NASA scientists at the Goddard Cosmic Ice Lab are studying a kind of
chemistry almost never found on Earth. The extreme cold, hard vacuum,
and high radiation environment of space allows the formation of an
unstructured form of solid water called amorphous ice.
Often particles and organic compounds are trapped in this ice that
could provide clues to life in the universe. Amorphous ice is so
widespread in interstellar space that it could be the most common form
of water in the universe. Left over from the age when the solar system
was born, it is scattered across vast distances, often as particles no
bigger than grains of dust. It has also been spotted in comets and icy
moons. The superthin ice can envelop building-block chemicals found in
space, including amino acids, which are key players in the chemistry of
life on Earth. Researchers have spent decades identifying an array of
amino acids in meteorites (including some involved in life), as well as
one found in a sample taken from a comet.
"We find that some amino acids could survive tens to hundreds of
millions of years in ice near the surface of Pluto or Mars and buried at
least a centimeter [less than half an inch] deep in places like the
comets of the outer solar system," says Cosmic Lab's Perry Gerakines.
"For a place that gets heavy radiation, like Europa, they would need to
be buried a few feet."
"This is not the chemistry people remember from high school," says
Reggie Hudson, who heads the Cosmic Ice Lab. "This is chemistry in the
extreme: bitter cold, harsh radiation and nearly non-existent pressure.
And it's usually taking place in gases or solids, because generally
speaking, there aren't liquids in interstellar space."
This cosmic ice needs such intense cold and low pressure to form that
the right conditions rarely, if ever, occur naturally on Earth. The
image below shows amorphous ice located in the iconic Horsehead Nebula,
part of a large, dark, molecular cloud, also known as Barnard 33. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. The darkness of the Horsehead is caused mostly by thick dust; the bright spots in the Horsehead Nebula's base are young stars in the process of forming.
These ultrathin layers turn out to be perfect for recreating some of
the key chemistry that takes place in space. In these tiny test tubes,
Gerakines and his colleagues in the Cosmic Ice Lab at NASA's Goddard Space Flight Center
in Greenbelt, Md., can reproduce reactions in ice from almost any time
and place in the history of the solar system, including some that might
help explain the origin of life.
The Cosmic Ice Lab is one of a few laboratories worldwide where
researchers have been studying the ultracool chemistry of cosmic ice.
With its powerful particle accelerator, the Goddard lab has the special
ability to mimic almost any kind of solar or cosmic radiation to drive
these reactions. And that lets them dig deep to study the chemistry of
ice below the surface of planets and moons as well as ice in space.
To simulate damage from solar wind particles and cosmic rays, scientists in Goddard’s Cosmic Ice Lab irradiate ice with a Van de Graaff accelerator. High voltage builds up in the 10-foot-long tube (left), culminating in the beam source (right) at the far end.
In a vacuum chamber about the size of a lunchbox, Gerakines recreates
a little patch of deep space, in all its extremes. He pumps out air
until the pressure inside drops to a level a billion times lower than
normal for Earth, then chills the chamber to minus 433 degrees
Fahrenheit (15 kelvins). To get ice, all that remains is to open a valve
and let in water vapor.
The instant the sprightly vapor molecules enter the chamber they are
literally frozen in their tracks. Still pointing every which way, the
molecules are transformed immediately from their gaseous state into the
disorderly solid called amorphous ice. Amorphous ice is exactly the
opposite of the typical ice on Earth, which forms perfect crystals like
those that make up snowflakes or frost needles. These crystals are so
orderly and predictable that this ice is considered a mineral, complete
with a rating of 2.5 on the Mohs scale of hardness—the same rating as a fingernail.
The secret to making amorphous ice in the lab, Gerakines finds, is to
limit the layer to a depth of about half a micrometer—thinner than a
strand of spider's silk.
"Water is such a good insulator that if the ice gets too thick, only
the bottom of the sample, closer to the cooling source, will stay
sufficiently cold," says Gerakines. "The ice on top will get warm enough
to crystallize."
"And because water is the dominant form of frozen material in the
interstellar medium and outer solar system," says Gerakines, "any amino
acids out there are probably in contact with water at some point."
For his current set of experiments, Gerakines makes three kinds of
ice, each spiked with an amorphous form of an amino acid (either
glycine, alanine or phenylalanine) that is found in proteins.
Earlier studies by other researchers have looked at ice chemistry
using ultraviolet light. Gerakines opts instead to look at cosmic
radiation, which can reach ice hidden below the surface of a planet or
moon. To mimic this radiation, he uses a proton beam from the
high-voltage particle accelerator, which resides in an underground room
lined with immense concrete walls for safety.
With the proton beam, a million years' worth of damage can be
reproduced in just half an hour. And by adjusting the radiation dose,
Gerakines can treat the ice as if it were lying exposed or buried at
different depths of soil in comets or icy moons and planets.
Gerakines tests the three kinds of water-plus-amino-acid ice and
compares them to ice made from amino acids only. Between blasts, he
checks the samples using a "molecular fingerprinting" technique called
spectroscopy to see if the amino acids are breaking down and chemical
by-products are forming.
As expected, more and more of the amino acids break down as the
radiation dose adds up. But Gerakines notices that the amino acids last
longer if the ice includes water than if they are left on their own.
This is odd, because when water breaks down, one of the fragments it
leaves behind is hydroxyl (OH), a chemical well-known for attacking
other.
The spectroscopy confirms that some OH is being produced. But
overall, says Gerakines, "the water is essentially acting like a
radiation shield, probably absorbing a lot of the energy, the same way a
layer of rock or soil would."
When he repeats the experiments at two higher temperatures, he is
surprised to find the acids fare even better. From these preliminary
measurements, he and Hudson calculate how long amino acids could remain
intact in icy environments over a range of temperatures.
"The good news for exploration missions," says Hudson, "is it looks
as if these amino acids are actually more stable than anybody realized
at temperatures typical of places like Pluto, Europa and even Mars."
These findings were reported in the journal Icarus in August 2012.
The Cosmic Ice Lab is part of the Astrochemistry Laboratory in
Goddard's Solar System Exploration Division and is funded in part by the
Goddard Center for Astrobiology and the NASA Astrobiology Institute.
Source: The Daily Galaxy via Goddard Space Flight Center
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