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Magnetization, Paleomagnetic Poles, and Polar Wander on Mars
Icarus 147, 26–34 (2000) doi:10.1006/icar.2000.6439, available online at http://www.idealibrary.com on
Magnetization, Paleomagnetic Poles, and Polar Wander on Mars Kenneth F. Sprenke and Leslie L. Baker College of Mines and Earth Resources, University of Idaho, Moscow, Idaho 83844 E-mail: [email protected] Received October 26, 1999; revised March 2, 2000
regions (Fig. 1). There, field intensities exceeding 1500 nT were measured at altitudes of near 100 km. The presence of these large anomalies over the ancient crust and their absence over younger terrain revealed that Mars, devoid of a global field at present, must have had an internal active dynamo in its ancient past. Connerney et al. (1999) presented analytical models of the extended magnetic sources in the Terra Sirenum and Terra Cimmeria region and their implications. Because of variations in orbital heights between and along tracks, direct comparison of the magnetic profiles is difficult. However, they noted that the polarities of the crustal magnetic anomalies were organized in extensive quasi-parallel east–west trending lineations from 1000 km to as long as 2000 km. Their quantitative analyses used four representative tracks of the MGS observations (Fig. 1) acquired from orbits with periapses near the 180◦ W meridian to explore models of crustal magnetization consistent with the observations. They noted that intense positive radial (vertical) features were separated by equally intense negative radial features, suggestive, in their opinion, of bands of crustal remanent magnetization of alternating sign. Although they interpreted the alternating magnetization to be suggestive of sea-floor spreading, they modeled the anomalies as resulting from unconstrained directions of remanent magnetization in the martian crust (Fig. 2). Although the Connerney et al. (1999) models are somewhat reminiscent of the magnetization directions in the terrestrial sea floor, they are very different in detail. On earth, paleomagnetic inclinations shift sign between reversals, but this shift occurs within a relatively short time interval as a 180◦ shift in field direction. In the Connerney et al. (1999) model, the transition between reversals takes as long as the reversed intervals, and the degree of change in the field direction appears to be random. For this paper, we have re-modeled the data presented by Connerney et al. (1999). We demonstrate that the anomalies can be modeled by well-constrained directions of remanent magnetization such as would be produced by an ancient areocentric dipole field either of constant or alternating polarity. Our interpretation does not require complex directions of magnetization, and it results in much smoother models of remanent magnetization in the martian crust. For the origin of the magnetic anomalies, we considered two different hypotheses, both constrained
Using the geometry of an areocentric dipole field, we have remodeled the anomalies observed in the Terra Sirenum and Terra Cimmeria regions by the Mars Global Surveyor (MGS). For the origin of the anomalies, we used both magnetic stripe and magnetic shell hypotheses. For magnetic stripe models, we assume that sea-floor spreading formed the anomalies in the presence of a reversing axial dipole field. We found that a simple magnetic stripe model with constant magnetization and crustal thickness results in poor agreement with the observed anomalies. However, by allowing the intensity of magnetization to vary freely between and along the stripes, a reasonable fit to the observed data can be found while maintaining the observed polarity along the stripes. Applying the simple magnetic stripe model to the Terra Sirenum and Terra Cimmeria anomalies results in best-fit spreading centers located well to the north of the surviving magnetic stripes. This could possibly explain the apparent lack of symmetry that has been noted in the anomalies. For the magnetic shell hypothesis, we assume that a monopolarity dipole field magnetized the early crust of Mars. The anomalies that persist today would then be the result of partial demagnetization of the original magnetic shell by more recent processes. The magnetic anomalies in Terra Sirenum and Terra Cimmeria are consistent with this model, especially if the magnetization of the original magnetic shell was laterally inhomogeneous. If the monopolarity magnetic shell hypothesis is correct, best-fit magnetic paleopole locations for the original magnetization lie in a sub-equatorial belt centered near the prime meridian. If the ancient magnetic pole of Mars coincided with the spin pole, this result implies that either apparent or true polar wander has occurred on Mars. Apparent polar wander would be consistent with plate tectonics having occurred after the formation of the magnetic anomalies. True polar wander has previously been proposed to explain certain morphologic features found on Mars. °c 2000 Academic Press Key Words: Mars, surface; magnetic fields.
INTRODUCTION
In 1997 and 1998, the Mars Global Surveyor (MGS) detected magnetic anomalies due to unknown features in the martian crust (Acu˜na et al. 1998, 1999). These anomalies were observed to correlate with the ancient cratered terrain of the martian highlands, particularly in the Terra Sirenum and Terra Cimmeria 26 0019-1035/00 $35.00 c 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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FIG. 1. Map of Mars showing the tracks of the four magnetic profiles of Connerney et al. (1999).
by directions of magnetization consistent with a simple areocentric axial dipole field. MODELS
The procedure we followed was similar to that outlined by Connerney et al. (1999) for their analysis of the same data. The sources of the magnetic anomalies in the Terra Sirenum and Terra Cimmeria regions are represented by a group of up to 40 uniformly magnetized horizontal crustal blocks each 30 km in depth and extended in the east–west direction. Our method differs from that of Connerney et al. (1999) in that, in all our models, we constrain the directions of magnetization in the blocks to be consistent with an areocentric dipole field. Several of our models further constrain the crustal blocks to be not only of constant thickness but also of constant magnetization.
FIG. 2. The near-random paleomagnetic inclinations inferred by the crustal model of Connerney et al. (1999). Paleomagnetic inclination is shown as a function of distance north of latitude 53◦ S and is defined in this case as a fourquadrant inverse tangent of J z/J x. This result is for track 1999 Day 15 P6.
For all our models, the magnetic field of each crustal block was computed using conventional formulae for a two-dimensional horizontal slab (e.g., Heiland 1940). In Fig. 3, the horizontal and vertical components of the magnetic anomaly 100 km above a single two-dimensional slab of thickness 30 km and width 100 km are shown. The slab in this example is uniformly magnetized (20 A/m) at an inclination angle of 45◦ in the direction of the profile (S–N). Readers unfamiliar with magnetic modeling
FIG. 3. The horizontal (Bx) and vertical (Bz) magnetic anomalies at a height of 100 km over a two-dimensional slab of material with magnetization contrast J = 20 A/m, inclination I = 45◦ , and strike angle α = 90◦ . Note that a single uniformly magnetized slab produces both positive and negative anomalies on both components. The same result would occur if J = +10 A/m and the magnetization of the surrounding material were J = −10 A/m.
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should note that uniform magnetization of a single slab produces both positive and negative magnetic anomalies. It is not necessary to reverse the polarity of the magnetization of the source to produce anomalies of alternating sign. It is also important to note that it is the lateral contrast in magnetization that causes an anomaly. The anomaly in Fig. 3 would be identical if the slab were magnetized 10 A/m within a country rock magnetized the same amount but in the opposite direction. The two-dimensional assumption in our models is critical because this forces the east component of the magnetic anomalies to be zero. Connerney et al. (1999) noted that the east component of the magnetic anomalies is small and that the magnetic anomalies are extended in the east–west direction. The 30-km slab thickness is arbitrary; in a thin slab model, the product of plate thickness and volume magnetization is the unknown being sought. For example, a 15-km-thick slab would require twice the magnetization to produce the same anomaly as a 30-km-thick slab. For each profile, the horizontal and vertical components of the observed field were fit to a crustal model consisting of up to 40 crustal blocks, uniformly magnetized, 30 km thick, and 100 km wide. The volume magnetization (J ) of each block (or for the whole ensemble for models with constant J ) is determined by a least-squares procedure that, under the constraints of the particular model, finds the best fit between the observed and calculated anomalies. The reader is referred to Connerney et al. (1999) for further technical details of the general inverse method that we have used. We considered two different hypotheses for the origin of the magnetic anomalies, a magnetic stripe model and a magnetic shell model. These hypotheses correspond to two different physical models for formation of the magnetic stripes, but from the perspective of magnetic modeling, the distinction between the magnetic stripe model and the magnetic shell model is subtle. They are similar in that, in both models, the anomaly is caused by a lateral contrast in magnetization, which works out to be more or less the same in both models. They are different in that, in the magnetic stripe model, the directions of magnetization from block to block are determined solely by the latitude of an assumed spreading center. For the magnetic shell model, the directions of magnetization vary according to the paleolatitude and paleolongitude of each crustal block relative to an assumed paleopole position. For all models, the direction of magnetization was constrained by the paleomagnetic inclination (I ) and paleomagnetic declination (D) of an areocentric dipole field, as calculated by the dipole equation. For the magnetic stripe model, the north magnetic pole was fixed at the present-day axis of rotation (D = 0◦ ), whereas, for the single-plate model, many trial paleopole locations were employed. As in all potential field inverse procedures, the results depend on the assumptions in the particular models; they are by no means unique. They show no more than a consistency between the observations and the model assumptions.
MAGNETIC STRIPE MODEL
The magnetic stripe model attempts to explain the anomalies as the result of sea-floor spreading in the ancient crust of Mars (Sagan and Pollack 1966, Sleep 1994). The assumption is that an ancient field of alternating polarity magnetized each crustal block at a central spreading center. Each block has then moved away from the center, retaining the magnetization directions acquired at the spreading center, resulting in magnetic stripes in the crust. Connerney et al. (1999) have proposed that sea-floor spreading on Mars could result in magnetic stripes of characteristic width 100 km if spreading rates on Mars were faster than those on Earth or if the rate of dipole reversals on Mars were less than on Earth. To implement this model, the intensity of magnetization and crustal thickness are held constant between and along the stripes. The stripes are presumed continuous in the east–west direction. The magnetic anomaly signature is thus controlled completely by the polarity of the field that magnetized each stripe. The directions of magnetization are constrained to those of a reversing areocentric axial dipole field at the spreading center. All the stripes in the model have either a positive or a negative magnetization vector in the same fixed direction. For the magnetic stripe model, 181 trial spreading center locations at 1◦ intervals of latitude along the 180◦ W meridian were tested. The magnetic stripes were assumed to continue unchanged in geometry, magnetization, and polarity across all four magnetic anomaly profiles. To obtain the best-fit magnetization for the stripes, a three-step inversion procedure was followed. First, for each trial spreading center latitude, a least-squares procedure was used to find the magnetization values for the 40 stripes that simultaneously best fit the combined four profiles. For this step, the magnetization values were free to be positive or negative but were fixed in inclination and declination. Second, the signs of the magnetization values were used to find the polarity of each stripe. Finally, a further least-squares procedure was used to find the best-fit constant magnetization for the entire ensemble with polarities as fixed in the second step. Results for the Magnetic Stripe Model The results for the simple magnetic stripe model are exemplified in Fig. 4, which shows a map of the crustal magnetizations as well as the observed and modeled results for one of the profiles. This result is for a possible spreading center at latitude 53◦ S, more or less in the midst of the profiles. The best-fit constant crustal magnetization for a spreading center at this location is a 7.3 A/m (30-km-thick crust). The magnetic stripes range from 100 to 400 km in width. For all profiles combined, the correlation coefficient (r 2 ) is 0.29. Although the fit is not very good, the general shape of the peaks and troughs in the magnetic profiles are somewhat reproduced by this simple magnetic stripe model. The variation in quality of fit with the assumed location of the spreading center is illustrated in Fig. 5. The best-fit spreading center is at 24◦ N, although possible centers at 20◦ S, 4◦ N, and
MAGNETIZATION AND POLOR WANDER ON MARS
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FIG. 4. Crustal magnetization map for the simple magnetic stripe model. The inset compares observed (dashed lines) and modeled (solid lines) horizontal (Bx) and vertical (By) magnetic anomalies along one track to illustrate the quality of fit; the other three tracks are similar. The origin of the distance axis is at latitude 53◦ S. The observed data, which we have resampled and smoothed to 100-km intervals, are from Connerney et al. (1999).
55◦ N also provide reasonable fits to the data, but even the best fit has a correlation coefficient less than 0.5. All these better fitting possible spreading centers are well to the north of the surviving magnetic stripes, thus possibly explaining the lack of symmetry that is apparent in the anomalies. Figure 6 illustrates a more general, but less geologically ideal, attempt to match the magnetic stripes to the observed anomalies. Here the polarity of the magnetization is maintained in each
stripe, but the intensity of magnetization is allowed to change freely both along each stripe and from stripe to stripe. The correlation coefficient for the combined data set is 0.68. This results in more acceptable fits to the anomalies, but the intensity of magnetization within and between stripes ranges widely, from 0 to ±35 A/m. The results of our magnetic stripe models suggest that, if the magnetic lineations are due to sea-floor spreading in the
FIG. 5. The quality of fit (r 2 ) between observed and calculated magnetic anomalies for a simple magnetic stripe model as a function of modeled spreading center latitude. For each peak, the best-fit mean magnetization for a 30-km-thick crust is also shown.
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FIG. 6. Crustal magnetization map for the more complex magnetic stripe model which allows crustal magnetization to vary along and between stripes. See the caption of Fig. 4 for other details.
ancient crust of Mars, then the ancient spreading center may have been located well to the north of the surviving magnetic stripes in Terra Sirenum and Terra Cimmeria. The ancient crust of the southern highlands may therefore have spread considerably south since its formation. The wide range of intensity of magnetization, within and between the stripes, required to match the observations would seem to argue against a magnetic stripe model. But, as Connerney et al. (1999) point out, the ancient highlands may have been reworked by impacts, fragmentation, and thermal events subsequent to their original magnetization. Such reworking might explain the inhomogeneous magnetization required by the model. MAGNETIC SHELL HYPOTHESIS
Our second model uses a magnetic shell concept. We presume the crust was magnetized in place as it cooled under a monopolarity areocentric axial dipole field, forming a magnetic shell around the planet. This hypothesis is consistent with a hot early Mars immediately after accretion, followed by rapid cooling and crust formation (Pepin and Carr 1992, Schubert et al. 1992). If the ancient dynamo operated for several hundred million years after accretion, sufficient time would have been available for the ancient iron-rich crust to form and to acquire a remanent magnetization. This magnetization, if undisturbed by reheating or reworking, would retain information on the direction and magnitude of the ancient dipole field. If undisturbed, such a shell would not produce an external field. However, the magnetization in much of this shell has been destroyed by impacts, younger volcanism, and perhaps metamorphic and hydrothermal activity, providing the contrast in magnetization that
results in the observed anomalies today. If, in certain areas, long fracture zones controlled this demagnetization, then long linear magnetic anomalies such as those observed in the southern highlands might result. This model does not preclude the possibility of plate tectonics on Mars prior to, during, or subsequent to the magnetization of the early crust. Sleep (1994), for example, has proposed a sea-floor spreading model of Mars to account for the relatively youthful northern plains and crustal dichotomy. To implement this model, we assume that magnetization of a ancient crustal shell, of constant 30-km thickness, was “broken” (somehow, by one of the demagnetization processes described above) into discrete east–west extended slabs, 100 km or more in width. Under the restriction that the directions of magnetization were constrained to a monopolarity areocentric dipole, we tested two variants of this model. In the first, the intensity of magnetization was allowed to change with location. In the second, the intensity of magnetization, like the crustal thickness, was held pervasively constant. For the magnetic shell models, some 600 trial pole positions at 10◦ spacing around the globe were tested. For each assumed paleopole position, a non-negative least-squares procedure was then used to find a distribution of magnetization that best fit the observations under the constants imposed by the paleopole position. For the constant magnetization models, the results of the above-described procedure were further refined. The median magnetization of the ensemble of the magnetized crustal blocks was calculated. Those crustal blocks with magnetization less than the median value were eliminated from the model. Those remaining were subjected to a further least-squares procedure that found the best-fit constant magnetization for the combined data for all four profiles.
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FIG. 7. Crustal magnetization map for the magnetic shell model. The shaded areas represent remnants of the original monopolarity magnetic shell. The surrounding white areas have been demagnetized. See the caption of Fig. 4 for other details.
Results for the Magnetic Shell Model The results of the magnetic shell model are illustrated by Figs. 7 and 8, which show maps of crustal magnetization and representative fits between the calculated and observed magnetic anomalies for two variants of the model. In the first, the magnetization and thickness of the shell are constant. In the second, the magnetization in the broken shell is laterally inhomogeneous. These particular results are for a trial paleomagnetic pole near Schiaparelli Crater. The physical dimensions and volume mag-
netization of the sources are, without doubt, much more complex than as modeled. The results exemplified by Fig. 7 show that a magnetized plate, of constant thickness and of monopolarity magnetization, can produce, if broken, a pattern of positive and negative anomalies similar to that observed on Mars. Alternating reversals in the direction of crustal magnetization are not required to explain the magnetic anomalies on Mars. The contrast in magnetization between the magnetized crustal blocks and the surrounding terrain, in which crustal magnetization has been destroyed, is sufficient
FIG. 8. Crustal magnetization map for a more complex magnetic shell model. The remnants of the original monopolarity magnetic shell have laterally inhomogeneous magnetization. See the caption of Fig. 4 for other details.
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FIG. 9. Map of Mars showing predicted magnetic paleopole locations (gray areas) based on our magnetic anomaly modeling. Also shown is a polar wander curve for the south pole of Mars (Schultz and Lutz 1986). The solid triangles are thought to represent stable spin pole positions based on geomorphology; UP0 is the oldest, followed by C0 , B0 , A0 , and AP0 . The open triangles are ancient spin pole positions inferred from the record of grazing impacts; 40 is the oldest, followed by 30 and 20 . The contours are RMS residuals between modeled and observed magnetic field anomalies in Terra Sirenum and Terra Cimmeria for 590 trial magnetic paleopole positions on the surface of Mars. The lowest residuals (gray areas) form a sub-equatorial belt near the prime meridian. The x’s indicate pole locations where the inverse process is unstable.
to reproduce the pattern of positive and negative anomalies observed. The correlation coefficient for the combined data set is 0.53. The best-fit constant crustal magnetization for the ancient magnetic shell is 17.8 A/m for a 30-km thickness. The results exemplified by Fig. 8 show that a better fit between the observed and calculated anomalies can be obtained if the monopolarity magnetization in the original shell was laterally inhomogeneous. Spatial variability in the original shell might arise from changes in mineralogy, petrology, and structure from place to place on early Mars. The correlation coefficient for the combined data set for this model is 0.86. The crustal magnetization required to fit the data well by this model, assuming a 30-km-thick crust, averages about 20 A/m, but ranges to as high as 70 A/m. Thinner or thicker crust would produce correspondingly higher or lower intensities of magnetization. Under the monopolarity magnetic shell hypothesis, a contour plot of the RMS errors for trial paleomagnetic pole positions on Mars indicates that the lowest residuals occur along a sub-equatorial belt centered at about 20◦ S latitude on the prime meridian (Fig. 9). This pattern holds for both variants of the magnetic shell model. Paleomagnetic poles anywhere within this belt produce a reasonably good fit with the observations. The better fitting paleopole locations do not include the present-day geographic poles of Mars. If the monopolarity magnetic shell hypothesis is correct, then either apparent polar wander or true polar wander has occurred on Mars. The limited extent and geometry of the four magnetic profiles available create some singularities with our analysis procedure for determining paleopole locations. For example, the cusps near the equator at longitudes 90◦ W and 270◦ W (marked
with x’s in Fig. 9) are artifacts. Trial paleopoles here happen to result in magnetic field directions parallel to the presumed east–west structures, resulting in computational insensitivity to magnetization changes. As more magnetic data from different regions of Mars become available, these artifacts should be less of a problem. Apparent Polar Wander The result, for the magnetic shell model, that the most probable paleopole positions do not include the present spin poles is intriguing. Apparent polar wander is certainly a viable possibility if sea-floor spreading, as proposed by Sleep (1994), formed the relatively young, northern lowlands. If plate tectonics has occurred on Mars, then the apparent paleopole near the equator at the prime meridian gives information about the plate motion. The lithospheric plate that includes the older magnetic crust in the Terra Sireum and Terra Cimmeria regions appears to have rotated through an angular distance of 90◦ about a plate rotation pole near the equator at 90◦ W longitude. If the ancient field was reversed polarity, the plate moved south from the northern hemisphere along the 180◦ W longitude to its present position. If the ancient field was normal polarity, the plate has moved from the southern hemisphere south along the prime meridian across the south pole to its present position. Additional magnetic data from Mars will greatly help establish the validity of plate tectonics on the planet. True Polar Wander If relative plate motion has not occurred on Mars, the possible paleomagnetic pole positions shown in Fig. 7 may coincide
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with ancient martian spin poles. Mars has long been considered a likely planet for true polar wander, a reorientation of an entire planetary surface with respect to its spin axis. True polar wander is thought to result from changes in the inertia tensor as a result of internal mass redistribution. Ward et al. (1979) and Melosh (1980) suggested that the spin pole of Mars moved as a result of the rise of Tharsis. McAdoo and Burns (1975) suggested that the Coprates Trough assemblage provides evidence of tensile stresses developed by polar wander on Mars, although Grimm and Solomon (1986) argue against this. The record of grazing impacts has been used to locate possible ancient orbital pole points (Schultz and Lutz-Garihan 1982). Geomorphic studies have shown that regions of mantled and layered terrain near the martian equator are similar to present polar deposits, providing additional proposed ancient paleopole locations (Schultz and Lutz 1988). If Terra Sirenum and Terra Cimmeria have not moved relative to the rest of the planetary surface, then our paleomagnetic pole locations appear to provide independent evidence for polar wander on Mars. A proposed wander path for the south pole of Mars presented by Schultz and Lutz (1988) is shown on Fig. 9. The solid triangles represent pole positions based largely on geomorphic evidence; the open triangles represent possible pole positions based on grazing impacts. Our possible paleomagnetic pole positions derived from the magnetic anomalies (the gray shaded areas in the equatorial regions near the prime meridian) agree remarkably well with the Schultz and Lutz (1988) model for polar wander. A possible paleomagnetic south pole position, compatible with the magnetic anomalies, that is also along the Schultz and Lutz polar wander path is near 45◦ E, 15◦ S, southeast of C0 . This ancient pole corresponds to the chaotic terrains of eastern Valles Marineris, and at the antipode, to the southeast slope of Elysium Mons, not far from the very old and heavily eroded deposits south of Elysium Planitia. Such highly eroded terrains might be indicative of polar geomorphology. The timing of a spin pole at this position would be somewhat earlier than the stable position of C0 proposed by Schultz and Lutz (1988). It would nonetheless be consistent with the emplacement of the highly magnetized rocks of the ancient cratered highlands, including Terra Cimmeria and Terra Sirenum, in early Mars. Another possible fit between the paleomagnetic data and the geomorphic data occurs southeast of Schiaparelli Crater near 20 , just south of the proposed stable pole at A0 on Fig. 9. However, according to Schultz and Lutz (1988) this stable pole position is contemporaneous with the development of the Tharsis fracture system and the beginning of Arsia Mons. It would seem to be too young to be related to the highly magnetized rocks in Terra Cimmeria and Terra Sirenum. CONCLUSIONS
We have applied an areocentric axial dipole hypothesis to evaluate the magnetic anomalies in southern highlands of Mars. We
have considered magnetic stripe and magnetic shell hypotheses for the origin of the anomalies. For the magnetic stripe models, we could not satisfactorily fit the magnetic data to simple magnetic stripes of alternating polarity and constant magnetic intensity. Only by allowing the intensity of magnetization to vary freely within and between stripes could a satisfactory fit be obtained. This suggests, that if the anomalies did form as sea floor stripes, they have been reworked greatly since their formation. The best-fit spreading centers for the magnetic stripe model are located well to the north of the southern highlands. This result possibly explains the observed lack of symmetry in the anomalies. For the magnetic shell model, fits are best if the original shell is allowed to vary in intensity of magnetization (or thickness). For this model, the most likely magnetic paleopoles fall on a subequatorial belt near the prime meridian. If the monopolarity magnetic shell model is correct, then either apparent or true polar wander has occurred on the planet since the anomalous material was magnetized. If apparent polar wander has occurred, the plate on which Terra Sirenum and Terra Cimmeria are located has moved through a 90◦ arc about a center of plate rotation near the equator at 90◦ W. If true polar wander has occurred, then the spin pole of Mars was near the equator close to the prime meridian at the time of the magnetization of the original magnetic shell. The limited crustal magnetic data released to date, as provided by Connerney et al. (1999), do little to resolve the controversy as to whether plate tectonics or true polar wander has occurred on Mars in the past. The implications of either plate tectonics (Sagan and Pollack 1966, Sleep, 1994) or true polar wander (Schulz and Lutz 1988) for understanding the planet are huge, and it appears that the paleomagnetic studies may provide the key to the geological history of Mars, just as they have on Earth.
ACKNOWLEDGMENTS We thank Norm Sleep and Robert Hargraves for helpful comments. Our research was partially supported by the NASA Idaho Space Grant Consortium and NSF POWRE Grant EAR-9973345 to L. Baker.
REFERENCES Acu˜na, M. H., and 19 colleagues 1998. Magnetic field and plasma observations at Mars: Initial results of the Mars Global Surveyor Mission. Science 279, 1676–1680. Acu˜na, M. H., and 12 colleagues 1999. Global distribution of crustal magnetization discovered by the Mars Global Surveyor MAG/ER experiment. Science 284, 790–793. Connerney, J. E. P., M. H. Acu˜na, P. J. Wasilewski, N. F. Ness, H. Reme, C. Mazelle, D. Vignes, R. P. Lin, D. L. Mitchell, and P. A. Cloutier 1999. Magnetic lineations in the ancient crust of Mars. Science 284, 794– 798. Grimm, R. E., and S. C. Solomon 1986. Tectonic tests of proposed polar wander paths for Mars and the Moon. Icarus 65, 110–121.
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Heiland, C. R. 1940. Geophysical Exploration, pp. 389–402. Prentice Hall, New York. McAdoo, D. C., and J. A. Burns 1975. The Coprates Trough assemblage: More evidence for martian polar wander. Earth Planet. Sci. Lett. 25, 347–354. Melosh, H. J. 1980. Tectonic patterns on a reoriented planet: Mars. Icarus 44, 745–751. Pepin, R. O., and M. H. Carr 1992. Major issues and outstanding questions. In Mars (H. H. Keiffer, B. M. Jakosky, C. W. Snyder, and M. S. Matthews, Eds.), pp. 147–183. Univ. of Arizona Press, Tucson. Sagan, C., and J. Pollack 1966. On the nature of the canals of Mars. Nature 212, 117–121.
Schubert, G., S. C. Solomon, D. L. Turcotte, M. J. Drake, and N. H. Sleep 1992. Origin and thermal evolution of Mars. In Mars (H. H. Keiffer, B. M. Jakosky, C. W. Snyder, and M. S. Matthews, Eds.), pp. 147–183. Univ. of Arizona Press, Tucson. Schultz, P. H., and A. B. Lutz-Garihan 1982. Ancient polar locations on Mars: Evidence and implications. J. Geophys. Res. Suppl. 87, A84–A96. Schultz, P. H., and A. B. Lutz 1988. Polar wander of Mars. Icarus 73, 91–141. Sleep, N. H. 1994. Martian plate tectonics. J. Geophys. Res. 99, 5639–5655. Ward, W. R., J. A. Burns, and O. B. Toon 1979. Past obliquity oscillations of Mars: The role of the Tharsis uplift. J. Geophys. Res. 84, 243– 259.
Magnetization, Paleomagnetic Poles, and Polar Wander on Mars Kenneth F. Sprenke and Leslie L. Baker College of Mines and Earth Resources, University of Idaho, Moscow, Idaho 83844 E-mail: [email protected] Received October 26, 1999; revised March 2, 2000
regions (Fig. 1). There, field intensities exceeding 1500 nT were measured at altitudes of near 100 km. The presence of these large anomalies over the ancient crust and their absence over younger terrain revealed that Mars, devoid of a global field at present, must have had an internal active dynamo in its ancient past. Connerney et al. (1999) presented analytical models of the extended magnetic sources in the Terra Sirenum and Terra Cimmeria region and their implications. Because of variations in orbital heights between and along tracks, direct comparison of the magnetic profiles is difficult. However, they noted that the polarities of the crustal magnetic anomalies were organized in extensive quasi-parallel east–west trending lineations from 1000 km to as long as 2000 km. Their quantitative analyses used four representative tracks of the MGS observations (Fig. 1) acquired from orbits with periapses near the 180◦ W meridian to explore models of crustal magnetization consistent with the observations. They noted that intense positive radial (vertical) features were separated by equally intense negative radial features, suggestive, in their opinion, of bands of crustal remanent magnetization of alternating sign. Although they interpreted the alternating magnetization to be suggestive of sea-floor spreading, they modeled the anomalies as resulting from unconstrained directions of remanent magnetization in the martian crust (Fig. 2). Although the Connerney et al. (1999) models are somewhat reminiscent of the magnetization directions in the terrestrial sea floor, they are very different in detail. On earth, paleomagnetic inclinations shift sign between reversals, but this shift occurs within a relatively short time interval as a 180◦ shift in field direction. In the Connerney et al. (1999) model, the transition between reversals takes as long as the reversed intervals, and the degree of change in the field direction appears to be random. For this paper, we have re-modeled the data presented by Connerney et al. (1999). We demonstrate that the anomalies can be modeled by well-constrained directions of remanent magnetization such as would be produced by an ancient areocentric dipole field either of constant or alternating polarity. Our interpretation does not require complex directions of magnetization, and it results in much smoother models of remanent magnetization in the martian crust. For the origin of the magnetic anomalies, we considered two different hypotheses, both constrained
Using the geometry of an areocentric dipole field, we have remodeled the anomalies observed in the Terra Sirenum and Terra Cimmeria regions by the Mars Global Surveyor (MGS). For the origin of the anomalies, we used both magnetic stripe and magnetic shell hypotheses. For magnetic stripe models, we assume that sea-floor spreading formed the anomalies in the presence of a reversing axial dipole field. We found that a simple magnetic stripe model with constant magnetization and crustal thickness results in poor agreement with the observed anomalies. However, by allowing the intensity of magnetization to vary freely between and along the stripes, a reasonable fit to the observed data can be found while maintaining the observed polarity along the stripes. Applying the simple magnetic stripe model to the Terra Sirenum and Terra Cimmeria anomalies results in best-fit spreading centers located well to the north of the surviving magnetic stripes. This could possibly explain the apparent lack of symmetry that has been noted in the anomalies. For the magnetic shell hypothesis, we assume that a monopolarity dipole field magnetized the early crust of Mars. The anomalies that persist today would then be the result of partial demagnetization of the original magnetic shell by more recent processes. The magnetic anomalies in Terra Sirenum and Terra Cimmeria are consistent with this model, especially if the magnetization of the original magnetic shell was laterally inhomogeneous. If the monopolarity magnetic shell hypothesis is correct, best-fit magnetic paleopole locations for the original magnetization lie in a sub-equatorial belt centered near the prime meridian. If the ancient magnetic pole of Mars coincided with the spin pole, this result implies that either apparent or true polar wander has occurred on Mars. Apparent polar wander would be consistent with plate tectonics having occurred after the formation of the magnetic anomalies. True polar wander has previously been proposed to explain certain morphologic features found on Mars. °c 2000 Academic Press Key Words: Mars, surface; magnetic fields.
INTRODUCTION
In 1997 and 1998, the Mars Global Surveyor (MGS) detected magnetic anomalies due to unknown features in the martian crust (Acu˜na et al. 1998, 1999). These anomalies were observed to correlate with the ancient cratered terrain of the martian highlands, particularly in the Terra Sirenum and Terra Cimmeria 26 0019-1035/00 $35.00 c 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
MAGNETIZATION AND POLOR WANDER ON MARS
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FIG. 1. Map of Mars showing the tracks of the four magnetic profiles of Connerney et al. (1999).
by directions of magnetization consistent with a simple areocentric axial dipole field. MODELS
The procedure we followed was similar to that outlined by Connerney et al. (1999) for their analysis of the same data. The sources of the magnetic anomalies in the Terra Sirenum and Terra Cimmeria regions are represented by a group of up to 40 uniformly magnetized horizontal crustal blocks each 30 km in depth and extended in the east–west direction. Our method differs from that of Connerney et al. (1999) in that, in all our models, we constrain the directions of magnetization in the blocks to be consistent with an areocentric dipole field. Several of our models further constrain the crustal blocks to be not only of constant thickness but also of constant magnetization.
FIG. 2. The near-random paleomagnetic inclinations inferred by the crustal model of Connerney et al. (1999). Paleomagnetic inclination is shown as a function of distance north of latitude 53◦ S and is defined in this case as a fourquadrant inverse tangent of J z/J x. This result is for track 1999 Day 15 P6.
For all our models, the magnetic field of each crustal block was computed using conventional formulae for a two-dimensional horizontal slab (e.g., Heiland 1940). In Fig. 3, the horizontal and vertical components of the magnetic anomaly 100 km above a single two-dimensional slab of thickness 30 km and width 100 km are shown. The slab in this example is uniformly magnetized (20 A/m) at an inclination angle of 45◦ in the direction of the profile (S–N). Readers unfamiliar with magnetic modeling
FIG. 3. The horizontal (Bx) and vertical (Bz) magnetic anomalies at a height of 100 km over a two-dimensional slab of material with magnetization contrast J = 20 A/m, inclination I = 45◦ , and strike angle α = 90◦ . Note that a single uniformly magnetized slab produces both positive and negative anomalies on both components. The same result would occur if J = +10 A/m and the magnetization of the surrounding material were J = −10 A/m.
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should note that uniform magnetization of a single slab produces both positive and negative magnetic anomalies. It is not necessary to reverse the polarity of the magnetization of the source to produce anomalies of alternating sign. It is also important to note that it is the lateral contrast in magnetization that causes an anomaly. The anomaly in Fig. 3 would be identical if the slab were magnetized 10 A/m within a country rock magnetized the same amount but in the opposite direction. The two-dimensional assumption in our models is critical because this forces the east component of the magnetic anomalies to be zero. Connerney et al. (1999) noted that the east component of the magnetic anomalies is small and that the magnetic anomalies are extended in the east–west direction. The 30-km slab thickness is arbitrary; in a thin slab model, the product of plate thickness and volume magnetization is the unknown being sought. For example, a 15-km-thick slab would require twice the magnetization to produce the same anomaly as a 30-km-thick slab. For each profile, the horizontal and vertical components of the observed field were fit to a crustal model consisting of up to 40 crustal blocks, uniformly magnetized, 30 km thick, and 100 km wide. The volume magnetization (J ) of each block (or for the whole ensemble for models with constant J ) is determined by a least-squares procedure that, under the constraints of the particular model, finds the best fit between the observed and calculated anomalies. The reader is referred to Connerney et al. (1999) for further technical details of the general inverse method that we have used. We considered two different hypotheses for the origin of the magnetic anomalies, a magnetic stripe model and a magnetic shell model. These hypotheses correspond to two different physical models for formation of the magnetic stripes, but from the perspective of magnetic modeling, the distinction between the magnetic stripe model and the magnetic shell model is subtle. They are similar in that, in both models, the anomaly is caused by a lateral contrast in magnetization, which works out to be more or less the same in both models. They are different in that, in the magnetic stripe model, the directions of magnetization from block to block are determined solely by the latitude of an assumed spreading center. For the magnetic shell model, the directions of magnetization vary according to the paleolatitude and paleolongitude of each crustal block relative to an assumed paleopole position. For all models, the direction of magnetization was constrained by the paleomagnetic inclination (I ) and paleomagnetic declination (D) of an areocentric dipole field, as calculated by the dipole equation. For the magnetic stripe model, the north magnetic pole was fixed at the present-day axis of rotation (D = 0◦ ), whereas, for the single-plate model, many trial paleopole locations were employed. As in all potential field inverse procedures, the results depend on the assumptions in the particular models; they are by no means unique. They show no more than a consistency between the observations and the model assumptions.
MAGNETIC STRIPE MODEL
The magnetic stripe model attempts to explain the anomalies as the result of sea-floor spreading in the ancient crust of Mars (Sagan and Pollack 1966, Sleep 1994). The assumption is that an ancient field of alternating polarity magnetized each crustal block at a central spreading center. Each block has then moved away from the center, retaining the magnetization directions acquired at the spreading center, resulting in magnetic stripes in the crust. Connerney et al. (1999) have proposed that sea-floor spreading on Mars could result in magnetic stripes of characteristic width 100 km if spreading rates on Mars were faster than those on Earth or if the rate of dipole reversals on Mars were less than on Earth. To implement this model, the intensity of magnetization and crustal thickness are held constant between and along the stripes. The stripes are presumed continuous in the east–west direction. The magnetic anomaly signature is thus controlled completely by the polarity of the field that magnetized each stripe. The directions of magnetization are constrained to those of a reversing areocentric axial dipole field at the spreading center. All the stripes in the model have either a positive or a negative magnetization vector in the same fixed direction. For the magnetic stripe model, 181 trial spreading center locations at 1◦ intervals of latitude along the 180◦ W meridian were tested. The magnetic stripes were assumed to continue unchanged in geometry, magnetization, and polarity across all four magnetic anomaly profiles. To obtain the best-fit magnetization for the stripes, a three-step inversion procedure was followed. First, for each trial spreading center latitude, a least-squares procedure was used to find the magnetization values for the 40 stripes that simultaneously best fit the combined four profiles. For this step, the magnetization values were free to be positive or negative but were fixed in inclination and declination. Second, the signs of the magnetization values were used to find the polarity of each stripe. Finally, a further least-squares procedure was used to find the best-fit constant magnetization for the entire ensemble with polarities as fixed in the second step. Results for the Magnetic Stripe Model The results for the simple magnetic stripe model are exemplified in Fig. 4, which shows a map of the crustal magnetizations as well as the observed and modeled results for one of the profiles. This result is for a possible spreading center at latitude 53◦ S, more or less in the midst of the profiles. The best-fit constant crustal magnetization for a spreading center at this location is a 7.3 A/m (30-km-thick crust). The magnetic stripes range from 100 to 400 km in width. For all profiles combined, the correlation coefficient (r 2 ) is 0.29. Although the fit is not very good, the general shape of the peaks and troughs in the magnetic profiles are somewhat reproduced by this simple magnetic stripe model. The variation in quality of fit with the assumed location of the spreading center is illustrated in Fig. 5. The best-fit spreading center is at 24◦ N, although possible centers at 20◦ S, 4◦ N, and
MAGNETIZATION AND POLOR WANDER ON MARS
29
FIG. 4. Crustal magnetization map for the simple magnetic stripe model. The inset compares observed (dashed lines) and modeled (solid lines) horizontal (Bx) and vertical (By) magnetic anomalies along one track to illustrate the quality of fit; the other three tracks are similar. The origin of the distance axis is at latitude 53◦ S. The observed data, which we have resampled and smoothed to 100-km intervals, are from Connerney et al. (1999).
55◦ N also provide reasonable fits to the data, but even the best fit has a correlation coefficient less than 0.5. All these better fitting possible spreading centers are well to the north of the surviving magnetic stripes, thus possibly explaining the lack of symmetry that is apparent in the anomalies. Figure 6 illustrates a more general, but less geologically ideal, attempt to match the magnetic stripes to the observed anomalies. Here the polarity of the magnetization is maintained in each
stripe, but the intensity of magnetization is allowed to change freely both along each stripe and from stripe to stripe. The correlation coefficient for the combined data set is 0.68. This results in more acceptable fits to the anomalies, but the intensity of magnetization within and between stripes ranges widely, from 0 to ±35 A/m. The results of our magnetic stripe models suggest that, if the magnetic lineations are due to sea-floor spreading in the
FIG. 5. The quality of fit (r 2 ) between observed and calculated magnetic anomalies for a simple magnetic stripe model as a function of modeled spreading center latitude. For each peak, the best-fit mean magnetization for a 30-km-thick crust is also shown.
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FIG. 6. Crustal magnetization map for the more complex magnetic stripe model which allows crustal magnetization to vary along and between stripes. See the caption of Fig. 4 for other details.
ancient crust of Mars, then the ancient spreading center may have been located well to the north of the surviving magnetic stripes in Terra Sirenum and Terra Cimmeria. The ancient crust of the southern highlands may therefore have spread considerably south since its formation. The wide range of intensity of magnetization, within and between the stripes, required to match the observations would seem to argue against a magnetic stripe model. But, as Connerney et al. (1999) point out, the ancient highlands may have been reworked by impacts, fragmentation, and thermal events subsequent to their original magnetization. Such reworking might explain the inhomogeneous magnetization required by the model. MAGNETIC SHELL HYPOTHESIS
Our second model uses a magnetic shell concept. We presume the crust was magnetized in place as it cooled under a monopolarity areocentric axial dipole field, forming a magnetic shell around the planet. This hypothesis is consistent with a hot early Mars immediately after accretion, followed by rapid cooling and crust formation (Pepin and Carr 1992, Schubert et al. 1992). If the ancient dynamo operated for several hundred million years after accretion, sufficient time would have been available for the ancient iron-rich crust to form and to acquire a remanent magnetization. This magnetization, if undisturbed by reheating or reworking, would retain information on the direction and magnitude of the ancient dipole field. If undisturbed, such a shell would not produce an external field. However, the magnetization in much of this shell has been destroyed by impacts, younger volcanism, and perhaps metamorphic and hydrothermal activity, providing the contrast in magnetization that
results in the observed anomalies today. If, in certain areas, long fracture zones controlled this demagnetization, then long linear magnetic anomalies such as those observed in the southern highlands might result. This model does not preclude the possibility of plate tectonics on Mars prior to, during, or subsequent to the magnetization of the early crust. Sleep (1994), for example, has proposed a sea-floor spreading model of Mars to account for the relatively youthful northern plains and crustal dichotomy. To implement this model, we assume that magnetization of a ancient crustal shell, of constant 30-km thickness, was “broken” (somehow, by one of the demagnetization processes described above) into discrete east–west extended slabs, 100 km or more in width. Under the restriction that the directions of magnetization were constrained to a monopolarity areocentric dipole, we tested two variants of this model. In the first, the intensity of magnetization was allowed to change with location. In the second, the intensity of magnetization, like the crustal thickness, was held pervasively constant. For the magnetic shell models, some 600 trial pole positions at 10◦ spacing around the globe were tested. For each assumed paleopole position, a non-negative least-squares procedure was then used to find a distribution of magnetization that best fit the observations under the constants imposed by the paleopole position. For the constant magnetization models, the results of the above-described procedure were further refined. The median magnetization of the ensemble of the magnetized crustal blocks was calculated. Those crustal blocks with magnetization less than the median value were eliminated from the model. Those remaining were subjected to a further least-squares procedure that found the best-fit constant magnetization for the combined data for all four profiles.
MAGNETIZATION AND POLOR WANDER ON MARS
31
FIG. 7. Crustal magnetization map for the magnetic shell model. The shaded areas represent remnants of the original monopolarity magnetic shell. The surrounding white areas have been demagnetized. See the caption of Fig. 4 for other details.
Results for the Magnetic Shell Model The results of the magnetic shell model are illustrated by Figs. 7 and 8, which show maps of crustal magnetization and representative fits between the calculated and observed magnetic anomalies for two variants of the model. In the first, the magnetization and thickness of the shell are constant. In the second, the magnetization in the broken shell is laterally inhomogeneous. These particular results are for a trial paleomagnetic pole near Schiaparelli Crater. The physical dimensions and volume mag-
netization of the sources are, without doubt, much more complex than as modeled. The results exemplified by Fig. 7 show that a magnetized plate, of constant thickness and of monopolarity magnetization, can produce, if broken, a pattern of positive and negative anomalies similar to that observed on Mars. Alternating reversals in the direction of crustal magnetization are not required to explain the magnetic anomalies on Mars. The contrast in magnetization between the magnetized crustal blocks and the surrounding terrain, in which crustal magnetization has been destroyed, is sufficient
FIG. 8. Crustal magnetization map for a more complex magnetic shell model. The remnants of the original monopolarity magnetic shell have laterally inhomogeneous magnetization. See the caption of Fig. 4 for other details.
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FIG. 9. Map of Mars showing predicted magnetic paleopole locations (gray areas) based on our magnetic anomaly modeling. Also shown is a polar wander curve for the south pole of Mars (Schultz and Lutz 1986). The solid triangles are thought to represent stable spin pole positions based on geomorphology; UP0 is the oldest, followed by C0 , B0 , A0 , and AP0 . The open triangles are ancient spin pole positions inferred from the record of grazing impacts; 40 is the oldest, followed by 30 and 20 . The contours are RMS residuals between modeled and observed magnetic field anomalies in Terra Sirenum and Terra Cimmeria for 590 trial magnetic paleopole positions on the surface of Mars. The lowest residuals (gray areas) form a sub-equatorial belt near the prime meridian. The x’s indicate pole locations where the inverse process is unstable.
to reproduce the pattern of positive and negative anomalies observed. The correlation coefficient for the combined data set is 0.53. The best-fit constant crustal magnetization for the ancient magnetic shell is 17.8 A/m for a 30-km thickness. The results exemplified by Fig. 8 show that a better fit between the observed and calculated anomalies can be obtained if the monopolarity magnetization in the original shell was laterally inhomogeneous. Spatial variability in the original shell might arise from changes in mineralogy, petrology, and structure from place to place on early Mars. The correlation coefficient for the combined data set for this model is 0.86. The crustal magnetization required to fit the data well by this model, assuming a 30-km-thick crust, averages about 20 A/m, but ranges to as high as 70 A/m. Thinner or thicker crust would produce correspondingly higher or lower intensities of magnetization. Under the monopolarity magnetic shell hypothesis, a contour plot of the RMS errors for trial paleomagnetic pole positions on Mars indicates that the lowest residuals occur along a sub-equatorial belt centered at about 20◦ S latitude on the prime meridian (Fig. 9). This pattern holds for both variants of the magnetic shell model. Paleomagnetic poles anywhere within this belt produce a reasonably good fit with the observations. The better fitting paleopole locations do not include the present-day geographic poles of Mars. If the monopolarity magnetic shell hypothesis is correct, then either apparent polar wander or true polar wander has occurred on Mars. The limited extent and geometry of the four magnetic profiles available create some singularities with our analysis procedure for determining paleopole locations. For example, the cusps near the equator at longitudes 90◦ W and 270◦ W (marked
with x’s in Fig. 9) are artifacts. Trial paleopoles here happen to result in magnetic field directions parallel to the presumed east–west structures, resulting in computational insensitivity to magnetization changes. As more magnetic data from different regions of Mars become available, these artifacts should be less of a problem. Apparent Polar Wander The result, for the magnetic shell model, that the most probable paleopole positions do not include the present spin poles is intriguing. Apparent polar wander is certainly a viable possibility if sea-floor spreading, as proposed by Sleep (1994), formed the relatively young, northern lowlands. If plate tectonics has occurred on Mars, then the apparent paleopole near the equator at the prime meridian gives information about the plate motion. The lithospheric plate that includes the older magnetic crust in the Terra Sireum and Terra Cimmeria regions appears to have rotated through an angular distance of 90◦ about a plate rotation pole near the equator at 90◦ W longitude. If the ancient field was reversed polarity, the plate moved south from the northern hemisphere along the 180◦ W longitude to its present position. If the ancient field was normal polarity, the plate has moved from the southern hemisphere south along the prime meridian across the south pole to its present position. Additional magnetic data from Mars will greatly help establish the validity of plate tectonics on the planet. True Polar Wander If relative plate motion has not occurred on Mars, the possible paleomagnetic pole positions shown in Fig. 7 may coincide
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MAGNETIZATION AND POLOR WANDER ON MARS
with ancient martian spin poles. Mars has long been considered a likely planet for true polar wander, a reorientation of an entire planetary surface with respect to its spin axis. True polar wander is thought to result from changes in the inertia tensor as a result of internal mass redistribution. Ward et al. (1979) and Melosh (1980) suggested that the spin pole of Mars moved as a result of the rise of Tharsis. McAdoo and Burns (1975) suggested that the Coprates Trough assemblage provides evidence of tensile stresses developed by polar wander on Mars, although Grimm and Solomon (1986) argue against this. The record of grazing impacts has been used to locate possible ancient orbital pole points (Schultz and Lutz-Garihan 1982). Geomorphic studies have shown that regions of mantled and layered terrain near the martian equator are similar to present polar deposits, providing additional proposed ancient paleopole locations (Schultz and Lutz 1988). If Terra Sirenum and Terra Cimmeria have not moved relative to the rest of the planetary surface, then our paleomagnetic pole locations appear to provide independent evidence for polar wander on Mars. A proposed wander path for the south pole of Mars presented by Schultz and Lutz (1988) is shown on Fig. 9. The solid triangles represent pole positions based largely on geomorphic evidence; the open triangles represent possible pole positions based on grazing impacts. Our possible paleomagnetic pole positions derived from the magnetic anomalies (the gray shaded areas in the equatorial regions near the prime meridian) agree remarkably well with the Schultz and Lutz (1988) model for polar wander. A possible paleomagnetic south pole position, compatible with the magnetic anomalies, that is also along the Schultz and Lutz polar wander path is near 45◦ E, 15◦ S, southeast of C0 . This ancient pole corresponds to the chaotic terrains of eastern Valles Marineris, and at the antipode, to the southeast slope of Elysium Mons, not far from the very old and heavily eroded deposits south of Elysium Planitia. Such highly eroded terrains might be indicative of polar geomorphology. The timing of a spin pole at this position would be somewhat earlier than the stable position of C0 proposed by Schultz and Lutz (1988). It would nonetheless be consistent with the emplacement of the highly magnetized rocks of the ancient cratered highlands, including Terra Cimmeria and Terra Sirenum, in early Mars. Another possible fit between the paleomagnetic data and the geomorphic data occurs southeast of Schiaparelli Crater near 20 , just south of the proposed stable pole at A0 on Fig. 9. However, according to Schultz and Lutz (1988) this stable pole position is contemporaneous with the development of the Tharsis fracture system and the beginning of Arsia Mons. It would seem to be too young to be related to the highly magnetized rocks in Terra Cimmeria and Terra Sirenum. CONCLUSIONS
We have applied an areocentric axial dipole hypothesis to evaluate the magnetic anomalies in southern highlands of Mars. We
have considered magnetic stripe and magnetic shell hypotheses for the origin of the anomalies. For the magnetic stripe models, we could not satisfactorily fit the magnetic data to simple magnetic stripes of alternating polarity and constant magnetic intensity. Only by allowing the intensity of magnetization to vary freely within and between stripes could a satisfactory fit be obtained. This suggests, that if the anomalies did form as sea floor stripes, they have been reworked greatly since their formation. The best-fit spreading centers for the magnetic stripe model are located well to the north of the southern highlands. This result possibly explains the observed lack of symmetry in the anomalies. For the magnetic shell model, fits are best if the original shell is allowed to vary in intensity of magnetization (or thickness). For this model, the most likely magnetic paleopoles fall on a subequatorial belt near the prime meridian. If the monopolarity magnetic shell model is correct, then either apparent or true polar wander has occurred on the planet since the anomalous material was magnetized. If apparent polar wander has occurred, the plate on which Terra Sirenum and Terra Cimmeria are located has moved through a 90◦ arc about a center of plate rotation near the equator at 90◦ W. If true polar wander has occurred, then the spin pole of Mars was near the equator close to the prime meridian at the time of the magnetization of the original magnetic shell. The limited crustal magnetic data released to date, as provided by Connerney et al. (1999), do little to resolve the controversy as to whether plate tectonics or true polar wander has occurred on Mars in the past. The implications of either plate tectonics (Sagan and Pollack 1966, Sleep, 1994) or true polar wander (Schulz and Lutz 1988) for understanding the planet are huge, and it appears that the paleomagnetic studies may provide the key to the geological history of Mars, just as they have on Earth.
ACKNOWLEDGMENTS We thank Norm Sleep and Robert Hargraves for helpful comments. Our research was partially supported by the NASA Idaho Space Grant Consortium and NSF POWRE Grant EAR-9973345 to L. Baker.
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