March 27, 2006
Part II: Could We Tell Life If We Saw It?
Potential for Early Life Hosted
in Basaltic Glass on a Wet Mars
Neil R. Banerjee
University of Alberta, University of Bergen, Norway
11 min. (slideshow requires QCShow Player)
Audio only (mp3 format)
View as a webpage (quicktime, real player) (notes)
Can we recognize the hallmarks of life if we see it, even if it is of a completely different biochemistry than anything we currently know? This is the question that will pervade our coming search for life in the universe.
We believe that the answer is yes, simply because we expect the physics of the Darwinian evolutionary process to be universal. Life inherently builds complex and highly refined structures. Andy Knoll of Harvard has said that a good biomarker should be something that is difficult to accomplish through inorganic processes. And Norm Horowitz of CalTech said much earlier, in 1956, that a good biomarker should display the fingerprint of natural selection.
Joe Kirschvink, in the previous lecture, emphasized the extraordinary quality of the magnetite crystals found in the Martian meteorite ALH84001, levels virtually impossible to achieve by inorganic means. This week's lecture is similar, but celebrates a completely different phenomenon: the etchings made by bacteria feeding on glass.
It wasn't until the early 1990's that the etchings in medieval church windows in Europe were first recognized to be caused by bacteria. That finding was almost immediately extended to natural basaltic glasses as well, where the same patterns were quickly discovered.
In this short but compelling talk given by Neil Banerjee to The Second Conference on Early Mars in October, 2004, Neil describes the bacterial process in both modern and very ancient terrestrial rocks. Evidence for early life on Earth has proven to be similarly controversial. Neil and his co-authors have recently discovered indicators of early life in the formerly glassy rims of ~3500 million year-old basaltic pillow lavas, essentially indistinguishable from those found in modern rocks. These ancient volcanic glasses represent a previously unexplored setting in the search for early life on Earth.
If this environment is true for Earth, it may well be true for Mars as well. The cratered surface of Mars likely hosts countless glassy basaltic impact breccias that may have been submerged in water for extended periods of time. Such rocks may well represent a viable habitat for early life on Mars, and Neil recommended at the end of his talk that such rocks be seriously considered for a future sample return mission.
As it occurs, that sample return mission may already have been accomplished — a century ago, in 1911, at Nakhla, Egypt.
In an article published this week in the journal Astrobiology, Martin Fisk and co-authors argue that a new study of the Nakhla Martian meteorite has revealed a series of microscopic tunnels that are similar in size, shape and distribution to tracks left on Earth rocks by feeding bacteria (see image at left).
Fisk, a professor of marine geology in the College of Oceanic and Atmospheric Sciences at Oregon State University and lead author of the study, said the discovery of the tiny burrows do not confirm that there is life on Mars.
"Virtually all of the tunnel marks on Earth rocks that we have examined were the result of bacterial invasion," Fisk said. "There are two possible explanations," he added. "One is that there is an abiotic way to create those tunnels in rock on Earth, and we just haven't found it yet. The second possibility is that the tunnels on Martian rocks are indeed biological in nature."
The igneous rock fragment from Nakhla — which weighs about 20 pounds — is 1.3 billion years in age. It is believed that the rock was exposed to water about 600 million years ago, based on the age of clay found inside the rocks. "It is commonly believed that water is a necessary ingredient for life," Fisk said, "so if bacteria laid down the tunnels in the rock when the rock was wet, they may have died 600 million years ago."
"Several types of bacteria are capable of using the chemical energy of rocks as a food source," he said. "One group of bacteria in particular is capable of getting all of its energy from chemicals alone, and one of the elements they use is iron – which typically comprises 5 to 10 percent of volcanic rock."
The igneous rocks from Mars are similar to many of those found on Earth, and virtually identical to those found in a handful of environments, including a volcanic field found in Canada. Although the tracks in the Nakhla meteorite do not appear to be as complex as they are in Neil Banerjee's terrestrial basaltic glasses, they are nonetheless strikingly similar.
— Wirt Atmar & Oregon State University press release
About the Speaker
Neil Banerjee's scientific interests include the biogeochemical effects of water-rock-microbial interactions, microbial alteration of modern and ancient oceanic crust, evidence for early life on Earth, formation and evolution of oceanic crust, geochemical cycling at mid-ocean ridges, element mobilization in magmatic-hydrothermal ore deposits, formation of massive sulfide deposits at mid-ocean ridges, trace-element modeling of minerals resulting from fluid-rock exchange, and the origin and emplacement of ophiolites.
Neil is a Staff Scientist at the Integrated Ocean Drilling Program, Texas A&M University as well as an Adjunct Assistant Professor, Department of Geology and Geophysics, Texas A&M University.
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