Imagine, if you will, a planet with atmosphere, oceans, rocks and life. On this planet, 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 also 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 helps 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 much 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 at the top of this post 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.
NASA recently compiled 10 years of MODIS data to produce an extraordinary movie of the Earth on fire. You can click on this image to the left (or here) to to go and view it.
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? And that also raises another interesting question, could oxygen producing marine organisms wage fiery war on their land-living counterparts (either for some advantage in resources, or inadvertently), while safely contained in their wet and fire resistant habitat?
As our telescopes reach out across the galaxy in search of "Earth-like" planets, perhaps we should consider looking for the signs of not just idyllic biospheres, but also those where such chemical imbalances are, at least temporarily, making a mess.
Anyone got any marshmallows?
This post is adapted (and updated) from the Life, Unbounded archives.