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 cratersas 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

1Astronomical 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

2Astronomical 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

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There are now about 170 impact meteoritic craters known on the Earth [http://en.wiki-pedia.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 21 0

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 Geospatial-Intelligence 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 hypothesis is that the Popigai structure is a chain of impact craters (such are known on the Moon but not yet on the Earth).

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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 Appli-cations", 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. 2. Second derivatives Tzz computed by complete EGM 08 in the area of Chixculub; scale [mE].

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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. 3. Second derivatives as in Figure 2, Tzz [mE], in parts of Mexico, Belize and Guatemala.

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THE END

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Figure. 5.

The second radial derivativeTzz [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 labelthe structures as Popigai I, II, III, and IV.

Figure 4.

The gravity anomalies ∆g [mGal] modelled by complete EGM 08 at Popigai, Siberia, Russia.