Lecture of the Week: Part II: Could We Tell Life If We Saw It?

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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. (requires QCShow Player)

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.

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