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Did respiration evolve before photosynthesis?

June 6, 2011

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.

Would contact with aliens be dangerous?

May 9, 2011

Humans have yet to encounter any extraterrestrial beings, and, although speculations abound in Hollywood and in science fiction, we really have no idea what contact with aliens would be like. In one scenario, a population of aggressive and malevolent extraterrestrials invade Earth in order to enslave us or eat us as part of their conquest of the galaxy. Still another hypothetical extraterrestrial civilization might welcome us with open arms into peaceful communication and help us solve many of our global problems. With such a wide range of possibilities for an as yet unknown encounter, can we say anything at all regarding the risk of contact with extraterrestrials?

We explore this question in a recently published article titled “Would contact with extraterrestrials benefit or harm humanity? A scenario analysis”. This article was written by Seth Baum, Shawn-Domagal Goldman, and myself and appears in the current issue of Acta Astronautica. Rather than focusing on one particular outcome of alien contact, we take a broad approach of categorizing a wide range of contact scenarios as either beneficial, neutral, or harmful to humanity. In doing so, we draw on scientific as well as ethical analysis to demonstrate that there are a wide range of responses to contact with extraterrestrials, which may depend at least in part on human actions in the near and distant future.

Our television and radio signals have been leaking away from our planet for decades now, detectable by any nearby extraterrestrials willing to listen. The light from our planet, too, shows not only signs of biological life but also signs of rapid warming and climate change. Although we cannot be certain that any of our behaviors will necessarily invoke the wrath–or solicit the aid–of advanced extraterrestrials, perhaps it would behoove us to give thought and care to our future trajectory, just in case someone is watching.

Sulfur Can’t Keep Mars Warm

June 28, 2010

Satellites and rovers sent to Mars keep giving us compelling geologic evidence that liquid water flowed on the surface of the red planet in the distant past. Three billion years ago, when oceans may have existed on parts of Mars, the sun was about 30% fainter. Mars today is well below the freezing point of water, so any lakes or oceans would be frozen over. In the distant past, then, this problem is even more pronounced: how was Mars able to stay warm enough to sustain liquid water?

Many attempted resolutions have been proposed to this problem, but none has provided a complete solution for a warm, wet early Mars. In a recent paper published in Earth and Planetary Science Letters, on which I am a co-author, we argue that greenhouse warming by sulfur dioxide could not have kept early Mars warm enough. Sulfur dioxide has been suggested in the literature because it is an effective greenhouse gas, similar to carbon dioxide or methane. However, we show that atmospheric photochemistry with sulfur dioxide leads to the production of sulfate aerosols in the upper atmosphere that absorb incoming sunlight and cool the surface. Thus, sulfur dioxide may have caused net cooling on early Mars, rather than warming.

We’re still trying other mechanisms to explain a warm, wet early Mars. Most likely, it was some combination of processes, including several greenhouse gases and warming by clouds. A negative result for sulfur dioxide is not as exciting as a solution to the early Mars problem, but it’s still a small step forward.

AbGradCon Ho!

April 13, 2009

My abstract on “Warming Early Mars with NO2″ has been accepted for a talk at this summer’s Astrobiology Graduate Conference! I’ll discuss a possible mechanism by which early Mars (3 billion years ago) could have sustained above-freezing temperatures as well as liquid water with a carbon dioxide/water vapor/nitrogen dioxide greenhouse effect. We are currently putting the finishing touches on our manuscript before submitting it to Earth & Planetary Science Letters in a week or so.

The organizers of AbGradCon09 have done a great job of putting together an excellent conference, including a field trip to Mt. St. Helens. Additionally, the conference will be broadcast live on Second Life, so if you can’t travel to Seattle you can still participate!

Where have all the aliens gone?

March 7, 2009

The Milky Way is old enough that a slightly more advanced civilization than us could conceivably have colonized the galaxy several times over by now. Known as the Fermi Paradox, the absence of extraterrestrial observations is often taken to imply either the rarity of life or the impossibility of interstellar travel.

In a paper published in the February issue of the Journal of the British Interplanetary Society titled “The Sustainability Solution to the Fermi Paradox”, we challenge this conclusion with the possibility that exponential growth is an unsustainable development. That is, even if an extraterrestrial civilization has colonized the galaxy, it would have done so through rapid unsustainable growth and collapsed upon reaching a physical resource limit. Not enough time has yet passed for a sustainable growth civilization to colonize the galaxy, so there is still promise in the search for extraterrestrial life. Furthermore, though the absence of extraterrestrial civilization does not imply the unsustainability of exponential growth, it does increase the probability that humanity should transition to sustainable development in order to prevent its collapse. A more detailed writeup is available on the Lifeboat Foundation blog.

In other news, NASA’s Kepler Mission successfully launched yesterday evening! Over the next three years, Kepler will observe 100,000 stars in a patch of the Milky Way in search of Earth sized planets. This is the first mission with the capability of detecting Earth at a distance, so with any luck we’ll soon have a better idea of just how common small rocky planets are in the galaxy.

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