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MICROMED:
A compact dust detector for Martian
airborne dust investigation
3M-S3 - Space Research Institute Moscow, Russia
October 8-12, 2012
F. Esposito1, C. Molfese1, F. Cortecchia1, F. Cozzolino1, S. Ventura2, F. D’Amato3, L. Gambicorti3
1 – INAF Osservatorio Astronomico di Capodimonte, Naples, Italy; 2 – ESA ESTEC, Noordwijk; 3 – Istituto Nazionale di Ottica,
Florence, Italy
MicroMED:
Physical quantities to be measured
The scientific goal is the characterisation of airborne dust
properties close to the Mars surface.
Measurements concerns the following physical quantities:

Atmospheric dust particle size
distribution (single grain detection)

Number density of particles vs.
size

Time evolution of the former
quantities, vs. short term variations
/ local events (e.g. wind, dust devils,
dust storms)
Dust in the atmosphere of Mars (1/10)
Martian atmosphere contains always a significant load of suspended dust
Airborne dust has important effects on morphological evolution of the
surface.
Aeolian erosion, redistribution of dust on the surface and weathering are
mechanisms which couple surface and atmospheric evolution.
The exchanges occur in the
planetary boundary layer,
i.e. the turbulent region
from surface to a few
hundred meters height.
Dust in the atmosphere of Mars (2/10)
Moreover, airborne dust severely impacts on the climate of Mars, influencing
the thermal behaviour of the troposphere.
•
Indeed, dust absorbs solar irradiation mainly in the VIS (re-emiting it in
the IR), locally warming the troposphere.
•
Even in case of moderate dust scenario, the influence of dust on Martian
thermal structure is critical, while during global dust storms > 80% of
sunlight is absorbed by dust.
The amount and size distribution of dust in the atmosphere is controlled by
lifting processes and wind transport.
Dust in the atmosphere of Mars (3/10)
Up to now, mechanisms for dust lifting and feedbacks on the atmospheric
circulation are not well understood.
As an example, the irregular occurrence and mechanisms for the growth of
global dust storm are not explained by models.
In order to understand
and proper model Martian
atmospheric
circulation
and meteorology it is
necessary to understand
how dust is lifted and
maintained
in
the
atmosphere and which is
the amount and size
distribution of the lifted
dust.
The amount and size distribution of airborne dust depends on the lifting
mechanisms.
Dust in the atmosphere of Mars (4/10)
Mechanisms for dust entrainment
Dust on Earth is generally emitted due
to the drag force of wind.
As wind speed increase, sand particles
of  100 mm size are first moved by
fluid drag.
After lifting they hop along the surface
 saltation
Saltation can mobilize particles of a wide
range of sizes (splashing, creep,
suspension).
Dust in the atmosphere of Mars (5/10)
Few wind measurements have
been performed from landers on
Mars.
These have shown that the light
Martian
atmosphere
rarely
exceed the saltation fluid
threshold (also confirmed by the
results of mesoscale and global
circulation models).
The ubiquitous sand dunes on
Mars appeared almost motionless
by
lander
and
orbiter
observations, and were supposed
to be formed in a previous
climate, in a thicker atmosphere.
Dust in the atmosphere of Mars (6/10)
Recent observations by high resolution
images (e.g. HiRISE) have revealed
widespread movements of dunes and
ripples at many locations on Mars.
Results show that the Martian thin
atmosphere blows sand in this dune field
at rates not much lower than Earth’s
much thicker atmosphere does on
terrestrial dunes.
Dust in the atmosphere of Mars (7/10)
This could be explained by the recent
results that, once initiated, saltation can
be sustained down to speed of only 10%
of the initiation threshold, thus allowing
saltation to occur at << wind speed.
On Mars lower g and air density imply
that grain trajectories are higher and
longer than on Earth  grains are
accelerated by wind for longer time 
impact threshold u*t_impact comparable
with that on Earth.
But lower air density  higher fluid
threshold on Mars ( 1 order of
magnitude)
 The ratio of impact to fluid threshold
on Mars is lower (10%) than on Earth
(80%).
Dust in the atmosphere of Mars (8/10)
Other suggested mechanisms for dust lifting on Mars
• MER MI and Curiosity MAHLI discovered
that dust often occurs as low-strength
sand-sized spheroid agglomerates possibly
due to electrostatic processes.
These structures need lower wind speeds to
be disrupted and lifted into the atmosphere.
•Thermophoresis.
Occurs when solar irradiation warms the
upper mm of the surface more than the
surrounding atmosphere. Gases embedded in
the pore spaces of the surface can transfer
enough momentum to dust particles to cause
them to lift.
Dust in the atmosphere of Mars (9/10)
Other suggested mechanisms for dust lifting on Mars
• Electric field.
Electric fields can be generated
during normal saltation and inside
dust devils vortices or dust storms.
Laboratory experiments demonstrated
that electric fields can enhance dust
lifting.
• Sublimation of CO2 ice below or around
dust particles may provide them with the
momentum required for the injection into
the atmosphere.
Dust in the atmosphere of Mars (10/10)
The amount of dust in the atmosphere and its size distribution depends on the
processes responsible for the dust lifting.
They are the key parameters influencing Martian climate and surface
weathering.
The information about size distribution of atmospheric dust has been mainly
retrieved from remote optical measurements.
Complex vertical distribution of dust
mixing ratio.
Peak @ 15-25 km observed by the
Mars Climate Sounder on MRO during
nothern spring and summer (McCleese
et al., 2010).
Another peak likely present in the
PBL (0-15 km).
Sharp decrease above ~ 25 km.
MicroMED:
Scientific objectives
The information that can be obtained by in situ measurements over
different time spans represents a key input in different areas of
interest:
in science
 to determine present climatic conditions at Mars surface
 to derive information about past history of Mars climate
 to derive information about dust loading mechanism (if coupled with
wind and other environmental measurements)
 to study dust storms properties and evolution
 to provide ground-truth for validation of data coming from orbiter
observations
in Mars operations
 to evaluate hazardous conditions due to Martian dusty environment
 to place constraints on operative conditions at Mars
 to support definition of future Mars exploration missions
Instrument concept:
From MEDUSA to MicroMED
MEDUSA configuration in Humboldt
Inlet
OS
Optical
fiber
connector
Vacuum
pump
• Optical Stage with laser diode source
in separated box coupled to the main
body with optical fiber
• Dust Accumulation Stage below the
Optical Stage
• Water Vapour Measure Stage in
external unit.
• Dust Deposition and Electrification
Sensor (DDES) as external unit.
Dust Accumulation
Stage
DDES
Water
Vapour
Stage
Optical
Fiber
Laser
Diode
Assemby
MEDUSA successfully passed the PDR
with TRL > 5.3
MicroMED configuration
Inlet
Optical
detection
Stage
Vacuum
Pump
Laser diode
mirror
Collimator optics
MicroMED Envelope 100 x 60 x 165 mm3
MEDUSA vs MicroMED
MEDUSA
MicroMED
2243 g
500 g (without ME) (TBC)
21.44 W
1.6 W (only PE)
Nozzle
Minimized tube
±33° in FW direction
±47° in BW direction
±65° at 90° direction
1.2 mm x 0.32 mm x 3 mm
1 mm x 1 mm x 0.08 mm
Detector(s)
Sensing Area: 50 mm2
Responsivity: 0.5 A/W
Sensitivity Area: 50 mm2
Responsivity: 0.5 A/W
Laser Diode
Wavelength: 808 nm
Optical Power: 1000 mW
Wavelenght: 850 nm
Optical Power: 100 mW
Maximum Flow Rate: 5.5 l/min
Maximum Flow Rate: 1.0 l/min
Microbalance for dust
Mass Sensitivity: 5.09·108
Hz/g/cm2
NO
Microbalance for water vapour
Mass Sensitivity: 2.47·109
Hz/g/cm2
NO
4 Channels
(2 for Forward Scattering + 2 for
Backward Scattering)
1 Channel with 2 outputs:
low gain 105, high gain 107
Total Mass
Total Maximum Power
Consumption
Sampling Head
Collecting mirrors aperture
Sampling Volume
Pump
Proximity Electronics
MicroMED working principles
•
Dust particles sampled by a pump cross a
collimated light beam emitted from an
infrared laser diode.
•
Dust particles sizes can be measured by
the scattered light collected with the
mirror on a photodiode.
•
MicroMED measures abundance and size
distribution of dust in Martian atmosphere
MicroMED fluid-dynamic design
Inlet and outlet shape design, sampling rate, sampling volume and air and dust flow
section have been designed in order to be compliant with the following technical
requirements:
• to have a small fraction of coincidence f (e.g. < 0.05), in order to have a single particle
counter, and a large number of particles detected in a short time (120 s)
• to concentrate the dust and air flux in a small area (e.g. 1 x 1 mm2) coincident with the
sampling volume generated by the laser beam
• to avoid turbulence inside the instrument
• not to alter the size distribution and volume density of sampled dust particles
• to be able to sample particles with size  20 mm
• inlet and outlet tubes protruding inside the MicroMED body shall not be invasive: they
shall not intercept the laser beam, nor produce shadow on the mirror or detector.
MicroMED fluid-dynamic design
Fluid-dynamic design has been tested in a Martian simulation chamber.
Preliminary tests results show that particles captured by MicroMED are conveyed
concentrated in a section 1 x 1 mm2 perpendicular to the flow, as predicted by Fluent
simulations.
MicroMED fluid-dynamic design
Experimental set-up
Inside sampling vol.
0.4 mm from the centre
At the edge of sampl. vol.
0.5 mm from the centre
Outside sampling vol.
0.6 mm from the centre
MicroMED optical design
Optical design is driven by the following requirements:
•
To concentrate the laser beam in a small area in order to obtain high power density
with a low power laser
•
To be able to detect particles in the size range 0.4 – 20 mm in diameter
•
To be able to detect single particles (having a small fraction of coincidence f (e.g. <
0.05))
•
To maximise the power density in the sampling volume
•
To produce a beam with uniform intensity inside the sampling volume (in order to avoid
that the same particle could produce different signal if intercepting the beam in
different points)
MicroMED optical design
MicroMED: performances evaluation
Signal vs. size
Scattered light
Evaluation considering:
• Spherical dust particles
(Mie theory)
• optical power: 100mW
• Radiation loss: 30%
(ZEEMAX simulations)
• angle of collected
scattered light: 130°
Coincidence fraction f = 1.210-4 in constant haze
and 4 10-3 during dust devils
Minimum detectable dust
particle size:
< 0.2 mm in radius
MicroMED: performances evaluation
Signal vs. size
Optical system is almost ready.
Optics have been produced and aligned.
Integration and performance tests foreseen in November 2012
Similar tests on the MEDUSA instrument
OS laboratory breadboard has been
able to detect 0.5 mm grains.
This is very close to the OS nominal
performances (det. Limit: 0.2 mm).
MicroMED Operations
MicroMED operations:
• Minimum 4 runs of 160 s
per SOL are foreseen.
For each Run
• 130 s laser diode ON
• 130 s Pump On
• 160 s PE on
Element Assembly
Power
Consumption
Proximity Electronics
0.170 W
Pump
0.5 W
Laser Diode
0.5 W
Total
1.560 W
(including DC/DC converter
dissipation)
Energy
/Run
0.058
Wh
Conclusions
 MicroMED is the lighter and reduced resources version of the MEDUSA
instrument on-board the Humboldt payload with TRL > 5.3.
 It is able to measure the size distribution and number density of dust particles in
the atmosphere of Mars.
 The information that will be provided by MicroMED are crucial for climate
modelling and for future missions planning.
 The performances are:
MicroMED performances and Technical specification
Detected Dust Size
0.4..20 mm diameter
Power
1.6 W
Mass
500 g
 MicroMED BB is under development.
 BB is foreseen to be completed and tested within Feb. 2013.