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Mangalyaan: Payload: Thermal Infrared Imaging Spectrometer

Mangalyaan (Mars Orbiter Mission)
Payload: Thermal Infrared Imaging Spectrometer (TIS)
6 More days to go !!!

 

Mars is a terrestrial planet which means that its bulk composition, like Earth consists of silicates, is metals and other elements that typically make up rock. Also like Earth, Mars is a differentiated planet, meaning that it has a central core made up of metallic iron and nickel surrounded by a less dense silicate mantle and crust. The planet’s distinctive red colour is due to oxidation of iron on its surface.

The knowledge on type of minerals present in any planetary system provides the information on the conditions under which minerals are formed and process by which they are weathered. Much of what we know about the elemental composition of Mars comes from orbiting spacecraft and landers. Most of these spacecraft carry spectrometers (A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials) and other instruments to measure the surface composition of Mars.

Thermal Infrared Spectrometer payload on MOM

 

Image: TIS
Source ISRO


 Thermal Infrared Spectrometer is one of the five instruments on MOM. TIS  weighing 3.2 kg can measure the thermal emissions and can be operated during both day and night. Temperature and emissivity are the two basic physical parameters estimated from thermal emission measurement. The TIS instrument measures thermal emissions from the Martian surface to deduce surface composition and mineralogy.


Science goals of TIS are:

  • To estimate ground temperature of Mars surface.
  • To map surface composition and mineralogy of Mars.
  • To detect and study the variability of aerosol/dust in Martian atmosphere.
  • To detect hot spots,which indicate underground hydrothermal systems.

 TIS will be useful in mapping mineral compositions and surface temperature during perigee imaging(The perigee is the point in a satellite's elliptical path around the earth at which it is closest to the center of the earth)and it will be used for assessment of global temperature distribution and aerosol turbidity in Martian atmosphere during apogee viewing(apogee is the point in the orbit of an artificial satellite most distant from the center of the earth).


Science behind the instrument
3D model of fore optics

 


Image source: univlyon1 (France)


The TIS instrument consists of a spectrometer that features a typical infrared grating spectrometer design. TIS consists of fore-optics, slit, collimating optics, grating and re-imaging optics. A 120X160 element bolometer array is placed at the focal plane of the re-imaging optics


Fiberport lens positions for collimating

 

Image source: thorlabschina


Sketch of multi wavelength re-imaging optics

 


Image source: aanda organization


In the common design, radiation is directed through an entrance slit (available light energy depends on light intensity of the source as well as the dimensions of the slit and acceptance angle( acceptance angle refer to the angle in an optical fiber below which rays are guided rays) of the system. The slit is placed at the effective focus of a collimator(A collimator is a device that narrows a beam of particles or waves,which means either to cause the directions of motion to become more aligned in a specific direction (i.e., collimated or parallel) or to cause the spatial cross section of the beam to become smaller.) that directs collimated radiation (focused at infinity) to a diffraction grating that acts as dispersive element. Another mirror refocuses the dispersed radiation onto a detector.


TIS uses a 120 by 160 element bolometer array detector. A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance


Principle of operation of a bolometer


Power P from an incident signal is absorbed by the bolometer and heats up a thermal mass with heat capacity C and temperature T. The thermal mass is connected to a reservoir of constant temperature through a link with thermal conductance G. The temperature increase is ΔT = P/G. The change in temperature is read out with a resistive thermometer. The intrinsic thermal time constant is τ = C/G.
Image source: wikimedia

A bolometer consists of an absorptive element, such as a thin layer of metal, connected to a thermal reservoir (a body of constant temperature) through a thermal link. The result is that any radiation impinging on the absorptive element raises its temperature above that of the reservoir — the greater the absorbed power, the higher the temperature. The intrinsic thermal time constant, which sets the speed of the detector, is equal to the ratio of the heat capacity of the absorptive element to the thermal conductance between the absorptive element and the reservoir. The temperature change can be measured directly with an attached resistive thermometer, or the resistance of the absorptive element itself can be used as a thermometer.

Bolometer receivers measure the energy of incoming photons. TIS is sensitive for an infrared wavelength range of 7 to 13 microns.


Microbolometer

 


Image source: www.wikimedia.com

The microbolometer array does not require cooling. Each pixel on the array consists of several layers including an infrared absorbing material and a reflector underneath it that directs IR radiation that passes through the absorber back to the absorbing layer to ensure a near complete absorption. As IR radiation strikes the detector, the absorbing material is heated and changes its electrical resistance which can be measured via electrodes connected to each microbolometer and processed into an intensity read-out in order to create an IR spectrum.

IR spectrum image

 

Image source: wikimedia

 

Conclusion
The analysis of TIS data would involve estimation of brightness temperature from observed and calibrated thermal radiance data. The retrival of surface temperature and emissivity spectra for different regions would be carried out. The estimated emissivity spectra would be compared with Mars analog mineral emissivity spectra. It is proposed to generate the emissivity spectra between 7-13 microns for minerals reported to exist in Martian surface.In this way,spectral library will be used to know the mineral composition on Mars using TIS data.

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Author
Ms Manasa Perikala
Intern at Indian Space Station,
IIT Varanasi

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