Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: formation and destruction of solid carbonic acid upon 1 keV electron irradiation

Context. Carbonic acid (H2CO3) is a weak acid relevant to astrobiology which, to date, remains undetected in space. Experimental work has shown that the β-polymorph of H2CO3 forms under space relevant conditions through energetic (UV photon, electron, and cosmic ray) processing of CO2- and H2O-rich ices. Although its α-polymorph ice has been recently reassigned to the monomethyl ester of carbonic acid, a different form of H2CO3 ice may exist and is synthesized without irradiation through surface reactions involving CO molecules and OH radicals, that is to say γ-H2CO3. Aims. We aim to provide a systematic set of vacuum ultraviolet (VUV) photoabsorption spectroscopic data of pure carbonic acid that formed and was destroyed under conditions relevant to space in support of its future identification on the surface of icy objects in the Solar System by the upcoming Jupiter ICy moons Explorer mission and on interstellar dust by the James Webb Space Telescope spacecraft. Methods. We present VUV photoabsorption spectra of pure and mixed CO2 and H2O ices exposed to 1 keV electrons at 20 and 80 K to simulate different interstellar and Solar System environments. Ices were then annealed to obtain a layer of pure H2CO3 which was further exposed to 1 keV electrons at 20 and 80 K to monitor its destruction pathway. Fourier-transform infrared (FT-IR) spectroscopy was used as a secondary probe providing complementary information on the physicochemical changes within an ice. Results. Our laboratory work shows that the formation of solid H2CO3, CO, and O3 upon the energetic processing of CO2:H2O ice mixtures is temperature-dependent in the range between 20 and 80 K. The amorphous to crystalline phase transition of H2CO3 ice is investigated for the first time in the VUV spectral range by annealing the ice at 200 and 225 K. We have detected two photoabsorption bands at 139 and 200 nm, and we assigned them to β-H2CO3 and γ-H2CO3, respectively. We present VUV spectra of the electron irradiation of annealed H2CO3 ice at different temperatures leading to its decomposition into CO2, H2O, and CO ice. Laboratory results are compared to Cassini UltraViolet Imaging Spectrograph observations of the 70−90 K ice surface of Saturn’s satellites Enceladus, Dione, and Rhea

Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: 1 keV electron irradiation of nitrogen- and oxygen-rich ices

Context. Molecular oxygen, nitrogen, and ozone have been detected on some satellites of Saturn and Jupiter, as well as on comets. They are also expected to be present in ice-grain mantles within star-forming regions. The continuous energetic processing of icy objects in the Solar System induces physical and chemical changes within the ice. Laboratory experiments that simulate energetic processing (ions, photons, and electrons) of ices are therefore essential for interpreting and directing future astronomical observations. Aims. We provide vacuum ultraviolet (VUV) photoabsorption spectroscopic data of energetically processed nitrogen- and oxygen-rich ices that will help to identify absorption bands and/or spectral slopes observed on icy objects in the Solar System and on ice-grain mantles of the interstellar medium. Methods. We present VUV photoabsorption spectra of frozen O2 and N2, a 1:1 mixture of both, and a new systematic set of pure and mixed nitrogen oxide ices. Spectra were obtained at 22 K before and after 1 keV electron bombardment of the ice sample. Ices were then annealed to higher temperatures to study their thermal evolution. In addition, Fourier-transform infrared spectroscopy was used as a secondary probe of molecular synthesis to better identify the physical and chemical processes at play. Results. Our VUV data show that ozone and the azide radical (N3) are observed in our experiments after electron irradiation of pure O2 and N2 ices, respectively. Energetic processing of an O2:N2 = 1:1 ice mixture leads to the formation of ozone along with a series of nitrogen oxides. The electron irradiation of solid nitrogen oxides, pure and in mixtures, induces the formation of new species such as O2, N2, and other nitrogen oxides not present in the initial ice. Results are discussed here in light of their relevance to various astrophysical environments. Finally, we show that VUV spectra of solid NO2 and water can reproduce the observational VUV profile of the cold surface of Enceladus, Dione, and Rhea, strongly suggesting the presence of nitrogen oxides on the surface of the icy Saturn moons

The parent bodies of the Quadrantid meteoroid stream

Aims.We attempt to prove or disprove the comet 96P/Machholz and asteroid 2003 EH1 as the parents of the Quadrantids. These two bodies have been regarded as the most probable candidates. Moreover, we investigate a possibility of an existence of their common progenitor, in the past. Methods.For the moments of several perihelion passages of each parent-body candidate under consideration, we model the theoretical streams around the orbit of the candidate and, via a numerical integration, monitor the dynamical evolution of these streams. The perturbations by eight major planets are taken into account. For the end of the evolution, corresponding with the present, we construct the distributions of orbital elements of that part of a given stream, in which the particles approach the Earth's orbit. These distributions are compared with the corresponding orbital elements of the photographically detected Quadrantids. Results.It is proved that at least one of 96P and 2003 EH1 is the parent body of the Quadrantid meteor stream. Due to an uncertainty in the orbit determination and unknown non-gravitational effects, it is impossible to decide which one of these two bodies is the dominant parent or whether both these bodies have significantly released the meteoroids into the stream. A large population of the Quadrantid stream had to be released from a parent (or parents) at least few millenia ago. If the Earth is also impacted with younger, several-century-old particles, these originate from the asteroid 2003 EH1. However, this young population can represent only a fraction of the entire Quadrantid-shower population. We also demonstrate some possibilities allowing an existence of a progenitor and its splitting to 96P and 2003 EH1. However, we suggest that, within a solely dynamical study, it is impossible to prove that the splitting event did actually happen. Neither of the other candidates considered, comet C/1939 B1 and asteroid 5496, associates any Earth-impacting meteor stream

The meteor-shower complex of 96P/Machholz revisited

Aims. The structure of the complex of meteoroid particles released from comet 96P/Machholz is studied to reveal a relationship among the meteor showers observed in the Earth’s atmosphere that belong to this complex. Methods. For eight perihelion passages of the parent comet in the past, we model theoretical streams associated with comet 96P and follow their dynamical evolution until the present. Subsequently, we analyze the orbital characteristics of the streams, especially of their parts approaching the Earth’s orbit. Results. The dynamics of the stream is controlled by Jupiter, which changes the initial orbits of the particles into the orbits situated within several specific corridors. It thus creates a filamentary structure of the complex. Six filaments approach the orbit of the Earth producing four well-known meteor showers and two showers, whose identification with κ-Velids and α-Cetids is not certain. The known showers, in order of the predicted abundance of meteors, are daytime Arietids, Southern δ-Aquarids, Quadrantids, and Northern δ-Aquarids. The filaments corresponding to the Arietids, δ-Aquarids S and N, and possibly α-Cetids constitute the ecliptical component and those corresponding to the Quadrantids and possibly κ-Velids constitute the toroidal component of the complex

Space weathering of asteroidal surfaces

Context. The surfaces of airless bodies in the Solar System are continuously altered by the bombardment of micrometeoroids and irradiation by solar wind, flares, and cosmic particles. Major effects of this process – space weathering – are darkening and “reddening” of the spectra of surface materials, as well as a “degrading” of absorption features. Aims. We studied the changes induced by energetic ion irradiation in the ultraviolet-visual-near-infrared (UV-Vis-NIR) (0.2–0.98 μm) reflectance spectra of targets selected to mimic the surfaces of airless bodies in the inner Solar System. Our chosen targets are olivine pellets, pure or covered by an organic polymer (polystyrene), which is transparent before irradiation. Polystyrene is used as a template for organic matter of low volatility that can be present on asteroidal surfaces. Moreover we measured the changes induced by ion irradiation in the absorption coefficient of the polymer. The purpose was to have a tool to better compare laboratory with observed spectra and distinguish between planetary objects with pure silicate surfaces and those whose surface is covered by organic matter exposed to cosmic ion bombardment. Methods. The samples were irradiated in vacuum, at room temperature, with 200 keV protons or 200–400 keV argon ions. Before, during, and after irradiation diffuse reflectance spectra were acquired. Polystyrene films were also deposited on quartz substrates and irradiated while transmittance spectra were recorded. Results. We measured the variations of the absorption coefficient of polystyrene as a function of ion fluence. We showed that after ion irradiation the diffuse reflectance spectra of the samples covered by organics exhibit a much more significant variation than those of pure silicates. The spectra of targets made of olivine plus polystyrene can be fitted by using the measured absorption coefficient of polystyrene. Conclusions. The results obtained for pure olivine extend to the UV the spectral range of previous experiments. The data concerning the absorption coefficient of polystyrene are available on our web site (http://web.ct.astro.it/weblab/dbindex.htm

Thermal and energetic processing of astrophysical ice analogues rich in SO 2

International audienc

The role of energetic processing on solid-phase chemistry in star forming regions

It is generally accepted that complex molecules observed in star forming regions are formed in the solid phase on icy grain mantles and are released to the gas-phase after desorption of icy mantles. Most of our knowledge on the physical and chemical properties of ices in star forming regions is based on the comparison between observations and laboratory experiments performed at low temperature (10–100 K). Here we present some recent laboratory experiments which show the formation of (complex) molecular species after ion bombardment of simple ices

Ion processing of ices and the origin of SO2 and O3 on the icy surfaces of the icy jovian satellites

International audienc

Synthesis of formamide and isocyanic acid after ion irradiation of frozen gas mixtures

Context. Formamide (NH2HCO) and isocyanic acid (HNCO) have been observed as gaseous species in several astronomical environments such as cometary comae and pre- and proto-stellar objects. A debate is open on the formation route of those molecules, in particular if they are formed by chemical reactions in the gas phase and/or on grains. In this latter case it is relevant to understand if the formation occurs through surface reactions or is induced by energetic processing. Aims. We present arguments that support the formation of formamide in the solid phase by cosmic-ion-induced energetic processing of ices present as mantles of interstellar grains and on comets. Formamides, along with other molecules, are expelled in the gas phase when the physical parameters are appropriate to induce the desorption of ices. Methods. We have performed several laboratory experiments in which ice mixtures (H2O:CH4:N2, H2O:CH4:NH3, and CH3OH:N2) were bombarded with energetic (30–200 keV) ions (H+ or He+). FTIR spectroscopy was performed before, during, and after ion bombardment. In particular, the formation of HNCO and NH2HCO was measured quantiatively. Results. Energetic processing of ice can quantitatively reproduce the amount of NH2HCO observed in cometary comae and in many circumstellar regions. HNCO is also formed, but additional formation mechanisms are requested to quantitatively account for the astronomical observations. Conclusions. We suggest that energetic processing of ices in the pre- and proto-stellar regions and in comets is the main mechanism to produce formamide, which, once it is released in the gas phase because of desorption of ices, is observed in the gas phase in these astrophysical environments