Our Solar System contains a huge number of dust grains, from tiny,
submicron-sized particles to centimeter-sized boulders.
Dust is ejected from cometary nuclei,
is produced by disintegration of asteriods,
comes from interstellar space.
The complex of dust particles forms a tenuos sheet, called Zodiacal Cloud,
which extends from the immediate vicinities of the Sun (solar
F-corona) to far beyond Pluto's orbit.
Dust grains are easily accessible to the observers: they reveal
themselves as Zodiacal Light,
give rise to microcraters on the lunar surface,
hit sensors onboard spacecraft.
Some of the particles enter the Earth's atmosphere and become visible as
shooting stars.
Some others bombard planetary satellites, which are not
shielded by atmospheres, and then...
Birth
... What happens if a dust particle - a
micrometeoroid
- meets a planetary
satellite on its way?
The speed of such a grain relative to the satellite is about tens
kilometers per second.
Hence each hypervelocity impact causes microexplosion and ejects
satellite's surface material.
The cumulative mass of ejecta is hundreds or thousand times the mass
of a projectile.
However, the speed of the ejected fragments is much lower than that of
the projectile.
As you know, a satellite is usually a small body having weak gravity.
That's why most of the ejected debris escape from
the parent satellite and enter the circumplanetary space...
Animation: Interplanetary meteoroids bombard a satellite (scheme).
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Life near the planet: the marvellous E-ring of Saturn
...When released from the satellite, dust particles are
subject to a
number of forces.
The planetary gravity usually dominates, but smaller debris
are also vulnerable to a large array of nongravitational forces -
solar radiation pressure, Lorentz force, circumplanetary plasma drag.
The dynamics of dust grains in this complex force field
were studied by many scientists, yielding a good understanding
of the most important features of several circumplanetary dust complexes.
A nontrivial interplay between the forces is problably best exemplified
by the E ring of Saturn, an ethereal dusty sheet extending from 3 to 8
saturnian radii from the planet.
Enceladus, a small icy moon of Saturn, is the likely source of the ring
material.
Some people believe in the mechanism described above, others think of
geysers or volcanoes.
The most striking observational fact is that the ring consists of
like-sized grains, 1 micron in radius!
Such a strange size distribution has been explained in a number
of scientific papers.
The idea is that the nongravitational forces acting on the grains are
size-dependent, and the size of 1 micron turns out to be dynamically
privileged.
Only micrometer-sized particles can spread over a
large region of the circumplanetary space and survive for a long time.
The orbits of these grains are stable, but nothing lasts too long...
Animation: Saturn's E ring during one Saturnian year
(our numerical model).
The shadow of Saturn on its inner dense rings shows
the direction of the incident sunlight.
Particles of three sizes are presented: 1.00 microns (green),
1.04 microns (blue), 1.24 microns (red).
The ring components, constituted by the grains of nearly identical sizes,
show quite different spatial distribution!
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Death
...The ejected particles have finite lifetimes.
Moving in the same region where the parent satellite orbits the planet,
sooner or later they collide it again.
In contrast to their birth due to the high-speed interplanetary impacts,
the recolliding particles are usually too slow to produce considerable
secondary ejecta. (However, alternative models also exist.)
Animation: Recollision of grains with the satellite (scheme).
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