Observation and Modeling of Mars Crustal Magnetism
Mars Global Surveyor's (MGS) magnetometer/electron reflectometer (MAG/ER) investigation mapped regions of intensely magnetized crust on Mars. The most intense crustal fields were observed over the ancient southern highlands but significant fields were observed over most of the planet. The most accurate magnetic field maps were compiled using data acquired during MGS mapping orbit at approximately 400 km altitude, owing to the large number of available observations and nearly complete global coverage. Interpretation of these maps yields valuable insight regarding crustal evolution on Mars, the role of plate tectonics, and a history of resurfacing by massive lava flows. However, one often desires knowledge of the field elsewhere - particularly nearer to the surface, directly sampled via aerobraking passes during pre-mapping (to 90 km altitude). The challenge is to find an appropriate technique for continuation of the vector magnetic field to lower altitudes. Methods used to infer magnetic fields include direct inversion (source modeling), Fourier techniques for downward continuation (as applied to survey data) and spherical harmonic modeling applied to global observations. We will discuss the pros and cons of each and present some promising results using an iterative Fourier technique for downward continuation.
Generation of Magnetic Fields in Terrestrial Planets
Only the Earth and Mercury possess an internally generated magnetic field at the present time. The absence of magnetic fields in the other terrestrial planets is thought to reflect a barrier to generating fields in a terrestrial planet. Several factor may contribute to this barrier. One factor is the high thermal conductivity of liquid iron. A high thermal conductivity is a necessary property of a good electrical conductor, but the disadvantage lies in the large heat flow carried by thermal conduction down the adiabatic gradient. This heat flow may represent a substantial fraction of the heat loss from the core because of the slow pace of cooling due convection in the overlying mantle. As a consequence, little of the heat flow through the core contributes directly to the generation of a magnetic field. A second factor is the low viscosity of liquid iron. A low viscosity ν relative to the magnetic diffusivity η (e.g. Pm ≡ ν/η ≈ 10-6) permits convection at very small scales. The flow at the smallest scales contributes little to the generation of the magnetic field because the effects of magnetic diffusion overwhelm the effects of magnetic induction. Thus any heat flow carried by small-scale convection is also unavailable to the dynamo. I give a brief review of the energy balance for planetary dynamos to illustrate the challenges in terrestrial planets. Numerical simulations are used to quantify the role of small-scale convection with the aim of refining the estimates of the power available to the dynamo.
Creating Abnormal Dynamo Models to Explain the Abnormal Magnetic Fields of Mars and Mercury
The Earth's magnetic field is dominated by its axial-dipole component and the strength of the field is in agreement with strong-field dynamo estimates. Several numerical dynamo models can reproduce magnetic fields with Earth-like characteristics by simulating convection in a thick shell geometry with a range of parameter values and boundary conditions. It appears that producing a dynamo model with strong axial-dipole dominated fields is relatively easy. However, there are several planetary magnetic fields that are non-Earth- like. For example, Mercury's observed field is weaker than anticipated for a strong-field dynamo, and the ice giant magnetic fields are non-axial, non-dipolar dominated. Mars' crustal magnetic field also presents puzzles as the strong fields are all concentrated in the southern hemisphere. Here we present numerical dynamo models that apply non-Earth-like scenarios to explain the abnormal magnetic fields of Mercury and Mars. For Mercury, we discuss the effects of convective shell thickness and the presence of stable layers. For Mars, we present our recent results of a single-hemisphere dynamo which can result in the presence of laterally homogeneous thermal boundary conditions at the core-mantle boundary.
Tidal Powering of the Core Dynamo of Mars
The giant impact basins of Mars formed at around 4 Ga provide evidence that there were a few satellites that orbited Mars in the early history with decaying orbital parameters. Once a satellite entered the Roche limit of Mars it ruptured and the large fragments impacted on Mars tracing a great circle. The orbital dynamics of four possible largest satellites show that each one of them could have orbited Mars for several hundred million years if it were a retrograde satellite. Continual elliptical straining of otherwise circular fluid streamlines of the liquid core of Mars by tidal deformation could have exerted strong strain that was large enough to overcome dissipation and excite the elliptical instability inside the core. We investigate the physical properties of the Martian core that are required to allow the tidal deformation to power the core dynamo, i.e., the growth time of the elliptical instability to become shorter than the dissipation time. The tidal energy dissipation rate inside Mars caused by one of the 4 largest satellites is found to be over two orders of magnitude greater than the magnetic energy dissipation rate in the core, indicating that if even only one of the 4 largest satellites were orbiting in retrograde sense, it would have likely powered the core dynamo of Mars for several hundred million years.
Concerning the Initial Temporal Evolution of a Hermean Feedback Dynamo
Various possibilities are currently under discussion explaining the observed weakness of the intrinsic magnetic field of planet Mercury. One of the possible dynamo scenarios is a dynamo with feedback from the magnetosphere. Due to its weak planetary magnetic field, Mercury exhibits a small magnetosphere whose sub-solar magnetopause distance is only about 1.7 Hermean radii. Hence, the magnetic field due to magnetopause currents cannot be disregarded in the dynamo region. Since the external field of magnetospheric origin is antiparallel to the dipole component of the dynamo field, a negative feedback results. For a simple α Ømega-dynamo two stationary solutions of such a feedback dynamo emerge, one with a weak and another with a strong magnetic field. The question, however, is how these two stationary solutions can be realized. To address this problem, we discuss various scenarios for a simple dynamo model and the conditions under which either of the stationary situations evolves. We find that the feedback mechanism quenches the overall field to a low value of about 100 nT if the dynamo is not driven too strongly.
Magnetic Properties, Processes and Minerals in Crustal Rocks of the Terrestrial Planets and Their Satellites
Earth is unique in having active plate tectonics which continually renews the crust. Deeply eroded ancient crust survives in Precambrian shields but retains little evidence of impact cratering. Continental plutonic rocks owe most of their magnetic signal to magnetite or hemoilmenite, whose potentially strong TRM and susceptibility give rise to both remanent and induced magnetic anomalies. Mafic gneisses and granulites of the middle and lower crust are also probable magnetic anomaly sources. High temperatures enhance the magnetite viscous induced signal but degrade the TRM through thermoviscous demagnetization. Hematite and iron oxyhydroxides are ubiquitous at Earths surface but their magnetic signals are overwhelmed by those of magnetite, whose spontaneous magnetization Ms is two orders of magnitude larger. The primary magnetic minerals of the oceanic crust are Ti-rich titanomagnetites with Ms values about 1/4 that of magnetite. They become more magnetic with phase-splitting to magnetite + ilmenite but less magnetic when altered by seawater to titanomaghemites. The Curie point isotherm of titanomagnetite is shallow, so that oceanic magnetic anomalies have predominantly near-surface remanent sources. Mars and other terrestrial planets have much smaller fields than Earths: their anomaly sources are entirely remanent. The very large anomalies in parts of the southern highlands crust of Mars suggest a deep Curie point isotherm, a high-Ms mineral, single-domain structure (associated with fine grain size) and perhaps also an unusual abundance of magnetic material. Fe-rich titanomagnetites fit the bill in all respects and form in abundance in synthesized basalts of Martian crustal composition. Large impact craters on both Mars and the Moon have low magnetic expression, probably due to shock demagnetization and/or randomization of remanence vectors. Iron-nickel minerals like kamacite and tetrataenite dominate in lunar rocks and meteorites. In most grain sizes they are unstable remanence carriers and lead to weak sample magnetizations and planetary crustal anomalies. Iron sulphides such as pyrrhotite and troilite are common in lunar samples and meteorites, and may well be significant in the Martian crust also.
Magnetization and Demagnetization Effects of Impact Shock Waves on the Moon and Terrestrial Planets
Clear evidence for magnetization and demagnetization effects of large-scale impacts has been observed on the Earth, Moon, and Mars. On the Earth, fields tend to be weaker than average over impact structures and central anomalies are often present. However, remanent magnetization effects on crustal fields are less easily distinguished from the effects of crustal permeability contrasts in the strong geodynamo field. On the Moon, crustal fields are especially weak within young large impact basins and this is attributable to shock demagnetization of relatively soft crustal magnetization (Halekas et al., MAPS, 2003). For older basins, central anomalies are sometimes present whose origin is not well understood. The strongest crustal magnetization on the Moon occurs diametrically opposite (antipodal) to the largest and youngest basins. The strong antipodal anomalies have been attributed mainly to shock remanent magnetization (SRM) associated with the impact of ejecta in a transient magnetic field amplified by the converging impact vapor-melt cloud (Hood & Artemieva, Icarus, 2008). Lunar ejecta materials contain abundant small metallic iron particles which are the main remanence carriers on the Moon. These materials could have acquired their magnetization rapidly in transient fields. The latter model allows but does not require the existence of a former lunar core dynamo. During the period of compressed field amplification, shock stresses are produced by the impacting ejecta within the range of 5-25 GPa where stable SRM of lunar soils has been found experimentally to occur (e.g., Fuller et al., Moon, 1974). In some cases, strong SRM was apparently also acquired by basin ejecta nearer to the basin itself (e.g., the Descartes Formation on the near side) and by pre-existing ejecta (e.g., peripheral to the South Pole Aitken basin). At Mars, crustal magnetic fields are weak within and near the youngest large basins; this is explained mainly by shock demagnetization and the absence of subsequent remagnetization processes within these basins. No antipodal anomalies are present. This may be due to the existence of a dense atmosphere, which would have opposed expansion around the planet of the vapor-melt cloud, and to the relatively long time required to form magnetic sources, which would not have favored magnetization in transient fields.