Alan R. Hildebrand

Chicxulub Crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico

We suggest that a buried 180-km-diameter circular structure on the Yucatan Peninsula, Mexico, is an impact crater. Its size and shape are revealed by magnetic and gravity-field anomalies, as well as by oil wells drilled inside and near the structure. The stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with, and overlain by, breccias that contain evidence of shock metamorphism. The andesitic rocks have chemical and isotopic compositions similar to those of tektites found in Cretaceous/Tertiary (K/T) ejecta. A 90-m-thick K/T boundary breccia, also containing evidence of shock metamorphism, is present 50 km outside the craters edge. This breccia probably represents the craters ejecta blanket. The age of the crater is not precisely known, but a K/T boundary age is indicated. Because the crater is in a thick carbonate sequence, shock-produced CO2 from the impact may have caused a severe greenhouse warming.

Size and morphology of the Chicxulub impact crater

The Chicxulub impact in Mexico has been linked to the mass extinction of species at the end of the Cretaceous period. From seismic data collected across the offshore portion of the impact crater, the diameter of the transient cavity is determined to be about 100 km. This parameter is critical for constraining impact-related effects on the Cretaceous environment, with previous estimates of the cavity diameter spanning an order of magnitude in impact energy. The offshore seismic data indicate that the Chicxulub crater has a multi-ring basin morphology, similar to large impact structures observed on other planets, such as Venus.

Proximal Cretaceous-Tertiary Boundary Impact Deposits in the Caribbean

Trace element, isotopic, and mineralogic studies indicate that the proposed impact at the Cretaceous-Tertiary (K-T) boundary occurred in an ocean basin, although a minor component of continental material is required. The size and abundance of shocked minerals and the restricted geographic occurrence of the ejecta layer and impact-wave deposits suggest an impact between the Americas. Coarse boundary sediments at sites 151 and 153 in the Colombian Basin and 5- to 450-meter-thick boundary sediments in Cuba may be deposits of a giant wave produced by a nearby oceanic impact. On the southern peninsula of Haiti, a ∼50-centimeter-thick ejecta layer occurs at the K-T boundary. This ejecta layer is ∼25 times as thick as that at any known K-T site and suggests an impact site within ∼1000 kilometers. Seismic reflection profiles suggest that a buried ∼300-km-diameter candidate structure occurs in the Colombian Basin.

Proximal impact deposits at the Cretaceous-Tertiary boundary in the Gulf of Mexico: A restudy of DSDP Leg 77 Sites 536 and 540

Restudy of Deep Sea Drilling Project Sites 536 and 540 in the southeast Gulf of Mexico gives evidence for a giant wave at Cretaceous-Tertiary boundary time. Five units are recognized: (1) Cenomanian limestone underlies a hiatus in which the five highest Cretaceous stages are missing, possibly because of catastrophic K-T erosion. (2) Pebbly mudstone, 45 m thick, represents a submarine landslide possibly of K-T age. (3) Current-bedded sandstone, more than 2.5 m thick, contains anomalous iridium, tektite glass, and shocked quartz; it is interpreted as ejecta from a nearby impact crater, reworked on the deep-sea floor by the resulting tsunami. (4) A 50-cm interval of calcareous mudstone containing small Cretaceous planktic foraminifera and the Ir peak is interpreted as the silt-size fraction of the Cretaceous material suspended by the impact-generated wave. (5) Calcareous mudstone with basal Tertiary forams and the uppermost tail of the Ir anomaly overlies the disturbed interval, dating the impact and wave event as K-T boundary age. Like Beloc in Haiti and Mimbral in Mexico, Sites 536 and 540 are consistent with a large K-T age impact at the nearby Chicxulub crater.

Gravity and magnetic field modeling and structure of the Chicxulub Crater, Mexico

The Chicxulub crater is an ∼180-km-diameter peak-ring crater based on drill hole logs and samples, potential fields, seismic reflection profiles, and surface fracture patterns. A structural cross section produced based on these constraints has the features expected for a large complex impact crater. The Bouguer-gravity anomaly consists of a broad ∼90-km radius, ∼30-mGal low with a central ∼20-km radius, ∼20-mGal high and two <5-mGal concentric lows at ∼35- and ∼60-km radius. The gravity anomaly is disrupted by large-scale basement anomalies and possibly by large-scale slumping and backwash erosion effects. The magnetic field anomaly over the crater consists of three zones, an outer zone from ∼45- to ∼90-km radius of low-amplitude, short-wavelength anomalies with an irregular perimeter, a middle zone from ∼20- to ∼45-km radius of high-amplitude, short-wavelength anomalies slightly elongated NNW-SSE, and an inner ∼20-km-radius single large-amplitude anomaly. Magnetic field modeling indicates that the melt pool averages ∼90 km in diameter and the melt volume in the crater is estimated at ∼20,000 km3. The melt pool size constrains the collapsed transient cavity diameter to ∼90 km. Gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but ∼40 km in diameter and underlies or protrudes into the melt pool. The preliminary structural cross section indicates that the inferred peak-ring is decoupled from the structural uplift. The geometry and Bouguer gravity signature of the crater indicate that no significant uplift of the Moho or relaxation of the crater has occurred.

Mapping Chicxulub crater structure with gravity and seismic reflection data

Abstract Aside from its significance in establishing the impact-mass extinction paradigm, the Chicxulub crater will probably come to exemplify the structure of large complex craters. Much of Chicxulub’s structure may be ‘mapped’ by tying its gravity expression to seismic-reflection profiles revealing an ∼180 km diameter for the now-buried crater. The distribution of karst topography aids in outlining the peripheral crater structure as also revealed by the horizontal gradient of the gravity anomaly. The fracturing inferred to control groundwater flow is apparently related to subsidence of the crater fill. Modelling the crater’s gravity expression based on a schematic structural model reveals that the crater fill is also responsible for the majority of the negative anomaly. The crater’s melt sheet and central structural uplift are the other significant contributors to its gravity expression. The Chicxulub impact released ∼1.2 × 1031 ergs based on the observed collapsed disruption cavity of ∼86 km diameter reconstructed to an apparent disruption cavity (Dad) of ∼94 km diameter (equivalent to the excavation cavity) and an apparent transient cavity (Dat) of ∼80 km diameter. This impact energy, together with the observed ∼2 × 1011 g global Ir fluence in the Cretaceous-Tertiary (K-T) fireball layer indicates that the impactor was a comet estimated as massing ∼1.8 × 1018 g of ∼16.5 km diameter assuming a 0.6 gcm−3 density. Dust-induced darkness and cold, wind, giant waves, thermal pulses from the impact fireball and re-entering ejecta, acid rain, ozone-layer depletion, cooling from stratospheric aerosols, H2O greenhouse, CO2 greenhouse, poisons and mutagens, and oscillatory climate have been proposed as deleterious environmental effects of the Chicxulub impact with durations ranging from a few minutes to a million years. This succession of effects defines a temperature curve that is characteristic of large impacts. Although some patterns may be recognized in the K-T extinctions, and the survivorship rules changed across the boundary, relating specific environmental effects to species’ extinctions is not yet possible. Geochemical records across the boundary support the occurrence a prompt thermal pulse, acid rain and a ∼5000 year-long greenhouse. The period of extinctions seems to extend into the earliest Tertiary.

Fireball passes and nothing burns¿The role of thermal radiation in the Cretaceous-Tertiary event: Evidence from the charcoal record of North America

High soot contents have been reported in Cretaceous-Tertiary (K-T) sedimentary rocks, leading to the suggestion that the amount of thermal power delivered from the Chicxulub impact was sufficient to have ignited wildfires. Soot cannot be used to indicate fire location, however, as soot from one large fire could spread globally. Sources other than biomass burning could also yield soot. Charcoal in nonmarine sedimentary rocks (here quantified in situ in polished blocks) provides a unique tool to record the distribution of wildfires and therefore assess the extent of any thermal radiation associated with the impact at Chicxulub. The K-T and lowermost Tertiary sedimentary rocks of six nonmarine sequences (Colorado to Saskatchewan) contain no charcoal or below-background levels of charcoal and a significant quantity of noncharred organic materials, revealing that there was no distinctive wildfire across the North American continent related to the K-T event. This finding indicates that the K-T impact cannot have delivered a peak irradiance of >95 kW.m - 2 of thermal power to the atmosphere and <19 kW.m - 2 to the ground. Therefore, the thermal power delivered from the impact to North America did not have the destructive potential previously predicted. High amounts of thermal radiation were not responsible for the environmental perturbations or extinctions associated with the K-T event.

Three-dimensional magnetic imaging of the Chicxulub Crater

Although few magnetization measurements are available for the structural elements of the Chicxulub impact crater, the magnetization intensities of the melt sheet, upper breccia unit, and central uplift are 3–4 orders of magnitude greater than the 3- to 4-km-thick carbonate and evaporite stratigraphy covering the Yucatan block. This allows three-dimensional modeling of the craters structure by inversion using a two-layer model. Two layers are separately inverted by dividing the craters magnetic field expression into 40-km wavelength components. The upper layer (average depth 2 km) models the distribution of highly-magnetized zones in the craters melt sheet. The lower layer (average depth 5 km) represents relief on the Yucatan blocks basement surface and effectively maps the craters ∼50-km-diameter central uplift and possibly the expression of the surrounding collapsed disruption cavity fill. The shallower magnetized zones consist of two generally concentric distributions, at radii of ∼20 and ∼45 km. These highly magnetized zones are thought to result from hydrothermal systems, localized at the edge of the central uplift and the collapsed disruption cavity, having produced magnetic phases during alteration of the melt sheet.

NEOSSat: a Canadian small space telescope for near Earth asteroid detection

Although there is some success in finding Near Earth asteroids from ground-based telescopes, there is a marked advantage in performing the search from space. The ability to search at closer elongations from the sun and being able to observe continuously, allowing quick revisits of new asteroids, are some of the unique benefits of a space platform. The Canadian Space Agency (CSA) together with Defense Research and Development Canada (DRDC) are planning a micro-satellite platform with a 15 cm telescope dedicated for near space surveillance. The NEOSSat (Near Earth Object Surveillance) spacecraft is expected to be able to detect 20 v magnitude objects with a 100 sec exposure, with a 0.85 deg FOV, on a 1024x1024 CCD, and sub arcsec pointing stability. For detection of NEO small bodies, it will be able to search an area from 45 degrees solar elongation and approximately 40 degrees north to south degrees in elevation. The observation strategy will be optimized to find as many asteroids as possible, based on recent models of asteroid population. Ground based telescopes will also be used to complement follow-ups for orbit determination when possible. The microsatellite is based on the CSA very successful MOST micro-satellite, operating since 2003. Baselined for launch in 2010, the NEOSSat is a shared project with DRDC to demonstrate the technology of an inexpensive space platform to detect High Earth Orbit (HEOSS) earth-orbiting satellites and debris.

Surface Heating from Remote Sensing of the Hypervelocity Entry of the NASA GENESIS Sample Return Capsule

An instrumented aircraft and ground-based observing campaign was mounted to measure the radiation from the hypervelocity (11.0 km/s) reentry of the Genesis Sample Return Capsule prior to landing on the Utah Test and Training Range on September 08, 2004. The goal was to validate predictions of surface heating, the physical conditions in the shock layer, and the amount and nature of gaseous and solid ablation products as a function of altitude. This was the first hypervelocity reentry of a NASA spacecraft since the Apollo era. Estimates of anticipated emissions were made. Erroneous pointing instructions prevented us from acquiring spectroscopic data, but staring instruments measured broadband photometric and acoustic information. A surface-averaged brightness temperature was derived as a function of altitude. From this, we conclude that the observed optical emissions were consistent with most of the emitted light originating from a gray body continuum, but with a surface averaged temperature of 570 K less than our estimate from the predicted heat flux. Also, the surface remained warm longer than expected. We surmise that this is on account of conduction into the heat shield material, ablative cooling, and finite-rate wall catalycity. Preparations are underway to observe a second hypervelocity reentry (12.8 km/s) when the Stardust Sample Return Capsule returns to land at U.T.T.R. on January 15, 2006.