A meteorite analyzed in the study at its collection site in Antarctica. Credit: Antarctic Search for Meteorites program, Case Western Reserve University. |
Creating
some of life’s building blocks in space may be a bit like making a
sandwich—you can make them cold or hot, according to new NASA research.
This evidence that there is more than one way to make crucial components
of life increases the likelihood that life emerged elsewhere in the
Universe, according to the research team, and gives support to the
theory that a “kit” of ready-made parts created in space and delivered
to Earth by impacts from meteorites and comets assisted the origin of
life.
In
the study, scientists with the Astrobiology Analytical Laboratory at
NASA’s Goddard Space Flight Center in Greenbelt, Md., analyzed samples
from fourteen carbon-rich meteorites with minerals that indicated they
had experienced high temperatures—in some cases, over 2,000 F. They
found amino acids, which are the building blocks of proteins, used by
life to speed up chemical reactions and build structures like hair,
skin, and nails.
Previously,
the Goddard team and other researchers have found amino acids in
carbon-rich meteorites with mineralogy that revealed the amino acids
were created by a relatively low-temperature process involving water,
aldehyde and ketone compounds, ammonia, and cyanide called
“Strecker-cyanohydrin synthesis.”
“Although
we’ve found amino acids in carbon-rich meteorites before, we weren’t
expecting to find them in these specific groups, since the high
temperatures they experienced tend to destroy amino acids,” said Dr.
Aaron Burton, a researcher in NASA’s Postdoctoral Program stationed at
NASA Goddard. “However, the kind of amino acids we discovered in these
meteorites indicates that they were produced by a different,
high-temperature process as their parent asteroids gradually cooled
down.” Burton is lead author of a paper on this discovery appearing
March 9 in Meteoritics and Planetary Science.
In
the new research, the team hypothesizes the amino acids were made by a
high-temperature process involving gas containing hydrogen, carbon
monoxide, and nitrogen called “Fischer-Tropsch” –type reactions. They
occur at temperatures ranging from about 200 to 1,000 degrees Fahrenheit
with minerals that facilitate the reaction. These reactions are used to
make synthetic lubricating oil and other hydrocarbons; and during World
War II, they were used to make gasoline from coal in an attempt to
overcome a severe fuel shortage.
Researchers
believe the parent asteroids of these meteorites were heated to high
temperatures by collisions or the decay of radioactive elements. As the
asteroid cooled, Fischer-Tropsch-type (FTT) reactions could have
happened on mineral surfaces utilizing gas trapped inside small pores in
the asteroid.
FTT
reactions may even have created amino acids on dust grains in the solar
nebula, the cloud of gas and dust that collapsed under its gravity to
form the solar system. “Water, which is two hydrogen atoms bound to an
oxygen atom, in liquid form is considered a critical ingredient for
life. However, with FTT reactions, all that’s needed is hydrogen, carbon
monoxide, and nitrogen as gases, which are all very common in space.
With FTT reactions, you can begin making some prebiotic components of
life very early, before you have asteroids or planets with liquid
water,” said Burton.
In
the laboratory, FTT reactions produce amino acids, and can show a
preference for making straight-chain molecules. “In almost all of the 14
meteorites we analyzed, we found that most of the amino acids had these
straight chains, suggesting FTT reactions could have made them,” said
Burton.
It’s
possible that both Strecker and FTT processes could have contributed to
the supply of amino acids in other meteorites. However, evidence for
the FTT reaction would tend to get lost because FTT reactions create
them in much lower abundances than Strecker synthesis. If an asteroid
with an initial amino acid supply from FTT reactions was later altered
by water and Strecker synthesis, it would overwrite the small
contribution from the FTT reactions, according to the team.
The
team believes the majority of the amino acids they found in the 14
meteorites were truly created in space, and not the result of
contamination from terrestrial life, for a few reasons. First, the amino
acids in life (and in contamination from industrial products) are
frequently linked together in long chains, either as proteins in biology
or polymers in industrial products. Most of the amino the amino acids
discovered in the new research were not bound up in proteins or
polymers. In addition, the most abundant amino acids found in biology
are those that are found in proteins, but such “proteinogenic” amino
acids represent only a small percentage of the amino acids found in the
meteorites. Finally, the team analyzed a sample of ice taken from
underneath one of the meteorites. This ice had only trace levels of
amino acids suggesting the meteorites are relatively pristine.
The
experiments showing FTT reactions produce amino acids were performed
over 40 years ago. The products have not been analyzed with modern
techniques, so the exact distributions of amino acid products have not
been determined. The team wants to test FTT reactions in the laboratory
using a variety of ingredients and conditions to see if any produce the
types of amino acids with the abundances they found in the 14
meteorites.
The
team also wants to expand their search for amino acids to all known
groups of carbon-rich meteorites. There are eight different groups of
carbon-rich meteorites, called “carbonaceous chondrites.” The new work
adds two additional groups to the three previously known to have
produced amino acids, leaving three groups to be tested. These three
remaining groups have a high metal content as well as evidence for high
temperatures. “We’ll see if they have amino acids also, and hopefully
gain some insight into how they were made,” says Burton. When the team
began looking for amino acids in carbon-rich meteorites, it was
considered somewhat of a long shot, but now: “We would be surprised if
we didn’t discover amino acids in a carbon-rich meteorite,” says Burton.
The
research was funded by the NASA Astrobiology Institute (NAI), the
Goddard Center for Astrobiology, and the NASA Cosmochemistry Program.
NAI is managed by NASA Ames Research Center in Mountain View, Calif. Dr.
Burton was supported by the NASA Postdoctoral Program, administered by
Oak Ridge Associated Universities through a contract with NASA.
Meteorite samples were provided by Dr. Kevin Righter of NASA’s Johnson
Space Center, Houston, Texas.
