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Interstellar ice

Interstellar ice consists of frozen mantles primarily composed of water (H₂O) and other volatile molecules, such as carbon monoxide (CO), carbon dioxide (CO₂), methanol (CH₃OH), and ammonia (NH₃), that form thin layers on the surfaces of microscopic dust grains in the cold, dense regions of the interstellar medium (ISM).^[1] These ices develop through the accretion and freeze-out of gas-phase atoms and molecules onto dust grains in molecular clouds, where temperatures drop to approximately 10 K and densities range from 10² to 10⁷ particles per cm³, enabling the buildup of amorphous icy layers up to hundreds of molecular layers thick.^[2] The composition varies by environment: in quiescent dense clouds, polar ices dominated by H₂O (with relative abundances of CO at 1–40%, CO₂ at 1–15%, CH₃OH at <4–10%, and NH₃ at 5–10%) prevail, while non-polar species like CO, N₂, and O₂ (each up to 10–40%) are more prominent in outer layers or warmer protostellar regions.^[1] Interstellar ices play a crucial role in astrochemistry, serving as sites for barrierless surface reactions and suprathermal processes driven by cosmic rays and ultraviolet photons, which synthesize complex organic molecules (COMs), including prebiotic compounds like amino acids, carbamic acid, and ammonium carbamate, potentially contributing to the origins of life on planets like Earth via delivery through comets and meteorites.^[2] Observations of these ices, primarily via infrared spectroscopy from telescopes like the Infrared Space Observatory^[3] and James Webb Space Telescope,^[4] reveal their evolution from simple volatiles to processed organics, influencing the chemical complexity of star-forming regions and protoplanetary disks.

Composition and Structure

Chemical Composition

Interstellar ices are predominantly composed of water ice (H₂O), which constitutes approximately 60-70% of the total ice abundance, serving as the primary matrix in which other molecules are embedded.^[5] Following water, the main components include methanol (CH₃OH) at 5-10% abundance, ammonia (NH₃) at 5-15%, carbon monoxide (CO) at 10-30%, and carbon dioxide (CO₂) at 10-20%, with variations depending on the local environment such as molecular cloud density and temperature.^[6] These abundances reflect the freeze-out of gas-phase species onto dust grains in cold interstellar regions, where water dominates due to its high cosmic abundance and favorable formation pathways. Trace components contribute to the organic complexity of interstellar ices, including formaldehyde (H₂CO), molecular hydrogen (H₂), carbonyl sulfide (OCS), and more complex species such as nitriles, ketones, esters, and polycyclic aromatic hydrocarbons (PAHs).^[7]^[8] These minor constituents, often at abundances below 5%, arise from photochemical processing and are detected in both observational spectra and laboratory analogs, enhancing the chemical diversity that may seed prebiotic molecules in star-forming regions.^[9] Isotopic variations in interstellar ices reveal segregation processes, particularly for carbon dioxide, where ¹³CO₂ is observed to separate from ¹²CO₂ in heated environments, as evidenced by distinct vibrational modes in protostellar envelopes.^[10] This segregation, driven by thermal diffusion at temperatures around 50-90 K, indicates dynamic evolution of ice mantles and provides insights into the thermal history of the surrounding medium.^[11] Laboratory simulations replicate these compositions by depositing mixtures such as H₂O:CO:CH₃OH at 10 K under ultra-high vacuum conditions to mimic interstellar grain surfaces, confirming the stability and spectroscopic signatures of these ices through infrared analysis.^[7] Such experiments demonstrate that polar molecules like H₂O and CH₃OH form compact matrices, while apolar CO segregates into distinct layers, aligning with astronomical observations. Recent advances in 2024-2025 have introduced machine learning tools, such as the Automatic Ice Composition Estimator (AICE), which predict fractional abundances of key components including H₂O, CO, CO₂, CH₃OH, and NH₃ directly from infrared spectra in the 2.5-10 μm range.^[12] These neural network-based methods enable rapid analysis of complex spectra from telescopes like JWST, improving accuracy in deriving ice compositions without extensive manual fitting.^[13]

Physical Structure

Interstellar ice primarily exists in the form of amorphous solid water (ASW), a non-crystalline phase that dominates at temperatures below 100 K in cold interstellar environments such as molecular clouds. This amorphous structure arises from the rapid freezing of water vapor onto cold dust grains, resulting in a disordered network of hydrogen-bonded water molecules without long-range order.^[14] Near protostars, where temperatures can exceed 100 K, ASW undergoes a phase transition to crystalline ice, typically hexagonal or cubic forms, driven by thermal annealing that reorganizes the molecular lattice.^[15] These ice layers form mantles coating interstellar dust grains, which are typically composed of silicate or carbonaceous cores with sizes ranging from 0.01 to 1 μm in diameter.^[16] The ice mantles can reach thicknesses of up to hundreds of monolayers (approximately 100–400 ML, or 30–120 nm), accumulating through successive adsorption events in dense regions.^[17] This coating alters the grains' effective size and optical properties, influencing further accretion and radiation interactions. The physical structure of interstellar ice exhibits significant porosity and density variations, with low-density amorphous ice (LDA) having a density of approximately 0.94 g/cm³, compared to more compact high-density forms.^[18] Laboratory studies from 2025 have refined these properties, including infrared band strengths for ASW at 2.82 × 10^{-16} cm molecule^{-1} for the O-H stretching mode at 10 K, which helps quantify ice column densities in observations.^[18] Porosity in ASW, often exceeding 50%, arises from the vapor-deposition process and affects trapping of volatiles and diffusion rates within the mantle.^[19] Recent James Webb Space Telescope (JWST) observations of the Chamaeleon I molecular cloud in 2024 have provided direct insights into grain-scale ice structures, revealing segregated icy layers through the detection of dangling OH groups at ~2.7 μm, indicative of porous, amorphous surfaces with distinct subsurface compositions.^[20] Thermal processing further modifies these structures; for instance, volatiles like CO desorb from ice mantles at temperatures between 20 and 40 K, leading to layering and potential compaction of the remaining water-dominated ice.^[21]

Formation Processes

Accretion onto Dust Grains

In cold, dense interstellar regions, the accretion of gas-phase molecules onto dust grains initiates the formation of ice mantles through physisorption, a process driven by van der Waals forces at low temperatures. This mechanism becomes dominant below approximately 100 K in environments with hydrogen densities exceeding $10^4 cm^{-3}, where molecular thermal velocities are sufficiently reduced to enable efficient adsorption without immediate desorption.^[22]^[7] In such conditions, prevalent in the interiors of molecular clouds, volatile species like H_2O, CO, and CO_2 condense onto the surfaces of silicate or carbonaceous dust grains, typically 0.1 \mum in size, building up thin icy layers over time. Dust grains play a crucial catalytic role in this accretion by providing heterogeneous surfaces that lower the activation energy barriers for molecule attachment, enhancing sticking compared to gas-phase interactions. For water molecules, the sticking probability approaches 1 at 10 K, reflecting near-complete retention upon collision due to the weak binding energies involved in physisorption (around 2000–5000 K for H_2O).^[23]^[7] This high efficiency ensures that even modest gas densities lead to rapid monolayer formation, with subsequent layers accumulating as the grain cools further. The layering of ices proceeds sequentially, beginning with polar molecules such as H_2O that form a stable base layer stabilized by hydrogen bonding networks. Less polar species, including CO, then accrete atop this foundation, creating an onion-like structure where apolar ices occupy outer positions due to their volatility and weaker surface interactions.^[24]^[25] In typical molecular clouds, complete mantle buildup to thicknesses of several monolayers requires timescales of $10^5 to $10^6 years, governed by the local density and depletion rates of gas-phase reservoirs.^[26] Early theoretical frameworks, notably those developed by Greenberg in 1991, emphasized the physical and chemical interactions between dust grains and accreting ices, laying the groundwork for understanding mantle heterogeneity and grain surface properties in interstellar environments.^[27] These models underscored how grain composition influences initial adsorption, contributing to the predominantly H_2O-rich inner mantles observed in such settings.

Thermal and Photochemical Evolution

Interstellar ices undergo thermal evolution primarily through desorption processes, where volatile components are released as temperatures rise in denser regions. Carbon monoxide (CO), a common volatile in ice mantles, desorbs at temperatures between 10 and 20 K in pure form, with sublimation occurring near 16 K under typical interstellar conditions.^[28] This selective desorption of volatiles like CO leads to the compaction of the remaining ice structure, transitioning porous amorphous water ice to more compact forms and influencing the overall mantle morphology. Water ice (H₂O), being more refractory, desorbs at higher temperatures ranging from 130 to 170 K, allowing it to persist longer in warmer environments before complete sublimation.^[29] Photochemical evolution is driven by ultraviolet (UV) irradiation, often induced by cosmic rays or radiation from embedded stars, which dissociates ice molecules into reactive radicals such as OH and HCO.^[9] These radicals facilitate the formation of complex organic molecules, including amino acids like glycine and alanine, through subsequent recombination reactions in the ice matrix. A 2025 review highlights how such photochemistry can produce complex organic molecules of astrobiological interest by processing simple precursors into more intricate structures under interstellar conditions.^[30] Cosmic ray processing further alters ice compositions through ionization and sputtering, which disrupt molecular bonds and eject surface atoms, leading to enriched refractory residues. Recent 2025 studies of the interstellar object 3I/ATLAS reveal spectral evidence of such processing, showing altered volatile-to-refractory ratios consistent with long-term galactic cosmic ray exposure during its journey.^[31] Laboratory experiments simulating these processes, such as UV irradiation of ethanol (CH₃CH₂OH)-CO ice mixtures at low temperatures, demonstrate the photostability of ethanol and the formation of refractory organic residues upon prolonged exposure. These analogs confirm that UV processing enhances the survival of complex organics while producing stable byproducts that mimic observed interstellar mantles.^[32]

Detection and Observation

Infrared Spectroscopy

Infrared spectroscopy serves as the cornerstone for detecting interstellar ices, primarily through absorption features in the mid-infrared (mid-IR) range of 2–20 μm, where vibrational modes of molecular bonds produce characteristic signatures. These features arise from the interaction of infrared light with the dipole moments of ice constituents, allowing identification of species like water (H₂O), carbon monoxide (CO), and carbon dioxide (CO₂). For instance, the O-H stretching mode in H₂O ice manifests as a prominent absorption band centered at approximately 3.05 μm, while the C-O stretching vibration in CO ice appears at 4.67 μm. Similarly, CO₂ ice exhibits a strong asymmetric stretch at 4.27 μm. These wavelengths correspond to fundamental vibrational transitions that are redshifted and broadened in the solid phase compared to gas-phase spectra, reflecting the amorphous or crystalline structure of the ices. The dominant observational technique is extinction spectroscopy, which measures the attenuation of infrared continuum emission from background sources passing through foreground interstellar clouds. This line-of-sight approach targets embedded young stellar objects (YSOs), background field stars, or other bright infrared sources, where the ice mantles on dust grains absorb specific wavelengths, creating dips in the spectrum. Observations are typically conducted at low spectral resolutions (R ≈ 50–2000) to capture broad band profiles, with column densities derived from the integrated optical depth using τ(λ) = ∫ N(s) σ(λ) ds, where N(s) is the molecular abundance along the path and σ(λ) is the absorption cross-section. This method has revealed ices in dense molecular clouds and protostellar envelopes, with typical H₂O column densities ranging from 10¹⁷ to 10¹⁹ cm⁻². Historically, ground-based telescopes such as the NASA Infrared Telescope Facility (IRTF) on Mauna Kea and the Gemini Observatory have enabled near- and mid-IR observations of ice features, overcoming atmospheric absorption windows in the 2–5 μm and 8–13 μm ranges through adaptive optics and high-altitude sites. Space-based platforms revolutionized the field by accessing the full mid-IR spectrum without terrestrial interference: the Infrared Space Observatory (ISO, launched 1995) provided the first comprehensive 2.5–45 μm coverage, followed by the Spitzer Space Telescope (launched 2003) with enhanced sensitivity in the 5–40 μm regime. A seminal survey by Gibb et al. (2004) utilized ISO's Short Wavelength Spectrometer to analyze 2.5–30 μm spectra toward 23 sources, including YSOs and diffuse ISM sightlines, confirming ubiquitous H₂O, CO, and CO₂ bands and establishing baseline ice abundances across environments. Accurate quantification of ice abundances relies on laboratory-derived band strengths (A), defined as the integrated absorption cross-section per molecule (in cm molecule⁻¹), which convert observed τ to column densities via N = τ / A. Recent 2025 laboratory measurements, incorporating porosity and temperature effects in amorphous ices, have refined these values; for example, the O-H stretch in H₂O ice at 10 K yields A = 2.82 × 10⁻¹⁶ cm molecule⁻¹, reducing prior column density estimates by up to 40% in JWST-era analyses. Similar updates for CO₂ ice emphasize density functional theory simulations to predict strengths for mixed mantles, ensuring consistency with astronomical data. A key challenge in mid-IR ice spectroscopy is the overlapping of absorption bands in multi-component ices, where features from H₂O (e.g., 6.0 μm bend), CH₃OH (e.g., 3.5–3.9 μm C-H stretches), and CO₂ (e.g., 15.2 μm bend) blend together, complicating species identification and abundance determinations. Deconvolution techniques, such as Gaussian fitting or multi-component modeling, are employed to isolate contributions, but matrix effects—like band shifts in H₂O-dominated versus pure ices—introduce uncertainties, often requiring laboratory analogs for validation. These issues are particularly acute in complex organic-rich ices, where minor species may contribute <1% to the total opacity yet drive astrochemistry.

Recent Observations with JWST

The James Webb Space Telescope (JWST) has revolutionized the study of interstellar ices through its high-sensitivity infrared observations, providing unprecedented detail on their composition and distribution in cold molecular clouds and protostellar environments. The Early Release Science program "Ice Age," initiated in 2023, conducted an inventory of ices in the dense Chamaeleon I molecular cloud, revealing the richest compositions observed to date in regions as cold as 10 K (-263°C). These observations detected a variety of simple and complex ices, including segregated layers of CO₂ alongside water (H₂O), carbon monoxide (CO), and methanol (CH₃OH), highlighting the initial "dark" chemical stages where ices form on dust grains without significant ultraviolet processing. Building on this, a 2024 study using JWST's Near-Infrared Camera (NIRCam) mapped the structure of water ice toward hundreds of background stars in the Chamaeleon I cloud, identifying elusive "dangling OH" absorption features at 2.703 μm and 2.753 μm. These features indicate grain-scale modifications in the amorphous water ice mantle, such as surface defects or partial crystallization, which alter the ice's spectroscopic signature and suggest dynamic processing even in quiescent dense clouds. The mapping revealed spatial variations in ice structure across the cloud, with stronger dangling OH signals in denser regions, providing insights into how ice morphologies evolve prior to star formation. The JWST Observations of Young protoStars (JOYS+) program, launched in 2023, extended these findings to embedded protostellar sources by detecting complex organic molecule (COM) ices for the first time in situ. Toward the low-mass protostar NGC 1333 IRAS 2A and the high-mass protostar IRAS 23385+6053, mid-infrared spectra identified icy COMs including methyl formate (CH₃OCHO), ethanol (CH₃CH₂OH), and acetic acid (CH₃COOH), with abundances up to several percent relative to water ice. These detections, spanning simple ions like HCOO⁻ and OCN⁻ to more complex species, demonstrate that COM formation occurs early in the ice lifecycle, likely through surface reactions on cold grains. In 2025, JWST observations further elucidated the chemical evolution of CO₂ ices in protostellar envelopes, revealing segregation between ¹²CO₂ and ¹³CO₂ isotopologues in sources like the low-mass Per-emb 35 and high-mass IRAS 20126+4102. Spectra of the 15.2 μm bending mode, 4.39 μm stretching mode, and 2.70 μm combination mode showed double-peaked profiles indicative of pure CO₂ ice layers separated from H₂O- and CH₃OH-rich mixtures, with segregation fractions of 15–20% attributed to thermal processing at ~100 K. The ¹²C/¹³C ratios (~90–132) in these ices closely match interstellar gas-phase values, implying minimal isotopic fractionation during formation and tracing the progression from mixed to layered ice mantles as protostars heat their surroundings. These JWST results complement earlier in situ evidence from the Rosetta mission's Philae lander, which in 2014 detected a hard, compact water ice layer beneath a porous dust mantle on Comet 67P/Churyumov-Gerasimenko using the MUPUS penetrator. This crystalline-like water ice, measured at temperatures around -170°C, serves as a preserved analog to interstellar ices transported from molecular clouds into the Solar System.

Distribution and Environments

In Molecular Clouds

In quiescent dense molecular clouds, where visual extinction exceeds 3 magnitudes (A_V > 3 mag), interstellar ices mantle nearly all dust grains, providing a stable coating that shields the grains from further gas-phase interactions.^[33] These ice layers form primarily through the freeze-out of gas-phase molecules onto cold grain surfaces, dominating in regions shielded from ultraviolet radiation.^[34] The prevalence of such ices underscores their role in the early stages of cloud evolution, prior to star formation. The composition of these ices in molecular cloud cores features high water-to-carbon monoxide (H₂O:CO) ratios, reaching up to 10:1, driven by the efficient freeze-out of abundant species in extremely cold environments (T < 15 K).^[35] Water ice forms the dominant component via successive hydrogenation of atomic oxygen on grain surfaces, while CO accretes later as a distinct layer due to its higher volatility.^[4] These ratios reflect the chemical stratification in shielded cores, where atomic hydrogen mobility enables water synthesis before CO condensation. Interstellar ices serve as primary reservoirs for a significant fraction, often >90%, of heavy elements such as carbon, nitrogen, and oxygen in these clouds, depleting the gas phase and altering cloud dynamics.^[36] This sequestration influences gas-grain interactions and sets the stage for complex molecule formation upon later processing. A 2024 study from the Open University, utilizing JWST NIRCam observations, mapped ices in the Chamaeleon I cloud, revealing uniform amorphous structures characterized by dangling OH features at ~2.7 μm, consistent with unprocessed, porous water-rich mantles across the region. Pre-JWST surveys provide key examples of ice distributions in such environments. In the Taurus molecular cloud, ground-based infrared observations detected the 3 μm water ice absorption feature toward background stars, indicating widespread ice mantles with column densities implying efficient formation on grains.^[37] Similarly, in the Orion molecular cloud, spectroscopic detections of H₂O ice confirmed its presence in dense regions, with abundances suggesting H₂O:CO ratios of 0.01-0.08, highlighting variations tied to local densities.^[38] These observations illustrate the baseline ice profiles in pre-star-forming clouds.

In Protostellar Regions

In protostellar regions surrounding young stars, interstellar ices undergo significant processing due to the heating from the central protostar, leading to partial sublimation and chemical alterations in the inner envelopes where temperatures exceed 100 K. This thermal desorption releases volatile species such as CO and N₂ into the gas phase, enriching the surrounding material with these molecules while leaving behind more refractory components like H₂O and CO₂ ices.^[28] The process occurs primarily in the warm inner regions of the envelope, where radiative heating drives the evaporation front outward over time.^[39] Near Class 0 protostars, the ices show elevated abundances of complex organic molecules (COMs), including methanol (CH₃OH) and formaldehyde (H₂CO), which can reach levels up to 10-20% relative to water ice in the outer envelopes. These higher abundances arise from the hydrogenation of CO on grain surfaces during the cold collapse phase, followed by partial desorption that concentrates the remaining COM-rich ices.^[40] Observations of sources like IRAS 16293-2422 and NGC 1333-IRAS 2A confirm these enrichments, with methanol ice fractions significantly higher than in quiescent molecular clouds.^[41] Recent James Webb Space Telescope (JWST) observations from 2023 to 2025 have revealed detailed structures of ices in embedded protostellar sources, highlighting transitions from amorphous to crystalline forms due to thermal processing. In embedded sources like the low-mass protostar Per-emb 35, segregated ¹²CO₂ and ¹³CO₂ ices are detected, with pure CO₂ features suggesting distillation from more volatile components.^[10] Similarly, toward protostellar cores in Chamaeleon I, such as Ced 110 IRS4, JWST data indicate CO₂ ice segregation and thermal processing, with the 15.2 μm bending mode showing double-peaked profiles characteristic of pure ice phases.^[42] High-mass IRAS sources like IRAS 20126+4104 also exhibit these segregated CO₂ features, underscoring the role of protostellar heating in ice evolution.^[43] Interstellar ices in these regions survive the protostellar infall phase, which typically lasts about 10⁵ years, allowing them to be incorporated into the emerging disk before further processing. During this period, the envelope material collapses inward, with ices in the outer layers remaining largely intact until the sublimation front reaches them.^[44] This timescale enables the inheritance of cloud-formed ices into the protostellar environment, where they contribute to the chemical complexity observed.^[45]

Astrophysical Importance

Role in Astrochemistry

Interstellar ices play a pivotal role in astrochemistry by providing low-temperature reaction sites on dust grain surfaces and within bulk mantles, where radical recombination drives the synthesis of complex organic molecules (COMs). These reactions, occurring at temperatures around 10–20 K, involve mobile radicals such as H, OH, HCO, and CH₃O that diffuse across amorphous solid water (ASW) surfaces to form bonds with minimal activation barriers. For instance, successive hydrogenation of CO leads to formaldehyde (H₂CO) and methanol (CH₃OH), which further recombine with radicals to produce prebiotic species like glycine (NH₂CH₂COOH) through pathways involving NH₂CH₂• and COOH• radicals, and ribose precursors via intermediates such as methyl formate (HCOOCH₃).^[46] A 2024 review highlights interstellar ices as a "factory" for origin-of-life molecules, emphasizing their capacity to form amino acids and other prebiotic compounds under interstellar conditions, paving the way for understanding life's chemical origins in space. Photochemical processing further enhances this complexity; ultraviolet (UV) irradiation and cosmic ray bombardment of ice analogs in laboratory simulations yield amino acids and nucleobases, including all pyrimidine bases (cytosine, uracil, thymine) and purine bases (adenine, hypoxanthine, xanthine) except guanine, with yields ranging from 1–4 ppm for pyrimidines and 38–136 ppb for purines in H₂O:CO:NH₃:CH₄O mixtures at 10 K.^[2]^[47] By locking up a dominant fraction of elemental oxygen as water ice—abundances reaching ~10⁻⁴ relative to H₂ in dense clouds—interstellar ices significantly deplete gas-phase atomic oxygen, limiting reactions like O + H₂ → OH and altering the overall chemical evolution of molecular clouds. Recent experimental work on H₂/CO-rich ice layers, with H₂/CO ratios of 0.1–0.7 at 4–15 K, demonstrates enhanced organic production (>3× yields at <10 K) of species like H₂CO, C₂H₂O, and HNCO upon irradiation, underscoring their contribution to COM formation in dense cloud interiors.^[48]^[49]

Evidence from Solar System Bodies

Studies of Solar System bodies provide compelling evidence that interstellar ices contributed significantly to the volatile inventory of our planetary system, preserving chemical signatures from the presolar interstellar medium. Analysis of the deuterium-to-hydrogen (D/H) ratio in water from enstatite chondrites, primitive meteorites thought to originate from the inner Solar System, reveals values matching those of Earth's oceans, suggesting that materials similar to enstatite chondrites, formed in the inner Solar System, could have delivered most of Earth's water, preserving presolar isotopic signatures from ion-molecule reactions in cold interstellar clouds.^[50] Recent JWST observations (as of 2025) of water ice and COMs in protoplanetary disks confirm the preservation of interstellar ice signatures into planet-forming environments, bridging to Solar System compositions.^[51] In situ observations of comets, remnants of the early Solar System, also demonstrate inheritance of interstellar ice compositions, often modified by subsequent processing. The Rosetta mission's encounters with Comet 67P/Churyumov-Gerasimenko in 2014 detected exposed water ice patches on the surface, including both amorphous and crystalline forms, consistent with initial formation as amorphous interstellar ice mantles on dust grains that underwent thermal crystallization during accretion and solar heating.^[52] These ices, comprising primarily water with traces of CO and CO₂, exhibit molecular abundances and isotopic ratios that mirror those observed in interstellar environments, underscoring minimal alteration from their primordial state despite billions of years of evolution.^[53] Interstellar objects passing through the Solar System offer direct samples of ices from other star systems, revealing processing akin to that in our interstellar medium. Observations of the third confirmed interstellar object, 3I/ATLAS (C/2025 N1), discovered in 2025, show spectral signatures of cosmic ray-altered ices, with an unusually high CO₂/H₂O ratio of approximately 7.6, indicative of irradiation in a dense molecular cloud before ejection from its host system.^[54] Eruptive CO₂ outgassing events observed during its perihelion passage suggest underlying pristine ice mantles, largely unprocessed since formation, providing a snapshot of interstellar ice chemistry beyond our Solar System.^[55] Primitive materials in the outer Solar System further attest to the longevity of interstellar ice signatures. Similarly, the water ice in Saturn's rings, estimated to be less than 400 million years old (as of 2023) based on dynamical modeling of meteoroid impacts, is dominated by nearly pure H₂O with minor organics. These findings highlight how Solar System bodies act as time capsules for interstellar ices. Looking ahead, proposed missions aim to expand this evidence through targeted sampling. A 2025 study by the Southwest Research Institute outlines a flyby mission concept for future interstellar comets like 3I/ATLAS, equipped with spectrometers to analyze ice compositions in situ, potentially confirming cosmic ray processing and volatile ratios indicative of diverse interstellar origins.^[56]

References

[1]
Evolution of Interstellar Ices | Space Science Reviews
Ices in molecular clouds are dominated by the very simple molecules H2O, CH3OH, NH3, CO, CO2, and probably H2CO and H2.
[2]
Interstellar Ices: A Factory of the Origin-of-Life Molecules
Jan 9, 2024 · Interstellar-ice chemistry paves the way for disclosing the mystery of the origin of life: amino acids and other prebiotic molecules are formed in space.Author Information · References
[3]
Interstellar Ice: The Infrared Space Observatory Legacy - IOPscience
We discuss the implications for our current understanding of ice processing. We briefly discuss the composition, origin, and evolution of interstellar ices.
[4]
An expected nonuniform ice mantle composition
excitation regions, ice is dominated by H2O, CO, and CO2, with water typically being the most abundant (∼60–70%). Water molecules formed by gas-phase ...
[5]
[PDF] Interstellar ice analogs: band strengths of H2O, CO2, CH3OH ... - arXiv
Jun 30, 2014 · Water and other simple molecules, such as methanol, carbon dioxide, carbon monoxide, methane, and ammonia, which are present in the interstellar ...
[6]
[PDF] evolution of interstellar ices
Ices in molecular clouds are dominated by the very simple molecules H2O, CH3OH, NH3, CO, CO2, and proba- bly H2CO and H2.
[7]
Interstellar/Precometary Organic Material and the Photochemical ...
Ices in molecular clouds contain the very simple molecules H2O, CH3OH ... H2CO, as well as more complex species including nitriles and ketones or esters.Missing: trace OCS
[8]
Photochemical Pathways to Interstellar Complex Organic Molecules
The interstellar medium is characterized by a rich and diverse chemistry. Many of its complex organic molecules are proposed to form through radical chemistry ...
[9]
JWST Observations of Segregated 12CO2 and 13CO2 Ices in ...
Jul 17, 2025 · Schematic overview of CO2 ice segregation in protostellar envelopes. The temperature and density zones are separated by the gray dashed lines ...Missing: variations | Show results with:variations
[10]
JWST observations of segregated $^{12}$CO$_2$ and $^{13 ... - arXiv
May 20, 2025 · The findings show that upon heating, CO_2 ice is likely segregating from mostly the water-rich ice layer and the CO_2-CH_3OH component becomes ...
[11]
A fast machine learning tool to predict the composition of interstellar ...
We present Automatic Ice Composition Estimator (AICE), a new tool based on artificial neural networks. Based on the infrared (IR) ice absorption spectrum ...
[12]
[2509.04331] A fast machine learning tool to predict the composition ...
Sep 4, 2025 · We present Automatic Ice Composition Estimator (AICE), a new tool based on artificial neural networks. Based on the infrared (IR) ice absorption ...
[13]
Energy Transfer and Restructuring in Amorphous Solid Water upon ...
Nov 16, 2022 · The main constituent of interstellar ices is amorphous solid water (ASW), (1) which is formed on dust grains in dark molecular clouds from ...
[14]
Crystallization Kinetics of Amorphous Solid Water - NASA/ADS
Amorphous solid water (ASW) is ubiquitous in space and has been identified as a major component in many astrophysical ices, including cometary nuclei, ...
[15]
THE EFFECTS OF GRAIN SIZE AND TEMPERATURE ...
The collapse models produce an ice mantle on the dust grains, with a varying thickness from 40 to 100 monolayers, from the smallest to largest grains. On ...
[16]
Ice Coverage of Dust Grains in Cold Astrophysical Environments
It was shown that water ice, having the nominal thickness of 400 ML or 130 nm (taking an approximate monolayer thickness of 0.3 nm [44] ), mixed with carbon ...
[17]
Interstellar water ice analogue properties as a function of temperature
With our new band strength of 2.82×10−16 (value at 10 K, for other temperatures see Table 3), the column densities are up to 40% smaller, and therefore the ...
[18]
Thermal collapse of porous interstellar ice - Astronomy & Astrophysics
This article aims at a quantitative characterization of the phase transition of porous amorphous solid water (ASW) to a non-porous, i.e., more compact structure ...
[19]
Detection of the elusive dangling OH ice features at ~2.7 μm in ...
We exploit the high sensitivity of JWST NIRCam to detect two dOH features at 2.703 and 2.753 μm along multiple lines of sight probing the dense cloud Chamaeleon ...Missing: segregated | Show results with:segregated
[20]
Evaporation of ices near massive stars: models based on laboratory ...
showed that CO desorbed from an H2O/CO ice mixture occurs in four temperature bands, rather than the assumed CO desorption at temperatures above about 25 K.
[21]
Computer Generated Realistic Interstellar Icy Grain Models
Apr 19, 2022 · These icy grain models allow studying the adsorption features (structure, binding energy, vibrational frequencies, etc.) of relevant species on a large variety ...
[22]
[PDF] Adsorption and desorption of model interstellar ices on a dust grain ...
At the low temperatures of molecular clouds, down to 10 K, gas phase species readily condense onto the surface of dust grains, forming icy mantles, where ...
[23]
Phase Transition of Interstellar CO Ice - IOPscience
Jul 6, 2021 · Dust grains in star-forming regions, the so-called dense molecular clouds, are covered by an ice mantle with an onion-like layered structure.
[24]
Using laboratory studies of CO–H2O ices to understand the non ...
We present results from laboratory experiments on layered CO–H2O-ice systems, carried out from sub-monolayer to multilayer CO coverages, and review recent ...
[25]
Interstellar matrices: the chemical composition and evolution of ...
They are made principally of dirty ices whose composition must be determined carefully to assess the importance of the solid state chemistry that takes place in ...Missing: definition | Show results with:definition
[26]
Physical, Chemical and Optical Interactions with Interstellar Dust
Photochemistry of interstellar ice forming complex organic molecules ... Greenberg, J.M. (1991). Physical, Chemical and Optical Interactions with ...
[27]
[PDF] Thermal desorption of interstellar ices - arXiv
Jan 19, 2022 · The temperature of gas and solids in the disk decays with radius leading to sequential freeze-out of volatile carriers of O (H2O, CO, CO2, ...
[28]
[PDF] Thermal Desorption of Interstellar Ices - HAL
Jul 15, 2024 · For instance, we can take the case of CO; in pure CO ice (Eb ≃ 900 K), it has a sublimation temperature near 16 K, while any CO bound to a ...
[29]
Photochemistry of interstellar ice forming complex organic molecules
Jul 4, 2025 · In addition to water, interstellar ice contains other simple species. In this Review we present our current knowledge on ice photochemistry ...
[30]
[PDF] Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic ... - arXiv
Oct 29, 2025 · With. 3I/ATLAS approaching perihelion in October 2025, immediate follow-up observations are critical to confirm this interpretation and ...
[31]
UV irradiation of ethanol-containing interstellar ice analogs - arXiv
Oct 31, 2025 · This proof-of-principle study aims to quantify how the ice composition influences the photostability of ethanol mixed with CO, from both ...
[32]
H2 formation on interstellar dust grains: The viewpoints of theory ...
This article has been written to present what we think we know about the H 2 formation in a variety of interstellar environments.Missing: H2O | Show results with:H2O
[33]
Interstellar Ices: A Factory of the Origin-of-Life Molecules - PMC
Jan 9, 2024 · Schematic representation of its composition and reactivity. In cold molecular clouds (∼10 K), interstellar dust grains are covered by water ice.
[34]
[PDF] Chemical reactions in H2O:CO interstellar ice analogs promoted by ...
Mar 15, 2022 · In laboratory, CO:H2O ice analogues have been exposed to ionizing irradiation to simulate cosmic irradiation of grain mantle existing in.
[35]
Cospatial ice mapping of H2O with CO2 and CO across a molecular ...
Mar 24, 2025 · Here we present cospatial maps of H 2 O, CO 2 and CO ice over the central region of the Chamaeleon I molecular cloud, using 44 lines of sight observed with the ...
[36]
Interstellar ice grains in the Taurus molecular clouds - Nature
May 19, 1983 · Ice may be widespread in the Taurus clouds, vindicating ideas on grain composition and growth first reported nearly 50 yr ago.
[37]
Evidence for interstellar H2O in the Orion molecular cloud
The H2O column density implies abundance ratios of (H2O)/(CO) = 0.01-0.08 or ... The gas-to-ice ratio is (H2O gas)/(H2O ice) = 0.06 with an estimated ...
[38]
Thermal Desorption of Interstellar Ices: A Review on the Controlling ...
Feb 16, 2022 · In this consensus evaluation, we focus on parameters controlling the thermal desorption of ices and how these determine pathways toward molecular complexity.
[39]
CH3OH abundance in low mass protostars - Astronomy & Astrophysics
the observations reveal clear evidence for high methanol abun- dances in the envelopes of two Class 0 protostars, IRAS 16293-. 2422 and NGC 1333-IRAS 2. At the ...
[40]
formaldehyde and methanol deuteration in protostars: fossils from a ...
Feb 27, 2012 · Analogously, several formaldehyde and methanol isotopologues have been detected in Class 0 low- mass protostars (Ceccarelli et al. 1998, 2001; ...
[41]
Pristine ices in a planet-forming disk revealed by heavy water - Nature
Oct 15, 2025 · The few existing observations probing water ice in disks cannot distinguish whether water is formed in the initial stages of star and planet ...
[42]
[PDF] Ice inventory towards the protostar Ced 110 IRS4 observed with the ...
(2023), reveal the presence of many ice species, including H2O, CO, CO2, NH3 and OCS, as well as CH3OH, which highlights the complex ice chemistry already ...
[43]
JWST observations of segregated 12CO2 and 13CO2 ices in ... - arXiv
May 20, 2025 · The findings show that upon heating, CO 2 ice is likely segregating from mostly the water-rich ice layer and the CO 2-CH 3OH component becomes ...
[44]
ICE EVOLUTION FROM CORES TO PROTOSTARS - IOP Science
Oct 4, 2011 · Star formation begins with the collapse of an interstellar cloud core to form a protostar, which grows from infall of envelope material and ...
[45]
Chemistry in low-mass protostellar and protoplanetary regions - PNAS
Aug 15, 2006 · When interstellar clouds collapse to form new stars and planets, the surrounding gas and dust become part of the infalling envelopes and ...
[46]
Radical reactions on interstellar icy dust grains - NIH
A dust particle with a typical diameter of ∼0.1 µm has a silicate or carbonaceous core. In the MC environment, the dust core is typically enveloped by an ice ...Missing: size | Show results with:size<|control11|><|separator|>
[47]
Nucleobase synthesis in interstellar ices | Nature Communications
Sep 27, 2019 · Thymine and other prebiotic molecules produced from the ultraviolet photo-irradiation of pyrimidine in simple astrophysical ice analogs.
[48]
Interstellar Water Chemistry: From Laboratory to Observations
We recently developed a model to simulate CO hydrogenation via H atoms on interstellar ice surfaces, the most important interstellar route to H2CO and CH3OH, ...
[49]
Organic Chemistry in the H2-bearing, CO-rich Interstellar Ice Layer ...
Feb 12, 2024 · Ice chemistry in the dense, cold interstellar medium (ISM) is probably responsible for the formation of interstellar complex organic molecules (COMs).
[50]
Earth's water may have been inherited from material similar to ...
Aug 28, 2020 · We show that EC meteorites contain sufficient hydrogen to have delivered to Earth at least three times the mass of water in its oceans.
[51]
Detection of exposed H2O ice on the nucleus of comet 67P ...
Detection of exposed H2O ice on the nucleus of comet 67P/Churyumov-Gerasimenko. as observed by Rosetta OSIRIS and VIRTIS instruments. M. A. Barucci1, G.
[52]
Unexpected and significant findings in comet 67P/Churyumov ...
Sep 7, 2016 · On 2014 November 12, Rosetta deployed the Philae lander on the surface of the comet nucleus. Hundreds of papers are reporting scientific results ...
[53]
Interstellar Comet 3I/ATLAS: Evidence for Galactic Cosmic ... - arXiv
Oct 30, 2025 · Spectral observations of 3I/ATLAS (C/2025 N1) with JWST/NIRSpec and SPHEREx reveal an extreme CO2 enrichment (CO2/H2O = 7.6+-0.3) that is 4.5 ...Missing: ices | Show results with:ices
[54]
https://arxiv.org/abs/2510.26308
[55]
SwRI-proposed mission could encounter and explore a future ...
Development study sets mission objectives and trajectory of journey to interstellar comet, as well as probability of success in locating a ...