Icy grain mantles are the main reservoir of the volatile elements that link chemical processes in dark, interstellar clouds with the formation of planets and the composition of their atmospheres. The initial ice composition is set in the cold, dense parts of molecular clouds, before the onset of star formation. With the exquisite sensitivity of the James Webb Space Telescope, this critical stage of ice evolution is now accessible for detailed study. Here we show initial results of the Early Release Science programme Ice Age that reveal the rich composition of these dense cloud ices. Weak ice features, including 13CO2, OCN−, 13CO, OCS and complex organic molecule functional groups, are now detected along two pre-stellar lines of sight. The 12CO2 ice profile indicates modest growth of the icy grains. Column densities of the major and minor ice species indicate that ices contribute between 2% and 19% of the bulk budgets of the key C, O, N and S elements. Our results suggest that the formation of simple and complex molecules could begin early in a water-ice-rich environment.
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Our raw data are publicly available at the STScI MAST JWST archive. Text files of our enhanced one-dimensional spectra are provided as part of our Early Release Science enabling product deliverables on Zenodo at the following URL: https://doi.org/10.5281/zenodo.7501239.
The ENIIGMA global fitting tool47 is publicly available on GitHub at https://github.com/willastro/ENIIGMA-fitting-tool.
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The Ice Age Early Release Science team thanks the support team at STScI (W. Januszewski, B. Sargent, N. Pirzkal and M. Engesser) for their technical suggestions and improvements to the programme since 2017. M.K.M. acknowledges financial support from the Dutch Research Council (NWO; grant VI.Veni.192.241). M.G.R. acknowleges support from the Netherlands Research School for Astronomy (NOVA). S.I., H.L. and E.F.v.D. acknowledge support from the Danish National Research Foundation through the Center of Excellence ‘InterCat’ (grant agreement number DNRF150). E.F.v.D. acknowledges support from ERC grant 101019751 MOLDISK. The research of L.E.K. is supported by a research grant (19127) from VILLUM FONDEN. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (D.C.L.). F.S. acknowledges funding from JWST/NIRCam contract to the University of Arizona, NAS5-02105. A.C.A.B. acknowledges support from the Space Telescope Science Institute for programme JWST-ERS-01309.019. J.E. acknowledges support from the Space Telescope Science Institute for programme JWST-ERS-01309.019. L.E.U.C.’s research was supported by an appointment to the NASA Postdoctoral Program at the NASA Ames Research Center, administered by Oak Ridge Associated Universities under contract with NASA. D.H. is supported by Center for Informatics and Computation in Astronomy (CICA) grant and grant number 110J0353I9 from the Ministry of Education of Taiwan. D.H. acknowledges support from the National Technology and Science Council of Taiwan through grant number 111B3005191. M.N.D. acknowledges the Swiss National Science Foundation (SNSF) Ambizione grant number 180079, the Center for Space and Habitability (CSH) Fellowship, and the IAU Gruber Foundation Fellowship. I.J.-S. acknowledges financial support from grant number PID2019-105552RB-C41 by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033. This work was supported by a grant from the Simons Foundation (686302, K.I.Ö.) and an award from the Simons Foundation (321183FY19, K.I.Ö.). J.K.J. acknowledges support from the Independent Research Fund Denmark (grant number 0135-00123B). Z.L.S. acknowledges financial support from the Royal Astronomical Society through the E. A. Milne Travelling Fellowship. J.A.N. and E.D. acknowledge support from French Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of the CNRS/INSU with the INC/INP, co-funded by the CEA and the CNES.
The authors declare no competing interests.
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Extended Data Fig. 1 Global fit of the combined spectrum for NIR38.
Combined NIRSpec and MIRI/LRS spectrum of the NIR38 source (black), with the ENIIGMA fitting tool model (green). Each component in the fit is colour-coded. Panel a shows the entire range between 2.5 and 13 μm and the residuals of the fit. Panels b-f show a zoom-in of selected ranges corresponding to the major ice components. Small insets show the fit of 12CO2 (Panel b), 13CO2 (Panel c), 13CO (panel d) and CH4 (panel e).
Extended Data Fig. 2 Global fit of the combined spectrum for J110621.
Combined NIRSpec and MIRI/LRS spectrum of the J110621 source (black), with the ENIIGMA fitting tool model (green). Each component in the fit is colour-coded. Panel a shows the entire range between 2.5 and 13 μm and the residuals of the fit. Panels b-f show a zoom-in of selected ranges corresponding to the major ice components. Small insets show the fit of 13CO2 (Panel c), 13CO (panel d) and CH4 (panel e).
Extended Data Fig. 3 Confidence interval analysis for the global fits to NIR38 and J110621.
Corner plot showing the confidence interval analysis of the coefficients in the linear combination. The grey-scale contours show the differences in the χ2 maps (Δ) which depends on the degree of freedom (ν) and the statistical significance (α). The yellow and red line contours indicate 2 and 3σ confidence intervals. The left and right plots are for NIR38 (AV = 60 mag) and J110621 (AV = 95 mag), respectively. Note that the ice species assigned to w1-w6 is automatically determined and differs between the left and right panels.
Extended Data Fig. 4 Observed absorption profile of the 13CO2 asymmetric stretching, around 4.39 μm, in NIR38 (left panel) and J110621 (right panel).
To demonstrate the ice chemical environment that best reproduces the observed feature peak, the coloured curves show the scaled profiles of 13CO2 in laboratory spectra of the following ice mixtures at 10 K: pure CO2 (blue), H2O:CO2 (orange), CO2:CO (green), and CO2:CH3OH (red). In all the ice mixtures, CO2 is diluted in a ratio of ~ 1:10, with 12CO2/13CO2 ~ 90.
Extended Data Fig. 5 Observed absorption profile of the 13CO stretching, around 4.78 μm, towards NIR38 (left panel) and J110621 (right panel).
The laboratory spectra of pure 13CO ice at 10 K are also shown in blue.
Extended Data Fig. 6 Silicate subtraction during optical depth calculation for NIR38 and J110621.
MIRI/LRS spectrum of the two background stars before (black) and after (blue) silicate subtraction. The grey dashed line is the synthetic silicate spectrum used to remove the silicate absorption toward the background stars.
Extended Data Fig. 7 Observed absorption profile of the OCN− feature around 4.62 μm, towards NIR38 (left panel) and J110621 (right panel).
A Gaussian fit using the parameters found in the literature62 is also shown.
Extended Data Fig. 8 Observed absorption profile of the C=O stretching of OCS, around 4.9 μm, towards NIR38 (left panel) and J110621 (right panel).
The coloured curves show the profile of the OCS in laboratory ice spectra of pure OCS(blue), H2O:OCS (orange), and CH3OH:OCS (green), all at 17.5 K.
Extended Data Fig. 9 Optical depths of the AV = 60 mag (NIR38, left) and AV = 95 mag (J110621, right) background sources in the 3.2–3.8 μm (3125 - 2631 cm−1) region.
Top: The red line shows the optical depths of CH3OH laboratory data at 15K scaled for the C–H stretching band around the 3.53 μm feature. Bottom: The blue Gaussian represents the likely NH3 ⋅ H2O component centred at 3.47 μm and the red line again displays the CH3OH laboratory data but both are simultaneously scaled so the sum (in green) fits the data from 3.40-3.65 μm.
Extended Data Fig. 10 Map of the column density distribution in the region inferred from the Herschel far-infrared maps from 70 to 500 μm.
The cyan plus-signs indicate the locations of the Class I protostar Ced 110-IRS4, the Class 0 protostar ChamI-MMS and the clump Cha1-C2 going from the north-east (top-left) to south-west (bottom-right). The yellow box and cross indicate the location of the background stars NIR38 (AV ≈ 60 mag) and J110621 (AV ≈ 95 mag), respectively. The contours indicate increasing H2 column densities in steps of 5 × 1021 cm−2, starting at a value of 5 × 1021 cm−2 for the lowest contour (yellow line).
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McClure, M.K., Rocha, W.R.M., Pontoppidan, K.M. et al. An Ice Age JWST inventory of dense molecular cloud ices. Nat Astron (2023). https://doi.org/10.1038/s41550-022-01875-w