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Sample 12002

Commentary Copyright © 2004 by Eric M. Jones.
All rights reserved.
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Last revised 8 December 2005.


PRELIMINARY EXAMINATION OF LUNAR SAMPLES, PART C :
CAUSE OF SECONDARY MAGNETIZATION IN LUNAR SAMPLES

G. W. Pearce (University of Toronto; Lunar Science Institute) and D. W.Strangway (NASA Manned Spacecraft Center)
(from the Apollo 16 Preliminary Science Report)

Introduction

All lunar samples that have been measured by various investigators (refs. 7-15 to 7-18) carry a natural remanent magnetization (NRM). This NRM is quite variable in its characteristics from sample to sample, although it is generally found to be a combination of two distinct components: a soft component unstable to alternating field demagnetization (AFD), and a hard component stable to AFD. In addition, many samples show a time-dependent effect.

The stable component is considered by most investigators to be of lunar origin. It is stable to AFD to 400 Oe, to temperatures of 750 C, and for times on the order of millions of years (ref. 7-19). This important component of the NRM is carried by single-domain-sized metallic iron grains and grains small enough to have only a few magnetic domains (i.e., grains with diameters from approximately 150 to 1000 Angstroms).

The unstable component can be eliminated by subjecting the sample to AFD in small fields of 20 to 50 Oe. It is much like an isothermal remanent magnetization (IRM) such as that induced by exposing the sample to steady fields of 10 to 50 Oe for a short time (ref. 7-20). Such fields might be produced by a permanent magnet, a coil, or a wire carrying direct current. For example, if the sample should be within a few inches of a wire carrying 10 A or more, such fields would be present. Because the field of a wire decreases linearly with distance from a wire carrying a current, a surge of 100 A would have a strong effect on a sample even a foot or more away. It is probable that there are many times that samples could experience such fields on their journey back from the Moon. This experiment was performed to test this probability.

The time-dependent or viscous remanent magnetization (VRM) is, in its more usual form, a strong magnetization that can be acquired in quite weak fields (a few oersteds) and that decays completely in approximately 8 hr if the sample is stored in a place with no magnetic field. This form of VRM is carried by very small particles (diameters of approximately 150 Angstroms) that are thermally unstable at room temperature (ref. 7-21).

Although all samples have been found to contain a stable component of NRM, the VRM component is found mainly in fragmental rock samples. The soft component has been found in igneous and fragmental samples from the Apollo 12, 14, 15, and 16 missions. The most complete documentation of the soft component is of the Apollo 12 samples: information on eight igneous rocks and one breccia is available.

All three containers used for returning the Apollo 12 samples (the two Apollo lunar sample return containers and the Surveyor bag) contained samples that had a soft component of magnetization. Because there is no one container that does not show the effect, it is difficult to determine the source of the field. It is also difficult to simulate fully the environment that the samples experience between collection on the Moon and delivery to the investigator.

Experiment

On the Apollo 16 mission, a tracer sample was returned to the Moon to be brought back with the Apollo 16 samples. The tracer chosen was a chip from Apollo 12 igneous rock 12002, which had a soft component after its original trip but had no VRM component (fig. 7-53). The chip was demagnetized to 400 Oe, leaving only the stable magnetization. It was then delivered inside the spacecraft in a μ-metal box and unpacked and installed approximately 30 hr before launch. Because samples normally do not make the outbound trip, it would have been more appropriate to keep the sample in the μ-metal box to protect it from magnetic fields until after lunar landing. However, as this would have been a difficult procedure, the location of the sample in the spacecraft was chosen to be similar on the outbound and inbound trips. During the outbound trip the chip was in the lunar module ascent stage; during the return trip, it was transferred to the command module with the other lunar samples.

The stowage location was quite similar to that of the Surveyor bag on the Apollo 12 spacecraft. The sample, inside a small Beta cloth bag, was attached to the flap of the interim stowage assembly (ISA), a bag of the type in which samples were returned on the Apollo 14, 15, and 16 missions. At recovery of the Apollo 16 command module, the sample was detached from the ISA and returned to the Manned Spacecraft Center (MSC) in one of the padded crates used for lunar samples. On arrival at MSC, the sample was taken to the magnetic properties laboratory at the Lunar Receiving Laboratory, where the NRM of the sample was measured several times during a period of 4 days. During this time, it was stored in a field-free room. No significant change occurred in the NRM during this storage test. Next, the sample was demagnetized by the AFD technique in steps to 100 Oe (fig. 7-53). It was possible to eliminate the soft, acquired component in the sample in an alternating field of 20 Oe. At this point, the direction and intensity of magnetization were approximately the same as they were in the sample before it left the Earth (fig. 7-54). In figure 7-55, the intensity of the stable component (2 X 10.6 emu/g) has been subtracted from the NRM so that the magnetization added by the Apollo 16 trip can be more easily compared with that of the tests described in the following discussion. The component added by the Apollo 16 trip is very similar in behavior to the original soft component of this rock when it first came back from the Moon. The intensity of the soft component was somewhat less than that found after the first (Apollo 12) trip.

In an attempt to simulate the soft component, a series of IRMs was induced in the sample by using steady fields of 10, 12, 15, 20, and 40 Oe for a period of 1 min each. This magnetization was then cleaned by the AFD technique. These demagnetization curves are compared with the curves of the original Apollo 12 NRM soft component and the Apollo 16 trip added component in figure 7-55. The IRM curves are similar to the curve of the trip-added magnetization. Thus, the trip-added magnetization is well simulated by an IRM that is induced by exposure to a steady field of 12 to 15 Oe applied for 1 min, and to simulate the original NRM would require exposure of the sample to 30- to 35-Oe fields.

Conclusions

During the return of lunar samples to Earth, at least some samples are subjected to magnetic fields of some tens of oersteds. These fields are capable of inducing in these samples a magnetization that can easily be removed upon AFD in relatively low fields (50 Oe or less). This magnetization is similar in behavior and intensity to the soft component of magnetization found in many lunar samples; therefore, the soft component is at least in part, if not totally, an artifact of the trip.

Acknowledgments

This experiment was conducted on short notice, and we thank the many people who helped the sample on its way.

References

7-1. Papanastassiou, D. A.; and Wasserburg, G. J.: Lunar Chronology and Evolution From Rb-Sr Studies of Apollo 11 and 12 Samples. Earth Planet. Sci. Letters, vol. 11, Aug. 1971, pp. 37-62.

7-2. Wasserburg, G. J.; and Papanastassiou, D. A.: Age of an Apollo 15 Mare Basalt; Lunar Crust and Mantle Evolution. Earth Planet. Sci. Letters, vol. 13, no. 1, Dec. 1971, pp. 97-104.

7-3. Murthy, V. R.; Evensen, N. M.; Jahn, B.; and Coscio, M. R., Jr.: Rubidium-Strontium and Potassium-Argon Age of Lunar Sample 15555. Science, voL 175, no. 4020, Jan. 28, 1972, pp. 419-420.

7-4. Turner, Grenville: 40Ar-39Ar Ages From the Lunar Maria. Earth Planet. Sci. Letters, vol. 11, Aug. 1971, pp. 169-191.

7-5. Hartmann, William K.: Lunar Cratering Chronology. Icarus, vol. 13, no. 2, Sept. 1970, pp. 299-301.

7-6. Trask, N. J.; and McCadiey, J. F.: Differentiation and Volcanism in the Lunar Highlands. Earth Planet. Sci. Letters, vol. 14, Mar. 1972, pp. 201-206.

7-7. Milton, D. J.; and Hodges, C. A.: Geologic Maps of the Descartes Region of the Moon, Apollo 16 Pre-Mission Map. U.S. Geol. Survey Misc. Geol. Inv. Map 1-748, sheets 1 and 2, 1972.

7-8. Wilhelms, D. E.; and McCauley, J. F.: Geologic Map of the Near Side of the Moon. U.S. Geol. Survey Misc. Geol. Inv. Map 1-703, 1971.

7-9. Apollo 15 Preliminary Examination Team: The Apollo 15 Lunar Samples: A Preliminary Description. Science, vol. 175, no. 4020, Jan. 28, 1972, pp. 363-374.

7-10. Hubbard, N. J.; Rhodes, J. M.; Gast, P. W.; Bans'd, B.M.; Wiesmann, H.; and Church, S. E.: Nonmare Basalts: Part II. Proceedings of the Third Lunar Science Conference, vol. 2, Dieter Heymann, ed., MIT Press (Cambridge, Mass.), 1972.

7-11. The Lunar Sample Preliminary Examination Team: Preliminary Examination of Lunar Samples From Apollo 12. Science, voh 167, no. 3923, Mar. 6, 1970, pp. 1325-1339.

7-12. The Lunar Sample Preliminary Examination Team: Preliminary Examination of Lunar Samples From Apollo 14. Science, vol. 173, no. 3998, Aug. 20, 1971, pp. 681-693.

7-13. Moore, C. B.; Gibson, E. K.; Latimer, J. W.; Lewis, C. F.; and Nichiporuk, W.: Total Carbon and Nitrogen Abundances in Apollo 11 Lunar Samples and Selected Achondrites and Basalts. Proceedings of tile Second Lunar Science Conference, vol. 2, A. A. Levinson, ed., Pergamon Press (New York), 1970, pp. 1375-1382.

7-14. Wahl, Walter: The Brecciated Stony Meteorites and Meteorites Containing Foreign Fragments. Geochim. Cosmochlm. Acta, vol. 2,1952, pp. 91-117.

7-15. Strangway, D. W.; Peatce, G. W.; Gose, W. A.; and Timme, R. W.: Remanent Magnetization of Lunar Samples. Earth Planet. Sci. Letters, vol. 13, no. l, Dec. 1971, pp. 43-52.

7-16. HargIaves, R. B.; and Dorety, N: Magnetic Property Measurements on Several Apollo 14 Rock Samples. Lunar Science-lll, Carolyn Watkins, ed. (Rev. abs. of the Third Lunar Science Conference (Houston, Tex.), Jan. 10-13, 1972), pp. 357-359.

7-17. Runcorn, S. K.; Collinson, D. W.; O'Reilly, W.; and Stephenson, A.: Magnetic Properties of Lunar Rocks and Fines. Lunar Science-Ill, Carolyn Watkins, ed. (Rev. abs. of the Third Lunar Science Conference (Houston, Tex.), Jan. 10-13, 1972), pp. 669-671.

7-18. Nagata, T.; Fisher, R. M.; and Schwerer, F. C.: Lunar Rock Magnetism. The Moon, vol. 4, nos. 1/2, Apr. 1972, pp. 160-186.

7-19. Pearce, G. W.; Strangway, D. W.; and Gose, W. A.: Remanent Magnetization of the Lunar Surface. Proceedings of the Third Lunar Science Conference, vol. 3, David R. Criswell, ed., MIT Press (Cambridge, Mass.), 1972.

7-20. Nagata, Takeshi: Rock Magnetism. Maruzen Co. (Tokyo), 1961.

7-21. Neel, L.: Theorie du Trainage Magnetique des Ferro- magnetiques en Grains Fin Avec Application aux Terres Cuites. Ann. Geophysique, vol. 5, 1949, pp. 99-136.


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