and Talbot (12.4 km diameter). The crater Talbot has a visible rim, but the ejecta blanket was embayed by mare basalt. Talbot's floor is flooded with mare basalt as well. Haldane is also as old or older than Talbot since sections of Haldane's rim are completely destroyed.

Explore the full NAC image, HERE.

Related LROC Featured Images:
Polka-dot Ejecta
Action Shot
Impact Art
Ejecta sweeps the surface

HDTV still from Japan's lunar orbiter SELENE-1 (Kaguya, 2007) captured the western interior of Mare Smythii, straddling the libration zone between the Moon's near and far sides. The small, fresh crater of interest (east-southeast of Haldane, on the lower far right) is not as readily visible at high (sunrise) solar illumination angles. From Earth, the Moon was a thin, early evening crescent [JAXA/NHK/SELENE].

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