Thursday, June 28, 2012

LROC: Copernicus looking straight down

Fractures and a collapse crater within impact melt rock on the floor of Copernicus crater. LROC Narrow Angle Camera (NAC) medium resolution montage from greater than 100 kilometers, field of view width is roughly 1800 meters [NASA/GSFC/Arizona State University].
Sarah Braden
LROC News System
(The LROC) Featured Image (released June 28, 2012), a 1.8 meter per pixel mosaic of Copernicus (9.62°N, 339.92°E, 93 km in diameter), compliments (the) fantastic oblique view of Copernicus Crater (released June 27).(This) view, looking straight down, highlights the central peaks as well as terraces, impact melt pools, and melt fractures.

The opening image features a linear fracture with aligned pits within the impact melt deposit on the floor, and a crater which may have formed by collapse of impact melt (collapse pit rather than an impact crater). The fracture may have formed as a tube collapsed. Lava tubes commonly form within basaltic volcanoes on Earth, as part of an underground plumbing system that moves magma away from a vent. The same type of tubes and pits probably formed in lunar mare (also basalt). Should we expect lava tubes in impact melt deposits? There is much evidence for such in the NAC images collected over the past few years. 

The NAC revealed collapse pits, often aligned in rows, in many impact melt deposits. These pits are similar to collapse pits found in lava tubes on the Earth (often called skylights). In one case two collapse pits side-by-side resulted in a natural bridge! But how did they form? What caused the melt to flow after it ponded in the crater floor? Perhaps slumps of wall material into the melt caused large-scale displacements of still molten subsurface melt to flow. Or perhaps over months and years the crater floor rebounded while melt was still cooling beneath a crust. Both likely happened, so it is not a big surprise that melt moved in subsurface tubes for quite a while after the impact event.

Comparison of today's mosaic with the oblique image of the central peaks. Top of the images is west. Images are 34 km across [NASA/GSFC/Arizona State University].
The interior of Copernicus contains dramatic impact melt features. The image below (a subsampled portion of the full mosaic) shows a section of the northern wall of Copernicus. The top of the image shows the edge of an impact melt pool emplaced on a small terrace. At some point, a portion of melt escaped the terrace and the liquid rock carved curved, sinuous channels as it flowed down the wall. Towards the bottom of the image you can see where one of the flows stopped, spread out, and deposited some of the impact melt. 

Impact melt flowed from terraces down the north wall of Copernicus, leaving behind curved channels. Field of view is 6480 meters [NASA/GSFC/Arizona State University].
The subsampled mosaic shows a dramatic view of the impact melt in Copernicus crater's floor. The melt in the eastern portion of the image shows several mounds, while the melt on the western half of the mosaic is noticeably smoother. Several terraces at different elevations along the northern wall also have melt ponds. How does Copernicus crater's impact melt compare to other large craters? Tycho crater has a similarly large sheet with a mix of chaotic and smooth melt. As does King crater, Necho crater, Giordano Bruno crater, and almost every other Copernican aged crater larger than 1 km in diameter!

Subsampled version of the Copernicus mosaic. Image field of view 36.5 km across [NASA/GSFC/Arizona State University].
Explore the entire Copernicus mosaic! And compare with the Copernicus peak oblique from yesterday's Featured Image. The mosaic images were taken only 8-10 orbits after the oblique image of Copernicus to maintain similar viewing geometry, making visual comparisons even easier!

Related Posts:

Failed Skylights of Copernicus (January 24, 2012)
The smooth anomaly in Copernicus (September 29, 2010)
Copernicus (September 23, 2010)
LOLA's Copernicus (April 23, 2010)

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