ISO detects a new molecule in interstellar space

ESA’s infrared space telescope has once again detected
a new molecule in the ‘chemistry labs’ of the Universe,
the clouds of gas and dust in the space amid the stars.
The newly-detected molecule is the methyl radical
CH₃, a so-called ‘free-radical’ whose existence in
the gas in space had been predicted by Nobel winner
Gerhard Herzberg, who died last year. ISO has already
made the first detections in space of at least ten new
molecules in the gas and solid-state phases. According to the authors –from Germany, the Netherlands and
Australia–, CH₃ is one of the most important tracers
for the formation of complex carbon-based molecules.

The ‘chemical cycle’ of the Universe begins in the stars, where a wealth of new chemical elements are produced in nuclear reactions. These elements combine in the surroundings of the stars to form various molecules, which in turn keep reacting as they travel through on interstellar space clumped in clouds of gas and dust. These so-called ‘molecular clouds’ work as space-chemistry laboratoriess; they close the cycle by contributing material to make new stars and planetary systems. Common molecules on Earth can therefore be common in space too, i.e. water, silicates or complex hydrocarbons –molecules made of carbon and hydrogen.

The new molecule detected by ISO, the methyl radical CH₃, is an intermediate product in reactions leading to the production of hydrocarbons. On Earth, for instance, it intervenes in the synthesis of all petroleum derivatives –which are hydrocarbons. CH₃ is called a ‘free radical’ because of its extremely reactive nature: it combines very easily with other molecules, and in fact in the labs on Earth its study is very difficult because its lifetime is only a few millionths of a second. Herzberg got the Nobel Prize in Chemistry in 1971 for discovering the electronic structure and geometry of free radicals, including CH₃.

The methyl radical was detected by pointing ISO towards the centre of our Galaxy. The spectrometer SWS on board the satellite unambiguously identified the ‘signature’ of the free radical, coming from very cold and thin molecular clouds. Astronomers estimate that the gas in these clouds is at only -256 degrees Centigrade (17* Kelvin), and its density is very low: around one thousand molecules and atoms per cubic centimeter. Most of these are hydrogen –only 13 molecules of CH₃ per one thousand million molecules of hydrogen are detected, a clear demostration of the SWS sensitivity.

These measurements provide the first estimates of the abundance of CH₃ in space, which will also help to derive also the abundance of the larger hydrocarbon molecules.

More than expected

But the detected abundance of CH₃ poses new problems too, since there is actually much more CH₃ than predicted by the traditional models describing molecular clouds. This implies that the models will have to be reviewed.

“The unexpected amount of CH₃ is a very important aspect. None of the models predict so much CH₃. This comes indeed as a surprise, since these models reproduce very well the abundance of many other molecules. So, what is wrong? Are we missing some important aspect of the chemistry?”, says Helmut Feuchtgruber, from the Max-Planck Institut f¸r Extraterrestrische Physik and lead author of the discovery. Feuchtgruber underlines also “how thrilled Herzberg would have been with this detection”.

To experts in space chemistry this discovery confirms the key role ISO is playing in this field. ISO has observed a wealth of molecules in space, both in the gaseous and solid form, e.g. water, ices and crystalline silicates. The methyl radical CH₃ itself was detected extraterrestrially for the first time by ISO on Saturn and Neptune in 1998 and 1999 respectively.

To Feuchtgruber, “ISO’s role in unveiling the chemical processes in space is unique. ISO spectrometers cover the important spectral range where the most abundant molecules have their infrared transitions. The fact that many molecules can be observed simultaneously with the same observatory, or even the same instrument, results in unprecedented accuracy of abundances for these molecules, and as a consequence the chemistry in space can be better understood. The unique sensitivity of the instruments allows also to detect minor species and species which cannot be detected because of the Earth’s atmosphere, extending our knowledge about the processes and raising new puzzling questions”.

The first detection of the free radical CH₃ in the interstellar space is reported in the paper by H. Feuchtgruber, F.P. Helmich, E.F. van Dishoeck and C.M. Wright, which appeared in ‘The Astrophysical Journal, 535: L111-L114, 2000 June 1’. The paper is based on data from both spectrometers on board ISO, the Short Wavelength Spectrometer, SWS, and the Long Wavelength Spectrometer, LWS.

Note for editors:

The European Space Agency’s infrared space telescope, ISO, operated from November 1995 till May 1998, almost a year longer than expected. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO successfully made nearly 30 000 scientific observations.

Contacts:

Helmut Feuchtgruber (Max-Planck Institut f. extraterrestrische Physik, Garching, Germany)
e-mail: fgb@mpe.mpg.de
Tel: +49 89 32 993290

Frank Helmich (SRON, Groningen, The Netherlands)
e-mail: f.p.helmich@sron.nl
Tel: +31 50 3634799

Ewine van Dishoeck (Leiden Observatory, Leiden, The Netherlands)
e-mail: ewine@strw.leidenuniv.nl
Tel: +31 71 5275814

Chris Wright (School of Physics, University College, Canberra, Australia)
e-mail: wright@phadfa.ph.adfa.edu.au
Tel: +61 2 6268 8694

Martin Kessler, ESA, ISO Project Scientist (ESA’s satellite tracking station at Villafranca, Madrid, Spain)
e-mail: mkessler@iso.vilspa.esa.es
Tel: +34 91 8131254