gravity at the surface, which indicate the density of the underlying rock. Knowledge of both sound velocity and density help todistinguish between different rock types.

The new analyses2 confirm the previouspicture and add some notable new featuresto it. The first is a low-velocity structure nearthe crater’s surface that is likely to be a disk of frothy impact melt mixed with brokenfragments of bedrock. The disk is about 1 kmthick and 80–100 km in diameter. It probablydoes not represent all the melt rocks lying in the crater: undetected denser melt mightlie deeper near the centre. The data alsoreveal a stratigraphic uplift in the crater’scentre that varies between 9 and 18 km, infair agreement with the expected uplift of 18–20 km for a crater of this size.

The most important new data concern the deepest part of the structure, the crust–mantle boundary, or Moho. The refractiondata show that the Moho is distorted beneaththe crater (Fig. 1). It rises about 1 km above its regional depth of 34 km right under thecentre, then sinks about 1.25–1.5 km deeperthan average at a distance of about 35 kmfrom the centre. This deepening coincideswith the projected trace of the outer ring fault and could reflect inward slip on the fault and flow of the deeper portions of the cratertowards the centre. This is the first time that a change in crustal thickness has been foundbeneath a terrestrial crater.

It might seem surprising that the Mohoshows such a tiny perturbation. The craterstructure requires that the original, or tran-sient, crater that collapsed to form the presentstructure was about 100 km in diameter. Atransient crater this large has a depth of about30 km, so shortly after the impact (a few minutes) the crater floor was almost at thedepth of the pre-impact Moho. If the craterwent nearly all the way through the crust, why is the Moho now nearly where it wasbefore the impact? This apparent conun-drum occurs frequently in discussions aboutcratering, but it has a simple solution.

Although the transient crater was briefly30 km deep, the material actually excavatedfrom the crust only extended to about 10 kmdepth. The material beneath the transientcrater, about 20 km of crust, was simplypushed down under the crater floor, notexcavated. When the crater collapsed thismaterial rebounded, the 10-km-deep exca-vation cavity was mostly filled in by the lateral inflow of rocky debris, and the Mohoresumed nearly its old position. At the same time, layers less deep than the Mohosquirted higher beneath the crater floor,finally producing the observed 9–18 km ofuplift at these higher levels.

Strange as this train of events might seem,it was first observed in small-scale, explosion-cratering experiments performed in a centri-fuge to increase the force of gravity. At firstthe experimenters believed that the apparent

lack of uplift of layers deep beneath the craterindicated that the crater did not go through adeep transient stage5. But later experiments,using photography to follow crater exca-vation and collapse, showed that the deeplayers were indeed first pushed down butthen nearly recovered their initial position6.Such continuity of layers deep beneath animpact crater (but shallower than the tran-sient crater) is also displayed on seismic profiles of the 3-km-diameter Steinheimcrater in Germany7.

At Chicxulub a small, but measurable,mantle uplift occurs beneath the crater. Thisis the only crater on Earth that shows such afeature. If Sudbury or Vredefort once hadmantle uplifts, flow in the lower crust andupper mantle has since erased them. Look-ing out into the Solar System, the largestcrater on Venus — Mead, 235 km in diameter— is associated with a negative gravityanomaly that rules out any substantial man-tle uplift beneath it8. In contrast, the muchlarger basins on the Moon not only possessmantle uplifts, but some of these uplifts arehigher than necessary to compensate for the excavation of material from the overlyingcrust9. Collapse of these monster craters wasapparently so vigorous that the lunar mantlewas caught up in the rebounding centralpeak and produced a positive gravity anom-aly in the crater centre.

Chicxulub’s status as the best-preservedlarge impact crater on Earth is due to its pro-tection, like that of an Egyptian mummy, bythick wrappings — in this case, of sedimen-tary material. As with many mummies, itsinterior is being revealed by modern tomo-graphic reconstructions. In the future we can look for further insights into Chicxulub, not only from improved seismic imagingmethods but also from the deep drilling project, organized by the International Continental Scientific Drilling Program, that commenced operations earlier thismonth at a site named Yaxcopoil-1. With this knowledge we will be better placed tointerpret what lies beneath the facial scars on our fellow planets. ■

Jay Melosh is in the Lunar and Planetary Lab-West,University of Arizona, 1040 East Fourth Street,Tucson, Arizona 85721-0077, USA.e-mail: jmelosh@lpl.arizona.edu1. Grieve, R. & Therriault, A. Annu. Rev. Earth Planet. Sci. 28,

305–338 (2000).

2. Christeson, G. L., Nakamura, Y., Buffler, R. T., Morgan, J. &

Warner, M. J. Geophys. Res. 106, 21751–21769 (2001).

3. Penfield, G. T. & Camargo, Z. Tech. Prog., Abstr. Biogr., Soc.

Exploration Geophysicists 51st Annu. Int. Mtg 37 (1981).

4. Hildebrand, A. R. et al. Geology 19, 867–871 (1991).

5. Schmidt, R. M. & Holsapple, K. A. Lunar Planet. Sci. Conf.

(Abstr.) XII, 934–936 (1981).

6. Schmidt, R. M. & Housen, K. R. Int. J. Impact Eng. 5, 543–560

(1987).

7. Reiff, W. Guidebook to the Steinheim Basin Impact Crater

(Geologisches Landesamt Baden-Württemberg, Steinheim am

Albuch, 1979).

8. Banerdt, W. B. et al. Icarus 112, 117–129 (1994).

9. Hood, L. L. & Zuber, M. T. in Origin of the Earth and Moon

(eds Canup, R. M. & Righter, K.) 397–409 (Univ. Arizona Press,

Tucson, 2000).

news and views

862 NATURE | VOL 414 | 20/27 DECEMBER 2001 | www.nature.com

Daedalus

Away with oxygen!The air-filled pneumatic rubber tyre hasrevolutionized transport, but is sadlyvulnerable to oxygen. When the polymeris oxidized it becomes inelastic, and itssurface wears. Vast numbers of wornrubber tyres are thrown out, and new onesbought. Piles of discarded tyres disfigurethe landscape, are thrown into the sea as‘artificial reefs’ for fish, or are destroyed by burning. Indeed, air defines the wholeproblem. Tyres run in air, which is 20%oxygen, and are inflated with it.DREADCO chemists now propose toinflate a new tyre with carbon dioxide.This gas is significantly soluble in rubber.It should saturate the polymer, keeping itelastic by displacing the troublesomeoxygen. In its slow leak outwards, it willsweep away external oxygen. A cartridgeinside will accept liquid carbon dioxide at a pressure of 70 atmospheres, and keep the tyre at its typical 2 atmospheres. It will maintain gas throughput and keep the tyre taut and perfect.

Marketing will be difficult. Moderntyres need little attention (Daedalus greatlyadmires the advertisement in which theMichelin Man in ‘Showboat’ dress sings“Ol’ Man Rubber, he just keeps rolling”).And non-leaky butyl rubber has eliminatedthe tradition of having your tyres pumpedup whenever you drive in for more petrol.But smart garages could use carbondioxide, possibly spiked with oxygen-scavenging additives such as nitric oxide, toinflate tyres. Motorists might reckon thatmore frequent pumping outweighed thehuge cost of changing worn tyres. The idealgas should save enough rubber to satisfythe market, and solve the terrible problemof endless worn tyres. Furthermore, a reallyimportant use for carbon dioxide would domuch to counter our fears about releasingthis greenhouse gas.

Rubber is also worn on shoes (punintended). Some training shoes have agas-blown sole for easier running. Apneumatic shoe pumped up with carbondioxide could give a very easy ‘ride’. Wemight even get rid of laces, a dreadfulinvention, by designing shoes thatbecome taut as they are inflated. Now that rubber has replaced leather as a solematerial, a wear-proof rubber sole andcan-pumped shoe could be attractive.Even better, a small shoe might withstandmuch more pressure than a big tyre.Narrow carbon dioxide channels couldhold the gas in almost liquid state underhigh pressure, combining long life with acomfortable ride. But such high pressuresmight make wearers nervous. David Jones

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