An artist's impression of the moment just before the impact 65 million years ago. Photo: NASA

Computer simulations have revealed that a vast region of the Yucatan Peninsula, Mexico, may have behaved like a fluid during the formation of the Chicxulub impact crater.

Gareth Collins, a postgraduate student in the TH Huxley School of the Environment, Earth Sciences and Engineering, Imperial College, announced the results of his computer modelling at the Geological Society of America annual meeting in Nevada on November 14. Mr Collins has used innovative computer modelling based on seismic data to analyse the distinctive complex crater formation at Chicxulub.

The Chicxulub crater is thought to have resulted from an impact with an asteroid or comet 65 million years ago. Many scientists believe that there is a link between the impact and the environmental change and mass extinction, including perhaps that of the dinosaurs, at the end of the Cretaceous period.

Gareth Collins explained, "Our understanding of complex crater formation and lack of a definitive model was limited by the absence of large pristine impact craters on Earth. This situation changed following the discovery of the buried Chicxulub impact structure in Mexico in 1990 which is the largest pristine crater known on Earth."

Mr Collins used the results from the Chicxulub Seismic Experiment, which provided new insight into the kinematics of complex crater formation, to design novel computer simulations of the Chicxulub crater.

The group at the Imperial College TH Huxley School hope that their study will further the understanding of crater collapse; the final stage in the impact crater formation process that produces the strange internal structures that characterize complex craters. Complex crater collapse has long been an enigmatic issue in planetary science.

Gareth Collins explained, "In order for the complicated internal structures to be produced that are observed at Chicxulub and many extra-terrestrial complex craters, the target material must behave as though it were a fluid."

"Of course the collapse process cannot be entirely hydrodynamic, as the end result would inevitably be a flat surface. Evidently, the fluid collapse must be frozen or suspended in some way to produce the observed complex crater morphologies. The mechanism driving this transient weakening, however, still remains a mystery - this phenomenon appears to violate current understanding of rock and debris mechanics."

The group at the TH Huxley School and their colleagues at the University of Arizona believe that one potential material weakening mechanism called Acoustic Fluidisation could come into action as the impact generated shock wave transforms the target into a sea of jostling granular material.

An artist's impression of the few days after the impact. Photo: NASA

Gareth Collins said, "We model the collapse stage of the cratering process which begins after the initial excavation of the cavity. Our simulations show that temporary weakening of the target by Acoustic Fluidisation allows the formation of internal peak and ring structures similar to those observed in terrestrial and extra-terrestrial craters. Our dynamic simulations of peak-ring formation at Chicxulub areremarkably consistent with observations from the seismic data."

His research group hopes to use this model for the generation of the peak-ring at Chicxulub to further their understandings of the geology of other cratered planets and satellites, such as Mercury, Venus and the Moon.

Dr Jo Morgan, Mr Collins' supervisor in the Geophysics Research Group, TH Huxley School, explained, "Improved understanding of large-impact crater formation will enable us to assess the environmental effects of such impacts and to determine whether this impact was the dominant force driving the mass extinction at the end of the Cretaceous period."