P33D-01
A View of the Prebiotic Earth's Atmosphere: Greenhouse Gases, Oxidized Organic Haze, and the Anti-Greenhouse Effect
Recent attempts to resolve the faint young sun paradox have focused on an early Earth atmosphere containing elevated levels of the greenhouse gases methane (CH4) and carbon dioxide (CO2) to provide adequate warming to the Earth's surface. However, the photolysis of CH4 and CO2 in equal ratios in the laboratory has been shown to produce significant aerosol mass, equivalent to or greater than the aerosol mass produced in Titan simulations. The haze layer generated by these aerosols could offset the warming by scattering incoming solar radiation, creating an antigreenhouse effect. The amount of CH4 in the prebiotic Earth's atmosphere could have been significantly less than the amount of CO2. Additionally, high amounts of H2 may have been present in the prebiotic Earth's atmosphere, and could affect the haze chemistry. In this work we examine haze formation in an early Earth atmosphere composed of CO2, H2, N2, and CH4, with a CO2/CH4 ratio of 10. Haze particles were generated using different concentrations of H2, with levels up to 15 percent by volume H2. To initiate aerosol formation a broad-spectrum ultraviolet (UV) energy source with emission at Lyman-α was used to simulate the solar spectrum. Aerosol composition and total aerosol mass produced as a function of reagent gas were measured using an Aerosol Mass Spectrometer (AMS). A Scanning Mobility Particle Sizer (SMPS) was also used to measure the aerosol mass, as well as the size distribution of the particles. Results show an order of magnitude decrease in haze production with the addition of H2, with no significant change in the chemical composition of the haze. We calculate that such a haze would not have a significant antigreenhouse effect. Further, the highly oxidized particles formed could be more biologically interesting than a Titan-like hydrocarbon haze. The presence of H2 on the early Earth could thus favor warmer surface temperatures while still allowing photochemical haze formation to deliver complex organic species to the early Earth's surface.
P33D-02 INVITED
The Global Nitrogen Budget and the Faint Young Sun Paradox
Models of the early Earth Climate invariably assume that the Archean atmospheric nitrogen inventory was equal to today's, or that Archean atmospheric pressure was 1 bar. Here, we relax this assumption and present radiative-convective climate model results for the late Archean atmosphere with 0.5, 1, 2 and 3 times the present atmospheric nitrogen inventory and varying carbon dioxide concentrations. Nitrogen is taken to be radiatively inert, but an increased nitrogen inventory causes a surface warming by pressure broadening of the adsorption bands of radiatively active species and through a pressure-lapse rate feedback. Assuming 25 times present carbon dioxide mixing ratio at 2.8 Ga, twice the present nitrogen inventory would give an 11K warming compared to the present nitrogen inventory, yielding a surface pressure of 286K. Thus it provides a possible solution to the Faint Young Sun paradox. Conversely, if the nitrogen inventory of the early atmosphere had been less, modeled surface temperature would be much colder and the Faint Young Sun paradox would be much more difficult to solve. The present atmospheric nitrogen inventory is 3.9 × 1018 kg N. Venus has three times this amount in its atmosphere, which suggests that Earth may have had more nitrogen than is currently obvious. The continental crust contains 1-2× 1018 kg N, which would have accumulated with continental growth, thought to have happened mostly since ~ 2.7 Ga. A larger hidden nitrogen reservoir may exist in the mantle. The present rate of transfer of nitrogen from the ocean crust and sediments to the mantle (subduction minus volcanism) has been reported to be ~ 7.5 × 108 kg yr-1, at which rate ~ 2 × 1018 kg N would have been transferred to the mantle since 2.7 Ga. Subduction rates in the past may well have been higher, so this may represent a lower bound. Thus twice the present atmospheric nitrogen inventory in the late Archean is a plausible estimate.
P33D-03
Mars: Volcanism and Young Atmosphere.
By using present-day observation and simple evolution models, we study the long term evolution of the Martian atmosphere influenced by volcanism degassing and atmospheric escape. Using a straightforward back integration based on the present state of the atmosphere, we show that even with the few data available we can propose a scenario for the evolution of the Martian atmosphere over the last three billion years. We focus on CO2 as the most likely main gas present in the atmosphere at that time and involved in large scale and long term processes. However we also apply our model to argon, nitrogen, sulphur and water. The model takes into account the effects of volcanic degassing, which constitutes an input of volatiles, and atmospheric escape into space. Atmospheric escape is assumed to have a major influence over planetary atmospheres. In the case of Mars in particular, where no evidence for carbonates has been found, escape seems to be the main mechanism for CO2 removal. Since hydrodynamic escape mostly takes place during the first few hundreds of million years, other processes for atmospheric escape have been considered in order to quantify the loss of volatiles during later periods. These processes are ionospheric outflow, ion pick-up, sputtering and dissociative recombination and are non-thermal. Their contributions are estimated by using data from ASPERA and Mars Express and several models such as those created by Leblanc (2001) or Chassefière, Leblanc and Langlais (2006). The input of gazes in the atmosphere is obtained by estimating the intensity of the volcanism over the history of Mars. Numerical models are used to quantify the melt production rate during the evolution of the planet. Here we used models from Breuer and Spohn (2006), Manga et al. (2006) and O'Neill et al. (2007). Depending on the volatile contents of the lavas, we have access to the amount of gazes released in the atmosphere. We show that with all our scenarios, the ancient primary atmosphere is efficiently lost around 3 billion years ago, leading in most of the cases to a time of thin atmosphere. The period of "high" CO2 pressures coincides with the formation of fluvial landforms (Mangold et al., 2004) and we show that the atmosphere at that time was probably able to sustain liquid water on the surface at least for short periods provided the surface temperature is high enough. However our model shows that over the past three billion years it is unlikely Mars could harbour a thick (>1 bar) atmosphere. The volcanism is then responsible for the preservation of what remains and, when the outgassing is sufficient, for the creation of a young secondary atmosphere. This implies that the present-day atmosphere might be recent rather than simply remnants of the primordial one, slowly eroded over 4.5 billion years. Our model shows this young atmosphere is likely to be 1 to 2 billion years old but might be even younger. We show that even with very low volcanic activity, the present-day Martian atmosphere is very likely composed of more than 50% to 70% of volcanic gazes.
P33D-04
Coupled Surface/Atmosphere/Interior Evolution of Venus: Current knowledge and future prospects
Venus is commonly thought to have experienced a transition, early in its history, from a wet, more Earth-like environment to its currently highly desiccated state. A more recent global transition is indicated by the sparse, randomly distributed and relatively pristine crater population, which implies a rapid decrease in volcanic resurfacing rate between 300 and 1000 Myr ago. The accompanying precipitous decline in outgassing rate would have caused large climate changes and globally synchronous plains deformation. These two transitions may have been causally related, as the loss of atmospheric water through evaporation, photodissociation and H escape caused the transition from plate tectonics to single plate behavior, and the cessation of subducting hydrated sediments caused the desiccation of the mantle and consequent loss of an asthenosphere. New data from Venus Express can help test these ideas by refining the timescale for water loss, constraining current outgassing rates, and improving models of climate, cloud formation and atmospheric dynamics, thus improving the sophistication of evolutionary climate models. I will discuss these efforts and review relevant plans for future missions.
P33D-05 INVITED
Titan's Hydrodynamically Escaping Atmosphere and the Structure of the Exobase Region
In a previous paper (Strobel, Icarus, 193, 588-594, 2008), I have argued that the upper atmosphere of Titan is undergoing hydrodynamic escape as a high density, slow outward expansion, driven principally by solar UV heating by CH4 absorption. The hydrodynamic mass loss, currently at a rate ~ (4-5)× 1028amu s-1, is essentially CH4 and H2 escape (Cui et al. J. Geophys. Res. in press, 2008; Yelle et al. J. Geophys. Res., in press, 2008) and limited by available solar UV power. The slow hydrodynamic expansion solutions below the exobase must be matched to an escape model in the exosphere, and there is still considerably controversy on the acceleration mechanism to achieve escape speeds. From Cassini INMS data, the structure of the "exobase region" on Titan will be explored and demonstrated to not be a thin transition level to an instanteously collisionless exosphere, as is normally assumed, but rather an extended region of ~ 1000 km, which is quasi-collisional. Processes that may considerably increase the current mass loss rate will be discussed.
P33D-06
The Cosmic Shoreline
Volatile escape is the classic existential problem of planetary atmospheres. The problem has gained new currency now that we can begin to study escape, or the cumulative effects of escape, from extrasolar planets seen in transit. Currently the published roster of transiting exoplanets consists of more than 30 "Jupiters" and one "Neptune." Already some intriguing patterns seem to be emerging. In particular, the transiting extrasolar planets appear to fit to a pattern already seen in our own Solar System. There is a clear hint that the boundary between planets with and without active volatiles --- the cosmic shoreline, as it were --- is surprisingly well-defined and therefore ought to be theoretically characterizable. The data show that (i) atmospheres are found where gravitational binding energy is high and solar heating low, and (ii) atmospheres are found where gravitational binding energy is high and impact velocities are low. In either case, to first approximation the boundary between planets with and without atmospheres is a single simple power law that extends from Pluto at one end to hot Jupiters at the other. Here we discuss how atmospheric escape can account for these patterns.
P33D-07
Examining the Ability of Sulfur-Bearing Gases to act as Biosignatures on Anoxic Planets
The search for life on extrasolar planets will depend on our ability to detect the presence of biogenic gases in the atmospheres of those planets using spectroscopy. For example, the presence of life on a planet similar to modern-day Earth could be detected by the simultaneous presence of molecular oxygen and methane in the planet's atmosphere, as indicated by their spectral features. However, for a significant portion of life's history on Earth no molecular oxygen was present in the atmosphere. Thus, the detection of a biosphere similar to the one that existed on Earth before the rise of atmospheric oxygen requires the study of novel biosignatures. Biogenic S-bearing gases may be able to act as biosignatures on such a planet. The two main sinks for these gases in the modern atmosphere are oxidation and photolysis, and they may build up to higher concentrations in atmospheres that have lower UV fluxes and a less oxidizing chemistry. These conditions may have been present on the early Earth, when the lack of molecular oxygen should have decreased the oxidation rate of these gases and an organic haze could have shielded them from photolysis. In this presentation, we will present the results of a 1-D photochemical model of the early Earth that includes the biogenic gases dimethyl disulfide (CH3-S-S-CH3), dimethyl sulfide (CH3-S-CH3), methyl mercaptan (CH3SH), carbon disulfide (CS2), and carbonyl sulfide (OCS). Specifically, we predict the mixing ratios and spectral features that could result when these gases are released into an anoxic atmosphere similar to the one thought to have existed on the early Earth. We also consider the potential for abiogenic sources of these species to create false positives for biology. The discussion will focus on the effects of atmospheric oxidation state and UV flux on the abundance of these gases and their corresponding ability to act as remotely detectable biosignatures.
P33D-08
Detecting Biosignatures in an Evolving Earth-like Atmosphere via New Worlds Observer
Over 300 extrasolar planets have been discovered in the last decade. Detection methods include indirect
means such as transits and dopplar shift and only one has been directly imaged. New Worlds Observer is a
mission that will revolutionize the direct detection of extrasolar planets by not only having the capability to
image terrestrial-sized planets close to the star, but will also analyze the spectrum of the planet's atmosphere
and surface. We have simulated what an 'Earth' will look like as a function of its atmospheric evolution. The
biosignatures of the Earth are shown to evolve significantly. New Worlds will be able to analyze exo-
atmospheres with a spectrograph and will specifically look for biosignatures. Levels of molecular Oxygen and
Methane, as low as 3% and 0.3% of the total atmosphere respectively, can be achieved with New World
technology. These levels correspond to detecting microbial life (methanogens) as early as 1.5 billion years
after the formation of a planet, or photosynthetic life on a more mature planet.
http://newworlds.colorado.edu