Tuesday, March 5, 2013

Copernicus collapse pit

Collapse feature in the impact melt on the floor of Copernicus, field of view 430 meters, from LROC Narrow Angle Camera (NAC) observation M168333206L, LRO orbit 9941, August 18, 2011; resolution 40 cm per pixel from 25.4 kilometers. Originally posted to illustrate "Failed Skylights of Copernicus," January 24, 2012 [NASA/GSFC/Arizona State University].
Sarah Braden
LROC News System

A fresh collapse feature within the impact melt floor of Copernicus crater is 330 meters across (that's about 3 American football fields wide!). At one point the impact melt in this area was flat, but then the area collapsed forming the feature here (located at 10.204°N, 339.998°E). The rim of the depression is still very fresh with outcrops and boulders.

An estimate of the feature's depth is approximately 50 meters, based on shadows from another NAC image of the same area. There are a few possible causes for the collapse.

A subsurface void may have formed as the impact melt flowed and cooled. Subsurface voids occur when melt emplaced shortly after the impact drains away deeper into the impact cavity. Perhaps a small bolide impacted the surface and instigated the collapse of the structurally weak void. Alternatively, the collapse might be due to seismic shaking from moonquakes disturbing the weak section of the melt deposit.

Oblique view of the featured collapse pit, from well to the east of Copernicus; a montage of the left and right frames of LROC NAC observation M193025138 (thumbnail at bottom), orbit 13472, May 30, 2012. Spacecraft slewed from 63.11° from nadir [NASA/GSFC/Arizona State University].
This collapse feature is much larger in diameter than the mare pits: the Mare Ingenii pit is about 130 meters in diameter, the Marius Hills pit is about 65 meters and the Mare Tranquillitatis pit is around 100 meter across.

Pits in mare basalt may have formed when a portion of a lava tube collapsed. The subsequent pit is a skylight that leads into the intact lava tube. Impact melt pits are common within impact melt deposits like the one here in Copernicus, however the depression in the Feature Image is larger in diameter than average impact melt pit. Perhaps in this case the subsurface void was larger, or the surface layer of impact melt was structurally weaker, resulting in a larger collapse area.

Arrow marks location of the featured collapse pit, near an north-south contact between distinct impact melt types, easier to visualize, perhaps, in color and other images highlighting lunar surface albedo. LROC Wide Angle Camera (WAC) monochrome mosaic M147109260C, orbit 6813, December 16, 2010; resolution 60 meters from 43 km, angle of incidence 78°
A previous Featured Image, "Copernicus Seen Looking Straight Down," featured a mosaic of Copernicus's floor (9.62°N, 339.92°E, 93 km in diameter), but today's Featured Image offers much higher resolution.

Explore the entire NAC frame for more high resolution impact melt deposits, HERE

Related Images:
Copernicus Seen Looking Straight Down
Copernicus Central Peak
Natural Bridge on the Moon!
Impact Melt Pit
Failed Skylights of Copernicus


Anonymous said...

Interesting. I am surprised to see what look like distinctive layers of rock in the walls of the pit. I would have guessed that impact melt was more-or-less monolithic, as it all formed at once. Why the layering?

Joel Raupe said...

More than one school of thought about that. The whole crater floor has a distinctly different surface in the northwest quadrant, which shows up clearly in telescopic images. As to the layering, the thought is melt arrives back (from what may have been a double impact) from different elevations, cooling at different rates (or heated to different temperatures and viscosities). One pool semi-cooled to be covered by subsequent runoff from off the walls or even the central peaks.

What has intrigued me are the multiple levels visible on the inside walls of the Tranquillitatis pit.