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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