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LABORATORY STUDIES on the FORMATION of OZONE (O3) on ICY SATELLITES and on INTERSTELLAR and COMETARY ICES Chris J

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LABORATORY STUDIES on the FORMATION of OZONE (O3) on ICY SATELLITES and on INTERSTELLAR and COMETARY ICES Chris J The Astrophysical Journal, 635:1362–1369, 2005 December 20 # 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A. LABORATORY STUDIES ON THE FORMATION OF OZONE (O3) ON ICY SATELLITES AND ON INTERSTELLAR AND COMETARY ICES Chris J. Bennett1 and Ralf I. Kaiser1,2,3 Received 2005 August 1; accepted 2005 August 25 ABSTRACT The formation of ozone (O3) in neat oxygen ices was investigated experimentally in a surface-scattering machine. At 11 K, solid oxygen was irradiated with energetic electrons; the chemical modification of the target was followed on-line and in situ via Fourier transform infrared spectroscopy (FTIR; solid state) and quadrupole mass spec- trometry (QMS; gas phase). The dominant product identified was the ozone molecule in the bent, C2v symmetric 1 structure, O3(X A1); the cyclic D3h isomer was not observed. The associated van der Waals complexes [O3 ...O] and [O3 ...O3] could also be detected via infrared spectroscopy, verifying explicitly the existence of oxygen atoms in the matrix at 11 K. Three different formation mechanisms of ozone were revealed. Two pathways involve the reaction 3 À of suprathermal oxygen atoms with molecular oxygen ½O2(X Æg ) at 11 K. Once the sample was warmed after the irradiation to about 38 K, a third, thermal reaction pathway involving the barrierless reaction of ground-state oxygen atoms with molecular oxygen sets in. During the warm-up phase, the inherent sublimation of oxygen and ozone was monitored by mass spectrometry and occurs in the ranges 28–43 and 58–73 K, respectively. Our data are of help to understand the mechanisms of ozone formation within apolar interstellar and cometary ices and could also be applicable to outer solar system icy bodies, such as the moons of Jupiter (Ganymede, Europa, and Callisto) and Saturn (Rhea and Dione), where ozone and/or condensed oxygen has been observed. Subject headinggs: astrochemistry — cosmic rays — ISM: molecules — methods: laboratory — molecular processes — planets and satellites: general 1. INTRODUCTION on an extrasolar planet using visible and radio frequencies is problematic (Le´ger 2000); thus, use of the mid-infrared region to The importance of the ozone layer in Earth’s atmosphere was detect molecular vibrations is the most promising. However, the realized in 1881 by Hartley (1881)—ozone (O ), during photo- 3 selection rules dictate that a vibration must change the molecular dissociation, absorbs ultraviolet (UV) light strongly between dipole moment to be infrared active. Since molecular oxygen has 200 and 320 nm (k ¼ 254 nm), which is extremely damaging max no dipole moment and is symmetric, the fundamental vibration is to life, particularly to DNA (kmax ¼ 260 nm; Okabe 1978; Thomas 1993). Ozone is produced within the terrestrial atmosphere in the infrared forbidden. On the other hand, the ozone abundance is fairly insensitive to the oxygen concentration and is expected gas phase by the following reaction scheme, similar to the one to be a logarithmic tracer for oxygen in planetary atmospheres originally proposed in 1930 by Chapman (1930): (Angel et al. 1986; Kasting & Catling 2003). Coupled with its 3 À 3 3 UV-shielding properties, it is hardly surprising that detection of O2(X Æg ) þ h (<240 nm) ! O( P) þ O( P); ð1Þ the strongly infrared active 9.6 m (1041 cmÀ1)bandofozone has been selected as one of the primary goals of both the Ter- 3 À 3 1 Ã O2(X Æg ) þ O( P) þ M ! O3(X A1) þ M : ð2Þ restrial Planet Finder (TPF; NASA) and Darwin (ESA) inter- ferometer space telescopes (Beichman et al. 1999; Le´ger 2000). Reaction (1) requires light of a wavelength below 240 nm, im- However, to be certain that molecular oxygen and hence ozone plying that the energy required to break the bond in molecular is really based on active life on an extraterrestrial planet, it has oxygen is 498 kJ molÀ1 (5.17 eV); thus, the heat of formation been recognized that we must also understand possible abiotic (Áf H ) for a single oxygen atom in its ground state is 249 kJ production processes of oxygen/ozone observed within our solar molÀ1 (2.58 eV). Since the production of ozone via equation (2) system to rule out the possibility of ‘‘false positive detections’’ À1 is exothermic (Áf H ¼143 kJ mol , À1.48 eV), the pres- (Selsis et al. 2002). For example, although ozone has been de- ence of a third body, M (N2 or O2 in our atmosphere), is crucial tected on Mars (Barth et al. 1973), there is not much evidence to in order to carry away some of the excess energy (107 kJ molÀ1, support that it has a biogenic origin. 1.10 eV) to stabilize the internally excited ozone molecule Astrochemical models of dark clouds predict that condensed formed via reaction (2).4 oxygen is likely to be a major component of apolar dust grains Owen (1980) suggested that a planet that harbors life should be within interstellar clouds (T 10 K), which will be chemically rich in oxygen (O2 ) and water. The detection of chemical species processed by irradiation from Galactic cosmic-ray particles (predominantly MeV–GeV protons) and the internal UV field (Tielens & Hagen 1982; Mathis et al. 1983; Greenberg 1984; 1 Department of Chemistry, University of Hawai’i at Manoa, Honolulu, HI Strazzulla & Johnson 1991). There is also some evidence of 96822. 2 condensed oxygen on the icy satellites of Jupiter and Saturn; the Department of Physics and Astronomy, The Open University, Milton 8 Keynes MK7 6AA, UK. 5773 visible absorption band arising from interacting pairs of 3 Corresponding author: [email protected]. O2 molecules has been detected on Ganymede (Spencer et al. 4 Values taken from http://webbook.nist.gov/chemistry. 1995) and, more recently, on Europa and Callisto (Spencer & 1362 FORMATION OF O3 ON ICY SATELLITES 1363 Fig. 1.—Top view of the experimental setup. Calvin 2002). There is some debate as to whether or not con- UV photons (Crowley & Sodeau 1989; Schriver-Mazzuoli et al. densed oxygen would be stable for long periods at the temper- 1995; Gerakines et al. 1996; Dyer et al. 1997), keV protons atures of these bodies (90–150 K). This led to the proposal of (Baragiola et al. 1999; Fama et al. 2002), and electrons (Lacombe bubbles of oxygen formed in ion tracks (Johnson & Jesser 1997); et al. 1997 and references therein) and will be discussed in this likewise, the signal could emanate from cooler icy patches, where context. oxygen may be stable (Calvin & Spencer 1997), or from an at- mospheric haze (Vidal et al. 1997). Evidence for chemical pro- 2. EXPERIMENTAL cessing comes from the detection of atomic oxygen in the UV The experimental setup is shown in Figure 1. Briefly, an ul- airglows of Europa and Ganymede from the 1304 and 1356 8 trahigh vacuum (UHV) chamber is evacuated down to a base emission lines (Hall et al. 1998); in addition, the detection of pressure of typically 5 ; 10À11 torr using oil-free magnetically ozone via its UVabsorption at 260 nm on Ganymede (Noll et al. suspended turbomolecular pumps. In the center of the cham- 1996) as well as on two icy satellites of Saturn, Rhea and Dione ber is a rotatable highly polished silver monocrystal, which is (Noll et al. 1997), is confirmed. These icy satellites are contin- cooled to 11:0 Æ 0:3 K by a closed-cycle helium refrigerator. ually bombarded by energetic particles from the magnetospheres The molecular oxygen (O2) frost was prepared by depositing of Jupiter and Saturn, with the highest energy flux coming from oxygen (99.998%) for 5 minutes at a pressure of 10À7 torr onto an energetic electron bombardment (Cooper et al. 2001). the cooled silver crystal. The condensed sample is inspected The use of energetic electrons to bombard our solid oxygen via a Nicolet 510 DX Fourier transform infrared spectrometer sample is twofold. First, we study the effects of energetic elec- (242 scans over 5 minutes from 6000 to 400 cmÀ1 with a reso- trons of the planetary magnetospheres interacting with oxygen lution of 2 cmÀ1) operating in absorption-reflection-absorption ices. Second, we have the additional benefits of simulating the mode (reflection angle ¼ 75). The gas phase is monitored by energetic electrons released via primary ionization processes in a quadrupole mass spectrometer (Balzer QMG 420) operating the track of the Galactic cosmic-ray particles in interstellar and in residual gas analyzer mode with the electron impact ioniza- cometary ices (Oort cloud). Most importantly, the averaged tion energy at 90 eV. linear energy transfer (LET) of keV electrons is comparable to The samples were then irradiated isothermally at 11:0 Æ 0:3K that of MeV protons (typically a few keV per m); therefore, we with 5 keV electrons generated with an electron gun (Specs EQ can also simulate the inelastic energy transfer processes occur- 22/35) at beam currents of 100 nA for 50 minutes by scanning ring without the need for a cyclotron (Johnson 1990). This paper theelectronbeamoveranareaof3:0 Æ 0:4cm2. In theory, this is the first in a series of a systematic research program that aims would mean that during the irradiation the sample would be ex- to investigate the formation of ozone using varied energetic par- posed to a total of 1:9 ; 1015 electrons (6:2 ; 1014 electrons cmÀ2); ticles (electrons, protons, and photons), target compositions however, not all of the electrons generated by our electron gun (neat oxygen and binary mixtures with water), and tempera- actually reach the target—the manufacturer states an extraction tures (10 K, relevant to molecular clouds and comets in the efficiency of 78.8%, meaning that the actual number of elec- Oort cloud, to 150 K, relevant to the Jovian satellites).
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