National Academies Press: OpenBook

Problems Related to Interplanetary Matter (1961)

Chapter: EXPLORATION OF THE MOON AND PLANETS

« Previous: MICROMETEORITE STUDIES FROM EARTH SATELLITES
Suggested Citation:"EXPLORATION OF THE MOON AND PLANETS." National Research Council. 1961. Problems Related to Interplanetary Matter. Washington, DC: The National Academies Press. doi: 10.17226/18683.
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Page 73
Suggested Citation:"EXPLORATION OF THE MOON AND PLANETS." National Research Council. 1961. Problems Related to Interplanetary Matter. Washington, DC: The National Academies Press. doi: 10.17226/18683.
×
Page 74
Suggested Citation:"EXPLORATION OF THE MOON AND PLANETS." National Research Council. 1961. Problems Related to Interplanetary Matter. Washington, DC: The National Academies Press. doi: 10.17226/18683.
×
Page 75
Suggested Citation:"EXPLORATION OF THE MOON AND PLANETS." National Research Council. 1961. Problems Related to Interplanetary Matter. Washington, DC: The National Academies Press. doi: 10.17226/18683.
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Page 76

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EXPLORATION OF THE MOON AND PLANETS A. R. Hibbs Jet Propulsion Laboratory California Institute of Technology Planned Program of the California Institute of Technology Jet Propulsion Laboratory The program for exploration of the Moon and planets is fairly well mapped out, for the next few years at least. In about a year's time there will be two test flights of Earth satellites very similar to each other. (These will be "space craft, " defined as the instrument package, or "pay- load, " plus structure, power supply, antennae, etc.) These satellites will be powered by solar batteries, and will have useful lifetimes of about two to three months. Their apogee will be between one and two million kilometers, so that they will have periods of approximately two months. Beginning in the second half of 1961, three space craft will be aimed at the Moon; payloads will be in a capsule equipped with retrorocket. The instruments, including a single-axis seismometer, are being designed to withstand impact of 70-100 meters/sec on a surface similar to concrete. The seismometer will report for several weeks. Before the release of the capsule, while approaching the lunar surface, a v-ray spectrometer, with 32-channel analyzer, will look for K40 activity. If the lunar surface has as much or more K40 as chondritic meteorites, it will be detectable. Vidicon photographs of the lunar surface will also be transmitted. Impact will be south and a little west of the crater Kepler. The exact location will be determined by radio-tracking. Now to somewhat more speculative plans. Venus will be in a favor- able position in August 1962, Mars in November of the same year. It is possible that attempts will be made to launch space craft at either or both of these targets. Their payloads will be designed for two types of experi- ments. In-flight experiments will examine properties of interplanetary space, while spectrograms, of the planetary atmosphere in the case of Venus and of the Martian surface, will be made on arrival at the planets. In 1963 or 1964, when it is hoped that the Centaur rocket will be ready, better-controlled soft landings on the Moon will be attempted. In 1965, if the Saturn rocket is available, it is very likely that devices can 73

be landed on the Moon capable of returning samples to the Earth. Further plans are quite nebulous. In terms of planetary exploration, it will be 1964 or later before a payload is landed on the surface of Mars or Venus, and not until 1966 or 1967 will remote-controlled experiments, of the type expected for the Moon in 1963, be possible for our planetary neighbors. The return of samples from Mars or Venus is much more difficult than from the Moon, so that planetary surface exploration by roving vehicles, etc., will be considerably in advance of actual delivery of samples. Mixing of the Lunar Surface Material In designing a program of lunar exploration, it is important to have some idea of the properties of the surface. One of the most significant of these properties is the amount of dust on the surface. Measurements of the flux of micrometeorites in regions of space near the Earth have been made (giving 8 x 10-3 particles cm-2 sec-1); coupled with a reasonable estimate of the size of these particles, it seems that the order of one meter thickness of micrometeorites should be deposited on the Moon's surface in a billion years. Also, there is good evidence for some long- term erosion processes on the Moon. [Hibbs engaged in an outline of the controversy between the lava and dust hypotheses of the origin of the lunar maria, essentially following the line of argument published by T. Gold (1955). Qlbert pointed out that he had made some calculations of the space charge and saltation distances to be expected for dust grains charged by Gold's postulated photoelectric effect, which indicate that extensive move- ment of dust might be expected, although the system represents a very complex problem in electro-hydrodynamics and the conclusions are some- what uncertain.] In any case, one would expect that deposition of micro- meteorites and erosion and transport of dust would obliterate the initial surface markings. Yet the ray systems, which may be billions of years old, and certain markings on the floors of the maria remain visible. The problem is then: to what extent can localized mixing of material from the original surface with the overlying dust layer, by means of meteoritic pitting, preserve the initial surface characteristics? Assign, for coding purposes, a "whiteness" to the initial surface, and a "blackness" to the material added. Then the fraction of white material on the present surface is W(x), where x is the depth of the dust layer. Two rates are involved: rm is the rate at which meteorites redeposit material from depth onto the present surface; re is the rate of deposit of "black" material. It may be assumed that the "color at a distance x from the original surface obeys the equation: /* ^ W(x) = /m • 4 / A(x-{)W(OdS (1) e m Jo 74

where V_ is the volume of the "average" meteorite pit (averaging done in some suitable but undefined way) and A (x - £) is the horizontal cross- sectional area at depth x - j; . The solution of equation (1) is particularly simple for a square-well type crater of average depth 1Q: W(x) = e-xx (2) where the characteristic distance X. is implicitly defined as: *4a-r + ? (l-e+VJh (3) e m Assuming rm = re, X = l.Z6Jfo . Clearly, material could only be mixed through a characteristic distance the order of the average crater depth under these conditions. Therefore, the preservation of the color contrasts in the maria by this mechanism would require craters large enough to be observable, if the postulated 100-meter depth of dust is indeed present. If one attempts to avoid this dilemma by increasing the efficiency of mixing material from depth, then it is necessary to assume that rm»re. In this case, an approximation may be made: re/rm^l/2\j^o . Thus, for mixing "white" material from depths ten times tne average"-crater depth, re must be about 5 percent of rm. This could only be valid if the material" brought in by the meteorites were negligible compared to the material stirred up, and if the rate at which dust is transported were small. [Con- sidering that the "average" depth is probably very nearly that of micro- meteorite craters, either r /r must be still smaller, or the depth of dust on those regions of the maria where apparently "original" surface marks remain must be much less than the order of 100 meters.] Regardless of the proper parameters to fit into these equations, such mixing processes must have occurred to some extent. Apparently, the lunar surface has had a complex history which should be explicitly con- sidered when analyzing the data which will be available in the next few years. Kohman: I wonder if there is really a net deposition of material on the Moon? Much of the incoming material must be simply vaporized. Hibbs: In any case, there is some redeposition. This may be seen in the rays associated with some of the prominent craters. Also, erosion from the "highlands" and mixing should occur in any case, although the rates are quite uncertain. Cameron: The rate of transport of eroded material, which is an important component of re, is especially sensitive to sintering. It may be that, considering a steady state balance between erosion and sintering, con- ditions strongly favor sintering of these fine particles, so that re is quite low. 75

Arnold: It is a difficult problem, because there are effects acting in both directions. For example, sintering should be slow in the absence of water, but accelerated by the hard vacuum and by the unsaturated bonds due to solar radiation. REFERENCE Gold, T. (1955) "The Lunar Surface, " Mon. Nat, of Roy. Astro. Soc. 115, 585. 76

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