Comets and comet-derived bodies

As we have noted in Chapter 1, the physical properties of comets are not at all well known (Lewis, 1996). Halley's comet in its most recent passage was the only one to have been studied by a 'fly-by' mission. From the studies of gases and dust emitted by comets as they pass close to the Sun, it is inferred that this particular comet contains fine-grain dirt and ice in approximately equal mass. However, it is thought that the density of this material varies from comet to comet. Some appear to contain at least 90 per cent of void space. Others are, possibly, firmly compacted and have a density of 1.5-2.0. These may be comprised of compressed permafrost with dirt and ice.

Comets break up even under relatively low gravitational disruptive forces. However, this disintegration may represent the parting of loosely bonded units of much stronger, dense 'permafrost' blocks of dust and ice (and possibly solid rock). When they enter the Earth's atmosphere, small units of cometary material are easily destroyed, so that only the more dense and stronger, larger, permafrost blocks are likely to penetrate nearer than 50 km from the Earth's surface.

As we have noted earlier, it is now held by those studying impact events that any impact crater with a diameter of more than 100 km is almost certainly attributable to meteor or meteor-derived bodies. Let us, therefore, assume that the conceptual model shown in Figure 7.38 and discussed briefly in Chapter 6, represents a large comet or comet-derived impacting body, which, in space, has a spherical form, with a diameter of several tens of kilometres. If much, or all, of the comet's carapace has been removed in multiple passages past the Sun, the remaining core could be composed of relatively strong chunks of permafrost which are only loosely bonded one to another. Provided the average dimension of the permafrost chunks is over 250 m in diameter, even such chunks that become disrupted from the original body would survive passage through the atmosphere. Though reduced in size, such bodies, on impact, could still measure about 150 to 200 m across.

The largest (non-giant size) comets, or comet-derived bodies, have a diameter of about 40 km (Taylor, 1992). Such a large cometary body could contain as many as 2,000,000 chunks of permafrost with an average diameter of 250 m. Even if the number of bodies of this size are as few as 200,000, this is a more complex situation than is modelled by a shotgun charge.

Let us first consider a relatively small comet with an initial diameter of 5 km and also assume that this comet splashes down in deep ocean. Then, even if the leading large permafrost bodies of this comet are reduced to a diameter of 150-200 m, they will still penetrate the water to a depth of 1.5-2.0 km before their speed can be reduced to near their terminal velocity in water. However, this situation would not arise. The earliest 'wave' of these 150- 200 m diameter blocks to hit the ocean's surface would disrupt the water by their individual shock waves, which would be so close to one another that they would quickly combine to push aside the upper levels of the ocean. In so doing, the first arrivals would experience a rapid reduction in their velocity, and would also begin to spread laterally one from another, as the oceanic water was either blown away in a radial direction, or burst through between these first arrivals as jets and fountains of water. Thus, the first arrivals would create a wide but shallow 'crater' in the water of the ocean.

The second wave of cometary blocks would not encounter the surface limits of the 'crater' of water until they were perhaps 0.5-1.0 km below the original oceanic surface, and would therefore maintain the impact a b

5 km

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