SPECIAL SECTION: MARS ORBITER MISSION CURRENT SCIENCE, VOL. 109, NO. 6, 25 SEPTEMBER 2015 1087 *For correspondence. (e-mail: kurian_mathew@sac.isro.gov.in) Methane Sensor for Mars Kurian Mathew*, S. S. Sarkar, A. R. Srinivas, Moumita Dutta, Minal Rohit, Harish Seth, Rajiv Kumaran, Kshitij Pandya, Ankush Kumar, Jitendra Sharma, Jalshri Desai, Amul Patel, Vishnu Patel, Piyush Shukla, S. Manthira Moorthi, Aravind K. Singh, Ashutosh Gupta, Jaya Rathi, P. Narayana Babu, Saji A. Kuriakose, D. R. M. Samudraiah and A. S. Kiran Kumar Space Applications Centre, Indian Space Research Organisation, Ahmedabad 380 058, India Methane Sensor for Mars (MSM), on-board Mars Orbiter Mission is a differential radiometer based on Fabry–Perot Etalon (FPE) filters which measures col- umn density of methane in the Martian atmosphere. It is the first FPE sensor ever flown to space. Spectral, spatial and radiometric performances of the sensor were characterized thoroughly during the pre-launch calibration. Geophysical calibration of the sensor was carried out using the data acquired over Sahara desert during Earth Parking Orbit phase. Retrieval algorithm for MSM, which is based on the lineariza- tion of radiative transfer equations, gets simultaneous solutions for CH 4 and CO 2 concentrations in the Mar- tian atmosphere. Keywords: Differential radiometer, Fabry–Perot Eta- lon, geophysical calibration, methane sensor, retrieval algorithm. Introduction MARS, the nearest planet to Earth, has many similarities with it. Like Earth, Mars is a terrestrial planet which has an atmosphere, hydrosphere, cryosphere and lithosphere. Thermal environment of Mars is suitable for the deve- lopment and evolution of life forms. Existence of life on Mars has been a recurrent question ever since the advent of modern astronomy. Recent detection of methane in the Martian atmosphere has generated a lot of interest among the scientific community, as it suggests the possibility of biological activities 1–3 . However, the presence of methane can be also due to geological activities 1–4 . So far, four research groups have reported measure- ment of methane in the Martian atmosphere. In 2003, Mumma et al. 5 detected methane plumes on Mars using high-resolution spectrometers at the Infrared Telescope Facility in Hawaii and Gemini South Telescope in Chile. Methane concentrations showed large spatial variations with a maximum value of about 250 ppb. In 2004, Formisano et al. 6 reported a peak methane concentration of 35 ppb based on measurements by Planetary Fourier Transform Spectrometer (PFS) on-board Mars Express Mission. Using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope, Krasnopolsky and co-workers 7,8 estimated the planetary average of methane to be about 10 ppb. Thermal Emission Spectrometer (TES) on-board Mars Global Surveyor (MGS) monitored methane on Mars from 1999 onwards for more than three years 9 and measured a peak concentration of about 70 ppb. Terrestrial as well as satellite measurements carried out during 1999–2006 showed large temporal variations in methane concentration 5–9 . Emission of CH 4 was episodic in nature with its concentration coming to minimum bet- ween episodes. Methane concentration also underwent seasonal and annual variations. Measurements by MGS– TES showed that methane concentration become maxi- mum during the autumn of the northern hemisphere and come to a minimum during winter. Then it gradually increased from winter to spring. Spatial distribution of methane in the Martian atmosphere is characterized by localized sources. TES measurements identified three methane hotspots located around Tharsis, Elysium and Arbia Terrae, where the former two are known to be volcanic regions 9 . Origin of methane as well as its temporal and spatial variations are yet to be understood. Spatial and temporal variations of methane indicate the presence of localized sources which are currently active. The origin of methane may be either due to geochemical or biologic process. Geological sources of methane on Earth are mainly volcanoes and hydrothermal hot spots. But no such geo- logical activities have been observed on Mars. It was suggested that methane could be generated through a process in which water reacts with serpentinized olivine rocks at low temperatures 4 . Even though large reservoirs of subsurface liquid water found on Mars give certain credence to this theory, direct evidences are yet to be obtained. Martian methane can also be exogenic in origin, whereby interactions of meteorites or comets with the Martian atmosphere produces methane. But this process will account for only a small fraction of the existing levels of methane concentration 3 .