Biomedical subjects

J F Kasting

Publications and source records attributed to J F Kasting.

54 records · Page 3Linked to original sources

PubMed1989

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.

Atmosphere↗

PubMed1989

Long-term stability of the Earth's climate.

Earth's climate has remained reasonably temperate for at least the last 3.5 billion years, despite a large increase in solar luminosity with time. The increase in solar flux has probably been offset by a decrease in atmospheric CO2 concentration caused by a negative feedback in the carbonate-silicate geochemical cycle. The same feedback mechanism implies that an Earth-like planet could remain habitable (i.e. possess liquid water) out to a least the orbit of Mars. The initial atmospheric CO2 concentration may have been much higher than the amount required to offset the lower solar output, in which case the Earth may have originally been much hotter than it is today. However, once the initial accretion period was over, Earth should have been stable against either a runaway greenhouse, that is, complete evaporation of the oceans, or against rapid loss of water. Long-term climatic evolution has thus far been studied only with one-dimensional, globally-averaged climate models. Although such models can provide a qualitative understanding of climate history, they rely on a number of assumptions that may not have been valid in the past. Some problems that deserve to be investigated with more sophisticated climate models are discussed.

Atmosphere↗

PubMed1988

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.

Atmosphere↗

PubMed1988

Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus.

A one-dimensional climate model is used to study the response of an Earth-like atmosphere to large increases in solar flux. For fully saturated, cloud-free conditions, the critical solar flux at which a runaway greenhouse occurs, that is, the oceans evaporate entirely, is found to be 1.4 times the present flux at Earth's orbit (S0). This value is close to the flux expected at Venus' orbit early in solar system history. Is is nearly independent of the amount of CO2 present in the atmosphere, but is sensitive to the H2O absorption coefficient in the 8- to 12-micrometers window region. Clouds should tend to depress the surface temperature on a warm, moist planet; thus, Venus may originally have had oceans if its initial water endowment was close to that of Earth. It lost them early in its history, however, because of rapid photodissociation of water vapor followed by escape of hydrogen to space. The critical solar flux above which water is rapidly lost could be as low as 1.1S0. The surface temperature of a runaway greenhouse atmosphere containing a full ocean's worth of water would have been in excess of 1500 degrees K--above the solidus for silicate rocks. The presence of such a steam atmosphere during accretion may have significantly influenced the early thermal evolution of both Earth and Venus.

Atmosphere↗

PubMed1988

How climate evolved on the terrestrial planets.

Planets with temperate, earthlike climates were once thought to be rare in our galaxy. Mathematical models now suggest that if planets do exist outside the solar system, many of them might be habitable.

Astronomical Phenomena↗

PubMed1987

The case for a wet, warm climate on early Mars.

Theoretical arguments are presented in support of the idea that Mars possessed a dense CO2 atmosphere and a wet, warm climate early in its history. Calculations with a one-dimensional radiative-convective climate model indicate that CO2 pressures between 1 and 5 bars would have been required to keep the surface temperature above the freezing point of water early in the planet's history. The higher value corresponds to globally and orbitally averaged conditions and a 30% reduction in solar luminosity; the lower value corresponds to conditions at the equator during perihelion at times of high orbital eccentricity and the same reduced solar luminosity. The plausibility of such a CO2 greenhouse is tested by formulating a simple model of the CO2 geochemical cycle on early Mars. By appropriately scaling the rate of silicate weathering on present Earth, we estimate a weathering time constant of the order of several times 10(7) years for early Mars. Thus, a dense atmosphere could have persisted for a geologically significant time period (approximately 10(9) years) only if atmospheric CO2 was being continuously resupplied. The most likely mechanism by which this might have been accomplished is the thermal decomposition of carbonate rocks induced directly and indirectly (through burial) by intense, global-scale volcanism. For plausible values of the early heat flux, the recycling time constant is also of the order of several times 10(7) years. The amount of CO2 dissolved in standing bodies of water was probably small; thus, the total surficial CO2 inventory required to maintain these conditions was approximately 2 to 10 bars. The amount of CO2 in Mars' atmosphere would eventually have dwindled, and the climate cooled, as the planet's internal heat engine ran down. A test for this theory will be provided by spectroscopic searches for carbonates in Mars' crust.

Atmosphere↗

PubMed1987

Europa, tidally heated oceans, and habitable zones around giant planets.

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. In our own solar system, Europa, one of the Galilean satellites of Jupiter, could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such case, sufficient solar energy could reach liquid water that organisms similar to those found under Antarctic ice could grow. In other solar systems, larger satellites with more significant heat flow could represent environments that are stable over an order of Aeons and in which life could perhaps evolve. We define a zone around a giant planet in which such satellites could exist as a tidally-heated habitable zone. This zone can be compared to the habitable zone which results from heating due to the radiation of a central star. In our solar system, this radiatively-heated habitable zone contains the Earth.

Biomass↗

PubMed1987

Theoretical constraints on oxygen and carbon dioxide concentrations in the Precambrian atmosphere.

Simple (one-dimensional) climate models suggest that carbon dioxide concentrations during the Archean must have been at least 100-1000 times the present level to keep the Earth's surface temperature above freezing in the face of decreased solar luminosity. Such models provide only lower bounds on CO2, so it is possible that CO2 levels were substantially higher than this and that the Archean climate was much warmer than today. Periods of extensive glaciation during the early and late Proterozoic, on the other hand, indicate that the climate at these times was relatively cool. To be consistent with climate models CO2 partial pressures must have declined from approximately 0.03 to 0.3 bar around 2.5 Ga ago to between 10(-3) and 10(-2) bar at 0.8 Ga ago. This steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data, which implies that pCO2 did not change appreciably during that time. Oxygen was essentially absent from the Earth's atmosphere and oceans prior to the emergence of a photosynthetic source, probably during the late Archean. During the early Proterozoic the atmosphere and surface ocean were apparently oxidizing, while the deep ocean remained reducing. An upper limit of 6 x 10(-3) bar for pO2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean. The establishment of oxidizing conditions in the deep ocean, marked by the disappearance of banded iron formations approximately 1.7 Ga ago, permitted atmospheric oxygen to climb to its present level. O2 concentrations may have remained substantially lower than today, however, until well into the Phanerozoic.

Animals↗

PubMed1986

Climatic consequences of very high carbon dioxide levels in the earth's early atmosphere.

The possible consequences of very high carbon dioxide concentrations in the earth's early atmosphere have been investigated with a radiative-convective climate model. The early atmosphere would apparently have been stable against the onset of a runaway greenhouse (that is, the complete evaporation of the oceans) for carbon dioxide pressures up to at least 100 bars. A 10- to 20-bar carbon dioxide atmosphere, such as may have existed during the first several hundred million years of the earth's history, would have had a surface temperature of approximately 85 degrees to 110 degrees C. The early stratosphere should have been dry, thereby precluding the possibility of an oxygenic prebiotic atmosphere caused by photodissociation of water vapor followed by escape of hydrogen to space. Earth's present atmosphere also appears to be stable against a carbon dioxide-induced runaway greenhouse.

Atmosphere↗

PubMed1986

A hybrid model of the CO2 geochemical cycle and its application to large impact events.

A hybrid model of the carbonate-silicate geochemical cycle is presented which is capable of calculating the partitioning of carbon dioxide between the atmosphere, ocean, and sedimentary rocks. The ocean is subdivided into a shallow, mixed layer, which remains in equilibrium with the atmosphere, and a massive, deep layer which does not. Gradients in dissolved carbon content are established between the mixed layer and the deep ocean as a consequence of downward fluxes of fecal matter and of dead planktonic organisms. The dissolved carbon content and alkalinity of the ocean as a whole are controlled by weathering and metamorphism of sedimentary rocks. Equilibrium solutions are derived for the preindustrial atmosphere/ocean system and for a system that may be similar to that existing during the Late Cretaceous Period. The model is then used to determine how the modern and ancient marine biospheres might be affected by an oceanic impact of a large asteroid or comet. Such an event could perturb the carbon cycle in several different ways. Global darkening caused by stratospheric dust veil could destroy most of the existing phytoplankton in a period of several weeks to several months. At the same time, dissolution of atmospheric NOx compounds synthesized during the impact would lower the pH of ocean surface waters and release CO2 into the atmosphere. Both effects might be enhanced by an influx of CO2 released from upwelling of deep ocean water near the hot impact site, from oxidation of dead organic matter, and from the comet itself. The net result could be to raise surface temperatures by several degrees and to make the surface ocean uninhabitable by calcareous organisms for as much as 20 yrs (the time scale for mixing with deep ocean). It appears unlikely, however, that an impact could create a "Strangelove ocean," in which surface waters remained corrosive to calcium carbonate for thousands or tens of thousands of years. Thus, disruption of the carbon cycle by an impact event cannot by itself explain the scarcity of calcium carbonate in sediments found within the first few centimeters above the K/T boundary.

Atmosphere↗

PubMed1985

Oxidant abundances in rainwater and the evolution of atmospheric oxygen.

A one-dimensional photochemical model has been used to estimate the flux of dissolved hydrogen peroxide (H2O2) and of other soluble species in rainwater as a function of atmospheric oxygen level. H2O2 should have replaced O2 as the dominant oxidant in rainwater at oxygen levels below 10(-3)-10(-2) times the present atmospheric level (PAL). The exact value of pO2 at which H2O2 becomes more important than O2 depends on the abundance of trace gases such as CO, CH4, and NO. H2O2 was probably an important oxidant even in an O2-free atmosphere, provided that CO2 levels were significant higher than today's. In model atmospheres containing free O2 the concentration of photochemically produced oxidants generally exceeds that of photochemically produced reductants. The oxidizing power of rainwater is therefore greater than that due to dissolved molecular O2 alone. The difference is small at present but becomes important at O2 levels less than 10(-3) PAL. At O2 levels between 10(-4) and 10(-5) PAL the oxidizing power of rainwater is almost independent of pO2. Precambrian soils in which a part or all of the Fe2+ in their source rocks has been oxidized to Fe3+ could therefore have developed in the presence of an atmosphere with very low values of pO2. On the other hand, the upper limit for pO2 during early and mid-Precambrian time suggested by the incomplete oxidation of FeO in soils developed on basaltic rocks is affected only slightly by the presence of photochemical products in rainwater.

Atmosphere↗

PubMed1985

Seafloor hydrothermal activity and spreading rates: the Eocene carbon dioxide greenhouse revisted.

A suggestion has been made that enhanced rates of hydrothermal activity during the Eocene could have caused a global warming by adding calcium to the ocean and pumping CO2 into the atmosphere (Owen and Rea, 1984). This phenomenon was purported to be consistent with the predictions of the CO2 geochemical cycle model of Berner, Lasaga and Garrels (1983) (henceforth BLAG). In fact, however, the BLAG model predicts only a weak connection between hydrothermal activity and atmospheric CO2 levels. By contrast, it predicts a strong correlation between seafloor spreading rates and pCO2, since the release rate of CO2 from carbonate metamorphism is assumed to be proportional to the mean spreading rate. The Ecocene warming can be conveniently explained if the BLAG model is extended by assuming that the rate of carbonate metamorphism is also proportional to the total length of the midocean ridges from which the spreading originates.

Atmosphere↗

PubMed1984

Effects of high CO2 levels on surface temperature and atmospheric oxidation state of the early Earth.

One-dimensional radiative-convective and photochemical models are used to examine the effects of enhanced CO2 concentrations on the surface temperature of the early Earth and the composition of the prebiotic atmosphere. Carbon dioxide concentrations of the order of 100-1000 times the present level are required to compensate for an expected solar luminosity decrease of 25-30%, if CO2 and H2O were the only greenhouse gases present. The primitive stratosphere was cold and dry, with a maximum H2O volume mixing ratio of 10(-6). The atmospheric oxidation state was controlled by the balance between volcanic emission of reduced gases, photo-stimulated oxidation of dissolved Fe+2 in the oceans, escape of hydrogen to space, and rainout of H2O2 and H2CO. At high CO2 levels, production of hydrogen owing to rainout of H2O2 would have kept the H2 mixing ratio above 2x10(-4) and the ground-level O2 mixing ratio below 10(-11), even if no other sources of hydrogen were present. Increased solar UV fluxes could have led to small changes in the ground-level mixing ratios of both O2 and H2.

Atmosphere↗

PubMed1984

The evolution of the prebiotic atmosphere.

One-dimensional radiative convective and photochemical models are used to estimate the vertical temperature structure and composition of the earth's prebiotic atmosphere. Greatly enhanced CO2 levels (100-1000 times present) are required to keep the mean surface temperature above freezing in the face of decreased solar luminosity during the earth's early history. Such high CO2 partial pressures would have affected the atmospheric oxidation state by facilitating the photochemical production of soluble species including H2O2 and H2CO. Oxidation of ferrous iron in the oceans by H2O2 dissolved in rainwater should have kept the atmospheric H2 mixing ratio above 2x10(-4) and the ground-level O2 mixing ratio below 10(-11), regardless of the magnitude of the rate of volcanic release of reduced gases.

Atmosphere↗

PubMed1984

Response of Earth's atmosphere to increases in solar flux and implications for loss of water from Venus.

A one-dimensional radiative-convective model is used to compute temperature and water vapor profiles as functions of solar flux for an Earth-like atmosphere. The troposphere is assumed to be fully saturated, with a moist adiabatic lapse rate, and changes in cloudiness are neglected. Predicted surface temperatures increase monotonically from -1 to 111 degrees C as the solar flux is increased from 0.81 to 1.45 times its present value. Surface temperatures corresponding to high solar fluxes may be underestimated, however, owing to neglect of H2O continuum absorption outside of the 8- to 12-micrometers window region. These results imply that the surface temperature of a primitive water-rich Venus should have been at least 80-100 degrees C and may have been much higher. The existence of liquid water at the surface depends on poorly known aspects of H2O continuum absorption and on uncertainties concerning relative humidity and cloudiness. In any case, water vapor should have been a major atmospheric constituent at all altitudes, leading to the rapid hydrodynamic escape of hydrogen. The oxygen left behind by this process was presumably consumed by reactions with reduced minerals in the crust. Both the loss of oxygen and the presently observed enrichment of the deuterium-to-hydrogen ratio are most easily explained if oceans of liquid water were initially present.

Atmosphere↗

PubMed1984

Comments on the BLAG model: the carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years.

A self-consistent method of determining initial conditions for the model presented by Berner, Lasaga, and Garrels (1983) (henceforth, the BLAG model) is derived, based on the assumption that the CO2 geochemical cycle was in steady state at t = -100 my (million years). This initialization procedure leads to a dissolved magnesium concentration higher than that calculated by Berner, Lasaga, and Garrels and to a low ratio of dissolved calcium to bicarbonate prior to 60 my ago. The latter prediction conflicts with the geologic record of evaporite deposits, which requires that this ratio remain greater than 0.5. The contradiction is probably caused by oversimplifications in the BLAG model, such as the neglect of the cycles of organic carbon and sulfur.

Atmosphere↗

PubMedDate not supplied

O2 concentrations in dense primitive atmospheres: commentary

Now that astronomers are, I understand, on the verge of detecting extrasolar planets, the question of whether such planets might be inhabited is beginning to be discussed in serious scientific circles. Specifically, astronomers such as Rosenqvist and Chassefiére (see the preceding article) are interested in whether spectroscopic measurements of free O2 in a planet's atmosphere might be used as evidence for life. As such, they have attempted to place constraints on the amount of O2 that might be found in the atmosphere of a lifeless planet or, more specifically, on a planet where oxygenic photosynthesis has not yet been invented. This question can be addressed by photochemical modeling, if one is careful about how one goes about it. The calculations presented here suggest an upper limit of approximately 10 mbar on the O2 partial pressure in a dominantly CO2 atmosphere.

Atmosphere↗

PubMedDate not supplied

The life span of the biosphere revisited.

A decade ago, Lovelock and Whitfield raised the question of how much longer the biosphere can survive on Earth. They pointed out that, despite the current fossil-fuel induced increase in the atmospheric CO2 concentration, the long-term trend should be in the opposite direction: as increased solar luminosity warms the Earth, silicate rocks should weather more readily, causing atmospheric CO2 to decrease. In their model, atmospheric CO2 falls below the critical level for C3 photosynthesis, 150 parts per million (p.p.m.), in only 100 Myr, and this is assumed to mark the demise of the biosphere as a whole. Here, we re-examine this problem using a more elaborate model that includes a more accurate treatment of the greenhouse effect of CO2, a biologically mediated weathering parameterization, and the realization that C4 photosynthesis can persist to much lower concentrations of atmospheric CO2(<10 p.p.m.). We find that a C4-plant-based biosphere could survive for at least another 0.9 Gyr to 1.5 Gyr after the present time, depending respectively on whether CO2 or temperature is the limiting factor. Within an additional 1 Gyr, Earth may lose its water to space, thereby following the path of its sister planet, Venus.

Atmosphere↗