Earth's Hazy Past
20 Feb 2007
(Source: Astrobiology Magazine)
by Henry Bortman
Saturn's giant moon Titan is enveloped by a thick layer of organic haze. Although Titan's haze layer is unique in the present-day solar system, a recent experiment by researchers at the University of Colorado at Boulder suggests that, billions of years ago, Earth was also covered by organic haze. The finding not only sheds light on our planet's early atmosphere, but could also help explain how life spread around the globe.
Titan's haze is the result of methane in the moon's atmosphere. Hundreds of miles above its surface, ultraviolet sunlight bombards methane gas molecules with high-energy photons, breaking them apart. This destructive process, known as photolysis, leaves behind free hydrogen and small hydrocarbon fragments (CH2). The fragments are like building blocks: they can combine with each other to form larger, chain-like molecules, such as ethane, propane and acetylene. Methane in Titan's atmosphere is a transparent gas, but the larger molecules condense into aerosol particles, which form the haze. Computer modeling studies by Alex Pavlov, currently a professor of planetary science at the University of Arizona at Tucson, predicted that if early Earth also produced methane, then perhaps it too was covered in haze. But early Earth's atmosphere also had a component that Titan's atmosphere lacks: carbon dioxide (CO2). Margaret Tolbert, a professor of chemistry at the University of Colorado in Boulder, and her graduate student Melissa Trainer set up a lab experiment to test Pavlov's model, to see if they could produce haze in an "atmosphere" simulating early Earth's.
Initially, they were skeptical. We thought "you would get these oxygen atoms coming off of the CO2," which would react with the CH2 fragments from the methane and prevent longer hydrocarbon chains from forming, says Trainer. "And so you wouldn't get big enough molecules to condense into particles." No particles, no haze. But "that's not what we found," says Tolbert. "Quite the contrary."
Their initial experiment replicated what takes place in Titan's atmosphere. They put methane gas in a sealed container (along with nitrogen, a major component of Titan's atmosphere, as it was of early Earth's), and irradiated the contents with very-short-wavelength, high-energy UV radiation. The researchers chose a radiation source that included a wavelength of 121 nm, which is powerful enough to crack apart methane molecules. "The sun's spectrum has a peak emission right at that wavelength," Tolbert explains. Under these conditions, a haze did form, just as it does on Titan.
They used a novel instrument known as an aerosol mass spectrometer (AMS) to catalogue the microscopic haze particles. Using the AMS allowed them to obtain results for particles "on the fly." Past experiments had to be run for hours, days, or even weeks, long enough for the particles to form a coating on the inside of the container, which was then scraped off and analyzed. But the AMS let Trainer and Tolbert assess the aerosol particles while they were still airborne in the container. "If we turn on our UV lamp, and there are gases flowing, within minutes we can get an aerosol mass spectrum telling us what the particles are made of," Tolbert says.
Then they ran the experiment again, adding increasing ratio of CO2 to the methane, to simulate early Earth. And they still got haze. In fact, says Tolbert, "when you have CO2 and methane in roughly equal amounts, you actually get even more haze than if you just had methane alone." As they increased the amount of CO2 further, the haze formation tapered off, "but we always saw some level of haze produced," Tolbert says. They're still trying to understand what happened. "We can't fully explain it," Tolbert admits.
But they have a hypothesis. When only methane is present, some of the hydrogen atoms that get split off from methane recombine with the CH2 fragments to reform methane. That limits the number of CH2 fragments available to bind together into longer-chain molecules. But when carbon dioxide is also present, it is split into carbon monoxide and oxygen atoms. The researchers suspect that this oxygen combines with hydrogen from the methane to make water. That leaves less hydrogen available to re-attach to the CH2 molecules, so more of the CH2 molecules bond to each other. The result: more long-chain molecules, and more haze.
If they're right, their findings could have important implications for the spread of life on Earth. When life first emerged, it most likely was restricted to isolated enclaves, because it was dependent on nutrients that were highly localized. But some of these early organisms were methanogens, which were belching out methane that eventually drifted up to the upper atmosphere. There this methane, along with CO2, was bombarded by high-energy UV radiation from the sun. Long hydrocarbon chains started to form, and the longer the chains got, the more readily they condensed into aerosol particles. Eventually, the Earth was covered by a haze of these particles, which rained down onto the surface.
In this process, CO2 was important for a second reason. Although much of the oxygen liberated from the carbon dioxide combined with hydrogen to form water - that's the process that let the haze form - some of it attached to the growing hydrocarbon chains. Oxygenated hydrocarbon chains are an energy-rich food source, and when these molecules began to rain down onto Earth, that food source became available planet-wide. "Life could be sustained in every little puddle on Earth," says Tolbert.
If they're right. Being right, however, requires that the atmosphere of early Earth was not brimming with oxygen, as it is today. Most researchers believe that the rise of oxygen in Earth's atmosphere didn't come until some 2 billion years after the origin of life. But if early Earth had an oxygen-rich atmosphere, haze formation could not have taken place.
On the other hand, it's possible that some purely chemical process on early Earth produced methane even before there were methanogens. There clearly is an abiotic source for atmospheric methane on Titan, and possibly on Mars as well. If that were the case, then haze chemistry, and a nutrient-rich rain, could have been in place before life got started, and the biosphere could have become a global phenomenon almost as soon as it began. Indeed, haze particles could have played a role in life's beginnings.
One way to sort this all out is to consult the rock record. Ancient rocks contain evidence of chemical reactions that were taking place in the atmosphere when the rocks formed. But this is tricky work. There are only a handful of places on Earth where rocks can be found that date back to life's earliest days. Over time these rocks have been extensively altered by heat and pressure, so their chemical signatures are degraded.
What's more, says Alex Pavlov, scientists have yet to establish what to look for. "The systematic analysis of what signature to expect hasn't been done so far." The first step, he says, is "to redo the experiment which Maggie did and analyze the bulk composition of these aerosols," the relative proportions of carbon, hydrogen, oxygen and nitrogen. That would give geologists a clearer idea of what type of chemical signature they were seeking.
Meanwhile, another of Tolbert's graduate students, Langley DeWitt, is conducting a follow-up experiment to test Tolbert and Trainer's hypothesis about why haze forms in the presence of CO2. "If that model is correct," Tolbert says, "adding hydrogen to the system should lead to less haze." The jury is still out.