Jafar Arkani-Hamed

The effects of serpentinization on density and magnetic susceptibility: a petrophysical model

Abstract New measurements are presented of initial magnetic susceptibility and density for serpentinized harzburgites from the Josephine Peridotite (Oregon). An inverse correlation between density and susceptibility, which is typical of these rocks and similar rocks from other localities, is not explained by simple serpentinization reactions. Calculations of density and susceptibility as a function of reaction progress for reactions that produce magnetite show susceptibilities that are larger than those observed at low degrees of serpentinization. Susceptibilities of Fe-bearing serpentine and brucite are calculated from a molecular field model initially developed for the olivine and orthopyroxene solid solutions. At high degrees of serpentinization, susceptibilities that are lower than those observed are predicted by reactions where iron is sequestered by serpentine and brucite. Observed density-susceptibility trends of serpentinized harzburgites fall between the values predicted by these limiting reactions. The observed properties may be modelled by multiple reactions involved in multi-stage serpentinization processes such that the production of magnetite increases with the degree of serpentinization. The empirical and calculated properties provide a complementary characterization of density and susceptibility values of serpentinites.

A 50‐degree spherical harmonic model of the magnetic field of Mars

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.

A study of lunar impact crater size-distributions

Discrepancies in published crater frequency data prompted this study of lunar crater distributions. Effects modifying production size distributions of impact craters such as surface lava flows, blanketing by ejecta, superposition, infilling, and abrasion of craters, mass wasting, and the contribution of secondary and volcanic craters are discussed. The resulting criteria have been applied in the determination of the size distributions of unmodified impact crater populations in selected lunar regions of different ages. The measured cumulative crater frequencies are used to obtain a general calibration size distribution curve by a normalization procedure. It is found that the lunar impact crater size distribution is largely constant in the size range 0.3 km ⩽D ⩽ 20 km for regions with formation ages between ≈ 3 × 109 yr and ≳ 4 × 109 yr. A polynomial of 4th degree, valid in the size range 0.8 km ⩽D ⩽ 20 km, and a polynomial of 7th degree, valid in the size range 0.3 km ⩽D ⩽ ⩽ 20 km, have been approximated to the logarithm of the cumulative crater frequencyN as a function of the logarithm of crater diameterD. The resulting relationship can be expressed asN ∼Dα(D) where α is a function depending onD. This relationship allows the comparison of crater frequencies in different size ranges. Exponential relationships with constant α, commonly used in the literature, are shown to inadequately approximate the lunar impact crater size distribution. Deviations of measured size distributions from the calibration distribution are strongly suggestive of the existence of processes having modified the primary impact crater population.

Scalar magnetic anomaly maps of Earth derived from POGO and Magsat data

A new 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.

Paleomagnetic pole positions and pole reversals of Mars

The paleomagnetic pole positions of Mars obtained by modeling 10 small-size, isolated magnetic anomalies show considerable clustering, 7 out of 10 are within a circle of 30 degrees radius centered at (230E, 25N). The directions of the magnetization of some nearby source bodies differ by ∼180 degrees and both north and south poles are found in the cluster, suggesting the magnetic pole reversals of Mars.

Global vector and scalar Magsat magnetic anomaly maps

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).

Fractal stochastic modeling of aeromagnetic data

We present a convolutional linear data model for the processing of aeromagnetic data. The model assumes that the data derive from the superposition of a deterministic system function and a stochastic innovation process. The two‐dimensional system function is described by a four‐pass autoregressive (AR) filtering procedure and is radially symmetric. The innovation process represents the distribution of near‐surface magnetic sources and is modeled as a spectrally self‐scaling (i.e., fractal) noise. The appropriate fractal noise is determined by examining aeromagnetic power spectra from various areas of the Canadian Shield. The AR coefficients of the system are determined using an iterative deconvolution procedure. For computational convenience, we make the traditional assumption of a spectrally white innovation, but modify the data prior to its deconvolution by prewhitening the assumed fractal innovation. The recovered system function is then removed from the original data in order to produce the fractal st...

Thermal evolution of Venus

Abstract The theory of three-dimensional and finite-amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified Boussinesq fluid assumption. The lateral dependences of the variables are resolved through their spherical harmonic representations, whereas their radial and time dependences are determined by numerical procedures. The theory is then applied to produce thermal evolution models for Venus. The emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet. The main conclusions achieved in this paper are (1) a significant portion of the present temperature in the mantle and heat flux at the surface of Venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation, (2) the effective Rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers, (3) the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer, and (4) a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust. The models presented in this paper suggest that Venus has been highly convective during its history until ∼ 0.5 Ga ago. The vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags, floating on the surface, in to the mantle. The rate of cooling of the planet was so high that its core has solidified. In the last 0.5 Ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer. The tectonic style on Venus has, consequently, changed from the recycling of crustal plates to hot spot volcanics. At the present time the planet is completely solid, except in the upper part of its mantle where partial melting may occur.

The lunar mascons revisited

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.

Contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world's oceans

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 worlds 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.