COSMIC RAYS AND A N C I E N T P L A N E T A R Y

MAGNETIC FIELDS

PAUL S. WESSON Astronomy Group, Dept. of Physics, Queen's University, Kingston, Ontario, Canada and

St. John's College, Cambridge, England

(Received 18 May, 1976)

Abstract. The possibility is discussed of using the latitude-dependent cutoff in the intensity and flux of cosmic ray particles reaching the surface of a planet to investigate ancient magnetic fields in the Moon, Mars and the Earth. In the last case, the method could provide a validity test for conventional palaeomagnetism.

1. Introduction

The flux and intensity of cosmic rays reaching the surface of the Earth varies in a systematic way from the geomagnetic equator to the poles due to the variation in magnetic rigidity. This is a well-known phenomenon (see, e.g., Lal, 1972a), and could in principle be used by an observer on the surface of any planet with a large-scale field to detect the positions of the magnetic poles. I wish to examine three ways (Section 2(i), (ii), (iii)) in which this effect could be used to find ancient pole positions by studying traces left by cosmic rays in rocks. It should also be useful as a test for checking pole positions found by conventional palaeomagnetic methods. Section 3

examines some possible difficulties affecting the feasibility of the test. Section 4 is a conclusion.

The cosmic ray flux as received on the Earth consists of protons and heavier nuclei, of solar and Galactic origin (the solar flux is more predominantly protons and is of average lower energy than the Galactic flux). Both components consist of protons and heavier nuclei with all energies from a few tens of MeV upwards, but the measure- ments as proposed below would only be sensitive to particles with energies > 500 MeV. (The effects of particles with energy < 1 GeV are felt at the poles on the Earth, but not at the equator, due to the geomagnetic cutoff.) On bodies with tenuous atmospheres, however, the cosmic ray flux as it can be studied in experiments like those of Lal (1972a, b) on etched rocks, is modified by interactions and comprises (1) protons from the Sun of energy > 10 MeV; (2) solar heavy nuclei (Z>20) of 0.5-10 MeV energy; (3) the integrated flux of protons and heavier nuclei of energy > 0.5 GeV; (4) Z > 20, medium-energy (100-200 MeV) particles. Detailed knowledge of the fluxes and energy spectra of the solar-derived cosmic rays can be obtained by studying meteorites (i.e., studying the isotopic changes and alteration of the crystalline structure of the rock by cosmogenic interactions). So far, data are only available on meteorites and lunar soil; no results have been reported for Earth-bound rocks. However, it seems that the

Astrophysics and Space Science 46 (1977) 321-326. All Rights Reserved Copyright �9 1977 by D. Reidel Publishing Company, Dordrecht-Holland

322 PAUL S. WESSON

technology of investigating tracks left in rocks by cosmic rays has just about reached the stage where a useful attempt could be made to analyse Earth-bound rocks.

While the test is probably of major interest in the case of the Earth, it can also be applied to the Moon and Mars as an investigative method to determine whether these bodies (and others in the Solar System) have ever supported notable multipole fields (dipole in particular). It will be a more sensitive indicator in the cases of the Moon and Mars because they have no or negligibly thin atmospheres compared to the Earth: in what follows, all discussion is based on the Earth, but applications to other bodies are indicated in (i), (ii), (iii) below. As stated, the method is based on the low- energy part of the cosmic ray flux impinging on the Earth: the dipole geomagnetic field at present causes the low-energy cosmic rays to be deflected from impact with the Earth in low latitudes, but allows unhindered impingement at the poles. The geomagnetic cutoff varies in a well-known and familiar way as the location varies, from the equator to the poles, causing a difference in the low energy cosmic ray flux. Palaeomagnetism is now a widely applied technique of determining ancient geomag- netic pole positions, but several objections have been made to its use on strata older than the Tertiary: explicitly, I have listed elsewhere (Wesson, 1972) eleven problems which at present make the application of palaeomagnetism to Palaeozoic and Pre- Cambrian rocks of doubtful validity: these include departures of the ancient field from that of a dipole, palaeoclimatic consistency tests, polar wandering (magnetic and geographical), the magnetic effects of laterite etc., magnetostriction and storm- induced electric currents. It also now appears that the effects of pressure and shocks on magnetic rocks (see Stacey, 1962; Shapiro, 1966; and Kapitsa, 1955 for theoretical considerations of this effect) can be considerable. One notable case showed permanent changes of order 10 gammas after an explosion and a pattern of changes of a few gammas across a nearby fault reminiscent of the lineations associated with sea floor spreading from ridges (Hasbrouck and Allen, 1972). The test discussed in the next section should be able to test the validity of palaeomagnetism as well as providing a possible way to detect ancient poles in other planets.

2. Rock Studies

There are three ways I can think of to test the expectation that theflux and energy of ancient cosmic rays was higher near palaeomagnetic poles than elsewhere; or, equiva- lently, that the flux was a minimum at a given ancient geomagnetic equator. All three ways assume that the cosmic ray flux was not varying on a short (< 106 yr) timescale in the geological past (this is known to be true of the medium-energy solar particles: Lavrukhina and Ustinova, 1971), and they all need specimens to be taken from ancient land surfaces as now represented by disconformities, transgression surfaces, littoral deposits, lava beds, etc. on the Earth. They all have in common the fact that an enormous amount of information can be derived from a small sample: the track density is 108-109 cm -2 at depths of about 10 -4 cm, to ~ 10 ~ at depths of 1-10 cm.

COSMIC RAYS AND ANCIENT PLANETARY MAGNETIC FIELDS 323

Both track densities and track lengths are very sensitive indicators of the type and energy of the original cosmic rays. The three suggestions comprising the test are:

(i) Spallationrecoilsinducedbycosmicrays. These can induce a sensible background, and in practice could, I think, be now observed although it would be a difficult task. Only samples of minerals which have been covered by less than the equivalent of 1 km of water for long periods of time and which have very low concentrations of uranium could be expected to exhibit spallation recoils from cosmic ray interactions. This means that acidic rocks, like obsidian, would not be of use due to their high U content. Basic and ultrabasic rocks do, however, offer a real hope of successful detection, and some types of mica (e.g., biotite, phlogopite) have already been use- fully examined (Price and Walker, 1966) by etching suitable crystals with acid. This sub-test can, in principle, be applied to the Earth, Mars, the Moon and other planets.

(ii) Fission of U impurities induced by nucleon-meson components of the cosmic ray flux, or fission events induced by the interaction of cosmic ray particles with heavy element impurities in the sample. This method seems to me to be less likely of success- ful application than (i) because of the very minute effect being looked for; but Lal (1972a) has given the rate of formation of tracks due to fission, from which it is possible to obtain the expected track density at the surface of the Earth (or, rather, at the bottom of the attenuating atmosphere). The equivalent depth is ~ 600 gm cm- 2 or the equivalent of 150 cm of lunar soil, and the resulting track density would be only ~ 1 track cm -2 (106 yr) -I. The contribution of impure fission events is small in some large crystals of silicates, and so it should be possible to study them and estimate the extent to which tracks have been contributed by fission of impurities. The non- spurious cosmic ray effect to be looked for is minute, though, and it is more likely that fission tracks induced by secondary cosmic ray particles (in favourable cases) might be seen. Thus, this effect may be detectable at mountain altitudes or in polar regions where the amount of shMding by rocks is less than 100 gm cm -2. These comments apply to the Earth; for any planet which supports a field and has a low- density atmosphere (of the type of Mars, say, where the atmospheric shielding is only about a tenth of that on Earth), the chance of successfully detecting the tracks is higher. The question of whether the Moon (Fleisher and Hart, 1973) ever had a large-scale field can almost certainly be investigated by studies of ancient cosmic ray tracks and (ii), like (i), can in principle be applied to all planets and satellites in the Solar System, including the Earth.

(iii) Low-energy cosmic rays cause isotope production at shallow depths in rocks. The presently observed proton and 0c-particle fluxes accomplish this for energies in the range 5-100 MeV per nucleon. Since cosmic rays appear to have been present since the origin of the Earth (Lal, 1972a, b) the study of isotope ratios at different places might give information that could be compared with palaeomagnetic pole positions to see if they match up. While Earth-bound and other data do give valuable knowledge on the existence of cosmic rays right back to the early history of the Solar

324 PAUL S. WESSON

System, the sub-test (iii) is not likely to be viable as regards application to the Earth's surface, because of the attenuating effect of the atmosphere (which screens out radia- tion of energy < 500 MeV). This objection to the use of (iii) does not apply to the Moon, where it could be used profitably in conjunction with data on the timescale of turn-over of the lunar regolith. It could be applied also to the investigation of a possible ancient field in Mars. The sub-test (iii) therefore could be used on satellites and planets with low-density atmospheres, but since only effects due to secondary mesons and neutrons can be expected in terrestrial rocks, (iii) is not appropriate to the Earth.

Of the three methods described above, the first is the most feasible, and should be within the domain of applicability of sensitive methods within a few years. One should also be able to detect ancient pole positions in other planets, especially in Mars and the Moon. Since the track-fading time in mica (for instance) is known to be greater than 10 s yr, I think there is a real possibility of being able to confirm or disprove palaeomagnetic pole positions back at least to Palaeozoic times in the Earth.

3. Difficulties of the Test

There are a lot of objections that can be thought of to the feasible employment of the methods (i), (ii), (iii) of the last section. Before the workability of the test can be established, the outline of Section 2 needs supplementing by (a) some data on cosmic ray effects in terrestrial rocks, particularly expected typical track densities or isotope concentrations; (b) specific data on the isotopes to be used in the spallation test, with an estimate of whether the effect would be detectable; (c) an estimate of the effect of spatial variations of the magnetic field on tracks and isotopes; (d) data on the observed cosmic ray effects in Moon rocks and how the variations of several orders of magni- tude in tracks and isotopcs might be averaged out to isolate the effect due to the mag= netic cutoff. Also, one should ask the following questions: (e) on the Earth, which isotopes should be examined (e.g., there are about 80 cosmogenic isotopes detected in meteorites) and how (f) would the abundance of atmospheric 3BAr dissolved in typical terrestrial rocks compare to that produced by cosmic rays ? (g) In view of the problem of detecting surface exposure times in Earth rocks, would it be better to attempt obtaining spectral information on the bombarding particles ? (h) How would the effect of altitude affect ancient cosmic ray samples ?

The objections (a)-(h) of the last paragraph are essentially topics that need practical laboratory work for their elucidation (Khan and Ahmad, 1975). The situation with them is roughly similar to that which existed when palaeomagnetism was a new discipline, and the large number of problems needing attention is a consequence of the previous scarcity of study on subjects relevant to cosmic ray tracks in geological strata.

In addition to the aforementioned problems, one should also mention the following: (i) To get information about the ancient magnetic fields, suitable rock samples must

COSMIC RAYS AND ANCIENT PLANETARY MAGNETIC FIELDS 325

be available. Since the methods proposed above measure the integrated flux back to some epoch, it would only be possible to determine the average field over a given time span unless the exposure of the sample can be accurately dated. Such averages would themselves be useful, and samples taken from all over the globe would locate the ancient pole if, as has been suggested (Wesson, 1972), it was trapped in some fixed location for long times in the remote past. This would be of particular importance in work directed at determining the position of a possible ancient lunar dipole. Alternatively, if the time of exposure of the sample could be determined accurately by measurements on the appropriate nuclide or by using the reversal history of the field itself as a standard of comparison, then more data on polar wandering might be obtained. (j) Rock samples for use in this test would have to possess roughly equal exposure times, since different exposure times would mimic the effect of different geomagnetic latitudes if numbers of tracks were being used as field indicators. Studies of the lengths of tracks and particle energies could perhaps be used to guard against this possibility, and in any case it should be feasible to select rock samples that were exposed for approximately equal times and then buried after an appropriate interval (burial of ancient lava flows by sediments during transgressions or by subsequently deposited layers of tuff, suggest themselves; considerable accuracy could be obtained by measuring numerous samples and analysing them statistically). The methods used to check for internal consistency in samples at present employed in palaeomagnetism are suitable for use here, and one must assume that relevant rock samples can be obtained in the absence, to date, of any laboratory study of the problem. One must also assume (k) that there have been no gross changes in the density of the Earth's atmosphere since the Palaeozoic (this assumption is well supported; see Meyerhoff, 1970), and that changes since epochs prior to this can be estimated by existing geo- chemical methods. (1) The three sub-tests given in Section 2 would probably be best applied in practice by determining the position of the ancient geomagnetic equator, and so inferring the positions of the poles. This is because the meson (secondary) and nucleon (primary) components of the cosmic ray flux affected by the Earth's magnetic field rise most steeply from a latitude of about 15 ~ levelling off at about 50 ~ The relative intensity of the meson component (for instance) at these latitudes is about 8:5. From 50 ~ to 90 ~ there is little variation, so it would seem more feasible, and certainly more accurate, to locate the equator (0 ~ rather than the poles (90~ In the test as outlined above, (m) the high-energy component of the primary cosmic ray flux (mainly Galactic in origin) is not modulated very much by the geomagnetic field, though it does produce secondary radiation that can reach the surface of the Earth. The solar cosmic ray flux is mainly protons, while the Galactic component contains an appreciable proportion of heavier nuclei (Z > 30). On a planet with an atmosphere, the two primary components could not be distinguished because the secondary components are similar; but on a planet with little or no atmosphere, the heavier nuclei (Z > 30) give ragged holes, rather than tracks, on interaction with rocks, so a means of relative discernment may be available.

326 PAUL S. WESSON

4. Conclusion

Provided the problems listed in the last section can be clarified by practical work,

the variat ion o f cosmic ray flux and intensity as a function o f geomagnetic latitude

should enable ancient pole positions to be located on planets that possess or possessed

large scale magnetic fields.

Acknowledgement

I am grateful to Professors R. R. Hillier, D. Lal and G. Bigazzi for helpful suggestions,

and to Professor A. Meyerhoff for checking the paper. This work was supported by

the Science Research Council o f England.

References

Fleisher, R. L. and Hart, H. R.: 1973, Nature 242, 104-105. Hasbrouck, W~ P. and Allen, J. H. : 1972, Bull. Seismol. Soc. Am. 62 (6), 1479-1487. Kapitsa, S. P. : 1955, Bull. Acad. Sci. USSR, Geophys., Ser. 6, 489-504. Khan, H. A. and Ahmad, I. N.: 1975, Nature 254, 126-127. Lal, D. : 1972a, Space Sci. Rev. 14 (1), 3-97. Lal, D.: 1972b, p. 60, in A. Elvius (ed.), From Plasma to Planet, J. Wiley-Interscience, London

(389 pp.). Lavrukhina, A. K. and Ustinova, G. K. : 1971, Nature 232, 462-463. Meyerhoff, A. A. : 1970, J. Geol. 78, 1-51. Price, P. B. and Walker, R. M. : 1966, J. Geophys. Res. 68 (16), 4847-4862. Rikitake, T. : 1968, Tectonophysics 6, 59-68. Shapiro, V. A. : 1966, Earth Physics 8, 61-73. Stacey, F. D.: 1962, Geomagnetica, Servito Meteorologico Naeional, Lisbon, 109-119. Wesson, P. S.: 1972, J. Geol. 80, 185-197.