[meteorite-list] Nanodiamonds are Forever? Maybe Not.

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:45 2004
Message-ID: <200303060036.QAA06360_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=390&mode=thread&order=0&thold=0

Nanodiamonds are Forever? Maybe Not.
Astrobiology Magzine
March 5, 2003

Summary: When nanodiamonds were discovered in 1989, they seemed to be
remnants of supernovas - tiny grains of physical history even older than the
solar system. Logically, comets should be full of these microscopic
diamonds. But a recent study found no nanodiamonds in comet dust, indicating
that the miniature crystals may have formed during the formation of our
solar system, and not before. If these tiny grains are as "young" as 4.5
billion years, what does that mean for the formation and development of
life?

Nanodiamonds are Forever? Maybe Not.

By: David Tenenbaum

Back when "nanotechnology" was still pure science fiction, "nanodiamonds"
from outer space became science fact. First discovered in meteorites in
1989, these tiny diamonds average 3 nanometers across - 300,000 of them
would fit side by side across the width of a human hair - and contain just a
few thousand carbon atoms.

By 1993, scientists discovered that in some nanodiamonds the ratio of
different isotopes of the inert gas xenon resembled those detected in
supernova explosions. Over time, scientists came to believe that nanodiamond
story began with a formation in star explosions that distributed the tiny
grains through the universe. One destination was the solar nebula that
coalesced into our solar system about 4.5 billion years ago.

Among other places, the tiny pre-solar (older than the solar system) grains
wound up in asteroids, which eventually broke up to form the meteorites in
which the first nanodiamonds were discovered. Meteorites originate chiefly
in the asteroid belt, which is relatively protected from the ravages of
sunlight, heat and chemical reactions that probably destroyed nanodiamonds
elsewhere in the solar system.

If that understanding was correct, nanodiamonds should be even more common
in regions like the Kuiper belt, home of many comets, which is even farther
from the Sun. And thus the surprise when, last summer, a new study found no
nanodiamonds in cometary debris.

The study turned the conventional wisdom inside out, says author John
Bradley, director of the Institute for Geophysics and Planetary Physics at
Lawrence Livermore National Laboratory. "If nanodiamonds are truly
pre-solar, they should get progressively more abundant as you go further
out, and they should be more abundant in comets than in asteroids. But we
did not find any in comets, which suggests that abundance falls off with
distance from the sun, rather than increases."

Bradley and colleagues published the study, "Possible in situ formation of
meteoritic nanodiamonds in the early Solar System," in the July 11, 2002,
issue of the journal Nature. The research analyzed meteorites, and
interplanetary dust particles trapped by a U2 plane in the stratosphere.

Nanodiamonds are detected using a series of acid baths in a process that has
been compared to "burning down a haystack to find the needle." Once the
diamonds are isolated, a spectrographic analysis is made of their isotopic
compositions.

As expected, the meteorites that came from asteroids carried nanodiamonds at
roughly one part per thousand by mass. A class of smaller dust particles
that were not clustered together, however, showed no trace of diamond.
Because these smaller particles appeared more "pristine" - less processed by
light and chemistry - the researchers concluded that they had come from
comets, not meteorites.

The absence of diamonds was surprising, says Bradley, since supernova
explosions should have "salted" nanodiamonds through the solar nebula: "This
is turning the picture around completely."

In retrospect, however, the finding does shed light on a certain
inconsistency in the link between nanodiamonds and supernovae. "It was
anomalous xenon isotopes that tagged them as pre-solar, but the bulk
isotopic composition was the same as the solar system," says Bradley. In
other words, although the rare xenon atoms trapped in the diamonds were
clearly non-solar, the far more numerous carbon atoms had run-of-the-mill
solar-system isotopic ratios.

The resolution of this conflict may lie in the fact that most nanodiamonds
do not contain even a single xenon atom, says Alan Boss, in the Department
of Terrestrial Magnetism at the Carnegie Institution of Washington. "It
could be that only one in a million nanodiamonds carries the xenon, and
maybe those diamonds are still from supernovae, but the rest of the diamonds
come from other processes."

But how could nanodiamonds form in the nascent solar system? One
possibility, Boss says, is shock caused by continual collisions in the
asteroid belt. A stronger possibility, however, is chemical vapor deposition
(CVD), a process used to make diamond film in the laboratory. "The
microstructural evidence from nanodiamonds indicates that they probably
formed by chemical vapor deposition," says Bradley.

For two reasons, CVD, in which gases undergo chemical reactions before
condensing, was until recently a doubtful source of nanodiamonds. First, CVD
works better in non-oxidizing conditions, and the solar nebula apparently
had a substantial amount of oxygen. Second, CVD is more efficient when the
carbon starts to crystallize on a substrate of atoms such as silicon, but
substrate atoms are not found in nanodiamonds. Now, it appears that diamonds
can form without substrates, under some oxidizing conditions.

More evidence for a stellar-nebula formation came shortly after the Bradley,
et al., report, in a study by Caroline Van Kerckhoven and colleagues.
"Nanodiamonds around HD 97048 and Elias 1," published in Astronomy &
Astrophysics, reported the detection of spectrographic signs of nanodiamonds
in two nebulae where stars (and conceivably planets) were forming.

As the conventional wisdom about nanodiamonds is revised, Bradley stresses
that the question of origins does not require an either/or answer. "My
personal view is that nanodiamonds probably form everywhere throughout the
galaxy, under all sorts of conditions."

For the search for life beyond Earth, the implications of a revised
nanodiamond theory could be momentous. If, due to solar heat and asteroid
impacts, early Earth was, as Boss puts it, "a molten body with a steam
atmosphere," then where did the oceans and organic compounds come from?

If the source was, as some suspect, a rain of comet fragments, then
understanding the circulation of material through the early solar system is
critical to understanding the origin of life. If the source or sources of
nanodiamonds can be pinned down, the little crystals could provide a rare
source of data on material flow through the primordial solar system, Boss
says.

Understanding the physics of the nebula is a tough job, he stresses. "This
detective story is difficult to unravel. We are 4.5 billion years after the
crime, and even though we are at the scene of the crime, we need every piece
of evidence to make a cohesive story."

For now, the nano-detectives may turn to laboratories rather than the usual
tools of spacecraft and telescopes. If most nanodiamonds formed via chemical
vapor deposition, researchers need to know more about artificial diamonds.
Processes that work today in the lab, after all, may also have worked in the
solar nebula 4.5 billion years ago.
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Received on Wed 05 Mar 2003 07:36:57 PM PST