Solar System Dust 625
FIGURE 4 An unusually strong meteor shower (Leonid) was
observed on 17 November 1966. The meteor trails seem to
radiate from the constellation Leo.
but form so-called comet trails along a short segment of the
comet’s orbit. Their different speeds will slowly spread the
particles out over the full orbit. Infrared observations by
theInfrared Astronomical Satellite (IRAS)identified many
such trails connected to short-period comets. Gravitational
interactions with planets and collisions with other cosmic
dust particles will scatter meteoroids out of the stream, and
they will become part of the sporadic background cloud
of meteoroids. The fact that some meteor showers display
strong variations of their intensities indicates that they are
young streams that are still concentrated in a small segment
of the parent’s orbit. The parent comet of the Leonids, the
periodic Comet Tempel–Tuttle, has the same periodicity of
33.3 years. The parent object of one of the strongest yearly
meteor showers, the Geminids, is 3200 Phaeton, which had
been previously classified as an asteroid because it shows
no cometary activity. However, its association with a me-
teor stream indicates that it is an inactive, dead comet
that at some time in the past emitted large quantities of
meteoroids. [SeePhysics andChemistry ofComets;
CometaryDynamics;Near-EarthObjects.]
Fewer than one out of ten thousand radar meteors has
been identified to be caused by interstellar meteoroids that
pass through the solar system on ahyperbolic orbit. Their
heliocentric speed is significantly higher than the solar sys-
tem escape speed, confirming that they are of truly inter-
stellar origin. The radius of these interstellar meteoroids
is about 20μm. These particles have been found to arrive
generally from southern ecliptic latitudes with enhanced
fluxes from discrete sources.
2.2 Interplanetary Dust Particles
There is another “window” through which extraterrestrial
material reaches the surface in a more or less undisturbed
state. Smallinterplanetary dust particles (IDPs)of a
few to 50μm in diameter are decelerated in the tenuous at-
mosphere above 100 km. At this height, the deceleration is
so gentle that the grains will not reach the temperature
of substantial evaporation (T∼ 800 ◦C), especially, since
these small particles have a high surface area-to-mass ratio
that enables them to effectively radiate away excessive heat.
These dust particles subsequently sediment through the at-
mosphere and become accessible to collection and scientific
examination. The abbreviation IDP (or “Brownlee particle”
after Don Brownlee, who first reliably identified their ex-
traterrestrial nature) is often used for such extraterrestrial
particles that are collected in Earth’s atmosphere.
Early attempts to collect IDPs by rockets above about
60 km were not successful because of the very low influx
of micrometeoroids into the atmosphere and the short res-
idence times of IDPs at these altitudes. More successful
were airplane collections in the stratosphere at or above
20 km. At this height, the concentration of 10-μm-diameter
particles is about 10^6 times higher than in space and terres-
trial contamination of this sized particles is still low. Only
micrometer- and submicrometer-sized terrestrial particles
(e.g., from volcanic eruptions) can reach these altitudes in
significant amounts. Another type of interference is caused
by man-made contamination: About 90% of all collected
particles in the 3- to 8-μm size range are aluminum oxide
spheres, which are products of solid rocket fuel exhaust.
Because of this overwhelming contamination problem for
small particles, the lower size limit of IDPs collected by
airplanes is a few micrometers in diameter.
Since 1981, IDP collection by airplanes has been rou-
tinely performed by NASA using high-flying aircraft, which
can cruise at 20 km for many hours. On its wings it car-
ries dust collectors that sweep huge amounts of air. Dust
particles stick to the collector surface that is coated with
silicone oil. After several hours of exposure, the collector
is retracted into a sealed storage container and returned to
the laboratory. There, all particles are removed from the
collector plate, the silicone oil is washed off, and the parti-
cles are preliminarily examined and catalogued. Individual
IDPs can be ordered for further scientific investigation. A
wide variety of microanalytic tools is used to examine and
analyze IDPs. Scanning electron microscopes (SEM) can
image atomic lattice layer structures. Focused ion beams in
combination with a SEM are used for sample preparation
and secondary ion mass spectrometers (SIMS) can measure
the distribution of individual elements and isotopes at sub-
micrometer resolution, deriving important information on
the mineralogy of the samples.
According to their elemental composition, IDPs come
in three major types: chondritic, 60% (cf. Table 2);