Sirenum Research Articles

Magnetization, Paleomagnetic Poles, and Polar Wander on Mars

Using the geometry of an areocentric dipole field, we have remodeled the anomalies observed in the Terra Sirenum and Terra Cimmeria regions by the Mars Global Surveyor (MGS). For the origin of the anomalies, we used both magnetic stripe and magnetic shell hypotheses. For magnetic stripe models, we assume that sea-floor spreading formed the anomalies in the presence of a reversing axial dipole field. We found that a simple magnetic stripe model with constant magnetization and crustal thickness results in poor agreement with the observed anomalies. However, by allowing the intensity of magnetization to vary freely between and along the stripes, a reasonable fit to the observed data can be found while maintaining the observed polarity along the stripes. Applying the simple magnetic stripe model to the Terra Sirenum and Terra Cimmeria anomalies results in best-fit spreading centers located well to the north of the surviving magnetic stripes. This could possibly explain the apparent lack of symmetry that has been noted in the anomalies. For the magnetic shell hypothesis, we assume that a monopolarity dipole field magnetized the early crust of Mars. The anomalies that persist today would then be the result of partial demagnetization of the original magnetic shell by more recent processes. The magnetic anomalies in Terra Sirenum and Terra Cimmeria are consistent with this model, especially if the magnetization of the original magnetic shell was laterally inhomogeneous. If the monopolarity magnetic shell hypothesis is correct, best-fit magnetic paleopole locations for the original magnetization lie in a sub-equatorial belt centered near the prime meridian. If the ancient magnetic pole of Mars coincided with the spin pole, this result implies that either apparent or true polar wander has occurred on Mars. Apparent polar wander would be consistent with plate tectonics having occurred after the formation of the magnetic anomalies. True polar wander has previously been proposed to explain certain morphologic features found on Mars.

Global distribution of crustal magnetization discovered by the mars global surveyor MAG/ER experiment

Vector magnetic field observations of the martian crust were acquired by the Mars Global Surveyor (MGS) magnetic field experiment/electron reflectometer (MAG/ER) during the aerobraking and science phasing orbits, at altitudes between approximately 100 and 200 kilometers. Magnetic field sources of multiple scales, strength, and geometry were observed. There is a correlation between the location of the sources and the ancient cratered terrain of the martian highlands. The absence of crustal magnetism near large impact basins such as Hellas and Argyre implies cessation of internal dynamo action during the early Naochian epoch ( approximately 4 billion years ago). Sources with equivalent magnetic moments as large as 1.3 x 10(17) ampere-meter2 in the Terra Sirenum region contribute to the development of an asymmetrical, time-variable obstacle to solar wind flow around Mars.

Martian surface mineralogy from 0.8 to 1.05 μm TIGER spectro-imagery measurements in Terra Sirenum and Tharsis Montes formation

New 0.8–1.05 μm reflectance spectra of Mars are derived from TIGER spectro-imaging measurements obtained at Mauna Kea CFH observatory during the 1990 opposition, from a systematic survey of the Tharsis volcanic region with spatial resolution of 200–250 km. Primary goals of this investigation are to identify the physico-chemical origins of the observed spectral variations, to map the surface units based on their spectroscopic properties, and to draw inferences in terms of compositional heterogeneities of bright and dark materials. Detailed information regarding the ferric and ferrous mineralogy of undocumented or poorly documented soils is presented, based on a reliable set of about 1500 highly-resolved spectra (R = λΔλ∼ 600) and with application of mixture modelling and spectral variable analyses to the dataset. The spectral signatures exhibit absorption features that can be distributed in three major groups: (1) the 0.85–0.89 μm bands, (2) the 0.90–0.94 μm bands, and (3) the 0.95–1.0 μm bands. Results of the mixture modelling reveal that at our spatial resolution, nearly 90% of the investigated terrains may be explained by various combinations of clinopyroxene-dominated with hematite-dominated minerals. The remaining 10% present subtle spectral variations that might result from slight differences in composition or from physical effects. Based on these combinations, the spectroscopic information shows that a linear mixture of low- and high-albedo soils mentioned in previous martian observations is not systematic to explain intermediate-albedo soils. Both ferric and ferrous characteristics are found to be present in the spectra of most dark and bright regions, for surface units of about 250 km × 250 km in areal coverage. In addition, spectral analyses show the occurrence of unusual associations of spectral properties that provide strong arguments to demonstrate the compositional heterogeneity of the soils. These heterogeneities are thought to be driven by variable mixtures of hematite-like minerals, hematite-pyroxene assemblages, and/or varied pyroxene phases. Relationships between spectral and thermal properties seem to be more complex than the classical bimodal distribution. Ferric and ferrous characteristics appear to be related to thermal inertias, though this is not systematic, with a few regions escaping from the general observed trend. The northern bright region of Tharsis, including Amazonis Planitia, is interpreted as spectrally uniform at a large spatial scale and covered by ferric-dominated materials, as demonstrated by the frequent occurrence of a hematite-related absorption centred at 0.86 μm. A surface alteration at large spatial scale is inferred from our spectral observations of this region. The southern dark terrains of Terra Cimmeria, Terra Sirenum, and south Daedalia Planum are spectrally documented and their spectral properties indicate a greater inherent variability with respect to the bright areas. The analysis of these areas suggests a composition of clinopyroxene-like minerals. Combined spectral and thermal properties also support the presence of bonded ferrous materials and crusts that remain partially exposed. Highly fractured bright areas of Noctis Labyrinthus, Noctis Fossae, northern Claritas Fossae, Ceraunius Fossae, and the erosional remnants of the plateau west of Amazonis are proposed to constitute isolated deposits of heavily altered fine-grained dust. The transition between the bright northern terrains of Tharsis and the southern cratered plateau is interpreted as variable in composition, according to the importance and spatial distribution of the alteration of the basaltic substrate. The proportion of exposed surface may also vary over the dark basaltic areas, and probably becomes less important towards north Tharsis.