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Velocity Impact Craters in Ice and Ice&Hyphen

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Velocity Impact Craters in Ice and Ice&Hyphen VOL. 84, NO. B14 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 30, 1979 Low-Velocity Impact Craters in Ice and Ice-Saturated Sand With Implications for Martian Crater Count Ages S. K. CROFT Departmentof Earth and SpaceSciences, University of California,Los Angeles,California 90024 S. W. KIEFFER U.S. GeologicalSurvey, Flagstaff,,Arizona 86001 T. J. AHRENS Divisionof Geologicaland PlanetarySciences, California Institute of Technology,Pasadena, California 91125 We produced a seriesof decimeter-sizedimpact craters in blocks of ice near 0øC and -70øC and in ice-saturatedsand near -70øC as a preliminary investigationof crateringin materialsanalogous to those found on Mars and the outer solar systemsatellites. The projectilesused were standard0.22 and 0.30 cal- iber bulletsfired at velocitiesbetween 0.3 and 1.5 km/s, with kineticenergies at impactbetween 109 and 4 x 10•ø ergs. Crater diameters in the ice-saturatedsand were -•2 timeslarger than cratersin the same energy and velocity range in competentblocks of granite, basalt and cement.Craters in ice were -•3 times larger. If this dependenceof crater size on strengthpersists to large hypervelocityimpact craters,then surfacesof geologicunits composedof ice or ice-saturatedsoil would have greatercrater count ages than rocky surfaceswith identical influx histories.The magnitudeof the correctionto crater countsrequired by this strengtheffect is comparableto the magnitudesof correctionsrequired by variations in impact velocity and surfacegravity usedin determining relative interplanetary chronologies.The relative sizesof cratersin ice and ice-saturatedsand imply that the tensile strengthof ice-saturatedsand is a strong in- verse function of temperature.If this is true, then Martian impact crater energy versusdiameter scaling may also be a function of latitude. INTRODUCTION where Eo is the muzzle energy, rn is the bullet mass, p is the densityof air, A is the cross-sectionalarea of the bullet, and Ca Impact cratering is recognized as an important processin is the coefficient of drag. Equation (1) was derived by in- planetary accretionand in shapingthe solid surfacesof plan- tegration of Newton's secondlaw using a low-viscosity,'V- ets and satellites in the solar system. Crater counting is fre- squared' drag force appropriatefor bullets [Albertsonet al., quently used,and is often the only techniqueavailable, for es- 1960]. The quantity rn?CaApestimated for each bullet is also timating both the relative and absolute ages of geologic given in Table 1. featureson other planets. Most surfacesin the inner solar sys- Three types of target blocks were used. They were prepared tem consist of rock materials and their weathered products. and characterized as follows: Consequently,terrestrial small-scale impact and explosionex- 1. 'Ice-saturatedsand' (ISS) blocksconsisted of water-sat- periments have been performed primarily in rock or soil. urated sand frozen to approximately -70øC. Containers --•27 However, becauseof the recognition of the probable domi- x 33 x 16 cm in size were filled with sand and then water un- nance of ice and ice-saturated soils, both at and far below the til the sand was covered by a thin water layer. The mixture melting point of water over large portions of Mars, the aster- was slowly stirred to remove air bubbles.The mixture was fro- oids, and particularly the satellitesof the outer solar system, zen in the container. we performed a seriesof low-velocity impact experimentsin 2. 'Supercooledice' (S-ice) blo,cks consisted of pure water ice and ice-saturated sand. The objective of these impact ex- ice frozen to about -70øC, with the same dimensions as the periments was to provide a preliminary survey of the mor- blocks of ice-saturated sand. To prevent the formation of phology and kinetic energy-dimensionalscaling of cratersin large single crystals or bubbles, these blocks were built up icy media comparedto impactsat similar kinetic energiesand layer by layer, adding first water and then crushed ice until velocities in rock and cohesionless sand. the ice was barely saturated. The water-ice mixture was then EXPERIMENTAL PROCEDURE frozen, producing blocks having a uniform fine grain phane- ritic texture with tiny bubbles(<0.1 mm) thinly distributedin The projectiles used were standard 0.22 and 0.30 caliber the interior. bullets fired at velocities between 0.3 and 1.5 km/s. Table 1 3. 'Temperate ice' (T-ice) blocks consistedof pure water givesthe ballistic data derived from manufacturer'sspecifica- ice near 0øC and were prepared in three ways. The first type tions for the bullets used. Impact kinetic energiesat the mea- of temperate ice blocksused were commercially producedand sured firing ranges(R) of 8-13 yd (7-12 m) were interpolated maintained in a freezer at a temperature of--•28øF (-2.2øC). from the ballistic data using the equation These blocks were --•36 x 20 x 20 cm in size. The commercial method of freezing produced a roughly tabular amorphous R= C•Ap core ('cloudy zone') imbedded in a matrix of elongated rod- like crystals(--•0.5-1 cm long and 0.2-0.3 cm thick) oriented Copyright¸ 1979by the American GeophysicalUnion. perpendicularto the face of the tabular cloudy zone. This pro- Paper number 9B 1305. 8023 0148-0227/79/009B- 1305501.00 8024 CROFTET AL.: SECONDMARS COLLOQUIUM TABLE 1. Bullet Ballistic Data Velocity, km/s Energy, ergs Bullet Mass,g Muzzle 100yd* Muzzle 100yd* m/Cap'l, cm 22 Short (22S)•- 1.88 0.334 0.275 1.04E95 7.05E8 5.36E4 22 Long (22L)•- 1.88 0.378 0.294 1.34E9 8.13E8 4.20E4 22 Long Rifle (22LR)•- 2.59 0.383 0.310 1.90E9 1.25E9 5.01E4 22 Hornet (22H)•- 3.24 1.25 NA 2.54E10 NA 4.86E4 (estimated) 30-06SPRG Accelerator 3.56 1.48 NA 3.88E10 NA 4.86E4 (SPRG)•- (estimated) 30-06PSP (PSP)•- 8.10 0.975 0.856 3.85E10 2.97E10 8.11E4 NA is not available. * 1 yd=0.91m. •-Abbreviationused in Table 2. $Read 1.04E9as 1.04x 109. vided for highly anisotropicmaterial propertieswhose effects flat end faces,which had vertical relief of <•3mm. All facesof on the craters are noted below. the commercial ice blocks were used; these surfaceswere also The second temperate ice blocks ('pressed blocks') were smooth.Bullet name, target type, range,and crater depth and prepared by compressingcrushed ice in a pressurevessel until diameterwere recordedfor each shot.Those data are grouped fusion. This produceda uniform but porphyritic texture with accordingto target compositionand listed by shot number in many millimeter-sizedbubbles. These blocks were cylinders order of increasing 'trapactenergy in Table 2. Crater 5 is with diametersand lengthsof--,20 cm. shown in Figure 1. Depths were measuredfrom the original The third temperateice ('pot') blockswere preparedby sat- target surface. Diameters are averages of the largest and urating a container filled with crushedice and then freezing. smallest diameters of each crater. This method produced nonuniform porphyritic textureswith RESULTS occasionallarge air pockets(which did not affect the results reported below, as we discardedsamples where the bullet ob- With the notable exception of craters formed in the com- viously hit an air pocket). Theseblocks were cylinders--,25 cm mercialice blocks,the craterswere hemiellipticalcups in cross in diameter and --•15 cm long. section.The subsurfacefracture systemswere both concentric The internal temperaturesof the blockswere made initially and radial in pattern similar to thosefound around --•5-cm-di- constant by prolonged residencein monitored refrigerators. ameter impact cratersin ArkansasNovaculite by Curran et al. The volumesof the refrigeratorsavailable severelylimited the [1977]. Radial fractureswere dominant in cratersin ice, while maximum practical size of the target blocks. The blocks re- fine concentric fractures predominated in the ice-saturated mained in the refrigeratorsuntil transferredto insulated boxes sand craters.Visible fracturing was concentratednear the im- for immediate transport to the firing range. Temperaturesin- pact site and near the rear face of the target block. Shots4 (S- side the insulated containers were monitored. On the basis of ice) and 9 (ISS), which were fired into blocks which each al- (1) the time between removal from the refrigeratorsuntil use ready had a 5-cm crater in them (whosevisible fracture zones at the firing range (a few hours), (2) the air temperaturesin- were small in comparisonto the block size), completelyshat- sidethe insulatedboxes at the time of target use (--•0øCfor the tered the blocks.Identical shots(3 in S-ice and 10 in ISS) into temperate ice blocks and -14øC for the ice-saturatedsand undamaged blocks produced measurablecraters and only and supercooledice blocks),(3) the thermal propertiesof ice, split the blocks.It is concludedthat the cratersproduced ex- and (4) the parameterizedtemperature history calculationsof tensive, less obvious interior failure beyond the visible frac- Schneider[1974], it is estimatedthat the surfacetemperatures ture systems. of the supercooledice and ice-saturatedsand blockshad risen The upper limit of usable impact energieswas set by the between 5 ø and 10øC, while those of the temperature ice target block sizeand composition.For blocksin the size range blocks had risen a few tenths of a degree. The temperature used,the upper energy limit for ice blocksis --,3 x 10•ø ergs, gradient near the surfaceof the supercooledice and ice-satu- becauseshot 16 at 3.7 x 10•ø ergs completely destroyed the rated sand blocks is estimated to have been --•0.5øC/cm from targetblock, while shot3, at 2.5 x 10•ø ergs, did not. The up- Schneider's [1974] calculations. Edges and corners of the per limit for the ice-saturatedsand blocksappears to be near blocks would have been a few degreeswarmer, but as the cra- 5 X 10iø ergs.In order to gain as large a rangein energyas ters were formed in the approximate centers of the block possiblefor scalinganalysis, craters were producedwith cra- faces,the influence of temperatureedge effectson crater for- ter/target block dimension ratios ranging from --,0.1 to 0.7.
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