U23E-0091
Climate transition on Mars: a simple solution.
Evidence for the presence of flowing water on the Martian surface at some time in the distant past is most persuasive. The current cold, dry climate and low atmospheric pressure precludes liquid surface water for any extended period of time, and these conditions seem to have generally prevailed since 1-2 Ga after Mars" formation. Water in unknown quantities (but presumably substantial) and CO2 in relatively small amounts are well established by many observations. The considerable difficulty of producing a warm, wet Mars under present solar output pales in comparison to producing these conditions when 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, namely the instability of these gases under ultraviolet light. This may be 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 especially 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 the 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.
U23E-0092
Simulation of Venus Atmosphere Dynamics With an Earth Climate GCM
We describe the results of initial simulations of the Venusian atmosphere, using the Community Atmosphere Model (CAM). The CAM model is a descendant of the NCAR Community Climate Model, and is defined as one of two "high-end" models designated by the US Climate Change Science Program for basic research. It may also be the most widely used 3D climate model in the US. CAM has grown substantially in complexity and Earth-specificity since the original version was released in 1983, and many of these Earth based physics parameterizations need to be adjusted to simulate the Venus atmosphere. Other groups are adapting CAM to simulate the atmospheres of Mars and Titan, thereby promising CAM simulation for all four terrestrial planets known to have substantial atmospheres. Studying these worlds together will provide calibration of Earth-centric studies of climate changes like global warming. It will also provide context for future searches for Earth-like planets orbiting other stars. In this work we will focus on Venus. The Venus atmosphere represents an extreme environment, strongly influenced by the greenhouse effect, and studying the Venus atmosphere may therefore be relevant to the possible future direction of the Earth's climate. The dynamical processes which occur in the Venusian atmosphere are not well understood, including the cause of the strong superrotation of the atmosphere, in which the planetary surface rotates with a period of around 243 days, but the atmosphere near the cloud tops has a rotational period of only around 4 days. We show the results of initial simulations of the dynamics of the Venus atmosphere, using a version of the CAM model with most of the Earth related processes, such as the cloud physics, removed. A simplified form of heating has been applied, similar to the thermal forcing approach used recently by other authors. We investigate the sensitivity of the model results to changes in the physics parameterizations we have used, including changes in the friction at the upper and lower boundaries, in the heating function, and in dissipation mechanisms, as well as the effects of introducing topography. We analyse the model results to determine the nature of the dynamical processes that produce the characteristics of the Venus atmosphere. We are implementing a self consistent model of the thermodynamic radiative forcing by a detailed calculation of the radiative fluxes at each level in the CAM model, in order to produce a more realistic representation of the thermal forcing which helps to generate the observed structure of the Venusian atmosphere. The radiation model is based on the Laboratoire de Meteorologie Dynamique Venus GCM, including parameterizations of the radiation at short and infrared wavelengths.
U23E-0093
Laboratory Measurements of Water Ice Cloud Formation on JSC-1 Mars Stimulant for Determination of Nucleation and Growth Conditions
It is believed that Martian Clouds, like those in our own atmosphere, play an essential role in the hydrologic cycle and balance of solar radiation. Since clouds contain visible signs and valuable clues to atmospheric processes, much has been done to model the role and effect of water ice clouds in the Martian climate. These models rely on fundamental microphysical properties that have been extrapolated from studies performed under terrestrial conditions, but have yet to be verified for Mars. In order to experimentally determine these properties, we have measured ice formation and growth on the standard JSC Mars-1 regolith stimulant on and subsets of that material under Martian temperatures and water partial pressures. We found that for a temperature of 175 K, nucleation of ice on JSC-1 did not occur until a saturation ratio of ~1.5 was reached. As temperatures are reduced, even higher saturations are required to initiate ice growth. A sample of JSC-1 was then centrifuged to separate several mineral fractions; we found that one fraction formed ice at lower saturation ratios and thus may be a better nucleator when removed from the whole sample. Another fraction exhibited nucleation properties which were very similar to those of the whole sample. In addition to nucleation studies, we are also exploring the effect of water partial pressure and temperature on the growth rate of ice after nucleation. The fractional sticking of water vapor onto ice appears to increase with reduced temperature, leading to an increased growth rate for a given partial pressure of water. The implications of these results for Mars climate models will be presented and their applicability to the polar mesospheric clouds on Earth and will be discussed.
U23E-0094
Contributions to Comparative Climatology by Venus Express
Venus Express is carrying six instruments dedicated to the study of the atmosphere of Venus and the interaction of the atmosphere with both the solar wind and the planetary surface. The spacecraft is orbiting in a highly elliptic polar orbit with an apocentre 66000 km above the South Pole and a pericentre about 250 km above the North Pole. The first two years of operation has taken place during the present solar minimum and the mission thus complements the Pioneer Venus mission well, both with respect to its orbit and to the phase of the solar cycle. A very important feature of Venus Express is its ability to probe the atmosphere in three dimensions by making use of the near-IR spectral windows. The early part of the mission has had the general objective of addressing a large number of questions, mainly related to atmospheric dynamics (3-D wind fields, cloud morphology, and the polar vortices), thermal structure, chemistry and escape. The instruments and mission parameters also lend themselves very well to studies related to problems of surface-atmosphere interaction and atmospheric evolution. With a large and still accumulating data base in all these areas, the focus of the mission can place increasing emphasis on addressing past and future climate change on Venus, leading to a better understanding of the comparative climatology of the terrestrial planets. This talk will summarize the most important results to date from Venus Express with relevance to climate studies, and will address some ongoing activities in this field.
U23E-0095
Climate Modeling of the Terrestrial Planets (and one Moon)
Venus, Earth, Mars, and Titan form a family of objects with relatively thick atmospheres overlying rocky surfaces. Comparative study of these objects allows access to a range of atmospheric and climate dynamics regimes that Earth by itself does not provide. The planets serve as rich and informative experiments and tests of our theoretical understanding over a wide range of conditions that can't be reproduced in the laboratory or field. In addition, detailed understanding of the processes operative in the modern climates of these bodies should improve reconstruction of past climates, which is often of greater interest to those outside of the atmospheric community. The family of "terrestrial atmospheres" inhabits a relatively broad range of parameter space, from fast rotating Earth and Mars to the extremely long Venusian day, from the roughly 2 day radiative timescale of Mars to the roughly 100 year timescale for Venus, and from temperatures hot enough to permit transition metal ore frost on Venus to those cold enough for methane and carbon dioxide to act as volatiles on respectively Titan and Mars. Despite this, all of the members of this family are amenable to study with suitably adapted versions of the three-dimensional General Circulation Model / climate modeling systems developed for the Earth. In this presentation, we describe the application of GCMs based on the NCAR global WRF (grid point) and CAM (spectral and finite volume) models to each of the planets of this family. Examples of our application of these models include the superrotation of the atmospheres of Venus and Titan; Methane cycling in Titan's atmosphere, including rainfall and patterns of aridity; Carbon dioxide and water cycles on Mars; and the cycle of Martian aerosol dust.