Copernicus crater central peak casts a long shadow to the west over a crater floor that was flooded with impact melt that cooled and hardened to form this spectacular landscape. LROC NAC M193025138LR, image width is 1350 m [NASA/GSFC/Arizona State University].

LROC imaged the interior of Copernicus crater pictured above; the central peaks immediately capture your eye, with the tallest peak rising one kilometer above the floor of the crater. For comparison, the Grand Canyon has an average depth of 1.6 km. During the impact that formed Copernicus crater, an unimaginable amount of kinetic energy was transferred instantaneously into the surface. After the excavation stage of the impact, the initial transient crater collapsed under the force of gravity causing the crater rim to move inward, and the central region rebounded (uplifts) to form the central peaks!

Central peaks only form in craters larger than 15-20 km in diameter on the Moon. The rock that forms the central peak originates from the greatest depth of all the material excavated by the crater. For that reason, scientists are very interested in the composition of central peaks, since the material tells us what lies deep beneath the surface of the lunar crust; studying central peaks of large craters is therefore one of the best ways, absent returned samples, to probe the composition of the lunar interior.

Read the whole story with additional images here.

Then the LROC team created a 1.8 m/pixel mosaic of Copernicus crater looking straight down, which highlights the central peaks as well as terraces, impact melt pools, and melt fractures. The image below 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.

Fractures and a collapse crater within impact melt rock on the floor of Copernicus crater. Image width is 1800 m [NASA/GSFC/Arizona State University].

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). The NAC has revealed collapsed 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.

Read more with additional images here.

Posted by: Soderman/NLSI Staff

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