Impactor Population

The first step in analyzing cosmic impacts is to determine the flux of comets and asteroids striking the Earth. The average total impact flux over the past 3 Gyr can be found from the crater density on the lunar maria (Hartmann et al., 1986). Given the absence of significant erosion or geological activity since the end of widespread volcanism on the Moon, these lava plains are a scorecard for the integrated flux of cosmic debris in near-Earth space. With adjustments for the Earth's greater gravity, the average flux at the top of the Earth's atmosphere can be derived from the lunar flux (Zahnle and Sleep, this volume). We can also estimate the contemporary impact rate on the Earth from a census of existing Earth-crossing asteroids and comets together with estimates of their dynamical lifetimes (Shoemaker, 1983; Shoemaker et al., 1990; Bottke et al., 2002, Stuart & Binzel, 2004; Steel, (1997). As surveys reveal more of the present population of near-Earth objects, this latter approach (estimating the current impact flux) has gained in favor over the lunar approach.

The Earth-crossing asteroids are primarily rocky or metallic fragments of main-belt asteroids or extinct comet nuclei (Wetherill, 1988; 1989). In addition, there are impacts from active comets. The bulk properties of the projectiles thus range from metallic (like iron-nickel meteorites) to stony (like chondritic meteorites) to cometary (low-density silicates, organics, and volatiles). The lunar cratering history does not distinguish among stony, metallic, or volatile-rich projectiles, but population estimates based on astronomical observations do, since comets are distinguished by the presence of an extended atmosphere or coma visible in telescopic images.

From the known asteroid discovery statistics and the lunar crater size distribution of 20 years ago, Shoemaker (1983) derived the population and size distribution for Earth-crossing asteroids larger than a given diameter (see also Morrison, 1992). He concluded that long-period and short-period active comets added about 5% of the asteroid flux. However, comets average significantly greater impact velocities than asteroids and, therefore, constitute a larger share (at least 10%) of the impact hazard.

More recent work based on a much larger observed population of both comets and asteroids generally confirms the Shoemaker estimates for the larger comets, those with nucleus diameters greater than a few kilometers (Weiss-man, 1997). However, we now recognize a significant shortfall in the population of smaller comets, relative to the number expected from a power-law size distribution. Recent surveys have discovered no comets with diameters of less than 1 km, in spite of their high brightness relative to asteroids of the same size, and the comets imaged by spacecraft (Halley, Borrelly, and Wild 2) are all between 4 and 8 km in average dimension. The low population of small craters inferred for the Galilean satellites, especially Europa, also argues for a paucity of small comets. Current arguments for the population of comets in the km and subkm range are summarized by D. Yeomans in Chapter 2 "Population Estimates" in Stokes et al. (2003). Note that in recent population estimates, extinct (nonoutgassing) comet nuclei in Earth-approaching orbits, if they exist, are counted with the population of near-Earth Asteroids (NEAs).

Steel has discussed evidence for possibly significant variations in the impact flux, noting that comet "showers" could lead to major short-term increases in the impact hazard (Steel et al., 1994). However, the current impact rate as derived from the known population of Earth-crossing comets and asteroids is apparently similar to the long-term average flux recorded by the lunar craters, suggesting that we do not live in a special time with respect to possible large-scale variations. For the purposes of this evaluation of the current impact hazard, it is adequate to treat impacts as occurring at their long-term average rate. Figure 9.1 summarizes the average terrestrial impact flux as a function of projectile kinetic energy, measured in megatons (1 MT = 4.2 x 1015 J). Also shown for comparison is the associated diameter for a stony asteroid striking the Earth with a velocity of 20 km/s. Note the steep power-law dependence of size on energy, a relationship equally apparent when we look at the size distribution of small lunar craters (A. Harris, private communication, 2003; Morrison et al., 2003; see also Bottke et al., 2002; Stokes, 2003; Stuart and Binzel, 2004).

Diameter (km)

Fig. 9.1. Impactor diameter (and kinectic energy in megatons) vs. Average terrestrial impact flux (1 MT = 4.2 x 1015 J).

Diameter (km)

Fig. 9.1. Impactor diameter (and kinectic energy in megatons) vs. Average terrestrial impact flux (1 MT = 4.2 x 1015 J).

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