The idea that comets and meteorites seeded an early Earth with
the tools to make life has gained momentum from recent observations of some of
these building blocks floating throughout the cosmos.
Scientists scanning a galaxy 12 million light-years away with
NASA's Spitzer Space Telescope detected copious amounts of nitrogen containing
polycyclic aromatic hydrocarbons (PANHs), molecules
critical to all known forms of life.
PANHs carry information for DNA and RNA and are an important component of
hemoglobin, the molecule that transports oxygen through the body. They also
make chlorophyll, the main molecule responsible for photosynthesis in plants,
and – perhaps most importantly – they're the main ingredient in
caffeine and chocolate.
"There once was a time that the assumption was that the
origin of life, everything from building simple compounds up to complex life,
had to happen here on Earth," said study leader Doug Hudgins of Ames Research
Center. "We've
discovered that some very biologically interesting molecules can be formed
outside our earthly environment and delivered here."
Wherever there's a planet ...
While organic compounds have been discovered in meteorites that
have landed on Earth, this is the first direct evidence for the presence of
complex, important biogenic compounds in space. So far evidence suggests that PANHs are formed in the winds of dying stars and spread all
over interstellar space.
"This stuff contains the building blocks of life, and now
we can say they're abundant in space," Hudgins said. "And wherever
there's a planet out there, we know that these things are going to be raining
down on it. It did here and it does elsewhere."
Using the Spitzer Space Telescope, Hudgins and his colleagues
detected the familiar chemical signature of regular polycyclic aromatic
hydrocarbons (PAHs) in the spiral galaxy M81, as well
as a similar, but unknown signature.
"There were a few anomalies in the spectrum that we
couldn't explain," Hudgins told SPACE.com.
The researchers compared their readings to the infrared signatures of similar
molecules, finally settling on nitrogen containing PANHs
because their data showed there was nitrogen in the regions they were
investigating.
"When we did that, we found that by putting a little
nitrogen in these molecules explained the troubling molecules," Hudgins
said. "This discovery takes this reservoir of molecules that we didn't
think were interesting and transforms all this stuff into something of biologic
interest."
The chicken wire of life
PAHs are flat, chicken-wire shaped
molecules made up of carbon and hydrogen, interesting to scientists because
life on Earth is carbon-based. However, PAHs are not
used in human biochemistry. In fact, they're better known as cancer-causing
carcinogens and environmental pollutants.
But swap a carbon atom with a nitrogen and a PAH becomes a
PANH, a class of molecules critical to humans. Without nitrogen, it would be
impossible to build amino acids, proteins, DNA, RNA, hemoglobin, and many other
important molecules.
Here on Earth, Nitrogen makes up 78 percent of the atmosphere
and is a key member of CHNOPS – carbon, hydrogen, nitrogen, oxygen,
phosphorus, and sulfur – the group of ingredients most important for
making life and staples of organic chemistry.
It's also the main component of ammonia, which is used in
fertilizers and explosives on Earth, but has also been detected in Jupiter's atmosphere
and possibly in Titan's icy
lakes.
PANHs aren't the first of life's
building blocks to be discovered in space – amino acids, the nuts and
bolts of proteins, have also been found in
the tails of comets. Meteorites
that have landed in Australia and Antarctica also contain amino acids and PANHs.
"This tells us that these things that we see out in space
can survive interstellar space and successfully be delivered to the surface of
a planet," Hudgins said.
Does not mean life
Some scientists even think that a Martian
meteorite found in Antarctica shows signs of extraterrestrial bacteria and
that sugar-loaded
asteroids may have fed early life on our planet.
While PANHs are abundant in
interstellar space, Hudgins says this doesn't prove that terrestrial life has
extra-terrestrial origins. But, to paraphrase Occam's
Razor, given two equally likely theories, choose the
simpler.
"This isn't proof that they were used, but a likely
suggestion," Hudgins said. "They were present in abundance at the
dawn of time and could have been useful in creating the first life form."
These findings are detailed in the Oct 10 issue of the Astrophysical Journal.
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