Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Tuesday, July 27, 2010

Planetary Prometheus

Imagine, if you will, a planet with atmosphere, oceans, rocks and life. On this planet then most chemical reactions are either slow and geophysical, or quick and biological but very localized. There is however an exception. Because of the particular nature of this world there is the ever-present potential for a type of chemical reaction that is not only fierce and destructive, but self-propagating. Once triggered it can spread across hundreds, even thousands, of square miles. It preferentially attacks and transforms living material - leaving behind a fragile deposit, stripped of most biomatter. It can only stop by either exhausting the supply of fresh reactants, or when its chemical energy is sucked away by an un-reactive medium.

This is a tricky planet. It forever teeters on the edge of letting this chemical storm get a grip, but its climate and varied topography help to confine outbreaks. The very compounds responsible are themselves critical ingredients for much of the life on this world, and cannot be eliminated. Indeed, the reaction itself serves a number of key roles in stabilizing populations, cycling elements between air, ground, and oceans, and is ancient enough to have been incorporated into the survival strategies of large numbers of species.

Imagine we could visit this world. Entering orbit we would scan it with our telescopes. Curiously, at any given time, we would observe tens of thousands of these intense chemical maelstroms dotted across the globe. Their signatures would be quite distinct, and we might be quite astonished that life existed in such a perilous environment.

Of course, this is no hypothetical planet. It is the Earth. The chemical reaction we know as fire is a strange and intriguing, and often overlooked, aspect of life here. The young Earth of 3 billion years ago, with little or no oxygen in its atmosphere, or flammable biomatter, would have probably only seen fire in volcanic settings. Somewhere along the line, maybe a billion or two years later, with enough free oxygen, perhaps some dried up mat of plant life on a tidal shore was the first victim of arson - possibly a result of lightning. Today, fires cover the globe. Satellite imagery, or remote sensing, tells the story. The image in the upper left shows thousands of fires scattered across south-central Africa, seen by the MODIS instrument on NASA's Aqua satellite. Many of these have been set by humans, following an ancient pattern of land-use. Humans have learnt to exploit this chemical fragility.

I think we tend to underestimate the chemical reactivity of our homeworld. Fire is an excellent example - it's so familiar to us that we (well, I) even have to pause to remember that it's something chemical, fiercely exothermic. It raises a number of interesting questions. Is a phenomenon like fire simply a consequence of the kind of chemical reactivity needed for a planet to harbor life? Life on Earth needs a lot of reduction-oxidation pathways. Can you propel a biosphere to the kind of richness we see today without taking this walk on the wild side - risking destruction for the chance to make hay with oxygen?

Evidence suggests that, for example, around 270 million years ago atmospheric oxygen levels were significantly higher than today - and that fire was much more frequent on a global scale. More oxygen and it becomes hard to avoid burning all flammable materials, clearly there could be a feedback mechanism at play - complicated by geography and climate. Just how fire-prone can a planet become before it wipes out its surface biosphere?

Anyone got any marshmallows?


nancyk said...

Hi, Caleb -
Fun ruminations. Of course, the land surface is not smooth and homogeneous, and for life to exist there has to be liquid water, so the fire would get stopped by the patchy distribution of vegetation, topography, and wet spots before it could wipe everything out. And fire-adapted ecosystems would be renewed by fire. And fire would be a negative feedback to the build-up of oxygen, so in a sense it would suppress itself from entirely taking over.

nancyk said...

Or...the other question is, since insects got so gigantic in times of high oxygen, could the planet get so badly infested by voracious insects that that would decimate all the vegetation and kill both themselves and everything else off? And then complex life would have to restart again from bacteria, so the planet would go through oscillations between complex and primitive life...

Caleb Scharf said...

I think the qn. of oscillations in biota is very interesting. Bacteria too can of course 'bloom' and throw systems completely out of whack, so I don't think it's something confined to 'complex' vs. 'simple' life - but the role that runaway situations (i.e. unstable situations) play is intriguing.

Eniac said...

This is an excellent train of thought.

I think, as you suggest, that fire is a necessary side effect of the conditions that permit the high energy density form of life we call animals. Animals require a reservoir of energy that is built up slowly by phototrophs (plants) or some other, possibly inorganic, mechanism. Any such reservoir will occasionally overflow, causing the uncontrolled release of energy. On Earth, the energy reservoir is the combination of biomass and oxygen, and the overflow is fire. It could be something completely different, elsewhere.

What I find exciting about this is that the presence of uncontrolled exothermic reactions is relatively easy to detect, and might be a valuable "astrobiomarker" one day. There is the possibility of false positives, of course, from inorganic processes such as lightening, but still, as astrobiomarkers go, this could be one of the better ones.

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