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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Understanding Microbial Contributions to Planetary Atmosphere

Should our search of distant, extrasolar planetary atmospheres encounter evidence of life, that evidence will most likely be the gaseous products of microorganisms. Our biosphere was exclusively microbial for over 80 percent of its history and, even today, microbes strongly influence atmospheric composition. Life's greatest environmental impact arises from its capacity for harvesting energy and creating organic matter. Microorganisms catalyze the equilibration of C, S and transition metal species at temperatures where such reactions can be very slow in the absence of life. Sunlight has been harvested through photosynthesis to create enormous energy reservoirs that exist in the form of coexisting reservoirs of reduced, organic C and S stored in Earth's crust, and highly oxidized species (oxygen, sulfate and ferric iron) stored in the crust, oceans and atmosphere. Our civilization taps that storehouse of energy by burning fossil fuels. As astrobiologists, we identify the chemical consequences of distant biospheres as expressed in the atmospheres of their planets. Our approach must recognize that planets, biospheres and atmospheres evolve and change. For example, a tectonically more active early Earth hosted a thermophilic, non-photosynthetic biosphere and a mildly reducing, carbon dioxide-rich and oxygen-poor atmosphere. Microorganisms acquired energy by consuming hydrogen and sulfide and producing a broad array of reduced C and S gases, most notably, methane. Later, diverse types of bacterial photosynthesis developed that enhanced productivity but were incapable of splitting water to produce oxygen. Later, but still prior to 2.6 billion years ago, oxygenic photosynthesis developed. We can expect to encounter distant biospheres that represent various stages of evolution and that coexist with atmospheres ranging from mildly reducing to oxidizing compositions. Accordinaly, we must be prepared to interpret a broad range of atmospheric compositions, all containing signatures of life. Remarkably little is known about the composition of our own earlier atmosphere, particularly prior to the rise of oxygen levels some 2.0 to 2.2 billion years ago. Thus, field and laboratory observations and theoretical simulations should be conducted to examine the relationships between the structure and function of microbial ecosystems and their gaseous products. Ecosystems that are analogs of our ancient biosphere (e.g., based upon chemosynthesis or non-oxygenic photosynthesis, thermophilic and subsurface communities, etc.) should be included. Because key environmental parameters such as temperature and levels of hydrogen, carbon dioxide and oxygen varied during planetary evolution, their consequences for microbial ecosystems should be explored.

DesMarais, David J.↗
NASA NTRS2000

Latest Word on Retreat of the West Antarctic Ice Sheet

The West Antarctic ice sheet during the Last Glacial Maximum (LGM) is estimated to have been three times its present volume and to have extended close to the edge of the continental shelf Holocene retreat of this ice sheet in the Ross Sea began between 11,000 and 12,000 years ago. This history implies an average contribution of this ice sheet to sea level of 0.9 mm/a. Evidence of dateable past grounding line positions in the Ross sector are broadly consistent with a linear retreat model. However, inferred rates of retreat for some of these grounding line positions are not consistent with a linear retreat model. More rapid retreat approximately 7600 years ago and possible near-stability in the Ross Sea sector at present suggest a slow rate of initial retreat followed by a more rapid-than-average retreat during the late Holocene, returning to a near-zero rate of retreat currently. This model is also consistent with the mid-Holocene high stand observations of eustatic sea level. Recent compilation of Antarctic bed elevations (BEDMAP) illustrates that the LGM and present grounding lines occur in the shallowest waters, further supporting the model of a middle phase of rapid retreat bracketed by an older and a more recent phase of modest retreat. Extension of these hypotheses into the future make subsequent behavior of the West Antarctic ice sheet more difficult to predict but suggest that if it loses its hold on the present shallow bed, the final retreat of the ice sheet could be very rapid.

Bindschadler, R.↗
NASA NTRS2000

Ubiquitous CM and DM

Ubiquitous is a real word. I thank a former Total Quality Coach for my first exposure some years ago to its existence. My version of Webster's dictionary defines ubiquitous as "present, or seeming to be present, everywhere at the same time; omnipresent." While I believe that God is omnipresent, I have come to discover that CM and DM are present everywhere. Oh, yes; I define CM as Configuration Management and DM as either Data or Document Management. Ten years ago, I had my first introduction to the CM world. I had an opportunity to do CM for the Space Station effort at the NASA Lewis Research Center. I learned that CM was a discipline that had four areas of focus: identification, control, status accounting, and verification. I was certified as a CMIl graduate and was indoctrinated about clear, concise, and valid. Off I went into a world of entirely new experiences. I was exposed to change requests and change boards first hand. I also learned about implementation of changes, and then of technical and CM requirements.

Crowley, Sandra L.↗
NASA NTRS2000

Water and Life on Mars

Mars appears to be cold dry and dead world. However there is good evidence that early in its history it had liquid water, more active volcanism, and a thicker atmosphere. Mars had this earth-like environment over three and a half billion years ago, during the same time that life appeared on Earth. The main question in the exploration of Mars then is the search for a independent origin of life on that planet. Ecosystems in cold, dry locations on Earth - such as the Antarctic - provide examples of how life on Mars might have survived and where to look for fossils. Although the Viking results may indicate that Mars has no life today, there is direct geomorphological evidence that, in the past, Mars had large amounts of liquid water on its surface - possibly due to a thicker atmosphere. From a biological perspective the existence of liquid water, by itself motivates the question of the origin of life on Mars. One of the martian meteorites dates back to this early period and may contain evidence consistent with life. From studies of the Earth's earliest biosphere we know that by 3.5 Gyr. ago, life had originated on Earth and reached a fair degree of biological sophistication. Surface activity and erosion on Earth make it difficult to trace the history of life before the 3.5 Gyr timeframe. Ecosystems in cold, dry locations on Earth - such as the Antarctic - provide examples of how life on Mars might have survived and where to look for fossils.

McKay, Christopher P.↗
NASA NTRS2000

Where is Extraterrestrial Life Likely to be Found

Mars appears to be cold dry and dead world. However there is good evidence that early in its history it had liquid water, more active volcanism, and a thicker atmosphere. Mars had this earthlike environment over three and a half billion years ago, during the same time that life appeared on Earth. The main question in the exploration of Mars then is the search for a independent origin of life on that planet. Ecosystems in cold, dry locations on Earth - such as the Antarctic - provide examples of how life on Mars might have survived and where to look for fossils. Although the Viking results may indicate that Mars has no life today, there is direct geomorphological evidence that, in the past, Mars had large amounts of liquid water on its surface - possibly due to a thicker atmosphere. From a biological perspective the existence of liquid water, by itself motivates the question of the origin of life on Mars. One of the martian meteorites dates back to this early period and may contain evidence consistent with Ye. From studies of the Earth's earliest biosphere we know that by 3.5 Gyr. ago, life had originated on Earth and reached a fair degree of biological sophistication. Surface activity and erosion on Earth make it difficult to trace the history of life before the 3.5 Gyr timeframe. Ecosystems in cold, dry locations on Earth - such as the Antarctic provide examples of how life on Mars might have survived and where to look for fossils.

McKay, Christopher P.↗
NASA NTRS2000

Life On Mars: Past, Present and Future

Mars appears to be cold dry and dead world. However there is good evidence that early in its history it had liquid water, more active volcanism, and a thicker atmosphere. Mars had this earth-like environment over three and a half billion years ago, during the same time that life appeared on Earth. The main question in the exploration of Mars then is the search for a independent origin of life on that planet. Ecosystems in cold, dry locations on Earth - such as the Antarctic - provide examples of how life on Mars might have survived and where to look for fossils. Although the Viking results may indicate that Mars has no life today, there is direct geomorphological evidence that, in the past, Mars had large amounts of liquid water on its surface - possibly due to a thicker atmosphere. From a biological perspective the existence of liquid water, by itself motivates the question of the origin of life on Mars. One of the martian meteorites dates back to this early period and may contain evidence consistent with life. From studies of the Earth's earliest biosphere we know that by 3.5 Cyr. ago, life had originated on Earth and reached a fair degree of biological sophistication. Surface activity and erosion on Earth make it difficult to trace the history of life before the 3.5 Cyr timeframe. Ecosystems in cold, dry locations on Earth - such as the Antarctic - provide examples of how life on Mars might have survived and where to look for fossils. Human exploration of Mars will probably begin with a small base manned by a temporary crew, a necessary first start. But exploration of the entire planet will require a continued presence on the Martian surface and the development of a self sustaining community in which humans can live and work for very long periods of time. A permanent Mars research base can be compared to the permanent research bases which several nations maintain in Antarctica at the South Pole, the geomagnetic pole, and elsewhere. In the long run, a continued human presence on Mars will be the most economical way to study that planet in detail. It is possible that at some time in the future we might recreate a habitable climate on Mars, returning it to the life-bearing state it may have enjoyed early in its history. Our studies of Mars are still in a preliminary state but everything we have learned suggests that it may be possible to restore Mars to a habitable climate.

McKay, Christopher P.↗
NASA NTRS2001

Higher Flux from the Young Sun as an Explanation for Warm Temperatures for Early Earth and Mars

Observations indicate that the Earth was at least warm enough for liquid water to exist as far back as 4 Gyr ago, namely, as early as half a billion years after the formation of the Earth; in fact, there is evidence suggesting that Earth may have been even warmer then than it is now. These relatively warm temperatures required on early Earth are in apparent contradiction to the dimness of the early Sun predicted by the standard solar models. This problem has generally been explained by assuming that Earth's early atmosphere contained huge amounts of carbon dioxide (CO2), resulting in a large enough greenhouse effect to counteract the effect of a dimmer Sun. However, recent work places an upper limit of 0.04 bar on the partial pressure of CO2 in the period from 2.75 to 2.2 Gyr ago, based on the absence of siderite in paleosols; this casts doubt on the viability of a strong CO2 greenhouse effect on early Earth. The existence of liquid water on early Mars has been even more of a puzzle; even the maximum possible CO2 greenhouse effect cannot yield warm enough Martian surface temperatures. These problems can be resolved simultaneously for both Earth and Mars, if the early Sun was brighter than predicted by the standard solar models. This could be accomplished if the early Sun was slightly more massive than it is now, i.e., if the solar wind was considerably stronger in the past than at present. A slightly more massive young Sun would have left fingerprints on the internal structure of the present Sun. Today, helioseismic observations exist that can measure the internal structure of the Sun with very high precision. The task undertaken here was to compute solar models with the highest precision possible at this time, starting with slightly greater initial masses. These were evolved to the present solar age, where comparisons with the helioseismic observations could be made. Our computations also yielded the time evolution of the solar flux at the planets - a key input to the climates of early Earth and Mars. Early solar mass loss is not the only influence that can alter the internal structure of the present Sun. There are minor uncertainties in the physics of the solar models and in the key observed solar parameters that also affect the present Sun's internal structure. It was therefore imperative to obtain an understanding of the effects of these other uncertainties, in order to disentangle them from the fingerprints that might be left by early solar mass loss. From these considerations, our work was divided into two parts: (1) We first computed the evolution of standard solar models with input parameters varied within their uncertainties, to determine their effect on the observable helioseismic quantities; (2) We then computed non-standard solar models with higher initial masses to test against the helioseismological observations.

Sackmann, I.-Juliana↗
NASA NTRS2001

Obliquity Modulation of the Incoming Solar Radiation

Based on a basic principle of orbital resonance, we have identified a huge deficit of solar radiation induced by the combined amplitude and frequency modulation of the Earth's obliquity as possibly the causal mechanism for ice age glaciation. Including this modulation effect on solar radiation, we have performed model simulations of climate change for the past 2 million years. Simulation results show that: (1) For the past 1 million years, temperature fluctuation cycles were dominated by a 100-Kyr period due to amplitude-frequency resonance effect of the obliquity; (2) From 2 to 1 million years ago, the amplitude-frequency interactions. of the obliquity were so weak that they were not able to stimulate a resonance effect on solar radiation; (3) Amplitude and frequency modulation analysis on solar radiation provides a series of resonance in the incoming solar radiation which may shift the glaciation cycles from 41-Kyr to 100-Kyr about 0.9 million years ago. These results are in good agreement with the marine and continental paleoclimate records. Thus, the proposed climate response to the combined amplitude and frequency modulation of the Earth's obliquity may be the key to understanding the glaciation puzzles in paleoclimatology.

Liu, Han-Shou↗
NASA NTRS1996

Comet Shoemaker-Levy 9, Jupiter, and Impact Shock Chemistry

Four years ago this month, a hitherto unknown comet in loose orbit around Jupiter passed so near the giant planet that it was torn apart into 20 fragments by tides. One orbit later, two years ago this month, the fragments of doomed comet P/Shoemaker Levy (SL9) fell into Jupiter. The enormous energies of these impacts (the largest fragments were nearly 1 km across and, hitting at 60 km/s, released some 2-4 x 10(exp 27) ergs) produced enormous explosions. Several of the ejecta plumes were imaged towering 3000 km above Jupiter's limb. The heat released when the plumes fell was considerable and easily observed on Earth. The impacts produced strong shocks, both promptly at the impact site and again, later, and over thousands of kilometers, when the ejecta plume reentered the atmosphere. The focus of this talk will be to discuss what the SL9 impacts taught us about impact shock chemistry - the processes, the ingredients, the results - and what inferences we may draw for impacts on early Earth. Shock chemistry generates a suite of molecules not usually seen on Jupiter. The most surprising report was of a huge amount of diatomic sulfur S2 at the site of the G impact. Other unusual products include CS, CS2, OCS, H2S, SO2, HCN, CO, and H2O; although H2S and H2O are doubtless abundant below the visible clouds. Hot or enhanced CH4 and NH3 were also detected. A general rule of shock chemistry is that CO forms until either C or O is exhausted. If O greater than C, the other products are oxidized, and excess O goes to H2O. If C greater than O, the other products are reduced, and excess C goes to HCN, C2H2, and a wide variety of more complicated organics. Ultimately, given time, the carbon would react all the way to graphite, but in practice the reactions are incomplete. The dark ejecta debris were probably composed in part of carbonaceous particles generated by the shocks. In a sense, the SL9 impacts performed the famous Miller-Urey experiment on a grand scale, with one result being the production of a lot of complex brown organic solids (called "tholins"). We use, a straightforward chemical kinetics model for the H, N, C, O, S system to follow the nonequilibrium chemistry behind the shocks. The model traces the evolving chemical composition of a parcel of gas by directly integrating the web of chemical reactions. Pressure and temperature histories of the parcels are patterned after those calculated by numerical hydrodynamic simulations of the ejecta plume. A given plume parcel is generally shocked twice; t.e a parcel shocked near the impact site is ejected at high velocity and is shocked again when it reenters the atmosphere. The final state of the gas depends mostly on the second shock, provided that the latter is hot enough. The chemical evidence is ambiguous, but most indications are that C greater than O in the shocked, reacting gas. Telltale signatures of abundant oxygen - SO2, SO, CO2, O2 - were not seen, while signatures of abundant carbon - CS, CS2, and HCN - were. On the other hand, abundant H2O would appear to require O greater than C, and two other observed sulfur species, S2 and OCS, appear to form more easily in a somewhat oxidized gas, presumable vaporized from the comet itself. Since on general principles one expects the -comet to have had a more-or-less cosmic composition, i.e. O greater than C, the production of CS, CS2, and HCN probably requires C greater than O in the shocked jovian air. This in turn implies that even the largest fragments released the bulk of their energy above the jovian water table, in all likelihood above 5 bars . There is no evidence in favor of the proposition that a significant amount of wet jovian air was shocked strongly enough to coax water to react; i.e. wet jovian air saw only temperatures significantly below 2000 K.

Zahnle, Kevin↗
NASA NTRS1999

Absolute Rovibrational Intensities for the Chi(sup 1)Sigma(sup +) v=3 <-- 0 Band of (12)C(16)O Obtained with Kitt Peak and BOMEM FTS Instruments

This work was initiated to compare absolute line intensities retrieved with the Kitt Peak FTS (Fourier Transform Spectrometer) and Ames BOMEM FTS. Since thermal contaminations can be a problem using the BOMEM instrument if proper precautions are not taken it was thought that measurements done at 6300 per cm would more easily result in satisfactory intercomparisons. Very recent measurements of the CO 3 <-- 0 band fine intensities confirms results reported here that the intensities listed in HITRAN (High Resolution Molecular Absorption Database) for this band are on the order of six to seven percent too low. All of the infrared intensities in the current HITRAN tabulation are based on the electric dipole moment function reported fifteen years ago. The latter in turn was partly based on intensities for the 3 <-- 0 band reported thirty years ago. We have, therefore, redetermined the electric dipole moment function of ground electronic state CO.

Chackerian, Charles, Jr.↗
NASA NTRS2002

Implementing Production Grids

We have presented the essence of experience gained in building two production Grids, and provided some of the global context for this work. As the reader might imagine, there were a lot of false starts, refinements to the approaches and to the software, and several substantial integration projects (SRB and Condor integrated with Globus) to get where we are today. However, the point of this paper is to try and make it substantially easier for others to get to the point where Information Power Grids (IPG) and the DOE Science Grids are today. This is what is needed in order to move us toward the vision of a common cyber infrastructure for science. The author would also like to remind the readers that this paper primarily represents the actual experiences that resulted from specific architectural and software choices during the design and implementation of these two Grids. The choices made were dictated by the criteria laid out in section 1. There is a lot more Grid software available today that there was four years ago, and various of these packages are being integrated into IPG and the DOE Grids. However, the foundation choices of Globus, SRB, and Condor would not be significantly different today than they were four years ago. Nonetheless, if the GGF is successful in its work - and we have every reason to believe that it will be - then in a few years we will see that the 28 functions provided by these packages will be defined in terms of protocols and MIS, and there will be several robust implementations available for each of the basic components, especially the Grid Common Services. The impact of the emerging Web Grid Services work is not yet clear. It will likely have a substantial impact on building higher level services, however it is the opinion of the author that this will in no way obviate the need for the Grid Common Services. These are the foundation of Grids, and the focus of almost all of the operational and persistent infrastructure aspects of Grids.

Johnston, William E.↗
NASA NTRS2003

Impact Craters of Venus with D Greater Than 5 km Classified Based on Degree of Preservation of the Associated Radar-Dark Deposits

This is a further continuation of work, which studied craters greater than or equal to 30 km in diameter. That work subdivided craters based on character of the associated radar dark deposits. It was suggested and then confirmed that the most pristine deposits of that sort are radar-dark parabolas. Non-parabolic radar-dark halos represent the next stage of the deposit evolution and then with time they disappear. So presence and character of crater-associated dark deposit can be used for estimates of the crater age and then for dating other features. Previous work classified craters into: 1) craters with dark parabola (DP), 2) with clear dark halo (CH), 3) with faint halo (FH) and 4) with no dark halo (NH). It was found that abundances of craters superposed on regional plains (whose mean age is close to the planet mean surface age T) and belonging to DP, CH, FH and NH classes were correspondingly 15, 30, 30 and 25%. From that it was concluded that DP craters are not older than 0.1-0.15T; CH craters formed during the time interval from approx. 0.5T until 0.1-0.15T ago, and the FH and NH craters formed prior to approx. 0.5T ago. It was shown that the DP, CH, FH and NH percentages show only slight apparent dependence on the crater geographic latitudes and no noticeable dependence on the crater size. The present study analyzes a much larger population (all D greater than or equal to 5 km craters) to investigate better the latitude effect and to study if within this larger crater population the size effect exists.

Basilevsky, A. T.↗
NASA NTRS2004

Recent Progress in Understanding the Sun's Magnetic Dynamo

100 years ago we thought that the Sun and stars shone as a result of slow gravitational contraction over a few tens of millions of years - putting astronomers at odds with geologists who claimed that the Earth was much, much older. That mystery was solved in the 1920s and 30s with the discovery of nuclear energy (proving that the geologists had it right all along). Other scientific mysteries concerning the Sun have come and gone but three major mysteries remain: 1) How does the Sun produce sunspots with an 11-year cycle? 2) What produces the huge explosions that result in solar flares, prominence eruptions, and coronal mass ejections? and 3) Why is the Sun's outer atmosphere, the corona, so darned hot? Recent progress in solar astronomy reveals a single key to understanding all three of these mysteries.The 11-year time scale for the sunspot cycle indicates the presence of a magnetic dynamo within the Sun. For decades this dynamo was though to operate within the Sun's convection zone - the outmost 30% of the Sun where convective currents transport heat and advect magnetic lines of force. The two leading theories for the dynamo had very different models for the dynamics of the convection zone. Actual measurements of the dynamics using the techniques of helioseismology showed that both of these models had to be wrong some 20 years ago. A thin layer of strongly sheared flow at the base of the convection zone (now called the tachocline) was then taken to be the seat of the dynamo. Over the last 10 years it has become apparent that a weak meridional circulation within the convection zone also plays a key role in the dynamo. This meridional circulation has plasma rising up from the tachocline in the equatorial regions, spreading out toward the poles at a top speed of about 10-20 m/s at the surface, sinking back down to the tachocline in the polar regions, and then flowing back toward the equator at a top speed of about 1-2 m/s in the tachocline itself. Recent dynamo models that include this meridional flow now appear to have some power for predicting the size of future sunspot cycles.

Hathaway, David. H.↗
NASA NTRS1998

Past, present, and future life on Mars

Although the Viking results indicated that the surface of Mars is dry and lifeless, there is direct geomorphological evidence that Mars had large amounts of liquid water on its surface in the past. From a biological perspective the existence of liquid water, by itself, motivates the question of the origin of life on Mars. One of the martian meteorites dates back to this early period and may contain evidence consistent with life. The Mars environment 3.5 to 4.0 Gyr ago was comparable to that on the Earth at this time in that both contained liquid water. Life had originated on Earth and reached a fair degree of biological sophistication by 3.5 Gyr ago. To determine if life similarly arose on Mars may require extensive robotic exploration and ultimately human exploration. Intensive exploration of Mars will require a continued presence on the Martian surface and the development of a self sustaining community in which humans can live and work for very long periods of time. A permanent Mars research station can obtain its life support requirements directly from the martian environment enabling a high degree of self-sufficiency. In the longer term, it is possible that in the future we might restore a habitable climate on Mars, returning it to the life-bearing state it may have enjoyed early in its history.

NASA Center ARC↗
NASA NTRS1993

Did surface temperatures constrain microbial evolution?

The proposition that glaciation may not have occurred before the Cenozoic--albeit not yet a consensus position--nevertheless raises for reconsideration the surface temperature history of the earth. Glacial episodes, from the Huronian (2.3 billion years ago; BYA) through the late Paleozoic (320 to 250 million years ago; MYA) have been critical constraints on estimation of the upper bounds of temperature (Crowley 1983, Kasting and Toon 1989). Once removed, few if any constraints on the upper temperature limit other than life remain. Walker (1982) recognized that life provides an upper limit to temperature in the Precambrian. We propose a more radical concept: the upper temperature limit for viable growth of a given microbial group corresponds to the actual surface temperature at the time of the group's first appearance. In particular, we propose here that two major evolutionary developments--the emergence of cyanobacteria and aerobic eukaryotes--can be used to determine surface temperature in the Precambrian, and that only subsequent cooling mediated by higher plants and then angiosperms permitted what may possibly be the earth's first glaciation in the late Cenozoic.

NASA Program Exobiology↗
NASA NTRS1994

Thinking of biology: asteroid impacts, microbes, and the cooling of the atmosphere

The authors examine the cooling of the Earth's surface from 3.75 to 1 billion years ago. Three effects of the bombardment of Earth by asteroids and comets that may have delayed surface cooling include time to form continents, volatilization of carbonate rocks which released carbon dioxide into the atmosphere, and inability of microbes to inhabit land masses during large impact events. Continental microbes may have helped reduce high temperatures from 3.75 to 3.5 billion years ago. If so, the evolutionary sequence of microbes is proposed to be anaerobic heterotrophs, chemoautotrophs, and then photoautotrophs.

Non-NASA Center↗
NASA NTRS1993

3.4-Billion-year-old biogenic pyrites from Barberton, South Africa: sulfur isotope evidence

Laser ablation mass spectroscopy analyses of sulfur isotopic compositions of microscopic-sized grains of pyrite that formed about 3.4 billion years ago in the Barberton Greenstone Belt, South Africa, show that the pyrite formed by bacterial reduction of seawater sulfate. These data imply that by about 3.4 billion years ago sulfate-reducing bacteria had become active, the oceans were rich in sulfate, and the atmosphere contained appreciable amounts (>>10(-13) of the present atmospheric level) of free oxygen.

NASA Discipline Exobiology↗
NASA NTRS1991

Box models for the evolution of atmospheric oxygen: an update

A simple 3-box model of the atmosphere/ocean system is used to describe the various stages in the evolution of atmospheric oxygen. In Stage I, which probably lasted until redbeds began to form about 2.0 Ga ago, the Earth's surface environment was generally devoid of free O2, except possibly in localized regions of high productivity in the surface ocean. In Stage II, which may have lasted for less than 150 Ma, the atmosphere and surface ocean were oxidizing, while the deep ocean remained anoxic. In Stage III, which commenced with the disappearance of banded iron formations around 1.85 Ga ago and has lasted until the present, all three surface reservoirs contained appreciable amounts of free O2. Recent and not-so-recent controversies regarding the abundance of oxygen in the Archean atmosphere are identified and discussed. The rate of O2 increase during the Middle and Late Proterozoic is identified as another outstanding question.

NASA Program Exobiology↗