Du méthane dans l’atmosphère terrestre

W

hen Marcel Migeotte obtained his doctorate in physics from the University of Liège in 1933, the study of the Earth's atmosphere was not really a priority. We knew, of course, that it was made up of nitrogen (mainly), oxygen and argon, its three main constituents. Add a few traces of water vapour and CO2 for good measure; ozone, meanwhile, was discovered at the turn of the century. In any case, the young doctor wasn't much interested in it either! Freshly graduated, he headed off to Paris, to the Institut d'Optique, where he earned another degree, this time as an optical engineer. Migeotte was not only a physicist, but also an engineer, particularly an optician, who never ceased to improve the instruments he worked with.

For the time being (1934-1935), Marcel Migeotte was in the United States, at the University of Michigan, where he specialized in infrared spectroscopy, another foundation, after optics, for his future work. On his return to Liège, he took up an assistant's post and naturally set about developing an infrared spectroscopy laboratory. However, he not only used an existing spectrograph, but also developed his own high-resolution instrument. He then used this instrument to observe the sun from the Cointe observatory (Liège)... during the war years. His first results, admittedly modest, concerned the spectrum of water vapor.

He had to wait until after the war to establish his reputation. He was then (1947-1948) visiting professor at Columbus University (Ohio, USA), where he discovered the progress made by the Americans in infrared techniques during the war. At the same time as teaching optics and electricity to young Americans, he fed the spectrograph at Columbus University with the sun's rays. And he made a double discovery: there was methane (_CH4) and carbon monoxide (CO) in the Earth's atmosphere! Migeotte's aim, it seems, was not to study the Earth's atmosphere, but rather the composition of the Sun's atmosphere. But radiation from the sun passes through the earth's atmosphere, hence the presence in the solar spectrum of what are known as telluric absorption lines... which reveal the composition of the atmosphere surrounding the earth.
For methane," explains Eric Gosset, astrophysicist at the University of Liège, "Marcel Migeotte is sure of his interpretation. For CO, he hesitated because Ohio is a highly industrial state and he thought it might simply be local (anthropogenic) pollution; in fact, at the time, he only published a short article on the subject. To his credit, this hesitation turned out to be unnecessary: he would later show that CO is indeed present in our atmosphere, and not just over the Ohio River, coming out of factory chimneys!

The discovery of methane caused a stir. Yet, as was often the case at the time, it was reported in just a few lines (around fifty!) in a letter with a title that could not have been more explicit, addressed to The Physical Review (1). A short, clear text (a more detailed article was to follow in The Astrophysical Journal (2)), whose historical importance is now fully understood when we consider the role played by methane in global warming! In the text, he even specifies the date on which he first saw the methane lines: January 10, 1948, a date we should perhaps remember... In 1950, he published a letter with Lucien Neven (Royal Observatory of Uccle) confirming the omnipresence of CO (3).

Back in Liège, Marcel Migeotte perfected his infrared spectrograph and continued to focus exclusively on the sun's infrared spectrum. To better study the latter, he installed the device at the International Scientific Station on the Jungfraujoch, in Switzerland, at an altitude of 3,580 m. this was a decisive step forward," exclaims Jean-Pierre Swings, Honorary Professor at the University of Liège, "because we mustn't forget that, at the time, there were still no satellites and most observations were still made from the ground - and observatories were often not built at altitude - or, at best, from balloons sent into the atmosphere. By installing his spectrograph at the Jungfraujoch, Migeotte considerably reduced the impact of the Earth's atmosphere (fewer telluric absorption bands and, above all, a much lower concentration of water vapor). It was at this time, in 1950 and 1951, that he studied the solar spectrum most closely, particularly at wavelengths between 2.8 and 23.7 microns. These were observations he made with Lucien Neven of the Royal Observatory of Belgium The result? The publication, in 1956, of a Photometric Atlas of the Solar Spectrum (4).
The work comprises 123 plates and lists 3624 absorption lines. eric Gosset explains: "It was to remain the reference in the field for a long time to come. Thanks to it, our knowledge of the solar and terrestrial atmospheres will take a spectacular leap forward. In 1958, the Jungfraujoch laboratory was equipped with an improved spectrograph, and observers were intensively present throughout the year. Migeotte first of all, but above all the various collaborators he had trained in Liège, continued to take measurements. The laboratory still exists today, and is largely automated: "First there was another atlas in 1963, followed by the Atlas du spectre solaire de 3000 à 10000 angströms by Luc Delbouille, Ginette Roland (both from the Université de Liège) and Lucien Neven, a new reference for spectroscopic work on the sun.

Text by Henri Dupuis

Scientific references

(1) Spectroscopic Evidence of Methane in the Earth’s Atmosphere, Marcel V. Migeotte, The Physical Review, Vol. 73, N°5, March 1, 1948, p.519.

(2) Methane in the Earth’s Atmosphere, Marcel V. Migeotte, Astrophysical Journal, Vol. 107, 1948, p.400.

(3) Detection of carbon monoxide in the earth's atmosphere at 3580 meters altitude, Marcel Migeotte, Lucien Neven, Physica, XVI, no.4, 1950, p.423.

(4) The solar spectrum from 2.8 to 23.7 microns, Part I: Photometric Atlas, M.V. Migeotte, L. Neven, J.W. Swensson, Mém. Soc. Roy. Sciences de Liège, Special volume no.1, Ed. Ceuterick (Louvain), 1956.