COVER STORY: Once Upon A Purple Earth

October 10, 2008

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With some scientists believing that the original life forms on our planet may have been purple, the search for life in other worlds may undergo some changes from the previous  telltale markers, writes Fatima Sajid.
Our planet has always been a special place — right from its formation to the time when single cells evolved into complex organisms, to the time of the dinosaurs and then humans populating the planet. It seems our planet was destined for life right from the beginning, with things happening at the right place at the right time.
 
A new study carried out by a team of German scientists from the Institute for Planetary Research, at the Garman Centre for Air and Space Travel in Berlin, has shed some light on the early days of the earth. Geological records show that water in its liquid form was present on earth when it was just in its infancy, about 3.7 billion years ago. According to estimates, the earth is approximately 4.5 billion years old. This indicates that for water in its liquid form to be present, temperatures must have been above its freezing point. Their latest study has recently been published in the journal Planetary and Space Science.
 
Warm and friendly
 
How warm the temperature actually was, is still not known but one thing is almost certain — our friendly planet has been ice-free since it was very young. Latest research indicates that in its early days, our solar system was quite different from what we see or know today. When the sun was very young, it did not provide much heat to the earth. But, interestingly, even then when things were a bit too chilly for comfort, earth's surface was free of ice.
 
Many theories have surfaced over the years for the “faint young sun problem” by scientists. Most theories rely on the assumption that initially the earth had a lot of carbon dioxide in its atmosphere with very high amounts of greenhouse gases to warm the atmosphere. But recently researches show that the levels of carbon dioxide in the atmosphere were much too low to keep the surface from freezing.
 
The team of German scientists delved deep into the earth's early years as a young and budding planet. The research was done using a new computer model examining the important events in the earth's history — events like the period called 'Late Heavy Bombardment', a time when frequent asteroid hit the earth, which has been estimated at 3.8 billion years ago; the first indication of oxygen in the atmosphere produced by cyanobacteria, 3.8 billion years ago; and the first oxidation occurrence estimated at two billion years ago.
 
“Our new model simulations suggest that the amount of carbon dioxide needed to keep the surface of the early earth from freezing is significantly less than previously thought,” the authors wrote in their research report.
 
The model showed that just a pressure of 2.9 millibars of carbon dioxide must have been enough to warm the earth to above freezing temperatures during the late Achaean period and the early Proterozoic period. In other words, the faint young sun was not such a big problem. Furthermore, the study also proves the important point of how big a role carbon dioxide plays in warming the planet and how increasing levels of carbon dioxide today, due to human activities, can bring about climate change.
 
The purple faze
 
Thus it seems our dear home may have been purple before it became green. Scientists are of the opinion that before the green life forms took over, the original life forms may have been purple. The microbes in question may have harnessed energy from the sun through a process that gave them a shade of violet. In that case, our search for “little green men” in the cosmos may be steered to “men in purple” as well. The ancient microbes might not have used chlorophyll to harness the energy from the sun.
 
Chlorophyll, that absorbs red and blue light from sunrays and gives plants their green colour, may have been preceded by another kind of microbe. It has puzzled scientists for sometime as to why chlorophyll, the basic pigment in plants, absorbs the red and blue pigment from the sun's rays, when the sun is known to transmit most of its energy in green in the visible spectrum.
 
Shil DasSarma, a microbial geneticist from the University of Maryland, stated, “Why would chlorophyll have this dip in the area that has the most energy?” The human vision has evolved as being most sensitive to green light, which is why night vision goggles are green. The mystery is why does photosynthesis not react in the same way?
 
According to DasSarma, it could be because chlorophyll came into being after another light-sensitive molecule called retinal, which was already present in the early days of the earth.
 
Today, retinal is found in the membrane of a photosynthetic microbe — halo bacteria — which absorbs green light and gives out red and violet light, which, when combined, give off the purple colour. Ancient microbes that used retinal to harness rays from the sun may have been the dominant molecules on earth during the early days, according to DasSarma, and covered the planet in a purple hue. Since microbes that used chlorophyll came later on, they could not directly overcome the ones using retinal, and survived by evolving and absorbing the wavelengths that retinal could not use.
 
“Chlorophyll was forced to make use of the blue and red light, since all the green light was absorbed by the purple membrane-containing organisms,” stated William Sparks, who also helped DasSarma in his research and is an astronomer at the Space Telescope Science Institute, (STScI) in Maryland.
 
Scientists and researchers assume that both kinds of molecules, the chlorophyll and retinal-based, coexisted for a certain period. “You can imagine a situation where photosynthesis is going on just beneath a layer of purple membrane-containing organisms,” DasSarma stated to LiveScience.
 
Scientists speculate that chlorophyll got the upper hand, as it is more efficient than retinal, and thus green took over from purple. “Chlorophyll may not sample the peak of the solar spectrum, but it makes better use of the light that it does absorb,” said Sparks.
 
Though the researchers do admit the entire theory is just speculation, the idea does have a strong case. It fits with the early earth scenario that scientists know about. The fact is that the structure of retinal is simpler than that of chlorophyll and would have been produced easily in the low-oxygen based atmosphere at that time. Moreover, retinal is produced under almost the same process as that of a fatty acid, which is one of the main ingredients in cell development.
 
“Fatty acids were likely needed to form the membranes in the earliest cells,” explains DasSarma. Then there are halo bacteria, a microbe that uses retinal and is even present today, which not a bacterium. The group of organisms it belongs to go back to the time when the earth did not have an atmosphere laden with oxygen. When all the evidence is put together, the case for retinal being present before chlorophyll took over, and the “purple earth” hypothesis put forward by the researchers, stays strong.
 
On the other hand, the idea needs to be further researched at the moment. “I'm a little cautious about who's using which wavelengths of light and making conclusions about how things were like three or four billion years ago,” said David Des Marais, geochemist at NASA's Ames Research Centre in California.
 
More evidence and research is certainly needed for the purple earth theory but if evidence does prove the hypothesis to a greater possibility to support the idea, then the search for life in other worlds will also undergo some changes from the previous telltale markers. “We should make sure we don't lock into ideas that are entirely centered on what we see on earth,” claimed Neil Reid who also works at the STScI.
 
“I think when most people think about remote sensing they're focused on chlorophyll-based life. It may be the more prominent one, but if you happen to see a plane that is at this early stage of evolution, and you're looking for chlorophyll, you might miss it because you're looking at the wrong wavelength,” further explained DasSarma.
 
No 'holes' Barred
 
The earth's surface during the early days was not as pretty as it is today. It was full of impact craters like puncture wounds due to constant meteorite impacts. According to the Earth Impact Database, which is maintained by the University of New Brunswick, Canada, the latest count of holes in the surface and even under the sea floor, is 170. And there would be many more if the earth did constantly 'remodel' its surface. The shifting of continental plates, eruption of volcanoes, formation of mountains and the erosions washing over the surface, have given our planet the similitude of a 'facelift' to make the surface interesting and attractive.
 
“If there was no erosion or tectonic activity, we would look like the moon. The moon is just pockmarked with impact craters,” stated geologist Lucy Thompson, from the University of New Brunswick.
 
Scientists estimate that the impacts were much more frequent and heavy when planets were being formed. That's kind of lucky for us humans and other life forms on the planet, as things have become quite stable of late. Among the most recent 'dents' on the surface is Arizona's Barringer Meteor Crater, which was formed around 50,000 years ago! Moreover, though tourists are pretty much awed and impressed by its three quarters of a mile diameter, according to geological calculations, it's quite tiny.
 
The Manicouagan Crater in Quebec is more than a hundred times wider than the Barringer. How's that for size? Thankfully it was made 200 million years ago.
 
“With large impacts, you have complex craters forming and instead of having a nice bowl shape, you get a central uplift. It's like if you drop something in water, you get rings forming, but the middle comes back up,” Lucy told SPACE.com. Scientists are puzzled as to how the rock behaves in this fashion without breaking or turning into liquid. The earth's largest verified crater is South Africa's Vredefort crater, 186 or 300 kilometres wide, and is thought to be the oldest at two billion years. Amongst the other interesting ones is the Chicxulub in Mexico, partly submerged off the Yucatan Peninsula. Its claim to fame is the possible extinction of the dinosaurs and other life on the planet. Still many more meteorite and asteroid impact craters are expected be discovered on the seafloor and other remote locations which surface changes might have missed.
 
It is ironical that a planet that remodels and refreshes itself to make itself more beautiful and habitable for humans is being destroyed today by the same species. And the devastation that is caused by extreme climatic conditions is only our own doing.