RESUMO
Earth appears to have been warm during its early history despite the faintness of the young Sun. Greenhouse warming by gaseous CO2 and H2O by itself is in conflict with constraints on atmospheric CO2 levels derived from paleosols for early Earth. Here we explore whether greenhouse warming by methane could have been important. We find that a CH4 mixing ratio of 10(-4) (100 ppmv) or more in Earth's early atmosphere would provide agreement with the paleosol data from 2.8 Ga. Such a CH4 concentration could have been readily maintained by methanogenic bacteria, which are thought to have been an important component of the biota at that time. Elimination of the methane component of the greenhouse by oxidation of the atmosphere at about 2.3-2.4 Ga could have triggered the Earth's first widespread glaciation.
Assuntos
Atmosfera/química , Planeta Terra , Evolução Planetária , Efeito Estufa , Metano/química , Dióxido de Carbono/análise , Clima , Euryarchaeota/metabolismo , Hidrogênio/análise , Hidrogênio/química , Metano/análise , Modelos Químicos , Fotoquímica , Temperatura , Água/análise , Água/químicaRESUMO
The nephelometer experiment carried on the Galileo probe was designed to measure the jovian cloud structure and its microphysical characteristics from entry down to atmospheric pressure levels greater than 10 bars. Before this mission there was no direct evidence for the existence of the clouds below the uppermost cloud layer, and only theoretical models derived from remote sensing observations were available for describing such clouds. Only one significant cloud structure with a base at about 1.55 bars was found along the probe descent trajectory below an ambient pressure of about 0.4 bar, although many indications of small densities of particle concentrations were noted during much of the descent.
Assuntos
Atmosfera , Meio Ambiente Extraterreno , Júpiter , Compostos de Amônio Quaternário/análise , Água/análise , Nefelometria e Turbidimetria , Pressão , Enxofre/análiseRESUMO
Images of Venus taken at 418 (violet) and 986 [near-infrared (NIR)] nanometers show that the morphology and motions of large-scale features change with depth in the cloud deck. Poleward meridional velocities, seen in both spectral regions, are much reduced in the NIR In the south polar region the markings in the two wavelength bands are strongly anticorrelated. The images follow the changing state of the upper cloud layer downwind of the subsolar point, and the zonal flow field shows a longitudinal periodicity that may be coupled to the formation of large-scale planetary waves. No optical lightning was detected.
