Occam’s Razor and Mars: Hertzian fractures, Gale and other domed and fractured craters. James Byous, A.T. Dowd Research Terms used in this paper can be found in Hertzian Fractures and Related Terms – A Glossary and The Physicalities of Hertzian Fractures.1 ©James Byous, A.T. Dowd Research, AAtR Publishing 2013, all rights reserved. Selections may be used for educational and non-commercial research purposes. ABSTRACT Gale crater is a 155 km diameter impact crater that is at its center a blend between a compound and a coved-shield-type Hertzian cone (HC). Its features include a tilted peak and dome and dual wings on the east and west in arcs running north to south which are typical of low-angle impacts that create Hertzian fractures (HF) in glass and ceramics. Gale’s mound strata were laid over the years and in the flash of one meteoric impact the sub-surface cone was created. The bowl-shaped crater area eroded to a level below the original crater floor as wind quarried and dispersed the dust of the surrounding upper plane. Around the impact-compressed cone-mound of Mount Sharp (Aeolis Mons) wind forces carved out and carried away uncompressed material enlarging the bowl to reveal the lower mound. Gale Crater is now eroded well below the original surface by aeolian reduction revealing the totality of the current structure. In the future Gale crater could include a still deeper bowl and taller mound as more material is extracted. The main peak-shaped point-of-impact (POI) of Mount Sharp stands at ~5 km above aqueous and aeolian deposited crater-floor strata that is comprised of sediment rock and dunes. A larger, second point-of-impact peak stands ~20 kilometers to the north forming the main upper mound that stands above the winged ridges to the ESE and WSW. This wider dome POI area on the mound’s center-north quadrant suggests at least two coincidental impacts by a fragmented projectile. Arroyos and canyons that cut through the mound hold similarities to features found in other Hertzian cones. The mound area of Mount Sharp contains the five classic features found in many HC and HF structures; point of impact, mirror, mist, hackles region and Wallner undulation lines. Yardang-shaped hillocks along the body of the mound appear to be eroded hackles as can be found on compound HC structures. Features formed by projectile impacts that are similar in shape to Mount Sharp can be found in the windshields of cars in countless mall and market parking lots around the world. Like the ding in a glass window, Mount Sharp is the structure of a common Hertzian fracture -- a projectile- impact-induced stack of compacted crystalline material. Gale is only one of innumerable and similarly created structures on Mars and other rocky planets and moons in the universe. INTRODUCTION Mound craters are like poorly designed sports arenas--seating on the outside with a dome over the central field of action. After the main impact blast, the mound is where the action took place and the rimming bowl is where transition and egress proceeds at a leisurely pace. Domes in craters are HC positive segments of Hertzian fractures and can contain the five main Hertzian features; point of impact, mirror, mist, hackle and Wallner lines as shown in figure 17. The overburden contains the mold section of the HF as shown in figure 1. The initial crater is a small tip-of-the-iceberg occurrence. Hertzian fractures in themselves are neutral; the void between the cone and the mold. The cone segment is many times larger than the inverse of the crater with fractures running deep into the target material, figure 29. As erosion progresses the original crater erodes into an expanding, downward and widening circle while surrounding strata are washed, worn or blown away along the lines of the fracture. Though the term Hertzian fracture was not used, Thomas J. Ahrens experiments with impacts of sandstone and granite display this structure. Cross-sectional cuts of these studies show the crater hollow above a vastly larger HC section of stone.2 The mound remains less effected by erosion and therefore intact due to compression during impact. It is be similar to compressed hammer or planer marks on wooden boards after processing. The blade-beaten, compressed fibers resist the sanding process if no sanding block is used.3 It is suspected that in aqueous strata the impact area creates fracturing beyond the crater walls the way oil and gas shale is “fracked” in North Dakota oil fields. It weakens the annular rock and promotes rapid erosion. Variation of central peak and domes in craters are variations of erosion or spall evacuation of surface material around the HC. 1 Byous, James D., The Physicalities of Hertzian Fractures, and Hertzian Fractures and Related Terms – A Glossary, A.T. Dowd Research, AAtR Publishing, Savannah, Ga. 2013. 2 Huirong Ai (with Thomas .J. Ahrens), Shock-Induced Damage in Rocks: Application to Impact Cratering, pp 115-116, Figure 5.2b. Caltech, 2006, and LaSalvia, J.C., McCuiston, R.C., Fanchini G., McCauley, J.W., Chhowalla, M., Miller H.T., and MacKenzie, D.E., Shear Localiztion in a Sphere-Impacted Armor-Grade Carbide. 23RD INTERNATIONAL SYMPOSIUM ON BALLISTICS TARRAGONA, SPAIN 16-20 APRIL 2007, pp 1331-1334, figures 1a,1b and figure 4. 3 Bob Flexner, Rules for Sanding Wood, Popular Woodworking Magazine, April 8, 2008, and Preparing Wood For Finish, August 2010, The Finishing Store.com. 1 Multi-ring craters are in the same manner created except that the spinner and POI are comminuted and flattened out to the laminae on the radius of the POI. Since primitive times Hertzian fractures, also know as conchoidal fractures, were used to sharpened edges of lithic blades and points. Their creation uses the same principle of physics that forms conical fractures in planets and moons when a speeding bolide strikes a planetoid surface. In a spear point, HF’s are created by using an indenter applied with angular pressure on a point of rock to create hollowed fluted sections, figure 10. The body of the blade is the equivalent of overburden and the flute is the mold (negative) section of the fracture. The partial cone (positive) section is shed in the process and is discarded. In ceramic dentures and glaciated-granite rock surfaces “flutes” can be found after a tooth slides along a ceramic denture implant or rock boulders in glaciers, figure 2 and 3, grind along granite walls and planes to create chatter marks.4 Figure 1 Diagram of a Hertzian fracture. The overburden is the main sheet. The projectile strikes the pane and creates a cone shown below the mold (void) in the overburden. If the impact is of high velocity the cone is fragmented but retains its shape Figure 2 Diagram adapted from Kim et al showing axis tilt in HF in ceramic and glass material due to low angle compression.. Figure 3 Chatter marks, semi HC’s, created by boulders being dragged across the surface of the granite creating tilted Hertzian fractures. Transient boulders serve as indenters creating results as described by Kim et al in ceramics and glass studies. The angle of the indenter or projectile pressure determines the angle of the HC. Gale Crater is a sample of a tilted HC. Photo, G.K. Gilbert USGS 1903. 4 Kim, J.-W., Kim, J.-H., Thompson, V.P., Zhang, Y., Sliding Contact Fatigue Damage in Layered Ceramic Structures, Department of Biomaterials and Biomimetics, New York University College of Dentistry. 2 . This semi-cone is caused by the inertia of the projectile transferring energy into the material, slanting the formation of the cone and fracture. The cone’s base tilts along the trajectory of the impacting projectile the way light refracts below the surface of glass or water. At very low velocity and angles >10° above the glass surface, the projectile can bounce off with a small amount of scuffing or by creating a circular fracture and/or a front side semi-cone. Around 20° to 30° the tilted cone can form below the surface. Spinning and rotation of a projectile will also affect the slant and shape of a cone in glass studies, therefore it is suggested that spinning bolides or fragments of bolides will have some influence on those feature in rock. Front side semi-cones can also be created in by low velocity projectiles as seen in figure 4. Figure 4 A low-velocity oblique impact in soda lime glass displays a front-side semi cone. Methods for HF creation For studies below 45 m/s a standard steel yoke and arm support sling was used. In studies between 45 to 91 m/s samples were created with a conventional air rifle.5 Velocity was adjusted by distance from the target, 7.62cm to 1 meter. Both release systems were hand held to allow for rotational variances in steel ball projectiles. Sling samples were found to be primarily on Y axis rotation while air rifle samples exhibited multi-rotational impact evidence. Two to three layers of glass were used to allow confined fractures for viewing of samples from the base. Angles varied generally from 45º to 60º and also true vertical at 0/360º. Figure 5 Configuration of impact sample set. GALE CRATER Configured in a sloping plane between the Naochian Age highlands’ southern region and the Hesperian Age lowlands northern edge, 155 km Gale Cater has a ~4% south to north grade due the regional tilt. The crater sits directly on the transition line between the two.