To study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low-luminosity YSOs (L $ 0:1Y10 L ) using 3Y38 m Spitzer and ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well-studied massive YSOs (L $ 10 5 L ). The long-known 6.0 and 6.85 m bands are detected toward all sources, with the Class 0Y type YSOs showing the deepest bands ever observed. The 6.0 m band is often deeper than expected from the bending mode of pure solid H 2 O. The additional 5Y7 m absorption consists of five independent components, which, by comparison to laboratory studies, must be from at least eight different carriers. Much of this absorption is due to simple species likely formed by grain surface chemistry, at abundances of 1%Y30% for CH 3 OH, 3%Y8% for NH 3 , 1%Y5% for HCOOH, $6% for H 2 CO, and $0.3% for HCOO À relative to solid H 2 O. The 6.85 m band has one or two carriers, of which one may be less volatile than H 2 O. Its carrier(s) formed early in the molecular cloud evolution and do not survive in the diffuse ISM. If an NH þ 4 -containing salt is the carrier, its abundance relative to solid H 2 O is $7%, demonstrating the efficiency of low-temperature acid-base chemistry or cosmic-rayYinduced reactions. Possible origins are discussed for enigmatic, very broad absorption between 5 and 8 m. Finally, the same ices are observed toward massive and low-mass YSOs, indicating that processing by internal UV radiation fields is a minor factor in their early chemical evolution.

This paper presents Spitzer-IRS λ/∆λ ∼ 600 spectroscopy of the CO 2 15.2 µm bending mode toward a sample of 50 embedded low-mass stars in nearby star-forming clouds, taken mostly from the "Cores to Disks (c2d)" Legacy program. The average abundance of solid CO 2 relative to water in low-mass protostellar envelopes is 0.32 ± 0.02, significantly higher than that found in quiescent molecular clouds and in massive star forming regions. It is found that a decomposition of all the observed CO 2 bending mode profiles requires a minimum of five unique components. In general, roughly 2/3 of the CO 2 ice is found in a water-rich environment, while most of the remaining 1/3 is found in a CO environment with strongly varying relative concentrations of CO 2 to CO along each line of sight. Ground-based observations of solid CO toward a large subset of the c2d sample are used to further constrain the CO 2 :CO component and suggest a model in which low-density clouds form the CO 2 :H 2 O component and higher density clouds form the CO 2 :CO ice during and after the freeze-out of gas-phase CO. The abundance of the CO 2 :CO component is consistent with cosmic ray processing of the CO-rich part of the ice mantles, although a more quiescent formation mechanism is not ruled out. It is suggested that the subsequent evolution of the CO 2 and CO profiles toward low-mass protostars, in particular the appearance of the splitting of the CO 2 bending mode due to pure, crystalline CO 2 , is first caused by distillation of the CO 2 :CO component through evaporation of CO due to thermal processing to ∼ 20 − 30 K in the inner regions of infalling envelopes. The formation of pure CO 2 via segregation from the H 2 O rich mantle may contribute to the band splitting at higher levels of thermal processing ( 50 K), but is harder to reconcile with the physical structure of protostellar envelopes around low-luminosity objects.

Abstract. Medium resolution (λ/∆λ = 5000−10 000) VLT-ISAAC M-band spectra are presented of 39 young stellar objects in nearby low-mass star forming clouds showing the 4.67 µm stretching vibration mode of solid CO. By taking advantage of the unprecedentedly large sample, high S/N ratio and high spectral resolution, similarities in the ice profiles from source to source are identified. It is found that excellent fits to all the spectra can be obtained using a phenomenological decomposition of the CO stretching vibration profile at 4.67 µm into 3 components, centered on 2143.7 cm −1 , 2139.9 cm −1 and 2136.5 cm −1 with fixed widths of 3.0, 3.5 and 10.6 cm −1 , respectively. All observed interstellar CO profiles can thus be uniquely described by a model depending on only 3 linear fit parameters, indicating that a maximum of 3 specific molecular environments of solid CO exist under astrophysical conditions. A simple physical model of the CO ice is presented, which shows that the 2139.9 cm −1 component is indistinguishable from pure CO ice. It is concluded, that in the majority of the observed lines of sight, 60−90% of the CO is in a nearly pure form. In the same model the 2143.7 cm −1 component can possibly be explained by the longitudinal optical (LO) component of the vibrational transition in pure crystalline CO ice which appears when the background source is linearly polarised. The model therefore predicts the polarisation fraction at 4.67 µm, which can be confirmed by imaging polarimetry. The 2152 cm −1 feature characteristic of CO on or in an unprocessed water matrix is not detected toward any source and stringent upper limits are given. When this is taken into account, the 2136.5 cm −1 component is not consistent with the available water-rich laboratory mixtures and we suggest that the carrier is not yet fully understood. A shallow absorption band centered between 2165 cm −1 and 2180 cm −1 is detected towards 30 sources. For low-mass stars, this band is correlated with the CO component at 2136.5 cm −1 , suggesting the presence of a carrier different from XCN at 2175 cm −1 . Furthermore the absorption band from solid 13 CO at 2092 cm −1 is detected towards IRS 51 in the ρ Ophiuchi cloud complex and an isotopic ratio of 12 CO/ 13 CO = 68 ± 10 is derived. It is shown that all the observed solid 12 CO profiles, along with the solid 13 CO profile, are consistent with grains with an irregularly shaped CO ice mantle simulated by a Continuous Distribution of Ellipsoids (CDE), but inconsistent with the commonly used models of spherical grains in the Rayleigh limit.

Measurements of trace gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. Here we report the apparent presence of phosphine (PH 3) gas in Venus's atmosphere, where any phosphorus should be in oxidized forms. Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH 3 at ~20 ppb abundance is inferred. The presence of PH 3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus's atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH 3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH 3 on Earth, from the presence of life. Other PH 3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas.