Wednesday, June 27. 2012
On May 5th, 2012 LRO slewed 63° to capture this LROC image of the interior of Copernicus crater (9.62°N, 339.92°E, 93 km in diameter). 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. Recent remote sensing studies using Moon Mineralogy Mapper spectra confirmed the presence of relatively unusual olivine-rich material in the central peaks of Copernicus. Are we sensing the upper portions of the mantle, or magma chambers that formed in the crust?
Copernicus crater also plays an important role in our understanding of the lunar geologic timescale. Scientists use the basic principles of stratigraphy and superposition to define relative ages for geologic terrains and features. Rays are young lunar features, and any geologic unit covered by a ray of Copernicus must be relatively older than the crater itself. In fact Copernicus crater is defined as the beginning of the youngest period of lunar geologic history, the Copernican period. But how young are Copernican materials? It wasn't until the first samples were brought back from the Moon, from the Apollo and Luna missions, that scientists were able to tie relative ages to absolute time within the lunar timescale. It is likely that Apollo 12 astronauts sampled material ejected from the impact that formed Copernicus crater. These samples were radiometrically age dated to be close to 800 million years old! So all materials mapped stratigraphically as Copernican are younger than 800 million years. Of course the sample collected at the Apollo 12 site is thought to be from Copernicus crater, not known to be. Many craters are much younger, so you can think of Copernicus as the oldest young crater on the Moon (see yesterday's LROC Featured Image on Giordano Bruno crater).
Copernicus crater was a candidate landing site for the Apollo 18 lunar landing mission, which was unfortunately cancelled. The Constellation Program also designated Copernicus crater as a region of interest. So perhaps in the future astronauts will visit Copernicus crater, but when? In the meantime scientists are using LRO data to understand the complex geology of this important crater and plan future exploration.
LROC took another oblique view of a much younger Copernican crater, Tycho (43.37°S, 348.68°E, 85 km in diameter), which is only about 110 million years old. Even though Tycho is smaller in diameter than Copernicus, the summit of Tycho's central peak is 2 km above the crater floor! That is twice as tall as Copernicus crater's central peak. The final form of a crater transitions with the size and speed of the impactor. Craters even larger than Copernicus do not have central peaks at all, but rather peak rings. You can compare the full resolution Tycho oblique view with the Copernicus oblique view.
Explore the entire amazing interior of Copernicus crater!