Collisions in the Solar System - IV. Cometary impacts upon the planets

The collision probability with each of the planets is calculated for all known comets, and hence the mean collision probability for each planet is determined; from the results the mean cratering rates may be estimated if it is assumed that the presently observed samples are diagnostic of the long-term populations. The mean collision probability with the Earth for the short-period comets is |~ 1.1 × 10⁻⁹\,yr⁻¹|⁠; relative to this the values for each of the planets are: Mercury, 0.6; Venus, 1.6; Mars, 0.1; Jupiter, 800; Saturn, 8.5; Uranus and Neptune, 0.01. If there are ~ 800 Earth-crossing short-period comets capable of producing craters larger than 10 km in diameter, then about one such body per 1.1 Myr hits our planet (i.e. one crater on land per 3 Myr). The mean collision probability with the Earth for the 613 known long-period comets is 3.3 × 10–9 per perihelion passage, somewhat higher than the value of 2.2 – 10–9 which results from theoretical distributions based upon the expected influx from the Oort Cloud. Relative to the Earth, the collision probabilities are: Mercury, 0.16; Venus, 0.7; Mars, 0.15; Jupiter, 160; Saturn, 40; Uranus and Neptune, 3. If about 133 long-period comets of q<1 AU and of sufficient size to produce 10 km craters pass perihelion each year then one such body would hit the Earth every 2.3 Myr. The production rate of such scars on land is therefore about 1 per 7 Myr by long-period comets only; the overall production rate (due to all comets and asteroids) is 15 to 20 times higher. The creation of large craters on the terrestrial planets is due to asteroids, short-period comets and long-period comets respectively in the following ratios: Mercury (42.5, 42.5, 15 per cent); Venus (70, 23, 7 per cent); Earth (82, 12, 6 per cent); Mars (94, 4, 2 per cent). In order to derive the actual impact rate with the giant planets, a much improved knowledge of the distribution of minor bodies in the outer solar system is required.