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The effects of atmosphere

When one looks at the Moon through even a low-powered telescope, one cannot but be impressed by the evidence of intense bombardment recorded on the surface of our satellite. The higher the degree of magnification used, the smaller are the diameters of craters that can be seen. Astronauts, from 'fly-by' and Moon landings, have recorded myriads of small craters with diameters of only a few metres. As the Moon is devoid of a significant atmosphere, erosion or covering of small craters will only result from subsequent, larger impacts, coverage by ejecta and the small degree of weathering that results from the monthly cycle of temperature changes. Many of the craters could have been in existence for 4000 Ma.

By comparison with the Moon, the cratering record of Earth is miniscule. This is in spite of the fact that the cross-sectional target area of Earth is 13 times greater than the Moon. Moreover, because of the much greater mass of Earth relative to that of its satellite, Earth should attract impacts by a further factor of about 20. Hence, Earth should have an impact record over 250 times greater than that of the Moon.

Unlike the Moon (see Figure 1.13), Earth has experienced extensive erosion and also widespread burial of older surfaces beneath extensive and sometimes thick sediments. In addition, if the impacts occurred in the ocean, obvious evidence of such impacts would not, in general, exist for more than about 150 Ma, for the oceanic lithosphere would be subducted, taking with it any evidence of impacts that may have existed. In addition, the Earth has a significant atmosphere (see Table 1.4). This atmosphere insures that smaller bodies will be heated to high temperatures and destroyed as it penetrates deeper and deeper into the atmosphere. Thus, the Earth has been saved from the massive bombardment of smaller missiles which continually take place on the Moon.

Figure 1.13 Photograph of the 'full Moon' revealing the intense development of cratering on its Earth-facing surface.
Table 1.4 Simple representation of the Earth's atmosphere.

Band

Height (km)

Atmosphere density (kg m3)

Temperature range (°K)

Exosphere

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