The instrument samples methane, carbon dioxide and atmospheric properties in a way that doesn’t allow for the methane data alone to be extracted.
The methane sensor instrument onboard ISRO’s Mars Orbiter Mission (MOM), which has been in orbit around the red planet since September 2014, may not be able to transmit some useful data to Earth, the digital magazine Seeker reported on December 7. Scientists have previously found that an as-yet unknown mechanism has been replenishing methane in the martian atmosphere; there are many satellites, including ISRO’s, currently in orbit around the planet trying to resolve this mystery.
The Methane Sensor for Mars (MSM) instrument was designed and built at the ISRO Satellite Centre (ISAC), Bengaluru. It is a Fabry-Pérot interferometer, an instrument that studies radiation by bouncing it between two partially silvered mirrors. With each bounce, some of the radiation escapes the mirrors and is collected at a point. Over multiple bounces, different parts of the same stream of radiation are collected as different waves of radiation, which are then made to interfere with each other to produce an interference pattern. The higher the reflectivity of the mirrors, the finer the pattern, and so the better it reveals important properties of the radiation. The MSM was the first Fabry-Pérot instrument in space when the MOM launched in November 2013.
It works by sampling two kinds of radiation: one that bounces off the Martian surface and is unimpeded by any methane molecules in the atmosphere (reference channel), and one that bounces off the Martian surface and is impeded by methane molecules (methane channel), which absorb the radiation. Michael J. Mumma, a senior scientist at the NASA Goddard Spaceflight Centre, Maryland, and one of the first scientists to report the presence of methane on Mars in 2003, told Seeker, “[ISRO scientists] did not design [MSM] properly for the detection of methane on Mars.”
Since both the methane and reference measurements require light to be incident on the Martian surface, MSM makes its measurements when the side of Mars it is on is also facing the Sun. As a result, the instrument is also able to measure the planet’s albedo, the measure of “the reflecting power of the surface“. “Changes in the albedo can reveal important information on seasonal changes owing to wind transport of dust, formation of water and carbon dioxide frosts, and so on,” Mumma told The Wire. “And these are fundamental to understanding the meteorology on Mars.”
However, he has said that the data MSM relays to Earth does not differentiate between radiation (in the methane channel) absorbed by methane – characterised by a particular frequency of the radiation being absent in the eventual interference pattern – and that absorbed by other gases, such as carbon dioxide, very well.
“Carbon dioxide signatures vary with topography, time-of-day, latitude and season,” he explained. “Temperature variations are very important – they cause the individual ro-vibrational spectral lines to vary in intensity.” Ro-vibrational stands for ‘rotational-vibrational’, a form of spectroscopy used to study the properties of gases. “The two instrument arms” – i.e. the methane and reference channels – “sample this variation in different ways, and accurate removal of the carbon dioxide signature from the difference signal is crucial to searching for methane, for example.”
He added that the Fraunhofer lines, spectroscopic measurements used to infer the composition of a star’s atmosphere, “are also sampled differently by the two arms, further complicating the process”. As a result, and as Mumma told Seeker, “The net effect is that there is no way that one can [cancel] out those two signals in order to retrieve a methane signal” alone.
A September 2015 paper by scientists from ISAC, including incumbent ISRO chairman A.S. Kiran Kumar, acknowledges that the MSM instrument is also sensitive to carbon dioxide, and that the gas absorbs up to 10% of radiation in both the reference and methane channels. Mumma and his colleague Geronimo Villanueva based their modelling of MSM results on parameters described in this paper to inform ISRO in February 2016 of the instrument’s issues. “I expect the instrument science team is evaluating and developing algorithms to include these effects, and I look forward to their publications on the results,” he concluded.
Though the MOM’s primary goal was to demonstrate ISRO’s ability to place a satellite around Mars, there have been few peer-reviewed scientific results published using data collected by the orbiter’s instruments.
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