FRAGMENT SHAPES PRODUCED BY HYPERVELOCITY IMPACT AND THEIRLUNAR SIGNIFICANCELeonard D. Jaffe Citation: Applied Physics Letters 11, 328 (1967); doi: 10.1063/1.1754998 View online: http://dx.doi.org/10.1063/1.1754998 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/11/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Detection of electromagnetic pulses produced by hypervelocity micro particle impact plasmas Phys. Plasmas 20, 092102 (2013); 10.1063/1.4819777 Observation of mass and velocity of projectile fragments produced by hypervelocity impact withlightweight ceramic targets AIP Conf. Proc. 1426, 891 (2012); 10.1063/1.3686421 A model for debris clouds produced by impact of hypervelocity projectiles on multiplate structures Appl. Phys. Lett. 93, 211905 (2008); 10.1063/1.3029747 MASS ANALYSIS OF IONS PRODUCED BY HYPERVELOCITY IMPACT Appl. Phys. Lett. 13, 89 (1968); 10.1063/1.1652528 Strong Plane Shock Produced by Hypervelocity Impact and LateStage Equivalence J. Appl. Phys. 37, 853 (1966); 10.1063/1.1708271

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Volume 11, Number 10 APPLIED PHYSICS LETTERS 15 November 1967

Fig. 6. Smoke stream flowing over bolt.

Also, holographic vibration analysis~"'; could be extended by permitting exposures while the object is in motion. Other possible applications include analysis of smoke stream flow patterns in wind tunnels; fluid flow; jet spray; large depth ill-motion microscopy; and simple storage of 3-D object di­menSIOns.

I A. D. Jacobson and F . .J. McClung, AJi/iI. Ojd;" 4, I !iO'! (I 'Hi'i). 'R. E. Brooks, L. O. HeHinger, R. F. Wuerker. and R. A.

Briones, ANII. Phy.,. l.flter.1 7, '!2 (I'Hi'i). "R. E. Brooks, L. O. Heninger, and R. F. Wucrkcr, IFFF

journal o/Qu(m/llm Eler/ronir.' QE-2, n!i (l'Uifi). 4 Agfa Cevaert, Munich, West (;ennany. "R. L. Powell and K. A. Stetson,.J OJil. Sot. A Ill. 55, 15'13 (I 'Iti5). "K. A. Haines and B. P. Hildebrand, ,/Nil. Ojd;" 5, 'i'I!i (l'Hili).

FRAGMENT SHAPES PRODUCED BY HYPERVELOCITY IMPACT AND THEIR LUNAR SIGNIFICANCE

L'IJIUlrri D. Jaltl' Jet Propulsion Laboratory. Calif(lrnia Institute of Technology

Pasadena, California (Received 22 September I 'Hi7)

Hypervelocity impacts against porous silicates and against polyethylene produced highly irregular fragments wit h re-entrant shapes, sometimes rather filamentary. A similar process may operate on the lunar surface.

The photometric function of the moon strongly suggests that the lunar surface is very rough, on a small scale. Thermal and radar data indicate that the density is considerably lower than that of solid rock. Many authors have thought that the surface consists of highly vesiculated rock with a con­tinuous solid phase. Surveyor results show, how­ever, that the surface, at least at one location, is made up of separate particles, with the bulk of the material in sizes smaller than 100 11-. 1

-4 Wes­

sclink" long ago suggested that the surface was covered with "powder." Hapke and van Horne6

pointed out that the optical properties were con­sistent with a "fairy castle" structure of near-spheri­cal particles, each contacting only 2 or 3 neighbors and held in place by forces at these contacts. So far it has proved somewhat difficult to suggest a suitable process by which a fairy-castle structure of spherical particles would develop on the lunar surface.

Most of the lunar surface particles were not re­solved in the Surveyor photographs and their shape is not yet determined. It is possible that the particles arc not near-spherical but of irregular re-entrant

shapes. Also, the particles may themselves be porous.7

If the particles are fibrousH or highly irregular in shape, most deposition processes would lead to

a particulate aggregate with a high void content and with thermal, optical, and radar properties generally similar to those observed on the moon. Such particles might be produced in several ways. One process that certainly occurs on the lunar surface is meteorite impact. Sytinskaya!l suggested that a rough surface might be formed by condensa­tion and precipitation from vapor produced by meteoritic impact. There is no experimental evi­dence that this process can occur, or even that rocks vaporize under impact at meteoritic velocities. Appreciable heating will, however, certainly take place, softening and perhaps melting the material. It seemed worthwhile to determine whether hyper­velocity impact upon silicates, especially glass­forming silicates, might produce filamentary or re­entrant fragments.

Accordingly, jets of solid copper were fired against samples at jet tip velocities of 6.7 -7.2 km/sec. These copper jets were produced by firing shaped

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Volume 11, Number 10 APPLIED PHYSICS LETTERS 15 November 1967

explosive charges with copper liners. The liners were in the form of cones of 45° apex angle, 2.4 em in base diameter and 0.06 cm thick, with the apex pointed into the high explosive. The standoff distance, from cone base to target, was 3.8 cm. Several techniques were tried for decelerating and recovering the fragments; the most successful, from the standpoint of obtaining interesting and presumably unaltered fragments, utilized soap lather as a decelerating material. With this tech­nique, firings were made against brick, glass, and polyethylene, in the form of slabs 2.5 to 5 cm thick, as well as against glass fibers to check the recovery method.

The brick was a commercial insulating firebrick (type 16-20) with a nominal composition 66% Si02 ,

27% AI 20:l> 1.5% Fe20:l, 1 % CaO, 1 % Ti02 , 0.5% MgO, 2.5% Na20 + K20 + Li20; its bulk density was 0.6 g/cm:l. The softening temperature of this material for slow heating (pyrometric cone equiv­alent) was 1585°C. The fragments produced were mostly fine and irregular. One of the coarser pieces is shown in Fig. lao It is very irregular, with many conical protuberances and re-entrant angles.

The glass was a commercial cellular glass with a composition: 74% Si02 , 6.9% AI20 a, 5.5% Na20, 5.3% CaO, 2.8% MgO, 2.4% K20, 1.8% BzO:l, 0.6% Sb20:!, 0.13% Fe20;h with small amounts of other oxides. It contained spherical non-connecting pores with diameters from I mm to lOp- or smaller, and had a bulk density of 0.14 g/cm:l. A porous material was selected partly because it resembles the present lunar surface in containing voids and partly be­cause a shock wave passing through a porous ma-

Fig. 1. Fragments produced by impact at about 7 km/sec. (a) On brick; (b) on glass; (c) and (d) on polyethylene.

terial produces more heating than does an equally strong shock passing through solid material. Thus the temperatures and thermal softening produced in this material by impact at 7 km/sec might simu­late those produced in a higher density or solid silicate by impacts at the higher velocities char­acteristic of primary meteorite impacts. Most of the glass fragments recovered were rough and faceted. One of the larger is shown in Fig. 1 b. It has three long fibrous "arms" which appear to di­vide into a multiplicity of fine filaments similar to those which might be produced by "pulling" hot glass in a conventional way. The "body" of the frag­ment appears to be hollow.

Polyethylene was used as a target because it softens at relatively low temperature and could thus simulate at 7 km/sec impact velocity the behavior of a higher-melting silicate at higher impact ve­locity. The material was commercial low-density polyethylene slab. Figure Ie shows typical fragmel1ls produced and recovered as described above. They tend to be highly filamentary. In Fig. ld are shown fragments of polyethylene produced by a slightly different technique: impact was by a jet of glass with a tip velocity of 7.6 km/sec, generated by using a glass liner for the shaped charge, and decelera­tion was by air, backed by a steel container. These fragments are not as filamentary as those in Fig. Ie, perhaps as the result of secondary effects when they struck the steel, but they are highly irregular, with surfaces re-entrant both on a scale comparable to the fragment dimensions and on a much smaller scale.

It is suggested that meteorite impact on an in­itially solid or void-containing lunar surface would tend to produce highly irregular fragments which would form a low-density surface layer. If the bulk density decreased during this process, later impacts would produce more heating and increase the ir­regular and filamentary nature of the resulting fragments. Lower velocity impacts from secondary particles would produce little heating and might tend to break the irregular fragments into smaller, more regular pieces. An equilibrium between these two processes would presumably be reached.

Gault, Heitowit, and Moore, in unpublished work,lO have obtained rather similar results and reached similar conclusions.

I thank L. Zernow and I. Lieberman of Shock Hydrodynamics, Inc., for designing and conducting the shaped-charge firings and fragment recoveries. This Letter presents results of one phase of re­search carried out under contract NAS 7-100,

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Volume 11, Number 10 APPLIED PHYSICS LETTERS 15 November 1967

sponsored by the National Aeronautics and Space Administration.

I I.. D. Jaffe et aI., Science 152, 1737 (1966). 2 I.. D. Jaffe,]. (;eojlhys. Res. 72, 773 (1967). "I.. D. Jaffe. I (;eojJhys. Res. 72,1727 (1967). 4 E. M. Christensen et aI., (unpublished).

5 A. J Wesselink. Bull. Astron. Inst. Netherlands 10, 351 (1948). 6B. Hapke and H. van Horne,I Geoj)hys. Res. 68,4545 (1963). 7V. S. Troitsky, Astron. Zh. 39, 73 (1962). Translated in Soviet

Astronomy-A] 6, 51 (1962). 8 A. R. Hibbs, Icarem 2, 181 (1963). "N. N. Sytinskaya, As/ron. Zh. 36, 315 (1959). Translated in

Soviet Astronomy-A] 3,310 (1959). 10D. E. Gault. E. D. Heitowit, and H. J. Moore, unpublished

work. Abstract in Trans. Am. Geoj)hys. Union 45,74 (1964).

A LOW-ENERGY ELECTRON DIFFRACTION STUDY OF THE EPITAXIAL SILICON LAYERS ON A Ge(lll) SURFACE

Yoshiyuki Takeishi, Isao Sasaki, and Kanji Hirabayashi Central Research Laboratory, Toshiba Electric Company

Kawasaki, Japan (Received 12 September 1967; in final form 23 October 1967)

Silicon atoms were evaporated onto cleaned and annealed Ge( III) surfaces at a rate of 2 X I {)14 atoms/cm2 min up to 40 min. Analyses of LEED patterns have revealed, on the substrate at 870o K, truncated tetrahedra consisting of reconstructed {311} and (III) planes, which have a :1 X I and a 7 X 7 superstructure, respectively. The 3 x I superstructure was observed on cleaned and annealed Si(3II) surfaces in an independent experiment.

Low-energy electron diffraction (LEED) has been proved to be a useful tool for studying early stages of epitaxial growth, as is shown by, for instance, Jona's recent work 1 concerning Si on clean Si sur­faces. We intended to look into another problem, Si on Ce, which is of particular interest as their lattice constants are slightly different from each other and both crystal surfaces have different super­structures. In this Letter we report an interesting result obtained when the substrate was at 870°K.

A chemically polished Ge substrate whose front face is parallel to the crystallographic (Ill) plane within a degree was installed into a LEED equip­ment. After evacuation down to I x 10-9 torr, the surface was thoroughly cleaned by Ar+ ion bombard­ment followed by anneal and the resultant surface shows well-defined 8 x 8 diffraction patterns, rep­resentative of the clean surface.2 -

4 Silicon was evap­orated onto the substrate from a source kept at 15300 K at a rate of 2 x 1014 atoms/cm2 min, esti­mated in a point-source approximation, under background pressures of .the order of 10-8 torr. The growth rate is 1.2 A/min on the substrate at :H)(fK in the range 150 to 300 A, measured by an interference microscope. The evaporation time was up to 40 min. The true growth rate in the early stages on the substrate at 8700 K has not been known. All LEED observations were done at room tempera­ture.

A diffraction pattern obtained after a 40-min

330

deposition onto the substrate at 8700 K is shown in Fig. 1. Features of the pattern are substantially same for the surfaces after an 8-min deposition. The pattern is of a threefold symmetry. When the incident beam energy is increased, most spots, for instance, {nm}, {(n!7)(m!7)}, and {(n/3)(m/3)} spots

Fig. 1. A LEED pattern (by 41·eV beam) obtained after a 40-min deposition of Si onto a Ge(lll) substrate at 870oK. See also Fig. 3.

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