early earth

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The outer edge of the habitable zone is traditionally defined as the outermost orbital distance at which a planet could sustain liquid water on its surface. At this distance orbit, Earth-like planets with plate tectonics (or a similar process for recycling volatiles) should build up dense carbon dioxide atmospheres that help offset the reduction in starlight. Carbon dioxide released from volcanoes provides additional greenhouse warming, although rainwater dissolves some of this. The amount of carbonic acid that dissolves in rainwater and reaches the ground depends upon the temperature: the colder it gets, the less carbon dioxide gets rained out of the atmosphere. This feedback is part of the carbonate-silicate cycle, which regulates an Earth-like planet’s carbon dioxide over geologic (million year) time scales.

In a recent paper published in The Astrophysical Journal, titled “Limit cycles can reduce the width of the habitable zone,” my co-authors and I examine the propensity of this carbonate-silicate cycle to cause a planet to oscillate between completely frozen and completely ice-free climate states. We update a simplified climate model to account for the increase in weathering that occurs as a planet builds up a dense carbon dioxide atmosphere. Beginning with a planet in completely ice-covered conditions, we allow volcanic outgassing of carbon dioxide to continue until the planet melts from the enhanced greenhouse effect. However, under certain conditions, the planet will then start to rain out and weather the atmospheric carbon dioxide at such a fast rate that the greenhouse effect decreases and the planet again plummets into global glaciation.

This type of climate cycle between glacial and ice-free states is not likely to occur on Earth today, but such cycles might have been possible on early Earth during the Hadean eon. Extrasolar planets may also be prone to this type of climate cycling, although predicting whether or not this should occur depends upon knowing a planet’s volcanic outgassing rate. Our climate calculations place boundaries on the conditions under which we should expect such climate cycles to occur for Earth-like planets orbiting a range of different stars.

Both Earth and Mars show geologic evidence of flowing liquid water in the distant past, nearly four billion years ago. The presence of liquid water on the surface of these planets is difficult to reconcile with the reduced luminosity of the sun at the time, so scientists have continued to search for possible explanations for the warm climates of early Earth and early Mars. A team of researchers recently suggested that early Earth may have had larger oceans and fewer clouds than today, which would have reflected away less incoming sunlight and might have allowed the planet to remain warm in spite of a fainter sun. Whether or not this solution will pan out for early Earth, it at least suggests the possibility of a similar mechanism on early Mars.

In a recent paper published in Astronomy & Astrophysics, on which I am co-author,
we examine the possibility that reduced reflectivity could have kept early Mars above freezing. We use a computer climate model to calculate the global average temperature at various values of ocean fraction and cloud coverage. We find that our model does indeed produce warm conditions for early Earth, but it fails to do the same for early Mars. In fact, our model can only produce warm conditions if early Mars were nearly entirely covered by oceans and also free of clouds, a result which is unlikely as well as inconsistent with geologic evidence. We conclude that some combination of climate and geochemical mechanisms, as yet unknown, may provide clues for understanding the stability of liquid water on early Mars.

If early Mars did harbor oceans, then the possibility remains that life could have developed. Examining the climates of both Earth and Mars in the past may therefore help in the quest to understand the origin of life. Future Mars exploration missions, as well as continued research on Earth, will slowly shed light on this mystery.

Animals today stay alive by breathing in oxygen-rich air through a process known as oxygenic respiration, which consumes oxygen (O2) and releases carbon dioxide (CO2) as a byproduct. Most plants, on the other hand, convert sunlight and carbon dioxide into energy through a process known as photosynthesis, which consumes CO2 and releases O2 into the atmosphere. Because photosynthesis is a source of oxygen, it seems intuitive that photosynthesis evolved first: once enough O2 was in the air, then respiration would be able to arise in the newly oxygen-enriched atmosphere. However, some biologists have argued since the 1970’s that respiration in fact evolved first. There are many reasons that this might be the case, and new measurements of bacterial respiration at very low levels of O2 have revived this “early-respiration” hypothesis.

In a recent paper written by myself and my two graduate advisers, we argue that small quantities of O2 could have reached the surface of early Earth through transport by atmospheric dynamics. This transport would primarily occur in the Wintertime hemisphere, where a “polar Winter vortex” develops near the polar region, because the lack of sunlight in Winter would allow for greatest amount of O2 to accumulate. Our calculations show that enough dissolved O2 could have accumulated in polar Winter waters to allow early forms of marine life (i.e. microbial life) to develop and use respiration–without needing to wait for photosynthesis to oxygenate the atmosphere. Although our model calculations cannot prove that respiration did in fact evolve first, they least demonstrate a proof-of-concept that the “early-respiration” hypothesis is in fact viable.

Our paper is titled “Availability of O2 and H2O2 on pre-photosynthetic Earth” and appears in the May issue of the journal Astrobiology.

My MS paper, “A Revised, Hazy Methane Greenhouse for the Archean Earth“, just appeared in the journal Astrobiology! You can view a PDF of the article on my research page.

We argue that the warm, ice-free climate of the early Earth (2.8 billion years ago) was maintained by a water vapor/carbon dioxide/methane/ethane atmospheric greenhouse effect that offset the ~20% reduction in solar luminosity from the faint young sun. Furthermore, a stabilizing feedback between life and the climate system may have resulted in a thin stratospheric organic haze that maintained above-freezing temperatures and shielded ultraviolet radiation. An excellent write-up of our work is available at The Planetologist.

I’ve given this talk several times over the past couple years, most recently on the Forum for Astrobiology Research (which should eventually be available as a podcast), and it feels good to finally see the paper come out.