The Earth suffered a double blow 65 million years ago. Not only was Mexico's Yucatan peninsula hit by a 10-kilometre diameter asteroid, but the rock hit at the worst possible angle, claim two American researchers. This may explain why the impact was so devastating, wiping out the dinosaurs. In paper to be published next month in the journal Geology, Peter Schultz of Brown Uni-versity in Providence, Rhode Island, and Steven D'Hondt of the University of Rhode Island argue that the killer asteroid struck the Yucatan from an angle of 20 degrees to 30B0 above the southeast horizon, splattering hot debris towards the northwest. Western North America suffered the worst devastation, but the effects were global.

The angle of impact is crucial, say Schultz and D'Hondt, because it affects the distribution of impact energy. A head-on impact concentrates energy downwards into the target rock, but a shallower blow scatters debris along the object's path.

Schultz argues that the Yucatan asteroid would have ejected a cloud of vaporised and molten rock that quickly engulfed the atmosphere over western North America, instantly wiping out any living things in its path. Over subsequent months, dust and debris in the upper atmosphere would have cooled the climate across the globe, finishing off the dinosaurs.

Most craters on the Earth and other planets are circular. But Schultz and D'Hondt claim that the Yucatan peninsula impact crater, long since buried, is elongated. They argue that distortions in the Earth's gravitational field around the impact site are caused by the compression and ejection of material when the asteroid struck. Contour lines in gravity data form concentric arcs to the southeast of the impact site, but to the northwest some contours fan outwards.

This mirrors the pattern of debris seen around asymmetric craters on the Moon. Experiments simulating shallow-angle impacts at NASA's Ames Research Center in California, in which a high-speed gun fires projectiles at rock, give a similar result.

Schultz and D'Hondt also note that quartz crystals containing shock flaws that are indicative of asteroid impacts, found in sediments dating to 65 million years ago, tend to be larger in North America than in Europe and the Pacific. Larger fragments fall to Earth first, which fits with the idea that debris was scattered to the northwest of the impact zone.

Schultz adds that the discovery of what seems to be a piece of the killer asteroid earlier this year fits this pattern, because the pebble came from a seabed core in the North Pacific, along the path of the debris.

Some scientists doubt that it is possible to determine the angle of impact, however. Richard Grieve of the Geological Survey of Canada in Ottawa thinks that the gravity contours could reflect irregularities that were present before the asteroid hit. He adds that the shocked quartz crystals are not conclusive, because no one has analysed similar crystals from South America. But D'Hondt also points to pollen data. These show that ferns suddenly proliferated just after the impact, presumably after spores colonised newly exposed ground. This 'fern spike' and extinctions of plants are strongest in western North America-as expected if low-flying debris ignited the region's flora. 'For me, this is the compelling thing,' says D'Hondt.

Material that was deposited rapidly from this low debris cloud could explain why the sediments in North America thought to be linked to the impact seem to be in two layers. The first layer would correspond to material from this cloud; the second to dust that flew high in the atmosphere.

New Scientist, 12 Oct 96, Volume 152, Issue 2051