They
haven't yet figured out how to draw blood from stones, but a group of French
researchers is offering new insight that could change how scientists search for
signs of life in Martian rocks.
By studying
the laboratory fossilization of microorganisms, scientists have caught a glimpse
into how early Earth and potential
Martian life might be preserved in rocks. The scientists focused on Pyrococcus
abyssi and Methanocaldococcus jannaschii, extremophiles which
thrive in piping hot (up to 176°F), oxygen-lacking (<0.2% of current levels)
environments.
Some
believe this sort of hot, anoxic environment is where life may have originated
on Earth, and perhaps even on Mars.
"Environmental
conditions were similar on the young terrestrial planets and traces of early
Martian life may have been similarly preserved as silicified microfossils," the
scientists write in their study, which appeared in the September issue of the
journal Geobiology.
On Earth,
some fossils are created when microorganisms are caught in concentrated flows
of silica and other minerals near hydrothermal sources. Instead of studying
existing hydrothermal ocean vents or hot springs, the scientists imitated this
natural fossilization process in the laboratory. This allowed them to more
closely mirror the oxygen-lacking environments of Mars
and the early Earth.
Effects
observed in the two organisms were quite different. Most M. jannaschii cells
collapsed and died within a week. They left behind only subtle organic traces
after inoculation with a silica solution. Many key features of P. abyssi
persisted for up to 4 months after inoculation. This created a larger window
for potential preservation.
"They do
fossilize a little bit, but do not survive the test of time," says Frances
Westall, a paleobiologist at the Centre National de la Recherche Scientifique in
Orleans, France, who supervised the research team's work.
Dorothy
Oehler of NASA's Johnson Space Center, who was not involved in the study, says
that even though the organisms in the study did not fossilize particularly
well, many ancient fossils have been found preserved in silica. This indicates that
other microbes studied under different conditions might offer a higher
potential for fossilization.
The
scientists did note in their study that a particular substance produced by the
microbes has more staying power. This substance could provide proof of life
even when the actual bodies are long gone.
Tell-Tale
Traces
The dead,
fossilized bodies of microorganisms can be difficult to find in ancient rocks.
Many rocks have been chewed up and spit out by Earth's tectonic recycling
program, which pulls rocks under the Earth's crust and heats them. Other rocks
that manage to avoid this geological processing are still subjected to
degradation by wind or rain weathering. Scientists can work around this by looking
for evidence of life that can survive over time, such as biomolecules. By
searching for substances associated with life, scientists can determine whether
or not life existed when the ancient rocks formed.
Some rocks are
made when dirt gets trapped in biofilms - sticky mat communities that are
created when microbes excrete starch-like substances called "extracellular
polymeric substances," or EPS. After the organisms in these biofilms die,
the dirt in their mats turns to stone ("lithifies"). Ancient lithified
biofilms could potentially still contain chemical evidence of the life that
created them.
This new
study notes that EPS excreted by the microbes fossilizes quickly, becoming part
of the rock. Because of this, EPS could be on the list of biological substances
that indicate life was once present.
Oehler points
out, however, that EPS in a rock could be the result of organisms entering the pores
of the rock long after it initially formed. This means that while EPS could be
a red flag indicating life, additional tests must be done to determine whether the
EPS is the same age as the rock or was introduced by colonizing microbes at a
much later date.
Timeline
for Life
Knowing
when life was present is important for scientists who study the emergence of life
on Earth. Many believe that life appeared 3.5 billion years ago or even earlier,
but fossils dating that far back in time are highly controversial. While some
argue that the microscopic forms show strong evidence of having once been life,
others say these so-called
ancient fossils are just non-biological mineral deposits.
Pinning
down the time of life's emergence on Earth has implications for the search for
life on Mars. The environments of early Earth and Mars are thought to have been
similar in the distant past. There is evidence that the Martian surface may
have been covered by oceans, lakes and rivers
of flowing water 3.5 billion years ago, and these water-rich conditions may
have persisted episodically on the Red Planet until about 3 billion years ago.
New findings show that frozen water sits below the Martian surface at this very
moment. If Mars was Earth-like around the same time that life emerged on Earth,
then it seems possible that life also could have emerged on Mars.
"If life
appeared on Mars, there is a high probability that the traces of those life
forms were preserved by fossilization with silica or other minerals," the
scientists write. "Since many ancient microfossils on Earth are preserved in a
form of silica called chert, this gives high hopes for the finding of possible
past Martian life forms."
Although
the rovers Spirit and Opportunity are still investigating the surface of Mars,
they are not able to study the rocks in great enough detail to find evidence of
life. "We had to use high-resolution imaging technology to study the
microfossils," says Westall. "We can't do this on Mars. The bottom line is that
we will probably need to study rocks that have been carefully chosen and
returned from Mars."
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