Astrochemistry and Cosmochemistry
Objects in space, not protected by atmospheres, are subject to irradiation by stellar ultraviolet photons and energetic electrons and ions. We are interested in understanding how the surfaces react to this irradiation. Recent research has included laboratory irradiation of low temperature (4 - 160 K) ices of water, oxygen, carbon dioxide, and hydrogen peroxide with accelerated ions and hard-UV photons (Lyman-alpha) and ion irradiation of minerals. The laboratory participates in NASA's Cassini mission to Saturn. A summary of observations follow.
- Photodesorption of interstellar and circumsolar ice mantles
We are studying the photodesorption of from ices of water and other gases induced by Lyman-alpha photons which simulate radiation from stars. We found that the photodesorption yields depend on irradiation dose and ice temperature. The existence of an incubation period before significant photodesorption occurs suggests that radicals produced by photolysis are important in desorption. The results where used to explain the puzzling absence of ice in diffuse interstellar clouds and to determine that photodesorption exceeds sublimation and erosion by energetic ions (sputtering) in the outer solar system. See our results in Nature 373, 205 (1995). Our new results for carbon dioxide and oxygen ices will be published soon.
By applying an array of experimental techniques we characterized the porosity of vapor-deposited amorphous solid water. We observed microporosity and mesoporosity depending on the incidence angle of the water flux. The behavior of dangling OH bonds in the internal surface of micropores with methane uptake revealed that multilayer condensation does not occur inside the micropores. Rather, filling of the core volume results from coating the pore walls with the first layer of methane, indicating pore widths below a few molecular diameters. We discovered that microporosity is destroyed by ion irradiation, which implies that impacts from cosmic rays can cause compaction in the icy mantles of the interstellar grains, explaining the absence of dangling bond features in the infrared spectrum of molecular clouds. The implications for astrochemistry and cometary science are the subject of work in progress. Relevant publications: Compaction of microporous amorphous solid water by ion irradiation, J. Chem. Phys. 126, (2007) and Astrophys. J. (in press). In related work we determined conditions for formation of nanopores and mesopores, and obtained an upper bound of 1nm for the width of the nanopores (see J. Chem. Phys. 127 (2007) 204713).
- Gas absorption by planetary ices
We do experiments to determine the changes in the optical reflectance (albedo) of ices due to particle bombardment. The results show that the peculiar appearance of some satellites of Jupiter can result from bombardment by energetic magnetospheric ions. Recent experiments study properties of ice with embedded oxygen, to interpret recent observations by the Hubble Space Telescope which suggest the presence of oxygen and ozone in Ganymede. See our results in Science 276 (1997) 1839, J. Geophys. Res. E 103 (1998) 25865, and 104 (1999) 14,183. We recently studied in detail the synthesis and sputtering of O2 in ice by irradiation (Phys. Rev. B 72 (2005) 245422) and discovered a synergistic effect of ion irradiation and water co-deposition that should operate at icy satellites, favoring the synthesis of ozone (see Astrophys. J. Letters 644 (2006) L141).
- Irradiation effects on icy satellites
We explained the formation of hydrogen peroxide in the satellite Europa (Icarus 180, 265 (2006) ) by ion irradiation of water ice and determined the chemical environment of the molecule by infrared spectroscopy (Geophys. Res. Lett. 32, L172023 (2005) ). Related to this topic we studied quantitatively the ion-induced decomposition of H2O2 into different products (J. Chem. Phys. 124, 104702 (2006)) and the distillation kinetics of highly concentrated solid H2O2 and its dihydrate in ultrahigh vacuum ( J. Phys. Chem 110 (2006) 6911)
To explain the spectacular plume of water vapor, ice particles and nitrogen gas of Enceladus we conducted experiments of irradiation of water-ammonia mixtures. During annealing, the irradiated samples eject particles, water, nitrogen and hydrogen in bursts and in slower outgassing. Microscopic observations lead to the proposal of micro-volcanoes consisting in bursting blisters containing radiolytic gas bubbles (see description in:: Enceladus: An Explanation for the Icy Outbursts and the Origin of Nitrogen, Astrophysics Journal Letters 649, L133 (2006)
Influence of radicals in the evolution of comets
We addressed the question of the evolution of ices that have been exposed to radiation from stellar sources and cosmic rays by studying the thermal evolution of a model ice sample: a mixture of water, hydrogen peroxide, dioxygen, and ozone produced by irradiating solid hydrogen peroxide with 50 keV protons at 17 K. Defects in the ice allow the normally forbidden infrared absorption in H 2 and O 2 , which disappears upon annealing. The temperature dependence of the formation of ozone indicates chemical reactions with radicals stored in the ice, triggered by the temperature increase. Relevant publications: Decomposition of Solid Amorphous Hydrogen Peroxide by Ion Irradiation, J. Chem. Phys. 124, (2005) 104702, A Model Study of Thermal Evolution of Astrophysical Ices, Astrophys. J. Lett. 639 (2006) L103,
- Ozone synthesis in the solid state
To understand the observation of ozone in some satellites of Jupiter and Saturn we conduct laboratory experiments where we irradiate different molecular gases with fast protons and Lyman-alpha (10.2 eV) photons. Ozone formation is determined by observing the Hartley band in the ultraviolet reflectance spectrum and by mass spectrometry. Our results, to be published, provide information that is also useful to constrain models of ozone formation in the upper atmosphere of the Earth. See our results in Nucl. Instr. Meth. Phys. Res. B157 (1999)233, Nucl. Instr. Meth. Phys. Res. 193(2002)775, Astrophys. J. Letters 644, L141-L144 (2006), and J. Chem. Phys. 127 (2007) 204713.
Carbon Dioxide Synthesis in Interstellar Grains
To understand the formation of carbon dioxide on cold grains in interstellar clouds, we exposed solid CO to a beam of cold oxygen atoms, producing about a third of a monolayer of CO2 at 20 K. Heating a film with a water capped showed that carbon dioxide production increased by 30%, indicating additional reactions between CO and O diffusing in the ice layer. See our results in Astrophys. J. Letters 737, L14 (2011).