Matters Arising

152 Nature Vol. 277 11 January 1979 matters arising

Was Venus born retrograde? and lower atmosphere is uncertain by to be Mesozoic, whereas the deposits in orders of magnitude, not by 30% as Ghana, Upper Volta, the Ivory Coast and INGERSOLL AND DOBROVOLSKIS 1 claimed by Kundt. Mali are Proterozoic or Infracambrian 3 ~8 • have disputed my theory2 of the atmos The thermal response time r is intro Their reference to kimberlites in Ghana is phere of Venus. The key quantity in both duced in ref. 1 to parameterise incorrect. Their location of a 'kimberlite' analyses is the accelerating atmospheric unmodelled effects which tend to damp in Ghana fits the location of a few torque TA, evaluated in equations (2) and the atmospheric tides. It is not necessarily diamonds in a stream panning exercise3 (22) of refs 1 and 2 respectively. The latter related to mass transport, and has nothing while the largest diamond fields of West expression is determined from observa to do with the speed of sound waves (an Africa (placer), located in southern tions to within some 30%, while the entirely separate phenomenon). The main Ghana, are omitted. Their 'kimberlite' in former contains the unknown 'thermal conclusions of ref. 1 (see our Fig. 2) are Nigeria is a lamprophyre dyke located response time' r for mass transport in the based on the undamped case r ~ oo. between Zaria and Sokoto (P. J. T. Ver atmosphere, for which the authors suggest Regarding the solid body torque, a heijen, personal communication). Like values between some weeks and infinity. value for Q of the order of 30 is not only wise, their postulated inland continua But sound waves can travel around the suggested by analogy with the Earth, tions of transform fault zones contradict planet within some 105 s, hence I expect r Moon and Mars, but is required (on the without explanation the generally accep to be about a day; and for this value, both average) if Venus has despun from an ted trends9 of these zones: the Chain and 9 determinations of T A give the same originally rapid rotation, whether pro Charcot into the Benue trough, the answer. grade or retrograde. Nevertheless, it is Romanche into the Keta basin bearing 9 10 The other important quantity is the true that the near-equality (in the opposite toward lbadan, ' and the St Paul enter braking solid body tidal torque T8 which sense) of the atmospheric and body tidal ing the Ivory Coast east of Sassandra 9 10 depends on the average value of the qual torque (TA""' T8 ) is a conjecture. This bearing ENE ' , Thus the ages and loca ity factor Q divided by the tidal Love hypothesis was originally advanced by tions of kimberlites as well as the positions 3 number k2 • Estimates for Q/4k2 range Gold and Soter to explain the nonsynch of transform faults and their postulated from 30 to :::;: 105 (refs 3, 2). In view of the ronous rotation period of Venus. landward continuations used by Williams different spin rate, chemistry and thermal These and similar questions are dealt and Williams2 disagree with those history of Venus, a comparison with Earth with in greater depth in a forthcoming established in the literature, with no need certainly not be trusted. An equality series of paper, as well as in ref. 4. explanation for this disagreement.

TA""' T8 can therefore at best be con The geographical pattern of kimberlite jectured. In ref. 1, this equality is sugges ANTHONY R. DOBROVOLSKIS pipes and the larger diamond placers can ted to hold within less than 2%, an ANDREW P. INGERSOLL be more accurately interpreted (Fig. 1) as unlikely event without independent California Institute of Technology, occurring along a primary arc running justification. Pasadena, California 91125 from Ghana through the Ivory Coast and 11 For this and the other reasons put for Liberia to Sierra Leone • In such a case ward in ref. 2, I still believe in the law of 1. Ingersoll, A. P. & Dobrovolskis, A. R. Nature 275, 37~38 the occurrence of young kimberlites is isochronism: that all Solar System secon (1978). brought into coincidence with all of the 2. Kundt, W. Astr. Astrophys. 60, 85~91 (1977). daries were born with a prograde spin 3. Gold, T. & Soter, S. Icarus II, 356~366 (1969). much older kimberlites and alluvial period between 5 and 10 h. 4. Dobrovolskis, A. R. thesis, California Institute of Tech deposits in the same general region. In nology (1978). addition there is a possibility of a second, WOLFGANG KUNDT roughly parallel arc extending from Institut fur Astrophysik An alternative hypothesis northern Ghana through Upper Volta, the der Universitiit Bonn, for the origin of Ivory Coast, Mali and Senegal. Known Auf dem Hugel 71 West African kimberlites ages of major deposits along the primary D-5300 Bonn 1, FRG arc include the diamonds contained in MARSH 1 suggested in 1973, that the alk and above Birimian meta-sediments 1. Ingersoll, A. P. & Dobrovolskis, A. R. Nature 275. 37 (2,209 Myr) in Ghana'.4, the Seguela and (1978). aline intrusives along the coast of south 2. Kundt, W. Astr. Astrophys. 60, 85 (1977). western Africa occurred where oceanic Kanangono kimberlites (1,429 Myr) in 5 7 3. Goldreich, P. & Peale, S. Astr. J. 75, 273 (1970). fracture zones extended under the the Ivory Coast ' , and the Sefadu and adjacent continents. Recently Williams Koidu kimberlites (140-92 Myr) in Sierra 2 6 INGERSOLL AND DOBROVOLSKIS and Williams have used this suggestion to Leone • Along the second arc, the REPLY-The questions which have been attempt to explain the origin of the West Kenieba kimberlites of Mali have been raised concerning the tidal torque on the African kimberlites. Their interpretation dated at 1,072 Myr(ref. 5). The geology of atmosphere of Venus1 deserve some rests heavily on the accuracy of kimberlite these deposits and the age determination clarification. and transform fault locations in their Fig. techniques and results are discussed The atmospheric torque evaluated in 1, on the assumption of Mesozoic ages for elsewhere 3 ~8 • Such a long history of pro equation (22) of ref. 2 depends on the all of their kimberlites, and ignores the gressive east to west arcuate diamond coefficient of viscosity in the upper important occurrences in the same region bearing belts requires more than an asso atmosphere of Venus. Because this is of pre-Mesozoic kimberlites and alluvial ciation between Mesozoic kimberlites and undoubtedly dominated by eddy momen diamond deposits. post-Palaeozoic ocean fractures. tum transport instead of molecular Williams and Williams' hypothesis An equally viable explanation might be diffusion, the coupling between the upper assumes West African diamond deposits the existence of a long-lived mantle