Timing of the Martian Core Dynamo

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, E03006, doi:10.1029/2003JE002195, 2004

Timing of the Martian core dynamo Jafar Arkani-Hamed Earth and Planetary Sciences, McGill University, Montreal, Quebec, Canada Received 7 October 2003; revised 27 January 2004; accepted 10 February 2004; published 18 March 2004.

[1] Using the radial component magnetic data acquired by Mars Global Surveyor during its mapping phase, a reliable magnetic anomaly map of Mars is derived. Many weak but reliable magnetic anomalies are now detected in the northern lowlands. Their source bodies have likely been partially demagnetized by the lowlands formation processes, and there has been no core field in the later times to remagnetize the bodies. No consistent anomalies are observed in the Hellas basin, implying that the core dynamo has been inactive at least since the impact event. The topography, the positive Bouguer anomaly at the center, and the strong negative Bouguer anomalies over the outskirts of Thaumasia plateau suggest that the plateau was the site of a large impact basin prior to the formation of the plateau. The lack of magnetic anomalies in the central part and the strong magnetic anomalies over the outskirts of the plateau resemble the magnetic signature of Hellas basin, implying a preexisting giant impact basin beneath the plateau and the absence of the core field since the impact time. The lack of magnetic edge effects of Valles Marineris, the absence of thermal demagnetization of Tharsis plain by the shield volcanoes, and the substantial distance between the paleomagnetic poles and the present rotation axis imply that a major part of Tharsis bulge was formed in the absence of the core field. INDEX TERMS: 1510 Geomagnetism and Paleomagnetism: Dynamo theories; 1512 Geomagnetism and Paleomagnetism: Environmental magnetism; 1517 Geomagnetism and Paleomagnetism: Magnetic anomaly modeling; 6225 Planetology: Solar System Objects: Mars; KEYWORDS: core dynamo of Mars, Mars magnetic field, Thaumasia plateau of Mars, impact demagnetization of Martian crust, Tharsis bulge of Mars, polar wander of Mars Citation: Arkani-Hamed, J. (2004), Timing of the Martian core dynamo, J. Geophys. Res., 109, E03006, doi:10.1029/2003JE002195.

1. Introduction strongly model-dependent. With reasonable parameter val- [2] The strong magnetic anomalies of the south hemi- ues it is possible to obtain drastically different scenarios for sphere detected by Mars Global Surveyor (MGS) indicate the thermal evolution of the planet [e.g., Stevenson et al., that Mars had a strong core field in the past. When the core 1983; Schubert and Spohn, 1990; Hauck and Phillips, 2002; dynamo ceased is a matter of debate. In analogy to Earth, if Breuer and Spohn, 2003]. the core dynamo of Mars were maintained by the solidifi- [3] There is some evidence from the Martian meteorites cation of a possible inner core, the dynamo must have that the core dynamo was active in the early history of the started several hundred million years after the formation of planet. Rock magnetic analyses of the oldest Martian the planet [Schubert et al., 2000]. On the other hand, if the meteorite (ALH84001) indicate that it was magnetized on core dynamo were maintained by thermal convection in a the Martian surface before 4 Gyr ago [e.g., Collinson, 1997; vigorously convecting liquid core, it may have started very Kirschvink et al., 1997; Weiss et al., 2002; Antretter et al., early in the history of Mars. Once a stagnant lid developed 2003]. Whether it was magnetized by the core field or by on the convecting mantle, and Mars became a one-plate the local crustal field is still debated. Nevertheless, the core planet, the lid hampered heat loss from the mantle and field must have existed prior to, or concurrent with, the subsequently from the core. The core dynamo ceased acquisition of magnetization by the meteorite. The young because of the reduction of heat loss from the core Martian meteorites have crystallization ages ranging from that decreased the vigor of core convection [Nimmo and 0.16 to 1.3 Gyr [Nyquist et al., 2001]. They seem to be Stevenson, 2000; Stevenson, 2001]. One of the major magnetized in a very weak field of less than 3000 nT [e.g., difficulties with the thermal evolution models, whether Cisowski, 1986; Collinson, 1986; McSween and Treiman, based on parameterized convection or a fully 3D convection 1998; Collinson, 1997], implying no appreciable core field with sophisticated algorithm is the lack of pertinent con- at the crystallization time. Rochette et al. [2001] suggest straining data. The results of convection calculations are Pyrrhotite as the major magnetic carrier of the Martian meteorites. They argue that Pyrrhotite becomes non- Copyright 2004 by the American Geophysical Union. magnetic at pressures about an order of magnitude lower 0148-0227/04/2003JE002195$09.00 than the impact shock pressures experienced by the mete-

E03006 1of12 E03006 ARKANI-HAMED: TIMING OF THE MARTIAN CORE DYNAMO E03006 orites when they were ejected from the Martian surface in zation of the Martian crust. This is especially important for the last few Myr. Pyrrhotite lost memory of its preimpact the magnetic anomalies that have been substantially weak- magnetization, and only regained ferromagnetic behavior ened by the tectonic and impact processes, such as the weak and became magnetized by the crustal field during pressure anomalies in the northern lowlands, the lack of magnetic release. Accordingly, the magnetization of the young signature inside giant impact basins, and the weak anoma- Martian meteorites reflects the magnetic field of Mars at lies in the outskirts of Thaumasia plateau. Mars Global the impact time, which was likely the crustal field, and not Surveyor has provided a huge amount of magnetic data at the original crystallization time, when a supposedly acquired within 360–420 km altitudes since it has been put strong core field existed. However, it is difficult to explain in the mapping phase orbit. The elevation/altitude referred the magnetic sources of the Martian crust by Pyrrhotite as a to in this paper is relative to a mean spherical surface of major magnetic carrier. The thermal evolution models of radius 3390 km. Although the high altitude of the spacecraft Mars [Hauck and Phillips, 2002; Arkani-Hamed,2003; limits the resolution of the data [Acuna et al., 2001; Breuer and Spohn, 2003] suggest high temperatures in the Connerney et al., 2001; Arkani-Hamed, 2002b], the huge crust in the early history of the planet when the magnetic amount of the data provides an opportunity to derive a source bodies were most likely magnetized. The low Curie highly repeatable and accurate magnetic anomaly map at temperature of Pyrrhotite, 320C, severely limits the thick- that altitude. For this purpose I use the radial component of ness of the hypothetical Pyrrhotite-rich magnetic layer on the magnetic field which is least contaminated by non- Mars. There is no evidence for a high concentration of crustal sources, and select the most common features on the Pyrrhotite that could have resulted in a bulk magnetization basis of covariance analysis. 5 (vertically integrated magnetization) of about 6 10 A, [7] The vast amount of the high-altitude data allows us to which is required to explain the magnetic anomalies of the divide the entire data into two almost equal sets, acquired south hemisphere [Connerney et al., 1999; Arkani-Hamed, during two different periods separated by about a year, and 2002a]. The major magnetic carriers of the Martian crust are to derive two separate magnetic maps. Only the nighttime poorly understood, and there is no consensus among rock data are used in order to minimize the effects of the external magnetic scientists to date, although many prefer magnetite magnetic field. Each data set almost uniformly covers the or ilmenite-hematite lamellae [e.g., Kletetschka et al., 2000; entire surface (except for the small areas near the poles) Hargrave et al., 2001; Robinson et al., 2002; Scott and with track separations of less than 15 km. Each set of data is Fuller, 2003]. binned over grid cells of 0.25 0.25 degree and the mean [4] The lack of magnetic anomalies inside giant impact data value of a given grid cell is calculated after removing basins Hellas, Argyre and Isidis suggests that the impacts the outliers (see Arkani-Hamed [2002b] for details). The have demagnetized the crust, and there has been no core altitude variations within a cell, <±5 km, are 80 times dynamo active to remagnetize it [Acuna et al., 1999; Mohit smaller than the mean elevation of the data points in the and Arkani-Hamed, 2001; Hood et al., 2003]. Likewise, the cell. The mean elevation of MGS changes by ±30 km over lack of strong magnetic anomalies in the northern lowlands, the entire globe, which is 13 times smaller than the mean and the similarity of the north-south magnetic dichotomy to altitude of the spacecraft. Moreover, the trend of the the north-south topographic dichotomy imply that the low- elevation changes is similar for both data sets, i.e., higher lands formation process has largely demagnetized the crust. elevations over the northern lowlands and lower elevations If the lowlands were formed as a result of the crustal over the southern highlands. Also, the difference in the thinning by a hot and giant mantle plume [e.g., Zuber et mean elevations of the two data sets over a given cell is less al., 2000], the major demagnetization of the crust is likely than 10 km. The altitude variations, either over a given cell due to the thermal demagnetization in the deeper parts or over the entire globe, have negligible effects on the [Arkani-Hamed, 2003]. The lowlands formation must have resulting magnetic anomaly maps. It is worth mentioning occurred in the absence of the core field, otherwise the that the final map will be used for identification purposes thermally demagnetized bodies would have acquired new only, and no source body modeling will be done on the magnetization as they cooled below their Curie temperature. basis of the map. The near-surface low-temperature hydration may not have [8] The two global maps thus obtained at the mean played a dominant role in the demagnetization of the low- elevation of 390 km are expanded in terms of the spherical lands [Arkani-Hamed, 2003]. harmonics using harmonics of degree up to 90, and then [5] An attempt is made in this paper to estimate the life downward continued to 100 km altitude. Their power span of the core dynamo, through the relationship between spectra and degree correlation coefficients (Figure 1) show the major tectonics of the planet and the observed magnetic that the two maps are almost identical over harmonics of anomalies. An ancient basin is discovered beneath Thau- degree up to 50, having degree correlation coefficients masia plateau, referred to as Thaumasia basin, with an higher than 0.87. The figure, however, displays distinct appreciably demagnetized floor. I also present evidence that characteristics over harmonics of degree higher than 60. the major part of the Tharsis plains formed in the absence of Not only do the two spectra appreciably differ from each a strong core field. other but also the general trends of the spectra change; the power increases significantly. Also, the correlation between the maps deteriorates, indicating that the higher degree 2. A New Magnetic Map of Mars harmonic coefficients are dominated by a non-crustal [6] A highly repeatable and reliable magnetic anomaly magnetic field. map of Mars is essential for the investigation of the [9] I also extracted radial magnetic maps from the orig- relationship between the tectonic features and the magneti- inal MGS data over Hellas basin and Thaumasia plateau and

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the covariance analysis further removes the components that are largely non-crustal. The method is not, however, capable of removing the quasi-steady component of the external magnetic field, such as the possible quasi-steady part of the magnetic field associated with the magnetic pile-up boundary and draping of the interplanetary magnetic field around the planet [Brain et al., 2003]. However, this component is independent of the crustal magnetic anomalies [Bertucci et al., 2003; Vennerstrom et al., 2003] and may not have appreciable contribution to the anomalies seen in Figure 2. [11] The strong magnetic anomalies seen in Figure 2 are almost identical to those in the magnetic anomaly maps derived through modeling the low-altitude data [e.g., Purucker et al., 2000, Arkani-Hamed, 2001a] and the high-altitude data [Arkani-Hamed, 2002b; Cain et al., 2003]. There are some differences over weak anomalies, especially over the places with no original low-altitude data (see below). The advantage of Figure 2 is that it is based on a huge amount of data acquired at almost constant elevation and there are no wide gaps in the data distribution, except over the polar regions. Even the weak anomalies seen in the figure are repeatable and thus reliable, as emphasized by the extremely large correlation coefficients, greater than 0.95, Figure 1. (a) The power spectra and (b) the degree over the highest degree harmonic coefficients retained (see correlation coefficients of the two radial component maps. Figure 1b). I also use more stringent criteria in selecting the n 2 nighttime data to assure minimum disturbance by the The power spectrum Pn is calculated from Pn = Sm=0 (Cnm 2 external field. The solar array current and spacecraft dy- +Snm), and the degree correlation coefficient hn is n 0 0 namic effects were removed from the data using the criteria determined from hn = Sm=0 (CnmC nm +Snm Snm)/ n 2 2 n 02 02 1/2 suggested by J. Connerney at Goddard Space Flight Center {[Sm=0 (Cnm +Snm)] [Sm=0 (C nm +Snm )]} . The 0 0 (personal communication, 2003). (Cnm,Snm) and (Cnm,Snm) are the fully normalized spherical harmonic coefficients of degree n and order m of the two maps. The curves denoted by cvr represent the co- 3. Constraints on the Life Span of the varying harmonics. The power is calculated at 100 km Core Dynamo altitude. [12] The strong magnetic anomalies in the south show no obvious correlation with the surface topography. Their made analyses similar to those mentioned above, but using source bodies are closely located, resulting in a significant Fourier-domain techniques. It turned out that the resolution overlap of their magnetic fields. To constrain the timing of of the regional maps did not improve appreciably compared the core dynamo I examine the magnetic signatures of the with that of the global maps. This is mainly because of the northern lowlands, the giant impact basins, Syria planum, densely distributed data with small cross-track distances. Thuamasia plateau, Valles Marineris, the shield volcanoes The resolution is actually limited by the elevation of the and the entire Tharsis bulge. I show that no remagnetization spacecraft rather than the cross-track distances. I will has occurred after the reduction of the magnetic signatures therefore focus on the global maps. by tectonic and impact events, indicating the lack of core [10] Figure 2 shows the radial component of the magnetic dynamo during or after the events. field of Mars at 100 km altitude derived by averaging the co- varying harmonic coefficients of the two global maps 3.1. Northern Lowlands (harmonics with positive degree correlation coefficients [13] The northern lowlands are characterized by having [see Arkani-Hamed, 2002b]). The color bar is saturated to no appreciable magnetic anomalies. The only anomalies better illustrate the weak anomalies, the magnetic anomalies reported so far are the two weak anomalies located near the are from 980 to +1250 nT. Also, to assure high accuracy of north pole [e.g., Acuna et al., 1999; Arkani-Hamed, 2001b; the final map, only harmonics of degree up to 50 are Hood and Zakharian, 2001]. Figure 2 shows many weak retained. Included in Figure 1 are the power spectra and anomalies in the lowlands which are close to the north-south degree correlation coefficients of the co-varying harmonics. topographic dichotomy, such as those centered at (38N, Their degree correlation coefficients are greater than 0.95 72E), (20N, 100E), (17N, 115E), (37N, 186E), (15N, 205E), over the entire harmonics retained, even the shortest wave- and (32N, 335E). Although the shortest wavelength retained lengths retained are highly repeatable and hence reliable. in the figure, 430 km, is 28 times longer than the typical Note that the procedure adopted in this study selects only the track spacing, the errors possibly introduced through the part of the magnetic data that had minor changes during spherical harmonic expansion may have minor effects on more than 2 years of data acquisition. The averaging of many these weak anomalies. The exact shape of the anomalies data values in a given grid cell measured at different times cannot be decided on the basis of the existing high-altitude suppresses the time varying non-crustal contributions, and magnetic data. The existing low-altitude data have many

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Figure 2. The radial component magnetic anomaly map of Mars at 100 km elevation derived from the high-altitude MGS data using harmonics of degree up to 50. The projection is Winkel Tripel, which is recommended for global maps. Units are in nT. Note that the color bar is saturated to better illustrate the weak anomalies considered in this paper. The magnetic anomalies are from 980 to 1250 nT. gaps parallel to the tracks and cannot be used to determine probably been demagnetized to a great extent by the thermal the exact shape of the anomalies either. Future magnetic effects of a giant mantle plume that thinned the crust explorations at lower elevations and shorter across-track beneath the lowlands and to a lesser extent by near-surface distances are required to better delineate the anomalies. low-temperature hydration [Arkani-Hamed, 2003]. It takes What is clear from Figure 2 is the existence of many up to 200 Myr, depending on their depth of burial, for the anomalies in the lowlands, though very weak and possibly thermally demagnetized source bodies to cool below their contaminated by data processing errors. Curie temperature and thus reacquire magnetization in the [14] The absence of magnetic anomalies in the central presence of a magnetic field. The lack of appreciable part of the lowlands and the proximity of the weak anoma- anomalies in the lowlands suggests that the core dynamo lies to the north-south topographic dichotomy suggest that may not have existed to remagnetize the source bodies after the lowlands formation has almost completely demagne- the event that gave rise to the dichotomy. tized the source bodies in the central part and partially demagnetized the source bodies near the boundary. Frey et 3.2. Impact Basins al. [2000, 2001] identified many quasi-circular depressions [15] The absence of appreciable magnetic anomalies in the lowlands with diameters larger than 200 km, inferred inside the giant impact basins and the presence of strong to be buried impact craters. The distribution of these ghost anomalies in the surroundings are used to argue for the craters is comparable to that of the prominent craters of the demagnetization of the crust by the impacts [Acuna et al., south, implying similar ages for the crust in the north and 1999; Mohit and Arkani-Hamed, 2001; Hood et al., 2003]. south [Frey et al., 2001, 2002]. There is no striking This suggests that the core dynamo was no longer active at evidence that the composition of the Martian crust had a the time of impacts [Acuna et al., 1999]. However, the distinct north-south dichotomy before the formation of the magnetic anomaly maps derived from the low-altitude data lowlands that would have prevented the formation of acquired at 100–200 km elevations during the science magnetic bodies in the north. The weak anomalies are most phase and aerobraking phase of MGS [Acuna et al., 1999; likely the relicts of stronger ones that existed prior to the Purucker et al., 2000; Arkani-Hamed, 2001a] show very formation of the lowlands. The magnetic source bodies have weak features inside Hellas basin. This may imply either the

4of12 E03006 ARKANI-HAMED: TIMING OF THE MARTIAN CORE DYNAMO E03006 core dynamo was active during and after the impact events strong negative Bouguer anomalies. This suggests that a but the post-impact materials in the basin were weakly pervasive magma chamber existed directly beneath the magnetic and acquired weak magnetization, or the core central vents for a geologically long period while contin- dynamo was in the later stages of its waning and thus could ued volcanism added more lava flows to the surround- not significantly magnetize the post-impact material. Al- ings. The isostatic compensation of the thick volcanic though it is not possible to rule out these scenarios, it is construct suppressed the crust into the mantle resulting in more likely that the weak magnetic signatures are largely the strong negative Bouguer anomalies. Also included in artifacts. To verify these weak magnetic features, I overlaid Figure 3 are the vertically averaged lateral variations of on Figure 2 the low-altitude data along the tracks passing density within a 50 km thick crust that are required to over the basin. No consistent commonality was detected give rise to the Bouguer anomalies. It is impossible to between the features seen in the low-altitude track data and determine the actual location of the density perturbations those in Figure 2. This inconsistency cannot be explained on the basis of the topography and gravity data alone, by the decay of short wavelength components at high because the gravity anomalies are related to the vertical altitudes. The decrease of the amplitude of a possible crustal integration of the lateral perturbations of density. The magnetic anomaly with increasing elevation from the low vertically averaged density perturbations presented in the elevations (100–200 km) to the high altitudes (390 km) figure represent the simplest model. It is likely that cannot alter the sign of the anomaly. Although the low- the Bouguer anomalies of Syria planum arise from the altitude data have higher resolution along the orbit tracks, undulation of the Moho due to compensation of surface the resolution of the low-altitude maps suffers from the topography. The undulations can easily be estimated from large data gaps parallel to the orbits. Moreover, and more the density perturbations. Assuming that the mantle importantly, the low-altitude data were acquired in daytimes density is greater than the crustal density by 600 kg/m3, and have appreciable external magnetic field contributions. the density perturbations can be converted to the undu- Because of the low elevations the external field has rela- lations of the Moho using a conversion factor of roughly tively negligible effects on the magnetic data over the strong 0.1 km/(kg/m3). For example a density perturbation of anomalies in the south, but it may have appreciable con- +100 kg/m3 in the figure is roughly equivalent to the tributions to the weak anomalies. upward displacement of the Moho by 10 km (Note that the conservation of mass requires a conversion factor of 3.3. Syria Planum 0.08. But more mass is needed at the Moho to satisfy [16] Located on the northwest of Thaumasia plateau and the Bouguer anomaly than the mass evenly distributed centered at 12S and 255E (Figure 3) [see also Anderson et throughout the 50 km thick crust used in the calculation al., 2001, Plate 1], Syria planum is the center of Tharsis of the density perturbations). topographic high, except for the shield volcanoes. It is a [17] Figure 3a shows no appreciable magnetic anomalies long-lived (from Noachian to early Amazonian) region of associated with Syria planum, implying no core dynamo volcanism on Tharsis [e.g., Tanaka et al., 1996; Dohm and was active during the long-lived volcanism that lasted from Tanaka, 1999; Hartmann and Neukum, 2001]. Detailed Noachian to early Amazonian. Individual flows were likely studies of the tectonic activities in the western hemisphere thin and cooled rapidly before thermally affecting the of Mars [Anderson et al., 2001] identify the planum as the underlying lava flows. The entire volcanic layer could have primary center of continuous tectonic activity that created acquired appreciable magnetization if the core dynamo were the pronounced radial pattern of normal faults and grabens. active. It is possible that the core dynamo had frequent pole Voluminous volcanic lavas were emplaced in this region reversals and magnetized different layers in opposite direc- during late Hesperian. Figure 3 shows the topography and tions, and hence the resulting volcanic construct does not Bouguer anomalies of Thaumasia region derived from produce an appreciable magnetic field despite significant MOLA data and from Yuan et al.’s [2001] spherical har- magnetization intensity of individual lava flows. But a very monic model of the gravity field of Mars. Only harmonics delicate tuning of the reversal pattern is required to explain of degree up to 50 are used. To determine the Bouguer the lack of magnetic anomalies in Syria planum as well as anomalies, the gravity field of the surface topography is over several places in Tharsis bulge (see below). The calculated using a crustal density of 2900 kg/m3. I adopted a absence of the core dynamo is a simple and more likely finite-amplitude topography technique in calculating the scenario. gravity field of the topography [Arkani-Hamed, 1999], [18] The lack of magnetic signatures on the outskirts of which takes into account the large dynamic ellipticity of Syria planum implies that the preexisting crust was not the Martian surface. Using the finite amplitude topography magnetized as an extended horizontal layer either. Figures technique avoids the error introduced by replacing the 3b and 3d indicate that the continued volcanism has created topography by an equivalent surface mass density as dem- a volcanic construct of about 20–25 km thickness. The onstrated by McGovern et al. [2002]. Calculations are made thermal blanketing effect of the construct would have on a global scale and the maps shown in Figure 3 are enhanced the temperature in the underlying crust by 200– extracted from the corresponding global maps. Note that the 250 degrees once the temperature approached a quasi- spherical harmonics of degree 2 are not retained in this steady condition. If the preexisting crust were magnetized, figure for better illustration of the local anomalies, although the thermal blanketing is expected to demagnetize an these harmonics are included in the calculations in order to appreciable part of the crust and create a magnetic edge take into account the dynamic bulge of Mars as mentioned effect (see section 3.5.1). The weak magnetic anomaly in above. Figure 3c shows a weak but negative Bouguer the southwest is probably the relict of a stronger anomaly anomaly at the center of the planum that is encircled by that has been reduced through partial demagnetization of its

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Figure 3. Thaumasia plateau (a) is the radial magnetic anomaly, (b) is the topography, (c) is the Bouguer anomaly, and (d) is the vertically averaged lateral density perturbations inside a 50 km thick crust. The horizontal axis is east longitude and the vertical axis is latitude, both in degrees. localized source body. The anomaly likely predates the anomalies, there is a positive Bouguer anomaly associated formation of the Syria planum. with the central part of the plateau, which is almost encircled by strong negative Bouguer anomalies. This resembles the 3.4. Thaumasia Plateau//Thaumasia Basin Bouguer anomaly associated with an impact basin such as [19] A giant impact basin, comparable to Hellas basin, Hellas (Figure 4). The distinct difference in shape between the seems to underlie the Thaumasia plateau. Unlike Alba Patera unfilled Hellas basin and the overfilled Thaumasia basin is and many terrestrial plateaus that have negative Bouguer most likely due to the later fillings of Thaumasia. The

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Figure 4. Hellas basin (a) is the radial magnetic anomaly, (b) is the topography, (c) is the Bouguer anomaly, and (d) is the vertically averaged lateral density perturbations inside a 50 km thick crust. The horizontal axis is east longitude and the vertical axis is latitude, both in degrees. topography and Bouguer anomaly of Hellas basin are almost of the plateau has sagged by about 2 km compared to its circular, whereas those of Thaumasia are quite modified. The outskirts, possibly due to subsequent partial compensation extensive volcanism of the nearby Syria Planum has depos- that also suppressed the mantle uplift. The density perturba- ited thick volcanic lava and produced a high surface topog- tions in the central part of Thaumasia are equivalent to a raphy over the northwestern part of Thaumasia basin, mantle uplift of 5 km, whereas those in the central part of resulting in the modified Bouguer anomaly. The central part Hellas are equivalent to a mantle uplift of 20 km.

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[20] There is good tectonic evidence for the preexisting general consensus that major part of the Tharsis rise has been Thaumasia impact basin from the radial normal faults and produced by basaltic flows. Here I present evidence from grabens associated with the primary center of continued several locations that the volcanic layers of Tharsis are not tectonism of Tharsis, the Syria planum, as mapped by significantly magnetized; they are formed either in the Anderson et al. [2001]. Two sets of these complex tectonic absence of a core dynamo or in the waning period of the structures are prominent. One set has resulted in the well- dynamo. My argument is based on the magnetization of developed Valles Marineris that extends from (5S, 260E) to Earth’s oceanic crust. The magnetization intensity of mid- (15S, 310E). The Valles-related faults may have occurred in ocean ridge basalt (MORB) on Earth declines from 20 A/m the early Noachian and the tectonics continued at least to when it is fresh to 5 A/m when it is older than 30 Myr, Hesperian. The other has produced the Claritas Fossae and and then remains almost unchanged for over 140 Myr [Bliel Thaumasia normal faults and complex grabens that extend and Petersen, 1983]. The high magnetization is largely due from (20S, 250E) to (40S, 255E) [see Anderson et al., 2001, to single domain titano-magnetite and titano-maghemite, and Plate 1; Anguita et al., 2001, Plate 1]. These tectonic the magnetization reduction is a result of low-temperature structures are formed or reactivated over a long time, from hydration by the circulating water in the oceanic crust [e.g., Noachian to Amazonian [e.g., Dohm and Tanaka, 1999]. Dunlop and Ozdemir, 1997]. The basaltic flows that formed These two sets are tangent to the rim of the suggested the Tharsis bulge must have been numerous and thin. At the preexisting Thaumasia basin, and thus are parallel to the low temperatures of the Martian surface, each flow most basin-encircling normal faults at the rim that were likely likely cooled rapidly before significantly affecting the un- created by the impact event and subsequent mantle uplift as derlying flows, and there is a good chance that rocks similar a response to the impact excavation. The reactivation of the to MORB on Mars contain single domain magnetic particles. impact-induced normal faults probably facilitated the de- Many places of Tharsis have been punctured by tectonic velopment of Valles Marineris, Claritas Fossae and Thau- activities, such as Valles Marineris and shield volcanoes, massia faults. Other Syria planum-induced radial faults that which would have created detectable magnetic edge effects are located between Valles and Claritas Fossae are not well if the Tharsis plains were appreciably magnetized. Here I developed [see Anderson et al., 2001, Plate 1]. A possible consider two places on Tharsis, Valles Marineris and shield explanation is that they are at high angle to the impact- volcanoes, in some detail. induced normal faults and have not been able to reactivate 3.5.1. Valles Marineris these faults. These tectonic relationships suggest that the [23] Figure 3a shows no distinct magnetic edge effects Thaumasia basin is probably as old as, or even older, than associated with Valles Marineris, which is in good agree- early Noachian. The sagging of the central parts of Thau- ment with the magnetic anomaly maps derived by many masia may have enhanced the tensional stresses required for authors [Acuna et al., 1999; Purucker et al., 2000; Arkani- the rupturing mechanism of Valles Marineris recently pro- Hamed, 2001a, 2002b; Cain et al., 2003]. The Valles has a posed by Wilkins and Schultz [2003]. mean depth of 5 km, a width of 100–400 km, and a length [21] Figure 3a shows that the Thaumasia plateau is sur- of 3500 km. The formation mechanism of this giant rounded in the east, south and west by strong magnetic canyon has been debated ever since it was discovered. anomalies that delineate the outskirts of the plateau, whereas Models include rifting, dike emplacement, normal faulting, no appreciable magnetic anomalies exist in the central part. tensional fracturing, removal of water-saturated debris, The magnetic signature of the plateau resembles that of and collapse of void space at depth [e.g., Schultz, 1997; Hellas basin (Figure 4a). It is plausible to suggest that the Anderson and Grimm, 1997; Tanaka, 1997; Wilkins and impact that created the Thaumasia basin also demagnetized Schultz, 2003]. The common feature of the proposed models the underlying crust, and no core dynamo was active after is some sort of mass wasting and crustal thinning. A the impact event to remagnetize the source bodies. The positive Bouguer anomaly of 150 mgal is associated with magnetic anomalies over the outskirts of the plateau are the central part of the canyon (Figure 3c), implying an likely the relicts of the preexisting stronger anomalies. excess mass in the crust equivalent to a mantle uplift of 15 km. Many of the models proposed for the formation of 3.5. Tharsis Bulge Valles Marineris imply that upper parts of the floor are [22] Figure 2 shows no significant magnetic anomalies porous, and the floor rocks have lower density than over Tharsis bulge. The bulge formed through major vol- 2900 km/m3, which I use in the calculation of the Bouguer canic activities in Noachian and early Hesperian, although anomalies [e.g., McKenzie et al., 2002; McGovern et al., minor volcanism likely continued to the recent past 2002]. The actual crustal thinning is probably more severe [Hartmann and Neukum, 2001]. Wise et al. [1979] than that deduced from the Bouguer anomaly in Figure 3c. suggested that Tharsis represents an uplift of the lithosphere The porous floor and possible long existing water make the resulting from a giant thermal anomaly or dynamics of the rocks more susceptible to low-temperature hydration. If the mantle. According to this model, the volcanic plains are thin Tharsis crust were magnetized prior to the formation of veneers on the older uplifted crust, implying that the preex- the canyon, it is to be expected that the crust in the canyon isting crust was not appreciably magnetized. On the other would be partly demagnetized by the near-surface low- hand, Solomon and Head [1982] relate the entire Tharsis to a temperature alteration, and partly by thermal demagnetiza- thick and vast volcanic construct. It is still debated whether tion in the deep crust due to the crustal thinning and mantle the Tharsis load is totally surficial or has some internal uplift. Figure 5 shows simplified models of the central part component [e.g., Willemann and Turcotte, 1982; Solomon of the canyon, which is approximated by a 400 200 km and Head, 1982; Banerdt and Golombek, 2000; Phillips et demagnetized rectangular zone of a 5 km thick infinite al., 2001; Lowry and Shong, 2003]. However, there is a horizontal layer. The bulk magnetization intensity of the

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Figure 5. The vertical component of the magnetic field of a 200 400 km rectangular demagnetized zone of an otherwise uniformly magnetized horizontal layer. The layer is horizontally (a) or vertically (b) magnetized with a uniform bulk magnetization of 5 104 A. The anomalies are calculated at 100 km elevation. The x and y axes denote distances (km). layer is set to 5 104 A, which is over 10 times weaker than altitude data led Purucker et al. [2000] to suggest lateral the bulk magnetization of the magnetic source bodies of offset for the formation of the canyon, as also proposed Mars [e.g., Connerney et al., 1999; Arkani-Hamed, 2002a]. by Anguita et al. [2001]. Wilkins and Schultz [2003] Both horizontal and vertical magnetization is considered. argued against this tectonic mechanism and proposed a The magnetic anomaly is calculated at 100 km elevation. No north-south extension, which seems compatible with the magnetic signature resembling those of Figure 5 exists over anomalies seen in Figures 3a and 3c. The elongated Valles Marineris in Figure 3a, emphasizing that the sur- northeast-southwest trending positive anomaly extending rounding plain is not magnetized, or is very weakly mag- from (5S, 315E) to (10S, 305E) shows a sharp cut by an netized. Adapting these simplified models to the size of east-west trending fault system on the magnetic map Syria planum reveals an appreciable magnetic anomaly on derived from the low-altitude data [see Purucker et al., the planum. The absence of such an anomaly supports the 2000, Plate 1], but no such a sharp cut is seen in Figure 3a. suggestion that the preexisting Tharsis plains beneath and The difference between the low-altitude map and Figure 3a immediately surrounding the planum are not significantly is probably due to the appreciable gaps in the low-altitude magnetized either. data over this area [see Purucker et al., 2000, Plate 1; [24] The effects of the formation of Valles Marineris Arkani-Hamed, 2001a, Figure 1], whereas the high-altitude are well presented in the magnetic anomaly and Bouguer data almost evenly cover the entire region. The detailed anomaly maps of Thaumasia (Figures 3a and 3c). The characteristics of the anomaly in the low-altitude map are Bouguer map shows that the negative anomaly over probably created by data interpolation procedures. It is also the outskirts of Thaumasia is cut apart by the canyon. possible that the effect of a possible sharp cut is smeared The magnetic anomalies in the eastern part of the canyon out in Figure 3a because of the decay of short wavelength (between 290E and 320E) show that, from east to west, magnetic anomalies at high altitudes. The low resolution of the almost circular positive magnetic anomaly outside the the anomalies in Figure 3a does not allow firm discrimi- canyon is not significantly affected by the canyon. The nation between the two tectonic scenarios. Nevertheless, neighboring northwest-southeast trending negative anom- both scenarios indicate that the magnetic anomalies in the aly overlapping the eastern part of the canyon crosses the eastern part of Valles Marineris are older than the tectonic canyon, but is weakened over the canyon. The positive activities that occurred during late Noachian to early anomaly to the west is divided into 2 parts, but there is a Hesperian, whether or not the anomalies were cut by weak connecting bridge. Finally, the next negative anom- lateral faults or were ruptured apart by extensional normal aly that overlaps a wider part of the canyon is entirely faults and grabens. Magnetic data from much lower ele- separated into two parts. It is reasonable to conclude that vations are essential to delineate the tectonic details of the these magnetic and Bouguer anomalies existed prior to canyon. the formation of Valles Marineris that ruptured apart the 3.5.2. Shield Volcanoes source bodies. The magnetic anomalies seen over this [25] The large shield volcanoes Olympus, Arsia, Pavonis, region on an earlier magnetic map derived from low- and Ascreaus have punctured the Tharsis plain and have

9of12 E03006 ARKANI-HAMED: TIMING OF THE MARTIAN CORE DYNAMO E03006 poured out a huge amount of molten lava. Hood and [28] The substantial distance between the paleomagnetic Hartdegen [1997] estimated the magnetic field that would pole positions and the present rotation axis of Mars suggests have been produced by the entire volcanic structure if it an appreciable polar wander of the planet. Arkani-Hamed were magnetized by a core field that is an order of [2001b] estimated the paleomagnetic pole position of Mars magnitude weaker than the Earth’s core field. Despite the on the basis of modeling 10 isolated magnetic anomalies conservative core field intensity, the predicted magnetic derived from the low-altitude MGS magnetic data. The pole field could have been easily detected by MGS. The lack positions are clustered within a circle of 30 degree radius of such a magnetic field indicates that no core dynamo was centered at 25N, 220E, despite the fact that the magnetic active during the formation of shield volcanoes, from early source bodies are distributed over the entire globe, ranging Hesperian to late Amazonian. The preexisting Tharsis plains from 65N to 35S and over the entire longitude. Hood and that are overlain by the shield volcanoes are not appreciably Zakharian [2001] modeled two magnetic anomalies near the magnetized either. If they were previously magnetized, the north pole and deduced a polar wander of up to 50 degrees. shield volcanoes could have demagnetized part of them and One of the anomalies was included in the 10 anomalies created low-magnetic patches, giving rise to detectable modeled by Arkani-Hamed and the pole positions deter- magnetic anomalies as illustrated in Figure 5. For example, mined by the investigators differ by less than 5 degrees. within a radius of about 150 km from the summit of Also, Hood and Richmond [2002] modeled two anomalies Olympus the original Tharsis plain is overlain by more than at low latitudes. The paleomagnetic pole position deter- 17 km of volcanic rocks, which includes the surface mined from one of the anomalies falls within the 30-degree topography and possible minor flexure of the underlying radius circle of Arkani-Hamed. Boutin and Arkani-Hamed crust. The temperature increase in the original crust beneath [2003] modeled 9 isolated magnetic anomalies using low- this region due to burial under the thick volcanic pile was altitude and high-altitude data simultaneously. The resulting likely more than 150C, which together with the thermal paleomagnetic pole positions show clustering similar, but effects of the underlying magma chamber could have not identical, to the previous one. The common feature of demagnetized an appreciable part of the crust, giving rise the paleomagnetic pole positions estimated by different to detectable magnetic anomalies. authors is that none of the poles coincide with the present rotation axis of Mars. Although a small mass such as Olympus mons emplaced on a spherically symmetric Mar- 4. Polar Wander of Mars tian model will move to the equator within 1 Gyr and [26] Theoretical studies of the rotational dynamics of result in a significant polar wander [Spada et al., 1996], the Mars suggest appreciable wandering of the rotation axis possible polar wander of Mars is actually controlled by since the emplacement of Tharsis bulge. The appreciable the huge mass associated with the Tharsis bulge. Once the polar wander is also supported by the paleomagnetic pole bulge reached the equator, adding relatively small extra positions estimated from the magnetic anomalies of Mars, masses at later times by the formation of the shield implying that the anomalies are older than the bulge. volcanoes could not result in an appreciable additional polar [27] There are several lines of evidence for the motion of wander. Assuming that the dipole axis of the core field that the rotation axis, the polar wander, of Mars. Murray and magnetized the magnetic source bodies coincided with the Malin [1973] proposed that the polar features of Mars bear rotation axis of Mars, which is almost the case for Earth and some relation to the past positions of the rotation axis, and Mercury at present, the paleomagnetic pole positions imply suggested a polar wander on the order of 10–20 degrees in an appreciable polar wander after the source bodies acquired the last 100 Myr. Melosh [1980] suggested the polar magnetization. This leads to the conclusion that magnetic wander of Mars resulted from the formation of Tharsis source bodies must have acquired magnetization prior to the bulge. He gradually removed the topography of the bulge formation of Tharsis bulge. while diagonalizing the moment of inertia tensor of Mars, [29] I would like to emphasize that the conclusions reached and deduced a polar wander of 25 degrees. The resem- in this study are mainly based on the high-altitude magnetic blance of Mesogaea deposits south of Olympus to the data. The high elevation of the data severely limits the deposits in the polar region led Schultz and Lutz [1988] resolution of the magnetic anomaly map, and details of to suggest a polar wander path with a total of 120 degrees possible tectonic and magnetic patterns of Mars are not wandering. The theoretical modeling of the polar wander of delineated on the map. The aeromagnetic and marine mag- Mars by Spada et al. [1996] showed that the rotational axis netic anomalies of Earth delineate major tectonic features, of Mars could have moved by as much as 70 degrees in such as continent-continent collision zones, and seafloor response to the emplacement of Tharsis mass. Using a wide spreading magnetic stripes. However, except for the Creta- range of mantle viscosity, 1021 to 1023 Pa s, they demon- cious quiet zones of oceans, the seafloor spreading magnetic strated that mantle viscosity has an important effect on the stripes are not visible at the high elevations of magnetometer rate of polar wander, the higher the viscosity the lower is satellites [e.g., Langel and Estes, 1982; Ravat et al., 1995]. the rate. However, the entire polar wander could have There is a strong suggestion that the core field of Mars had easily occurred even for the high-viscosity mantle model. reversed its polarity [Arkani-Hamed, 2001b]. If there are Stiefelhagen [2002] further improved the theoretical model seafloor-spreading type Martian lithosphere [Sleep, 1994], by incorporating an elastic lithosphere on a viscous mantle. and the core field reversals were frequent, the resulting He concluded that a thick and rigid lithosphere enhances magnetic anomalies are short wavelength and cannot be the rate of polar wander by preventing the isostatic com- detected at the high altitudes of MGS. Accordingly, the pensation of the surface mass and retaining it at the surface magnetically quiet regions of Mars may resemble the Jurassic far from the axis of rotation. magnetic quiet zones of Earth. Future low-elevation magnetic

10 of 12 E03006 ARKANI-HAMED: TIMING OF THE MARTIAN CORE DYNAMO E03006 measurements are essential to better delineate details of the source bodies were broken apart by the formation of the magnetic anomalies, and the tectonic pattern of Mars. canyon, which is in good agreement with the broken feature [30] Last but not least, the relationship between the of the Bouguer anomaly. The magnetic source bodies were magnetic anomalies and tectonic features of Mars presented magnetized prior to the formation of Valles marineris. in this paper does not entirely rule out the possibility of Finally, the lack of magnetic edge effects associated with intermittent activity of the core dynamo. Although a possi- Valles Marineris, the Tharsis plains expected to be demag- bly vigorous thermal convection in the liquid core may have netized by the shield volcanoes, and Syria planum is good declined in the early history of Mars and hence killed the evidence that a major part of the Tharsis plains is not dynamo, further cooling of the core could nucleate a solid magnetized. It is formed in the absence, or in the later inner core and regenerate a strong core field by non-thermal, stages of waning, of the core dynamo. The relationship chemical convection in the outer liquid core. The absence of between the observed magnetic anomalies and tectonic a core field at present does not provide any constraint on the features on Mars suggests that core dynamo was likely thermal evolution of the planet and the state of its core. decaying during early Noachian and no continuously active Neither a completely solidified core nor a completely strong core dynamo existed from Noachian to late Amazo- molten but slowly convecting core can sustain a strong nian, and possibly to the present. dynamo. Also, a core with a solidifying inner core may not sustain a dynamo if the liquid part is in eutectic composition [32] Acknowledgments. This research was supported by the Natural and slowly convecting. The solar tide Love number of Mars Sciences and Engineering Research Council (NSERC) of Canada. I would like to thank Jack Connerney at Goddard Space Flight Center for suggest- recently determined from the dynamics of MGS suggests ing a more stringent criterion to select the least contaminated nighttime that the upper parts of the core is likely liquid [Yoder et al., MGS data. I would also like to thank Andrew Hynes at McGill, and the two 2003]. In the absence of diagnostic geophysical data such as reviewers, Norm Sleep and an anonymous one, for helpful and constructive seismic and heat flow it is implausible to rule out a scenario comments and suggestions. that the inner core nucleated some times after the formation References of the shield volcanoes, and the outer core has only recently Acuna, M. H., et al. (1999), Global distribution of crustal magnetization reached a eutectic point and hence lost the ability of discovered by the Mars Global Surveyor MAG/ER experiment, Science, supporting a core dynamo. However, a simple scenario that 284, 790–793. the core dynamo was active only in the very early history of Acuna, M. H., et al. (2001), Magnetic field of Mars: Summary of results from aerobraking and mapping orbits, J. Geophys. Res., 106, 23,403–23,417. the planet seems more likely. The seismic and heat flow Anderson, F. S., and R. E. Grimm (1997), Rift processes at Valles Mar- measurements by future Mars missions are essential to ineris: Rate dependent strength evolution versus necking, Proc. Lunar investigate the timing of the Martian core dynamo more Planet. Sci. Conf. 28th, 1558. Anderson, R. C., et al. (2001), Primary centers and secondary concentra- quantitatively. tions of tectonic activity through time in the western hemisphere of Mars, J. Geophys. Res., 106, 20,563–20,585. Anguita, F., et al. (2001), Tharsis dome, Mars: New evidence for Naochian- 5. Conclusions Hesperian thick-skin and Amazonian thin-skin tectonics, J. Geophys. Res., 106, 7577–7589. 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