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Planetary Ring Around Earth - Part 3 of 3

Lou Varricchio schrieb:

> Subject: Saturn-Like Ring Around the Earth
>
> The April 1999 issue of DISCOVER science magazine, p. 20, has a
> story about two U.S. scientists proposing that a ring of ejecta by
> the so-called K-T impactor created a temporary ring around the
> Earth whose shadow had a profound affect on climate change at
> the end of the Cretaceous.


Meteoritics 32-4, 1997, p. 124:

Prolonged iridium deposition after the Cretaceous-Tertiary Boundary: the
primary signal from an impact-generated temporary ring around the Earth?

M.Stage(1) and K.L.Rasmussen(2)
(1) Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400
Copenhagen, Denmark (stage@gandalf.natmus.min.dk)
(2) Carbon-14 Dating Laboratory, National Museum, Ny Vestergade 11,
DK-1471 Copenhagen, Denmark.

The Ir distribution with depth at the Cretaceous-Tertiary boundary
worldwide exhibits shoulders stratigraphically up and down from the
high-Ir layer. The shoulders have been interpreted as a secondary signal
from bioturbation and/or diffusion [1,2]. Although the Ir distributions
do not have exactly the same shape globally, they exhibit the same
general features. In the Cretaceous they gradually increase toward the
boundary, followed by a maximum at the basal boundary layer. The rate of
increase is fast and might seem exponential, as expected from diffusion
processes. The decrease in the Ir enrichment in the Tertiary is
prolonged (i.e., a wider distribution) and does not resemble an
exponential decrease expected for a diffusional process. Instead the
decrease is more noisy. We interpret the Ir distribution shoulder after
the boundary as a primary accretion signal from a temporary planetary
ring around the Earth, created by a major impact (i.e., the formation of
the Chicxulub crater).
We have constructed a new three-dimensional computer model, simulating
the orbital decay due to atmospheric drag of impact ejecta inserted into
Keplerian orbits around the Earth. The typical size of the ejecta
particles is estimated to be 0.4 mm in diameter, adopted from Melosh
[3], who estimates the size of condensing droplets from a vaporized
ejecta after a terrestrial impact with a 10-km impactor. We have
monitored the number of interparticle collisions in such a ring. The
particles experience very few collisions in their lifetime, thus
collisions are omitted from our present model. With the model we
estimate the lifetime of a dilute planetary ring around the Earth, and
construct accretion profiles, depicting the amount of accreting material
as a function of time.
Figure 1 shows the particles’ mean residence time in the ring as a
function of the initial mean distance to the center of the Earth. The
particles stay in the ring thousands of years at the shown heights. A
timespan of thousands of years is in good agreement with Hansen et al.
[4], who reported a deposition time for the Ir of 10-40,000 yr. The
deposition time was estimated from Milankovitch cycles in the magnetic
susceptibility signal across the K/T boundary.
Figure 2 shows the number of particles accreted onto the Earth as a
function of time, calculated by our model. The accretion of ring
material will start after the impact that created the maximum Ir
concentration. A large fraction of the ring material will acerete just
after the impact, followed by a slow decrease in the amount of accreting
ring particles. In this situation the accretion ends after ~ 18,000 yr.
This scenario will, in the sediment, be reflected as a maximum
concentration of Ir at the boundary, followed by a shoulder of the Ir
peak.
Thus we hypothesize that debris from a more or less oblique impact was
injected into the atmosphere, constituting a very dilute ring around the
Earth for thousands of years. The ring material slowly accretes onto the
Earth in thousands of years after the impact, prolonging the Ir
enrichment into the Tertiary. The impact itself creates the Ir maximum
together with the other well-known impact features described, such as
Ni-rich spinels, shocked quartz and the Chicxulub impact crater.

References:

[1] Robin D. et al. (1991) LPS XXII, 1125-1126.
[2] Alvarez W. et al. (1990) Science 250, 1700-1702.
[3] Melosh H.J. (1989) Impact Cratering: A Geologic Process, Oxford    
Univ.
[4]Hansen H.J. et al. (1992) Meteoritics, 27, 230.


Best wishes,

Bernd


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