Nature © Macmillan Publishers Ltd 1997 letters to nature NATURE | VOL 388 | 28 AUGUST 1997 857 This would involve very dense Fe-rich melts 15 , particularly before the completion of core formation, and some of these melts may have partially sunk with core-forming metal towards the martian interior. Cumulate and residue flotation depends on the density crossover between solid and liquid phases 27 , which is poorly con- strained for Mars. However, if the outer portions of the planet were rich in depleted residual solids during the magma ocean stage, the W and Nd isotope compositions would be expected to be radiogenic and to survive homogenization. The sizes of inner Solar System planetary bodies seem to increase with accretion and differentiation intervals, from the most primitive and undifferentiated materials such as equilibrated chondrites (6 Myr) to the eucrite parent body (10 Myr) to Mars (10– 30 Myr) and to the Earth–Moon system (50 Myr) (refs 2, 3). The data are consistent with early termination of accretion of asteroids and Mars, and longer-lived accretion of larger bodies resulting from late planet-scale impacts. The trend corresponds to a transition in the dominant mechanisms of heating from decay of short-lived nuclides to the release of accretional energy. Whereas a late giant impact on Earth may have been sufficiently energetic to effectively homogenize the W isotope composition and destroy any proto- core 2,3 , the W and Nd isotope data are inconsistent with an impact sufficiently energetic to melt and homogenize Mars later than 4.53 Gyr (ref. 22).  Methods. All samples were powdered in an aluminum oxide mortar after surface saw marks and fusion crusts had been removed through mild acid leaching (1M HCl) and hand-picking. The samples were digested sequentially with concentrated HF, 8M HNO 3 and 6M HCl. Roughly 10% of the solution was separated and spiked with 178 Hf and 186 W, whereas the remaining solution was dried and redissolved in 8ml of 4M HF. The method of chemical separation of W was adapted from the first column of the Hf chemistry developed by Salters and Hart 29 but on a reduced scale, with 3.5 ml of Bio-Rad AG1  8 (200–400 mesh) anion resin. The Hf and W were eluted and collected sequentially using a mixed solution of 6M HCl + 1M HF. The same chemical procedure was used for the spiked solutions, but the column volume was further reduced (1ml), and Hf and W were collected together. The total W procedural blank was 0.4ng. Tungsten isotopic measurement by multiple collector inductively coupled-plasma mass spectrometry (MC-ICPMS) has been previously described by Lee and Halliday 30 . The NIST-3163 W standard was run between every sample to monitor the performance of MC-ICPMS and to check for memory effects, which were negligible. The W isotopic measure- ments were normalized to 186 W= 184 W ¼ 0:927633 (ref. 30). The quoted 2j standard errors all refer to the least significant figures. The concentrations of Hf and W were determined by isotope dilution, and the uncertainty is typically 0.2% or better. All the W data are blank-corrected, which is negligible for all analyses except for Juvinas which has been corrected by 1.5 e unit. Received 3 April; accepted 15 July 1997. 1. Harper. C. L. & Jacobsen, S. B. Evidence for 182 Hf in the early Solar System and constraints on the timescale of terrestrial accretion and core formation. Geochim. Cosmochim. Acta 60, 1131–1153 (1996). 2. Lee, D.-C. & Halliday, A. N. Hafnium–tungsten chronometry and the timing of terrestrial core formation. Nature 378, 771–774 (1995). 3. Halliday, A. N., Rehka ¨mper, M., Lee, D.-C. & Yi, W. Early evolution of the Earth and Moon: new constraints from Hf–W isotope geochemistry. Earth Planet. Sci. Lett. 142, 75–89 (1996). 4. 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Precise determinations of the isotopic compositions and atomic weights of molybednum, tellurium, tin and tungsten using ICP magnetic sector multiple collector mass spectrometry. Int. J. Mass Spec. Ion Proc. 146/147, 35–46 (1995). Acknowledgements. We thank M. Lindstrom, L. Nyquist, G. MacPherson, C. Perron and M. Wadhwa for access to their meteorite collections at NASA, Smithsonian Institution of Washington, Museum National d’Histoire Naturelle at Paris, and Field Museum in Chicago. We also thank J. Christensen,E. Essene, H. Pollack, M. Rehka ¨mper, P. van Keken and Y. Zhang for their comments, M. Johnson and C. Hall for their assistance, and K. Righter and M. Drake for access to unpublished papers. This work was supported by NSF, DOE, NASA and the University of Michigan. Correspondence and requests for materials should be addressed to D.-C.L. (e-mail: dclee@umich.edu) Effect of microgravity on the crystallization of a self- assembling layered material Homayoun Ahari*, Robert L. Bedard†, Carol L. Bowes*, Neil Coombs‡,O ¨ mer Dag*, Tong Jiang*, Geoffrey A. Ozin*, Srebri Petrov*, Igor Sokolov*, Atul Verma*, Gregory Vovk* & David Young* * Materials Chemistry Research Group, Lash Miller Chemical Laboratories, University of Toronto, Toronto, Ontario M5S 3H6, Canada † UOP, Research Division, 25 E. Algonquin Road, Des Plaines, Illinois 60017, USA ‡ Imagetek Analytical Imaging, 32 Manning Avenue, Toronto, Ontario M6J 2K4, Canada ......................................................................................................................... In microgravity, crystals of semiconductors and proteins can be grown with improved crystallinity, offering the prospect of improved structural analyses (for proteins) and better electronic properties (for semiconductors) 1–3 . Here we study the effect of a microgravity environment on the crystallization of a class of materials—layered microporous tin(IV) sulphides 4–11 —whose crystal structure is determined by weak interlayer interactions (electrostatic, hydrogen-bonding and van der Waals) as well as strong intralayer covalent bonds. We find that the crystals grown in microgravity (on board the Space Shuttle Endeavour) show improved crystal habits, smoother faces, greater crystallinity, better optical quality and larger void volumes than the materials grown on Earth. These differences are due at least in part to the