Molecules vital to life have been detected
in outer space and isolated in meteorites and comets. Some of this material
that rained down on Earth may have jump-started biology. If so, these space
seeds also may have planted a particular molecular orientation, or
"handedness," that spread to the world's first creatures. New
research is studying how this handedness could arise in space.
Amino
acids, the building blocks of proteins, exist in two so-called "chiral" forms
that are mirror
reflections of each other, like a left and right hand. For some unknown
reason, organisms use left-handed amino acids almost exclusively in making
proteins (the other mirror image, while rare, is sometimes used in other processes).
"Outside
of biology the ratio of these chiral forms is 50-50, so we want to understand
the starting point of life's preference for left-handed amino acids," says
Orlando Santos of NASA Ames Research Center.
Santos and his colleagues are designing a
small satellite that would carry up biologically relevant molecules to see what
effects space has on a sample's handedness, and whether this could explain the origins
of homochirality.
"Other
researchers in this field have tried to reproduce space conditions in a
lab," Santos says. "But artificial systems are just that. We want to
test the theories in a natural environment."
The project
is part of the Astrobiology Science and Technology Instrument Development and
Mission Concept Studies. In a follow-up story, Astrobiology Magazine will
profile another NASA-supported experiment that hopes to address how handedness
in space might be delivered to the ground.
Meteor
impact
The
evidence for a space origin for homochirality comes from meteorites. Several of
these space rocks contain amino acids, and in a few cases the left-handed amino
acids have outnumbered the right-handed ones by as much as 15 percent.
"There
is no doubt about the left-handed excess in meteorites," says Sandra
Pizzarello of Arizona State University, who has done extensive
analysis of various meteorite samples. She has found that they contain
higher than normal levels of the isotopes deuterium and carbon-13, which would
argue that the molecules came from space and are not simply contaminants from
Earth.
"The
isotope ratios imply that at least parts of each molecule formed in a cold
space environment at less than 50 degrees above absolute zero," says
George Cooper of NASA Ames.
Space
factors
However, it
is unknown what mechanism
in space could impart handedness to amino acids or any other molecules.
The most
popular candidate is circularly polarized ultraviolet light. Amino acids are
broken apart by ultraviolet light, but if this light is circularly polarized
(rotating in a clockwise or counterclockwise direction), the destruction of one
hand will be faster than the other.
"Both
hands decompose in the light, so it turns into a race toward total
destruction," Pizzarello explains. "An excess of one hand can be made
this way, but there will only be a small percentage of the original material
left over."
The maximum
excess that has been generated in the lab has been 9 percent, according to
Pizzarello. This is considerably less than what is observed in the meteorites,
so it's unclear what would make up the difference.
Another
concern is that circularly polarized light has never been detected in
ultraviolet wavelengths coming from an astronomical source. It has only been
inferred to exist around neutron stars and inside dense molecular clouds where
stars are forming.
An
alternative possibility is that magnetic fields can help to promote one chiral
form over the other. Louis Pasteur, who was the first to observe homochirality
in a biological sample, imagined that magnetic fields might play an important
part.
Later
theoretical work showed that a magnetic field lined up with ultraviolet light
can destroy one chiral form more readily that the other, just as in the case of
circularly polarized light.
"In
our solar system, you are more likely to encounter a magnetic field than
circularly polarized light," Cooper says.
However,
many scientists have tried but failed to create higher concentrations of one
chiral form with magnetic fields. It wasn't until 1997 that anyone succeeded in
showing the magnetic effect, but it took a huge field of more than a Tesla,
which is 10,000 times the Earth's magnetic field.
A
go-and-see policy
With no
clear mechanism at play, it may be that several factors in space work together
to produce an excess of one hand over the other.
"The
meteorites were exposed to a variety of effects - some that we don't even know
yet," Cooper says.
Cooper is
working with Santos, Arthur Weber and others in the NASA Ames Exobiology Branch
to develop a simple satellite based on the design of another experiment
called O/OREOS. This new satellite - entitled Exposure of Organics On a
Small Satellite (EOOSS) - will be used specifically study chiral effects in
space. The plan is to carry up amino acids and other organic compounds and
expose them to UV light from the sun and the magnetic field from the Earth.
"The
light and magnetic field can be produced in a lab, but we think other factors
could be important like microgravity," Santos says. "The lack of
gravity might help orient molecules, so you could get away with a smaller
magnetic field."
An on-board
polarimeter will record the polarity of light going through the samples, and
thereby determine if one hand of molecules begins to dominate the other. Data
will be collected in real-time and beamed down to Earth.
The Earth's
magnetic field probably had little influence on organic compounds falling onto
the planet from space. However, it can serve as a proxy for magnetic fields
that presumably existed billions of years ago when these compounds formed.
"We
want to simulate the early solar system environment of these organics as best
we can," says Cooper.
Devil in
the details
"It's
always good to go and test something in a real setting," says Pizzarello,
who is not involved with the project.
However,
she is not sure that the Earth's magnetic field will be strong enough.
"The devil's in the details," she says.
Right now,
Santos and his colleagues are working on their design, and they expect in
January 2010 to have a final decision on whether to go forward and build their
satellite. If all goes well, a launch could happen in 2011-12, Santos says.
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