User:Nicjordan009/Occator (crater)

Between 2015 and 2017, the Occator impact crater has had five separate attempts to date the impact’s age on Ceres. The age dating models of the lobate flows and crater ejecta range from 200 million years to 78 million years and 100 million years to 6.09 million. The age ranges have different chronology models, image data at verifying resolution, and different methods to evaluate the data. The current data estimates an age of impact at ~20 to 24.5 million years; however, the estimates are of the sample areas with some uncertainty and variability due to arbitrary cratering and the use of different models to date the impact. Thermal evolution of a large cryo-magmatic chamber below Occator Carter constrained the age of the impact is closer to 18 million years, this is evident in the difference between impact geology and formation of the Cerealia Facula (bright spot).

Central Depression (pit) and Dome Structure
The Ac-9 Occator quadrangle is located on an elevated equatorial region and is the brightest region of the dwarf planet Ceres. The central feature located in the northwest region of the Ac-9 quadrangle, the 92km Occator impact crater. The possible Occator impactor (igneous rock) was approximately 5 km in diameter through Experimental simulation with an estimated velocity range of 4.8 km/sec to 7.5 km/sec and a target surface lithology of icy-rock material. The simulation variables produced an 80 km impact crater with a central peak and a crater depth of 15 – 30 km. The Ac-9 shows heavily fractured crater floors and is consistently shallow compared to similar size non-fractured crater floors. The Occator crater floor is covered in linear impact fractures from the southwest to the central depression. These fractures cross over the northeast lobate flow deposits at the base of the crater wall that extends into the central depression.

The large number of impact craters on the surface of Ceres in the range of 70 to 150 km in size have central depression rather than central peaks. Before the Dawn mission arrived at Ceres, the impact craters between 10 – 300 km in size have prominent conical central peaks. These prominent conical central peaks are a prominent morphological feature on major icy bodies such as Saturn’s moons Tethys or Dione. The smaller craters less than 40 km are virtually indistinguishable from the craters on other icy bodies; however, larger craters tend to have central depressions. Occator Crater’s central depression is ~9 km across and ~1 km in depth from the crater floor. The northern and southern edges of the convex profile of the crater are rimless with slops of <10°, while the eastern and western edges of the crater’s depressions are dominated by irregular high standing massifs that formed an incomplete rim around the crater edge.

Discovered in March 6, 2015 during the early stages of mapping of Ceres’s surface, the Dawn mission located a luminous region on the Occator crater floor. This incandescent material was determined to have a dominant composition of sodium (Na) carbonates, Aluminum (Al) phyllosilicates, and Ammonium chloride (NH4Cl). A review of high-resolution images from the Dawn mission framing camera (FC) revealed structural differences with the central peak of the impact. Occator crater is currently the most prominent well-preserved impact crater on Ceres’s surface, classified as a 92 km-wide complex crater. Occator collapsed central peak, now a 9-10km wide depression, the rim’s Crater average relief is ~3.75 km and has a crater slope between 30°- 50°. Occator crater’s central 1km deep depression displays a pronounced luminous feature names Cerealia Facula. Occator crater’s central depression is filled with a 700 m (2 km wide) dome, encompassed by several dense fractures along the dome flanks. This data indicates that the magnesium sulfide (MgS) deposits were in place after the central peak and dome’s uplift and collapse.

The crater floor comprises three central morphological units, which divide the crater into zones. The outermost unit or terrace zone along the crater wall forms a circumferential pattern. This unit contains hummocky and angular material with small to large, tilted fault blocks that vary in size up to ~10km in diameter and up to 2 km in height. The interior zone of the crater is divided into two different units that have two different morphological characteristics. The Northwestern Interior Zone is primarily hummocky material similar to the terrace zone material. This northwestern unit topography is formed of irregular mounds and uneven ridges and laterally blends into the hummocky faulted terrace unit along the crater wall, making this section very difficult to distinguish between the terrace and interior zones. The material within these zones shows significant displacement from direct relation to the crater wall slumping and floor uplift during the impact event.

The southern half of the crater interior zone is primarily a flat, low-lying topography of lobate deposits covering an estimated 1/3rd of the interior crater floor. Most of the southern u-shaped zone is formed around the central dome and opens to the structure’s northwest. The local relief of the topography within the lobate deposits of the southern half of the interior zone constraints within ~100 m. The topography relief of the western half of the interior zone is has a gentle increase of the slope ~500 m.

The asymmetrical change in relief of the lobate deposits located in the southern half of the interior indicates two significant factors. First, the impactor made an oblique angle impact trending from the southeast to the northwest. Second, the target had variations in composition or topography that altered the impact. Near the central depression and slightly offset from the center is an ~ 3km wide dome structure with an upper surface densely covered in cross pattern fractures. These fractures become less evident along the flanks and are believed not to extend into the walls of the depression (pit) structure. The bright material deposits extend to the inward-facing wall of the depression and transition to the dome structure’s exterior wall. This deposition pattern indicates the deposits formed within the contiguous geological unit and that the uplift and fracturing formed before deposition.