Towards a Dynamical History of ‘Proto-Encke‘ |
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Authors: | DJ Asher SVM Clube |
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Institution: | (1) Communications Research Laboratory, Japan;(2) University of Oxford and Armagh Observatory, United Kingdom |
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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 (∼0.2 kyr, ∼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 ϖ pE ≈ ϖ J or ϖ J + π. 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.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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Keywords: | Comets orbital evolution zodiacal dust climatic cycles |
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