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W.M. Napier, Senior Research Fellow

Zodiacal Cloud

The zodiacal cloud is a disc of dust orbiting within the inner planetary system. It is supplied erratically by the disintegration of comets entering the solar system, and by dust feeding in from collisions in the asteroid belt. The cloud can be seen at certain times of the year as a second `Milky Way' along the ecliptic. Existing models of the zodiacal cloud have always adopted an equilibrum assumption, in which the input from comet and asteroid disintegration balances the output from collisional grinding between the dust particles and their removal by the Poynting-Robertson effect. A long-term project by Bill Napier has been the construction of a model zodiacal cloud in which the assumption of a steady-state balance is no longer made.

The significance of this study is that it puts into quantitative form the suggestion that the cometary component of the zodiacal cloud is highly variable, and that in the wake of giant comet entry into a short-period, near-Earth orbit, the dust influx to the Earth's atmosphere may acquire a climatically significant optical depth. The output from the programme is thus intended to be an input to Mie scattering studies of radiative transfer in the stratosphere and mesosphere, and to models of global climatic change.

This model was developed throughout 1999 to the state where quantitative results were being obtained. The cloud is specified by a three-dimensional grid, each element of which contains the numbers of dust particles as a function of semi-major axis, eccentricity and mass. The evolutionary pathways of dust particles due to radiation pressure are described by fixed transition probabilities connecting the grid elements. Other elements are absorbing states representing infall to the Sun or ejection to infinity: particles entering these states are removed from the system. Mass is injected from the breakup of comets entering short-period, high-eccentricity orbits at random times, and removed through collisional disintegration, the Poynting-Robertson effect and radiation pressure.

The detailed results confirm preliminary analyses that the annual flux of cometary dust on to the Earth may on occasion approach a million tons a year, almost two powers of ten higher than current values. Thus the zodiacal cloud is confirmed to be a highly variable entity, a fact which will inform future studies of climatic effect of cometary and asteroidal dust over long time-scales.

Quasar Redshifts

Continuing a long-standing interest in the statistical aspects of `anomalous redshift' claims, Bill Napier, in collaboration with Geoffrey Burbidge (University of California at San Diego), has been investigating claims that there is a periodicity of 0.089 in the $\log (1+z)$ frequency distribution of quasars, with peaks occurring at redshifts $z = 0.06, 0.30, 0.60, 0.96, 1.41, \ldots, 4.46$. This claim is about 30 years old but a possible rationale has recently emerged in that such oscillations may be a generic feature of scalar-tensor theories of gravity in the post-inflation phase of expansion. The claim, if confirmed, would therefore have profound implications for cosmology. Selection effects are the major problem with this periodicity claim, and samples were chosen with a view to minimizing these. By the end of 1999 a definitive answer to the issue had still not been obtained.