A Faint Young Sun Paradox Solution for Mars

Transient reducing greenhouse warming on early Mars

Authors:

Wordsworth et al

Abstract:

The evidence for abundant liquid water on early Mars despite the faint young Sun is a long-standing problem in planetary research. Here we present new ab initio spectroscopic and line-by-line climate calculations of the warming potential of reduced atmospheres on early Mars. We show that the strength of both CO2-H2 and CO2-CH4 collision-induced absorption (CIA) has previously been significantly underestimated. Contrary to previous expectations, methane could have acted as a powerful greenhouse gas on early Mars due to CO2-CH4 CIA in the critical 250-500 cm^-1 spectral window region. In atmospheres of 0.5 bar CO2 or more, percent levels of H2 or CH4 raise annual mean surface temperatures by tens of degrees, with temperatures reaching 273 K for pressures of 1.25-2 bar and 2-10% of H2 and CH4. Methane and hydrogen produced following aqueous alteration of Mars' crust could have combined with volcanically outgassed CO2 to form transient atmospheres of this composition 4.5-3.5 Ga. This scenario for the late Noachian climate can be tested via future in situ and orbital studies of the martian crust.

Methane Did NOT Warm the Early Earth

For at least a billion years of the distant past, planet Earth should have been frozen over but wasn't. Scientists thought they knew why, but a new modeling study from the Alternative Earths team of the NASA Astrobiology Institute has fired the lead actor in that long-accepted scenario.

Humans worry about greenhouse gases, but between 1.8 billion and 800 million years ago, microscopic ocean dwellers really needed them. The sun was 10 to 15 percent dimmer than it is today--too weak to warm the planet on its own. Earth required a potent mix of heat-trapping gases to keep the oceans liquid and livable.

For decades, atmospheric scientists cast methane in the leading role. The thinking was that methane, with 34 times the heat-trapping capacity of carbon dioxide, could have reigned supreme for most of the first 3.5 billion years of Earth history, when oxygen was absent initially and little more than a whiff later on. (Nowadays oxygen is one-fifth of the air we breathe, and it destroys methane in a matter of years.)

"A proper accounting of biogeochemical cycles in the oceans reveals that methane has a much more powerful foe than oxygen," said Stephanie Olson, a graduate student at the University of California, Riverside, a member of the Alternative Earths team and lead author of the new study published September 26 in the Proceedings of the National Academy of Sciences. "You can't get significant methane out of the ocean once there is sulfate."

Sulfate wasn't a factor until oxygen appeared in the atmosphere and triggered oxidative weathering of rocks on land. The breakdown of minerals such as pyrite produces sulfate, which then flows down rivers to the oceans. Less oxygen means less sulfate, but even 1 percent of the modern abundance is sufficient to kill methane, Olson said.

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Simulating the Earth's Hazy Archean Atmosphere

An atmospheric haze around a faraway planet -- like the one which probably shrouded and cooled the young Earth -- could show that the world is potentially habitable, or even be a sign of life itself.

Astronomers often use the Earth as a proxy for hypothetical exoplanets in computer modeling to simulate what such worlds might be like and under what circumstances they might be hospitable to life.

In new research from the University of Washington-based Virtual Planetary Laboratory, UW doctoral student Giada Arney and co-authors chose to study Earth in its Archean era, about 2 ½ billion years back, because it is, as Arney said, "the most alien planet we have geochemical data for."

The work builds on geological data from other researchers that suggests the early Earth was intermittently shrouded by an organic pale orange haze that came from light breaking down methane molecules in the atmosphere into more complex hydrocarbons, organic compounds of hydrogen and carbon.

"Hazy worlds seem common both in our solar system and in the population of exoplanets we've characterized so far," Arney said. "Thinking about Earth with a global haze allows us to put our home planet into the context of these other worlds, and in this case, the haze may even be a sign of life itself."

Arney and co-authors will present their findings Nov. 11 at the American Astronomical Society's Division of Planetary Sciences conference in National Harbor, Maryland.

The researchers used photochemical, climate and radiation simulations to examine the early Earth shrouded by a "fractal" hydrocarbon haze, meaning that the imagined haze particles are not spherical, as used in many such simulations, but agglomerates of spherical particles, bunched together not unlike grapes, but smaller than a raindrop. A fractal haze, they found, would have significantly lowered the planetary surface temperature.

However, they also found the cooling would be partly countered by concentrations of greenhouse gases that tend to warm a planet. They saw that this combination would result in a moderate, possibly habitable average global temperature.

Such a haze, the researchers found, also would have absorbed ultraviolet light so well as to effectively shield the Archean Earth from deadly radiation before the rise of oxygen and the ozone layer, which now provides that protection. The haze was a benefit to just-evolving surface biospheres on Earth, as it could be to similar exoplanets.

The researchers also found that, based on the early Earth data, it's unlikely such a haze would be formed by abiotic, or nonliving means. So for exoplanets with Earthlike amounts of carbon dioxide in their atmospheres, Arney said, "organic haze might be a novel type of biosignature. However, we know these hazes can also form without life on worlds like Saturn's moon Titan, so we are working to come up with more ways to distinguish biological hazes from abiotic ones."

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28 Potential Archean PaleoAtmospheric Greenhouse Gases Examined

Radiative forcings for 28 potential Archean greenhouse gases

Authors:

Byrne et al

Abstract:

Despite reduced insolation in the late Archean, evidence suggests a warm climate which was likely sustained by a stronger greenhouse effect, the so-called Faint Young Sun Problem (FYSP). CO2 and CH4 are generally thought to be the mainstays of this enhanced greenhouse, though many other gases have been proposed. We present high accuracy radiative forcings for CO2, CH4 and 26 other gases, performing the radiative transfer calculations at line-by-line resolution and using HITRAN 2012 line data for background pressures of 0.5, 1, and 2 bar of atmospheric N2. For CO2 to resolve the FYSP alone at 2.8 Gyr BP (80% of present solar luminosity), 0.32 bar is needed with 0.5 bar of atmospheric N2, 0.20 bar with 1 bar of atmospheric N2, or 0.11 bar with 2 bar of atmospheric N2. For CH4, we find that near-infrared absorption is much stronger than previously thought, arising from updates to the HITRAN database. CH4 radiative forcing peaks at 10.3, 9, or 8.3 Wm-2 for background pressures of 0.5, 1 or 2 bar, likely limiting the utility of CH4 for warming the Archean. For the other 26 HITRAN gases, radiative forcings of up to a few to 10 Wm-2 are obtained from concentrations of 0.1-1 ppmv for many gases. For the 20 strongest gases, we calculate the reduction in radiative forcing due to overlap. We also tabulate the modern sources, sinks, concentrations and lifetimes of these gases and summaries the literature on Archean sources and concentrations. We recommend the forcings provided here be used both as a first reference for which gases are likely good greenhouse gases, and as a standard set of calculations for validation of radiative forcing calculations for the Archean.

Composition & Density of the Archean Atmosphere Based on 3+ Billion Year old Fuild Inclusions in Hydrothermal Quartz

Nitrogen Isotopic Composition and Density of the Archean Atmosphere

Authors:

Marty et al

Abstract:

Understanding the atmosphere's composition during the Archean eon is a fundamental issue to unravel ancient environmental conditions. We show from the analysis of nitrogen and argon isotopes in fluid inclusions trapped in 3.0-3.5 Ga hydrothermal quartz that the PN2 of the Archean atmosphere was lower than 1.1 bar, possibly as low as 0.5 bar, and had a nitrogen isotopic composition comparable to the present-day one. These results imply that dinitrogen did not play a significant role in the thermal budget of the ancient Earth and that the Archean PCO2 was probably lower than 0.7 bar.

Beyond Extraordinary Claim: Gravitational Constant is Variable (& its a solution to the faint sun paradox!)

Can a variable gravitational constant resolve the Faint Young Sun Paradox ?

Authors:

Sahni et al

Abstract:

Solar models suggest that four billion years ago the young Sun was about 75% fainter than it is today, rendering Earth's oceans frozen and lifeless. However, there is ample geophysical evidence that Earth had a liquid ocean teeming with life 4 Gyr ago. Since L⊙∝G7M5⊙, the Sun's luminosity L⊙ is exceedingly sensitive to small changes in the gravitational constant G. We show that a percent-level increase in G in the past would have prevented Earth's oceans from freezing, resolving the faint young Sun paradox. Such small changes in G are consistent with observational bounds on ΔG/G. Since LSNIa∝G−3/2, an increase in G leads to fainter supernovae, creating tension between standard candle and standard ruler probes of dark energy. Precisely such a tension has recently been reported by the Planck team.

DId the Young Faint Sun Allow Mars to be Warmer?

Much like the Grand Canyon, Nanedi Valles snakes across the Martian surface suggesting that liquid water once crossed the landscape, according to a team of researchers who believe that molecular hydrogen made it warm enough for water to flow.

The presence of molecular hydrogen, in addition to carbon dioxide and water, could have created a greenhouse effect on Mars 3.8 billion years ago that pushed temperatures high enough to allow for liquid water, the researchers state in the current issue of Nature Geoscience.

The team includes Ramses M. Ramirez, a doctoral student working with James Kasting, Evan Pugh Professor of Geosciences, Penn State.

Previous efforts to produce temperatures warm enough to allow for liquid water used climate models that include only carbon dioxide and water and were unsuccessful. The researchers used a model to show that an atmosphere with sufficient carbon dioxide, water and hydrogen could have made the surface temperatures of Mars warm to above freezing. Those above-freezing temperatures would allow liquid water to flow across the Martian surface over 3.8 billion years ago and form the ancient valley networks, such as Nanedi Valles, much the way sections of the Grand Canyon snake across the western United States today.

"This is exciting because explaining how early Mars could have been warm and wet enough to form the ancient valleys had scientists scratching their heads for the past 30 years," said Ramirez. "We think we may have a credible solution to this great mystery."

The researchers note that one alternative theory is that the Martian valleys formed after large meteorites bombarded the planet, generating steam atmospheres that then rained out. But this mechanism cannot produce the large volumes of water thought necessary to carve the valleys.

"We think that there is no way to form the ancient valleys with any of the alternate cold early Mars models," said Ramirez. "However, the problem with selling a warm early Mars is that nobody had been able to put forth a feasible mechanism in the past three decades. So, we hope that our results will get people to reconsider their positions."

Ramirez and post-doctoral researcher Ravi Kopparapu co-developed a one-dimensional climate model to demonstrate the possibility that the gas levels from volcanic activity could have created enough hydrogen and carbon dioxide to form a greenhouse and raise temperatures sufficiently to allow for liquid water. Once they developed the model, Ramirez ran the model using new hydrogen absorption data and used it to recreate the conditions on early Mars, a time when the sun was about 30 percent less bright than it is today.

"It's kind of surprising to think that Mars could have been warm and wet because at the time the sun was much dimmer," Ramirez said.

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Simulating the Archean Earth's Atmosphere and Surface to Solve the Young Faint Sun Paradox

Investigating the early earth faint young sun problem with a general circulation model

Authors:

M. Kunze, M. Godolt, U. Langematz, J.L. Grenfell, A. Hamann-Reinus, and H. Rauer

Abstract:

The faint young Sun problem, i.e. the contradiction of a reduced solar luminosity by 15–25% during the Archaean and the geological evidence for relatively high surface temperatures that allowed the presence of liquid water, is still mostly open. It is suggested that the cooling induced by a fainter Sun was e.g. offset by higher levels of greenhouse gases (GHGs) during the Archaean, but achieving the amounts of carbon dioxide (CO2) that are necessary to solve the problem can not be supported by proxy data and the estimates of other additional GHGs diverge.

In our study we investigate this problem by using the Climate model EMAC with a spectrally resolved irradiance dataset valid for the Archaean epoch of the Earth. Our experimental setup contains a series of model runs which allow the investigation of the role of the continents, the ozone and oxygen content of the atmosphere, the solar luminosity, and the CO2 concentration on the climate of the Archaean.

Replacing the present day continents with a global ocean lead to a warming at the surface by ∼3 K and an intensified hydrological cycle. The generation of planetary waves and their propagation to the middle atmosphere is reduced, which intensifies the polar night jet and decelerates the Brewer-Dobson circulation. Slightly lower global annual mean temperatures can be found for an anoxic atmosphere. The absent ozone heating in the middle atmosphere, leads to very low temperatures in the middle atmosphere and a vanishing polar night jet, whereas the subtropical jets and the Hadley circulation are intensified. The reduction of the solar luminosity to 82% of the present value leads to a globally ice-covered planet and very dry conditions. Prescribing 10 times the present atmospheric level of CO2 with the same solar luminosity lead to a broad belt of liquid surface water throughout the year, although the global annual mean temperature is below the freezing point of water. On reducing the solar luminosity to 77% of the present value with the same amount of CO2, the area of ice-free ocean water narrows, but still suggesting a habitable environment during the Archaean for a CO2 concentration consistent with paleosol data.

Young Faint Sun Paradox May Partially Be Artifact of Bad Modeling

Solving the "faint young sun paradox" -- explaining how early Earth was warm and habitable for life beginning more than 3 billion years ago even though the sun was 20 percent dimmer than today -- may not be as difficult as believed, says a new University of Colorado Boulder study.

In fact, two CU-Boulder researchers say all that may have been required to sustain liquid water and primitive life on Earth during the Archean eon 2.8 billion years ago were reasonable atmospheric carbon dioxide amounts believed to be present at the time and perhaps a dash of methane. The key to the solution was the use of sophisticated three-dimensional climate models that were run for thousands of hours on CU's Janus supercomputer, rather than crude, one-dimensional models used by almost all scientists attempting to solve the paradox, said doctoral student Eric Wolf, lead study author.

"It's really not that hard in a three-dimensional climate model to get average surface temperatures during the Archean that are in fact moderate," said Wolf, a doctoral student in CU-Boulder's atmospheric and oceanic sciences department. "Our models indicate the Archean climate may have been similar to our present climate, perhaps a little cooler. Even if Earth was sliding in and out of glacial periods back then, there still would have been a large amount of liquid water in equatorial regions, just like today."

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"In our opinion, the one-dimensional models of early Earth created by scientists to solve this paradox are too simple -- they are essentially taking the early Earth and reducing it to a single column atmospheric profile," said Toon. "One-dimensional models are simply too crude to give an accurate picture."

Wolf and Toon used a general circulation model known as the Community Atmospheric Model version 3.0 developed by the National Center for Atmospheric Research in Boulder and which contains 3-D atmosphere, ocean, land, cloud and sea ice components. The two researchers also "tuned up" the model with a sophisticated radiative transfer component that allowed for the absorption, emission and scattering of solar energy and an accurate calculation of the greenhouse effect for the unusual atmosphere of early Earth, where there was no oxygen and no ozone, but lots of CO2 and possibly methane.

The simplest solution to the faint sun paradox, which duplicates Earth's present climate, involves maintaining roughly 20,000 parts per million of the greenhouse gas CO2 and 1,000 ppm of methane in the ancient atmosphere some 2.8 billion years ago, said Wolf. While that may seem like a lot compared to today's 400 ppm of CO2 in the atmosphere, geological studies of ancient soil samples support the idea that CO2 likely could have been that high during that time period. Methane is considered to be at least 20 times more powerful as a greenhouse gas than CO2 and could have played a significant role in warming the early Earth as well, said the CU researchers.

There are other reasons to believe that CO2 was much higher in the Archean, said Toon, who along with Wolf is associated with CU's Laboratory for Atmospheric and Space Physics. The continental area of Earth was smaller back then so there was less weathering of the land and a lower release of minerals to the oceans. As a result there was a smaller conversion of CO2 to limestone in the ocean. Likewise, there were no "rooted" land plants in the Archean, which could have accelerated the weathering of the soils and indirectly lowered the atmospheric abundance of CO2, Toon said.

Another solution to achieving a habitable but slightly cooler climate under the faint sun conditions is for the Archean atmosphere to have contained roughly 15,000 to 20,000 ppm of CO2 and no methane, said Wolf. "Our results indicate that a weak version of the faint young sun paradox, requiring only that some portion of the planet's surface maintain liquid water, may be resolved with moderate greenhouse gas inventories," the authors wrote in Astrobiology.

"Even if half of Earth's surface was below freezing back in the Archean and half was above freezing, it still would have constituted a habitable planet since at least 50 percent of the ocean would have remained open," said Wolf. "Most scientists have not considered that there might have been a middle ground for the climate of the Archean.

"The leap from one-dimensional to three-dimensional models is an important step," said Wolf. "Clouds and sea ice are critical factors in determining climate, but the one-dimensional models completely ignore them."