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Stratigraphic chronology—a problem in extraterrestrial manned geologic exploration
I~.RUS ~ 4~r--4~ (1967)
Stratigraphic Chronology A Problem in Extraterrestrial Manned Geologic Exploration W A Y N E A. R O B E R T S
The Boeing Company, ~eattle, Washlngton Communicated by Zden~k Kopal Received November 7, 1966 One of the problems that must be solved before the geologic history of a planetary body without atmosphere can be established is the development of techniques for the determination of the time of formation of layered ejecta units. One approach to the solution involves the determination of the exposure periods of each unit in a layered sequence. The exposure period is that length of time a unit is at the planetary surface and exposed to the flux of meteoroids and radiation from the regions surrounding the body. For a method to be useful, the extent of consequent effects during this exposure must be measurable, related to the total dose received, and an estimate of exposure possible from the estimated flux and laboratory calibration of these changes. Promising potential methods are related to the transmutation of calcium which may be present in extraterrestrial rocks and to the possible change in oxidation state at the surface by the solar wind. All possible methods need to be studied for feasibility, experimentally checked, the need for new measurement techniques determined, conceptual design specified for any apparatus, and computational methods selected for the determination of the exposure period and establishment of the age of each unit encountered. geologic history of the body investigated. INTRODUCTION I t is the objective of this report to il- I n order to establish the geologic history of lustrate the need for feasibility study and the planet, it will be necessary to establish, experimental programs to determine ap- in addition to the relative ages, the abplicable methods for m e a s u r e m e n t of the solute age, in so far as possible, of not only length of exposure of p l a n e t a r y surfaces the oldest rocks noted, other igneous outto e x t r a p l a n e t a r y or exotic environmental crops, or fragments selected from the unfactors and to select the most useful consolidated deposits encountered, but also methods for eventual use either in situ, if of the layered clastic units overlying a n y applicable, or on samples returned to rigid subsurface rocks. T h e ages of these E a r t h or to an extraterrestrial-based stratigraphic units encountered in extralaboratory. I t is a secondary objective to terrestrial manned exploration m u s t be state the need to determine analytically determined to relate epochs or events to the procedures necessary for an o p t i m u m the history of the p l a n e t a r y body; howsampling p r o g r a m to determine surface ever, new or unique techniques for determining the absolute stratigraphic age m u s t exposure periods. One of the m a j o r objectives of manned be developed for those p l a n e t a r y bodies lunar and p l a n e t a r y exploration, utilizing where stratigraphic techniques developed techniques developed within the various for E a r t h will not apply. L a y e r e d rock sequences will p r o b a b l y earth sciences, will be to determine the 427
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occur on all planetary bodies where a lithic surface is exposed to a gaseous atmosphere or to an extraplanetary environment. The quantity and type of the atmosphere surrounding the body will determine the sedimentologic processes active at present or during the past. On bodies with little to no atmosphere, the layers will be related to catastrophic events characterized by deposition rapid relative to the atmospheric sedimentary processes resulting from the movement of fluids; however, on these bodies, layered sequences will probably not be as extensive vertically as on the atmosphere-covered bodies, and, locally, the sequence may contain only a few units. Such stratigraphic units will be ejecta layers related to specific impact events, and probably intrusives and extrusives related to volcanic events. Noting the superpositional relationships of these various layers and correlation of these layers over a portion or portions of the planetary surface will determine the order of events and a relative time scale. In order to determine the absolute time coordinates of the events illustrated in the sequential stratigraphic column, the 'tge of each unit, not the age of the fragments in the clastic deposits, must be determined where possible. I t may also be possible to relate events in the galactic history by the radiation effects to an absolute time scale (Kopal, 1966). On planetary bodies without atmospheric and life cycles similar to those on Earth, biotic chronologic, or paleobiotic, determinations will not be possible. Where biotic forms and evolutionary processes are evident, they will eventually be classified and dated by biochronologic techniques. Where unaltered igneous rocks are present, standard teehniqu(,s such as K40-Ar4° and other isotopic ratios of radioaetive parents and daughter products may be used, but e ieeta layers on bodies with little atmosphere may require new and different techniques to determine the stratigraphie ages. I t is possible that ejeeta layers may be the only stratigraphie units encountered in some locales or on some planetary bodies. Here, the environmental exposure
and the consequent property changes must be used to estimate the stratigraphic ages. APPLICABLE METHODS
Standard techniques of geochronology will reflect the age of mineralogical formation or the age of last melting generally. Techniques for measurement of spallation products, as a function of exposure, particularly tile He:~ concentration derived from H:~ produced by high-energy cosmic rays, are well known. Large penetration distances, however, may limit the usefulness of this technique except for dense materials. The age to be measured must be related to the effects of catastrophic elastic formation or to the volcanism with fractionation of the components that mark the time of origin of each stratigraphic unit. Techniques for determining this age must rely on changes or effects that commence at this time. On planetary bodies such as the Moon, methods other than paleontologic or isotope ratios will be necessary to determine these formational ages. Techniques based on radiation spallation or capture products, thermqluminescence, oxidation state changes, formation and retrogression of shock metamorphic minerals, structural (mechanical) breakdown of rocklike materials due to total radiation dose, thickness and sorting of material pulverized by a micrometeorite flux or radiation darkening could all be applicable to the determination of length of exposure of a planetary surface. DISCUSSION
The basic hypothesis for exposure dating of rock or fragmental layers is that, except for intrusives and some interstitial chemical deposits, rock layers are exposed for a period of time after formation upon the surface of the planet. This surface layer is exposed to the environment which includes both particulate and electromagnetic irradiation as well as bombardment by micrometeorites. Certain changes, depending on the nature, energy spectrum, and flux of impinging radiation and the isotopic content of the surface material, will occur and some of the changes will hope-
STRATIGRAPHIC CHRONOLOGY
fully be detectable and quantitative and can be used to estimate with fair accuracy, the length of exposure time. Where layers are subsequently covered by younger rock or fragmental layers, the irradiation and shock pressure environments are sharply curtailed, the extent of protection depending upon the thickness of the protecting blanket which is now absorbing the impinging extraplanetary sources of energy. An example of a typical layer is, of course, the shock breccia deposit which will surround the point of impact and the shock crater produced by the impact of a meteorite. Adjacent impacts could produce a stratified system of ejecta layers, each layer exposed at the surface for a discrete period of time. It is possible that dispersed within this column of ejecta layers will be flows of rock or ash which have been exposed to the environment and possibly dikes or sills, laccoliths, lopoliths, or other intrusives which would be protected from the radiation and meteorite bombardment. The exposed surface of these major layers, regardless of origin, will present essentially fresh unexposed rock to the radiation and meteorite flux. Lava or ash would be derived from magmas well below the surface of the planet. Ejecta from craters is deposited in an order the inverse of the preshock in situ sequence and will present the best protected material to the greatest flux of radiation. For ejecta layers several feet thick this will mean that very little material affected by radiation prior to the impact will be found at the surface and exposure ages determined from radiation effects in the surface layer will represent the age calculated from a true post-impact flux. The greatest concentration of shockaltered material will probably occur at the top of each unit, aiding in the location of intershock ejecta horizons. Unfortunately, continued impact by micrometeorites will tend to erode and move the more susceptible uppermost layer, complicating interpretation. The flux of electromagnetic and particulate radiation will range from X-ray, ultraviolet, visible, and infrared, electrons, possible gamma rays, and very low energy
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protons in the solar wind, through mediumenergy solar particle event radiation, to high-energy solar and galactic cosmic rays. Meteorites will range in size from submicron to asteroidal and will impact at velocities ranging up to about 70 km/sec. Consequent with this particulate and electromagnetic influx, the exposed rocks may be changed structurally, texturally, chemically, and isotopically and the extent of those changes will depend partly upon the total energy influx, or "dose." An experimental study should be designed to determine which of the possible changes, upon measurement, will provide a workable method for establishing the absolute ages of the formational layers that may be found during planetary exploration. The planetary bodies most exposed to the environmental energy influx will be Mercury, the Moon, and Mars. Plateau basalts are composed of the following elements, listed in an approximate order of abundance: oxygen, silicon, aluminum, iron, calcium, magnesium, sodium, titanium, hydrogen, potassium, phosphorus, and manganese. Of these elements, calcium appears to offer the most useful potential for nuclear reactions which could be used for geochronology. Calcium-40 is the most abundant isotope. The following reactions occur for neutron and proton capture: C a 4° + n - * C a 4' - ~ K 4' -5 t~+(K c a p t u r e ) Ca40 -{- p --* Sc 4, --, C a 4, -5 8+ --~ K 4, ~-/3 +.
Thus, Ca 41 is the product of both neutron and proton capture (although the cross section is low.) Ca 41 is radioactive with a half-life of 1.2 X 10~ years. K 4~ is stable-thus Ca 41 would be an excellent isotope to consider for short periods of protection up to about one million years. Ca 44, about 2% of the calcium present on Earth, yields stable Se45 for either neutron or proton capture. Scandium is a relatively rare element, about 5 ppm in the Earth's crustal rocks, and minute quantities could probably be detected by chemical concentration of large samples and subsequent mass spectrometric analysis of both calcium and scandium. The Ca44-Sc4~ reaction could be
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used to determine extremely long periods of exposure. Present estimated moderate energy, 8+MeV, proton flux values (Webber, 1966), are such that, on cursory examination, this reaction appears to be marginal for incremental times of about 0.1 cons. Scandium may be present in lower concentrations in the protected rocks of Mercury, the Moon, and Mars, increasing the sensitivity of this approach. Another type of potential change in some minerals in the exposed rocks concerns electron displacement and subsequent luminescence of these minerals when heated. The total luminescence is also a function of total irradiation dose and could conceivably be used to estimate the length of exposure to a known radiation flux. This method when applied to surfaces also exposed to a flux of micrometeorites could have serious limitations, however, since high-velocity impacts produce both pressure and heat. These effects would be predominant in the uppermost layer of the surface where the thermoluminescence effects would be concentrated and the production of heat could destroy the luminescent qualities of susceptible minerals. This method, then, might measure only that period of time required to blanket the surficial layer with impact-induced temperatures on the order of a few hundred degrees Centigrade. This deficiency can be readily checked in a laboratory having both irradiation and impact capabilities. A third potential change in the exposed surface layer concerns the interaction of the low-energy solar protons with the silicate crystals. Meer et al. (1964) found that heated basaltic rocks exposed to hydrogen removed about 0.5% of the oxygen to form water. Thus, the solar protons (atomic hydrogen) could alter the oxidation state of the rocks and in effect produce oxygen-deficient surface material (Barton and Cotton, 1966). This could in turn mean lowering the Fe3+:Fe0"÷ ratio, the production of metallic iron, magnesium, or silicon. Plateau basalt on Earth contains 3.5% by weight of Fe203 and 9.78% by weight of FeO. Thus Fe 3÷ is about ~ of the total Fe ions. With a flux of about 108
p/sec cm 2, significant changes in this ratio are possible. Metallic iron, magnesium, and silicon are for all practical purposes nonexistent in the silicate rocks of Earth. Methods based on the change in the Fe3+: Fe '~÷ or the quantity of unoxidized metals (excluding meteoritic iron-nickel) as a function of depth from a vertical array of samples may possibly be derived to calculate a probable solar wind exposure period. Shock induced by impact of meteorites can produce high-pressure polymorphs of silica and possibly silicate minerals. A polymorph of silica, stishovite, has been concentrated from an ejecta sample from Meteor Crater, Arizona, and is currently being irradiated to determine retrogressive metamorphic effects of neutron and proton bombardment. Retrogression seems probable and, since the retrograde product is probably amorphous silica, stishovite:silica glass ratios could provide an additional working method for the determination of exposure periods. Spallation products, principally He3, a decay product of tritium, have been used to indicate exposure ages of meteorites since the helium can be easily concentrated from a specimen. Since this method is well known, it need not be investigated. Other possible techniques concern the mechanical breakdown of material from both irradiation and micrometeorite bombardment. Such structural crystalline degradation may be inseparable in origin; however, a simulated environment, at higher flux rates, of course, could be used to determine combined breakdown rates, resulting thickness, and Sorting of pulverized material. There should be spectral infrared reflectance and emittance changes as well as changes in the visible as a function of total radiation dose and type. As the rocks are irradiated, these changes should be documented at dose intervals. The acquisition of proper samples from the surficial deposits of extraterrestrial bodies will be difficult and part of the experimental program should be directed toward analytical determination of the
STRATIGRAPHIC CHRONOLOGY
depth-effect function and proper depths of effect and control samples. High-energy particles produce cascades of secondary energetic particles and radiation effects could vary in a nonlinear fashion below the surface. Peak effect volumes for certain changes could occur at some depth below the surface. This effect of secondary particle production should be calculated analytically for several spectral flux assumptions, a maximum, a minimum, and a probable spectrum at the lunar surface. The age assigned to any unit will depend upon the exposure periods of all the overlying units. Should the age of any of the clastic units exceed the age of the underlying rigid rocks, determined by superposition to be the oldest and dated by standard geochronologic techniques, this would indicate that estimates of the average present and past flux were too low. Radiation flux of moderate to high energy particulate and electromagnetic radiation could possibly have varied widely during the past and such variations could be related to solar or galactic events. Wide divergence of exposure periods determined by independent techniques could indicate the approximate dates of these events and detailed study of the appropriate units could indicate the nature of these events. APPROACH
The approach toward determining workable methods for measurement or estimate of the age of stratigraphic units composed of clastic ejecta should probably be done in three distinct phases. These phases, in the preferred order of performance are as follows: (1) a study of feasibility phase; (2) an experimental phase based on the results of the feasibility study; (3) a data reduction phase to determine workable techniques for stratigraphic chronology based on the feasibility and experimental phases. During the first phase, or feasibility study, the anticipated extraplanetary meteorite and radiation environment should be specified or assumed on the basis of the latest data available. A maximum and a minimum of these fluxes, in addition to the
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probable average flux at the lunar surface, should be estimated to determine the potential effects or changes on typical silicate rocks as a function of depth beneath the surface. Later, long-term spectral measurement of the radiation environment will be made by telemetering instrumental complexes located on the lunar surface. Several types of terrestrial silicate rocks, analogous to possible extraterrestrial material, should be assumed. Proton reaction cross sections are probably available, but some, of reactions with minor isotopic constituents of rocks, may need to be determined to establish the feasibility. Each potential change should be evaluated, the magnitude estimated, and the feasibility of using this change as a measure of exposure period on the surface determined. Analysis of each feasible approach will yield the determinative experiments and apparatus required for experimental evaluation. The literature should be concurrently searched for the development of applicable experimental equipment and subsequent experimental work. The cascading effect of producing energetic secondaries from high-energy galactic and solar cosmic particles could produce a nonlinear response of the rocks as a function of depth. Using the estimate of the maximum, minimum, and probable average of the low-, intermediate-, and high-energy radiation fluxes, the probable changes as a function of depth can be estimated and used to indicate sampling and procedures and programs during manned extraterrestrial exploration. The experimental program will depend upon the results of the study phase. The facilities required will depend upon the methods that appear feasible after study. Test specimens of a suite of rock types should be selected and irradiated, if such proves necessary to demonstrate feasibility of a workable hypothesis, with electrons, protons, or electromagnetic radiation or with combinations at appropriate energies. Measurement of changes as a function of total dose, analogous to time once the flux rates have been estimated, will indicate the sensitivity of these
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methods to various size time increments. crystalline material, are all derived from New techniques will probaby be required older rocks and the fragments within these for tile rapid measurement of the oxidation units may maintain the age characteristics state of the rock specimens and should of the parent material. Hence the formabe investigated. In addition to measuring tional or stratigraphic age must be apspectrally induced radioactivity, isotopic proached by different techniques. On the bodies without atmosphere at changes, changes in the oxidation state, the infrared spectral reflectance and emittance, present that presumably had little atmosthe thermoluminescence should be deter- phere for any appreciable period during their history, no life forms or fossils or mined. After the experimental program has sediments transported and deposited by demonstrated the feasibility of any effect fluids will be present. On these bodies, the or effects for use in determining the ex- layered rock sequences will probably be posure period, the most practical methods predominantly catastrophic clastics formed can be selected and the sampling and com- by impact and may contain both intrusive putational procedures derived for deter- and extrusive effusive igneous rocks. Here, mining the exposure-protection periods of standard paleontologic stratigraphic techrocks exposed to the estimated lunar en- niques will not be possible and new, unique vironment. This would be essentially a methods will be required. The best possidata reduction period devoted to the de- bility for use on these types of planetary velopment of techniques of surface chro- bodies appears to be the measurement of the nology for the methods deemed preferable extent of effects or changes related to the environment to which the rocks have been by feasibility and experimental studies. Techniques for the acquisition, handling, exposed. To convert these effects to equivand treating of samples should also be alent exposure periods, the possible techdetermined as a result of the early phases. niques must first be calibrated. To do this, The thickness of individual effect layers the average probable environment must be may be very thin and difficult to handle determined and the changes in this enand special techniques may be needed. It vironment during the past must be posshould also be possible to estimate the tulated. Laboratory measurement of the probability of success of each technique changes in rocklike materials must be made based on the terrestrial rocks selected to relate the extent of these changes or effects to the environment and to the length and on the flux estimates. of exposure to the environment. The environmental factors that appear at SUMMARY present to be of prime importance for the Two types of ages are necessary to es- production of effects that will represent tablish the geologic history of extraterres- time are the fluxes, particles or fragments, trial bodies. The first type of age, and and radiation. The particles will range hence the time of formation, is related to in size from submicron to asteroidal dimenthe last crystallization of igneous rocks sions and tile flux of micro- to macroand of rock and mineral components of meteoroids will produce, in addition to composite rocks derived from the parent craters, shock breccia or shock-fragmented, igneous masses. The second type of age, vaporized, melted, comminuted, and phasethe concern of this report, is related to transformed material related to the charunits, beds, or formations, of layered rock acteristics of the target and the missile. sequences. This stratigraphic age must be The radiation flux is composed of both determined in addition to the absolute particulate and electromagnetic radiation ages of the rock masses or fragments dat- ranging from the extremely low energies of ing from the last melting or fractionation the solar wind to the extremely high enerof the contained molecular species. These gies of the galactic cosmic rays. Composilayered rocks, if composed of fragmented tional as well as physical changes will
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result from this flux, hopefully in measurable extent. Those potential changes that appear to offer the most promise are related to proton capture and the consequent transmutation of the calcium isotopes and to a change in the oxidation state of the exposed rocks due to removal of oxygen by the solar flux of low-energy protons, or hydrogen nuclei. Both effects will be greatest in the surficial layer exposed and will decrease in intensity with depth beneath the surface, and both will require multiple sampling as a function of depth. Oxidation state changes will probably be more restricted vertically than transmutation. Calcium 44 is transmuted to scandium-45 by both neuron and proton capture. Since scandium is a rare element, its concentration, enhanced chemically, may be used to indicate exposure age, providing the moderate-energy proton flux is large enough for a given exposure time increment to produce scandium in quantities measurable relative to the natural abundance. Calcium40, the most abundant calcium isotope, is transmuted to stable potassium-41. Inter-
mediate calcium-41 is radioactive with a half-life of slightly greater than 105 years and may be used to establish short periods of protection up to about one million years. Both methods need to be studied critically and to be checked experimentally. The results of the feasibility studies and the experimental testing may then be used to list the probable order of success within established laboratory capabilities, new laboratory or experimental techniques that must be developed, and workable methods for determining the stratigraphic chronology. REFERENCES BARTON, J. A., A~D CovroN, J. E. (1966). Oral communication, June. KOPAL, Z. (1966). Oral communication, September. MEER, F. C., el al. (1964). "Extraction of Water and Oxygen from Rocks." Boeing Document D2-23424, Boeing Sci. Res. Lab., Seattle, Washington. WEBBER, W. R. (19~). "An Evaluatioa of SolarCosmic-Ray Events During Solar Minimum." Boeing Document D2-84274-1, Boeing Sci. Res. Lab., Seattle, Washington.
Stratigraphic Chronology A Problem in Extraterrestrial Manned Geologic Exploration W A Y N E A. R O B E R T S
The Boeing Company, ~eattle, Washlngton Communicated by Zden~k Kopal Received November 7, 1966 One of the problems that must be solved before the geologic history of a planetary body without atmosphere can be established is the development of techniques for the determination of the time of formation of layered ejecta units. One approach to the solution involves the determination of the exposure periods of each unit in a layered sequence. The exposure period is that length of time a unit is at the planetary surface and exposed to the flux of meteoroids and radiation from the regions surrounding the body. For a method to be useful, the extent of consequent effects during this exposure must be measurable, related to the total dose received, and an estimate of exposure possible from the estimated flux and laboratory calibration of these changes. Promising potential methods are related to the transmutation of calcium which may be present in extraterrestrial rocks and to the possible change in oxidation state at the surface by the solar wind. All possible methods need to be studied for feasibility, experimentally checked, the need for new measurement techniques determined, conceptual design specified for any apparatus, and computational methods selected for the determination of the exposure period and establishment of the age of each unit encountered. geologic history of the body investigated. INTRODUCTION I t is the objective of this report to il- I n order to establish the geologic history of lustrate the need for feasibility study and the planet, it will be necessary to establish, experimental programs to determine ap- in addition to the relative ages, the abplicable methods for m e a s u r e m e n t of the solute age, in so far as possible, of not only length of exposure of p l a n e t a r y surfaces the oldest rocks noted, other igneous outto e x t r a p l a n e t a r y or exotic environmental crops, or fragments selected from the unfactors and to select the most useful consolidated deposits encountered, but also methods for eventual use either in situ, if of the layered clastic units overlying a n y applicable, or on samples returned to rigid subsurface rocks. T h e ages of these E a r t h or to an extraterrestrial-based stratigraphic units encountered in extralaboratory. I t is a secondary objective to terrestrial manned exploration m u s t be state the need to determine analytically determined to relate epochs or events to the procedures necessary for an o p t i m u m the history of the p l a n e t a r y body; howsampling p r o g r a m to determine surface ever, new or unique techniques for determining the absolute stratigraphic age m u s t exposure periods. One of the m a j o r objectives of manned be developed for those p l a n e t a r y bodies lunar and p l a n e t a r y exploration, utilizing where stratigraphic techniques developed techniques developed within the various for E a r t h will not apply. L a y e r e d rock sequences will p r o b a b l y earth sciences, will be to determine the 427
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occur on all planetary bodies where a lithic surface is exposed to a gaseous atmosphere or to an extraplanetary environment. The quantity and type of the atmosphere surrounding the body will determine the sedimentologic processes active at present or during the past. On bodies with little to no atmosphere, the layers will be related to catastrophic events characterized by deposition rapid relative to the atmospheric sedimentary processes resulting from the movement of fluids; however, on these bodies, layered sequences will probably not be as extensive vertically as on the atmosphere-covered bodies, and, locally, the sequence may contain only a few units. Such stratigraphic units will be ejecta layers related to specific impact events, and probably intrusives and extrusives related to volcanic events. Noting the superpositional relationships of these various layers and correlation of these layers over a portion or portions of the planetary surface will determine the order of events and a relative time scale. In order to determine the absolute time coordinates of the events illustrated in the sequential stratigraphic column, the 'tge of each unit, not the age of the fragments in the clastic deposits, must be determined where possible. I t may also be possible to relate events in the galactic history by the radiation effects to an absolute time scale (Kopal, 1966). On planetary bodies without atmospheric and life cycles similar to those on Earth, biotic chronologic, or paleobiotic, determinations will not be possible. Where biotic forms and evolutionary processes are evident, they will eventually be classified and dated by biochronologic techniques. Where unaltered igneous rocks are present, standard teehniqu(,s such as K40-Ar4° and other isotopic ratios of radioaetive parents and daughter products may be used, but e ieeta layers on bodies with little atmosphere may require new and different techniques to determine the stratigraphie ages. I t is possible that ejeeta layers may be the only stratigraphie units encountered in some locales or on some planetary bodies. Here, the environmental exposure
and the consequent property changes must be used to estimate the stratigraphic ages. APPLICABLE METHODS
Standard techniques of geochronology will reflect the age of mineralogical formation or the age of last melting generally. Techniques for measurement of spallation products, as a function of exposure, particularly tile He:~ concentration derived from H:~ produced by high-energy cosmic rays, are well known. Large penetration distances, however, may limit the usefulness of this technique except for dense materials. The age to be measured must be related to the effects of catastrophic elastic formation or to the volcanism with fractionation of the components that mark the time of origin of each stratigraphic unit. Techniques for determining this age must rely on changes or effects that commence at this time. On planetary bodies such as the Moon, methods other than paleontologic or isotope ratios will be necessary to determine these formational ages. Techniques based on radiation spallation or capture products, thermqluminescence, oxidation state changes, formation and retrogression of shock metamorphic minerals, structural (mechanical) breakdown of rocklike materials due to total radiation dose, thickness and sorting of material pulverized by a micrometeorite flux or radiation darkening could all be applicable to the determination of length of exposure of a planetary surface. DISCUSSION
The basic hypothesis for exposure dating of rock or fragmental layers is that, except for intrusives and some interstitial chemical deposits, rock layers are exposed for a period of time after formation upon the surface of the planet. This surface layer is exposed to the environment which includes both particulate and electromagnetic irradiation as well as bombardment by micrometeorites. Certain changes, depending on the nature, energy spectrum, and flux of impinging radiation and the isotopic content of the surface material, will occur and some of the changes will hope-
STRATIGRAPHIC CHRONOLOGY
fully be detectable and quantitative and can be used to estimate with fair accuracy, the length of exposure time. Where layers are subsequently covered by younger rock or fragmental layers, the irradiation and shock pressure environments are sharply curtailed, the extent of protection depending upon the thickness of the protecting blanket which is now absorbing the impinging extraplanetary sources of energy. An example of a typical layer is, of course, the shock breccia deposit which will surround the point of impact and the shock crater produced by the impact of a meteorite. Adjacent impacts could produce a stratified system of ejecta layers, each layer exposed at the surface for a discrete period of time. It is possible that dispersed within this column of ejecta layers will be flows of rock or ash which have been exposed to the environment and possibly dikes or sills, laccoliths, lopoliths, or other intrusives which would be protected from the radiation and meteorite bombardment. The exposed surface of these major layers, regardless of origin, will present essentially fresh unexposed rock to the radiation and meteorite flux. Lava or ash would be derived from magmas well below the surface of the planet. Ejecta from craters is deposited in an order the inverse of the preshock in situ sequence and will present the best protected material to the greatest flux of radiation. For ejecta layers several feet thick this will mean that very little material affected by radiation prior to the impact will be found at the surface and exposure ages determined from radiation effects in the surface layer will represent the age calculated from a true post-impact flux. The greatest concentration of shockaltered material will probably occur at the top of each unit, aiding in the location of intershock ejecta horizons. Unfortunately, continued impact by micrometeorites will tend to erode and move the more susceptible uppermost layer, complicating interpretation. The flux of electromagnetic and particulate radiation will range from X-ray, ultraviolet, visible, and infrared, electrons, possible gamma rays, and very low energy
429
protons in the solar wind, through mediumenergy solar particle event radiation, to high-energy solar and galactic cosmic rays. Meteorites will range in size from submicron to asteroidal and will impact at velocities ranging up to about 70 km/sec. Consequent with this particulate and electromagnetic influx, the exposed rocks may be changed structurally, texturally, chemically, and isotopically and the extent of those changes will depend partly upon the total energy influx, or "dose." An experimental study should be designed to determine which of the possible changes, upon measurement, will provide a workable method for establishing the absolute ages of the formational layers that may be found during planetary exploration. The planetary bodies most exposed to the environmental energy influx will be Mercury, the Moon, and Mars. Plateau basalts are composed of the following elements, listed in an approximate order of abundance: oxygen, silicon, aluminum, iron, calcium, magnesium, sodium, titanium, hydrogen, potassium, phosphorus, and manganese. Of these elements, calcium appears to offer the most useful potential for nuclear reactions which could be used for geochronology. Calcium-40 is the most abundant isotope. The following reactions occur for neutron and proton capture: C a 4° + n - * C a 4' - ~ K 4' -5 t~+(K c a p t u r e ) Ca40 -{- p --* Sc 4, --, C a 4, -5 8+ --~ K 4, ~-/3 +.
Thus, Ca 41 is the product of both neutron and proton capture (although the cross section is low.) Ca 41 is radioactive with a half-life of 1.2 X 10~ years. K 4~ is stable-thus Ca 41 would be an excellent isotope to consider for short periods of protection up to about one million years. Ca 44, about 2% of the calcium present on Earth, yields stable Se45 for either neutron or proton capture. Scandium is a relatively rare element, about 5 ppm in the Earth's crustal rocks, and minute quantities could probably be detected by chemical concentration of large samples and subsequent mass spectrometric analysis of both calcium and scandium. The Ca44-Sc4~ reaction could be
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used to determine extremely long periods of exposure. Present estimated moderate energy, 8+MeV, proton flux values (Webber, 1966), are such that, on cursory examination, this reaction appears to be marginal for incremental times of about 0.1 cons. Scandium may be present in lower concentrations in the protected rocks of Mercury, the Moon, and Mars, increasing the sensitivity of this approach. Another type of potential change in some minerals in the exposed rocks concerns electron displacement and subsequent luminescence of these minerals when heated. The total luminescence is also a function of total irradiation dose and could conceivably be used to estimate the length of exposure to a known radiation flux. This method when applied to surfaces also exposed to a flux of micrometeorites could have serious limitations, however, since high-velocity impacts produce both pressure and heat. These effects would be predominant in the uppermost layer of the surface where the thermoluminescence effects would be concentrated and the production of heat could destroy the luminescent qualities of susceptible minerals. This method, then, might measure only that period of time required to blanket the surficial layer with impact-induced temperatures on the order of a few hundred degrees Centigrade. This deficiency can be readily checked in a laboratory having both irradiation and impact capabilities. A third potential change in the exposed surface layer concerns the interaction of the low-energy solar protons with the silicate crystals. Meer et al. (1964) found that heated basaltic rocks exposed to hydrogen removed about 0.5% of the oxygen to form water. Thus, the solar protons (atomic hydrogen) could alter the oxidation state of the rocks and in effect produce oxygen-deficient surface material (Barton and Cotton, 1966). This could in turn mean lowering the Fe3+:Fe0"÷ ratio, the production of metallic iron, magnesium, or silicon. Plateau basalt on Earth contains 3.5% by weight of Fe203 and 9.78% by weight of FeO. Thus Fe 3÷ is about ~ of the total Fe ions. With a flux of about 108
p/sec cm 2, significant changes in this ratio are possible. Metallic iron, magnesium, and silicon are for all practical purposes nonexistent in the silicate rocks of Earth. Methods based on the change in the Fe3+: Fe '~÷ or the quantity of unoxidized metals (excluding meteoritic iron-nickel) as a function of depth from a vertical array of samples may possibly be derived to calculate a probable solar wind exposure period. Shock induced by impact of meteorites can produce high-pressure polymorphs of silica and possibly silicate minerals. A polymorph of silica, stishovite, has been concentrated from an ejecta sample from Meteor Crater, Arizona, and is currently being irradiated to determine retrogressive metamorphic effects of neutron and proton bombardment. Retrogression seems probable and, since the retrograde product is probably amorphous silica, stishovite:silica glass ratios could provide an additional working method for the determination of exposure periods. Spallation products, principally He3, a decay product of tritium, have been used to indicate exposure ages of meteorites since the helium can be easily concentrated from a specimen. Since this method is well known, it need not be investigated. Other possible techniques concern the mechanical breakdown of material from both irradiation and micrometeorite bombardment. Such structural crystalline degradation may be inseparable in origin; however, a simulated environment, at higher flux rates, of course, could be used to determine combined breakdown rates, resulting thickness, and Sorting of pulverized material. There should be spectral infrared reflectance and emittance changes as well as changes in the visible as a function of total radiation dose and type. As the rocks are irradiated, these changes should be documented at dose intervals. The acquisition of proper samples from the surficial deposits of extraterrestrial bodies will be difficult and part of the experimental program should be directed toward analytical determination of the
STRATIGRAPHIC CHRONOLOGY
depth-effect function and proper depths of effect and control samples. High-energy particles produce cascades of secondary energetic particles and radiation effects could vary in a nonlinear fashion below the surface. Peak effect volumes for certain changes could occur at some depth below the surface. This effect of secondary particle production should be calculated analytically for several spectral flux assumptions, a maximum, a minimum, and a probable spectrum at the lunar surface. The age assigned to any unit will depend upon the exposure periods of all the overlying units. Should the age of any of the clastic units exceed the age of the underlying rigid rocks, determined by superposition to be the oldest and dated by standard geochronologic techniques, this would indicate that estimates of the average present and past flux were too low. Radiation flux of moderate to high energy particulate and electromagnetic radiation could possibly have varied widely during the past and such variations could be related to solar or galactic events. Wide divergence of exposure periods determined by independent techniques could indicate the approximate dates of these events and detailed study of the appropriate units could indicate the nature of these events. APPROACH
The approach toward determining workable methods for measurement or estimate of the age of stratigraphic units composed of clastic ejecta should probably be done in three distinct phases. These phases, in the preferred order of performance are as follows: (1) a study of feasibility phase; (2) an experimental phase based on the results of the feasibility study; (3) a data reduction phase to determine workable techniques for stratigraphic chronology based on the feasibility and experimental phases. During the first phase, or feasibility study, the anticipated extraplanetary meteorite and radiation environment should be specified or assumed on the basis of the latest data available. A maximum and a minimum of these fluxes, in addition to the
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probable average flux at the lunar surface, should be estimated to determine the potential effects or changes on typical silicate rocks as a function of depth beneath the surface. Later, long-term spectral measurement of the radiation environment will be made by telemetering instrumental complexes located on the lunar surface. Several types of terrestrial silicate rocks, analogous to possible extraterrestrial material, should be assumed. Proton reaction cross sections are probably available, but some, of reactions with minor isotopic constituents of rocks, may need to be determined to establish the feasibility. Each potential change should be evaluated, the magnitude estimated, and the feasibility of using this change as a measure of exposure period on the surface determined. Analysis of each feasible approach will yield the determinative experiments and apparatus required for experimental evaluation. The literature should be concurrently searched for the development of applicable experimental equipment and subsequent experimental work. The cascading effect of producing energetic secondaries from high-energy galactic and solar cosmic particles could produce a nonlinear response of the rocks as a function of depth. Using the estimate of the maximum, minimum, and probable average of the low-, intermediate-, and high-energy radiation fluxes, the probable changes as a function of depth can be estimated and used to indicate sampling and procedures and programs during manned extraterrestrial exploration. The experimental program will depend upon the results of the study phase. The facilities required will depend upon the methods that appear feasible after study. Test specimens of a suite of rock types should be selected and irradiated, if such proves necessary to demonstrate feasibility of a workable hypothesis, with electrons, protons, or electromagnetic radiation or with combinations at appropriate energies. Measurement of changes as a function of total dose, analogous to time once the flux rates have been estimated, will indicate the sensitivity of these
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methods to various size time increments. crystalline material, are all derived from New techniques will probaby be required older rocks and the fragments within these for tile rapid measurement of the oxidation units may maintain the age characteristics state of the rock specimens and should of the parent material. Hence the formabe investigated. In addition to measuring tional or stratigraphic age must be apspectrally induced radioactivity, isotopic proached by different techniques. On the bodies without atmosphere at changes, changes in the oxidation state, the infrared spectral reflectance and emittance, present that presumably had little atmosthe thermoluminescence should be deter- phere for any appreciable period during their history, no life forms or fossils or mined. After the experimental program has sediments transported and deposited by demonstrated the feasibility of any effect fluids will be present. On these bodies, the or effects for use in determining the ex- layered rock sequences will probably be posure period, the most practical methods predominantly catastrophic clastics formed can be selected and the sampling and com- by impact and may contain both intrusive putational procedures derived for deter- and extrusive effusive igneous rocks. Here, mining the exposure-protection periods of standard paleontologic stratigraphic techrocks exposed to the estimated lunar en- niques will not be possible and new, unique vironment. This would be essentially a methods will be required. The best possidata reduction period devoted to the de- bility for use on these types of planetary velopment of techniques of surface chro- bodies appears to be the measurement of the nology for the methods deemed preferable extent of effects or changes related to the environment to which the rocks have been by feasibility and experimental studies. Techniques for the acquisition, handling, exposed. To convert these effects to equivand treating of samples should also be alent exposure periods, the possible techdetermined as a result of the early phases. niques must first be calibrated. To do this, The thickness of individual effect layers the average probable environment must be may be very thin and difficult to handle determined and the changes in this enand special techniques may be needed. It vironment during the past must be posshould also be possible to estimate the tulated. Laboratory measurement of the probability of success of each technique changes in rocklike materials must be made based on the terrestrial rocks selected to relate the extent of these changes or effects to the environment and to the length and on the flux estimates. of exposure to the environment. The environmental factors that appear at SUMMARY present to be of prime importance for the Two types of ages are necessary to es- production of effects that will represent tablish the geologic history of extraterres- time are the fluxes, particles or fragments, trial bodies. The first type of age, and and radiation. The particles will range hence the time of formation, is related to in size from submicron to asteroidal dimenthe last crystallization of igneous rocks sions and tile flux of micro- to macroand of rock and mineral components of meteoroids will produce, in addition to composite rocks derived from the parent craters, shock breccia or shock-fragmented, igneous masses. The second type of age, vaporized, melted, comminuted, and phasethe concern of this report, is related to transformed material related to the charunits, beds, or formations, of layered rock acteristics of the target and the missile. sequences. This stratigraphic age must be The radiation flux is composed of both determined in addition to the absolute particulate and electromagnetic radiation ages of the rock masses or fragments dat- ranging from the extremely low energies of ing from the last melting or fractionation the solar wind to the extremely high enerof the contained molecular species. These gies of the galactic cosmic rays. Composilayered rocks, if composed of fragmented tional as well as physical changes will
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result from this flux, hopefully in measurable extent. Those potential changes that appear to offer the most promise are related to proton capture and the consequent transmutation of the calcium isotopes and to a change in the oxidation state of the exposed rocks due to removal of oxygen by the solar flux of low-energy protons, or hydrogen nuclei. Both effects will be greatest in the surficial layer exposed and will decrease in intensity with depth beneath the surface, and both will require multiple sampling as a function of depth. Oxidation state changes will probably be more restricted vertically than transmutation. Calcium 44 is transmuted to scandium-45 by both neuron and proton capture. Since scandium is a rare element, its concentration, enhanced chemically, may be used to indicate exposure age, providing the moderate-energy proton flux is large enough for a given exposure time increment to produce scandium in quantities measurable relative to the natural abundance. Calcium40, the most abundant calcium isotope, is transmuted to stable potassium-41. Inter-
mediate calcium-41 is radioactive with a half-life of slightly greater than 105 years and may be used to establish short periods of protection up to about one million years. Both methods need to be studied critically and to be checked experimentally. The results of the feasibility studies and the experimental testing may then be used to list the probable order of success within established laboratory capabilities, new laboratory or experimental techniques that must be developed, and workable methods for determining the stratigraphic chronology. REFERENCES BARTON, J. A., A~D CovroN, J. E. (1966). Oral communication, June. KOPAL, Z. (1966). Oral communication, September. MEER, F. C., el al. (1964). "Extraction of Water and Oxygen from Rocks." Boeing Document D2-23424, Boeing Sci. Res. Lab., Seattle, Washington. WEBBER, W. R. (19~). "An Evaluatioa of SolarCosmic-Ray Events During Solar Minimum." Boeing Document D2-84274-1, Boeing Sci. Res. Lab., Seattle, Washington.