Kevin J. Zahnle

Carbon dioxide cycling and implications for climate on ancient Earth

The crustal Urey cycle of CO2 involving silicate weathering and metamorphism acts as a dynamic climate buffer. In this cycle, warmer temperatures speed silicate weathering and carbonate formation, reducing atmospheric CO2 and thereby inducing global cooling. Over long periods of time, cycling of CO2 into and out of the mantle also dynamically buffers CO2. In the mantle cycle, CO2 is outgassed at ridge axes and island arcs, while subduction of carbonatized oceanic basalt and pelagic sediments returns CO2 to the mantle. Negative feedback is provided because the amount of basalt carbonatization depends on CO2 in seawater and therefore on CO2 in the air. On the early Earth, processes involving tectonics were more vigorous than at present, and the dynamic mantle buffer dominated over the crustal one. The mantle cycle would have maintained atmospheric and oceanic CO2 reservoirs at levels where the climate was cold in the Archean unless another greenhouse gas was important. Reaction of CO2 with impact ejecta and its eventual subduction produce even lower levels of atmospheric CO2 and small crustal carbonate reservoirs in the Hadean. Despite its name, the Hadean climate would have been freezing unless tempered by other greenhouse gases.

Environmental perturbations caused by the impacts of asteroids and comets

We review the major impact-associated mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary. We then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general. Our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions, but we also stress that there are many uncertainties. We conclude that impacts with energies less than about 10 Mt are a negligible hazard. For impacts with energies above 10 Mt and below about 104 Mt (i.e., impact frequencies less than one in 6 × 104 years, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively), blast damage, earthquakes, and fires should be important on a scale of 104 or 105 km², which corresponds to the area damaged in many natural disasters of recent history. However, tsunami excited by marine impacts could be more damaging, flooding a kilometer of coastal plain over entire ocean basins. In the energy range of 104–105 Mt (intervals up to 3 × 105 years, corresponding to comets and asteroids with diameters up to 850 m and 1.4 km, respectively) water vapor injections and ozone loss become significant on the global scale. In our nominal model, such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects, but if a higher fraction of pulverized rock than we think likely reaches the stratosphere, stratospheric dust (causing global cooling) would also be important in this energy range. Thus 105 Mt is a lower limit where damage might occur beyond the experience of human history. The energy range from 105 to 106 Mt (intervals up to 2 × 106 years, corresponding to comets and asteroids up to 1.8 and 3 km diameter) is transitional between regional and global effects. Stratospheric dust, sulfates released from within impacting asteroids, and soot from extensive wild-fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10. Moreover, the ejecta plumes of these impacts may produce enough NO from shock-heated air to destroy the ozone shield. Between 106 and 107 Mt (intervals up to 1.5 × 107 years, corresponding to comets and asteroids up to 4 and 6.5 km diameter), dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis. Ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km², and the resulting smoke would reduce light levels even further. At energies above 107 Mt, blast and earthquake damage reach the regional scale (106 km²). Tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the worlds ocean basins. Fires would be set globally. Light levels may drop so low from the smoke, dust, and sulfate as to make vision impossible. At energies approaching 109 Mt (>108 years) the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids. The Cretaceous-Tertiary impact in particular struck evaporate substrates that very likely generated a dense, widespread sulfate aerosol layer with consequent climatic effects. The combination of all of these physical effects would surely represent a devastating stress on the global biosphere.

Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event

It has been suggested that a decrease in atmospheric methane levels triggered the progressive rise of atmospheric oxygen, the so-called Great Oxidation Event, about 2.4 Gyr ago. Oxidative weathering of terrestrial sulphides, increased oceanic sulphate, and the ecological success of sulphate-reducing microorganisms over methanogens has been proposed as a possible cause for the methane collapse, but this explanation is difficult to reconcile with the rock record. Banded iron formations preserve a history of Precambrian oceanic elemental abundance and can provide insights into our understanding of early microbial life and its influence on the evolution of the Earth system. Here we report a decline in the molar nickel to iron ratio recorded in banded iron formations about 2.7 Gyr ago, which we attribute to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time. We measured nickel partition coefficients between simulated Precambrian sea water and diverse iron hydroxides, and subsequently determined that dissolved nickel concentrations may have reached ∼400 nM throughout much of the Archaean eon, but dropped below ∼200 nM by 2.5 Gyr ago and to modern day values (∼9 nM) by ∼550 Myr ago. Nickel is a key metal cofactor in several enzymes of methanogens and we propose that its decline would have stifled their activity in the ancient oceans and disrupted the supply of biogenic methane. A decline in biogenic methane production therefore could have occurred before increasing environmental oxygenation and not necessarily be related to it. The enzymatic reliance of methanogens on a diminishing supply of volcanic nickel links mantle evolution to the redox state of the atmosphere.

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 Earths 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 waters 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.

ATMOSPHERIC SULFUR PHOTOCHEMISTRY ON HOT JUPITERS

We develop a new one-dimensional photochemical kinetics code to address stratospheric chemistry and stratospheric heating in hot Jupiters. Here we address optically active S-containing species and CO2 at 1200 ≤ T ≤ 2000 K. HS (mercapto) and S2 are highly reactive species that are generated photochemically and thermochemically from H2S with peak abundances between 1 and 10 mbar. S2 absorbs UV between 240 and 340 nm and is optically thick for metallicities [S/H]>0 at T ≥ 1200 K. HS is probably more important than S2, as it is generally more abundant than S2 under hot Jupiter conditions and it absorbs at somewhat redder wavelengths. We use molecular theory to compute an HS absorption spectrum from sparse available data and find that HS should absorb strongly between 300 and 460 nm, with absorption at the longer wavelengths being temperature sensitive. When the two absorbers are combined, radiative heating (per kg of gas) peaks at 100 μbars, with a total stratospheric heating of ~8 × 104 W m–2 for a jovian planet orbiting a solar-twin at 0.032 AU. Total heating is insensitive to metallicity. The CO2 mixing ratio is a well behaved quadratic function of metallicity, ranging from 1.6 × 10–8 to 1.6 × 10–4 for –0.3 < [M/H] < 1.7. CO2 is insensitive to insolation, vertical mixing, temperature (1200 < T < 2000), and gravity. The photochemical calculations confirm that CO2 should prove a useful probe of planetary metallicity.

Initiation of clement surface conditions on the earliest Earth

In the beginning the surface of the Earth was extremely hot, because the Earth as we know it is the product of a collision between two planets, a collision that also created the Moon. Most of the heat within the very young Earth was lost quickly to space while the surface was still quite hot. As it cooled, the Earths surface passed monotonically through every temperature regime between silicate vapor to liquid water and perhaps even to ice, eventually reaching an equilibrium with sunlight. Inevitably the surface passed through a time when the temperature was around 100°C at which modern thermophile organisms live. How long this warm epoch lasted depends on how long a thick greenhouse atmosphere can be maintained by heat flow from the Earths interior, either directly as a supplement to insolation, or indirectly through its influence on the nascent carbonate cycle. In both cases, the duration of the warm epoch would have been controlled by processes within the Earths interior where buffering by surface conditions played little part. A potentially evolutionarily significant warm period of between 105 and 107 years seems likely, which nonetheless was brief compared to the vast expanse of geological time.

MATERIAL ENHANCEMENT IN PROTOPLANETARY NEBULAE BY PARTICLE DRIFT THROUGH EVAPORATION FRONTS

Solid material in a protoplanetary nebula is subject to vigorous redistribution processes relative to the nebula gas. Meter-sized particles drift rapidly inward near the nebula midplane, and material evaporates when the particles cross a condensation/evaporation boundary. The material cannot be removed as fast in its vapor form as it is being supplied in solid form, so its concentration increases locally by a large factor (more than an order of magnitude under nominal conditions). As time goes on, the vapor-phase enhancement propagates for long distances inside the evaporation boundary (potentially all the way into the star). Meanwhile, material is enhanced in its solid form over a characteristic length scale outside the evaporation boundary. This effect is applicable to any condensible (water, silicates, etc.). Three distinct radial enhancement/depletion regimes can be discerned by use of a simple model. Meteoritic applications include oxygen fugacity and isotopic variations, as well as isotopic homogenization in silicates. Planetary system applications include more robust enhancement of solids in Jupiters core formation region than previously suggested. Astrophysical applications include differential, time-dependent enhancement of vapor phase CO and H2O in the terrestrial planet regions of actively accreting protoplanetary disks.

Earth’s Earliest Atmospheres

Earth is the one known example of an inhabited planet and to current knowledge the likeliest site of the one known origin of life. Here we discuss the origin of Earths atmosphere and ocean and some of the environmental conditions of the early Earth as they may relate to the origin of life. A key punctuating event in the narrative is the Moon-forming impact, partly because it made Earth for a short time absolutely uninhabitable, and partly because it sets the boundary conditions for Earths subsequent evolution. If life began on Earth, as opposed to having migrated here, it would have done so after the Moon-forming impact. What took place before the Moon formed determined the bulk properties of the Earth and probably determined the overall compositions and sizes of its atmospheres and oceans. What took place afterward animated these materials. One interesting consequence of the Moon-forming impact is that the mantle is devolatized, so that the volatiles subsequently fell out in a kind of condensation sequence. This ensures that the volatiles were concentrated toward the surface so that, for example, the oceans were likely salty from the start. We also point out that an atmosphere generated by impact degassing would tend to have a composition reflective of the impacting bodies (rather than the mantle), and these are almost without exception strongly reducing and volatile-rich. A consequence is that, although CO- or methane-rich atmospheres are not necessarily stable as steady states, they are quite likely to have existed as long-lived transients, many times. With CO comes abundant chemical energy in a metastable package, and with methane comes hydrogen cyanide and ammonia as important albeit less abundant gases.

Mass fractionation of noble gases in diffusion-limited hydrodynamic hydrogen escape

Mass fractionation by hydrodynamic hydrogen escape is a promising mechanism for explaining the observed elemental and isotopic abundance patterns in terrestrial planet atmospheres. Previous work has considered only pure hydrogen winds. Here, the theory of mass fractionation by hydrogen escape is extended to atmospheres in which hydrogen is not the only major constituent. Analytical solutions are derived for cases in which all relevant atmospheric constituents escape; both analytical and numerical solutions are obtained for cases in which important heavy constituents are retained. In either case the fractionation patterns that result can differ significantly from those produced by pure hydrogen winds. Three applications of the theory are discussed: (1) The observed fractionation of terrestrial atmospheric neon with respect to mantle neon can be explained as a by-product of diffusion-limited hydrogen escape from a steam atmosphere toward the end of accretion. (2) The anomalously high Martian (SNC) 38Ar/36Ar ratio is attributed to hydrodynamic fractionation by a vigorously escaping, nearly pure hydrogen wind. (3) It is possible that the present high Martian D/H ratio was established during the same hydrodynamic escape phase that fractionated argon, but the predicted degree of D/H enrichment is sensitive to other, less well constrained parameters.

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 °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.