Abiotic formation of O2 and O3 in high-CO2 terrestrial atmospheres

Previous research has indicated that high amounts of ozone (O3) and oxygen (O2) may be produced abiotically in atmospheres with high concentrations of CO2. The abiotic production of these two gases, which are also characteristic of photosynthetic life processes, could pose a potential "false-positive" for remote-sensing detection of life on planets around other stars.We show here that such false positives are unlikely on any planet that possesses abundant liquid water, as rainout of oxidized species onto a reduced planetary surface should ensure that atmospheric H2 concentrations remain relatively high, and that O2 and O3 remain low. Our aim is to determine the amount of O3 and O2 formed in a high CO2 atmosphere for a habitable planet without life. We use a photochemical model that considers hydrogen (H2) escape and a detailed hydrogen balance to calculate the O2 and O3 formed on planets with 0.2 of CO2 around the Sun, and 0.02, 0.2 and 2 bars of CO2 around a young Sun-like star with higher UV radiation. The concentrations obtained by the photochemical model were used as input in a radiative transfer model that calculated the spectra of the modeled planets. The O3 and O2 concentrations in the simulated planets are extremely small, and unlikely to produce a detectable signature in the spectra of those planets. We conclude that with a balanced hydrogen budget, and for planets with an active hydrological cycle, abiotic formation of O2 and O3 is unlikely to create a possible false positive for life detection in either the visible/near-infrared or mid-infrared wavelength regimes.Comment: 27 pages, 15 figures, Astronomy & Astrophysics accepte

Venus: The case for a wet origin and a runaway greenhouse

To one interested in atmospheric evolution, the most intriguing aspect of our neighboring planet Venus is its lack of water. Measurements made by Pioneer Venus and by Several Venera spacecraft indicate that the present water abundance in Venus' lower atmosphere is of the order of 20 to 200 ppmv, or 3 x 10( exp -6) to 3 x 10 (exp -5) of the amount of water in Earth's oceans. The exact depletion factor is uncertain, in part because of an unexplained vertical gradient in H2O concentration in the lowest 10 km of the venusian atmosphere, but the general scarcity of water is well established. The interesting question, then, is: Was venus deficient in water when it formed and, if not, where did its water go? The conclusion that Venus was originally wet is consistent with its large endowment of other volatiles and with the enhanced D/H ratio in the present atmosphere. The most likely mechanism by which Venus could have lost its water is by the development of a runaway or moist greenhouse atmosphere followed by photodissociation of water vapor and escape of hydrogen to space. Climate model calculations that neglect cloud albedo feedback predict the existence of two critical transitions in atmospheric behavior at high solar fluxes: (1) at a solar flux of approximately 1.1 times the value at Earth's orbit, S(o), the abundance of stratospheric water vapor increases dramatically, permitting rapid escape of hydrogen to space (termed a moist greenhouse) and (2) at a solar flux of approximately 1.4 S(o), the oceans vaporize entirely, creating a true runaway greenhouse. If cloudiness increases at high surface temperatures, as seems likely, and if the dominant effect of clouds is to cool the planet by reflecting incident solar radiation, the actual solar flux required to create moist or runaway conditions would be higher than the values quoted above. Early in solar system history, solar luminosity was about 25 percent to 30 percent less than today, putting the flux at Venus' orbit in the range of 1.34 S(o) to 1.43 S(o). Thus, it is possible that Venus had liquid water on its surface for several hundred million years following its formation. Paradoxically, this might have facilitated water loss by sequestering atmospheric CO2 in carbonate rocks and by providing an effective medium for surface oxidation

Climatic effects of enhanced CO2 levels in Mars early atmosphere

Results are presented of one-dimensional radiation convection modeling of the early Mars atmosphere. Up to 5 bars of CO2 would have been required to raise the surface temperature (orbitally and globally averaged) above the freezing point, although at the equator at perihelion, 1 bar would have sufficed. Such an atmospheric CO2 invertory, the author argued, is not inconsistent with any known constraint on Mars' degassed volatile inventory

Evolution of the atmosphere

Theories on the origin of the Earth atmosphere and chemical composition are presented. The role of oxygenic photosynthesis on the determination of the Earth's origin is discussed. The research suggests that further analysis of the geologic record is needed to more accurately estimate the history of atmospheric oxygen

Warming the early Earth - CO2 reconsidered

Despite a fainter Sun, the surface of the early Earth was mostly ice-free. Proposed solutions to this so-called "faint young Sun problem" have usually involved higher amounts of greenhouse gases than present in the modern-day atmosphere. However, geological evidence seemed to indicate that the atmospheric CO2 concentrations during the Archaean and Proterozoic were far too low to keep the surface from freezing. With a radiative-convective model including new, updated thermal absorption coefficients, we found that the amount of CO2 necessary to obtain 273 K at the surface is reduced up to an order of magnitude compared to previous studies. For the late Archaean and early Proterozoic period of the Earth, we calculate that CO2 partial pressures of only about 2.9 mb are required to keep its surface from freezing which is compatible with the amount inferred from sediment studies. This conclusion was not significantly changed when we varied model parameters such as relative humidity or surface albedo, obtaining CO2 partial pressures for the late Archaean between 1.5 and 5.5 mb. Thus, the contradiction between sediment data and model results disappears for the late Archaean and early Proterozoic.Comment: 53 pages, 4 tables, 11 figures, published in Planetary and Space Scienc

Venus: A search for clues to early biological possibilities

The extensive evidence that there is no extant life on Venus is summarized. The current atmospheric environment, which is far too hostile by terrestrial standards to support life, is described. However, exobiologists are interested in the possibility of extinct life on Venus. The early history of Venus is discussed in terms of its ability to sustain life that may now be extinct

Impact production of NO and reduced species

It has recently been suggested that a reported spike in seawater (87)Sr/(86)Sr at the K-T boundary is the signature of an impact-generated acid deluge. However, the amount of acid required is implausibly large. Some about 3 x 10 to the 15th power moles of Sr must be weathered from silicates to produce the inferred Sr spike. The amount of acid required is at least 100 and probably 1000 times greater. Production of 3 x 10 to the 18th power moles of NO is clearly untenable. The atmosphere presently contains only 1.4 x 10 to the 20th power moles of N-sub 2 and 3.8 x 10 to the 19th power moles of O sub 2 If the entire atmosphere were shocked to 2000 K and cooled within a second, the total NO produced would be about 3 x 10 to the 18th power moles. This is obviously unrealistic. A (still to short) cooling time of 10th to the 3rd power sec reduces NO production by an order of magnitude. In passing, we note that if the entire atmosphere had in fact been shocked to 2000 K, acid rain would have been the least of a dinosaur's problems. Acid rain as a mechanism poses poses other difficulties. Recently deposited carbonates would have been most susceptable to acid attack. The researchers' preferred explanation is simply increased continental erosion following ecological trauma, coupled with enchanced levels of CO-sub 2

Partitioning of carbon dioxide between the atmosphere and lithosphere on early Mars

It is pointed out that in addition to the 1 to 5 bar CO2 total inventory, a high level of global volcanism was needed to keep the CO2 from being drawn away permanently by weathering of igneous rocks; the volcanism would continually decompose the carbonate resulting in steady efficient recycling