Periodic Comet Shoemaker/Levy 9 will impact Jupiter in late July 1994. The comet, which broke into more than 20 telescopically detectable fragments when it passed with the Roche lobe of Jupiter on July 8, 1992, is captured in a highly eccentric orbit about Jupiter. The 21 recognized nuclei will be spread out in a train of the order 7 x 10(exp 6) km long at the time of impact, and the impacts will be spread in time over about 5 1/2 days centered on about July 21.2 UT. In addition to the train of recognized bright nuclei, the comet consists of 'wings' of unresolved bodies that are the source of a very broad composite dust tail. The linear extent of the wings is about an order of magnitude greater than that of the train of recognized discrete nuclei. Collision of the wings will be spread in time over several months. Thus the impact of P/S-L 9 with Jupiter will be an event of appreciable duration

Shoemaker, C. S.

Shoemaker, E. M.

English

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Precambrian era. Also, the amount of cometary volatile material
that has been deposited on Earth may be more than is found at
present, since some of the volatile material may have been removed
by impact erosion. In addition, the UV absorption cross sections of
cometary H20 and NH 3 may be larger than the values measured in
the laboratory, since in comets they could be in the form of cluster
molecules [8]. Finally, the continuous introduction of H20 and NH 3
at the top of the atmosphere would not only supress O 3 production
by UV absorption, but the photolyzed products would catalytically
destroy 03 by the well-known reactions at work in the stratosphere _
at present [9].
Since suppression of the O 3 atmosphere by comets in the inner
solar system appears to be possible, it is of interest to note that the _
same mechanism would resolve two other current problems involv-
ing the history of Earth and Mars. The likelihood of NH 3 in the
Precambrian atmosphere was suggested [ 10] in order to provide the
necessary greenhouse effect when solar luminosity was less than its
present value, and thus reconcile calculated ocean temperatures
with the observation that the oceans had not frozen. However,
calculation of NH 3 photolysis [ I 1] indicated a lifetime for NH 3 in the
atmosphere that was significantly less than that which was required.
The presence of NH 3 and H20 in the inner solar system according
to the scenario presented here could reduce the rate of NH 3 photoly-
sis in the Earth's atmosphere to a level that would permit the small
amount of NH 3 that had been suggested [10]. In a similar way the
efficient greenhouse nature of NH 3, in conjunction with its shielding
from solar UV radiation by cometary outgassing in the inner solar
system, could account for the warmer temperatures on Mars that are
needed to explain the fluvial features that have been observed there.
To summarize the main suggestion proposed here: Discontinuities
in the history of Earth's biota can be explained by the single unifying
suggestion that low levels ofO 3production are controlled by cometary
activity. Precambrian biological activity is explained along with
mass extinctions. Where mass extinctions since the Cambrian are
concerned, comet showers from the remnants of the nebular disk
lasting for thousands to millions of years provide a model consistent
with the paleontological record, which shows biological degrada-
tion lasting for similar periods and becoming increasingly destruc-
tive with time until the event suddenly ends. Since the model
proposed here appears to answer some outstanding questions it
should be investigated further.
Acknowledgments: This work was supported by NSF grant_
ATM-9206417.
References: [I] Rather M. I. and Walker J. C. G. (1972)
J. Atmos. Sci., 29, 803. [2] Levine J. S. (1982) J. Molec. Evol., 18,
161. [31 Oort J. H. (1950) Bull. Inst. Neth., 11, 91. [4] Hills J. G.
(1981) Ap. J., 86, 1730. [5] Fernandez J. A. (1980)Icarus, 42,406.
[6] Oro J. et al. (1990) Annu. Rev. Earth Planet. Sci., 18, 317.
[7] Delsemme A. H. (1992) Origins of Life and Ev61ution of the
Biosphere, 21,279. [8] Frank L. A. and Sigworth J. B. (1993) Rev.
Geophys., 31, 1. [9] Turco R. P. (1985) The Photochemistry of
Atmosphere (J. S. Levine, ed.), 77, Academic. [10] Sagan C. and
Mullen G. (i972) Science, 177, 52. [i i] Kuhn W. RI afid_ireya
S. K. (1979) Icarus, 37, 207.
" -" Contribution No. 825 I 13
N94- 28313
THE CRASH OF P/SHOEMAKER-L 9 INTO JILTER ./ _.
AND ITS IMPLICATIONS FOR COMET BOMBARDMENT !-
ON EARTH. E. M. Shoemaker and C. S. Shoemaker, Lowell
Observatory, 1400 West Mars Hill Road, Flagstaff AZ 86001, USA.
Periodic Comet Shoemaker/Levy 9 will impact Jupiter in late
July 1994 [ 1,2]. The comet, which broke into more than 20 telescopi-
cally detectable fragments [3I _vhen it passed within the Roche lobe
of Jupiter on July 8, 1992, is captured in a highly eccentric orbit
about Jupiter. The 21 recognized nuclei will be spread out in a train
of the order 7 x 106 km long at the time of impact, and the impacts
will be spread in time over about 5 1/2 days centered on about July
21.2 UT [4]. In addition to the train of recognized bright nuclei, the
comet consists of "wings" of unresolved bodies that are the source
= of a very broad composite dust tail. The linear extent of the wings
: is about an order of magnitude greater than that of the train of
recognized discrete nuclei. Collision of the wings will be spread in
-time over several months. Thus the impact of P/S-L 9 with Jupiter
_will be an event of appreciable duration. =_
Sizes of the recognized nuclear fragments of P/S-L 9 are not yet
firmly established. Photometry from high-resolution images ac-
quired by the Hubble Space Telescope suggests that the 11 largest
nuclear fragments range from 2.5 to about 4.3 km in diameter, and
that the precursor body, before breakup, was about 8 km in diameter
or larger [5]. A preliminary dynamical solution for the development
of the observed train of nuclei by Scotti and Melosh [6] suggests that
the precursor body was only about 2 km in diameter. However, a
later solution by Chodas and Yeomans [4], based on orbits of the
individual nuclei, indicates that the precursor body was about 9 km
in diameter. We conclude that the precursor of P/S-L 9 was of the
order 9 km diameter and that the total impact energy will be of the
order 10 Xmegatons TNT.
Observations of comets discovered shortly after escape from
jovicentric orbit, plus the discovery of P/S-L 9, indicate that the
frequency of collision of objects orbiting Jupiter with the estimated
impact energy of P/S-L 9 is of the order once per millenium [7].
Collisions with Jupiter of comets on free heliocentric orbits are
several times more frequent. These collision rates are an order of
magnitude higher than those predicted with the use of Opik's
equations, which fail for very-low-velocity encounters with Jupiter.
The development of a train of cometary debris by tidal breakup
leading to multiple impacts on a planet has direct relevance to the
impact history of Earth. Both periodic and long-period Sun-grazing
comets are subject to tidal disruption. A catastrophically disrupted
periodic comet on a free heliocentric orbit can be expected to form
a compact debris stream somewhat similar to but much narrower
than an ordinary meteoroid stream. If such a stream intersects
Earth's orbit at one of the nodes, Earth will be subject to repeated
impacts as a result of its annual passage through the stream [8-10].
High-inclination periodic comets tend to be driven to Sun-grazing
orbits [ I I], and the formation of compact debris streams should
occur fairly frequently. Breakup of very large (several hundred
kilometers in diameter) periodic comet nuclei should have occurred
occasionally during the last 0.5 Ga, leading to comet showers with
durations of the order of i05 yr. If one such breakup occurred when
the distance to a node was near I AU, an intense pulse of bombard-
ment lasting a few decades may have occurred. We postulate that
just such an event happened at K/T boundary time.
https://ntrs.nasa.gov/search.jsp?R=19940023810 2020-06-16T15:00:29+00:00Z
I 14 1(7"Event and Other Catastrophes
References: [1] Carusi A. et al. (1993) IAU Symposium I60,
9. [2] Chodas P. W. and Yeomans D. K. (1993) Bull. A.A.S., 25,
1042. [3] Jewitt D. et al. (1993) Bull. A.A.S., 25, 1042. [4] Chodas
P. W. and Yeomans D. K. (1993) Bull. A.A.S., 25, 1042.
15] Weaver H. A. et al. (1993) Science, in press. [6] Scotti J. V. and
Melosh H. J. (I 993) Nature, in press. [7] Shoemaker E. M. et al.
(1993) Bull. A.A.S., 25, 1042. [8] Shoemaker E. M. (1991) ACM
'91,194, [9] Shoemaker E. M. and lzett G. A. (1992)LPS XXIII,
1293-1294. [10] Shoemaker E. M. and Izett G. A. (1992) LPI
Contrib. No. 790, 66-68. [I 1] Bailey M. E. et at. (1992) Astron.
Astrophys., 257, 315-322.
CRYSTALS, LITHICS, AND GLASSY EJECTA AT THE KT
BOUNDARY: IMPLICATIONS FOR LITHOLOGY OF THE
CRUST AT THE IMPACT SITE. H. Sigurdsson 1, S. Smith I,
S. D'Hondt I, S, Carey _, and J.-M. Espindola 2, 1Graduate School of
Oceanography, University of Rhode Island, Narragansett R102882,
USA, 21nstituto de Geofisica, Universidad Nacional Aut6noma de
Mrxico, Cd. Universitaria, Mexico City 04510, Mexico.
Ejecta from bolide impact may contain melt quenched to glass
droplets as well as rock fragments and fractured mineral grains from
the impact terrane. In distal sections of the ejecta deposit, an impact
origin for glass and shocked minerals may generally be established,
whereas the origin of lithics and unshocked crystal fragments found
in association with impact glass is uncertain, due to possible con-
tamination from other sources. In addition to 1-8-ram-diameter
glassy ejecta spherules, the 0.5-1-m-thick KT boundary impact
10 A1203
44 46 48 50 52 54 56 58 60 62 6,1 66 68 70
SiO2 wt%
Fig. 1. Compositional trends in KT boundary glassy ejecta spherules,
illustrating the smooth compositional trends of A 1203 and CaO, from high-Ca
yellow glasses to the more abundant high-silica glasses. In total, the dataset
represents analyses of 140 sphemles. The compositional range from 44-
62 wt% SiO 2 can be modeled as a simple binary mixing trend [I], whereas
the high-silica glasses reflect a more heterogenous source. Curves define best-
fit regression lines through the data, with correlation coefficient of 0.992 for
CaO and 0.91 for AI203.
• gl 0
5 •
o• Is
o• 8
44 46 48 50 52 54 56 58 60 62 64 66 68 70
SiO2 wt%
Fig. 2. Two distinct trends of FeO content in KT boundary impact glasses.
The trend of decreasing Fe content with increasing silica inthe high-silica glasses
may reflect heterogeneity in the source region, whereas the trend be-
tween high-Ca glasses (44-54 wt% SiO:) and high-silica glasses can be mo-
deled as a mixing trend.
ejecta layer in the Beloc pelagic carbonate sediment formation in
Haiti contains silicate mineral fragments and rock fragments that
may provide clues about the nature of the Earth's crust at the impact
site. Because of the monotonous and relatively pure lithology of the
enclosing upper Createceous and lower Tertiary carbonate sedi-
ments, the Haiti exposures of the KT boundary layer provide an
opportunity to detect mineral ejecta present in only trace amounts,
Fig. 3. Scanning electron micrograph of a crystal of amphibole from the KT
boundary ejecta layer. The 400-pro euhedral crystal isentirely defined by ( I10 }
cleavage planes and formed by breakage. Mineral chemistry is consistent with
derivation from 9-12 km depth in the crust.


The crash of P/Shoemaker-Levy 9 into Jupiter and its implications for comet bombardment on Earth

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