It was Valentine's Day, 1990 when a sleeping eye awoke after nearly nine year's of inactivity, and for a few brief moments, from a distance of nearly four billion miles, took its last look at the cosmic neighborhood from whence it came. Voyager 1, now the most distant human made object from Earth, sent back an image now known as "the Pale Blue Dot."
Astronomer and exobiologist Carl Sagan eloquently reminded us that this small, inconsequential speck amidst the cosmic background is home to everyone, every idea, and every plant, animal or microbe that has ever existed on our planet. And so far, this small, pale blue dot is the only place in the universe that we are certain has life.
I was kneeling at a depth of 120 feet in McMurdo Sound, Antarctica awaiting the arrival of my buddy, a telepresent ROV, that was being driven from a console at NASA's Ames Research Center over 10,000 miles away. It had been lowered into the water via a dive hole that had been made through the eight feet of sea ice above us. As I watched the graceful robotic device dropping through the column of clear, dark water, I noticed that the somewhat distant dive hole appeared as a small, pale blue dot against the darker ice that surrounded it.
Recalling the image of Earth taken at the edge of our solar system, I wondered if this is what it would be like to view the Earth, or another Earth-like planet, from the vantage point of Voyager several years earlier. In the years that had I worked and dived in the oceans and lakes of Antarctica, I had never thought about the dive hole quite this way, but there it was, like a painting or postcard sent to me by Voyager.
Looking at Mars
As we prepare for more intense investigations of the planet Mars, we try to focus the questions that we wish to have answered, and refine the tools that are required to make finding the answers possible. There are many ways to go about this task, and one way is to study life on Earth in a range of environments that we think are analogous in some ways to the conditions that existed previously.
To this end, our work in polar regions has been fruitful in several ways. Microbial mat communities composed of cyanobacteria and diatoms have formed on the bottoms of many of the perennially ice-covered lakes in the McMurdo Dry Valleys located about 700 miles from the South Pole. What makes these ecosystems so special is that there are no 'higher' life forms such as crustaceans, insects, or fish to disturb or disrupt the formation of the complex mat communities. An exclusion mechanism of sorts keeps all but the microbes from colonizing the lake environments. Typically this exclusion mechanism might be high salt concentrations (like North America's largest salt works, the Exportadora de Sal, in Guerrero Negro, Baja California), or the hot water around thermal springs (Yellowstone), or in the case of the Antarctic, thick ice-covers, extremes in chemistry, geographic isolation and the other polar circumstances that are unsuitable for many life forms.
All the life in the lakes is microbial in nature, the largest organisms being nematodes, rotifers and gastrotrichs. There are rather few instances of large ecosystems dominated by microorganisms, and the benthic microbial mats beneath the thick ice-covered lakes are among these unusual examples. When we descend through the tunnel of ice into the lake water below, we are, in a way, traveling back in time six hundred million to three billion years, to a much younger Earth prior to the develop of multicellular life. For most of Earth's history, vast microbial mat communities colonized aquatic environments and dominated the planet. Our studies show us how microbial mats grow, diversify and function, and this helps with the interpretation of the geological record of Earth's earliest life forms.
Chilling Out
While the earliest life on Earth did not (for the most part) exist beneath ice, there were times that it did. There were several periods of glaciation in the Precambrian, and perhaps if a Snowball Earth (a period when the entire planet was ice-covered) occurred, then at the very least microbial mats similar to what we see could have survived near the equators if the ice was not much thicker than what we have in the Dry Valley lakes.
Not only do these benthic environments provide us with a glimpse of the evolutionary trail that shaped our own past, they may allow us to see how life could have thrived on an earlier, more clement Mars. Results from robotic missions to Mars have provided compelling evidence that liquid water once existed at or near the surface of the red planet during the time when life is known to have existed on Earth. It may also be the case that Mars was never a really warm place – its mean annual temperature may never have been much different from what we find in the McMurdo Dry Valleys where the mean annual temperature is about –4 F. The more we understand about the physical, chemical and biological processes that are occurring in the Antarctic lakes, the easier it will be for us to understand the conditions that probably impacted any life that may have existed on Mars in a similar setting, and how the record of this life was preserved – particularly if the martian lakes or oceans were covered with thick ice.
We have also been investigating a series of perennial springs located on Axel Heiberg Island in the Canadian high arctic. These are among the northernmost springs in the world, and occur in a region of thick, continuous permafrost. Permafrost is simply ground that has had a temperature below freezing for more than two years in a row. The mean annual temperature of Axel Heiberg near the springs is –4.8 F, which is slightly warmer than what we have measured in the McMurdo Dry Valleys. Permafrost depths reach 2,000 feet or more, making it a very effective barrier for water movement either in our out of the ground.
Again, Mars may have been quite cold throughout its history, and the permafrost may have always been thick. Our current work at Axel Heiberg, in conjunction with astrobiologists from NASA and McGill University in Montreal, has elucidated the mechanisms by which the water can flow upward through the permafrost. We have also been cataloging the microbial communities that exists in the water and sediments associated with the springs. The springs form barrage pools, troughs and terraces of carbonate over which the water flows. With the help of sophisticated measurements, we hope to decipher the role the bacteria play in the mineralization process, and whether or not they leave a record of their existence in the sediments and mineral deposits.
The origins of life and the evolution of the biosphere here on Earth, and perhaps on Mars, are still mysteries seeking answers. Many of the questions that have emerged from studies of terrestrial analogs have answers within our grasp, and our continuing investigations of the pale blue dot will one day lead us to other life, on other worlds.
Andersen is a Principal Investigator for the SETI Institute.
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