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This paper presents a 50-degree and order spherical harmonic model of the magnetic field of Mars derived at 120-km altitude using the three orthogonal components of the vector magnetic field data acquired by the Mars Global Surveyor within the 80- to 200-km-altitude range. The downward continued vector magnetic field components to the surface of Mars delineate details of the Martian magnetic field. The strong magnetic anomalies in the southern hemisphere do not show compelling evidence for magnetic lineations similar to those associated with the seafloor spreading on Earth. The anomalies are more or less equidimensional. The magnetic signatures surrounding the impact basins Hellas, Argyre, and Isidis show that the prominent magnetic anomalies are older than the impact events. An average magnetization of ∼2 A/m is estimated for the upper crust surrounding Hellas basin. Four isolated magnetic anomalies are modeled by vertical prisms with circular cross sections of 170- to 190-km radius and 20- to 30-km thickness. The magnetization of the model prisms is 5 A/m more than that of the surroundings.

This paper provides an estimate of the thermal state of Martian lithosphere established since the formation of the shield volcanoes. Comparison of the spherical harmonic models of Martian topography derived using harmonics truncated at degrees 10, 11, 12, and 14 shows that the volcanoes have almost no contribution from harmonic coefficients of the topography less than degree 11. Therefore intermediate-scale surface topography and gravity anomalies of Mars, specified by harmonics of degree 11-50, are considered in this study. These harmonics are dominated by shield volcanoes, Alba Patera, and Isidis basin. The central part of Valles Marineris is also well defined by these harmonics. The strong correlation of the topography and gravity anomalies over these harmonics and the positive Bouguer anomalies of the volcanoes suggest significant excess masses beneath these surface features in addition to their topographic masses. This indicates that the Martian lithosphere has been strong enough to support both the topography and the internal excess masses. We determine the rigidity of the lithosphere beneath a given surface feature. The lithosphere is modeled by a thin elastic spherical shell that overlies an inviscid interior of higher density and is subjected to both surface and internal loads. The thickness of the shell and the magnitude of the internal load are calculated such that the deformed structure resulting from the flexure of the shell under both surface and internal loads gives rise to the observed topography and gravity anomaly. It is shown that the lithosphere beneath Elysium Mons, Alba Patera, and the central part of Valles Marineris can be modeled by elastic shells subjected to topographic loads alone, whereas the lithosphere beneath Olympus and Tharsis Montes cannot. Internal excess masses of at least 40%, and more likely 50%, of the topographic masses are required, and the lithosphere beneath must have an elastic core of ∼100-80 km to support these volcanoes. The mean thermal gradient beneath this region of Tharsis is estimated to be ∼7-10 K/km on the basis of the strength envelopes calculated using diabase rheology for the crust of 50 km thickness and olivine rheology for the underlying mantle.

By lowering the altitude of Lunar Prospector to about 10–20 km the spacecraft will provide measures of the Moon's gravity field at very high resolution, enabling us to derive a spherical harmonic representation that includes harmonics of degree 360 and demanding precise calculation of the gravity field of the topography at such low altitudes. We present a method to determine the gravity field of the topography at low altitudes, which is very efficient even for higher degree harmonics and is also applicable for topography with laterally and radially varying density. The method is applied to three simple models; namely, an unfilled basin, a basin with mare filling, and a topography specified by a tesseral harmonic of degree 60 and order 30, before applying it to the actual topography of the Moon. It is shown that the gravity of the topography calculated by the surface-mass density approximation (a conventional method) is sufficient for harmonics of degree up to about 100 but becomes increasingly inadequate as the degree of the harmonics increases. The method is also applied to internal density interfaces, such as a possible Moho undulation, and it is concluded that the surface-mass density approximation becomes less appropriate once the degree of the harmonics exceeds about 20. We also calculate the free air gravity anomaly of the lunar topography at 10 km altitude using this method and the available 90 degree spherical harmonic model of the topography. Although the surface topography of Venus has been expanded in terms of the spherical harmonics of degree up to 360 [Rappaport and Plaut, 1994], its gravity field is measured at altitudes greater than 150 km [Sjogren et al., 1997], which is much larger than the surface topography or possible undulation of the crust-mantle boundary. The results of our method and the conventional method may not differ significantly for Venus.

The equipotential surface hypothesis suggests that lunar maria floors lie on a surface parallel to the selenoid. This is examined using the spherical harmonic representations of the Clementine topography and Lunar Prospector gravity data. It is demonstrated that the floors of both circular and noncircular maria significantly deviate from an equipotential surface. Deeper circular maria and the deeper part of the noncircular Maxe Tranquillitatis have been subsided under larger mass loads in the crust. We calculate the mass beneath the maria to be in excess to the mass required for isostatic compensation of the topography at 60 km depth. A global map of this excess mass shows that the noncircular maria are isostatically compensated, unlike the circular maria. The map also reveals seven new sizable mascons: the three largest are associated with Mendel-Rydberg, Mare Humboldtianum, and Mare Moscoviense.

Regional studies have shown that remanent magnetization of the Cretaceous quiet zones (CQZs), created during the long period of geomagnetic normal polarity at 118–84 Ma, produce well-defined magnetic anomalies at satellite altitudes. We investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale. We consider entire oceanic areas because oceanic lithosphere formed during periods other than CQZ, but with a predominant polarity, also have appreciable contributions to the satellite anomalies. The magnetic anomalies of the world's oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map, plate relative motions, and the apparent polar wander path of Africa. Three magnetization models are examined: thermoremanent magnetization confined to extrusive layer 2A, and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km. Although all models lead to rather similar anomaly distributions, the amount of magnetization required suggests the need for deeper sources. All models produce the satellite anomalies associated with the CQZ in the North Atlantic and parts of the Indian and Pacific Oceans, with slight differences in location, depending on the models. Low-amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the Indian and Pacific Oceans are successfully modeled. A major difference between models and observation is the north-south elongated model anomalies associated, for instance, with the CQZs in the South Atlantic and the fast spreading East Pacific Rise. These elongated anomalies are systematically absent on the observed map, probably because they were removed by along-track filtering in the early stages of processing the satellite data. A careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4, 0, 1, and 0.6 A/m for the extrusive basalts, intrusive basalts, gabbros, and peridotites, respectively.

The entire spectra of the surface topography and gravity field of lunar mascons are well presented by the high-resolution Clementine data. The negative correlation between the topography and gravitational potential of the mascons suggests that they are dynamically supported. Both elastic support and viscous decay models are examined. For the elastic support models, the spectral characteristics and the lateral variations of the thickness of the elastic layer are determined on the basis of the thin spherical shell flexure formulation and using only the antivarying harmonics of the topography and potential. An elastic layer thickness of about 50 km is required to support Imbrium, Serenitatis, and Nectaris mascons, about 35 km for Crisium mascon, and about 30 km for Smythii and Humorum mascons. A layer of about 20 km thickness can support Orientale mascon. The strength envelopes of the upper 100 km of the Moon within 4–3 Gyr ago show that the elastic layer was not thick enough to support the mascons in the early history. They decayed through viscous deformation of the lunar interior. A lower limit of 6×1024 Pa s is estimated for the lunar viscosity within 3.6–3 Gyr ago. We also determine the thicknesses of the crust and mare flows of the mascon basins on the basis of the assumption that the basins were isostatically compensated prior to the mascon formation. The crust beneath the mascon basins is about 30–40 km except for Crisium and Orientale, where the crust is about 20 km. The mare flows of about 3–6 km are obtained for almost all of the mascon basins. The topography and gravitational potential of Aitken basin shows that the basin is compensated at 52–54 km depth. The mantle beneath the basin has rebounded upward by about 20–30 km and is overlain by about 20–25 km of a mixture of the excavated crustal and mantle material. The lack of a mascon and pervasive mare flows in the basin brings into question the current models of the mascon formation.

We have used spherical harmonic representations of the Venus topography and geopotential, obtained from Magellan data, to evaluate isostatic support in several areas within the Ishtar Terra highlands, including the Lakshmi plateau, its surrounding mountain belts, namely Akna and Freyja, and Maxwell Montes, and the Fortuna Tessera province. We find that topography in Ishtar is largely isostatically compensated (>80%). Regional geoid-topography variations in the subregions can be explained by a combination of Airy (crustal thickening) and thermal (lithospheric thinning) mechanisms, provided Venus has a thick reference thermal lithosphere (∼300–400 km). With the exception of eastern Fortuna, low elevation areas (h 4 km above MPR) with small GTRs are almost certainly Airy compensated via thickened crust. Relatively large (>60 km) total Airy crustal thicknesses obtained in the western Ishtar mountain belts, together with a probable basalt-eclogite phase change, suggest a possible silicic composition for these structures, provided they are older than ∼25–50 Ma. Lakshmi Planum seems essentially thermally supported, with the thermal lithosphere thinned to ∼100 km. We suggest, as one possibility, that the lithospheric thinning process under Lakshmi is delamination of a dense eclogite lower lithosphere layer into the mantle. The decrease in GTR observed in Ishtar between Lakshmi to the west (GTR ∼ 20 m/km), Maxwell and west Fortuna (GTR ∼ 8 m/km), and eastern Fortuna (GTR ∼ 4 m/km) may correspond to a decay in thermal compensation attributed to lithospheric delamination, which would be fairly recent (∼100 Ma) in Lakshmi, partially decayed in west Fortuna, and absent in east Fortuna, where a mostly Airy-supported topography is essentially relaxed with no thermal uplift. Alternatively, if surficial concentrations in radiogenic elements were prevalent throughout the crust, partial melting of a thickened crust could account for the thermal uplift in Lakshmi and west Fortuna. The zero-elevation basaltic crustal thickness H ∼ 24 km obtained for the east Fortuna Tessera region may be representative of the ambient crustal thickness in the Venus lowlands. Our findings support multicomponent models for tectonic and volcanic activity in Ishtar. The thick ambient crust and thermal lithosphere implied by this study agree with observational constraints such as support of extreme elevations, large topographic slopes, unrelaxed craters, and the thick elastic lithosphere suggested by flexure studies. If the ambient thermal lithosphere on Venus were to be relatively thin (∼100–200 km), with a cold mantle and radiogenic elements concentrated in the crust, then thermal evolution on Venus may be in quasi-steady state, with the geodynamic evolution in monotonic decline. However, if the ambient thermal lithosphere is very thick (∼300–400 km), as suggested by our thermal model fits, then it is consistent with the predictions of strongly unsteady state thermal evolution models and an interior which is currently heating up. This would support the view that catastrophic resurfacing on Venus might be episodic.

The relationship between the surface topography and gravitational potential of Venus and the Earth is investigated over the spherical harmonics of degree and order up to 75. The covarying harmonics of the topography and the potential, i.e. the harmonics having nonnegative degree/order correlation coefficients, are treated separately from the antivarying harmonics, those with negative degree/order correlation coefficients. The covarying harmonics provide reliable estimates of the compensation depth of the surface topography. The surface topography of the Earth specified by spherical harmonics of degree higher than 11 is essentially compensated isostatically at the Moho discontinuity, whereas the lower-degree harmonics are largely supported by mantle dynamics. For Venus the harmonics of degree lower than 9 are probably supported by mantle dynamics, while those of degree higher than 35 are most likely supported isostatically at a depth of about 45 km. There is a gradual increase in the compensation depth of the surface topography as the degree of harmonics decreases from 35 to 9, implying a combination of isostatic and dynamic support of these harmonics. Part of the topography specified by antivarying harmonics of degree greater than about 20 cannot be maintained by an isostatic compensation or a steady state dynamic support ; it is more likely time dependent. Assuming that this part of the topography arises from viscous deformation of mantle under surface loading, the gravity and topography relationship provides a first-order estimate of 5x10 24 Pa s for the viscosity of the Venusian mantle. This high viscosity results in a Rayleigh number of about 12000 for the mantle, which is only about 17 times greater than the critical Rayleigh number for convection, implying a very slow quasi-steady convection in the mantle.

The apparent depth of compensation (ADC) of the topography of Venus, determined on a global scale using the Airy isostasy model, shows that the topography specified by the spherical harmonics of degree lower than 10 is compensated at depths greater than 150 km, that of degree higher than about 35 is compensated at a constant depth of about 35 km, and the compensation depths of the harmonics of degree 10–35 shoal as the harmonic degree increases. Based on these characteristics of the globally determined ADC, three different maps of the topography and gravitational potential of the Greater Ishtar Terra are derived using the harmonics of degree 1–75 (LW), 10–75 (IW), and 30–75 (SW), respectively. The LW maps of the topography and potential do not correlate well, but the IW and SW maps show good correlation, emphasizing that the long-wavelength components of the gravitational potential over the Ishtar Terra have contributions from sources broader than the Terra. The energy of the gravitational potential of the topography is about an order of magnitude greater than that of the observed potential, indicating that the topography is strongly compensated. The ADC of the topography is estimated by both spectral and space domain analysis. In the spectral domain, three independent procedures, the energy spectrum, the statistical method, and the wavelength-dependent method, are employed. The first two yield a mean ADC value for the entire spectrum, whereas the last one calculates the ADC for different wavelengths. The ADC decreases as the longer-wavelength components are excluded. The mean ADC values determined for the LW, IW, and SW maps by the energy spectrum method are 115, 65, and 40 km, respectively. Those calculated through the statistical method are 85, 55, and 34 km, respectively. These values reflect the upper and lower limits, because the energy spectrum method overestimates and the statistical method underestimates the ADC. The wavelength-dependent method shows that the ADCs of the wavelengths 500–1350 km are similar for all the maps but those of the longer wavelengths significantly differ among the maps. The mean ADC values also differ from the ADC of local topographic features. For example, the ADC of Maxwell Montes is about 55 km in both the LW and IW maps. In the space domain, the ADC is determined for Pratt and Airy isostasy models through fitting a linear and a quadratic function to the geoid height versus topography data, respectively. The ADC values determined show good agreement with the above-mentioned values. In addition, the relationship between the high-resolution topography and the line-of-sight acceleration residuals of the western Ishtar Terra specified by wavelengths of 300–500 km suggests that the high-resolution topography is compensated at a depth of about 25 km. We also determine the lateral density perturbations inside a surface layer so that together with the topography of the Greater Ishtar Terra they give rise to the observed gravitational potential over the Terra. Three layer thicknesses of 50, 100, and 200 km are examined. The resulting density perturbations are too large to be interpreted in terms of lateral temperature variations alone. This suggests lateral variations in the rock type; the crust beneath the mountain belts may contain considerable amounts of low-density material.

A new Polar Orbit Geophysical Observatory (POGO) scalar magnetic anomaly map at 400 km altitude is presented which consists of spherical harmonics of degree 15-60. On the basis of the common features of this map with two new Magsat anomaly maps, dawn and dusk, two scalar magnetic anomaly maps of the Earth are presented using two selection criteria with different levels of stringency. These selection criteria suppress the noncrustal components of the original maps by different amounts. The more stringent selection criteria seek to eliminate as much contamination as possible, at the expense of suppressing some anomaly signal. This map is represented by spherical harmonics of degree 15-60. The less stringent selection criteria seek to retain as much crustal signal as possible, at the expense of also retaining some contaminating fields. This map is represented by spherical harmonics of degree 15-65. The resulting two maps are highly correlated with degree correlation coefficients greater than 0.8.

We have performed a detailed analysis of the skewness of marine magnetic anomalies in Indian Ocean basins created between 85 and 40 Ma as a result of the northward motion of India. Visual and semiautomated methods of skewness determination were applied to the data. Both provide consistent results, but the visual method is prefered for its ability to deal with noisy data. Plots of apparent effective remanent inclination (or skewness corrected for present geomagnetic field inclination) versus time for conjugate basins display the combination of three effects: a gradual increase with time, related to the northward motion of the ridges attached to India in the geomagnetic reference frame; a gap between conjugate curves, which represents anomalous skewness senso stricto; and short-period fluctuations, which represent the sequence effect, i.e., the effect of neighboring magnetic sources on the skewness of a given anomaly. The anomalous skewness decreases with faster spreading rate and completely disappears above 50 km/m.y., an observation which negates geomagnetic field behavior as a possible cause of the observed anomalous skewness.

The effects of thermal coupling of the core and mantle on the thermal evolution and mantle dynamics of terrestrial planets are investigated using three-dimensional thermal convection models in a spherical shell. Three pairs of models are presented to study mantle models of different viscosities and internal heating. In each pair, one model adopts a constant temperature at the core-mantle boundary, and the other allows the core to thermally couple to the mantle and cool. In the first and second pairs, the mantle is heated only from below, whereas in the third pair, internal heating is due to radioactive elements. The mantle viscosity of the first and third pairs is 1021 Pa s, while the mantle of the second pair has a viscosity of 1022 Pa s. The thermal coupling of the core and mantle has strong effects on the thermal boundary layer near the core-mantle boundary but only minor effects on the thermal boundary layer near the surface. It cools the core, substantially weakens the thermal boundary layer near the core-mantle boundary, and thus reduces the vigor of convection in the mantle. The models with cooling cores result in a substantially colder and relatively less dynamic mantle than the models with a fixed temperature at the core-mantle boundary which overestimates temperatures in the mantle.

We present a new approach to producing scalar Magsat magnetic anomaly maps based on correlation coefficient filtering and the use of almost all of the available orbits. Our method differs from earlier techniques with respect to the following: (1) Passes are selected based on their variance properties rather than planetary indices such as Kp. (2) The core field model is least squares fit to individual passes and subsequently removed instead of substracting the model directly. This technique replaces band pass filtering or polynomial trend removal methods. (3) Each selected pass is sorted geographically and by local time, placed into one of four different altitude bands, and correlation coefficient filtered with the two adjacent passes. The filtering is the second step toward isolating the static lithospheric signal from the more dynamic external field signals. (4) Least squares collocation is used to grid the correlated passes; subsequently, the dawn and dusk maps are also correlation filtered providing another step toward removal of external fields. (5) The four resultant total field maps are continued to a common altitude and again correlation filtered for the static lithospheric anomalies. (6) The filtered results are then averaged together to provide a new total field map of the lithosphere south of 40 deg S latitude. Our total field map differs from previous efforts over the crustal blocks of West Antarctica. We obtained a positive anomaly over Edward VII Peninsula, extending into the Byrd subglacial basin and obtained a negative anomaly over the Ellsworth Mountains and parts of the Byrd subglacial basin. Also, a positive anomaly extending from the Ross Sea to offshore Wilkes Land is present in our map; however, this feature is absent in other maps. Positive anomalies marking the Weddell Sea in previous efforts are not present in our map. Prominent external field anomalies in the quadrant offshore of Wilkes Land are present in all previous efforts; however, these signatures are reduced in our total field map. This map significantly advances efforts for the identification and interpretation of magnetic anomalies of the south polar lithosphere.

[1] In the paper “Possible crippling of the core dynamo of Mars by Borealis impact” by Jafar Arkani‐Hamed (Journal of Geophysical Research, 115, E12021, doi:10.1029/ 2010JE003602, 2010) the impact heating of Mars by the Borealis impact, which likely created the northern lowland basin, was calculated. Using the crater scaling laws [Holsapple, 1993; Melosh, 1989] the basin diameter was related to the size of the impactor. Following the impact, a nearly uniform shock pressurewas generated inside an isobaric region in the mantle, outside which the shock pressure decreased exponentially with distance from the isobaric center following the average model of Pierazzo et al. [1997]. After crossing the core‐mantle boundary, the shock wave travelled in the core where the shock pressure again decayed with distance travelled from the isobaric center according to Pierazzo et al.’s average model but with the physical parameters of the core. The corresponding impact temperature increase is obtained using the Foundering model of Watters et al. [2009]. [2] The impact‐heated core stratified within a few tens of years and the possible pre‐existing core dynamo decayed with the characteristic time of 10–15 kyr [Arkani‐Hamed and Olson, 2010]. Shortly after the stratification, convection started in a thin layer in the outer core, the thickness of which increased in time as convection penetrated to deeper parts of the core until the entire core convected on a global scale. It took ∼60 Myr for the core to develop global convection and power a new core dynamo. [3] Here I describe a correction required in the calculation of the shock pressure distribution in the core. This correction is important because it increases the crippling time of the Martian core dynamo by a factor of larger than 4. The source of the required correction is the transmission coefficient of the shock wave across the core‐mantle boundary (CMB), the ratio of the shock pressure at the top of the core to that at the base of the mantle. The transmission coefficient was determined on the basis of seismic ray theory [Aki and Richards, 2002, section 5.2]. It resulted in a sudden drop of shock pressure as passing from the mantle to the core, in contrast to the sudden jump of the pressure according to recent Hydrocode models [Ivanov et al., 2010; Bierhaus et al., 2011].

A petromagnetic forward model of induced magnetization of Iceland Plateau is presented. Magnetic susceptibilities of Icelandic rocks from the literature are placed in a vertical stratigraphic context according to a model of igneous and metamorphic layering of the Icelandic crust that is compatible with seismic and other geophysical data. Curie isotherm depths for the magnetic forward model are based on the geothermal gradients measured in drill holes and those derived from magnetotelluric surveys. The petromagnetic model is used to predict the satellite altitude (400 km) magnetic field of Iceland Plateau. Comparison of calculated and Magsat anomalies shows that the total induced magnetization (the vertically integrated induced magnetization) in the surrounding oceanic lithosphere is ≈2300 A. This value is very similar to independent estimates based on rock magnetic models of the oceanic crust Rockall Plateau is a Precambrian microcontinent largely surrounded by oceanic crust a few hundred kilometers south and east of Iceland. The size, age, and location of this plateau are ideally suited to characterize the ocean-continent magnetization contrast The total induced magnetization contrast between the plateau and its surroundings estimated from the Magsat anomaly of Rockall Plateau is ≈14,000 A.

One interpretation of the Magellan data suggests that the cratering record on Venus was erased by a global resurfacing event, or events, the latest ending about 500 m.y. ago. In this global-resurfacing model the resurfacing was followed by minor volcanism and tectonism that has been concentrated primarily in the equatorial highland regions characterized by extensive fracture belts and rifts. A thermal evolution model of Venus that can explain these observations is one in which a deformable lithosphere, capable of being incorporated in mantle circulations, provides an almost stress-free condition at the surface. Mantle convection with an almost stress-free boundary at the surface cools the interior more efficiently. Rapid cooling decreases the Rayleigh number of mantle convection below a transition value required for oscillatory convection, and the vigor of convection diminishes as the mantle changes to a quasi-steady circulation after about 500 m.y. ago.

Satellite magnetic anomaly maps of west Africa display persistent anomalies that are spatially related to the geological structure of the West African Craton. The Reguibat and Man shields to the north and south of the Taoudeni Basin have positive magnetization contrasts whereas the basin, which was disrupted by rifting and volcanism in the Late Proterozoic and Mesozoic, has a negative magnetization contrast. A forward model of the craton is developed, based on the surface geology and including aspects of the evolution of deep-seated thermal, petrological, and magnetization contrasts. Anomalies calculated from the model correspond fairly well to the most reliable observed anomalies. The anomaly associated with the basin is due partly to the anomalies arising from the shields and partly to intra-basin deep-seated demagnetization resulting from Mesozoic thermal and metasomatic demagnetization. Another similar demagnetized zone probably contributes to the Reguibat Shield anomaly. The Man Shield anomaly results primarily from the Archean segment of this shield. The largest magnetization contrast required, for a 70-km-thick shield slab, is + 0.3 A m−1.

[1] This paper examines the magnetization that can be acquired by the lower crust of Mars in the absence of a core field but in the presence of the magnetic field of the upper crust that is magnetized by the core field. A uniformly magnetized vertical prism of a square horizontal cross section is considered to be a magnetic source in the upper crust for simplicity. Three thicknesses of 40, 60, and 80 km and six horizontal dimensions of 180, 300, 600, 900, 1200, and 1500 km are considered for the prism. To estimate the upper limit for the contribution of the lower crust to the observed magnetic anomalies, a potentially magnetic layer of 100 km thickness is adopted. Coarse-grain hematite is uniformly distributed in the lower crust with 4, 10, 20, or 30% concentration of hematite. The first is equivalent to the oceanic extrusive basalt as far as its thermo-remanent magnetization (TRM) is concerned. It is shown that the magnetization of the lower crust has minor effects on the strong magnetic anomalies of Mars over the southern hemisphere. However, if the lower crust has 20 to 30% coarse-grain hematite, which is highly unlikely, it may have appreciable contributions to small magnetic anomalies of Mars with horizontal dimensions smaller than 200 km.

[1] Using a suite of numerical models, we show that impact-induced convection in the Moon can explain the formation of lunar mare basalts, the depth range of their source region, the observed delay between impact basin formation and starting time of mare flows, and the long duration of the basaltic flows. The effects of an impact on the thermal evolution of the Moon and melt production in the mantle are investigated using convection calculations in an axisymmetric cylindrical coordinate system. An ascending mantle plume is allowed to melt as it crosses the depth of the solidus temperature. We consider two different models: permeable and impermeable. Five different viscosity models and three different impact basin sizes are examined. The total amount of melt produced by the permeable model with 1000 times viscosity contrast across the computation domain is comparable to the observed mare flows in Imbrium and Orientale basins. Moreover, the starting time of major melting in the mantle and its duration are also compatible with the observations. The model also allows a rigid lithosphere to develop beneath the basins that is capable of supporting the mascons largely created during the peak mare flow period. The model for South Pole Aitken basin results in a substantial amount of melting in the mantle, which does not seem to be compatible with the observations. A potassium, rare earth elements, and phosphorus (KREEP) layer with high concentration of radioactive elements incorporated directly beneath the crust has minor effects on the melt production in the mantle.

[1] Magnetization of Martian crust has been modified by impact-induced shock waves and viscous decay since the cessation of the core dynamo of Mars at around 4 Gyr ago. Thermal evolution models of Mars suggest that the potentially magnetic layer was about 85 km thick during the active period of the core dynamo, assuming magnetite as the major magnetic carrier. The lower boundary of the magnetic layer has gradually decreased, by a total of about 30 km, through viscous decay of magnetization. The large impacts that created the giant basins Hellas, Argyre, and Isidis have almost completely demagnetized the crust beneath the basins. The shock wave pressure produced by impacts that created craters of diameters 300–1000 km is expected to significantly demagnetize the crust beneath the craters. However, except for a few craters, there is no signature of appreciable demagnetization. This implies that either the magnetic carriers have high coercivity and have resisted demagnetization, or magnetic source bodies are deep seated, or they have acquired magnetization after the intensive impact cratering period. An alternate possibility is that the scaling laws proposed for small craters do not apply to the large craters considered in this paper.

[1] The strong magnetic anomalies of Mars require highly magnetic sources in the crust. The bottom of the potentially magnetic layer is constrained by the Curie temperature of its magnetic carriers, and the top of the layer is constrained by the thickness of the uppermost crust that has been demagnetized by the impacts. This paper presents a systematic study of the thermal evolution of the Martian crust and the effects of eight major physical parameters on the thickness of the potentially magnetic layer in the crust. It is shown that the initial upper mantle temperature, the mantle viscosity, and the total radioactive content of Mars are the major parameters that have substantial effects on the thermal state of the crust in the first 1 Gyr of the planet's history. The magnetic source bodies that have been magnetized by the core field during the first 500 Myr are located in the upper about 100–90, 90–80, or 55–45 km of the crust if hematite, magnetite, or pyrrhotite is the major magnetic carrier of the source bodies, respectively. The shock pressures induced in the crust by impacts can demagnetize the uppermost part of the crust. It is demonstrated in this paper that impacts that create craters of diameters larger than ∼200 km are capable of demagnetizing the entire crust, and those that create craters of diameters less than ∼50 km can demagnetize the upper 10–20 km of the crust. Detailed studies of the secondary thermal remanent magnetization acquired by deeper parts of the crust, in the absence of the core field but in the presence of the magnetic field of the upper crust, suggest that the secondary magnetization has minor effects on the observed magnetic anomalies of Mars.

[1] It is shown in this paper that five giant impact basins of Mars, Argyre, Hellas, Isidis, Thaumasia, and Utopia, are located on a single great circle. This suggests that the projectiles' orbital planes coincided with the equator of Mars at the time of impact. The projectiles were larger fragments of a heliocentric asteroid that broke apart as it entered the Roche limit of Mars. The mass of the asteroid was at least ∼9.8 × 1020 kg, ∼1.5 × 10−3 times the mass of Mars.

[1] Using rock magnetism and thermal modeling, we evaluate the candidate minerals responsible for strong magnetic anomalies in the Terra Sirenum and Terra Cimmeria regions of Mars' southern highlands. We assume an early global dynamo field similar in strength to the present Earth's field, enduring about 500 Myr after accretion and core formation, and a basaltic crust containing no more than 4-7 weight% of magnetic minerals. Thermal evolution models with a wide variety of initial crustal thicknesses, distributions of radioactive elements, and thermal expansion coefficients all yield similar thermal histories for the crust: warming in the first ∼1000 Myr (due mainly to radioactive heating) followed by monotonic cooling for the remainder of Mars' history. Primary thermoremanent magnetization (TRM) acquired by intrusive and extrusive bodies during the first 500 Myr was in part thermally demagnetized by general crustal warming after the dynamo field disappeared, from 500 to 1000 Myr. The Curie point isotherms around 1000 Myr established the maximum depth of TRM-bearing crust. Shock and heating due to impacts demagnetized the uppermost ∼10 km of the crust around the same time, resulting in potential magnetic layer thicknesses of 15-20 km for pyrrhotite, 40-50 km for magnetite, and 50-60 km for hematite. Other magnetic phases, such as iron and finely exsolved low-Ti titanohematite, are possible but less likely in a basaltic crust under oxidizing conditions. The prime candidates, in order of likelihood, are single-domain magnetite (0.2-0.4 volume% or 0.4-0.8 weight% required), single-domain pyrrhotite (1-2 volume% or 2-4 weight%), and either multidomain (>15 μm) or 5-15 μm single-domain hematite or a mixture of both (1.5-3 volume% or 3-6 weight%). A composite source with different combinations of these minerals at different depths is entirely possible. Viscous decay of TRM is difficult to assess without detailed knowledge of the distribution of minerals and blocking temperatures with depth but would increase the amounts of magnetic material required.

[1] This paper shows that single-domain or pseudo-single-domain (SD/PSD) magnetite grains can exist in the upper 30–45 km of the Martian crust. The upper ∼50 km of the Martian crust have likely formed by volcanic eruptions on a stagnant lithosphere. A given lava flow cools very rapidly at the surface but is then gradually heated up through burial heating as it is overlain by other successive flows. The thermal evolution models of a lava flow show that its temperature remains below the magnetic blocking temperatures of magnetite, 480–580°C, until it reaches ∼30–45 km depth. The highly magnetic SD/PSD magnetite grains can easily explain the strong magnetic anomalies of Mars; less than 1% by volume of magnetite particles is sufficient.

[1] The huge amount of data acquired by Mars Global Surveyor during its mapping period provides a unique opportunity to reassess the paleomagnetic pole positions of Mars previously determined on the basis of the limited low-altitude magnetic data. We identify nine small and isolated magnetic anomalies on the basis of the global magnetic maps and model each anomaly using a vertical prism of elliptical cross section. Both high-altitude (360–430 km) and low-altitude (100–200 km) magnetic data are used simultaneously. We calculate a paleomagnetic pole position assuming that the body is magnetized by a dipole core field. Although the new pole positions do not cluster as closely as the old ones, the new cluster overlaps the older cluster. The clustering suggests that Mars' rotation axis has likely wandered by ∼50°–60° in the last ∼4 Gyr. The number of north and south poles in the cluster suggests at least one reversal of the core field during the time the source bodies acquired magnetization.

[1] The immense amount of the mapping-phase Mars Global Surveyor (MGS) magnetic data allows us to derive a highly accurate magnetic map of Mars. The data acquired at nighttimes within the first ∼3 years of the mapping phase are divided into two almost equal parts, and each part is expressed in spherical harmonics of degree up to 90. The two models are almost identical over harmonics of degree up to 62 but show appreciable differences over higher-degree harmonics, indicating that the higher-degree harmonic coefficients have appreciable noncrustal contribution. The most repeatable components of the two models are combined to produce an accurate 62-degree harmonic model of the magnetic field of the Martian crust. It is further demonstrated that the altitude of MGS is the major limiting factor of the resolution of the mapping-phase data. The small-scale features of the data, with wavelengths shorter than ∼400 km, have significant contribution from noncrustal sources. They are not useful for delineating the details of the magnetic source bodies in the crust.

[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.

[1] This paper investigates the thermoremanent magnetization (TRM) of the Martian lithosphere assuming that the upper part of the lithosphere acquired TRM in the early history of the planet and in the presence of the core field (the primary magnetization), whereas the lower part has been gradually magnetized by the magnetic field of the upper part as it has cooled below the Curie temperature (secondary magnetization). The secondary magnetization is relatively weak if the magnetic properties of the lower lithosphere are taken to be similar to that of the extrusive basalt near the oceanic ridge axes on Earth. However, if the Martian lower lithosphere is more magnetic, e.g., by an order of magnitude, the secondary magnetization can be significant, although still weaker than the primary magnetization of the upper part. Such a highly magnetic lower lithosphere does not seem plausible by terrestrial standards, and it remains to be confirmed whether it is geologically plausible for Mars. I also examine a lithospheric model with a strongly magnetic layer beneath the crust, 25 times more magnetic than the extrusive basalt, representing a possible ilmenite-rich layer with a considerable amount of hematite-ilmenite lamellae. I assume that the layer was not magnetized by the core field because it was hotter than the Curie temperature of the lamellae when the core dynamo was active. It acquired secondary magnetization later in the presence of the magnetic field of the crust. Despite adopting an extraordinarily high magnetic layer which seems implausible by terrestrial standards, the secondary magnetization of the layer has minor contributions to the observed magnetic anomalies. In all models considered the primary magnetization is ∼20–30 A/m and has the dominant contribution to the observed magnetic anomalies. It is worth noting that the radial components of the primary and secondary magnetization are parallel, whereas the tangential components are in opposite directions. Because the strong magnetic anomalies in the south likely arise from the tangential magnetization, considering the secondary magnetization requires even stronger primary magnetization.

[1] This paper presents the lateral variations of the vertically averaged magnetization model of the Martian crust determined from the degree 50 spherical harmonic model of the magnetic potential. The model potential is derived using the Mars Global Surveyor low-altitude magnetic data acquired within elevations of 85–200 km. The potential model is first verified by comparing the three orthogonal components of the corresponding vector field with the observed high-altitude (∼380 km) data. Using the paleopole positions calculated through modeling 10 isolated, small magnetic anomalies, the model potential is inverted to determine the vertically averaged magnetization of a nominal 50 km thick crust. The crust is highly magnetic, with a magnetization contrast (peak-to-trough) as high as 35 A/m. The lack of distinct magnetic signatures for the giant impact basins, Hellas, Argyre and Isidis, and the absence of a magnetic anomaly associated with the north-south topographic dichotomy indicate that the upper parts of the crust are low magnetic and that the strong magnetic source bodies are in the lower parts.

[1] This paper presents a 50-degree spherical harmonic model of the magnetic field of Mars derived from both low- and high-altitude magnetic data through covariance analysis. Three sets of models are first calculated by using the high-altitude data alone, which include harmonics of degree 55, 60, and 65. Each set consists of four models, depending on whether the north-south and west-east components data are used in a model in addition to the radial component. The models correlate strongly over harmonics of degree up to about 50, except the model that is calculated by using the radial component data alone, which shows appreciable differences from the others. The high-altitude models are then correlated with the corresponding models that were previously derived from the low-altitude data. The correlation between the corresponding models is again very high over harmonics of degree 4–50, confirming the reliability of the spherical harmonic coefficients over these harmonic degrees. The slightly lower correlation, ∼0.7, over harmonics of degree lower than 4 implies some contribution from quasi-steady external field. A final 50-degree model is determined from the covarying harmonics of the low- and high-altitude models that use all three components of the magnetic data. The corresponding radial and tangential components of the magnetic anomalies are presented at the surface of Mars, along with their error limits.

[1] Depth-dependent density perturbations are determined inside the elastic part of the Martian lithosphere (assumed to be 300 km thick) using a thick elastic shell model, such that the deformed model under the loads of surface topography and the density perturbations gives rise to the observed topography and gravity anomalies of Mars specifies by spherical harmonics of degree 3–50. Large positive density perturbations, as high as ∼150 kg/m3, exist in the crust that are associated with Hellas, Isidis, Argyre, and Utopia basins, reflecting the mantle uplift in response to the impact excavation. Also, large positive density perturbations exist in the crust beneath Olympus (∼350 kg/m3) and Tharsis monts (∼200 kg/m3), indicating that these shield volcanoes are not isostatically compensated. On the other hand, negative density perturbations are associated with Alba Patera, Elysium rise, and the highland surrounding Hellas basin. The density perturbations gradually diminish with depth, becoming less than 20 kg/m3 by 250–300 km depth. The resulting stress differences within the crust and upper mantle are dominated by Olympus (up to 100 MPa), whereas other shield volcanoes create lesser stress differences. We estimate the brittle-ductile transition temperature in the lithosphere, adopting the creep law of dry olivine for the rheology of the Martian lithosphere. The brittle-ductile transition temperature beneath Olympus is ∼800C throughout the lithosphere, indicating that the upper parts of Mars must have been cold to support this volcano for 1–2 Gyr.

Several models are calculated in order to assess the effects of different physical parameters on the thermal evolution of Venus. The models are based on three-dimensional thermal convection calculations in an incompressible mantle of infinite Prandtl number using a modified Boussinesq approximation. The mantle is assumed to have a temperature- and pressure-dependent viscosity, temperature-dependent thermal conductivity, depth-dependent thermal expansion coefficient, and time-dependent internal heat generation rate. The physical parameters considered are the initial temperature distribution, a possible D″-like layer at the base of the mantle, the temperature at the core/mantle boundary, the core solidification, the decrease of thermal expansion coefficient with depth, the rate of internal heat generation, the radially dependent viscosity, and the velocity boundary condition at the surface. A constant temperature at the core/mantle boundary develops a strong thermal boundary layer at the base of the mantle, resulting in highly oscillatory mantle convection. Allowing the core to cool suppresses the boundary layer and reduces the amplitude of the oscillations substantially. The initial temperature distribution, the core solidification, and a D″-like layer have minor effects on the overall cooling of the mantle, although the enhanced heat of fusion of the core hampers the cooling of the core. The decrease of the thermal expansion coefficient with depth lowers the slope of the adiabatic temperature gradient in the mantle and reduces the temperature in the lower part of the mantle and the core appreciably. The heat generation rate has a significant effect on the present thermal state of the mantle; a higher rate of heat generation enhances the mantle temperature, Similarly, a higher mantle viscosity decreases the convection velocities and hampers the heat loss from the mantle. However, the most important parameter that controls the thermal evolution of the planet is the velocity boundary condition at the surface. A stress-free (we examined semifree) surface allows mantle material to approach the surface and cool efficiently, whereas a rigid (we examine semirigid) surface hampers heat loss from the planet, resulting in a hot planet even when the internal heat sources are reduced by about an order of magnitude.

Magsat anomalies over the Pacific Ocean are related to volcanic plateaus and the edge effect of the Pacific Cretaceous Quiet Zone (PCQZ). Four prominent plateaus, Shatsky Rise, the Mid-Pacific Mountains, Hess Rise, and Manihiki Plateau, are considered in this study. The major lateral magnetization contrasts associated with these plateaus are considered to arise from the topographic swells and the crustal roots, with root thicknesses estimated from topography using an Airy isostatic model. Shatsky Rise and the Mid-Pacific Mountains are older than the Cretaceous Quiet Period and their natural remanent magnetization (NRM) is not a significant component of the magnetization contrasts that cause the Magsat anomalies associated with these plateaus. Their magnetic properties are characterized by effective magnetic susceptibility contrasts of 0.07 (±0.01, S.I., per unit volume) averaged over their topographic swells and crustal roots, equivalent to 0.04 if averaged over the entire crustal thicknesses (swell plus root plus intervening layer). This value agrees well with the estimates of previous investigators for these plateaus and for similar plateaus in other oceans. Two plateaus of Cretaceous Quiet Period age within the PCQZ, Hess Rise and Manihiki Plateau, have origins similar to those of Shatsky Rise and the Mid-Pacific Mountains, and they probably have similar effective susceptibility contrasts but they may also carry Cretaceous Quiet Period NRM. Magnetic anomalies due to Hess Rise and Manihiki Plateau, calculated from the effective susceptibility contrast deduced for Shatsky Rise and the Mid-Pacific Mountains, are substantially larger than the observed Magsat anomalies of these plateaus. There are also Magsat anomalies over parts of the Pacific where no plateaus exist Some of these anomalies are demonstrated as probably due to a contrast in NRM between the PCQZ and its surroundings. The total remanent magnetization contrast (magnetization contrast in amperes per meter times thickness in meters, equivalent to the dipole moment per unit area) of the PCQZ is 14,000–20,000 amperes (±30%), which cannot be simply explained by the absolute values of remanent magnetization predicted by conventional models of oceanic crustal magnetic properties. Modelling of the Cretaceous quiet zones in the northern Atlantic indicates a similar total remanent magnetization contrast. These results may be explained by including uppermost mantle remanent magnetization, with an intensity of about 1–2 A m−1 if magnetization persists to a depth of ≈7 below the seismic Moho, i.e, down to the depth of the crustal roots of the plateaus. Including NRM in the uppermost mantle of the PCQZ also serves to reduce the calculated amplitude of the Hess Rise and Manihiki Plateau Magsat anomalies if the NRM of the crustal roots is less than the NRM of the surrounding uppermost mantle. On this basis the estimated intensity of NRM in the uppermost 7 km of the PCQZ mantle is ≈1–3 A m−1, in very good agreement with the intensity estimated from the PCQZ edge effect anomalies. These estimates of uppermost mantle NRM are probably a maximum limit, recognizing that other sources may also contribute to the total remanent magnetization contrasts of the Cretaceous quiet zones.

The aeromagnetic anomalies of the oceanic crust northeast of Newfoundland, Canada, are strongly skewed, with an anomalous skewness of 33–52°, with respect to the anomalies calculated using the standard sea-floor spreading model. This paper examines the possibility that the anomalous skewness is on account of an oceanic crust with a thermo-viscous remanent magnetization, where unlike the standard model the lower crust is magnetic. The thermo-viscous remanent magnetization of the lower crust beneath the Labrador Sea has dominant contributions to broad-scale marine magnetic anomalies and to the negative Magsat anomaly of the Sea.

A low-pass filtered aeromagnetic map of southern Vancouver Island is inverted into a magnetization-contrast map that illustrates the lateral variations in the magnetization of the crust. The Island Intrusions and the West Coast Crystalline Complex are characterized by high magnetization relative to the other lithologic groups. The Island Intrusions form an almost continuous pluton in the deeper part of the crust. The low-magnetic Leech River Formation thickens northward. The almost uniform magnetic signature of the Leech River Formation in the east–west direction indicates that the crustal structure revealed from the Lithoprobe seismic results is typical throughout the formation west of the Survey Mountain fault. A low-magnetic zone, elongated in the northwest–southeast direction, cuts through the high-magnetic Metchosin basalts from the southern tip of the island to the Leech River fault. It coincides with the Sooke gabbros exposed in the southern tip of the island, implying that the gabbros probably cut the entire Metchosin basalts at depth.

Magnetic susceptibility anomalies associated with the lithosphere in Africa and South America show that the lithosphere underlying sedimentary basins within aulacogens which originated during the rupture of the Gondwana continent have lower magnetic susceptibility than the lithosphere underlying the cratons. This can be explained by the demagnetization effects of high temperature asthenospheric material possibly intruded into the aulacogens during the rupture. Thermal evolution of different intrusion models implies that (a) thermal demagnetization may last for about 300 Ma, (b) there is no close relationship between the early subsidence rate of a basin and the magnetization of the underlying lithosphere, (c) surface heat flow anomalies do not necessarily correlate with the magnetic susceptibility anomalies, and (d) once an intrusion of more magnetic material cools to the ambient temperature the total magnetic susceptibility of the lithosphere may increase. High-magnetic susceptibilities associated with sedimentary basins over old crustal rifts can be explained by this phenomenon.

A crustal scalar magnetic anomaly map of Canada and the northern United States is derived using data collected from the MAGSAT satellite. The anomalies are correlated to geological features. Basins associated with failed arms of old rifts have high magnetic anomalies. The Rocky Mountains, the Appalachian Mountains, the suture zone of the Grenville province, modern hotspots and ocean ridges have low magnetic anomalies.

To investigate the presence and nature of the lateral heterogeneities in the earth's mantle, geophysical data with global distribution are analyzed and correlated. Surface heat flow, seismic travel-time residuals, and crustal thickness data are expanded in spherical harmonics. Using these, and the coefficients of similar expansions of surface topography of the earth, and the gravitational potential variations as revealed by satellite data, both total and degree correlation coefficients are obtained. From the interpretation of these results, jointly with available laboratory data on velocity-density relationships and the temperature coefficient of velocity, the following are concluded: (1) Lateral mass anomalies and/or density variations in the earth's mantle are larger by at least an order of magnitude from what is required to explain the satellite data. Earth's surface topography and crustal structure introduce loads which are compensated in the upper, and perhaps lower, mantle giving rise to lateral density anomalies in the mantle. (2) Surface heat flow variations are controlled primarily by the shallow structure and tectonic features of the earth. (3) At long wavelengths (n ≤ 6), gravitational, heat flow, and seismic travel-time variations are not correlated with topographic elevations.

The effects of a giant impact on the thermal evolution of the Moon are investigated. It is found that an impact similar to that of Imbrium creates lateral temperature variations of more than 200 deg within the upper 200 km of the Moon. Starting with a common lithosphere of 70 km thickness, the lithosphere beneath the basin grows to 200 km in thickness within 0.5 b.y. after the impact, while that beneath the highlands reaches to only 100 km in thickness during this period. The model presented for the thermal evolution of the Moon is compatible with (1) the existence of mascons for more than 3 b.y., (2) the late magmatization and subsequent volcanic activities during 4 to 3 b.y. ago and (3) the negative gravity rings around the large mascons.

Empirical and analytical techniques for modeling ionospheric fields in Magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies. This task has been accomplished for equatorial ΔX, ΔZ, and ΔB component and polar ΔZ and ΔB component measurements. The techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data-processing techniques developed during the last decade. Data-processing techniques include retention of common signal in a correlation analysis of adjacent passes ; analysis of field differences between dawn and dusk data at points where their orbits cross ; and retention of common signal in a covariant spherical harmonic analysis procedure. Results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere (vector components as well as scalar).

Interpretations of magnetic anomalies often assume negligible remanent magnetization because detailed information about magnetic properties is rarely available. This paper attempts to assess the effects of thermal remanent magnetization (TRM) on magnetic anomalies caused by intrusives. Several three-dimensional axisymmetric and two-dimensional intrusives of different sizes are emplaced at different depths and at different times, and their thermal evolutions are calculated. On the basis of these thermal evolutions, we determine their TRM patterns, assuming reversals of the Earth's magnetic field have occurred and using typical magnetic blocking temperatures for igneous rocks. Results indicate that an intrusive with a uniform distribution of magnetic minerals acquires a layered pattern of TRM. Near-surface intrusives acquire stronger magnetization than deep-seated ones. The contribution of TRM to magnetic anomalies is quite different from the contribution of induced magnetization, especially for near-surface intrusives. Interpretations of magnetic anomalies caused by deep-seated intrusives in terms of induced magnetization are more reliable than those of the anomalies which are associated with near-surface intrusives.

The total magnetic field of the earth over Canada and the northern United States is sampled along individual orbits (passes) of the magnetometer satellite (Magsat) on the basis of data extracted from the Magsat chronicle tapes. The main magnetic field at a given sample point is calculated using the main field model by Langel et al. (1982a) and then subtracted from the sampled value. The external magnetic field contribution is partially removed by a quadratic function fitted to each individual pass. The resulting magnetic residuals are resampled at 55 by 55 km grid points on an equal-area projection and subsequently contoured. The dawn passes yield a magnetic map that is different than the dusk passes, implying that the remaining external magnetic field component has a dawn-dusk asymmetry. The component is further suppressed by applying a band-pass filter in the frequency domain. The magnetic anomaly map derived by averaging the dawn and the dusk band-pass-filtered maps shows the main features seen on the aeromagnetic maps of Canada and the United States. The map also shows that the Appalachian and the Cordilleran orogens and the Grenville-Superior suture zone are magnetically low regions, whereas the craton basins such as the Michigan, the Thelon, and the NE British Columbia Basin are magnetically high areas.

Remanent magnetization patterns of the oceanic lithosphere determined on the basis of the thermal evolution of the lithosphere, while taking into account both thermal and viscous magnetizations, give rise to magnetic anomalies similar to those observed by Magsat over the Cretaceous quiet zones of the Atlantic Ocean and by marine magnetic surveys over anomaly 33. Viscous magnetization acquired during a given core field polarity period is not removed completely by that acquired in the next period of opposite polarity because of cooling of the lithosphere and the strong temperature dependence of viscous magnetization. The magnetization pattern of layer 2A resembles that of the standard seafloor spreading magnetic model, consisting of normally and reversely magnetized zones separated by vertical boundaries, and produces symmetric magnetic anomalies. The magnetization pattern of the middle crust is similar to that of layer 2A for high magnetic blocking temperature models, but it is slightly skewed with respect to layer 2A for low magnetic blocking temperature models. The magnetization pattern of the lower crust is strongly skewed relative to that of layer 2A and has diffuse boundaries with pronounced slopes between normally and reversely magnetized zones. It produces magnetic anomalies with appreciable skewness, suggesting that observed skewness of marine magnetic anomaly 33 arises from the magnetization of this layer. The magnetization pattern of the upper mantle is appreciably skewed and almost devoid of small-scale variations. It produces strongly skewed magnetic anomalies at satellite altitudes which may explain the observed skewness of Magsat anomalies over the Cretaceous quiet zones of the Atlantic Ocean.

The magnetometer satellite (Magsat) data are separated into two distinct sets: the dawn and the dusk sets. The two sets are analyzed independently because of the dawn-dusk asymmetry of the external magnetic field components of the data. Two scalar magnetic anomaly maps, the dawn and the dusk maps, are derived using the spherical harmonic expressions of the respective data set. The significant correlation of the two maps in the harmonic range of 18–41 indicates the repeatability of the Magsat data in these harmonics. The shortest repeatable wavelength is about 920 km. A band-limited global scalar magnetic anomaly map is derived at about 410 km altitude by arithmetic averaging of the dawn and the dusk maps. The map is then downward continued to about 10 km altitude. The downward continued magnetic anomalies are relatively more localized. The main features of the anomalies are as follows: (1) Continents have relatively stronger magnetic anomalies than oceans, especially the younger oceans, (2) many circum-Pacific subduction zones have magnetic signatures, and (3) there is no overall correlation between anomalies and major shields.