The subsurface nature of Mercury and Mars from thermal microwave emission

Abstract

Detailed numerical modeling techniques are applied to the analysis of microwave observations of Mercury and Mars. The model calculations include the effects of orbital-axial resonance and dependence of regolith properties (e.g. specific heat and thermal conductivity) on temperature in the case of Mercury, and for the effects of seasonally varying CO_2 frost caps in the case of Mars. Variations of geocentric aspect from one observation period to the next are treated for both planets. The dielectric properties of the subsurfaces of these planets are treated as independent of temperature and homogeneous with depth and location on the planet. Observations of Mercury were made at 3.71, 6, and 18 cm, and previously published observations at .31, .33, and 3.75 cm are also employed in the analysis. The Mercury data appear to be consistent with the presence of a dry, porous regolith in which the radiative transport of heat is important in the total thermal conductivity. The ratio of radiative to contact thermal conductivity, χ, is normally evaluated at T = 350° K, and it is found that these data limit χ thus defined to the range 0.4 < χ < 1.0. A value for the effective subsurface dielectric constant is determined from interferometric measurements at 3.71 cm presented here. This value (e = 2.0 ± .16 ) is then corrected for the effects of surface roughness to yield a value for the dielectric constant of the regolith of e = 2.4 ± .3. Final values of other parameters are: tan Δ = regolith loss tangent = .0075 ± .002 γ = thermal inertia = .0014^(+.0021_(-.0008) cal cm ^(-2) deg ^(-1) sec ^(-½). In a similar way, expected microwave spectra of Mars are computed using accurate aspect geometry and a thermal model that includes seasonal polar cap effects. It is found that for a range of loss tangents characteristic of dry particulate geological materials (.003 < tan Δ < .015), and for values of other surface parameters determined independently, the observable spectrum of Mars in the microwave region is flat from 0.1 to 21 cm to within the accuracy of the present data , and that a regolith of homogeneous, lunar-like properties is completely consistent with the existing data set when polar cap effects are considered. This result differs from that predicted by the analytical theory in common use which is in apparent conflict with the observed spectra for values of the surface parameters similar to those found for the Moon or Mercury. Final values of other relevant parameters are: γ = thermal inertia = .006 cal cm ^(-2) deg ^(-1) sec ^(-½) e = regolith dielectric constant = 2 .5 ± .3 A = bolometric Bond albedo = .25 E = infrared emissivity = .90.