First highly sampled ice mapping of most abundant ices across a molecular cloud obtained using a specialised technique with the NIRCam instrument on the JWST.

Ever since its launch, JWST observations of cold, dense regions of the galaxy have been revolutionising our understanding of how simple molecules form as interstellar ices on dust grains and continue to evolve during the star and planet formation process. So far, the JWST observations collecting spectra that contain signatures of these icy dust particles have primarily studied individual forming stars, revealing in incredible detail each forming stars’ evolved chemical and physical environment. However, to understand the initial chemical and physical environments within which these stars form and ultimately determine what quantities of these simple molecules are available to be delivered to their daughter planetary systems, it is critical to probe the chemistry of whole molecular clouds, the interstellar nurseries where stars are born. To do this, we need tens or hundreds of observations of these ices across different parts of the molecular cloud. These are obtained by measuring the light emitted by stars that are aligned with the cloud but are a great distance behind it, called background stars. This light interacts with the interstellar icy grains as it passes through the cloud before being captured by the JWST’s large mirror. These interactions create the signatures we use to discover where and how much ice is present towards each probed position of the molecular cloud. One of the main aims of the IceAge program, led by Dr. Melissa McClure from Leiden Observatory, was to demonstrate that a specialised mode on the NIRCam instrument of the JWST could be used to probe the Chamaeleon I molecular cloud at tens of positions simultaneously in a single, efficient observation. This allowed the team to build comprehensive maps of the ice chemistry across the cloud. 

In the latest publication of the IceAge team (Smith et al. 2025), we present the first highly spatially sampled maps of the most abundant ice species - water, carbon dioxide and carbon monoxide - across any molecular cloud. These ices are the key chemical precursors that will set the initial conditions for the chemistry of the protoplanetary disks that will form around new stars in these regions and, ultimately, contribute to the potential habitability of the exoplanets that may form within a disk. As such, it is critical to finely map how this ice chemistry changes across the region of the molecular cloud which will collapse into the disk material. These ice maps are the first ever from JWST and were obtained using a specialised mode of the NIRCam telescope (Wide Field Slitless Spectroscopy) that required the development of a custom piece of software to extract the infrared spectral data. The mapping technique represents a step change in our ability to observe and detect ice in space, and is a novel (and currently under-exploited) use of the technical capacity, sensitivity, spatial and spectral resolution of JWST. Our technique, combining high sensitivity infrared spectroscopy with the custom extraction method, allows us to map ices on spatial scales close to those routinely used to map gas phase chemistry in clouds with radioastronomy. We can thus probe changes in both the ice and dust, adding the missing piece of the puzzle to fully understand the chemistry of the whole cloud.

Lead scientist Dr. Zak Smith developed the software necessary to build these ice maps during his PhD, supervised by Dr. Helen Fraser and Dr. Hugh Dickinson at the Open University. 

“From the very beginning of my PhD, the prospect of developing a piece of software for the world’s most advanced telescope was daunting but incredibly exciting. Now, a number of years later, this software has successfully unlocked the molecular components of mysterious icy grains within a molecular cloud by observing an unmatched number of background stars, giving me and my colleagues access to the missing information required to understand the initial chemical conditions from which stellar and planetary systems may form. Looking ahead, I’m even more excited about what this tool will allow the community to do in the future. Astronomers will go on to map many more clouds with JWST, exploring how chemical diversity among clouds may create life-friendly, habitable chemical conditions in the planets that may form there.”

These results have uncovered that the local astrophysical phenomena that occur within molecular clouds impact the relative abundances of the most simple yet essential ices, including water. Prior to JWST, the small number of observations that had been made of these icy clouds suggested that the more dust was present, the more ices would grow. Our maps, which sample the Chamaeleon I cloud more completely, reveal that those broad generalisations about ice formation must now be treated with caution when studying chemistry in molecular clouds, since we see local variations in ice abundances. The work opens a new window on studying star-forming regions since the mapping technique can be applied to observations of many other clouds or other regions of the same cloud. In the upcoming CHEERIO program led by Smith, now at Leiden Observatory, the edge of the Chamaeleon I cloud will be mapped to search for the initial formation of molecules, like water, at the transition from diffuse to dense regions where chemistry is impacted by high energy radiation from the interstellar medium. Ultimately, ice mapping observations will allow us to understand the chemical origins of these cloud ice abundance variations as well as how icy molecules go on to evolve throughout their journey from molecular clouds to protoplanetary disks to planets.

Link to the published article:

https://www.nature.com/articles/s41550-025-02511-z

Contact:

Zak Smith, zsmith@strw.leidenuniv.nl

Reference:

Smith et al. ‘Cospatial ice mapping of H₂O with CO₂ and CO across a molecular cloud with JWST/NIRCam,’ Nature Astronomy 2025, doi: 10.1038/s41550-025-02511-z