Towards a Dynamical History of ‘Proto-Encke’

Abstract

There are too few active comets to account for the observed zodiacal dust. Rather we look to the collisional fragmentation and erosion of sub-kilometre meteoroids in orbit close to the ecliptic. Since 1975 we have also been aware of an apparently massive meteoroidal swarm in probable 7:2 mean motion resonance with Jupiter, seemingly at the heart of the Taurid Complex and connecting therefore with the near-ecliptic system through the so-called Štohl Stream. The notable absence of pre-1786 apparitions of 2P/Encke took on a new significance with the 1983 detection by IRAS of its asymmetric trail inside this resonance. Thus it was possible all these meteoroidal components were ultimately derived from a continuously eroded, substantially dormant, librating progenitor within the trail whose more volatile inclusions are exposed from time to time and expelled either singly or severally as independent comets. A Taurid progenitor of this kind (proto-Encke) dominating the inner Solar System environment probably then accounts for most of the recorded enhancements of the larger meteoroid flux to Earth, including ‘Tunguska’ bodies as well. Terrestrial dust insertions which control mean temperature and hence climate are also inferred based upon the libration and nodal precession half-periods of proto-Encke (~0.2kyr, ~2.5 kyr respectively) albeit the longer of these cycles was not at first evident in the terrestrial record (Asher & Clube 1993). Recently however this cycle appears to have been confirmed as a significant (long term) global warming/meridional atmospheric circulation/iceberg calving cycle with the correct phase producing the so-called mini-Heinrich and Heinrich events of the Holocene and late Upper Pleistocene respectively, i.e., during the past ~60 kyr BP. The comparative stability of this terrestrial cycle, in contrast with the weakness of the observed resonance, suggests a fairly recent diversion therefore from a much stronger sungrazing 7:2 Jovian resonance in which proto-Encke’s and Jupiter’s longitudes of perihelion are related by ${\varpi _p}E \approx \varpi J\,or\,\varpi J + \pi$. Thus both the Hephaistos Stream and the Taurid Complex could have formed together during a recent close planetary encounter, say with Mercury ~5 kyr BP. It follows that we envisage a single large progenitor in 7:2 Jovian sungrazing resonance for 50 kyr or so which undergoes repeated tidal stress: a continuous dust-induced major glaciation is thus sustained on Earth for most of this dynamical timescale before a disruptive planetgrazing event finally brings its sungrazing status to an end and produces the present meteoroidal complex. This evolutionary sequence almost certainly requires that the original sungrazing stream still exists (without its source): a potentially significant fact because it may have a direct bearing on both the observed zodiacal bands and the original progenitor orbit as well as the known periodic variation of solar radiance and convected magnetic field, of possible relevance to the solar cycle. While these aspects have to be further explored, the purpose of the present investigation is to describe some preliminary modelling with a view to inferring the likely dynamical history of proto-Encke.