Mangalyaan: Payload: Lyman Alpha Photometer

Mars Orbiter Mission: Mangalyaan
Payload: Lyman Alpha Photometer

Instrument and its interpretation
Lyman Alpha Photometer (LAP) is one of the scientific instruments of the payload on MOM spacecraft, which is India’s maiden mission to the red planet,Mars.

Why is it called Lyman Alpha Photometer?

When electron in a hydrogen atom makes transition from n=2 energy level to n=1 energy level, a photon is released and this type of emission of photon is known as Lyman Alpha emission. Photometer is an instrument for measuring intensity of light.Lyman Alpha Photometer is an absorption cell photometer.


Lyman Alpha Emission:


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What is an absorption cell photometer?

An absorption photometer for measuring the absorption by conducting the light to a thin flow cell in which a liquid sample flows,wherein the sample light for measuring the absorption peak is superimposed on the reference light selected from the transparent(window) range of the liquid and the absorbance is detected by separating the sample light and reference light after transmission of the flow cell.changes in the light path conditions can be mentioned accurately,and therefore high accuracy measurement immune to noises is made possible even using an elongated flow cell.     

Absorption cell photometer:

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LAP measures the relative abundance of deuterium and hydrogen from Lyman-alpha emission in the Martian upper atmosphere .Measurement of D/H (Deuterium to Hydrogen abundance ratio) will improve our understanding of the process involved in the loss of water from the planet.The estimated D/H ratio will be used in MGCM(Mars General Circulation Model) algorithms to asses the present Water escape rate from Martian Exosphere.

Fundamentals and Principle of working of the instrument:

When the planet has  no or little intrinsic magnetic field,direct interaction between the solar wind and the atmosphere occurs and causes the escape of atmosphere through thermal and non-thermal heating process.

Escape of atmosphere on Mars:

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In upper atmosphere hydrogen and deuterium atoms are produced by photo dissociation from H2O and HDO molecules.In the escape of these atoms,the D/H ratio in the atmosphere increases with time because escaping ratio of H atoms is expected to be greater than that of D atoms because of the mass difference.

LAP operates on the principle of resonant scattering and absorption at Lyman alpha wavelengths of H and D i.e., 121.56 nm, 121.53 nm respectively.Thermally dissociated H2 and D2 molecules in the cells absorb the incoming H2/D2 Lyman alpha incident on the cell.

Technical specifications of LAP

The fore-optics comprising of a plano-convex lens collects the input radiation and transmits to the gas cells.Gas filled cells of the instrument works as an effective narrow band-pass rejectionfilter at hydrogen and deuterium alpha wavelengths.Tungstun filament is used to thermally dissociate the gases in to atoms.There aroms will resonantly absorb the incoming hydrogen/deuterium lyman alpha radiation at their wavelengths.A 15 nm bandwidth lyman alpha filter placed in the front of the detector cuts-off the undesirable radiation that lies outside the wavelength range of interest and a solar-blind side-on type photo multiplier tube(PMT) is selected for photon detection.

Photo multiplier tube(PMT)

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Range of operation : 3000 km-periapsis-3000 km
Size (cubic meter)    : 27.6 X 138 X 100.5
Mass (kg)                  : 1.97
Total power (watt)  : 8


My sincere thanks to

L.M.Viswanathan , scientist (LEOS)-ISRO
Raja V.L.N. Sridhar, scientist (LEOS)-ISRO


Manasa Perikala, IIT-Varanasi

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