Webb peers into the frozen core of the molecular cloud–revealing the darker side of prestellar ice chemistry

Webb identified immobilized forms of a wide range of molecules, including carbon dioxide, ammonia, and methane.

An international team of astronomers announced using[{” attribute=””>NASA’s James Webb Space Telescope. This result allows astronomers to examine the simple icy molecules that will be incorporated into future exoplanets, while opening a new window on the origin of more complex molecules that are the first step in the creation of the building blocks of life.

This image by NASA’s James Webb Space Telescope’s Near-Infrared Camera (NIRCam) features the central region of the Chamaeleon I dark molecular cloud, which resides 630 light years away. The cold, wispy cloud material (blue, center) is illuminated in the infrared by the glow of the young, outflowing protostar Ced 110 IRS 4 (orange, upper left). The light from numerous background stars, seen as orange dots behind the cloud, can be used to detect ices in the cloud, which absorb the starlight passing through them. Credit: Image: NASA, ESA, CSA, Science: Fengwu Sun (Steward Observatory), Zak Smith (The Open University), IceAge ERS Team, Image Processing: M. Zamani (ESA/Webb)

James Webb Space Telescope Unveils Dark Side of Pre-stellar Ice Chemistry

If you want to build a habitable planet, ices are a vital ingredient because they are the main source of several key elements — namely carbon, hydrogen, oxygen, nitrogen, and sulfur (referred to here as CHONS). These elements are important ingredients in both planetary atmospheres and molecules like sugars, alcohols, and simple amino acids.

An international team of astronomers using NASA’s James Webb Space Telescope has obtained an in-depth inventory of the deepest, coldest ices measured to date in a molecular cloud.^[1] In addition to simple ice like water, the team was able to identify frozen forms of a wide range of molecules, from carbonyl sulfide, ammonia and methane, to the simplest complex organic molecule, methanol. (The researchers considered organic molecules to be complex when six or more atoms are present.) This is the most comprehensive census yet of the icy ingredients available to form future generations of stars and planets, before they were heated during the formation of young stars.

said Melissa McClure, an astronomer at the Leiden Observatory in the Netherlands, who is the principal investigator for the observation program and lead author of the paper describing the finding. “These observations open a new window on the formation pathways of the simple and complex molecules needed to make the building blocks of life.”

Annotated version of the image above. The two background stars used in this study, NIR38 and J110621 are indicated on the image in white. Credit: NASA, ESA, CSA, and M. Zamani (ESA/Webb); Science: F. Sun (Stward Observatory), Z. Smith (Open University), and Ice Age ERS team

In addition to the molecules they identified, the team found evidence of molecules more complex than methanol, and although they haven’t definitively attributed these signals to specific molecules, this proves for the first time that complex molecules form in the icy depths of molecular clouds before stars are even born. .

added Will Rocha, an astronomer at the Leiden Observatory who contributed to the discovery. “This could mean that the presence of precursor molecules for prebiotics in planetary systems is a common consequence of star formation, rather than a unique feature of our solar system.”

By detecting sulfur-containing icy carbonyl sulfides, researchers have been able to estimate the amount of sulfur present in icy prestellar dust grains for the first time. While the measured amount is larger than previously observed, it is still less than the total amount that would be expected to be present in this cloud, based on its density. This is true of other CHONS as well. The main challenge for astronomers is understanding where these elements are hiding: in ice, soot-like material, or rocks. The amount of CHONS in each type of material determines how much of these items end up being processed[{” attribute=””>exoplanet atmospheres and how much in their interiors.

“The fact that we haven’t seen all of the CHONS that we expect may indicate that they are locked up in more rocky or sooty materials that we cannot measure,” explained McClure. “This could allow a greater diversity in the bulk composition of terrestrial planets.

Astronomers have taken an inventory of the most deeply embedded ices in a cold molecular cloud to date. They used light from a background star, named NIR38, to illuminate the dark cloud called Chamaeleon I. Ices within the cloud absorbed certain wavelengths of infrared light, leaving spectral fingerprints called absorption lines. These lines indicate which substances are present within the molecular cloud.
These graphs show spectral data from three of the James Webb Space Telescope’s instruments. In addition to simple ices like water, the science team was able to identify frozen forms of a wide range of molecules, from carbon dioxide, ammonia, and methane, to the simplest complex organic molecule, methanol.
In addition to the identified molecules, the team found evidence for molecules more complex than methanol (indicated in the lower-right panel). Although they didn’t definitively attribute these signals to specific molecules, this proves for the first time that complex molecules form in the icy depths of molecular clouds before stars are born.
The upper panels and lower-left panel all show the background star’s brightness versus wavelength. A lower brightness indicates absorption by ices and other materials in the molecular cloud. The lower-right panel displays the optical depth, which is essentially a logarithmic measure of how much light from the background star gets absorbed by the ices in the cloud. It is used to highlight weaker spectral features of less abundant varieties of ice.
Credit: Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI), Science: Klaus Pontoppidan (STScI), Nicolas M. Crouzet (LEI), Zak Smith (The Open University), Melissa McClure (Leiden Observatory)

Chemical characterization of the ices was accomplished by studying how starlight from beyond the molecular cloud was absorbed by icy molecules within the cloud at specific infrared wavelengths visible to Webb. This process leaves behind chemical fingerprints known as absorption lines which can be compared with laboratory data to identify which ices are present in the molecular cloud. In this study, the team targeted ices buried in a particularly cold, dense, and difficult-to-investigate region of the Chamaeleon I molecular cloud, a region roughly 500 light-years from Earth that is currently in the process of forming dozens of young stars.

“We simply couldn’t have observed these ices without Webb,” elaborated Klaus Pontoppidan, Webb project scientist at the Space Telescope Science Institute in Baltimore, Maryland, who was involved in this research. “The ices show up as dips against a continuum of background starlight. In regions that are this cold and dense, much of the light from the background star is blocked, and Webb’s exquisite sensitivity was necessary to detect the starlight and therefore identify the ices in the molecular cloud.”

This research is part of Ice Age project, one of Webb’s 13 Early Release programs. These observations are designed to showcase Webb’s observational capabilities and to allow the astronomical community to learn how to get the best out of its instruments. The Ice Age team has already planned more observations, and hopes to trace the ice’s journey from its formation to the gathering of icy comets.

“This is only the first time in a series of spectral snapshots we will get to see how ices evolve from their initial composition to comet-forming regions of protoplanetary disks,” McClure concluded. “This will tell us which mixture of ices – and therefore which elements – could eventually be delivered to the surfaces of terrestrial exoplanets or incorporated into the atmospheres of gas giants or icy planets.”

These results were published in the January 23 issue of natural astronomy.

Notes

1. A molecular cloud is a huge interstellar cloud of gas and dust in which molecules, such as hydrogen and carbon monoxide, can form. Cool, dense clumps in molecular clouds with higher densities than their surroundings could be the sites of star formation if they collapsed to form protostars.

Reference: “Ice Age JWST Inventory of Dense Molecular Cloud Snow” by M.K. McClure, D. . Qasim, MJ Rasheed, ZL Smith, Fengo Sun, Tracy L. Beck, ACA Bogert, W. Brown, P. Caselli, S.B. Charnley, Herma M. Cobbin, H. Dickinson, M.N. Drozdovskaya, Egami, J. Erkal, H. Fraser RT Garrod, DeHarsono, S. Iopoulou, I Jimenez-Serra, MJin, JK Jorgensen, Lee Christensen, DC Lees, MRS McCostra, Brett A McGuire, JG Melnick, Karen I Oberg, May Palumbo, T. Shimonishi, J.A. Storm, EF Van Dishoek and H. Lennarts, Jan. 23, 2023, Available here. natural astronomy.
DOI: 10.1038/s41550-022-01875-w

The James Webb Space Telescope is the world’s premier space science observatory. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and explore the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners ESA (European Space Agency) and the Canadian Space Agency.