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One element of the Martian environment,
which attracts much scientific interest is the fine
reddish dust that covers the planetary surface and
is always present in the atmosphere. This dust is studied
both for its influence on atmospheric dynamics, for
the clues it may hold to the planets past history,
and for the hazard it poses to future human or robotic
explorers of the planet. A Danish-built experiment*
on the two American rovers focuses on studying this
wind-borne dust.
Based on the observation from previous
Mars missions that a large fraction of the Martian
dust is highly magnetic, this experiment consists of
a suite of magnets that attract dust from the air.
The dust buildup on the magnets is studied by imaging
the magnets with the rover cameras and dust composition
can be studied by bringing several spectrometers on
the rover’s robotic arm into close contact with
the dusty magnets. Before landing on Mars, a lot of
effort goes into characterization of the magnets and
the detailed process by which a dust grain is captured
on a magnet. This is done both by wind-tunnel experiments
and by CFD simulation using STAR-CD.
The STAR-CD simulations are run
in simple geometries with the objective of understanding
the dependence of the capture process on such factors
as wind-speed, grain magnetization and grain size.
The Martian pressure of only ~ 6mBar makes most wind-flows
fairly regular and makes it possible to run the simulations
in an uncoupled steady-state mode. First a steady solution
is found to the flow field, next the grain tracks are
computed using the computed wind velocities and mean
turbulence levels as input. The effect of the magnetic
force on the grains is handled through the STAR-CD
user subroutine DROMOM, which allows for addition of
extra momentum source terms to the equation of motion
for the grains.
It is apparent from the simulations
that even for strong magnets only grains passing very
close above the magnetic surface will be captured by
the magnetic force. Nonetheless both simulations and
experiments show that the low Martian pressure makes
capture significantly easier than it is under Terrestrial
conditions. This effect is due to the lower atmospheric
density causing a lower drag on the grains. Whether
the Martian gravity of ~40% Earth gravity has a significant
effect is also studied by CFD; while easy to simulate
by CFD, a change in gravity is obviously nontrivial
to simulate in a laboratory experiment.
*Delivered by the Danish Mars Group,
Niels Bohr Institute, Copenhagen.
For more information contact: kjartan@phys.au.dk
Image 1:
Presence of dust in the Martian atmosphere: Two images from the Hubble
Space Telescope show Mars before and after the emergence of a global
dust storm during the summer of 2001. The dust storm obscures all surface
features.
Image 2:
A computer-generated image of one of NASAs two Mars rovers. Two of the
Danish magnets are visible at the foot of the central white camera
mast.
Image 3:
Result of a simulation of 500 magnetic dust grains passing the surface
of a strong magnet. The magnet only captures grains passing within
half a centimeter of the surface. Wind speed is 1.3 m/s.
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