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The cosmos is laced with tiny specks of dust
that decide the fate of young stars and planets. Now, NASA scientists
can study the properties of far-flung space dust in the lab.
by Patrick Barry
Space is a vacuum, right?
Well, almost... The space between the stars is about as empty as
the best artificial vacuums created by scientists on Earth, but
throughout space, the void is faintly sprinkled with gas molecules
and dust grains.
These extremely sparse crumbs of matter drifting in lonely spaces
between the stars may seem utterly obscure and insignificant, but
they turn out to play an important role in the formation of stars
and planets and many other astrophysical phenomena.
To better understand how dust grains
respond to conditions in space, researchers at NASA's Marshall Space
Flight Centre (MSFC) have built an apparatus in the Dusty Plasma
Laboratory (DPL) that can suspend individual dust grains in a near
vacuum. Once a dust grain is captured, scientists can bombard it
with forms of radiation found in space and see what happens.
"What we're doing here is taking one particle and exposing it to
these space-like environments and studying what happens to its (electrical)
charge and other properties," said Catherine Venturini, who worked
on the project for more than four years while pursuing her master's
degree in physics at the University of Alabama in Huntsville.
The electrical charge on dust grains in
space can, among other things, determine how small particles of dust
stick together and grow into larger-sized grains that lead to the
formation of stars and planets. Gravity pulls dust and gas in interstellar
clouds together, but because the electrostatic force over short distances
is so much stronger than gravity, even a small electrostatic repulsion
between dust grains can influence (and possibly prevent) a cloud's
collapse.
Stars
in the Keyhole Nebula began to form about 3 million years
ago. Tiny grains of interstellar dust influence how rapidly
the nebula can collapse. They also enrich the cloud with heavy
elements that are important for the formation of rocky Earth-like
planets.
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Also, the gas in the cloud tends to heat
up as it compresses. If the cloud cannot radiate that heat away, the
expansive force of the heat will resist further compression. Dust
grains in the cloud are able to radiate this heat as infrared light,
cooling the cloud and allowing collapse to continue.
If a nebula does collapse into a stellar system, dust provides many
of the elements such as carbon, iron, magnesium and silicon that comprise
the planets. (Unlike household dust, which is largely composed of
dead skin cells and other organic debris, cosmic dust probably consists
mostly of heavier elements.)
The role cosmic dust plays in planet and star formation is only
one of many reasons astronomers are interested in better understanding
its properties.
Dust also plays a role in crafting some of the most beautiful features
of the cosmos: planetary rings (like those around Saturn), the long
tails of comets and the spectacular, colourful clouds of nebulae.
Saturn's rings are marked by strange dark radial
features called spokes. Since they have been observed on both sides
of the ring plane, spokes are thought to be microscopic dust grains
that have become charged and are levitating away from the ring plane.
Another possibility is that a meteoroid punched through Saturn's
rings, lifting dust particles away from the plane of the rings.
When the Voyager spacecraft first observed these spokes, their movements
seemed to defy gravity and had the scientists very perplexed. Since
the spokes rotate at the same rate as Saturn's magnetic field, it
is likely that electromagnetic forces are at work. This is still
an unsolved puzzle.
Comet tails also contain a large amount of dust
expelled by gases released when the comet passes close to the Sun.
Because comets are composed mainly of dust and ice, studying the
properties of cosmic dust will help scientists understand comets
better, says Venturini.
Interstellar nebulae are laced with dust, too. The percentage of
dust in nebulae is much less than in comets - less than 1 percent
- but still the dust has important effects on the properties of
the cloud.
For example, dust influences the way the cloud reflects, absorbs
or emits light.
Understanding the optical properties of dust is especially important
when an interstellar cloud upstages some other astronomical object
that scientists are trying to study.
Dark nebulae completely block light coming from stars behind them,
creating a dark patch in the sky. Some nebulae shine with reflected
light from nearby stars (like clouds in Earth's atmosphere illuminated
by the Sun), while other nebulae emit their own radiation in the
form of infrared light.
"Most of the infrared light observed from the
sky results from space dust," said Dr. James Spann, a DPL principal
investigator. "Many times astronomers wish it was not there, but
it is. They have to remove the contribution of the dust so that
they can study other objects they are interested in."
This image,
from a picture captured by Dave Palmer, shows the Milky Way
in the constellation of Sagittarius. The Centre of our galaxy
lies near the middle of the image, but we can't see it because
dust grains in intervening clouds block starlight coming from
the core of the Milky Way. The dark areas in this image are
places where the dust concentration is high. (Airplanes caused
the lines through this picture.)
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The Dusty Plasma Lab allows scientists at MSFC
to study the optical characteristics of the dust grains trapped
in their apparatus.
Venturini said that other experiments
will measure how dust grains of different sizes and materials scatter,
absorb and emit light of different frequencies. Experiments with
the DPL have already measured the effects of ultraviolet radiation
and an electron beam on simulated cosmic dust grains. The electron
beam is used to mimic the plasma in which cosmic dust is sometimes
immersed - hence the name "Dusty Plasma Laboratory."
To complement these experiments, the
MSFC investigators are working on a collaborative arrangement with
Auburn University's Space Plasma Laboratory (SPL). The SPL, which
is coordinated by Dr. Edward Thomas, performs experiments on large
groups of dust particles rather than individual grains.
"The characteristics that we will see from the individual dust particles
can be calibrated and correlated and investigated in connection
with (Thomas' results)," explained Dr. Mian Abbas, an MSFC scientist
and the principal investigator for planned experiments dealing with
optical characteristics of dust grains and their condensation processes.
"We look at single particles and Dr. Thomas looks at large collections
of particles, so the two are sort of the complements of each other."
Laboratory work is only one aspect of better understanding cosmic
dust, Venturini continued.
"It's another facet to understanding the whole picture. You have
the modeling, you've got the theory, you've got the observation
from satellites, and then you need the lab work to help verify the
other components."
Satellite missions with instruments for measuring interstellar dust
- such as Cassini, Galileo, and STARDUST - have fuelled a surge
of interest in such research over the last decade or so, Venturini
said.
"Because of (the satellite data) they're realizing, 'Hey, these
little dust particles are playing a much more important role than
we thought before,'" Venturini said.
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