P33C-01
Constraints on the concentration of heat sources in the Martian interior from viscous relaxation of topography
Crustal thickness variations induce lateral pressure gradients which can drive flow in the lower crust if the temperatures there are sufficiently high. Gravity and topography data imply that the thickness of the Martian crust is not constant and that large lateral thickness variations are associated with ancient structures such as the Hellas impact basin. The abundance of radioactive isotopes in the Martian interior has a significant influence on the thermal structure of the crust and their concentration can therefore be constrained from the bounds on lower crustal temperatures placed by topographic relaxation considerations. We have used parametrized thermal evolution models which include contributions from secular cooling to determine the thermal structure of the crust underneath Hellas. We have calculated relaxation times for isostatically supported topography and the influence of radiogenic heating and hydrothermal crustal cooling on lower crustal temperatures have been investigated. Our results indicate that hydrothermal cooling of the crust is not necessary to explain the absence of lower crustal flow. Even a brecciated upper crust would be sufficiently conductive if the pore spaces were filled with ice or water. If global scale hydrothermal cooling did occur, radiogenic heating after core formation could have been larger than 60 pW kg-1 without initiating lower crustal flow, but the question of the presence and/or extent of hydrothermal crustal cooling remains unresolved. If no additional crustal cooling occurred, the absence of lower crustal flow at the Hellas impact basin implies that radiogenic heating after core formation cannot have exceeded 45 pW kg-1 and that a thin crust or low initial temperatures are required if heating exceeded 30 pW kg-1, which corresponds to the chondritic heat production rate. In this case, compositional models for Mars with greater than chondritic abundances of K, U, and Th, e.g., the model by Lodders and Fegley (1997), are difficult to reconcile with the observations.
P33C-02
Mars' internal viscosity structure and response to short-period loads
Observations of the rate of secular evolution of the orbit of Phobos indicate that the interior of Mars is strongly dissipative relative to Earth. In this work, we report on possible models for Mars' interior viscosity structure that can satisfy the observed disspiation rate. We further examine the response of such a body to short-period surface mass loads arising from seasonal variations in polar cap masses. The simplest model we consider is that of a homogeneous Maxwell viscoelastic body, characterized by density ρ, rigidity μ, and viscosity ν. The latter two parameters are adjusted to fit the tidal response. For this model to reproduce the degree 2 Love number estimates and the secular acceleration of Phobos, we require μ = (4.6 + 2.0) 1010 Pa and ν = (8.7 + 0.6) 1014 Pa s, with a corresponding Maxwell relaxation time of just over 5 hours. A further constraint is that Mars has significant long-lived topography, which we accommodate in layered models via an outer elastic shell that supports loads for long times. In order to produce the observed tidal effects, the effective viscosity of the mantle and core must then have correspondingly lower viscosities. We explore a number of different scenarios for accommodating this requirement. ~ An additional set of constraints on Mars' internal structure can be derived from the response to annual surface loads, associated with the seasonal transport of mass into and out of the polar caps. Previous treatments of this phenomenon have considered that the surface upon which dust and volatiles are deposited is perfectly rigid. We note that at least the very longest wavelength components of this process are likely to reflect a finite yielding of the surface in response to seasonally varying loads. As a result, the gravitational, topographic, and rotational responses will together provide joint constraints on the surficial mass transport, and the internal structure. The observed time varying gravitational signal, for example, represents contributions from the volatile masses on the surface and in the atmosphere, and the crustal deformation induced by these loads. We discuss a range of internal structure models which are consistent with these constraints.
P33C-03
Temporally Transitional Mantle Convection: Implications for Mars
The thermal evolution of terrestrial planets such as Earth, Mars and Venus is strongly dominated by the convective processes in the planet's silicate mantle. The actual planform of convection controls the efficiency of heat transport and thus, the cooling behavior and thermal evolution of the whole planet. In the present study we investigate the heat transport properties of variable viscosity convection. Here, the focus is on the temporally transitional behavior discovered recently (Loddoch et. al, 2006). While the difference of the newly found convective regime to the already known stagnant lid and episodic behavior has been elaborated in our previous study, the present work investigates the implications of the observed intermittent behavior on the thermal evolution of terrestrial planets. A 3D numerical mantle convection code is applied and calculations are carried out in the parameter range for which the temporally transitional behavior has been found. Using the described approach it is possible to investigate the transition from a (temporarily) mobilized towards a stagnant surface in a fluid dynamically consistent manner. While such a scenario has repeatedly been suggested for Mars' early history, it has so far been investigated only by means of parameterized convection models. We show, that the sporadic surface mobilization events occur on time scales relevant for Mars. In order to assess their influence on the subsequent thermal evolution of planetary bodies, an internal heating of the mantle and a secular cooling of the core are additionally taken into account. The obtained results are compared to the findings of thermal evolution studies employing parameterized convection models.
P33C-04
Evidence for Material Removal and/or Subsidence of the Martian Lithosphere from a Global Dataset of Surface Faults.
The normal faults associated with the formation of Tharsis extend over almost half of Mars, and hence have been the major focus for studies of lithospheric deformation, e.g., the membrane and flexure model of Banerdt and Golombek [2000] and the gravitational potential model (GPE) of Dimitrova et al [2006]. Recent orbital exploration has led to the creation of expanded fault data sets and we extend these studies to use the global fault data from Knapmeyer et al [2006] to evaluate the suitability of the data for lithospheric stress models and constraints on the crustal evolution on Mars. We perform a Kostrov moment tensor summation to estimate the total strain tensor associated with the fault segments, where we assume a uniform amount of slip for each fault as a first approximation. We use an objective inner product measure to estimate the misfit of the GPE associated model of stress to the calculated strain. We perform inversions minimizing the surface integral of the misfit for additional stresses due to GPE variations or membrane displacements. The normal faults mapped to-date are clustered in the western hemisphere around Tharsis, and hence, are (1) well fitted by the GPE model and (2) provide little to no constraint on processes elsewhere. On the other hand, the reverse faults are much more uniformly distributed and provide better constraints; in particular, reverse faults in high topography areas are ill fitted by the GPE model. Inversions for minimized misfit to the reverse faults show that a GPE model with additional variations in crustal density alone of ± 57 kg.m-3, mantle density alone of ± 207 kg.m-3, or both crustal and mantle density of ± 34 kg.m-3, can fit the majority of the reverse and normal faults. A GPE model with additional membrane displacement model fails to improve significantly the fit to the reverse faults even for spherical degree and order 18 cut-off. A GPE model with additional variation in densities (± 69 kg.m-3 for the crust, ± 212 kg.m-3 for the mantle, ± 185 kg.m-3 for both the crust and the mantle) and membrane displacement of ± 185 m provides the best fit. Inversion results for both normal and reverse faults show correlated changes in GPE and membrane displacements, requiring additional GPE and/or upward vertical displacement under Tharsis and Margaritifer Terra and Meridiani Planum, and Argyre Planitia in the western hemisphere and Hellas and Utopia Planitia in the eastern hemisphere. While the results for Tharsis may reflect deviations from the average crustal density as proposed by Neumann et al [2004], which will significantly affect the base GPE model, the remaining areas can be explained by a combination of removal of material and subsidence after the fault formation. This interpretation is also consistent with these areas associated with very large craters and/or networks of outflow channels.
P33C-05
Improvement Of The Ephemeris And Mass Of The Martian Moons From MEX Precise Orbit Determination
Because of the much larger eccentricity of the MEX orbit than that of the MGS orbit (eccentricity of 0.6 versus 0.01), MEX is more sensitive to the gravitational attraction of the Martian moons Phobos and Deimos. From the 2-way Doppler and range data provided by the MEX Radio Science (MaRS) experiment over the last three years, we perform MEX Precise Orbit Determination (POD) in order to improve both moons' ephemeris and mass. We use the GINS software developed by CNES and further adapted at ROB for planetary geodesy applications. We obtain an average precision of 20 meters or less on the MEX positioning around Mars. This represents an improvement by more than a factor 2 with respect to the MEX navigation orbit provided by the flight dynamics team of ESOC. Here, we show the impact of the improved MEX orbit on the Martian Moon ephemerides by using images of the moons taken by the camera (SRC) onboard MEX. We obtain estimates of the masses of Phobos and Deimos with formal errors that are comparable or better than previous estimates from MGS and Mars Odyssey. By using our accurate orbits of MEX, we also estimate the second order coefficients of the gravity field of Phobos, which can be used as a constraint on the internal mass distribution of Phobos.
P33C-06
Effects of the Martian crustal magnetic fields on the geographic distribution of energetic charged particles
The strong crustal magnetic fields at Mars should affect the distribution of energetic charged particles (e.g., solar energetic particles and galactic cosmic rays) in the Martian atmosphere and across the Martian surface. We present preliminary results from our attempts to model these interactions. We discuss the computational framework used (based on the GEANT4 software toolkit) and the different model components necessary to track the propagation of the particles from interplanetary space down to below the Martian surface. We explore the impact of several significant parameters such as the distribution of the crustal magnetization (i.e., depth, thickness, coherence scale, magnetization strength and direction, including both strong and weak crustal field regions), the atmospheric conditions, and the solar activity level. Within this large parameter space, we present preliminary results and discuss the implications for robotic and human exploration of Mars.
P33C-07
Aerosol Charging by Ion Attachment and Electrical Conductivity in the Lower Atmosphere of Mars
Aerosol in the atmosphere of Mars is a topic of considerable interest since their effect on the climate has been recognized. The aerosols interact with both visible and infrared radiation and modify atmospheric heating rates which are responsible for the atmospheric circulation, dust storms etc. In the present work, the charging of aerosols and the conductivity of the lower atmosphere of Mars during the day and night-time are calculated. Galactic cosmic rays are the dominant ionizing process in the lower atmosphere producing molecular ions and ion clusters. These ion clusters get attached to the aerosols and charging occurs during the night-time. Solar UV photons are an additional ionizing agent during the day-time. Solar photons of energy less than 6 eV reach the surface of Mars as those with energies greater than 6 eV are absorbed by the atmospheric molecules before they reach the lower atmosphere. Those photons, which reach the lower atmosphere, ionize the aerosols as the ionization potential of most of the aerosols is less than 6 eV and produce electrons. Aerosols become charged by the attachment of ions and electrons during the day-time. The ion-aerosol and electron-aerosol attachment coefficients are calculated. The neutral atmospheric properties required to calculate the aerosol charging and the conductivity are obtained from Magalhaes et al. (1999). The aerosols have a concentration and effective radius of 2.26 cm-3 and 1.9 mm, respectively, at the surface. The charge distribution of aerosols is obtained by the simultaneous solution of the ion-electron-aerosol charge balance equations. Both the steady state and time dependent concentration of charged aerosols are calculated. It was observed that about 80% of the aerosols close to the surface become charged during the night-time (Michael et al., 2007). In addition to ions, electrons are also present during the day-time. More charging occurs and most of the aerosols become charged during the day-time. The conductivity of the lower atmosphere is also estimated and it was found that the presence of aerosols reduces the conductivity. The effect of different temperature structures, solar conditions and aerosol concentration on the conductivity will be presented. The variation in conductivity during the dust storm of 2001 will also be discussed. References Magalhaes, J.A., Schofield, J.T., and A. Seiff, Results of the Mars Pathfinder atmospheric structure investigation, J. Geophys. Res., 104, 8943-8955, 1999. Michael, M., M. Barani, and S. N. Tripathi, Numerical predictions of aerosol charging and electrical conductivity of the lower atmosphere of Mars, Geophys. Res. Lett., 34, L04201, 10.1029/2006GL028434, 2007. http://home.iitk.ac.in/~snt
P33C-08
Solar control of the Martian Ionosphere as Detected by MARSIS Active Ionospheric Sounder
The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), aboard the ESA spacecraft Mars Express, was deployed 17 June 2005 and commissioned in July 2005. MARSIS includes an Active Ionospheric Sounding (AIS) mode, which targets the topside ionosphere of Mars. Between August 2005 and July 2007, the spacecraft performed approximately 2500 orbits. From these orbits we have selected for analysis some 30,000 ionograms, about one-half of which were acceptable for inversion to an electron density profile. We bin these data by various quantities and plot the peak plasma densities and their corresponding altitudes from each bin as a function of solar zenith angle. Each plot can then be fit to a Chapman model to determine characteristic values of the subsolar plasma density peak, subsolar density peak altitude, and neutral scale height for that bin. The results from each bin can then be plotted as a function of the binned quantity to infer trends in the martian ionosphere. We detect an increase in electron density in the martian ionosphere with solar activity as indexed by F10.7 flux and XUV flux from SORCE XPS, both corrected for the changing distance of Mars from the Sun. We see an average change of about 6% in the subsolar peak electron density and a 6 km increase in the subsolar peak density altitude during periods when we can infer the presence of solar energetic particles from MARSIS AIS surface reflection data. We also see variation in the Chapman parameters due to seasonal and areodetic variation. In particular, at high latitudes near the summer solstice in both northern and southern hemispheres, we note an increase in the neutral scale height and decrease in the electron density peak altitude.