Phoenix Lidar Observations of the Cloud Topped Boundary Layer on Mars
The NASA Phoenix mission to Mars landed on 25 May 2008 and operated for five months. The LIDAR instrument on Phoenix observed water ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed up to heights of 4 to 6 km by the summer daytime turbulence and convection. The water ice clouds were detected at the top of the PBL and near the ground each night in late summer after the air temperature started decreasing. The interpretation is that water is transported downward by precipitation at night, and then upward to the top of the PBL by turbulent mixing during the day. The clouds and precipitation act to confine water within the planetary boundary layer.
Experimental Results of Fractionation of HDO and H2O with simulated Martian Dust: Implications for the interpretation of past climate on Mars
Climate change on Mars has been greatly debated in recent years. This has been motivated by the results from the Mars Reconnaissance Orbiter, Phoenix Lander and ground-based spectroscopic studies which have found mounting evidence that not only may Mars have had a wet and warm past, but those conditions inclement to life may also have been present more recently. On Mars, this is largely a story of water transport and, as on the Earth, isotopic analysis presents a key to understanding and decoding the Martian paleoclimate. For Mars, the major fractionation observed is in HDO, analogous to the Oxygen-18 cycle on Earth, and observations have shown that the D/H ratio of the planet is enriched by a factor of 5 to 6 from comparable terrestrial values. The conventional explanation is that a great deal of water has been lost to space over geologic time. However, previous studies have not taken into account the ability of present-day Mars to fractionate water as it moves from the polar caps to the polar layered deposits through the atmosphere, potentially masking any climate signal which may exist. In this presentation, we shall report on a series of Mars analogue experiments completed at the relevant ranges of pressure and temperature. Two different scenarios were simulated: the sublimation of dusty water ice and the sublimation of clean water ice through a simulated regolith/dust lag. In both cases, we have found that the system is dominated by adsorption of water. However, the simulant dust (JSC-1) appears to be an extremely efficient vehicle for fractionating water at cold temperatures, as different desorption rates have been recorded for HDO and H2O. This, when coupled with the relatively small amount of water exchanging today implies heavy fractionations in the current Mars system without requiring significant water losses to space.
Climate Transition on Mars: Solution and Implications
Evidence for the presence of flowing water on the Martian surface is persuasive. The current cold, dry climate and low atmospheric pressure preclude liquid surface water for any extended period of time, and these conditions seem to have generally prevailed for the last 3+ billion years of Martian history. Water in unknown (but presumably substantial) quantities and CO2 in relatively small amounts, are well established by many observations. The considerable difficulty of explaining a warm, wet Mars under present solar output pales in comparison to explaining these conditions early in Martian history when the standard model of stellar evolution predicts that the sun was less luminous. Attempts to model such a climate using CO2 as the main greenhouse gas lead to very high atmospheric concentrations, which should have left an obvious signature. The absence of significant carbonate mineral deposits suggests that CO2 was not likely ever present in the atmosphere at the necessary concentrations. Alternative greenhouse gases, such as methane and ammonia, have met with the same objections applied to similar discussions of the early climate of Earth, but perhaps are slightly less problematical for Mars given the greater distance from the sun. However, if the major reservoir for surface carbon on Mars was as organic compounds, the resolution to the climate problem could lie in the regeneration of methane (and perhaps ammonia) by volcanically-driven hydrothermal processing of these organics in the presence of water. Maintenance of sufficiently high levels of these potent greenhouse gases would have depended on continued thermal activity, especially early in Mars' history when the faint young sun required extra high levels, and when thermal sources were undoubtedly greater than at present. The climate transition from warm and wet to cold and dry (in spite of increasing luminosity of the sun) was the result of the exponential decay of thermal activity on the relatively small Mars, and consequent reduction of atmospheric concentrations of reduced greenhouse gases. Recently discovered methane emissions on Mars are most likely remnants of this hydrothermal activity, rather than biological processes.
Dust Lifting Processes in GM3
We are currently working on the dust lifting processes in the Mars general circulation model GM3 (the Global Mars multiscale model) developed at York University. The dust lifting initiated by surface wind stresses and convective processes has been used to simulate Martian dust cycle, for example, by Newman et al.  and Basu et al. . For the surface wind based dust lifting, we apply the dust mobilization and dry deposition schemes from DEAD (The mineral Dust Entrainment And Deposition [Zender at al., 2003]) model to GM3. The dust mobilization scheme is based on near-surface wind stress and saltation sandblasting using the boundary layer meteorology conditions from GM3. The fine dust particles can be ejected into the atmosphere as a result of saltating sand size particles on the surface. Dust in the Martian atmosphere can also be lifted by dust devils. For our next work, we will add dust devils based on thermal convective scheme into GM3.
Analysis of Dust Devils on Mars using CFD
Recent Mars missions have reported evidence of the existence of dust devils. A detailed study of vortex dynamics will provide a better understanding of this swirling flow of the Martian atmosphere. Further, it is believed that there is a relationship between dust devils and water transport. Recently, the Phoenix Mars mission, designed to investigate ice water and natural events on Mars, has successfully finished. The Phoenix Surface Stereo Imager (SSI) camera captured images of the passage of dust devils over or close to the lander. Additionally, dustless devils, which have similar vortex characteristics but insufficient strength to raise dust from the surface, have been detected in the lander's pressure measurements. It was found that dust devils occur mainly in the early afternoon. Because of this, numerical models of a vortex generator are used to study the physics of this complex swirling flow and the effect of dust devils on the transport of water vapour from the regolith. Characteristic parameters such as core radius and swirl ratio are being explored for scaling factors. Scaling factors will be studied and tested, comparing the small and large scales of numerically generated vortices and laboratory generated vortices. Small scale of numerical models of atmospheric vortices are studied using a commercial software package, ANSYS/CFX11.0 with finite volume method (FVM). Large eddy simulations (LES) of planetary boundary layers are based on NCAR LES code to simulate convective vertical vortices that naturally form in quiescent convective boundary layers (CBL) over homogeneous flat surfaces. This will help to find the approximate location and physical characteristics of the vortices on the surface. The numerical models of atmospheric vortices and the experimental vortex generator validations will help to define the water vapour cycle on Mars.
Potential Vorticity as a Diagnostic for the Mars Polar Vortex
Polar vortices dominate the dynamics of the winter mid- and polar latitudes in the martian atmosphere as well as in the terrestrial stratosphere. Polar vortices have also been observed on Venus (Taylor, 2002), Jupiter (Orton, 2002), Saturn (Fletcher, 2008), and Titan (Teanby, 2008). Potential vorticity is the analysis quantity of choice for the terrestrial polar vortices because its vertical component distills the most important features of the wind and temperature fields into a single scalar variable; because it is a conserved tracer under adiabatic conditions; because it serves as the medium for Rossby waves; and because steep potential vorticity gradients are observed to be correlated with steep gradients in the concentrations of chemical species. Using potential vorticity derived from Mars Global Surveyor Thermal Emission Spectrometer (TES) temperature soundings, we compare the structure of the martian polar vortices to those of the earth. We find that the northern martian winter polar vortex, just like its terrestrial conterpart, is bounded by a region of very steep potential vorticity gradients and is surrounded by a "surf zone", a region of low potential vorticity and very low potential vorticity gradients. The surf zone concept, as first described for the terrestrial stratosphere by McIntyre and Palmer (1983), implies persistent Rossby wave breaking. In the vicinity of the northern polar vortex, the TES data set provides some examples of local gradient reversals that are suggestive of wave breaking. There is also one case of possible large-scale wave breaking accompanied by an abrupt polar warming. The martian southern polar vortex, unlike its terrestrial counterpart and unlike the northern martian polar vortex, lacks a distinct boundary between the polar vortex and a surf zone. Instead, the potential vorticity field is highly disorganized with local gradient reversals throughout the middle and polar latitudes. In the zonal mean, the southern winter potential vorticity gradient ends up being relatively uniform, although it is somewhat enhanced near 60 degrees latitude.
A TW in Mars' Atmosphere and Implications for the Aerobraking region
A solar terminator wave (TW) is found in high-resolution general circulation model (GCM) simulations of Mars' atmosphere. In the horizontal plane at 160 km the wave fronts precede the westward-moving dusk terminator, exhibit a horizontal wavelength of order 15°-30° or 900-1800km, and are oriented about 10°-30° with respect to the terminator. The disturbance originates in the lower atmosphere due to dust insolation absorption, propagates upward with an effective vertical wavelength of order 60 km, and increases in amplitude as the dust extends further away from the surface. The TW density amplitudes for low and elevated dust layers (both with opacities = 1.0) are of order ± 15-20% and ± 30% at 160 km, which are sufficiently large to pose a concern for aerobraking operations. Temperature and wind perturbations for the former case are of order ± 10-20 K and ± 30-75 m/s. The Mars TW shares many common features with a TW recently observed in Earth's thermosphere and simulated with a GCM.
The Emergence of Zonal Jets in a New Anelastic Model of Rapidly Rotating Spherical Convection in Gas Giants
We study the emergence and evolution of large-scale zonal flows, as observed on the gas giant planets, using a newly developed 3-D GCM in spherical shell geometry. This new model is specified in terms of a grid-point based methodology which employs a hierarchy of tessellations derived from successive dyadic refinements of the spherical icosahedron. One major advantage of this multi-grid methodology is that it allows for nearly linear growth of complexity in operation count as opposed to the spectral transform models, which by their nature are at least quadratic in computational cost. Another potential advantage is the absence of pole problems, and therefore the ability of the code to capture important features of the dynamics in the polar regions. An added bonus of this new methodology is the possibility for greater local control over the computational mesh. The physical basis of the model is the anelastic approximation of the hydrodynamic equations of motion, continuity, and classical thermodynamics. We describe a comparative investigation of the convective response of a layer of Boussinesq fluid and density stratified fluid in rapidly rotating, three-dimensional spherical shell geometry subject to isothermal temperature boundary conditions. The physical scaling is determined by the 3 non-dimensional parameters: Ekman, Prandtl and Rayleigh numbers, while the depth of the shell is a variable parameter. We present results from long runs of the model in the Boussinesq and fully anelastic approximation for two different relative shell depths (10 % and 25 %) and compare the formation and evolution of zonal jets, which are driven by vigorous convection and strong Coriolis force. These test cases are of particular relevance to the outer layers of the gas giant planets where a number of open questions associated with the formation and evolution of coherent structures await solution. The numerical experiments are performed in the high Rayleigh number (≥ 106), low Ekman number (≤ 3 × 10-4) regime, with the Prandtl number fixed to unity. Mixed mechanical boundary conditions for velocity (free at the top and rigid at the bottom of the shell) are employed in both experiments. In these experiemnts Reynolds stresses are balanced only by weak viscous forces and drive strong eastward jets at low latitudes at the outer surface and weaker oscillatory jets at high latitudes as observed in the weather layer of the gas giant planets. In order to resolve the fine-grained structure of the zonal jets observed by the Galileo probe for Jupiter and most recently by the Cassini spacecraft for Saturn, very high spatial resolution will be required. A pronounced feature of our experiments is that we invariably observe strong convection developing inside the tangent cylinder where the effect of the Coriolis force is small, as opposed to the powerful zonal flows which develop outside the tangent cylinder. Clearly, simulations over a wider range of shell depths and more extreme values of the Rayleigh and Taylor numbers must be carried out to determine which spherical shell geometries and control parameter values best fit available equatorial jet observations. In addition, the effects of density stratification in the fully anelastic version of the code together with the proper mechanical boundary conditions for velocity deserve further study in order to better understand the mechanism whereby large scale zonal flows on the gas giant planets develop from strongly forced rapidly rotating convection.