Ultraviolet photolysis of anthracene in H2O interstellar ice analogs: Potential connection to meteoritic organics

Abstract— The polycyclic aromatic hydrocarbon (PAH) anthracene was oxidized by exposure to ultraviolet (UV) radiation in H₂O ice under simulated astrophysical conditions, forming several anthracene ketones (9‐anthrone, 1,4‐anthraquinone, and 9,10‐anthraquinone) and alcohols (1‐anthrol and 2‐anthrol). Two of the ketones produced have been detected in the Murchison meteorite but, to our knowledge, there has been no search for the alcohols or other oxidized anthracenes in meteorites. These results seem consistent with the possibility that interstellar ice photochemistry could have influenced the inventory of aromatics in meteorites. Since quinones are also fundamental to biochemistry, their formation in space and delivery to planets is relevant to studies relating to the habitability of planets and the evolution of life.     

Ultraviolet photolysis and proton irradiation of astrophysical ice analogs containing hydrogen cyanide

Hydrogen cyanide (HCN) has been identified in the gas phase of the interstellar medium as well as in the comae of several comets. Terrestrially, HCN is a key component in the synthesis of biologically important molecules such as amino acids. In this paper, we report the results of low-temperature (18 K) ice energetic processing experiments involving pure HCN and mixtures of HCN with H₂O and NH₃. Ice films, 0.1 to several microns in thickness, were exposed to either ultraviolet photons (110-250 nm) or 0.8-MeV protons to simulate the effects of space environments. Observed products include HCNO (isocyanic acid), NH₄⁺ (ammonium ion), CN⁻ (cyanide ion), OCN⁻ (cyanate ion), HCONH₂ (formamide), and species spectrally similar to HCN polymers. Product formation rates and HCN destruction rates were determined where possible. Results are discussed in terms of astrophysical situations in the ISM and the Solar System where HCN would likely play an important role in prebiotic chemistry. These results imply that if HCN is present in icy mixtures representative of the ISM or in comets, it will be quickly converted into other species in energetic environments; pure HCN seems to be polymerized by incident radiation.     

Interstellar water and interstellar ice

There are two ways that water ice can form in the interstellar medium: H₂O molecules can form in the gas phase and then freeze out onto dust grain surfaces, or O and OH can be converted at the surfaces of grains to form H₂O, which is then retained. Bergin et al. (1998) have recently shown that shocks passing through interstellar clouds sufficiently frequently can make the first method effective. However, we present results from a similar chemical model which indicate that this requires significant optical shielding because of the high ionization fraction in regions exposedto a high UV flux. We deduce, therefore, that grain surface reactions probably represent the main source of H₂O ice on lines of sight with visual extinction up to about 6 magnitudes to an embedded source or 12 magnitudes to a background object. This revised version was published online in July 2006 with corrections to the Cover Date.     

On water ice formation in interstellar clouds

A model is proposed for the formation of water ice mantles on grains in interstellar clouds. This occurs by direct accretion of monomers from the gas, be they formed by gas or surface reactions. The formation of the first monolayer requires a minimum extinction of interstellar radiation, sufficient to lower the grain temperature to the point where thermal evaporation of monomers is just offset by monomer accretion from the gas. This threshold is mainly determined by the adsorption energy of water molecules on the grain material; for hydrocarbon material, chemical simulation places this energy between 0.5 and 2 kcal mol⁻¹, which sets the (true) visible extinction threshold at a few magnitudes. However, realistic distributions of matter in a cloud will usually add to this an unrelated amount of cloud core extinction, which can explain the large dispersion of observed (apparent) thresholds. Once the threshold is crossed, all available water molecules in the gas are quickly adsorbed, because the grain cools down and the adsorption energy on ice is higher than on bare grain. The relative thickness of the mantle, and, hence, the slope of  τ₃( A _v)  depend only on the available water vapour, which is a small fraction of the oxygen abundance. Chemical simulation was also used to determine the adsorption sites and energies of O and OH on hydrocarbons and study the dynamics of formation of water molecules by surface reactions with gaseous H atoms, as well as their chances to stick in situ.     

The heterogeneous condensation of interstellar ice grains

Processes of heterogeneous condensation are investigated. We consider the situation that vapor and solid grains of which material is other than that of the vapor coexist and the saturation of the vapor increases with time. If the contact angle of the condensate to the solid is small (?90°), the condensate will be formed as mantles coating the solid grains but only a fraction of the solid grains will be coated by mantles. The ratio of core-mantle grains to bare solid grains in number will be smaller as the density of the vapor and the time scale of saturation increase are large. Actual calculations are made by adopting H₂O ice as the mantle material and magnesium silicate as the core material.     

Thermal desorption of water ice in the interstellar medium

Water (H₂O) ice is an important solid constituent of many astrophysical environments. To comprehend the role of such ices in the chemistry and evolution of dense molecular clouds and comets, it is necessary to understand the freeze-out, potential surface reactivity and desorption mechanisms of such molecular systems. Consequently, there is a real need from within the astronomical modelling community for accurate empirical molecular data pertaining to these processes. Here we give the first results of a laboratory programme to provide such data. Measurements of the thermal desorption of H₂O ice, under interstellar conditions, are presented. For ice deposited under conditions that realistically mimic those in a dense molecular cloud, the thermal desorption of thin films (≪50 molecular layers) is found to occur with zeroth-order kinetics characterized by a surface binding energy, E _des, of 5773 ± 60 K, and a pre-exponential factor, A , of 10^30 ± 2 molecules cm⁻² s⁻¹. These results imply that, in the dense interstellar medium, thermal desorption of H₂O ice will occur at significantly higher temperatures than has previously been assumed.     

Diffuse component of the cosmic far UV radiation and interstellar dust grains

The diffuse far UV radiation ( 1350–1480 Å) observed in the sky region ofl ^II180°, 0°b ^II40° is analyzed in connection with the distributions of stars and dust grains as well as with optical properties of grains. Its intensity (starlight+scattered light) is about 6×10^–7 erg cm^–2 sec^–1 sr^–1 Å^–1 in the direction ofb ^II0° andl ^II180°. The latitude dependence of the intensity is in approximate agreement with the plane parallel slab model of the galaxy with a reasonable set of parameters. The interstellar scattering gives an albedo close to unity and forward phase function of about 0.6, which are not inconsistent with the model of interstellar grains of Wickramasinghe. The upper limit of the extragalactic UV is 2×10^–8 erg cm^–2 sec^–1 sr^–1 Å^–1 in the same region of wave-length.     

Far UV radiation transfer and H2CO lifetime in dense interstellar clouds

The UV radiation transfer within spherical interstellar dust clouds is analyzed using the method of successive scatterings. The results are used to determine the lifetime of interstellar H₂CO against photo-destruction. The effectiveness of this process is compared with those of chemical mechanisms.     

Far UV extinction and the interstellar 4430 Å band in the large magellanic cloud

The central depths of the interstellar 4430 Å band and far UV extinction for a sample of reddened LMC members are presented here.     

Solar cycle variations in ice acidity at the end of the last ice age: Possible marker of a climatically significant interstellar dust incursion

Hammer et al. (Climatic Change 35 (1997) 1) report the presence of regularly spaced acidity peaks (H⁺,F^-,Cl^-) in the Byrd Station, Antarctica ice core. The event has a duration of about one century and falls at the beginning of the deglacial warming. Volcanism appears to be an unlikely cause since the total acid deposition of this event was about 18 fold greater than the largest known volcanic eruption, and since volcanic eruptions are not known to recur with such regularity. We show that the recurrence period of these peaks averages to 11.5±2.4 years, which approximates the solar cycle period, and suggest that this feature may have an extraterrestrial origin. We propose that this material may mark a period of enhanced interstellar dust and gas influx modulated by the solar cycle. The presence of this material could have made the Sun more active and have been responsible for initiating the warming that ended the last ice age.     

The physical and infrared spectral properties of CO2 in astrophysical ice analogs

Both laboratory measurements and theory indicate that CO2 should be a common component in interstellar ices. We show that the exact band position, width, and profile of the solid-state 12CO2 infrared bands near 3705, 3600, 2340, and 660 cm-1 (2.70, 2.78, 4.27, and 15.2 micrometers) and the 13CO2 band near 2280 cm-1 (4.39 micrometers) are dependent on the matrix in which the CO2 is frozen. Measurements of these bands in astronomical spectra can be used to determine column densities of solid-state CO2 and provide important information on the physical conditions present in the ice grains of which the CO2 is a part. Depending on the composition of the ice, the CO2 asymmetric stretching band was observed to vary from 2328.7 to 2346.0 cm-1 and have full widths at half-maxima (FWHMs) ranging from 4.7 to 29.9 cm-1. The other CO2 bands showed similar variations. Both position and width are also concentration dependent. Absorption coefficients were determined for the five CO2 bands. These were found to be temperature independent for CO2 in CO and CO2 matrices but varied slightly with temperature for CO2 in H2O-rich ices. For all five bands this variation was found to be less than 15% from 10 to 150 K, the temperature at which H2O ice sublimes. A number of parameters associated with the physical behavior of CO2 in CO2- and H2O-rich ices were also determined. The CO2-CO2 surface binding energy in pure CO2 ices is found to be (delta Hs/k) = 2690 +/- 50 K. CO2-H2O and CO-H2O surface binding energies were determined to be (delta Hs/k) = 2860 +/- 200 K and 1740 +/- 100 K, respectively. Under our experimental conditions, CO2 condenses in measurable quantities into H2O-rich ices at temperatures up to 100 K, only slightly higher than the temperature at which pure CO2 condenses. Once frozen into an H2O-rich ice, the subsequent loss of CO2 upon warming is highly dependent on concentration. For ices with H2O/CO2 > 20, the CO is physically trapped within the H2O lattice, and little CO2 is lost until the sublimation temperature of the H2O matrix is reached. In contrast, in ices having H2O/CO2 < 5, the CO2 remains only to temperatures of about 90 K. Above this point the CO2 readily diffuses out of the H2O matrix. These results suggest that two different forms of H2O lattice are produced. The implications of these data for cometary models and our understanding of cometary formation are considered.     

High resolution spectroscopy in the far UV: Observations of the interstellar medium by IMAPS on ORFEUS-SPAS

The Interstellar Medium Absorption Profile Spectrograph (IMAPS) is an objectivegrating, echelle spectrograph built to observe the spectra of bright, hot stars over the spectral region 950–1150Å, below the wavelength coverage of HST. This instrument has a high wavelength resolving power, making it especially well suited for studies of interstellar absorption lines. Following a series of sounding rocket flights in the 1980's, IMAPS flew on its first Shuttle-launched orbital mission in September 1993, as a partner in the ORFEUS-SPAS program sponsored by the US and German Space Agencies, NASA and DARA.On ORFEUS-SPAS, IMAPS spent one day of orbital time observing the spectra of 10 O- and early B-type stars. In addition to outlining how IMAPS works, we document some special problems that had an influence on the data, and we explain the specific steps in data reduction that were employed to overcome them. This discussion serves as a basic source of information for people who may use archival data from this flight, as well as those who are interested in some specific properties of the data that will be presented in forthcoming research papers.IMAPS is scheduled to fly once again on ORFEUS-SPAS in late 1996. On this flight, 50% of the observing time available for IMAPS and two other spectrographs on the mission will be available to guest observers.     

UV photolysis, organic molecules in young disks, and the origin of meteoritic amino acids

The origin of complex organic molecules such as amino acids and their precursors found in meteorites and comets is unknown. Previous studies have accounted for the complex organic inventory of the Solar System by aqueous chemistry on warm meteoritic parent bodies, or by accretion of organics formed in the interstellar medium. This paper proposes a third possibility: that complex organics were created in situ by ultraviolet light from nearby O/B stars irradiating ices already in the Sun’s protoplanetary disk. If the Sun was born in a dense cluster near UV-bright stars, the flux hitting the disk from external stars could be many orders of magnitude higher than that from the Sun alone. Such photolysis of ices in the laboratory can rapidly produce amino acid precursors and other complex organic molecules. I present a simple model coupling grain growth and UV exposure in a young circumstellar disk. It is shown that the production may be sufficient to create the Solar System’s entire complex organic inventory within 10⁶ yr. Subsequent aqueous alteration on meteoritic parent bodies is not ruled out.     

Infrared spectroscopy of Triton and Pluto ice analogs: the case for saturated hydrocarbons

The infrared transmission spectra and photochemical behavior of various organic compounds isolated in solid N2 ices, appropriate for applications to Triton and Pluto, are presented. It is shown that excess absorption in the surface spectra of Triton and Pluto, i.e., absorption not explained by present models incorporating molecules already identified on these bodies (N2, CH4, CO, and CO2), that starts near 4450 cm-1 (2.25 micrometers) and extends to lower frequencies, may be due to alkanes (C(n)H2n+2) and related molecules frozen in the nitrogen. Branched and linear alkanes may be responsible. Experiments in which the photochemistry of N2:CH4 and N(2):CH4:CO ices was explored demonstrate that the surface ices of Triton and Pluto may contain a wide variety of additional species containing H, C, O, and N. Of these, the reactive molecule diazomethane, CH2N2, is particularly important since it may be largely responsible for the synthesis of larger alkanes from CH4 and other small alkanes. Diazomethane would also be expected to drive chemical reactions involving organics in the surface ices of Triton and Pluto toward saturation, i.e., to reduce multiple CC bonds. The positions and intrinsic strengths (A values) of many of the infrared absorption bands of N2 matrix-isolated molecules of relevance to Triton and Pluto have also been determined. These can be used to aid in their search and to place constraints on their abundances. For example, using these A values the abundance ratios CH4/N2 approximately 1.3 x 10(-3), C2H4/N2 < or = 9.5 x 10(-7) and H2CO/N2 < or = 7.8 x 10(-7) are deduced for Triton and CH4/N2 approximately 3.1 x 10(-3), C2H4/N2 < or = 4.1 x 10(-6), and H2CO/N2 < or = 5.2 x 10(-6) deduced for Pluto. The small amounts of C2H4 and H2CO in the surface ices of these bodies are in disagreement with the large abundances expected from many theoretical models.     

Limits on the trapping of atmospheric CH4 in martian polar ice analogs

Recent detection of methane (CH₄) on Mars has generated interest in possible biological or geological sources, but the factors responsible for the reported variability are not understood. Here we explore one potential sink that might affect the seasonal cycling of CH₄ on Mars - trapping in ices deposited on the surface. Our apparatus consisted of a high-vacuum chamber in which three different Mars ice analogs (water, carbon dioxide, and carbon dioxide clathrate hydrates) were deposited in the presence of CH₄ gas. The ices were monitored for spectroscopic evidence of CH₄ trapping using transmission Fourier-Transform Infrared (FT-IR) spectroscopy, and during subsequent sublimation of the ice films the vapor composition was measured using mass spectrometry (MS). Trapping of CH₄ in water ice was confirmed at deposition temperatures <100 K which is consistent with previous work, thus validating the experimental methods. However, no trapping of CH₄ was observed in the ice analogs studied at warmer temperatures (140 K for H₂O and CO₂ clathrate, 90 K for CO₂ snow) with approximately 10 mTorr CH₄ in the chamber. From experimental detection limits these results provide an upper limit of 0.02 for the atmosphere/ice trapping ratio of CH₄. If it is assumed that the trapping mechanism is linear with CH₄ partial pressure and can be extrapolated to Mars, this upper limit would indicate that less than 1% is expected to be trapped from the largest reported CH₄ plume, and therefore does not represent a significant sink for CH₄.     

Bremsstrahlung gamma radiation and interstellar electron spectrum in the local interstellar medium

We present a detailed study of the bremsstrahlung gamma-ray emissivity of the galactic disk. We show that there are large uncertainties in the production spectrum of photons in the medium energy range (10–100 MeV) due to our lack of knowledge of the interstellar electron spectrum below a few hundred MeV. In fact, gamma-ray observations can be of great help in determining this spectrum. At present, the spectral shape of the local gamma-ray emissivity above 30 MeV is available, thanks to the SAS-II and the COS-B satellites. Comparing it to our calculations, we determine the local interstellar electron flux in the 50–500 MeV range; the corresponding integrated gamma-ray emissivity above 100 MeV is equal to 2.4×10^–25 photons s^–1 (H-atom)^–1, 60% higher than previously accepted values.     

Ultraviolet photolysis of amino acids in a 100 K water ice matrix: Application to the outer Solar System bodies

We report the rates of decomposition by ultraviolet (UV) photolysis of four amino acids in millimeter-thick crystalline water ice matrices at 100 K to constrain the survivability of these important organic molecules within ice lying near the surfaces of outer Solar System bodies. We UV-irradiated crystalline ice samples containing known concentrations of the amino acids glycine, aspartic acid, glutamic acid, and phenylalanine, then we measured the surviving concentrations using high performance liquid chromatography (HPLC) with fluorescence detection. From these experiments, we determine photolytic decomposition rates and half-lives. The half-life varies linearly with the ice thickness for all acids studied here. For example, glycine is the most resistant to photolytic destruction with a half-life of 50, 12, and 3.7 h in 1.6, 0.28, and 0.14 mm thick ices, respectively. We explain this linear variation of half-life with thickness as a consequence of extinction, mostly due to scattering, within these macroscopically thick ice samples. Applied to low latitude surface ice on Jupiter's satellite Europa, this analysis indicates that the concentration of any of these amino acids within the top meter of similar ice will be halved within a ∼10 year timescale.     

Water ice clouds in the Martian atmosphere: Two Martian years of SPICAM nadir UV measurements

The SPICAM instrument onboard Mars Express has successfully performed two Martian years (MY 27 and MY28) of observations. Water ice cloud optical depths spatial and temporal distribution was retrieved from nadir measurements in the wavelength range 300–320 nm. During the northern spring the cloud hazes complex distribution was monitored. The clouds in the southern hemisphere formed a zonal belt in the latitude range 30–60°S. The edge of the retreating north polar hood merged with the northern tropical clouds in the range 250–350°E. The development of the aphelion cloud belt (ACB) started with the weak hazes formation (cloud optical thickness 0.1–0.3) in the equatorial region. At the end of the northern spring, the ACB cloud optical thickness reached already values of 0.3–1. The ACB decay in the end of the northern summer was accompanied with a presence of clouds in the north mid-latitudes. The expanded north polar hood merged with the north mid-latitude clouds in the eastern hemisphere. The interannual comparison indicates a decrease in cloud activity immediately after a strong dust storm in southern summer of MY28. The strong dust storms of the MY28 may also be a reason of the observed north polar hood edge shifting northward by 5°.     

Abundances in the interstellar medium

Summary Recent developments in the theory of element production and the chemical evolution of the galaxy are presented. Following this, observational data and their interpretation are given. A case by case analysis of results for D, He, Li and CNO isotope data in the disk and center of our galaxy is presented; previous results for element gradients are also summarized.The primordial abundances of D and He cannot be directly obtained from observations; corrections for stellar processing are discussed. From these data and the Li abundances, it appears that the abundance of the light elements is consistent with the standard big bang. In agreement with previous results, the range of, the baryon to photon ratio, is 5–8 10^–10. If the amount of non-baryonic matter is small, these results indicate an open universe, in the standard big bang model.New data show a gradient in the (¹²C/¹³C) and (¹⁶O/¹⁸O) ratios with galactocentric distance, D_GC. The presence of a gradient in the (¹⁴N/¹⁵N) ratio is less clear and there is no measurable gradient in the (³²S/³⁴S) ratio. In the interstellar medium near the sun, the carbon isotope ratio is –20 percent lower than the solar system ratio. This indicates that there has been only a moderate amount of enrichment of the nearby interstellar medium since the formation of the solar system. These results and previously determined galactic element gradients are interpreted in the framework of chemical evolution models. Delayed recycling of nucleosynthesis products is essential for the correct interpretation of the results. Comparisons of data with galactic evolution models are discussed.This article was processed by the author using the Springer-Verlag T_EX AAR macro package 1991     

Fluctuations in interstellar grain temperatures

An analysis has been made of temperature fluctuations expected for various grain materials exposed to the interstellar radiation field. It is shown that in all but the densest clouds average grain temperatures in the 5–10 K range are of little statistical significance because of large fluctuations produced by absorbed photons from the interstellar radiation field. At higher average temperatures large fluctuations may still be present for certain grain materials and grain radii. The effect of these fluctuations on the simple problem of H atom recombination on grain surfaces is discussed.