REPORTS measured at 247 points on 90 individual trees (23, 30), an experimentally observed mechanistic 16. w.K. Burton, N. Cabrera, F. C. Frank, Nature 163, 398 (Fig. 4C). magnitude of the Burgers vector, a line can be fit that this dislocation-driven nanowire growth mech- 1. So c ndon 243. 299, 951 rank, Philos rans. to the data as plotted, with the slope represent- anism proposed for Pbs trees is likely general to 18. G W. Sears, Acta MetalL. 1. 457(1953) ing b from Eq. I above. This can be more and is underappreciated in the synthesis of ID 19. G W. Sears, Acta Metall. 3, 361(1955) directly seen in a histogram of the calculated nanostructures, particularly in cases where the 20. R S Wagner, W.C. Ellis, KA Jackson, S M. Arnold Burgers vectors(Fig. 4D). A Gaussian fit to these growth mechanism is inconclusively explained 21. D.R. Veblen, ]. E. Post, Am. Mineral. 68 component of the Burgers vector(the projection nism is likely to occur in materials that are prone 23.12 Hfor k 1996), chaps. 22 andy s- data yields the average magnitude of the screw and especially when free of catalysts. Besides the (1983) component of the Burgers vector b=6+2A. analogous PbSe for which we have found pre-22.DB.Williams,CB.Carter, Transmis Because the Burgers vector direction is con- liminary evidence of similar growth phenomena, firmed to be [110] by TEM, a 6 A screw the dislocation-driven nanowire growth mecha- of b onto the dislocation line u [100) is ap have screw dislocations, such as SiC, GaN, 24. A Foitzik, W. Skrotzki, P Haasen, Physica Status Solidi A proximately equal to the lattice constant of ZnO, and Cds, both in vapor-phase growth and 121.81(1990) PbS,a=5.94 A. It is known that smallest b in solution-phase synthesis. However, we caution 25. E Bauser, H Strunk, /. Cryst Growth 51. 362(1981) allowable is the shortest lattice translation vector that postgrowth mechanical perturbation could 27. Edge dislocations do not produce such distortions. Mixed crystals is y110), whose screw component is one might not be able to observe dislocations in mponents. When the cross sectio v100)(half the lattice constant a). Given various the final nanowire products if samples are not circular (which is often the case). the cross-section area sources of errors in this estimate (27), it is sat- handled properly. n be used together with a small alculation. When the dislocation line is not at the center isfying to see that no data were observed sub of the cylinder, a small correction factor is applied tantially below the theoretical minimal vector References and Notes 28. R D. Dragsdorf, W. W. Webb, ). Appl. Phys. 29, 817 and the average estimated b value of twice the 1.Y. xia et al. Adv Mater. 15. 353(2003) 29. G. W. Sears, Chem. Phys. 31, 53 (1959) tionally, theory predicts that left-handed disloca- 3. A. M. Morales, C.M. Lieber, Science 279, 208 (1998). 30. F. R N. Nabarro, Theory of Crystal Dislocations tion spirals lead to right-handed Eshelby twists 4. R.S.Wagner,W.. Ellis, Appl.Phys. Lett., 89 (1964) 31.5.J. thanks NSF(CAREER DMR-0548232) Research and vice versa(23, 26); therefore, the equal prob- 5. Z w. Pan, Z R. Dai, Z L. Wang, Science 291, 1947 ability of twist handedness implies equal prob-(2001) rofessor Grant, and 3M Nontenured Faculty Award for bility of Burgers vector sense(sign) 6. I. J Trentler et al. Science 270, 1791(1995) upport M ]. B. was partially supported by an Air Products Fellowship. We thank R Selinsky for assistance with the The observation of Eshelby twist in these pine illustrations in Fig. 2 aL., Not Mater. 3, 380(2004) 四5ooE ee nanowires is a clear demonstration and val- idation of Eshelby's theory on dislocations. The 9.DWang, FQian, C.Yang, Z HZhong, C.M.Lieber, Nano Lett. 4, 871(2004) results also provide evidence for a catalyst-free 10. A Dong, R. Tang, W E. Buhro, J Am. Chem. Soc. 129, upporting Online Material wwsciencemag. org/cgi//full/1157131/DC1 dislocations and imply that VLS and screw aterial on Science online Figs. 51 to $9 dislocation-driven nanowire growth can coexist. 12. M.]. Bierman, Y.K. A Lau, S Jin, Nano Left. 7. 2907 Because of the distinct morphology difference o0033SEo=o from the hyperbranched nanowires, it is unlikely 13 ). Zhu et al, Nano Lett. 7, 1095(2007) that the dislocation is a result of cool-down or other M. Fardy, A. L Hochbaum, J. Goldberger, M. M. Zhang, 2008: accepted 16 April 2008 postgrowth perturbation. Although some general 15.1.BHannon,S.Kodambaka,FM.Ross,R.M.Tromp nce.1157131 discussions on the origins of dislocations exist tue440,69(2006) information when citing this paper Detection of silica-Rich sulfate-rich sedimentary rocks at Meridiani Planum (4). The rover Spirit recently investigated the Deposits on Mars Eastem Valley between Home Plate and the Mitcheltree/Low Ridge complex(Fig. 1)in Gusev crater. Here we describe the discovery of silica- S.W. Squyres, R.E. Arvidson, 2 S. Ruff R. Gellert, R.V. Morris, 'D. W. Ming, 'L. Crumpler, rich deposits in the Eastem Valley and farther east 1. D. Farmer,D. ] Des Marais, A. Yen, S. M. McLennan, W. Calvin, 01. F. Bell ll, 1 near sulfate-rich soil depos BC.Clark,A Wang, 2 T ] McCoy, M. E. Schmidt, 2P. A de Souza ]r 3 Home plate consists of laminated to-cross- bedded tephra that shows evidence for a volcani Mineral deposits on the martian surface can elucidate ancient environmental conditions on the explosive origin, including a bomb sag produced planet. Opaline silica its (as much as 91 weight percent Sio2) have been found in association when an ejected -4-cm clast fell into deformable with volcanic materials by the Mars rover Spinit. The and as bedrock. We interpret these materials to have deposits are present both as light-toned soils rid e posited seas of theme latdge for understanding the past habitability of Mars because hydrothermal environments on Eart ant eroded synclinal structures that expose tephra upport thriving microbial ecosystems. deposit of vesicular basalt boulders. Soils in the Inner Basin -250 m to the north(Samra) and -50 m to the east(tyrone)(Figs. I and 2)of O silica deposits are an indicator of of amorphous silica on rocks(1, 2), Home Plate contain hydrated ferric sulfate de- aqueous activity. Some regions of this interpretation is not unique (3). posits(6, 7). The mobility of ferric iron under Mars exhibit a thermal infrared spectral rom the Mars rover Opportunity have apparently oxidizing conditions, leading to ferric gnature that has been interpreted to result from that opaline silica could be present in sulfates and oxides, is suggestive of low pH con- www.sciencemag.orgSciEnceVol32023May2008 1063measured at 247 points on 90 individual trees from 16 synthetic batches (Fig. 4C). To extract the magnitude of the Burgers vector, a line can be fit to the data as plotted, with the slope represent￾ing b from Eq. 1 above. This can be more directly seen in a histogram of the calculated Burgers vectors (Fig. 4D). A Gaussian fit to these data yields the average magnitude of the screw component of the Burgers vector b = 6 ± 2 Å. Because the Burgers vector direction is con￾firmed to be [110] by TEM, a 6 Å screw component of the Burgers vector (the projection of b onto the dislocation line u [100]) is ap￾proximately equal to the lattice constant of PbS, a = 5.94 Å. It is known that smallest b allowable is the shortest lattice translation vector in a material (23), which in the case of rock salt crystals is ½〈110〉, whose screw component is ½〈100〉 (half the lattice constant a). Given various sources of errors in this estimate (27), it is sat￾isfying to see that no data were observed sub￾stantially below the theoretical minimal vector, and the average estimated b value of twice the minimal theoretical length is reasonable. Addi￾tionally, theory predicts that left-handed disloca￾tion spirals lead to right-handed Eshelby twists and vice versa (23, 26); therefore, the equal prob￾ability of twist handedness implies equal prob￾ability of Burgers vector sense (sign). The observation of Eshelby twist in these pine tree nanowires is a clear demonstration and val￾idation of Eshelby’s theory on dislocations. The results also provide evidence for a catalyst-free nanowire growth mechanism driven by axial screw dislocations and imply that VLS and screw dislocation-driven nanowire growth can coexist. Because of the distinct morphology difference from the hyperbranched nanowires, it is unlikely that the dislocation is a result of cool-down or other postgrowth perturbation. Although some general discussions on the origins of dislocations exist (23, 30), an experimentally observed mechanistic understanding is currently lacking. We suggest that this dislocation-driven nanowire growth mech￾anism proposed for PbS trees is likely general to and is underappreciated in the synthesis of 1D nanostructures, particularly in cases where the growth mechanism is inconclusively explained and especially when free of catalysts. Besides the analogous PbSe for which we have found pre￾liminary evidence of similar growth phenomena, the dislocation-driven nanowire growth mecha￾nism is likely to occur in materials that are prone to have screw dislocations, such as SiC, GaN, ZnO, and CdS, both in vapor-phase growth and in solution-phase synthesis. However, we caution that postgrowth mechanical perturbation could work the dislocation out of the nanowires, and one might not be able to observe dislocations in the final nanowire products if samples are not handled properly. References and Notes 1. Y. Xia et al., Adv. Mater. 15, 353 (2003). 2. C. M. Lieber, Z. L. Wang, MRS Bull. 32, 99 (2006). 3. A. M. Morales, C. M. Lieber, Science 279, 208 (1998). 4. R. S. Wagner, W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964). 5. Z. W. Pan, Z. R. Dai, Z. L. Wang, Science 291, 1947 (2001). 6. T. J. Trentler et al., Science 270, 1791 (1995). 7. A. I. Persson et al., Nat. Mater. 3, 677 (2004). 8. K. A. Dick et al., Nat. Mater. 3, 380 (2004). 9. D. Wang, F. Qian, C. Yang, Z. H. Zhong, C. M. Lieber, Nano Lett. 4, 871 (2004). 10. A. Dong, R. Tang, W. E. Buhro, J. Am. Chem. Soc. 129, 12254 (2007). 11. Materials and methods are available as supporting material on Science Online. 12. M. J. Bierman, Y. K. A. Lau, S. Jin, Nano Lett. 7, 2907 (2007). 13. J. Zhu et al., Nano Lett. 7, 1095 (2007). 14. M. Fardy, A. L. Hochbaum, J. Goldberger, M. M. Zhang, P. Yang, Adv. Mater. 19, 3047 (2007). 15. J. B. Hannon, S. Kodambaka, F. M. Ross, R. M. Tromp, Nature 440, 69 (2006). 16. W. K. Burton, N. Cabrera, F. C. Frank, Nature 163, 398 (1949). 17. W. K. Burton, N. Cabrera, F. C. Frank, Philos. Trans. R. Soc. London A 243, 299 (1951). 18. G. W. Sears, Acta Metall. 1, 457 (1953). 19. G. W. Sears, Acta Metall. 3, 361 (1955). 20. R. S. Wagner, W. C. Ellis, K. A. Jackson, S. M. Arnold, J. Appl. Phys. 35, 2993 (1964). 21. D. R. Veblen, J. E. Post, Am. Mineral. 68, 790 (1983). 22. D. B. Williams, C. B. Carter, Transmission Electron Microscopy: A Textbook for Materials Science (Plenum, New York, 1996), chaps. 22 and 25. 23. J. P. Hirth, J. Lothe, Theory of Dislocations (McGraw-Hill, New York, 1968). 24. A. Foitzik, W. Skrotzki, P. Haasen, Physica Status Solidi A 121, 81 (1990). 25. E. Bauser, H. Strunk, J. Cryst. Growth 51, 362 (1981). 26. J. D. Eshelby, J. Appl. Phys. 24, 176 (1953). 27. Edge dislocations do not produce such distortions. Mixed dislocations can be evaluated as separate screw and edge components. When the cross section of the nanowire is not circular (which is often the case), the cross-section area can be used together with a small correction factor for this calculation. When the dislocation line is not at the center of the cylinder, a small correction factor is applied. 28. R. D. Dragsdorf, W. W. Webb, J. Appl. Phys. 29, 817 (1958). 29. G. W. Sears, J. Chem. Phys. 31, 53 (1959). 30. F. R. N. Nabarro, Theory of Crystal Dislocations (Oxford Univ. Press, London, 1967). 31. S. J. thanks NSF (CAREER DMR-0548232), Research Corporation Cottrell Scholar Award, DuPont Young Professor Grant, and 3M Nontenured Faculty Award for support. M.J.B. was partially supported by an Air Products Fellowship. We thank R. Selinsky for assistance with the illustrations in Fig. 2. Supporting Online Material www.sciencemag.org/cgi/content/full/1157131/DC1 Materials and Methods Figs. S1 to S9 References Movie S1 29 February 2008; accepted 16 April 2008 Published online 1 May 2008; 10.1126/science.1157131 Include this information when citing this paper. Detection of Silica-Rich Deposits on Mars S. W. Squyres,1 * R. E. Arvidson,2 S. Ruff,3 R. Gellert,4 R. V. Morris,5 D. W. Ming,5 L. Crumpler,6 J. D. Farmer,3 D. J. Des Marais,7 A. Yen,8 S. M. McLennan,9 W. Calvin,10 J. F. Bell III,1 B. C. Clark,11 A. Wang,2 T. J. McCoy,12 M. E. Schmidt,12 P. A. de Souza Jr.13 Mineral deposits on the martian surface can elucidate ancient environmental conditions on the planet. Opaline silica deposits (as much as 91 weight percent SiO2) have been found in association with volcanic materials by the Mars rover Spirit. The deposits are present both as light-toned soils and as bedrock. We interpret these materials to have formed under hydrothermal conditions and therefore to be strong indicators of a former aqueous environment. This discovery is important for understanding the past habitability of Mars because hydrothermal environments on Earth support thriving microbial ecosystems. Opaline silica deposits are an indicator of past aqueous activity. Some regions of Mars exhibit a thermal infrared spectral signature that has been interpreted to result from coatings of amorphous silica on rocks (1, 2), although this interpretation is not unique (3). Results from the Mars rover Opportunity have suggested that opaline silica could be present in sulfate-rich sedimentary rocks atMeridiani Planum (4). The rover Spirit recently investigated the Eastern Valley between Home Plate and the Mitcheltree/Low Ridge complex (Fig. 1) in Gusev crater. Here we describe the discovery of silica￾rich deposits in the Eastern Valley and farther east near sulfate-rich soil deposits. Home Plate consists of laminated–to–cross￾bedded tephra that shows evidence for a volcanic explosive origin, including a bomb sag produced when an ejected ~4-cm clast fell into deformable ash deposits (5). Mitcheltree Ridge and Low Ridge, located east of Home Plate, are partially eroded synclinal structures that expose tephra deposits (including lapillistones) capped by a deposit of vesicular basalt boulders. Soils in the Inner Basin ~250 m to the north (Samra) and ~50 m to the east (Tyrone) (Figs. 1 and 2) of Home Plate contain hydrated ferric sulfate de￾posits (6, 7). 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