COHERENT CATASTROPHISM
Implicating Taurid progenitor
COHERENT CATASTROPHISM
D. J. Asher, S. V. M. Clube, W. M. Napier and D. I. Steel
Vistas in Astronomy, Vol. 38, pp. 1-27, 1994
ABSTRACT
We review the theoretical and observational evidence that, on timescales relevant to mankind, the
prime collision hazard is posed by temporally correlated impacts (coherent catastrophism, At
102-104 yr) rather than random ones (stochastic eatastrophism, At ~ 105-10 s yr). The mechanism
whereby coherent incursions into and through the terrestrial atmosphere occur is described as being
the result of giant cometary bodies arriving in orbits with perihelia in the inner solar system. Hierarchical
fragmentation of such large (100 kin-plus) bodies -- due to thermal stresses near perihelion,
collisions in the asteroid belt, or passages through the Jovian Roche radius -- results in numerous
~kilometre-sized objects being left in short-period orbits, and appearing in telescopic searches as
Apollo-type asteroids. Many more smaller objects, in the 10-100 metre size range and only recently
observed, by the Spacewatch team, are expected to be in replenished clusters in particular orbits as
a result of continuing disintegrations of large, differentiated, cometary objects. Gravitational perturbations
by Jupiter bring these clusters around to have a node at 1 AU in a cyclic fashion, leading to
impacts at certain times of year every few years during active periods lasting a few centuries, such
periods being separated by intervals of a few millennia. Furthermore, fragmentations within the hierarchy
result in significant bombardment commensurabilities (At ~ 10-102 yr) during active periods
occurring at random intervals (At ~ 102-10 a yr).
It appears that the Earth has been subject to such impacts since the break-up of such a comet
~2 x 104 years ago; currently we are not passing through a high-risk epoch, although some phenomena
originating in the products of this break-up have been observed in the 20th century. This most recent
hierarchical disintegration, associated with four well-known meteor showers and termed the Taurid
Complex, is now recognized as resulting in a dozen apparently asteroidal objects -- almost 10% of the
discovered Apollo population -- as well as Comet P/Encke. A substantial asteroidal remnant of the
Taurid progenitor may still be present in resonance with Jupiter. The implications of the existence
of any asteroidal or meteoroidal cluster due to the progenitor are briefly discussed. We emphasize
finally the relative importance of active and inactive comets in the cratering record, pointing out
the potentially significant contribution by disintegrating large comets to the population of inactive
Earth-crossers.