[meteorite-list] Building Life From Star-Stuff

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Sep 8 12:29:56 2005
Message-ID: <200509081606.j88G6Sk12264_at_zagami.jpl.nasa.gov>

http://www.physorg.com/news6305.html

Building Life From Star-Stuff
physorg.com
September 08, 2005

Life on Earth was made possible by the death of stars. Atoms like carbon
and oxygen were expelled in the last few dying gasps of stars after
their final supplies of hydrogen fuel were used up.

How this star-stuff came together to form life is still a mystery, but
scientists know that certain atomic combinations were necessary. Water -
two hydrogen atoms linked to one oxygen atom -was vital to the
development of life on Earth, and so NASA missions now search for water
on other worlds in the hopes of finding life elsewhere. Organic
molecules built mostly of carbon atoms are also thought to be important,
since all life on Earth is carbon-based.

The most popular theories of the origin of life say the necessary
chemistry occurred at hydrothermal vents on the ocean floor or in some
sunlit shallow pool. However, discoveries in the past few years have
shown that many of the basic materials for life form in the cold depths
of space, where life as we know it is not possible.

After dying stars belch out carbon, some of the carbon atoms combine
with hydrogen to form polycyclic aromatic hydrocarbons (PAHs). PAHs -- a
kind of carbon soot similar to the scorched portions of burnt toast --
are the most abundant organic compounds in space, and a primary
ingredient of carbonaceous chondrite meteorites.

Although PAHs aren't found in living cells, they can be converted into
quinones, molecules that are involved in cellular energy processes. For
instance, quinones play an essential role in photosynthesis, helping
plants turn light into chemical energy.

The transformation of PAHs occurs in interstellar clouds of ice and
dust. After floating through space, PAH soot eventually condenses into
these "dense molecular clouds." The material in these clouds blocks out
some but not all of the harsh radiation of space. The radiation that
does filter through modifies the PAHs and other material in the clouds.

Infrared and radio telescope observations of the clouds have detected
the PAHs, as well as fatty acids, simple sugars, faint amounts of the
amino acid glycine, and over 100 other molecules, including water,
carbon monoxide, ammonia, formaldehyde, and hydrogen cyanide.

The clouds have never been sampled directly -- they're too far away --
so to confirm what is occurring chemically in the clouds, a research
team led by Max Bernstein and Scott Sandford at the Astrochemistry
Laboratory at NASA's Ames Research Center set up experiments to mimic
the cloud conditions.

In one experiment, a PAH/water mixture is vapor-deposited onto salt and
then bombarded with ultraviolet (UV) radiation. This allows the
researchers to observe how the basic PAH skeleton turns into quinones.
Irradiating a frozen mixture of water, ammonia, hydrogen cyanide, and
methanol (a precursor chemical to formaldehyde) generates the amino
acids glycine, alanine and serine -- the three most abundant amino acids
in living systems.

Because UV is not the only type of radiation in space, the researchers
also have used a Van de Graaff generator to bombard the PAHs with
mega-electron volt (MeV) protons, which have similar energies to cosmic
rays. The MeV results for the PAHs were similar although not identical
to the UV bombardment. A MeV study for the amino acids has not yet been
conducted.

These experiments suggest that UV and other forms of radiation provide
the energy needed to break apart chemical bonds in the low temperatures
and pressures of the dense clouds. Because the atoms are still locked in
ice, the molecules don't fly apart, but instead recombine into more
complex structures.

In another experiment led by Jason Dworkin, a frozen mixture of water,
methanol, ammonia and carbon monoxide was subjected to UV radiation.
This combination yielded organic material that formed bubbles when
immersed in water. These bubbles are reminiscent of cell membranes that
enclose and concentrate the chemistry of life, separating it from the
outside world.

The bubbles produced in this experiment were between 10 to 40
micrometers, or about the size of red blood cells. Remarkably, the
bubbles fluoresced, or glowed, when exposed to UV light. Absorbing UV
and converting it into visible light in this way could provide energy to
a primitive cell. If such bubbles played a role in the origin of life,
the fluorescence could have been a precursor to photosynthesis.

Fluorescence also could act as sunscreen, diffusing any damage that
otherwise would be inflicted by UV radiation. Such a protective function
would have been vital for life on the early Earth, since the ozone
layer, which blocks out the sun's most destructive UV rays, did not form
until after photosynthetic life began to produce oxygen.

>From space clouds to the seeds of life

Dense molecular clouds in space eventually gravitationally collapse to
form new stars. Some of the leftover dust later clumps together to form
asteroids and comets, and some of these asteroids clump together to form
planetary cores. On our planet, life then arose from whatever basic
materials were at hand.

The large molecules necessary to build living cells are:

* Proteins

* Carbohydrates (sugars)

* Lipids (fats)

* Nucleic acids

Meteorites have been found to contain amino acids (the building blocks
of proteins), sugars, fatty acids (the building blocks of lipids), and
nucleic acid bases. The Murchison meteorite, for instance, contains
chains of fatty acids, various types of sugars, all five nucleic acid
bases, and more than 70 different amino acids (life uses 20 amino acids,
only six of which are in the Murchison meteorite).

Because such carbonaceous meteorites are generally uniform in
composition, they are thought to be representative of the initial dust
cloud from which the sun and solar system were born. So it seems that
nearly everything needed for life was available at the beginning, and
meteorites and comets then make fresh deliveries of these materials to
the planets over time.

If this is true, and if molecular dust clouds are chemically similar
throughout the galaxy, then the ingredients for life should be widespread.

The downside of the abiotic production of the ingredients for life is
that none of them can be used as "biomarkers," indicators that life
exists in a particular environment.

Max Bernstein points to the Alan Hills meteorite 84001 as an example of
biomarkers that didn't provide proof of life. In 1996, Dave McKay of
NASA's Johnson Space Center and his colleagues announced there were four
possible biomarkers within this martian meteorite.

ALH84001 had carbon globules containing PAHs, a mineral distribution
suggestive of biological chemistry, magnetite crystals resembling those
produced by bacteria, and bacteria-like shapes. While each alone was not
thought to be evidence for life, the four in conjunction seemed compelling.

After the McKay announcement, subsequent studies found that each of
these so-called biomarkers also could be produced by non-living means.
Most scientists therefore are now inclined to believe that the meteorite
does not contain fossilized alien life.

"As soon as they had the result, people went gunning for them because
that's the way it works," says Bernstein. "Our chances of not making an
error when we come up with a biomarker on Mars or on Europa will be much
better if we've already done the equivalent of what those guys did after
McKay, et al., published their article."

Bernstein says that by simulating conditions on other planets,
scientists can figure out what should be happening there chemically and
geologically. Then, when we visit a planet, we can see how closely
reality matches the predictions. If there's anything on the planet that
we didn't expect to find, that could be an indication that life
processes have altered the picture.

"What you have on Mars or on Europa is material that's been delivered,"
says Bernstein. "Plus, you have whatever has formed subsequently from
whatever conditions are present. So (to look for life), you need to look
at the molecules that are there, and keep in mind the chemistry that may
have happened over time."

Bernstein thinks chirality, or a molecule's "handedness," could be a
biomarker on other worlds. Biological molecules often come in two forms
that, while chemically identical, have opposite shapes: a "left-handed"
one, and its mirror image, a "right-handed" one.

A molecule's handedness is due to how the atoms bond. While handedness
is evenly dispersed throughout nature, in most cases living systems on
Earth have left-handed amino acids and right-handed sugars. If molecules
on other planets show a different preference in handedness, says
Bernstein, that could be an indication of alien life.

"If you went to Mars or Europa and you saw a bias the same as ours, with
sugars or amino acids having our chirality, then people would simply
suspect it was contamination," says Bernstein. "But if you saw an amino
acid with a bias towards the right, or if you saw a sugar that had a
bias towards the left -- in other words, not our form -- that would be
really compelling."

However, Bernstein notes that the chiral forms found in meteorites
reflect what is seen on Earth: meteorites contain left-handed amino
acids and right-handed sugars. If meteorites represent the template for
life on Earth, then life elsewhere in the solar system also may reflect
that same bias in handedness.

Thus, something more than chirality may be needed for proof of life.
Bernstein says that finding chains of molecules, "such as a couple of
amino acids linked together," also could be evidence for life, "because
in meteorites we tend to just see single molecules."
Received on Thu 08 Sep 2005 12:06:28 PM PDT