U34B-01 INVITED
Solar Evolution and Climate on the Terrestrial Planets
Venus, Earth, and Mars followed different evolutionary paths, partly because of their relative distance from the Sun, and partly because of the differences in their masses. Venus was too close to the Sun to retain its water, despite reduced solar luminosity early in Solar System history (1). The loss of water, followed by the buildup of CO2 in its atmosphere, led to the atmosphere that we see today. Earth was within the liquid water regime throughout its history. However, it must have had a larger greenhouse effect in the past in order to compensate for the faint young Sun. A combination of CO2, H2O, CH4, and C2H6 may have helped keep it warm (2,3). Mars' surface appears to have been wet early in its history, although opinions differ on how warm it must have been (4-6). CO2 and H2O alone could not have kept Mars' surface above freezing during Mars' early history when most of the large-scale fluvial features are thought to have formed (7). SO2 has been suggested as an additional greenhouse gas (8), but new calculations show that it would likely have been insufficient. Other mechanisms for warming early Mars may exist, however. Mars' albedo could have been significantly lowered by the presence of trace gases that absorb visible sunlight. NO2, which has a broad absorption peak centered at 400 nm, is a good candidate. A 3- bar CO2 atmosphere containing 30 ppm of NO2 could have kept Mars' mean surface temperature well above the freezing point of water at 3.8 Ga. Plausible sources of nitrogen oxides on early Mars include lightning and impacts. Other visible/UV-absorbing trace gases may have added to this warming. Thus, a complex mixture of gases could have helped keep early Mars warm. References: 1. J.F. Kasting, Icarus 74, 472 (1988). 2. A.A. Pavlov et al., J. Geophys. Res. 105, 11 (2000). 3. J.D. Haqq-Misra et al., Astrobiol. (in press). 4. J.B. Pollack et al., Icarus 71, 203 (1987). 5. T.L. Segura, O.B. Toon, A. Colaprete et al., Science 298, 1977 (2002). 6. C.P. McKay, J. Phys. IV 121, 283 (2004). 7. J.F. Kasting, Icarus 94, 1 (1991). 8. I. Halevy et al. Science 318, 1903 (2007).
U34B-02 INVITED
Using Terrestrial GCMs to Understand Climate on Venus, Mars and Titan
For many decades now, General Circulation Models have been developed for the Earth to model our climate. Using these tools, which complexity and efficiency increase with years and computers power, many features of the Earth's circulation may be analyzed and interpreted. But the Earth is a unique case, with rapid rotation rate, with seasons, with water oceans. The Earth GCMs have many parameters finely tuned to reproduce the many observations that are available. How robust are these models under evolving conditions ? To what point may we trust them when exploring Earth's future ? Earth, Venus, Mars and also Titan have dense atmospheres that present many differences, but also similarities. How mechanisms that are at work in the Earth's atmosphere do adapt under these different conditions ? Why are Venus and Titan's atmospheres in super-rotation ? Are the Martian storms produced by processes that also happen on the Earth ? Using the GCMs developed for the Earth for these different atmospheres is very appealing to study these questions. Though the amount of available data is much less for extraterrestrial atmospheres, adapting the terrestrial GCMs to these different environments is worth the effort. Even if the dynamical core may be used almost as it is, adapting the physical parameterizations is not straightforward, but based on the increasing amount of observational data, it may be done with more and more accuracy. These extraterrestrial GCMs may now be used to understand the different features of these climates, but also to compare the different behaviors of these atmospheres under different forcings. It explores the robustness of this kind of tools under widly different conditions, putting strength and confidence in our exploration of Earth's atmosphere future evolution. In this talk, we will present the adaptations that have been necessary to develop GCMs for Mars, Titan and Venus from the LMDZ Earth GCM. These models have then followed their own developments, with many successes in understanding these climates. We will also give exemples of mutual benefits, related to the common base for all these models.
U34B-03 INVITED
Comparative Climate Studies of Earth, Venus and Mars
One of the most promising area in Solar System science is the comparative study of the three terrestrial planets (Venus, Earth, Mars). Why did the three planets evolve in such different ways, from relatively comparable initial states? The small size of Mars, favoring atmospheric escape, certainly played a role in making the present Mars so inhospitable. Venus has almost the same size and density as the Earth, and was probably initially endowed with similar amounts of volatile material. The absence of water in significant amounts on Venus, generally explained by intense primitive atmospheric escape, remains poorly constrained and understood. Two specific problems, related to Mars and Venus climate evolution, will be discussed in this talk. One major challenge of Mars studies is to determine the nature of the present Martian atmosphere: is it the residual of an old atmosphere, progressively lost by escape, or is it young, at steady state equilibrium between outgassing and escape? The possible detection of methane in Martian atmosphere suggests that methane currently is being produced, possibly signing outgassing, since methane is the thermodynamically favoured form of carbon, as released by potential volcanism or hydrothermalism, at low Martian temperature and pressure. Although no typical volcanic gas, like SO2, has ever been detected on Mars, the existence of recent lava flows (a few million years old) shows that a residual volcanism is still episodically active. Another possible tracer of outgassing, that is radon 222, seems promising, and could have been detected (although indirectly) in the Martian atmosphere from recent APXS (MER rovers) and Gamma Ray Spectrometer (Mars- Odyssey) measurements. The lack of any isotopic fractionation of carbon and oxygen in Martian CO2, as shown by existing measurements, suggests that the atmosphere of Mars is young, since it should have been fractionated by escape if it is old. This possibility of a young Martian atmosphere, permanently supplied from the planetary interior, will be discussed. Concerning Venus, we will focus on the question of the fate of its primitive ocean. The possibility that Venus was born 'dry', massively losing its atmosphere to space during the first ten million years of its life, will be assessed. It will be shown that massive primitive escape doesn"t necessarily result in strong isotopic fractionation of atmospheric species, and that the few existing information about noble gas isotopic ratios in Venus atmosphere (Ne, Ar) are consistent with a primitive episode of intense hydrodynamic escape.
U34B-04 INVITED
Remote Sensing Methods for Monitoring the Climates of Venus, Earth and Mars
A wide range of remote sensing methods have been used to study the climates of Venus, Earth, and Mars. In some cases, techniques pioneered for Earth were subsequently used to study the climates of Venus and Mars. For example, the thermal infrared limb sounders used on NIMBUS 7 (LIMS, SAMS) and UARS (ISAMS, CLAES) were the precursors of the Mars Reconnaissance Orbiter Mars Climate Sounder (MRO MCS). In other cases, methods first used to study planetary environments, were then used to study the Earth's climate. The Pioneer Venus Orbiter Cloud Photopolarimeter (PV OCPP) was a precursor to the POLDER instruments on ADEOS and PARASOL, and the Aerosol Polarimetry Sensor (APS) on the Glory spacecraft. Similarly, hyperspectral imagers that have long been used for studying planetary environments (NIMS, VIMS, OMEGA, VIRTIS) have only recently been used for studying the Earth (EO1 Hyperion). High spectral resolution solar remote sensing methods like those being developed for measuring CO2 and other greenhouse gases, such as those on the NASA Orbiting Carbon Observatory (OCO) and the Japanese Greenhouse Gases Observing Satellite (GOSAT) provide new tools for measuring surface pressures, trace gas abundances, and the dust and ice distributions in the Martian atmosphere. Active radar and lidar sounders, like those deployed on the CloudSat and CALIPSO spacecraft, provide new methods for studying the vertical structures of the H2SO4 clouds of Venus as well as dust and ice clouds on Mars. These and other opportunities will be reviewed here.
U34B-05 INVITED
Cosmic Impacts and Climates of the Terrestrial Planets
Cosmic impacts have an unsavory reputation in Earth science from their frequent use as dei ex machinis. But unwelcome as they may be, they do happen, and Earth, Venus, and Mars have all seen bigger impacts than the K/T event. The lunar crater record indicates that impacts were big and frequent when Earth was young. Impacts coeval with the lunar basins would have had drastic effects on Earth. The faint early Sun suggests that without an abundant greenhouse gas (e.g., 3 bars of CO2 would suffice), early Earth would have been very cold and icebound, a Hadean snowball. A Hadean snowball would differ from Paleoproterozoic and Neoproterozoic snowballs in that geothermal heat flow would surely have been high enough on a wide enough scale that the ice could not everywhere have been thick. If there were a Hadean snowball, impacts like those the late bombardment would have melted the ice, and so created transient "impact springs." Because Venus's atmosphere is very thick, only the biggest impacts can change it. Under current conditions the most interesting possibility is a big wet impact adding enough water to the atmosphere to ramp up the greenhouse effect. The effect of adding water to Venus's atmosphere has been discussed previously in the context of giant volcanoes. For Mars the chief effect of the late bombardment on the climate may be existential: impact erosion has been a leading hypothesis to explain why Mars has a thin atmosphere. A more controversial suggestion is that the evidence for warm wet climates on ancient Mars is really just another aspect of the impact cratering record. Impacts are capable of recycling and vaporizing buried martian volatiles --- water in particular --- but it remains to be shown that they do so often enough, or that the effects last long enough, to account for the apparent endurance of warm wet episodes.
U34B-06 INVITED
Volcanic Forcing of Climate on Earth, Mars and Venus
While shaping the surfaces of terrestrial planets, geological activity also influences the evolution of their atmospheres. In particular, geochemical cycles establish the abundance of radiatively important volatiles such as CO2 and sulfur gases. On Earth, the carbonate-silicate cycle results in a steady-state CO2 abundance of about 300 ppm, determined by a balance between the removal of CO2 into carbonate layers, and its eventual cycling back to the atmosphere through volcanism, on a time scale of about half a million years. Large scale deviations from average rates of volcanic activity, such as the formation of the large ingeous provinces, were associated with perturbations to the atmosphere, changes in greenhouse forcing, and enhanced surface temperatures. On Mars, outgassing associated with largest shield province in the solar system, Tharsis, would have produced an enormous input of water vapor, CO2, and sulfur gases into the atmosphere. One result of this era (most likely in the late Noachian) is the formation of massive sulfate beds such as seen at Meridiani, and the ubiquitous martian high-sulfur dust and sand. Acid-sulfate chemistry apparently dominated this era in martian history, preventing the drawdown of CO2 into carbonate (as happened on Earth) and propping up an early martian CO2-H2O greenhouse. On Venus, ongoing volcanism provides the ingredients for the clouds. Rapid loss of SO2 to carbonates at the surface and H2O to space strongly implies an active source for these gases on a time scale of 10's of My. The stability of Venus' climate is therefore dependent upon active volcanism and the sulfur cycle. Geological evidence for dramatic changes in resurfacing rate imply large amplitude climate changes which may have left a record of synchronous global deformations and other climatically forced geological signatures. Volcanism has played a significant role in the stability of terrestrial planet climates, but has also acted as an agent of global change due to atmospheric radiative and chemical processes.
U34B-07 INVITED
Atmospheric collapse on Venus-like planets orbiting M-dwarfs
The term "anti-greenhouse effect" refers to the ability of an atmosphere to cool the surface of a planet under circumstances when a sufficient proportion of the incident shortwave radiation is absorbed in the upper atmosphere and re-radiated there as infrared. It is a well-known factor in the effect of Titan's haze clouds on surface temperature. Grey-gas models show that in a particularly extreme form, the anti-greenhouse effect can lead to a deep isothermal layer which can be as cold as the skin temperature. If this happened in a dense CO2 atmosphere, it could lead to atmospheric collapse and the formation of a CO2 ocean. In some sense, our own Venus is not far from this threshold, since the deep convective layer is maintained by a relatively small trickle of solar radiation reaching the surface. Many near-habitable planets of the Super-Earth class have been discovered orbiting M-dwarf stars. These cool stars have a higher proportion of their output in the near-infrared, and therefore are subject to an enhanced anti-greenhouse due to strong absorption of incident radiation by CO2 and water vapor. The effect is quantified using a real-gas radiation model, and the prospects for atmospheric collapse for planets orbiting such stars is discussed. The calculations also have implications for the runaway greenhouse threshold in M-dwarf systems, and these implications will also be discussed.
U34B-08 INVITED
Comparative Planetology and the Search for Habitable Extrasolar Planets
In the last decade, comparative planetology has grown to encompass not just the planets in our own Solar System, but also the more than 300 planets that are now known to orbit other stars in our Galaxy. The vast majority of the planets discovered so far are gas or ice giants, but a growing fraction have masses less than 10 Earth masses, and so may be terrestrial. Over the next two decades, NASA and ESA are planning to build large space-borne telescopes that will enable statistical studies and the first direct detection and characterization of terrestrial planets beyond our Solar System. These missions will allow the study of planets formed under diverse initial conditions, and at stages of evolution that are billions of years younger or older than the Earth. Ultimately though, these missions will finally provide the technical capability to search for habitable environments and life on planets beyond our Solar System. The scientific foundation that will guide this search is built on comparative climate studies of the planets in our own Solar System. From the perspective of extrasolar planet studies, the evolution of the climates of Venus, Earth and Mars inform the definition and characteristics of planetary habitability. Climate and chemistry models, developed initially to be flexible enough for Venus, Earth and Mars studies, and validated against measurements and observations of these planets, are now being modified to model a diversity of plausible extrasolar planetary environments. Specifically, these models have been used to better understand the interaction between the parent star, and the global environment and biosphere of a terrestrial planet for planetary systems unlike our Solar System. Additionally, planetary radiative transfer models developed for Venus, Earth and Mars studies can be used to predict the spectroscopic appearance of distant planetary environments and to simulate a telescopic view of the Earth as an extrasolar planet. This presentation will discuss the relevance of Solar System based comparative climatology to extrasolar planet studies, and show examples of its application to a range of simulated extrasolar terrestrial planet environments.