PLEASE NOTE:
*
CCNet 19/2002 - 2 February 2002
-------------------------------
(1) CARBON AT THE K/T: SOME FACTS AND THE ROLE OF C IN DUST
Iain Gilmour <I.Gilmour@open.ac.uk>
(2) EXPLODED PLANET HYPOTHESIS FAILED MAJOR TEST
David Tholen <tholen@IfA.Hawaii.Edu>
(3) ESTIMATES OF EFFECTS OF THE DEEP IMPACT INTO TEMPEL 1
Keith Holsapple <holsapple@aa.washington.edu>
and
Kevin Housen <kevin.r.housen@boeing.com>
(4) MOONLIGHTING
Mark Kidger <mrk@ll.iac.es>
============
(1) CARBON AT THE K/T: SOME FACTS AND THE ROLE OF C IN DUST
>From Iain Gilmour <I.Gilmour@open.ac.uk>
Dear Benny:
Tom Van Flandern's comment on CCNET (01/02/02) repeats some (10
year
old) inaccuracies on the nature of carbon recovered from K/T
boundary
sediments that need correcting. I'd also like to highlight the
role of C
in the atmosphere as an efficient absorber of sunlight.
Carbon has been recovered from Chicxulub ejecta in several forms:
relatively amorphous elemental carbon, spheroidal soot-like
particles and
impact-produced diamonds. The soot and elemental carbon have been
recovered globally, while diamonds have only been recovered from
North
American K/T sites. Soot may have played an important role in any
extinciton scenario (see below). At the outset, I should point
out that
the occurrence of impact-produced diamond in Chicxulub ejecta is
very
strong evidence for an impact, and while Van Flandern's comment
cites
them as such, his statement that "The diamonds found at the
K/T boundary
are confirmed to be of extraterrestrial origin, not
shock-generated or
terrestrial" is inaccurate.
The inaccuracies reproduced in Van Flandern's comment concern the
occurrence of diamond in the upper and lower K/T boundary clay
layers in
North America and Northern Mexico. He cites the initial report by
Carlisle and Bramman [1] who suggested that these diamonds may
have been
derived from the impactor suggesting that this was a CM2
(incidentally
based on an erroneous calculation of the diamond/Ir ratio in CM2
chondrites by Carlisle and Bramman). They later reported [2] a
carbon
isotope composition of -39 permil claiming that this matched the
carbon
isotopic composition of the interstellar diamond component in CM2
chondrites. It does not, interstellar diamonds in CM2 chondrites
display
a range of carbon isotope compositions of between -26 and -42
permil
depending on grain size [3] with a mean of between -33 and -36
permil.
More importantly, however, they contain a highly anomalous
nitrogen
isotope composition of -200 to -250 permil [3], consistent with
an origin
in C-rich AGB stars. Detailed studies of the carbon and nitrogen
isotope
compositions of K/T diamonds (ranging in size from 6 nanometre
crystallites to 30 micron polycrystalline diamonds) from North
American
and Mexico showed a range in carbon isotope compositions greater
than
that shown by CM2 chondrite interstellar diamonds and, most
importantly,
no anomalous nitrogen [4,5]. The nitrogen they did contain was
very
terrestrial in its signature [4]. Larger polycrystalline diamonds
isolated from N.E. Mexico K/T sequences display a hexagonal
morphology,
reflecting the hexagonal crystal structure of the precursor
graphite.
The diamonds in Chicxulub ejecta are similar in all respects to
impact-produced diamonds recovered from the Popigai, Ries,
Sudbury,
Lappajarvi, Gardnos, and other craters. They have the same
origin: they
are produced by the shock-transformation of graphite during the
impact
event. We should not be surprised, impact-produced diamonds are
chemically and morphologically quite distinctive and have been
known
about since they were first reported in a terrestrial impact
structure by
Victor Masaitis in 1972.
The carbon isotope compositions of Chicxulub impact diamonds
therefore
reflect the carbon isotope composition of the target rocks. As
such,
this appears to rule out target rock-derived carbon as a source
for the
global distribution of soot and elemental carbon [6] (the
isotopic
composition of K/T soot is very restricted at around -26 permil
and
typical of C3 biomass) reinforcing the argument that the soot is
derived
from a global distribution of wildfires. Soot itself may play an
important role in the K/T extinction mechanism. Soot absorbs
sunlight
more effectively and settles more slowly than does rock dust and
would
have been a contributing mechanism to the shutdown of
photosynthesis, a
point recognised in Kevin Pope's paper. In our original 1988
paper [6] we
calculated an optical depth of around 1800, prior to coagulation,
based
on the integrated abundance of soot in K/T boundary clays - even
if this
is an over-estimate, soot is an efficient absorber of sunlight.
Combined
with sulfate aerosols, we are not short of mechanisms to explain
the
evidence for a shutdown of photosynthesis observed in the
geological
record.
[1] D. B. Carlisle, D. R. Braman, Nature 352, 708-709 (1991).
[2] D. B. Carlisle Nature 357, 119-120 (1992).
[3] Vechovsky et al. (1998) Science, 281, 1165-1168.
[4] I. Gilmour et al., Science 258, 1624-1626 (1992).
[5] R. M. Hough, I. Gilmour, C. T. Pillinger, F. Langenhorst, A.
Montanari, Geology, 25, 1019-1022 (1997).
[6] W. Wolbach, I. Gilmour, E.Anders, C. Orth, R. Brooks, Nature,
334,
665-669 (1988).
Iain
--
Dr. Iain Gilmour
Planetary and Space Sciences Research Institute
The Open University
Milton Keynes MK7 6AA
United Kingdom
+44 190 865 5140 (direct)
+44 190 865 2883 (secretary)
+44 190 865 5910 (fax)
http://pssri.open.ac.uk
============
(2) EXPLODED PLANET HYPOTHESIS FAILED MAJOR TEST
>From David Tholen <tholen@IfA.Hawaii.Edu>
Benny,
In the 2002 February 1 edition of CCNet 18/2002, Tom Van Flandern
has
resurrected his so-called "Exploded Planet Hypothesis",
or EPH for
short, in the final sentence of the following paragraph:
The point about the inland seas may be another
telltale clue. Among
other indicators of this, apparently an
intra-continental sea
covered the middle of North America during the
Cretaceous period,
but disappeared near the K/T boundary. [16]
Evaporation of a large,
distant body of water would not be an expected
consequence of an
impact event. Neither is a single global fire.
However, both are
predicted consequences of heating of the
biosphere by a massive,
prolonged, heavy bombardment of meteors, as
would follow for example
the explosive break-up of a planet-sized body
elsewhere in the solar
system. [17]
What he failed to note is that his EPH failed a major test.
On
1997 February 21, Van Flandern posted on the sci.astro USENET
forum
the following prediction:
If the NEAR rendezvous with Eros shows it to
be an
isolated, single body, or even a simple
"binary
asteroid", but without a debris field
orbiting it, I
will publicly concede before the next Division
of
Planetary Sciences meeting that the hypothesis
leading
to that prediction has failed.
The hypothesis that led to the prediction was the EPH. No debris
field
was found orbiting Eros. It should also be noted that the public
concession never took place.
Dave Tholen
Institute for Astronomy
University of Hawaii
============
(3) ESTIMATES OF EFFECTS OF THE DEEP IMPACT INTO TEMPEL 1
>From Keith Holsapple <holsapple@aa.washington.edu>
and
Kevin Housen <kevin.r.housen@boeing.com>
Re CCNet 23 Jan 2002:
"The Deep Impact mission hopes to reveal
the nature of the threat
and how to deflect it safely. On American
Independence Day 2005,
Deep Impact will reach its target, the
six-kilometre diameter comet
Tempel 1. The space probe will release a
350-kilogram (770 lbs)
projectile into the heart of the comet at 10
kilometres per second
(six miles per second). It is expected to blow
a crater the size of a
football field and 20 metres (65 feet) deep.
The comet will survive
but should reveal the nature of its interior
to add to scientific
knowledge and to guide any future plans to
deflect a killer comet
with a nuclear nudge."
These crater estimates have been made by Peter Schultz, and have
been
quoted by several recent contributions. We would like to go on
record
as noting that these estimates of the effects are vastly
different than
ours. As Schultz also notes in [1] referenced below, the actual
result
may simply be a compression crater with little ejecta or surface
expression, or it may be very large. Schultz's estimates are
based on
the large assumption. We favor a much lower value: we would
predict a
crater on the order of 10 meters or less, not the 100 meters of a
football field.
These estimates differ by factors of about 1000 in crater volume,
and
corresponding differences in the amount of ejecta. Why this large
discrepancy? It arises from the simple fact that we cannot
perform
experiments at the size scale of interest, with 10km/s impact
velocity,
on the actual (unknown) material, and at the low gravity of the
surface
of Tempel. Therefore, one does the experiments that are possible,
and
extrapolates to the conditions of interest. Those extrapolations
use
scaling theories: theories about how the answer depends on the
parameters of the problem. Here those parameters are the impact
velocity, the impactor size, and the surface gravity. Schultz has
done
experiments in low density porous materials at 1 G and various
velocity,
and then applied scaling theories. Housen [2] has done
experiments in a
low density material also, and at variable gravity but at a
single
velocity.
We would not extrapolate Schultz's results the same way he does.
While
not going into details here, basically he has assumed that his
results
should be gravity-scaled, while we believe they should use
strength-scaling. Using his approach, the crater sizes increase
as a
power law (about -1/2) of gravity. When extrapolated back to the
.08
cm/sec^2 gravity on the surface of the comet, he obtains his
large
estimate. Our scaling approach would imply that the results are
essentially independent of surface gravity, so do not increase at
low
gravity compared to the 1G experiments. (For the definitions of
these
scaling regimes, one can consult, for example, Holsapple [3]
listed
below.)
At the LPSC conference in March in Houston, Housen will be
presenting
his results and interpretations [2], and Schultz will be
presenting his
[1]. The large uncertainties illustrate the need for missions of
this
type. Here the real results will be known in 2005. It is nice to
have a
debate about something where the correct answer is forthcoming.
References:
[1] Schultz, P. H. ,Anderson, J. L. B. and J. T, Heineck, Impact
crater
size and evolution: Expectations for Deep Impact, Lunar and
Planetary
Science XXXIII, March 2002.
[2] Housen, K. R., Does gravity scaling apply to impacts on
porous
asteroids?, Lunar and Planetary Science XXXIII, March 2002.
[3] Holsapple K. A. (1993) The scaling of impact processes in
planetary
sciences. In Ann. Rev. Earth Planet. Sci. 21 , Wetherill, Albee
and
Burke, eds), 333-373.
==============
(4) MOONLIGHTING
>From Mark Kidger <mrk@ll.iac.es>
Benny:
Thinking about the Open Letter (I've just read it because I was
at a
conference all last week), it might have been even more effictive
to point out that Gordon Garradd is an amateur astronomer (I
believe
that this is still true). In other words, the Earth's defenses
rely to
a significant degree on a man who is, quite literally,
moonlighting.
Mark
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