1
This work has been prepared in summer 2008 in the frame of grant ESA PECS C 98056
“GOCE –specific tasks on fine gravity field structure of the Earth”
item 4: Detection of impact (meteoritic) structures...
for a purpose of popularization of our results obtained during testing
the new EGM 08 gravity field model
See also presentation at IAG conf. in Chania, Crete, Greece, June 2008
Chicxulub and Popigai impact craters
as seen by the newest gravitational model
of the Earth EGM 08
as multiple structures
Jaroslav Klokočník1, Jan Kostelecký3,4, Pavel Novák3,5, Ivan Pešek4, Carl A.
Wagner6, Christian Gruber1, Ales Bezděk1, Jan Vondrák2
1
Astronomical Institute, Academy of Sciences of the Czech Republic,
CZ-251 65 Ondřejov Observatory, Czech Republic,
jklokocn@asu.cas.cz, gruber@asu.cas.cz, bezdek@asu.cas.cz
2
Astronomical Institute, Academy of Sciences of the Czech Republic,
CZ-140 00 Praha 4, Boční II, Czech Republic, vondrak@ig.cas.cz>
3
Research Institute for Geodesy, Topography and Cartography,
CZ-250 66 Zdiby 98, Czech Republic, kost@fsv.cvut.cz
4
Department of Advanced Geodesy, Czech Technical University,
CZ-166 29 Praha 6, Thákurova 7, Czech Republic, pesek@fsv.cvut.cz
5
Department of Math., Faculty of Applied Sciences, University of West Bohemia,
CZ-306 14 Plzeň (Pilsen), Univerzitní 8, Czech Republic, pavel.novak@pecny.cz
6
Laboratory for Satellite Altimetry, CZNOAA, Silver Spring, MD 20910 - 3226, USA,
carl.wagner@noaa.gov
2
There are now about 170 impact meteoritic craters known on the Earth [http://en.wikipedia.org/wiki/List_of_impact_craters_on_Earth, www.unb.ca/pass/ImpactDatabase],
although up to 600 other potential impact crater sites may exist according to some
authorities [http://web.eps.utk.edu/ifsg.htm].
We have computed the gravitational signals (anomalies and gradients) at about 30
localities with the largest impact craters and at some other promising places. Here we
give two examples, Chicxulub and Popigai (for more details see Klokočník et al 2008).
The novelty of our approach is from the use of a very detailed global gravitational
field model EGM 08 (Pavlis et al., 2008 a, b) with theoretical resolution 8 [km] cos φ
on the ground (where φ is latitude) and from computing two anomalous parameters of
the gravitational field, the gravity anomaly ∆g and the second radial derivatives Tzz of
that disturbing potential, not available from ground surveys. The second derivative has
increased sensitivity to smaller features showing greater detail over the area surveyed.
The gravity signal itself of circular or ring-like structures has a tendency to circularity
of positive and negative values of ∆g or Tzz, but circular-like signals are not proof of an
impact origin, just an indication. Additional data is needed such as magnetic anomalies,
seismic profiles and deposits of shock-metamorphic minerals (stishovite, coesit,
diamond, etc), shatter cones, impact breccias, etc.
Chicxulub – a double crater ?
The Chicxulub structure on north Yucatan Peninsula, Mexico (center at latitude 210
20’ N and longitude 2700 30’ E) is a multi-ringed impact crater buried partly under a flat
surface and partly under a shallow sea. It was discovered by gravity and magnetic
anomalies (available to an oil company) disclosing concentric, ring-like patterns. Such
structures are often marked also by the presence of shock metamorphosed minerals and
by specific seismic profiles.
The size of the crater remains unresolved. Earlier investigations (e.g., Hildebrand et al
1995, 1998) found that there are two rings with diameters around 80 and 170 km,
although others (e.g., Sharpton et al 1993) identified two more-distant rings in their
gravity profiles, and they interpret a 300km-wide crater. Bottke et al (2007) discovered
that Chicxulub is a result of the impact of a fragment (of carbonaceous chondrite) from
the Baptistina asteroid family, the first such specific identification of the origin of an
Earth impactor.
Now, for crater identification, we can employ comprehensive detailed global gravity
field models using extensive satellite as well as surface data (with advantages over
exclusive ground surveys mentioned above). One such older model, EGM 96 (Lemoine
et al 1998), providing 50 km resolution at the latitude of Chicxulub) showed a negative
anomaly in the Chicxulub area, but revealed no further details. Newer models,
computed with the help of data from more recent satellite missions, CHAMP and
GRACE, unfortunately did not reveal much more detail for this location. However, the
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most recent model, EGM 08 (Pavlis et al 2008), officially issued at EGU Vienna 2008,
with spherical harmonics complete to degree and order 2160, has theoretical resolution
about 8 km. In addition to the satellite data from GRACE in this model, there are 5’x 5’
area mean surface free-air gravity anomalies and gravity anomalies derived from
satellite altimetry nearly worldwide, prepared mostly by NGA (National GeospatialIntelligence Agency). Their precision for Yucatan and many other places is 1-3
milliGals (mGals). But in some localities the precision is below 20 mGals and
resolution is much lower – e.g. in Antarctica.
We used EGM 08 to 2160 and computed ∆g and Tzz (Holmes et al 2006). Our results
are summarized in a progress report for ESA; here we show two examples.
Figures 1 and 2 show ∆g and Tzz for the Chicxulub area. Two circular-like structures
are clearly visible with strong negative values of Tzz , and a central positive part and
two rings with positive anomalies. The outer ring has a diameter of 160-180 km. Larger
but fainter and fragmented are outer “circles” of minimum and maximum gradient with
a possible diameter of roughly 250 km. North-east of the Chicxulub impact we can also
see a less pronounced circular-like feature, partly interfering with the outer ring of the
“original” Chixculub. This smaller crater seems to have two rings with the diameter for
the outer ring reaching about 100 km. It is fair to note that the existence of the second
crater might have been anticipated (but was not) already from older maps of the gravity
anomalies from Fig. 1 of Shapton’s paper, p. 1565.
Figure 2 is in fact a detail of Figure 3 which shows the gradient over the whole of
the Yucatan to Guatemala. The ring structures Chicxulub “I” and “II” are visible here as
well, but in many places the gradient signal is much stronger than at Chicxulub proper
and is not related to any impact. The “waves” near the sea-shore at the town of
Villahermosa may be an artifact of the spherical harmonic expansion truncated here at
degree 2160. They are easily distinguished from the circular features.
Our result using EGM 08 is that the Chicxulub crater is larger that 170 km in
diameter (in agreement with Sharpton and others and in disagreement with Hildebrand
and others) and that it may be a multiple, as shown in Figure 1-3.
Popigai – a chain of craters?
Popigai (φ = 71° 39’ N, λ = 111° 11’ E) is a very large impact structure, (diameter
about 100 km, age only 36 My) located in Siberia near a seashore, on an old Siberian
platform. Popigai is the best example yet of the formation of an impact crater of this
type visible on the surface. Three other craters are larger, but they are either buried
(Chicxulub), strongly deformed (Sudbury), or deformed and severely eroded
(Vredefort). The shock pressures from the impact instantaneously transformed graphite
in the ground into diamonds within a 13.6 km radius of ground zero. Coesit and
stishotive are also present (www.mines.edu/academic/geology/faculty/klee).
Figures 4 and 5 show ∆g and Tzz for the Popigai area. We think EGM 08 clearly
reveals more craters, Popigai I, II, III, and may be IV oriented along a NW-SE line. Our
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hypothesis is that the Popigai structure is a chain of impact craters (such are known on
the Moon but not yet on the Earth).
Concluding remarks
The intriguing hint of a tendency to “double craters” on the Earth may interest
astronomers to explain it due either to the disintegration of the impactor near the Earth
(from internal weakness under the forces of atmospheric drag) or alternately due to the
prevalence of asteroids (impactors) traveling in tandem or close groups.
Our search is just beginning. It is not yet a systematic screening of the whole globe for
‘circular’ gravitational and (where present) corresponding topographic features. We
believe that we can find further candidates for impact craters, some hidden under the
land surface or sea bottom. Of course such candidates must then be subjected to further
on-site inspection by geologists to confirm their origin.
Acknowledgements
We are grateful to Jianling Huang and Nikolaos K. Pavlis from the IAG Evaluation
team for EGM 08 harmonic geopotential coefficients and for various consultations.
This work has been supported by ESA PECS grant # C 98056. We also thank our Czech
colleagues David Vokrouhlický, Petr Pravec, Václav Cílek, Ivanka Charvátová and
others.
References
Hildebrand, A. R., Pilkington, M., Connors, M., Ortiz-Aleman, C. & Chavez, R. E.
(1995), Size and structure of the Chicxulub crater revealed by horizontal gravity
gradients and cenotes. Nature, 376, 415-417
Hildebrandt, A.R. et al. (1998), Mapping Chicxulub crater structure with gravity and
seismic reflection data, In: Grady, M.M. et al. (eds) Meteorites: Flux with Time and
Impact Effects, Geolog. Soc. London, Spec. Publs, 140, 15-176.
Klokočník J., Novák P., Pešek I., Kostelecký J., Wagner C.A. (2008), EGM 08: Tests of
the model and simulations for GOCE. Presented at IAG Int. Symp. GGEO 2008, 23-27
June 2008, Chania, Crete, Greece.
Sharpton, V. L., and 9 others (1993), Chicxulub multiring impact basin: Size and other
characteristics derived from gravity analysis. Science, 261, 1564-1567.
Bottke, W.F., Vokrouhlický, D., Nesvorný, D. (2007), An asteroid breakup 160 Myr
ago as the probable source of the K/T impactor, Nature 449, 48-53.
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Pavlis N.K., Holmes S.A., Kenyon S.C., Factor J.K. (2008a), An Earth Gravitational
Model to Degree 2160: EGM 2008, presented at Session G3: "GRACE Science
Applications", EGU Vienna.
Pavlis N.K., Holmes S.A., Kenyon S.C., Factor J.K. (2008b), EGM2008: An Overview
of its Development and Evaluation. Presented at IAG Int. Symp. GGEO 2008, 23-27
June 2008, Chania, Crete, Greece.
Lemoine F., et al. (1998), The Development of the Joint NASA GSFC and the NIMA
Geopotential Model EGM 96, NASA/TP-1998-206861, NASA GSFC Greenbelt.
Holmes, S.A., et al. (2006),. A FORTRAN PROGRAM FOR VERY-HIGH-DEGREE
HARMONIC SYNTHESIS (HARMONIC_SYNTH), version 05/01/2006.
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Fig. 1. The gravity
anomalies ∆g [mGal]
based on complete
EGM 08 model for
the area of Chicxulub.
Possible double crater
(see the arrow)
Fig. 2. Second derivatives Tzz computed by complete EGM 08
in the area of Chixculub; scale [mE].
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Fig. 3. Second derivatives as in Figure 2, Tzz [mE], in parts of Mexico, Belize and
Guatemala.
8
Figure 4.
The gravity anomalies ∆g
[mGal] modelled by
complete EGM 08
at Popigai, Siberia, Russia.
Figure. 5.
The second radial derivative
Tzz [mE] using the complete EGM 08 at
Popigai, Siberia, Russia.
Not only one crater is seen, but more
circular-like structures, candidates for
impact craters, less pronounced than the
original crater, but visible in the SE
direction from the original one. We label
the structures as Popigai I, II, III, and IV.
THE END