Mars (and Mercury) Atmosphere and Interior
Presiding: J A Jernsletten, University of Bergen; S Ismail, NASA Langley Research Center
P23A-01 13:30h
Core Composition and the Magnetic Field of Mercury
The density of Mercury suggests a core of approximately 1800 km radius and a mantle of approximately 600 km thickness. Convection in the mantle is often claimed to be capable of freezing the core over the lifetime of the solar system if the core is nearly pure iron. The thermal history calculations of Stevenson et al. (1983) and Schubert et al. (1988) suggest that about 5 weight-% sulphur are required to lower the core liquidus sufficiently to prevent complete freezing of the core and maintain a significant fluid outer core shell. Other candidates for a light alloying element require similarly large concentrations. The requirement of a significant concentration of volatile elements in the core is likely to be at variance with cosmochemical arguments for a mostly refractory, volatile poor composition of the planet. We have re-addressed the question of the freezing of Mercury's core using parameterized convection models based on the stagnant lid theory of planetary mantle convection. We have compared these results to earlier calculations (Conzelmann and Spohn, 1999) of Hermian mantle convection using a finite-amplitude convection code. We find consistently that the stagnant lid tends to thermally insulate the deep interior and we find mantle and core temperatures significantly larger than those calculated by Stevenson et al. (1983) and Schubert et al. (1988). As a consequence we find fluid outer core shells for reasonable mantle rheology parameters even for compositions with as little as 0.1 weight-% sulphur. Stevenson, D.J., T. Spohn, and G. Schubert. Icarus, 54, 466, 1983. Schubert, G. M.N. Ross, D.J. Stevenson, and T. Spohn, in Mercury, F. Vilas, C.R. Chapman and M.S. Matthews, eds., p.429, 1988. Conzelmann, V. and T. Spohn, Bull. Am. Astr. Soc., 31, 1102, 1999.
P23A-02 13:45h
Enriched, insulating crustal units and runaway crustal growth on Mars.
Crustal growth can either cool or heat the mantle, as the crust not only depletes the mantle of heat producing elements, but thermally insulates it. We employ mantle convection models including thickened crustal units of variable heat production to show that for increasing extents of thickened crust, mantle temperatures and melt production can either increase of decrease, depending on the degree of enrichment of the crust with respect to the mantle, the total heat production, and the Rayleigh number. The formation of the continents on Earth would have efficiently cooled the upper mantle, resulting in lower subsequent rates of melt production and continental formation. In contrast, the growth of the Martian highlands would have raised the temperatures of the Martian mantle, increasing rates of melt production, and leading to runaway crustal growth. This would have continued as long as the lithosphere of Mars was mobile.
P23A-03 14:00h
The Construction of Interior Structure Models of Mars
Based on available chemical models of the planet (Wanke and Dreibus, 1994; Dreibus and Wanke, 1989; Lodders and Fegley, 1997; Sanloup et al., 1999; Lodders, 2000), a new set of global models of the Martian interior has been constructed. The model comprises four submodels - a model of the outer porous layer, a model of the crust, a model of the mantle and a model of the core. The first 10- 11 km layer is considered as an averaged transition from regolith to consolidated rock. The mineral composition of the crustal basaltic rock varies with depth because of the gabbro-eclogite phase transition. As a starting point for mantle modeling there have been used experimental data obtained by Bertka and Fei (1997,1998) along the areotherm, iron content of the mantle being varied. New high P-T measurements of the density of Fe, FeS and FeH enable us to refine the core model. Taking into account available chemical models and the fact that noticeable amount of hydrogen could enter the Martian core during its formation (Zharkov, 1996), such parameters as ferric number of the mantle (Fe&35;), sulfur and hydrogen content in the core are varied. If there is no hydrogen in the core, a model produces a Fe/Si ratio that is smaller than the chondritic value of 1.71. The presence of hydrogen in the core significantly increases the Fe/Si ratio up to about 1.7, and reduces the melting temperature of the core material. To satisfy the bulk chondritic ratio, more than 50 mol % of hydrogen must be incorporated into the core. Then, a problem of consistency of the cosmochemical DW model with the internal structure model of the planet is solved. It will confirm the idea that terrestrial planets were formed from chondritic material. This is a fundamental problem on the formation of Mars and its evolution. The determination of the core radius continues to be of great importance, in case of a reliable determination of the core radius uncertainties concerning the composition of Mars will be resolved. From cosmochemical point of view, it is difficult to assume that the core contains more than 20 wt % of sulfur. The radius of such core is about 1600-1700 km. Therefore, if the core of Mars turns out to be larger, hydrogen could be such an admixture element. According to numerical modeling hydrogen increases the core radius and decreases Fe&35; of the mantle.
P23A-04 14:15h
Gravity Models of the Argyre and Isidis Impact Basins, Mars: The Relative Importance of Surface Loading and Mantle Uplift
Several large martian impact basins, including Argyre and Isidis, have large positive free-air gravity anomalies, and in this regard are similar to the lunar mascon basins. Such gravity highs are not intuitively expected over topographic basins. Previous modeling emphasized two possible causes, flexurally-supported surface loads emplaced after basin formation and super-isostatic uplift of the crust-mantle interface. Quantitative constraints on the relative importance of the two mechanism have not previously been presented for Mars. Distinguishing between these two models is important, because the super-isostatic uplift model places significant constraints on both the impact process, which presumably produces the uplift, and on the post-impact thermal environment, which controls the relaxation of the uplift. Recent work on the morphometry of large impact structures on Mars constrains the expected basin depth for pristine impact structures as a function of basin diameter (Howenstine and Kiefer, Lunar and Planetary Science Conference, 2005). Basins that are shallower than expected are interpreted as being partially filled by post-impact deposition of volcanic or sedimentary material. These results indicate that there is 2.6 ± 1.0 km of post-impact fill on Argyre's floor and 2.9 ± 0.7 km of fill on the floor of Isidis, providing important new constraints on the causes of their mascon gravity anomalies. The gravity high in Argyre, 142 mGal, can be explained by post-impact deposition alone provided that this load is less than 40-50% compensated. Super-isostatic moho uplift is not required at Argyre, although it is permitted if the surface load is close to isostatic compensation or relatively thin. The gravity high in Isidis is much larger, 415 mGal, and thus harder to explain by surface loading alone. In the limiting case of uncompensated basaltic fill of the maximum allowed thickness, the surface load can reproduce no more than 75% of the peak gravity anomaly. Thus, some super-isostatic moho uplift (at least 5-6 km) is required at Isidis. However, it is likely that at least part of the fill on the Isidis floor is sedimentary (Vastitas Borealis Formation). This reduces the mean fill density and increases the required amount of super-isostatic moho uplift.
http://www.lpi.usra.edu/science/kiefer/home.html
P23A-05 14:30h
A Sensitive Search for SO2 in the Martian Atmosphere: Implications for Seepage and Origin of Methane
We observed Mars near the peak of the strongest SO2 band at 1364-1373 cm-1 with resolving power of 7×104 using the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility. The spectrum shows absorption lines of three CO2 isotopomers and three H2O isotopomers. The water vapor abundance derived from the HDO lines assuming D/H = 5.5 times the terrestrial value is 13±1.0 pr. Îœm, in agreement with the simultaneous MGS/TES observations of 14 pr. Îœm at the latitudes (50°S to 10°N) of our observation. Summing of spectral intervals at the expected positions of eleven SO2 lines puts a 2σ upper limit on SO2 of 0.8 ppb. SO2 may be emitted into the martian atmosphere by seepage and is removed by three-body reactions with OH and O. The SO2 lifetime, 2 years, is longer than the global mixing time 0.5 year, so SO2 should be rather uniformly distributed across Mars. Seepage of SO2 is less than 14,000 tons per year on Mars which is smaller than the volcanic production of SO2 on the Earth by a factor of 700. CH4/SO2 is typically 10-4 - 10-3 in volcanic gases on the Earth, and this does not support seepage as a possible source of the recently discovered methane on Mars and makes even more plausible its biogenic origin. Possible productions of ethane and propane are weaker than that of methane, and these gases should be additionally depleted photochemically by factors of 25 and 250 relative to methane on Mars.
P23A-06 14:45h
A Remote Raman and Laser-Induced Fluorescence Spectrometer and its Application for Lidar Remote Sensing of Martian Atmosphere
A combined remote Raman and Laser Induced Fluorescence (RLIF) spectrometer was proposed as a mast-mounted instrument for the Mars Science Laboratory (MSL). This remote RLIF system is capable of conducting reconnaissance of fluorescence materials and minerals with high sensitivity (e.g., carbonates, sulfates, phosphates, quartz, etc.) that can be recorded with a single 532 nm (35 mJ) laser pulse of 8 ns half-width. The RLIF system is also capable of identification of mineral, organic, and biogenic materials and is sitable for atmospheric studies of Mars. This instrument design is based on a prototypes that was developed with partial support from NASA's Planetary Instrument Definition and Development Program (PIDDP) at the University of Hawaii. This prototype instrument has been modified to operate in the lidar mode to obtain Mie-Rayleigh scattering profiles in the atmosphere for studying meteorological processes in the marine atmosphere. Application of RLIF to obtain range-resolved atmospheric backscattering profiles using the AOTF technique are capable of providing atmospheric backscatter profiles. Data from RLIF can be used to retrieve optical properties of dust aerosols and clouds, including the profiling of scattering intensity, location of cloud base and thickness, atmospheric extinction, and de-polarization. These measurements can be made at high vertical resolution up to altitudes >5 km. Simultaneous measurements can be made of atmospheric CO2 and its variations; surface CO2-ice and water-ice; and surface and subsurface hydrated methane on Mars. Capability of RLIF and examples of atmospheric measurements applicable to RLIF will be presented in this paper.