Cosmic Dust Collections at Various Latitudes

Summary Airborne dust collections, lasting over several months, have been made at Barbados and the Isles of Scilly. Microscopic (100μ) flakes of metallic iron and of nickel have been found. When allowance is made for their masses, their arrival date suggests that they are associated with meteor showers. However, these flakes show no sign of melting or of heavy oxidation; and, since they are more abundant than the Van der Hulst's (1947) zodiacal estimate, there is a possibility that they may be spiralling in on grazing orbits from a dust shell surrounding the Earth. At certain times, particularly at midsummer in the northern hemisphere, this shell would have to give enhanced precipitation. The tabloidal or wafer shape of some fragments, especially the nickel, together with the occasional sandwiching of metal between layers of some reddish material, implies that these types have been torn from a layered primary. Metallic flakes embedded within a pale yellow‐green carbonaceous substance have also been found. No silicon has been detected in these other materials and with the high purity of the nickel flakes, this suggests a non‐meteoritic origin. If comets are responsible, then in parts of them the metal could exist as extensive sheets. Metallic fragments have also been found in Antarctic meltwater; and the size distributions for the iron fragments at the various latitudes leads to an accretion law df = (˜ 10 −6 ) dm/m s where df is the number of fragments of mass mμg incident on a square metre per second, in the mass interval dm . Here s varies slightly with latitude but the mean value of 1.53 agrees with the Van der Hulst's distribution for the zodiacal cloud; and it also agrees with the distribution of chips of shattered brick. Knowing approximately the total mass of the very finest iron fragments and extrapolating the above law, the minimum fragment size existing in the solar system is about 10μ. The nickel fragments follow a similar law but are about 10 to 20 times less numerous than the iron. An alternative interpretation, involving possible ablation at special mass values appropriate to particular meteor showers, is also discussed. Hollow magnetic spherules arrive at the same time as the flakes and, in contrast with metallic centred deep‐sea spherules, could be bubbles of boiling oxide swept off the surface of perhaps millimetre‐sized fragments of iron. The finding of one airborne metallic centred spherule allows the rate of deep sea sedimentation to be estimated. This is of order 1 mm per 1 000 years; and using the airborne nickel collections some 10 per cent of the nickel in oceanic deposits could be extraterrestrial.