Anaerobic methanotrophy and the rise of atmospheric oxygen.

In modern marine sediments, the anoxic decomposition of organic matter generates a significant flux of methane that is oxidized microbially with sulphate under the seafloor and never reaches the atmosphere. In contrast, prior to ca 2.4Gyr ago, the ocean had little sulphate to support anaerobic oxidation of methane (AOM) and the ocean should have been an important methane source. As atmospheric O2 and seawater sulphate levels rose on the early Earth, AOM would have increasingly throttled the release of methane. We use a biogeochemical model to simulate the response of early atmospheric O2 and CH4 to changes in marine AOM as sulphate levels increased. Semi-empirical relationships are used to parameterize global AOM rates and the evolution of sulphate levels. Despite broad uncertainties in these relationships, atmospheric O2 concentrations generally rise more rapidly and to higher levels (of order approx. 10(-3) bar versus approx. 10(-4) bar) as a result of including AOM in the model. Methane levels collapse prior to any significant rise in O2, but counter-intuitively, methane re-rises after O2 rises to higher levels when AOM is included. As O2 concentrations increase, shielding of the troposphere by stratospheric ozone slows the effective reaction rate between oxygen and methane. This effect dominates over the decrease in the methane source associated with AOM. Thus, even with the inclusion of AOM, the simulated Late Palaeoproterozoic atmosphere has a climatologically significant level of methane of approximately 50ppmv.

Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth.

The low O2 content of the Archean atmosphere implies that methane should have been present at levels approximately 10(2) to 10(3) parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH4 is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH4 implies that hydrogen escapes to space (upward arrow space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O2 and H, and methanogenesis transfers the H into CH4. Hydrogen escape after CH4 photolysis, therefore, causes a net gain of oxygen [CO2 + 2H2O --> CH4 + 2O2 --> CO2 + O2 + 4H(upward arrow space)]. Expected irreversible oxidation (approximately 10(12) to 10(13) moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized.

High-resolution calculations of asteroid impacts into the Venusian atmosphere.

We present results from a number of 2D high-resolution hydrodynamical simulations of asteroids striking the atmosphere of Venus. These cover a wide range of impact parameters (velocity, size, and incidence angle), but the focus is on 2-3 km diameter asteroids, as these are responsible for most of the impact craters on Venus. Asteroids in this size range are disintegrated, ablated, and significantly decelerated by the atmosphere, yet they retain enough impetus to make large craters when they meet the surface. We find that smaller impactors (diameter <1-2 km) are better described by a "pancaking" model in which the impactor is compressed and distorted, while for larger impactors (>2-3 km) fragmentation by mechanical ablation is preferred. The pancaking model has been modified to take into account effects of hydrodynamical instabilities. The general observation that most larger impactors disintegrate by shedding fragments generated from hydrodynamic instabilities spurs us to develop a simple heuristic model of the mechanical ablation of fragments based on the growth rates of Rayleigh-Taylor instabilities. Although in principle the model has many free parameters, most of these have little effect provided that they are chosen reasonably. In practice the range of model behavior can be described with one free parameter. The resulting model reproduces the mass and momentum fluxes rather well, doing so with reasonable values of all physical parameters.

Airburst origin of dark shadows on Venus.

A simple analytic model for the catastrophic disruption and deceleration of impactors in a thick atmosphere is used to (1) reproduce observed Venusian cratering statistics and (2) generate radar-dark disks by the impact of atmospheric shock waves with the surface. When used as input to Monte Carlo simulations of Venusian cratering, the model nicely reproduces the observed low diameter cutoff. Venusian craters are found to be more consistent with an asteroidal rather than a cometary source. The radar-dark "shadows" of the title are surface features, usually circular, that have been attributed to airbursting impactors. A typical craterless airburst is the equivalent of a approximately 10(6) megaton explosion. The airburst is treated as a massive, extended explosion using a thin-shell, isobaric cavity approximation. The strong atmospheric shock waves excited by the airbust are then coupled to surface rock using the usual impedance matching conditions. Peak shock pressures experienced by surface rock typically exceed 0.2 GPa for distances 15-30 km from ground zero (the place on the surface immediately beneath the site of the airburst), and 1 GPa for 10-20 km. These high shock pressures are felt to considerable depth, often more than a kilometer. Beneath the airburst the shock could reduce surface rocks to fine rubble, while at greater distance the weaker shock would leave fields of broken blocks, perhaps in part accounting for radar-bright halos that often surround the dark shadows.

Ignition of global wildfires at the Cretaceous/Tertiary boundary.

An impressive amount of evidence supports the proposal of Alvarez et al. that the Cretaceous era was ended abruptly by the impact of a comet or asteroid. The recent discovery of an apparently global soot layer at the Cretaceous/Tertiary boundary indicates that global wildfires were somehow ignited by the impact. Here we show that the thermal radiation produced by the ballistic re-entry of ejecta condensed from the vapour plume of the impact could have increased the global radiation flux by factors of 50 to 150 times the solar input for periods ranging from one to several hours. This great increase in thermal radiation may have been responsible for the ignition of global wildfires, as well as having deleterious effects on unprotected animal life.

Annihilation of ecosystems by large asteroid impacts on the early Earth.

Large asteroid impacts produced globally lethal conditions by evaporating large volumes of ocean water on the early Earth. The Earth may have been continuously habitable by ecosystems that did not depend on photosynthesis as early as 4.44 Gyr BP (before present). Only a brief interval after 3.8 Gyr exists between the time when obligate photosynthetic organisms could continuously evolve and the time when the palaeontological record indicates highly evolved photosynthetic ecosystems.

Sulfur, ultraviolet radiation, and the early evolution of life.

The present biosphere is shielded from harmful solar near ultraviolet (UV) radiation by atmospheric ozone. We suggest here that elemental sulfur vapor could have played a similar role in an anoxic, ozone-free, primitive atmosphere. Sulfur vapor would have been produced photochemically from volcanogenic SO2 and H2S. It is composed of ring molecules, primarily S8, that absorb strongly throughout the near UV, yet are expected to be relatively stable against photolysis and chemical attack. It is also insoluble in water and would thus have been immune to rainout or surface deposition over the oceans. The concentration of S8 in the primitive atmosphere would have been limited by its saturation vapor pressure, which is a strong function of temperature. Hence, it would have depended on the magnitude of the atmospheric greenhouse effect. Surface temperatures of 45 degrees C or higher, corresponding to carbon dioxide partial pressures exceeding 2 bars, are required to sustain an effective UV screen. Two additional requirements are that the ocean was saturated with sulfite and bisulfite, and that linear S8 chains must tend to reform rings faster than they are destroyed by photolysis. A warm, sulfur-rich, primitive atmosphere is consistent with inferences drawn from molecular phylogeny, which suggest that some of the earliest organisms were thermophilic bacteria that metabolized elemental sulfur.

Evolution of a steam atmosphere during Earth's accretion.

We have modeled the evolution of an impact-generated steam atmosphere surrounding an accreting Earth. The model assumes Safronov accretion; it includes degassing of planetesimals upon impact, thermal blanketing by a steam atmosphere, interchange of water between the surface and the interior, shock heating and convective cooling of Earth's interior, and hydrogen escape, both by a solar extreme ultraviolet (EUV) powered planetary wind and by impact erosion (atmospheric cratering). The model does not include atmophiles other than water, chemical reaction of water with metallic iron, core formation, compression, and spatial and temporal inhomogeneity of accretion. If the incoming planetesimals were too dry or the EUV flux too high, very little water would accumulate at the surface. Essentially all water retained by such a planet would be through rehydration of silicates. If rehydration were inefficient, very little water would be retained in any form. Degassing of wetter planetesimals produces a steam atmosphere over a magma ocean, the energy of accretion being sufficient to maintain a runaway greenhouse atmosphere. The mass of the atmosphere is limited by water's solubility in the (partial) melt. This type of solution is produced for a wide range of model parameters. During accretion, approximately 30 bars of water could have kept the surface at 1500 degrees K. As the accretional energy input declined below the runaway greenhouse threshold, the steam atmosphere rained out. Outgassing of dissolved water at the close of accretion is quantitatively important. These models can leave from approximately 100 to more than 300 bars of water at the surface at the close of accretion. In general, most of the water accreted remains dissolved in the mantle. H2 could have escaped as rapidly as it formed only if the planetesimals were relatively dry. Consequently H2 should have accumulated until it reached chemical equilibrium with water vapor. Impact erosion (escape caused by impact) is a critical but poorly understood process. It can prevent the accumulation of a steam atmosphere if the planetesimals are sufficiently dry, or for wetter impactors if it is much more effective than we have assumed. Impact erosion of a steam atmosphere is less important; it is equivalent to a slightly drier rain of impactors. If a hypothetical Moon-forming impact took place before the collapse of the runaway greenhouse, relatively little water (approximately 30-100 bars) would have been in the atmosphere; hence little could have been lost. If the event took place later, the potential damage could have been greater.

Lunar nodal tide and distance to the Moon during the Precambrian.

The pace of tidal evolution for the past approximately 450 Myr implies an Earth/Moon collision some 1,500-2,000 Myr BP, an event for which there is no corroborating evidence. Here we present the first direct determination of the lunar distance in the Precambrian. We interpret a 23.3 +/- 0.3-yr periodicity preserved in a 2,500 Myr BP Australian banded iron formation (BIF) as reflecting the climatic influence of the lunar nodal tide, which has been detected with its modern 18.6-yr periodicity in some modern climate records. The lunar distance at 2,500 Myr BP would then have been about 52 Earth radii. The implied history of Precambrian tidal friction is in accord with both the more recent palaeontological evidence and the long-term stability of the lunar orbit. The length of the Milankovitch cycles that modulate the ice ages today also evolve with the Earth-Moon system. Their detection in the Precambrian sedimentary record would then permit an independent determination of the lunar distance.