Footprints in the Dust: The Lunar Surface and Creationism
Perhaps the strongest, most appealing claim that the creationists have put forth against evolutionary timescales is the rate of infall in interplanetary dust into the terrestrial atmosphere. Frank Awbrey, in this journal, has detailed the arguments put forward in the creationist tracts on the age of the earth. There are, however, a few points which should be added, especially concerning the nature of the lunar surface, since it is also a useful lesson in the workings of scientific argument and the reasons for space exploration in the first place.
Measurements of the rate of infall of meteoritic material were first accomplished using the U-2 as a collection device, and by measuring the amount of contamination by interplanetary material in dust falling atop Mauna Loa.1 The rate which he got, now known to be about an order of magnitude too high, was the first serious attempt to derive the number density of interplanetary material by something other than the brightness of the Zodiacal light (the bright band of solar illuminated interplanetary dust which is a permanent resident feature of the solar system). The current rate, which is quoted by Hartmann2 in his recent review text on the solar system, is 108+1 kilograms per year for the Earth and about 4 x 106 kilograms per year for the Moon, the latter being a directly measurable quantity.
First, notice that the figure is considerably higher (and more uncertain) for the Earth. The reason is Newtonian gravitation—the accretion radius of the Earth is correspondingly larger due to its greater mass. The fact that the scaling works quite well is an indication that the measurement can be trusted. The second is that the rate of overall infall is in very good agreement with the cratering characteristics of the lunar material. I should explain this a bit more, since it is a powerful argument and one which the creationists have never bothered to worry about.
The surfaces of airless bodies are constantly pummeled by infalling debris. which, because there is no atmosphere to slow the infalling particles to terminal velocity, arrives at the surface at essentially the escape velocity. For the Earth, this would mean that the infall would arrive at about 10 kilometers per second. Even a piece of fine dust at this ballistic velocity would pit any rock, with a known "crater" to particle radius ratio. From counting the rate of production of craters on every piece of lunar material as a function of the size of the crater, it has been concluded that the surface of the Moon is literally saturated by the infall—the primary source of erosion on the surface is due to this debris infall. Now in order for this to occur, there must be a definite spectrum of infalling particles so that by averaging over the observed crater distribution, it is possible to calculate the total rate of infall of the dust and larger bodies and to compare this with the actual observed rate. The two agree very well.
In the case of Mercury, although we have not been able yet to set even a mechanical foot on the surface, we can still count the larger craters and observe that they derive from the same distribution as is responsible for the lunar surface morphology. If the cratering has been going on for only 10,000 years, as the more avid literalist creationists would have one believe, the rate of cratering must have been astonishingly high at an instant, or else we are measuring far too little infall at present, because the surface we see is in effect in equilibrium. This is absolutely impossible with the information that the creationists use. There is simply not enough time in their model to provide for the surface as we see it. In fast, it even leaves open a stronger violation of observation. If the Earth and everything else were as young as they claim, there should be many more large craters on the Earth's surface, since erosion would not have had sufficient opportunity to remove them on this atmosphere-dominated planet. Again, it is impossible to imagine under what conditions the laws of physics which we see in operation now could have been superseded in the so-recent past.
Well, never mind, they will say. The point is that the best calculations of the lunar surface indicated that there should be an enormous dust layer on the Moon into which the astronauts would sink—the very fact that the best evolutionary calculations gave an unambiguous answer that there should be several meters of dust on the surface which was subsequently not found should itself be an indication that the theory is clearly wrong.
Let me make one minor aside here, since as in all of the creationist arguments this one on the nature of the infall clearly points to a hidden agenda: "Since the scientific theory leads to some disagreement, and there can be no dissection in matters of truth, the scientific approach must be wrong and consequently misleading and useless. On the other hand, Genesis is the word of God which, being infallible, is an absolute meter-stick for measuring truth." One should never lose sight of this attack on the scientific method in all of the creationist writings. It may not be stated as such, but it always lurks beneath the surface.
Now back to our tale. In 1965, a conference was held on the nature of the lunar surface.3 The basic conclusion of this conference was that both from the optical properties of the scattering of sunlight observed from the Earth, and from the early Ranger photographs, there was no evidence for an extensive dust layer. In fact, some three years before we set foot on the moon, there was already hard evidence that the dust layer must be quite thin. The creationists would of course point to the fact that this was stated only after we had sent probes to the Moon. But wait, there is another source for the same statement. In a conference held in late 1963, on the Lunar Surface Layer, McCracken and Dubin4 state that
"The lunar surface layer thus formed would, therefore, consist of a mixture of lunar material and interplanetary material (primarily of cometary origin) from 10 cm to 1 m thick. The low value for the accretion rate for the small particles is not adequate to produce large scale dust erosion or to form deep layers of dust on the moon, for the flux has probably remained fairly constant during the past several billion years." (p. 204)
They also state that the rate of infall of material with masses smaller than 10 kg has been about 1 gram per square centimeter during the past 4.5 billion years, a result which is completely in agreement with the modem measurement. The most one could expect was a layer some 1 meter deep, not the norm of the predictions. All of the participants at the earlier conference agreed that the rate of infall should in fact be low. They also argued that the lunar surface should be eroded on a small scale, and that the jagged appearance hypothesised by the earlier ground-based observers should be modified to allow for the results of dust and solar wind impact. To be sure there were predictions of a deeper layer, notably by Gold using the erosion from the solar wind (assuming properties of the lunar material which have since been shown to be too fragile) and by Salisbury and Smalley5 in the 1963 conference, which suggested that a debris (rubble) layer could be an average of 60 cm thick. The essential point of these measurements from the ground was that we would know once the space probes reached the Moon.
The thing to keep in mind is that the predictions of the surface properties of the Moon were based on very questionable laboratory simulations and on quite preliminary data from terrestrial measurements. Using these numbers, the values which resulted from the theory of surface morphological changes could vary widely. The reasons for sending probes up was to determine what the rate of infall is, to achieve at least some hard numbers which everyone could agree on and use for subsequent predictions in their models.
It should also be mentioned that the fact that the surface cannot be very dusty was also known from the infrared observations6 of the moon during eclipse. The rate of cooling of a surface depends on the conductivity, or poricity, and therefore on whether it is dusty or not. The fact that craters on the surface stay hotter longer into the eclipse (totally improperly attributed by the creationists to the actual heat flow from the interior of the body) is a direct evidence that the floors of the craters are debris-strewn. That the lunar surface as a whole cools very quickly was already known as well years before we set foot on the surface, and was consistent with the layer of dust which was finally shown to be present.
In short, this addendum and expansion on Awbrey's article is simply meant to place the question of influx in its proper perspective. The number for the rate of meteoritic infall is important, but is at best a backwater in the far greater question of the origin and development of the solar system. The creationists have contributed nothing to this argument, and in fact have even argued among themselves about the value, and so we should simply leave this to the historians as an interesting sidelight in the race for the Moon, and go on with the business of planetary exploration. Our footprints have been left in the dust that was expected, and the question is settled.
1. Petterson, H. 1960, Scientific Amer., 202 (Nr. 2), 123.
2. Hartmann, W. K. 1983, Moon and Planets: 2nd Edition (CA, Wadsworth), p. 161 f.
3. Hess, W. N., Menzel, D. H. and O'Keefe, J.A. (eds.) 1966, The Nature of the Lunar Surface (Johns Hopkins Univ. Press). See especially p. 99 ff, p. 141 ff and p. 287 im passim. A very excellent recent discussion of the rate of infall, which was about 10 years after the conference, is by: Smith, D., Adams, N. G. and Kahn, H. A. 1974, Nature, 251, 379: "Flux and Composition of Micrometeroids in the Diameter Range 1 to 10 Microns." See also Short, N. M. 1975, Planetary Geology (Prentice Hall) p. 42f. For one of the most complete discussions of the infall measurements in the vicinity of the Earth see: Fechtig, H. 1982, in Comets: IAU Colloquium 61 (ed. L. L. Wilkening) (Univ. Arizona Press) p. 370ff "Cometary Dust in the Solar System," which mainly discusses the dust production by comets, but also includes a useful bibliography.
4. Salisbury, J. W. and Glaser, P. E. (eds.) 1964, The Lunar Surface Layer: Materials and Characteristics (Academic Press).
5. In the earlier conference, there was indeed some disagreement, but this was not on the depth of the dust layer but actually on the nature of the material of the surface layer.
6. Shorthill, R. W. and Saari, J. M. 1972, Advances in Astron. and Astrophys., 9, 149: "Infrared Observations on the Eclipsed Moon." See also, in the same issue, the article by Winter, D. F. "Infrared Emission from the Surface of the Moon."
7. Phillips, P. G. 1978, in Origins and Change: Selected Readings from the Journal of the American Scientific Affiliation (ed. D. L. Willis) (Elgin, IL, ASA) p. 74: "Meteoritic Influx and the Age of the Earth."