A Time-Dependent Model of Radiative and Conductive Thermal Energy Transport in Planetary Regoliths with Applications to the Moon and Mercury

Modeling thermal energy transfer in planetary regoliths involves treating four processes: visible radiative transfer, thermal radiative transfer, conductive transfer, and heat storage. We explicitly treat these processes while considering time-dependent problems, and we apply this model to the regoliths of the Moon and Mercury. Fitting the model to observational data allows us to constrain the radiative resistivity and thermal inertia parameters of these regoliths and hence constrain their conductivities, thermal extinction coefficients, and average grain sizes. It is also found that water ice would be stable in the polar subsurfaces of both bodies, even in areas which receive sunlight during the day.     

Laboratory studies into the effect of regolith on planetary X-ray fluorescence spectroscopy

In the analysis of X-ray fluorescence spectra from planetary surfaces, it is traditionally assumed that the observed surface is a plane-parallel, smooth, and homogeneous medium. The spectral and spatial resolutions of the instruments that have been used to measure X-ray emission from planetary surfaces to date have been such that this has been a reasonable assumption, but a new generation of X-ray spectrometers will provide enhanced spectral and spatial resolutions when compared with previous instrumentation. In light of these improvements in performance, it is important to assess how the requirements on the methodology of analysis of spectra may change when the surface is considered as a regolith. At other wavelengths, varying physical properties of planetary regoliths, such as the packing density, are known to have an effect on the observed signal as a function of viewing geometry. In this paper, the results from laboratory X-ray fluorescence measurements of regolith analogue materials at different viewing geometries are presented. Characteristic properties of the regolith such as particle sizes and packing density are found to affect the measured elemental line ratios. A semiempirical function is introduced as a tool for fitting fluorescent line intensity dependences as a function of viewing geometry. The importance of the results is discussed and recommendations are made for the future analysis of planetary X-ray fluorescence data.     

On the impact of multiply charged heavy solar wind ions on the surface of Mercury, the Moon and Ceres

We have studied the impact of multiply charged solar wind O⁷⁺ and Fe⁹⁺ ions on the surfaces of Mercury, the Moon and on a Ceres-size asteroid using a quasi-neutral hybrid model.The simulations showed that heavy O⁷⁺ and Fe⁹⁺ ions impact on the surface of Mercury non-homogenously, the highest flux being near the magnetic cusps—much as in the case of impacting solar wind protons. However, in contrast to protons, the analyzed heavy ions do not create high ion impact flux regions near the open-closed magnetic field line boundary. Dawn-dusk asymmetry and the total ion impact flux were each found to increase with respect to the increasing mass per charge ratio for ions, suggesting that the Hermean magnetic field acts as a mass spectrometer for solar wind ions. The Moon, in contrast, does not have a global intrinsic magnetic field and, therefore, solar wind ions can freely impact on its surface when this body is in the solar wind. The same is true for a, non-magnetized, Ceres-size asteroid.The impact of multiply charged ions on a solid surface results in a large variety of physical processes, of often intimately inter-related atomic reactions, e.g. electron exchange between solid and approaching projectile, inelastic scattering of projectile, electronic excitation in the projectile and/or the solid, ejection of electrons, photons, neutral and iodized surface particles, and eventual slowing down and stopping of the projectile in the solid. The electron transfer process between impacting heavy ions and surface constituents can result in soft X-ray (E<1 keV) and extreme ultraviolet (EUV) photon emissions. These processes will eventually damage the target surface. Analysis of the hybrid Mercury model (HYB-Mercury) suggests that, at this planet the damaging processes result in non-homogenous ageing of the surface that is controlled by the intrinsic magnetic field of the planet and by the direction of the interplanetary magnetic field. In the corresponding Lunar model (HYB-Moon) and in the non-magnetized asteroid model (HYB-Ceres), surface ageing is demonstrated to take place on that side of the body that faces toward the flow of the solar wind.     

Potential relations between Caloris basin and Mercury's sodium exosphere

We have developed a 1D thermal model of Mercury's regolith, in order to simulate the heat diffusion in the upper subsurface (first 10 m). We assume in our model that the thermophysical properties of the Hermean regolith are similar to those of the lunar regolith. We apply our thermal model to the Caloris basin which slopes induce distortions of the surface temperature compared to results obtained for a perfect spherical planet. This thermal model is then coupled with a 3D Monte Carlo model of Mercury's sodium exosphere [Leblanc, F., Johnson, R.E., 2003. Icarus 164, 261-281; Leblanc, F., Delcourt, D., Johnson, R.E., 2003b. J. Geophys. Res. 108 (E12), doi:10.1029/2003JE002151/.5136], in order to describe the signatures of Caloris basin on Mercury's sodium exosphere in term of temporal and spatial variabilities. In particular, we find a motion of the maxima of sodium density in the exosphere towards the Northern hemisphere similar to the one observed by Potter et al. [Potter, A.E., Morgan, T.H., Killen, R.M., 1999. Planet. Space Sci., 47, 1441-1449] but did not reproduce the observed change of the emission brightness. The main conclusion of this study is that the Caloris basin-exosphere relations might be observable from the Earth which we hope will motivate new observations of Mercury's exosphere.     

Radar imagery of Mercury’s putative polar ice: 1999-2005 Arecibo results

We present an updated survey of Mercury’s putative polar ice deposits, based on high-resolution (1.5-km) imaging with the upgraded Arecibo S-band radar during 1999-2005. The north pole has now been imaged over a full range of longitude aspects, making it possible to distinguish ice-free areas from radar-shadowed areas and thus better map the distribution of radar-bright ice. The new imagery of the south pole, though derived from only a single pair of dates in 2005, improves on the pre-upgrade Arecibo imagery and reveals many additional ice features. Some medium-size craters located within 3° of the north pole show near-complete ice coverage over their floors, central peaks, and southern interior rim walls and little or no ice on their northern rim walls, while one large (90 km) crater at 85°N shows a sharp ice-cutoff line running across its central floor. All of this is consistent with the estimated polar extent of permanent shading from direct sunlight. Some craters show ice in regions that, though permanently shaded, should be too warm to maintain unprotected surface ice owing to indirect heating by reflected and reradiated sunlight. However, the ice distribution in these craters is in good agreement with models invoking insulation by a thin dust mantle. Comparisons with Goldstone X-band radar imagery indicate a wavelength dependence that could be consistent with such a dust mantle. More than a dozen small ice features have been found at latitudes between 67° and 75°. All of this low-latitude ice is probably sheltered in or under steep pole-facing crater rim walls, although, since most is located in the Mariner-unimaged hemisphere, confirmation must await imaging by the MESSENGER orbiter. These low-latitude features are concentrated toward the “cold longitudes,” possibly indicating a thermal segregation effect governed by indirect heating. The radar imagery places the corrected locations of the north and south poles at 7°W, 88.35°N and 90°W, 88.7°S, respectively, on the original Mariner-based maps.     

The morphology of Mercury’s Caloris basin as seen in MESSENGER stereo topographic models

A digital terrain model (1000-m effective spatial resolution) of the Caloris basin, the largest well-characterized impact basin on Mercury, was produced from 208 stereo images obtained by the MESSENGER narrow-angle camera. The basin rim is far from uniform and is characterized by rugged terrain or knobby plains, often disrupted by craters and radial troughs. In some sectors, the rim is represented by a single marked elevation step, where height levels drop from the surroundings toward the basin interior by approximately 2 km. Two concentric rings, with radii of 690 km and 850 km, can be discerned in the topography. Several pre-Caloris basins and craters can be identified from the terrain model, suggesting that rugged pre-impact topography may have contributed to the varying characteristics of the Caloris rim. The basin interior is relatively smooth and shallow, comparable to typical lunar mascon mare basins, supporting the idea that Caloris was partially filled with lava after formation. The model displays long-wavelength undulations in topography across the basin interior, but these undulations cannot readily be related to pre-impact topography, volcanic construction, or post-volcanic uplift. Because errors in the long-wavelength topography of the model cannot be excluded, confirmation of these undulations must await data from MESSENGER’s orbital mission phase.     

Mercury's spectrophotometric properties: Update from the Mercury Dual Imaging System observations during the third MESSENGER flyby

Presented here are analyses of the photometric measurements acquired by the imaging system on the MESSENGER spacecraft during its three flybys of Mercury, in particular the dedicated sequence of photometric measurements obtained during the third flyby. A concise, analytical approach is adopted for characterizing the effects of scattered light on the images. This approach works well for wavelengths shorter than 700 nm but breaks down at the longer wavelengths where the scattering behavior of the imaging system is more complex. Broadband spectral properties are commensurate with ground-based observations for spectra acquired at phase angles less than 110°; photometric corrections to a common illumination and viewing geometry provide consistent results for those phase angles. No phase reddening is apparent in the image-derived spectra. A bolometric albedo of 0.081 is derived over the wavelength range of the imaging system.     

Solar quadrupole moment and purely relativistic gravitation contributions to Mercury's perihelion advance

The perihelion advance of the orbit of Mercury has long been one of the observational cornerstones for testing General Relativity (G.R.).The main goal of this paper is to discuss how, presently, observational and theoretical constraints may challenge Einstein's theory of gravitation characterized by β=γ=1. To achieve this purpose, we will first recall the experimental constraints upon the Eddington-Robertson parameters γ,β and the observational bounds for the perihelion advance of Mercury, Δω_obs. A second point will address the values given, up to now, to the solar quadrupole moment by several authors. Then, we will briefly comment why we use a recent theoretical determination of the solar quadrupole moment, J ₂=(2.0 ± 0.4) 10^-7, which takes into account both surfacic and internal differential rotation, in order to compute the solar contribution to Mercury's perihelion advance. Further on, combining bounds on γ and J ₂ contributions, and taking into account the observational data range for Δω_obs,we will be able to give a range of values for β. Alternatively, taking into account the observed value of Δω_obs, one can deduce a dynamical estimation of J ₂ in the setting of G.R. This point is important as it provides a solar model independent estimation that can be confronted with other determinations of J ₂ based upon solar theory and solar observations (oscillation data, oblateness...). Finally, a glimpse at future satellite experiments will help us to understand how stronger constraints upon the parameter space (γω J ₂) as well as a separation of the two contributions (from the quadrupole moment, J ₂, or purely relativistic, 2α²+2αγ–β) might be expected in the future. This revised version was published online in July 2006 with corrections to the Cover Date.     

Seasonal and daily variation of mercury evasion at coastal and off shore sites from the Mediterranean Sea

Dissolved gaseous mercury (DGM) was measured continuously using two newly developed techniques and a manual technique. The continuous techniques were based on the equilibrium between the aqueous and gaseous phase (DGM = Hg_extr / H', Hg_extr is the measured mercury concentration in the gas phase, H' is the Henry's Law coefficient at the desired temperature). In order to calculate the annual mercury evasion from the Mediterranean Sea, diurnal and seasonal measurements of DGM, total gaseous mercury in air (TGM), water temperature and wind speed were performed. During August 2003, March–April 2004 and October–November 2004 measurements of these parameters were conducted on board the RV Urania. The continuous measurements of DGM showed a diurnal variation in concentration, at both coastal and off shore sites, with higher concentrations during daytime than nighttime. The concentration difference could be as large as 130 fM between day and night. The degree of saturation was calculated directly from the measurements, S = Hg_extr / TGM and was found to vary between the different seasons. The highest average degree of saturation (850%) and the largest variation in saturation (600–1150%) was observed during the summer. The spring showed the lowest variation (260–360%) and the lowest average degree of saturation (320%). The autumn also showed a large variation in saturation (500–1070%) but a lower average (740%) compared to the summer cruise. This might be explained by the temperature difference between the different seasons, since that parameter varied the most. The flux from the sea surface was calculated using the gas exchange model developed by Nightingale et al. [Nightingale, P.D., Malin, G., Law, C.S., Watson, A.J., Liss, P.S., Liddicoat, M.I., Boutin, J., Upstill-Goddard, R. C., 2000. In situ evaluation of air–sea gas exchange parameterization using novel conservative and volatile tracers. Global Biogeochemical Cycles, 14(1):373–387]. The evasion varied between the different seasons with the highest evasion during the autumn, 24.6 pmol m^− 2 h^− 1. The summer value was estimated to 22.3 pmol m^− 2 h^− 1 and the spring to 7.6 pmol m^− 2 h^− 1. Using this data the yearly evasion from the Mediterranean Sea surface was estimated to 77 tons.