Bland, PA

Cintala, MJ

Cressey, G

Horz, F

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SURVIVABILITY OF METEORITE PROJECTILES Ð RESULTS FROM IMPACT EXPREIMENTS.P. A. Bland1, M. J. Cintala2, F. Hrz2 and G. Cressey3. Planetary and Space Sciences Research Institute, The OpenUniversity, Milton Keynes MK7 6AA, U.K.,   p.a.bland@open.ac.uk  ; 2Planetary Sciences Branch, Code SN2, NASAJohnson Space  Center ,  Houston,  Texas  77058,  U.S.A. ,    mark.j.cintala1@jsc.nasa.gov  ,friedrich.p.horz1@jsc.nasa.gov  ; 3Department of Mineralogy, Natural History Museum, Cromwell Road, LondonSW75BD, U.K.,   gmc@nhm.ac.uk   .Introduction:  Extra-terrestrial material impactsthe EarthÕs surface at speeds ranging from 0.1km/s forsmall meteorites to cosmic velocities for crater formingobjects. As the numerous samples in our collectionsattest, terminal velocity on Earth is insufficient to dis-rupt most impacting meteorites. However, the situationfor other solar system bodies is less well known. Mod-elling suggests that small (<150g) meteorites enteringMars atmosphere may approach a terminal velocity,and impact the surface at speeds of 1-2 km/s [1-3].Lacking an atmosphere, the moon has a minimum im-pact velocity related to its escape velocity and thegravitational effect of the Earth at lunar distance: 2.78km/s (although this will be modified by orbital velocityof the moon about the Earth, ~1 km/s) [4]. Are theseminimum impact speeds for Mars and the Moon sur-vivable for meteoritic material? Numerous hyperve-locity impact studies have focussed on effects in thetarget, with little attention being paid to the projectile.We therefore chose to explore this question in the cur-rent work.Methodology:  Using the Vertical Impact Facilityat NASA JSC, a combination of terrestrial and extra-terrestrial materials were launched into fines, coveringa velocity range of 0.2-1.8 km/s. Samples of iron(Gibeon) and chondritic (Kernouve, a low-shock H6)meteorite were prepared as cylindrical projectiles(4mm diameter by 5mm long), launched in four-piece,plastic sabots. Similar preparations were made formono-mineralic samples, and terrestrial basalt. Targetmaterials were quartz sand and JSC MARS-1, a Marssoil simulant [5]; grain sizes were <250 µm and 125-250 µm respectively. Impact was normal to the targetin all cases, making these worst-case scenarios for im-pactor survivability. All major projectiles were recov-ered by sieving with meshes larger than typical targetgrain size. Selected sub-samples of both projectile andtarget were prepared for X-ray diffraction (XRD)analysis.Results: Iron meteorite samples remained intact atall velocities, although deformation was increasinglyapparent at impact speeds >1 km/s. Chondritic meteor-ite appears to be more resistant than the mono-mineralic or basaltic samples (see Table 1). At thehighest velocities (~1.8 km/s) in the Kernouve experi-ments, fragments 3.7% and 3.1% of the original im-pactor mass remained intact, recovered from quartzsand and simulant respectively. Interestingly, theselargest surviving fragments were bigger than in the 1.4km/s shots (2.0% and 2.7% of the original impactor).The intermediate 1.6 km/s shots saw large pieces ofmaterial, 10.8% and 13.3% of the original impactormass, remaining intact. A similar (though less pro-nounced) feature also appears to be present in the ba-salt and olivine experiments (see Table 1). This sur-prising result is illustrated in Figure 1, which showsthe similarities in fragmentation behaviour at differentvelocities for Kernouve shots into quartz sand andMars simulant.TargetMaterialProjec-tile typeImpactormass (mg)Impactvel. (km/s)Largestfrag. (mg)q. sand Kernouve 286.91 1.806 10.661q. sand Kernouve 281.50 1.647 30.500q. sand Kernouve 279.83 1.412 2.738simulant Kernouve 281.49 1.804 8.814simulant Kernouve 271.00 1.618 36.007simulant Kernouve 281.16 1.414 7.559q. sand Basalt 218.18 1.679 6.936q. sand Basalt 224.16 1.487 5.141q. sand Basalt 218.19 1.394 5.576simulant Basalt 234.12 1.686 4.114simulant Basalt 230.16 1.519 28.307simulant Basalt 228.20 1.347 23.777q. sand Olivine 272.53 1.765 2.863q. sand Olivine 267.85 1.605 3.978q. sand Olivine 249.03 1.448 1.323Table 1. Data for impacts of chondrite and basalt intosand and Mars soil simulant, and olivine into sand. Themass of the largest surviving fragment is indicated foreach experiment.In addition to studying the fragmentation of theprojectile, we used X-ray diffraction to look for shock-induced distortion of mineral lattices within the pro-jectile, and any effects of low-velocity impacts on theMars soil simulant. No effect was observed in the tar-get (aside from a marked comminution of particles inthe simulant), but differences between projectiles wereLunar and Planetary Science XXXII (2001) 1764.pdfSURVIVABILITY OF METEORITE PROJECTILES:  P.A. Bland, M.J. Cintala, F. Hrz and G. Cresseyobserved. Although the results are preliminary, someline-broadening was observed in mono-mineralic oli-vine at the higher impact speeds. We are carrying outfurther analyses in the hope of defining an instrumentalmethod for determining the shock level that a meteor-ite has experiencedFigure 1. Number of surviving fragments greater thana given mass for shots of Kernouve chondrite intoquartz sand (a) and JSC MARS-1 simulant (b) at 3different velocities. See Table 1 for data.Discussion and conclusions: Meteoritic materialmay survive impact on a planetary surface directly, ifthe velocity is low enough, and also in some cases byspallation from the rear of the projectile (by stresswave interference) during a hypervelocity impact [6].We have shown that large fragments of the impactor(>10% at 1.6 km/s and >3% at 1.8 km/s) may remainafter direct impact of our small projectiles into fines.An additional factor to consider, although not ex-plored in the current study, is the angle (or obliquity)of impact - more oblique trajectories can be correlatedwith greater projectile integrity after the impact [7].Lower-angle trajectories on Mars will entail more se-vere atmospheric effects during entry, complicating thesituation and forcing the trajectory toward the vertical,but any obliquity to the impact angle will typicallylower the overall shock stress felt by the projectile.Thus, our experimental results can be considered todefine worst-case, maximum levels of fragmentationfor chondritic meteorites. As such, intact fragmentscomprising a significant portion of the impactor massmay remain at speeds up to, or even beyond, 2km/s.Our work suggests that mm-sized meteorite frag-ments found on the moon, such as Hadley Rille [8],may be emplaced by direct infall at velocities <3 km/s,as well as by stress wave interference. Although suchspeeds apply to only a small fraction of lunar impac-tors, the absence of weathering means that any mete-oritic material added to a lunar soil will last indefi-nitely. On Mars, minimum impact speeds are muchlower (approximately 1.2 km/s [1]). Our data indicatethat a significant fraction of this material, includingmeteorites with impact speeds up to or exceeding 2km/s, will survive intact, or as large discrete fragments.This finding supports previous suggestions that accu-mulations of meteorites should be present on thatplanet [1]. Finally, encounter velocities between aster-oids may also be low Ð the predicted impact velocitydistribution for Eros has a peak near 1 km/s [9]. Ourwork would suggest that polymict brecciation wouldstill be feasible at significantly higher velocities.The most intrigueing result is the departure from asimple picture of increasing velocity producing in-creasing fragmentation, in the region of 1.6 km/s. Itmay be that enhanced survivability at this speed is re-lated to shock wave interference toughening the sam-ple, possibly by partially removing its porosity. Moreinvestigation is needed to clarify this point.The marked comminution of the Mars soil simulantthat we observe is also noteworthy. It suggests thatimpacts of small, low velocity projectiles may be asignificant factor in the small scale evolution of Mars,including its global dust budget.Finally, the suggestion of peak-broadening in ourpreliminary X-ray diffraction survey may point theway to an instrumental assessment of a meteoriteÕsshock level.References: [1]ÊBland P. A. and Smith T. B. (2000)Icarus, 144,  21-26. [2]ÊHrz F. P. et al. (1999) Science,285, 2105-2107. [3]ÊDycus R. D. (1969) Pub. Astron.Soc. of the Pacific, 91, 399-414. [4] Zook H. A. (1975)Proc. LPSC VI, 1653-1672. [5] Allen C. C. et al.(1998) Space 98, Proc. Am. Soc. Civ. Eng., 469-476.[6] Melosh H. J. (1989) Impact cratering, Oxford Uni-versity Press, Oxford. [7] Gault D.E. and WedekindJ.A. Proc. LPSC 9, 3843-3875. [8] Rubin A. E. (1997)MAPS, 32, 135-141. [9] Barnouin-Jha O. S. and ChengA. F. (2000) LPSC XXXI, 1255.1101 10(b)1.8041.6181.414Mass (mg)1101.6471.8061.412Fragments(a)Lunar and Planetary Science XXXII (2001) 1764.pdf

Survivability of meteorite projectiles – results from impact experiments.

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