Cold, interstellar molecular clouds are places where stars and planetary systems form – the solar system also once formed in such a cloud. Now, for the first time, astronomers have looked into the icy center of such a cloud and, using the James Webb Space Telescope, have identified the elements and molecules present there. The spectral data reveal the presence of key elements of all life building blocks, which are present there as water, methane and nitrogen ice, but also as more complex organic molecules. This provides first insights into the chemical raw materials that are already present in the "primeval clouds" at the beginning of star and planet formation.
At the beginning of new stars and planetary systems is an interstellar cloud of cold molecular gases and ice-covered dust. When the dense, cool gas of such clouds collapses under its own gravity, new stars form at their centers. A disc of gas and dust rotates around this, forming the breeding ground for new planets. Astronomers and planetary researchers have long suspected that these protoplanetary discs not only contain the raw materials for the planets and their gas envelopes, but also the first chemical precursors for the building blocks of life - biomolecules that form the basis of all cells and organisms. The ice layer of the interstellar dust grains in the cold molecular clouds is considered to be a particularly productive medium for the chemical reactions that produce such precursor molecules. However, it has so far only been possible to partially determine which elements and molecules are present in the center of these clouds, because the dust blocks the view.
Spectra reveal icy molecules
With the help of the James Webb Space Telescope, a team of astronomers led by Melissa McClure from Leiden University has now succeeded in recording the existence of these icy building blocks of life in a dark molecular cloud. To do this, they used the telescope to focus on the Chameleon I molecular cloud, 630 light-years from Earth, in which dozens of new stars are currently forming. “We recognize the different ice molecules based on their so-called absorption spectrum. They leave this chemical fingerprint in the background starlight that shines through the cloud onto the telescope,” explains co-author Maria Drozdovskaya from the University of Bern. "These measurements were only possible with Webb's high-precision infrared spectrographs (NIRSpec and MIRI), which can precisely detect and break down radiation at these wavelengths."
The result is a first inventory of the icy building blocks of life in the center of an interstellar molecular cloud. "This is the first time that researchers have been able to study the composition of so-called prestellar ice species near the center of a molecular cloud," says McClure. The spectral data revealed that the ice-covered dust at the center of Chameleon I contains large fractions of water ice, as well as frozen carbon dioxide and carbon monoxide, as expected. In addition to frozen ammonia and methane, the nitrogen compound cyanate (OCN) and the sulfur compound carbonyl sulfide (OCS) are also found there, as the astronomers report. They were also able to detect more complex organic molecules in the center of Chameleon I, including the alcohols methanol and ethanol and the hydrocarbons acetone and acetaldehyde. "The identification of more complex organic molecules such as methanol, and likely ethanol, suggests that the stars and planetary systems developing in this cloud will inherit chemicals that are already relatively advanced," says McClure's colleague Will Rocha.
key elements of life
The astronomers have thus identified important key elements of life in the cold molecular cloud: carbon, hydrogen, oxygen, nitrogen and sulphur, also known as CHONS, are the raw materials from which almost all biomolecules are made. "These elements are important components of prebiotic molecules such as simple amino acids - and thus, so to speak, ingredients of life," explains co-author Maria Drozdovskaya from the University of Bern. According to the research team, the occurrence of even more complex organic molecules suggests that precursors of prebiotic molecules in space are not the exception, but rather the rule. "This could mean that the presence of prebiotic molecules in planetary systems is a common result of star formation and not just a unique feature of our solar system," says McClure.
However, the measurement results also raise questions. Because the amounts of elements detected by the spectra are actually too small compared to the density of the cloud. As the team explains, this could indicate that the elements and molecules are not only present in the icy constituents of the molecular cloud, but also in the dust itself. "The fact that we are 'missing' part of the CHONS budget could mean that CHONS are trapped in rocky dust particles,” explains McClure. The team has already planned further observations with the James Webb telescope as part of their "Ice Age" project. "This is just the first in a series of spectral snapshots that we plan to take," explains McClure. "They should show us how these ices evolve from their initial formation to the comet-forming regions of the protoplanetary disks."
Source: Melissa McClure (Leiden University) et al., Nature Astronomy, doi: 10.1038/s41550-022-01875-w