 space science news home |
|
|
space science news home |
|
May
29, 1998: As astrophysicists turn their telescopes to probe
the origins of stars and planets, they will start giving more
attention to the smallest of astronomical bodies - dust particles
- which both make them and also obscure the view.
"We're developing an experimental method to measure scattering and extinction cross sections for dust particles in the solar system," said Dr. James Spann of NASA's Marshall Space Flight Center. Spann is leading development of the Dusty Plasmas Laboratory. In it, a single grain of dust is suspended by static electricity while it is bombarded with electrons and light and its reactions measured.
At right, above, a dust storm sweeps across the north pole of Mars in images taken by the Hubble Space Telescope on Sept. 18 and Oct. 15, 1996.
Dust might seem like a lowly object to receive such attention, but it's an important factor in the vacuum between planets and stars. Dust particles drift through space where they absorb and scatter light.
How rapidly they extinguish light over the millions or billions of miles of "empty" space determines how visible the source will be.
"We think we can devise an experiment that replicates the environment of these particles in planetary or preplanetary atmospheres," Spann said.
The observations planned by Spann and another Marshall scientist, Dr. Mian Abbas, will balance between two well known areas of optics, Rayleigh scattering and geometrical optics. Rayleigh scattering, where an object is much smaller than a wavelength of light, is why the sky is blue. Geometrical optics, where an object is much larger than a wavelength of light, is why lenses bend light.
Between these two is the Mie theory covering light scattered by objects that are about the same size as a wavelength of light.
"It's a very beautiful theory," Spann said. "It's incredibly fascinating for a lot of reasons."
One of those reasons is how infrared light is scattered by dust grains which are much larger than visible light, but about the size of longer-wavelength infrared.
Little work has been done in this area - it's mostly extrapolated from visible light observations or from the bulk properties of dust. The work won't be easy.
"Part
of the challenge in this experiment is that these grains are irregularly
shaped," Spann explained. "Unless you're dealing with
liquid droplets, which are spherical, the orientation of the grain
is important." Thus, a grain may be larger than a wavelength
of light across its length, but much smaller across its width.
Right: Dr. Jim Spann and Cathy Venturini with the Dusty Plasmas Laboratory. Links to 1,176x1,349-pixel, 273KB JPG. Photo credit: NASA/Marshall Space Flight Center.
Interplanetary dust particles range from 5 to 100 microns in length; 30 microns is typical. They can be spherically or irregularly shaped, and made of silicate or carbonaceous materials. In total, it's a complex range of particles that Spann and Abbas will try to measure in detail.
With the Dusty Plasmas Laboratory, Spann and Abbas will be able to make unique measurements of how dust particles polarize light - convert its vibrations so they are all in one plane - and the angles at which the light is reflected.
"We can make significant contributions to planetary missions," Spann said.
"All planetary atmospheres have dust, aerosols and grains hanging in the atmosphere." Even Mars with its tenuous atmosphere has months-long dust storms that obscure the surface.
Results from the Dusty Plasmas Laboratory will also help in understanding what is seen in the thick dust clouds in deep space where planets are slowly condensing. Infrared telescopes can see little of what is happening because the view is obscured by the very dust that eventually will become planets, comets, asteroids, or just the dust that, as in our solar system, reflects sunlight back to give the sky a slight glow along the plane of the planets.
| Related links |
Abstract: SH52C-03 Experimental Determination of Infrared Extinction Cross Sections of Interplanetary Dust Particles. J F. Spann and Mian Abbas, NASA/Marshall Space Flight Center. Knowledge of the distribution of interplanetary and circumsolar dust particles and their physical and optical characteristics provides valuable information about many issues dealing with the origin and formation of the solar system. Interplanetary dust particles (IDPÌs) are considered to have their origin in cometary, asteroidal, and meteoritic sources, along with possible contributions from planets and the pre-solar molecular cloud. Dust particles in the interplanetary medium are produced by a variety of sources and have a diverse size range. Particles ranging from 5 to 100 microns contribute to most to the zodiacal light, with a typical particle size of 30 microns. The major constituents of the spherical or irregularly shaped circumsolar and IDPÌs are believed to be silicates and carbonaceous materials, as indicated by analyses of stratospheric dust particles of interplanetary origin. An experimental technique being developed in the laboratory at Marshall Spaced Flight Center for measurements of scattering and extinction cross sections of some commonly known interplanetary and circumsolar dust particles will be presented. This technique is based on irradiating a single charged dust particle suspended by electrodynamic balance in a cavity and measuring the scattered radiation as a function of angle. Comparison with Mie theory calculations leads to simultaneous determination of the particle radius, the complex refractive index, and the scattering and extinction cross sections. An application of this technique will also be discussed for investigation of rotational bursting phenomena whereby large size cosmic and interplanetary particles are believed to fragment into smaller dust particles. |
|
Dust to dust: New lab studies death of stars, origin of planets. New lab tool makes dust grains the center of attention. (April 6, 1998) American Geophysical Union Spring meeting. Hubble Space Telescope images of dust storms on Mars (1996) and 1997. www.spaceweather.com - up-to-date space weather and news from NASA and NOAA |
Join
our growing list of subscribers - sign
up for our express news delivery and you will receive
a mail message every time we post a new story!!!
Headlines
Copyright © 2000 SpaceRef Interactive Inc. All rights reserved. Privacy Policy
