Unveiling the origin of radial grabens on Alba Patera volcano by finite element modelling

The Tharsis region is an 8000-km-wide structural dome that incorporates a concentration of the main volcanic and tectonic activity on the Planet Mars. The area of structural doming is characterised by giant radial graben-dike systems. Nested on a set of these giant dikes to the northern side of Tharsis, is Alba Patera, one of the largest volcanoes in the planetary system. The regional dikes there are in arcuate arrangement and imply an E W to NW SE regional extension at Alba Patera. To assess the influence of regional and local tectonics, we studied the dike orientations on the volcano with Viking mosaic data and simulated plausible stress fields with finite element modelling. We found that the influence of a NW SE regional extension was strong near the volcano centre but decreased rapidly in importance towards the northern pole, i.e., far from the Tharsis centre. By combining this regional stress with a broad uplift that is due to a buoyancy zone of about 1400 km in lateral extent and centred under Alba Patera, we reproduced the radial pattern of dike swarms that diverge from the Tharsis trend. Regional tectonics may have dominated the early stages of dike injection. During the evolution of Alba Patera, however, local updoming controlled the dike pattern, supporting the idea of a hotspot under Alba Patera. The well-expressed dike geometry and characteristics of Alba Patera provide an ideal example for comparative study with analogue hotspots on Earth where plate tectonics and active erosion may complicate the reconstruction of volcanic and tectonic history and the understanding of involved geodynamic processes

A Geodynamic Model of Alba Patera; Hotspot Tectonics and Volcanism Under the Tharsis Stress Field

Alba Patera, one of the largest volcanoes in the planetary system, is situated at the northern periphery of the Tharsis rise and is characterised by numerous graben-dike systems. A study of the fault or dike geometry combined with finite element modelling and analogue experiments allows to assess the influences of regional and local tectonics on the volcano. The graben configuration reflects a change of regional stress orientation and magnitude from the Tharsis centre to the periphery. To the south of Alba Patera, the branch of oldest grabens indicates a significant regional E-W extension. At higher latitudes, the direction of the regional extension turned towards NW-SE. Its influence on the structural pattern was important near the volcano centre and decreased toward the north. However, volcanism and tectonics at Alba Patera were largely uncoupled from the Tharsis activity. Broad uplift centred on Alba Patera better explains the radial pattern of dike swarms that occurred from the edifice centre to the northern pole along 1000 km distance. Coupled with the giant dike swarms, the widespread volcanism of Alba Patera's early phase is similar to the flood basalt provinces commonly associated to hotspot and continental rifting episodes on Earth. Local uplift was followed by subsidence of smaller wavelength responsible for the formation of concentric grabens on the upper and mid flanks of the volcano. The circular fracturing represents a long-term mechanism active during several ten or even hundreds of Myrs accounting for mantle dynamic processes. An increase of density of the mid and lower crust by intrusion and subsequent cooling below Alba Patera probably formed a local stress field that superposed on the regional tectonics. The study of Alba Patera allows to reconstruct the successive tectonic and magmatic events from the birth to the death of an hotspot

Planetenkrusten Vorlaeufiger Arbeitsbericht

SIGLETechnische Informationsbibliothek Hannover: AC 6378. / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

MaRS: Mars Express Orbiter Radio Science

The Mars Express Orbiter Radio Science (MaRS) experiment will employ radio occultation to (I) sound the neutral martian atmosphere to derive vertical density, pressure and temperature profiles as functions of height to resolutions better than 100 m, (II) sound the ionosphere to derive vertical ionospheric electron density profiles and a description of the ionosphere through its diurnal and seasonal variations with solar wind conditions; MaRS will also (III) determine the dielectric and scattering properties of the martian surface in target areas by a bistatic radar experiment, (IV) determine gravity anomalies for the investigation of the structure and evolution of the martian crust and lithosphere in conjunction with observations of the High Revolution Stereo Camera as a base for 3D topography, and (V) sound the solar corona during the superior conjunction of Mars with the Sun. The radio carrier links of the spacecraft Telemetry, Tracking and Command subsystem between the Orbiter and Earth will be used for these investigations. Simultaneous and coherent dual-frequency downlinks at X-band (8.4 GHz) and S-band (2.3 GHz) via the High Gain Antenna will permit separation of contributions from the classical Doppler shift and the dispersive media effects caused by the motion of the spacecraft with respect to the Earth and the propagation of the signals through the dispersive media, respectively. The investigation relies on the observation of the phase, amplitude, polarisation and propagation times of radio signals transmitted from the spacecraft and received with antennas on Earth. The radio signals are affected by the medium through which they propagate (atmospheres, ionospheres, interplanetary medium, solar corona), by the gravitational influence of the planet on the spacecraft and, finally, by the performances of the various systems aboard the spacecraft and on Earth

LunarStar Subsatellite "GAUSS" - A Proposal for the Complete Gravity Field Determination of the Moon

GAUSS will be provided by Germany as a contribution to LUNARSTAR Mission in the NASA Discovery program. The main scientific objectives of the LUNASTAR mission are the exploration of a lunar atmosphere, the study of lunar geochemistry and the surface evolution of the Moon In connection with a determination of the total gravity field with high resolution (typically 70 - 70), considerable new insight will be gained in the following areas: Global shape model of the Moon, near/far-side dichotomy, structure and evolution of large impact craters and basins, evolution of the lithosphere, comparative planetology. It is proposed to provide a subsatellite with a total mass of ~ 16 kg dedicated only to gravity field determination. GAUSS shall relay phase coherent microwave signals (S-band) from LUNARSTAR to Earth with an overall Doppler accuracy of better than 1 mm/s. The frequency source (USO) onboard the Lunastar main spacecraft. From the Doppler data collected at the German and US groundstations both local and global gravity information will be derived. The time duration for the gravimetry mapping part of the LUNARSTAR has reached its 100 km altitude polar circular orbit. GAUSS will remain in the eccentric polar lunar transfer orbit (200 km - 5000 km). Launch is forseen in October 2001