Xkm

Vol 77krri

5 km

Figure 5.39 (a) Schematic representation of a 5 km diameter ice-free comet travelling at velocity V0 which may be of the order of 30 km s-1. The body is likely to have an initial porosity of about 30 per cent and a volume of about 65 km3. (b) The possible change of sectional form as the comet enters the Earth's atmosphere and changes from a sphere to a flattened front with trailing elements containing winnowed and incandescent smaller particles. The body approximates to an oblate spheroid, perhaps 6.5 km in diameter and 3.5 km front to back, with a volume of about 77 km3. The increase in volume can be attributed to spreading of the major rock units because of atmospheric resistance. The 'continental' impact velocity (l\) may be significantly less than 10 km s_1.

Figure 5.39c If the comet impacts in the ocean, the increase in resistance of the ocean is likely to cause a further significant flattening and broadening of the impacting ice-free comet, causing the body to have a width of perhaps 10 km and a front-to-back thickness of only 2 km, so that the volume of the oblate spheroid increases to about 100 km3. (SL=sea level, SF=sea floor, WJ=water (or steam) jet.)

then the ends of this line are approximately 2 km above the first impact point. If we assume the impact velocity to be 5 km s-1, the impact at X and X will occur at about 0.4 seconds after the initial contact at GZ. This, we suggest, makes Snowball a reasonable representation of a cometary impact.

The rate of slowing of a body in the Earth's atmosphere will mainly be determined by the mass and velocity of the body and its cross-sectional area. For a 5 km diameter stony meteorite travelling at an initial velocity of 15 km s-1, the ratio of the momentum to air-resistance will be relatively large. However, if a block 100 m in diameter, which forms part of a de-iced comet, enters the atmosphere, the ratio of momentum to resistance is very much smaller, so that this and comparable components of the meteor will tend to reduce their velocity quite appreciably.

Indeed, one can specify minimum dimensions of ice, stony and iron bodies that are able to penetrate the Earth's atmosphere and reach the surface. Thus, an ice body must have an initial diameter in excess of about 150 m, while stony and iron meteorites require diameters respectively in excess of 60 m and 20 m.

A solid, stony meteorite, with a diameter of about 5 km, travelling at a velocity of about 15 km s-1, will generally survive this passage through the Earth's atmosphere and experience relatively little diminution in its dimensions, so is likely to give rise to a crater with a diameter of about 40 km.

In contrast, the core of a de-iced comet with an initial diameter of 5 km and entering the Earth's atmosphere at 50 km s-1, because it is comprised of relatively small elements, would be significantly

Figure 5.39 (a) Schematic representation of a 5 km diameter ice-free comet travelling at velocity V0 which may be of the order of 30 km s-1. The body is likely to have an initial porosity of about 30 per cent and a volume of about 65 km3. (b) The possible change of sectional form as the comet enters the Earth's atmosphere and changes from a sphere to a flattened front with trailing elements containing winnowed and incandescent smaller particles. The body approximates to an oblate spheroid, perhaps 6.5 km in diameter and 3.5 km front to back, with a volume of about 77 km3. The increase in volume can be attributed to spreading of the major rock units because of atmospheric resistance. The 'continental' impact velocity (l\) may be significantly less than 10 km s_1.

i Vol "lOOkrrr

10 km

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