It
is now clear that organic chemistry has run rampant through the
solar system and beyond.
Carl
Sagan, Scientific American, 1997
May 21, 1999: When the Galileo spaceprobe flew by Jupiter's
moon Callisto earlier this month, the detection of life on that strange
and distant world was not among the scientific objectives. After
all, Callisto's heavily cratered surface is a frigid -220o
F and is scarcely protected from the ravages of space by a extraordinarily
thin CO2 atmosphere. Indeed most astrobiologists concur
that Callisto is an unlikely abode for life.
But even if Callisto was wet and warm and teeming with life,
would Galileo have noticed? The question brings to mind an earlier
Galileo flyby of another curious planet -- Earth.
Above: This view of Earth's southern
hemisphere centered on the South Pole was created using images
from the Galileo spacecraft taken during the December 1990 flyby.
More
information.
When the Galileo spaceprobe swooped by Earth in 1990, all its
instruments were pointed towards us. As Galileo flew toward our
planet, the Earth was centered in the windshield and then again
in the rear-view mirror as Galileo continued on its journey to
Jupiter.
Galileo's close encounter with Earth framed one of the most difficult
questions in astrobiology:
Can a modern space instrument tell if the Earth, or any planet,
is a good candidate for harboring life?
Above A candidate line-up: How would
a spacecraft flyby pick out the lone and unusual suspect that
harbors life? As Cornell Professor, J.R. Vallentyne, put the
matter in his opinion in 1965: "Apparent inherent limitations
on temperature, pressure or chemical environment for living matter
are geocentric myths."
To put the 1990 flyby in perspective, the late
Carl Sagan and his colleagues published a 1993 Nature
article on this question. According to Sagan, the Galileo spacecraft
found clear signs of life during its flight past the earth including:
- strong absorption of light at the red end of the visible
spectrum, particularly over the continents. The light-absorbing
pigment that causes this is chlorophyl, a molecule essential
to plant life and photosynthesis. (Plants appear green because
chlorophyl reflects green light and absorbs red and blues.)
- spectral absorption features caused by molecular oxygen in
Earth's atmosphere. The amount of O2 in our atmosphere
is many orders of magnitude greater than is found on any other
planet in the Solar System. An oxygen-rich atmosphere
is a curiosity because oxygen slowly combines with rocks on the
earth's surface. Maintaining the oxygen content requires some
replenishing mechanism, in this case photosynthesis by plants
-- the action of life.
- infrared spectral lines caused by methane in the atmosphere.
Although the amount of methane Galileo saw was miniscule
-- about 1 part per million -- it is still important. In a oxygen-rich
atmosphere like Earth's, methane should rapidly oxidize into
water and CO2. Not a single molecule of methane would
remain in equilibrium. Biological action such as bacterial metabolism
in bogs replenishes the supply.
- modulated narrowband radio transmissions. These emissions
look nothing like natural sources of radio waves like lightning
and plasma instabilities in Earth's magnetosphere. They are clear
signs of a technological civilization.
Galileo's flyby of Earth was just the beginning of the first-ever
control experiment in astrobiological remote sensing. The second
part happened two years later, in 1992, when Galileo returned
for a flyby of the moon.
Right: The false-color image of
the Moon was taken in 1992 by the Galileo spacecraft enroute
to Jupiter. The Sea of Tranquillity (Mare Tranquillitatis) is
the blue area at right, the Ocean of Storms (Oceanus Procellarum)
is the extensive blue and orange area on the left, and white
lines radiate from the crater Tycho at bottom center. Three filters
were used to make three separate exposures, combined in an exaggerated
color scheme to emphasize composition differences - blue hues
reveal titanium rich areas while orange and purple colors show
regions relatively poor in titanium and iron. More
information.
While Earth is known to be teeming with life, the Moon is believed
to be the exact opposite -- cold, barren, and lifeless throughout
its long geological history. What did Galileo see when it passed
by the moon?
"Nothing," says David Noever, a NASA astrobiologist.
"There was no evidence for life. No chlorophyll, no oxygen-methane
atmosphere, no artificial radio transmissions. It was just as
we would have expected, and consistent with the Sagan criteria."
Caveat Lunar
The Galileo flybys showed that we know how to identify life
at a distance, at least the kinds of life we're familiar with
here on Earth. However, things may not be as simple as they seem.
Organic compounds have been discovered in some unlikely -- and
almost certainly lifeless -- places, including amino acids in
meteorites, organic molecules in interstellar clouds, and organic
compounds called porphyrins in lunar soil.
Left: Unloading of Apollo 12 lunar
soil and rocks in November 1969
The example of porphyrins
on the Moon is particulary intriguing in the context of the
Galileo flybys and Sagan's subsequent criteria for life. Porphyrins
are the building blocks of brightly pigmented biomolecules such
as hemoglobin and chlorophyll which reflect only certain wavelengths
of visible light. Chlorophylls, for example, are greenish pigments
which contain a porphyrin ring. This is a stable ring-shaped
molecule around which electrons are free to migrate. Because
the electrons move freely, the ring has the potential to gain
or lose electrons easily, and thus the potential to provide energized
electrons to other molecules. This is the fundamental process
by which chlorophyll captures or harvests the energy of sunlight--a
kind of powerstation molecule underlying all life seen on earth.
The first 3 of Sagan's 4 criteria for life, as gleaned from Galileo's
Earth flyby, are all related to porphyrins through the action
of chlorophyll. Chlorophyll and photosynthesis are responsible
for the spectral colors of plant-covered continents, for the
oxygen content of the atmosphere and for its methane balance.
Galileo didn't detect porphyrins during its flyby of the Moon,
but they were there in quantities too small to see.
Right:
Porphyrin molecules seem fully capable of biological wizardry
on Earth. Put an iron atom in a porphyrin and the closely related
oxygen-carrying blood molecule, hemoglobin, results. Put a magnesium
atom in a porphyrin and the closely-related light-harvesting
molecule, chlorophyll, is made. Put lunar soil specimen, 12023,
into the lab for chemical analysis, and porphyrin shows up on
the moon.
Here on Earth porphyrin organic compounds are useful biomarkers.
For example, petroleum hunters look for porphyrins as markers
of oil deposits and thermal maturity. They can be detected remotely
without extracting organic matter to reveal oil shales and source
rock that came from the decay of green plants.
Does the presence of porphyrins mean that there is or has been
life on the Moon?
Not at all. The 1969 discovery of lunar porphyrins probably says
less about the chances for biochemistry there, than about how
common their generation may be elsewhere in the universe. In
1978 Simionescu et al. were able to produce porphyrins
under laboratory conditions similar to those of primaeval Earth,
before the genesis of life. They summarized the results in the
journal Origins of
Life:
"Experiments with gas mixtures intended to simulate the
primaeval atmosphere of the Earth yielded many biologically important
chemicals. Investigations into the synthesis of porphyrin-like
compounds from methane, ammonia and water vapour were carried
out by using high frequency discharges. Microanalyses of porphyrins
showed that porphyrin-like pigments were formed in this way.
The presence of divalent cations in the reaction system increased
the yield of porphyrin-like pigments also involving the direct
synthesis of their metal complexes. The ready formation of these
compounds in abiotic conditions is significant, suggesting the
possibility of their appearance during the early stage of chemical
evolution."
Left: A close-up view of Apollo
12 lunar
sample no. 12025, called Core Sample 1, and collected on
the lunar surface, about 225 meters below the point where the
Apollo 12 Lunar Module touched down. Soil sample 12025 is closely
spaced in collection catalogs with the porphyrin-like pigments
in Apollo 12 lunar soil sample 12023. Far Left: A brightly
orange pigmented pebble-like lunar sample, Apollo 17 collection
catalog.
The idea that the "stuff of life" is common even in
lifeless places like the Moon is gaining momentum. On February
19th of this year an article in Science magazine reported
one group's attempt to mimic an organic chemistry lab in outer
space. The research team included a new breed of astrochemists--including
Scott Sandford at the NASA Ames Research Center and the SETI
Institute, both in Mountain View, CA, and lead author of the
Science paper, Max Bernstein of Stanford University. Their
experiments involved a class of complex carbon and hydrogen molecules,
called polyaromatic hydrocarbons, or "PAHs." Like the
porphyrins, these molecules are also part of the so-called CHNOPS
elements--carbon, hydrogen, oxygen, nitrogen, phosphorus and
sulfur.
To reproduce the chemistry of an interstellar molecular cloud,
Bernstein's group followed a simple recipe:
- mix carbon and hydrogen molecules, the PAHs, with water ice
at minus 440 degrees Fahrenheit, the temperature inside an interstellar
cloud;
- place these ice grains in a vacuum;
- shine ultraviolet light on them, the same type of radiation
put out by nearby stars and re-radiated by glowing hydrogen gases.
Because of the extreme conditions, the likelihood of
more complex, biologically useful molecules being formed seemed
as remote as space itself. Instead, about 10 percent of the PAHs
were converted to more biologically useful molecules such as
alcohols, ketones and esters.
"These experiments take molecules that only an astrophysicist
could love and transform them into something that ought to fascinate
astrobiologists," comments Thomas Wdowiak, an astrophysicist
at University of Alabama at Birmingham. "This shows there
is a process that takes a rather abundant substance that exists
in the universe and converts it to the kinds of things that are
susceptible to the origin-of-life scenario."
Earth as we, the aliens, might see it....
On Christmas Eve 1968, Apollo 8 completed 10 orbits around
the Moon and returned live television pictures back to our planet.
Over half a billion people watched as Earth rose on the Moon's
horizon. For many observers it was a transforming perspective.
Poet Archibald MacLeish wrote: ". . . to see the earth as
it truly is, small and blue and beautiful in that eternal silence
where it floats, is to see ourselves as riders on the earth together...."
For astrobiologists, the Galileo flyby invoked a similar transformation
-- a first-time view of the Earth as an alien world. It affirmed
that standards of proof may be the most interesting -- and vexing
-- piece of the puzzle in the search for life among the stars.
Meanwhile, scientists continue to push the limits of their understanding
of both the biological and pre-biotic envelope for life, as we
might know it, and how we might see it remotely from space--even
when looking back directly at ourselves.
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