SCIENTIFIC CORRESPONDENCE made. It would also be possible to assess longing the lifetime of the clouds. The The australites in southern Australia these ideas by incorporating them into an introduction of ice-forming nuclei into occur in two sub-parallel lines represent­ established cloud modeL which treats the colder clouds could increase or decrease ing their original lines of in fall'. The salient microphysical processes explicitly. their lifetimes, depending on the circum­ distance between these two lines relative The most propitious situation for effecting stances. to the oceanic extension of the australite the proposed radiative changes would JOHN lATHAM sub-strewn field suggests that this tektite involve fields of stable boundary-layer Department of Pure and Applied Physics. could be a part of one or more such lines clouds with low droplet concentrations, University of Manchester Institute of of infall, the existence of which has hither­ occurring under highs over the tropical Science and Technology, to been undetected in the oceans because oceans. PO Box 88, Manchester M60 1QD, UK of poor sampling. This finding does not Increasing the droplet concentration in conform to Von Koenigswald's' obser­ clouds is likely to enhance their colloidal 1 Slingo, A. Nature343, 49-51 (19901 vation of latitudinal morphological simi­ stability, making the coalescence process 2 Pruppacher, H.R. & Klett, D. MicrophySICS of Clouds and larities between Australasian tektites, and Precipitation (Reidel, Dordrecht, 1978). that leads to rainfall more difficult to initi­ 3 Charlson, R.J.. Lovelock. J.E .. Andreae, M.O. & Warren, also Ford's' proposed southeasterly dis­ ate and thereby, in some situations, pro- S.G. Nature326, 655-661 (1987). tribution of Australasian tektites and Chapman's'" suggestion that aerodynam­ ically ablated australites do not occur Tektites far and wide north of 23° S. SIR-Meteorite impacts on terrestrial buttons from southern Australia, and its M. SHY AM PRASAD rocks produce glassy ejecta called tek­ density is greater than average for austra­ P. S. RAO tites'. These have been found strewn in lites. By extrapolating the curvature of its National Institute of Oceanography, four fields, of which one in Australasia is core and estimating a volume loss of about Dona Paula, both the youngest (0.7 Myr) and the 62 per cent during ablation, we deduce the Goa 403004, India largest (covering about 1110 of the Earth's pre-ablation shape of the tektite to have surface'; a in the figure). Flanged austra­ been a sphere of 11 mm diameter. The. 1. Taylor, S.R. Earth Sci. Rev. 9, 101-123 (1973). 2. Glass, B.P .. Swincki. M.S. & Zwart, PA Proc. Lunar lite buttons (aerodynamically ablated tektite has a vitreous lustre, and the planet. Sci. Con f. 10, 2535-2545 (1979). tektites) have so far been found only in excellent preservation of the fine mark­ 3. Baker, G. 1n Tektites (ed. O'Keefe, J.A.) 1-24 (University of Chicago Press, 1963). southern Australia, never (although ings of the schlieren structures on the core 4. O'Keefe, JA Tektites and the" Origin (Elsevier, Amster• expected) in the ocean. We have exclude long-distance transport. The dam. 1976) recovered a flanged australite button from sedimentation rate in this area is 2m Myr·' 5. Udintsev, G.B. Geological-Geophysical Atlas of the Indian Ocean (Pergamon, New York, 1975). the surficial sediments of the Central (ref. 5), so that, given the 0.7-Myr age of 6. Chalmers, R.O., Henderson, E.P. & Mason, B. Smithson. Indian Basin (12° 37' S; 78° 30' E), at a the Australasian tektite field, this tektite Contrib. EarthSci.11, 1-46 (1976). 7. McColl, B.H. & Williams. G.E. Nature 226, 154-155 water depth of 5,300 m. should have been buried under 1.4 m of (1970). Our tektite ( b in the figure) measures sediment. Its near-surface retention could 8. Von Koenigswald, G.H.R. Proc. K. Ned. Akad. Wetensch. 70, 104-112 (1967). 12.054 x 4.092 mm, with a flange width of be due to bioturbation and/or removal of 9. Ford, R.J. Austr. I Earth Sci. 35,483-490 (1988). 2.352 mm, and weighs 0.6633 g. It is sediments by bottom currents; alternat­ 10. Chapman, D.R.J. J. geophys. Res. 16, 6309-6338 similar to the perfectly formed and well ively, the Australasian tektites might have (1971). preserved Port Campbell specimens'. The fallen more recently'. specific gravity of the tektite is 2.508 and Our finding signifies a large westward its refractive index is 1.528, corresponding extension of the australite sub-strewn field More light on PCR to a silica content' of 66 per cent. The into the Indian Ocean, increasing its area specimen is smaller than typical flanged by about 4.3 million km' (a in the figure). contamination SIR-In our recent Scientific Correspon­ dence1we showed that ultraviolet light was mmi AUSTRALITE SUB-STREWN FIELD a potent inactivator of a 750-base pair iZ2Z3 OCEANIC EXTENSION OF AUSTRALITE segment of DNA when it contaminated SUB- STREWN FIELD • LOCATION Of PRESENT SPECIMEN I:;> reagents. All reagents, including Taq 0 ® LOCATION OF KNOWN MICROTEKTITE . )) polymerase, could be decontaminated OCCURRENCES e but, for obvious reasons, contamination ~~0i~~~~EA~~~~~~ANCES~~-- @""-...... of the template DNA cannot be elimin­ ~//______~ ',,',,, ated. Despite a 100,000-fold inactivation of contaminant, the integrity of all the , ;q ~\ reagents was preserved to the extent that a a· normal intensity of amplified product could be seen in a standard 30-cycle poly­ ®® lQ~:~~--~ \ merase chain reaction of human genomic DNA. Cimino et a!.' did not appreciate / :l7.o7 Ill 0 ~~ some of the above points. They presented a theoretical argument and reported an ~ '0:. ~.. ):~ experiment indicating that a 121-bp seg­ ,, ~ ~ a / /}t:l ...... ®@ ® ~ // <./ ment is markedly less susceptible to ------// ultraviolet inactivation than an unrelated ------®------~' 500-bp segment. We have performed experiments which support and extend this finding. Sensitiv­ a, The Australasian tektite strewn field (from ref. 2; australite distribution in southern Australia is from ref. 7) ity to ultraviolet varies enormously(> 10') and the oceanic extension suggested by our finding. b, Top: posterior view of the flanged australite button, in the 11 segments examined. Sequence is showing well-preserved, schlieren structures on the core and smooth texture on the flange. Middle: anterior important as well as size as a 151-bp seg­ side, showing a bubble pit in the top half. A thin circular groove runs along this side; further ablation would ment was at least 25-fold more sensi­ have led to flange shedding along the groove, resulting in the formation of a lens. Bottom: oblique view from posterior side, showing curvature of the core. The diameter of the tektite is 12.054 mm. tive than a 430-bp segment. Two main 340 NATURE · VOL 347 · 27 SEPTEMBER 1990 © 1990 Nature Publishing Group