MICROMED: A compact dust detector for

Martian airborne dust investigation

3M-S3 - Space Research Institute Moscow, RussiaOctober 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

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)

MicroMED:Physical quantities to be measured

Martian atmosphere contains always a significant load of suspended dustAirborne 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.

Dust in the atmosphere of Mars (1/10)

The exchanges occur in the planetary boundary layer, i.e. the turbulent region from surface to a few hundred meters height.

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 (2/10)

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.

Dust in the atmosphere of Mars (3/10)

The amount and size distribution of airborne dust depends on the lifting mechanisms.

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 saltationSaltation can mobilize particles of a wide range of sizes (splashing, creep, suspension).

Mechanisms for dust entrainment

Dust in the atmosphere of Mars (4/10)

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 (5/10)

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 (6/10)

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 (7/10)

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 (8/10)

• Sublimation of CO2 ice below or around dust particles may provide them with the momentum required for the injection into the atmosphere.

• 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.

Other suggested mechanisms for dust lifting on Mars

Dust in the atmosphere of Mars (9/10)

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.

Dust in the atmosphere of Mars (10/10)

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.

The information that can be obtained by in situ measurements over different time spans represents a key input in different areas of interest:

in scienceto determine present climatic conditions at Mars surfaceto derive information about past history of Mars climateto 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

observationsin Mars operations

to evaluate hazardous conditions due to Martian dusty environmentto place constraints on operative conditions at Marsto support definition of future Mars exploration missions

MicroMED:Scientific objectives

Instrument concept:From MEDUSA to MicroMED

• 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. Water

Vapour Stage

Laser Diode Assemby

Optical Fiber

InletOS

Vacuum pump

Dust Accumulation Stage

Optical fiber connector

MEDUSA configuration in Humboldt

MEDUSA successfully passed the PDR with TRL > 5.3

DDES

MicroMED configuration

Inlet

Vacuum Pump

Optical detection Stage

MicroMED Envelope 100 x 60 x 165 mm3

mirror

Laser diode

Collimator optics

MEDUSA MicroMED

Total Mass 2243 g 500 g (without ME) (TBC)

Total Maximum Power Consumption 21.44 W 1.6 W (only PE)

Sampling Head Nozzle Minimized tube

Collecting mirrors aperture ±33° in FW direction±47° in BW direction

±65° at 90° direction

Sampling Volume 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/WSensitivity Area: 50 mm2

Responsivity: 0.5 A/W

Laser Diode Wavelength: 808 nmOptical Power: 1000 mW

Wavelenght: 850 nmOptical Power: 100 mW

Pump 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

Proximity Electronics4 Channels

(2 for Forward Scattering + 2 for Backward Scattering)

1 Channel with 2 outputs: low gain 105, high gain 107

MEDUSA vs MicroMED

• 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 working principles

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

MicroMED fluid-dynamic design

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

Experimental set-up

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 optical design

Scattered light Evaluation considering:• Spherical dust particles

(Mie theory)• optical power: 100mW• Radiation loss: 30%

(ZEEMAX simulations)• angle of collected

scattered light: 130°

Minimum detectable dust particle size: < 0.2 mm in radius

MicroMED: performances evaluation

Signal vs. size

Coincidence fraction f = 1.210-4 in constant haze and 4 10-3 during dust devils

MicroMED: performances evaluation

Signal vs. sizeOptical system is almost ready.

Optics have been produced and aligned.

Integration and performance tests foreseen in November 2012

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).

Similar tests on the MEDUSA instrument

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

Energy/Run

Proximity Electronics

0.170 W

Pump 0.5 W

Laser Diode 0.5 W

Total (including DC/DC converter dissipation)

1.560 W 0.058 Wh

MicroMED Operations

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 BB is under development.

BB is foreseen to be completed and tested within Feb. 2013.

MicroMED performances and Technical specificationDetected Dust Size 0.4..20 mm diameter

Power 1.6 W

Mass 500 g

Conclusions