Mangalyaan: Payload: Martian Exospheric Neutral Composition Analyser

Martian Exospheric Neutral Composition Analyser


       Image source: gisrescource

Martian Exospheric Neutral Composition Analyser, MENCA payload weighing 3.56 kg, is a quadrupole mass spectrometer based scientific payload on MOM, capable of measuring relative abundances of neutral constituents,in the mass range of 1-300 amu .The core objective of MENCA is to study the exospheric neutral density and composition at altitudes as low as 372 kilometers above the Martian surface.The instrument examines radial,diurnal,and seasonal variations in the Martian exosphere.with Mangalyaan in its operational orbit, MENCA is to estimate the upper limits of the neutral density distribution and composition around mars.Studying Martian exosphere will provide valuable data on the present conditions.

Science goal
MENCA  would provide the first ever insitu measurement of the neutral composition  and density distribution of the Martian exosphere (atmosphere ~ 500 km and beyond from the Martian surface).

Explanation on what happens in a mass spectrometer

Atoms can be deflected by magnetic fields-provided the atom is first turned into an ion.Electrically charged particles are affected by a magnetic field although electrically neutral ones aren’t. The atom is ionised by knocking one or more electrons off to give a positive ion.This is true for things which you would normally expect to form negative ions(chlorine for example) or never form ions at all( ex: argon). The ions are accelerated so that they all have the same kinetic energy.The ions are then deflected by a magnetic field according to their masses.The lighter they are,the more they are deflected.The more the ion is charged, the more it gets deflected.The beam of ions passing through the machine is detected electrically.
Principle of working:

Quadrupole rods

A Quadrupole Mass Spectrometer consists of four parallel metal rods with opposing rods being connected electrically.A radio frequency voltage is applied between the two pairs of rods and a direct current voltage is applied between the two pairs of rods and a direct current voltage is then superimposed on the RF voltage. Ions entering the instrument travel down the quadrupole between the rods.Depending on their mass-to-charge ratio, ions either enter unstable trajectories and collide with the rods or make it through to the detector (detectorsbeing used in MENCA are channel electron multiplier(CEM) and Faraday Cup (FC)  )

Electron multiplier


Image source: wikimedia



Image source: masspec

                  The m/z of ions reaching the detector is a function of the voltage setting which allows the operator to select an ion with a particular mass-to-charge ratio to measure its abundance or run the instrument through a range of voltages to scan for a number of species that might be present.
Ions are generated via electron ionization. Electrons are produced through thermionic emission. The electrons are accelerated in an electric field and focused into a beam by a trap electrode. The atoms and molecules enter the ion source perpendicular to the electron beam. As high-energy electrons pass by and collide with the particles, large fluctuations in the electric field around the neutral molecules are caused leading to ionization and fragmentation.
The MENCA instrument operates at an m/z range of 1 to 300 amu (atomic mass unit) with a mass resolution of 0.5u which allows detailed detection of species. The instrument can operate at the low partial pressure found in the upper Martian atmosphere.

Additional instruments in MENCA payload
                     MENCA has an in-built pressure gauge for the measurement of total pressure.The instrument has a provision to study the time-evolution of a set of selectable species in the  mode of operation.The primary science goal  of MENCA is the in-situ measurement of neutral composition and distribution of the martian upper atmosphere and exosphere and to examine its radial, diurnal and possibly seasonal variations.The instrument has telecommand, telemetry and data interface to the space craft.optical combination of operating parameters which can be chosen through telecommands will be used to control the instrument at different observation phases so that best possible scientific data could be derived.

Telecommand architecture

Image source: delangdelong


Image source: raptorresearchcenter

Due to various thermal and non-thermal processes, Mars lost its atmosphere deserting it in its present form.Study of the composition and the distribution of the Martian Exosphere by MENCA  may help in understanding the thermal escape of the Martian atmosphere.

Manasa Perikala
IIT, Varanasi

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