M. J. Loeffler

ENERGETIC PROCESSING OF INTERSTELLAR SILICATE GRAINS BY COSMIC RAYS

While a significant fraction of silicate dust in stellar winds has a crystalline structure, in the interstellar medium nearly all of it is amorphous. One possible explanation for this observation is the amorphization of crystalline silicates by relatively ‘‘low’’ energy, heavy-ion cosmic rays. Here we present the results of multiple laboratory experiments showing that single-crystal synthetic forsterite (Mg2SiO4) amorphizes when irradiated by 10 MeV Xe ions at large enoughfluences.Usingmodeling,weextrapolatetheseresultstoshowthat0.1Y5.0GeVheavy-ioncosmicrayscan rapidly (� 70 Myr) amorphize crystalline silicate grains ejected by stars into the interstellar medium. Subject headingg cosmic rays — dust, extinction Online material: color figures

Cosmic Ray Compaction of Porous Interstellar Ices

We studied the compaction of microporous vapor-deposited ice films under irradiation with different ions in the 80Y400keVenergyrange.Wefoundthatporositydecreasesexponentiallywithirradiationfluence,withameancompactionareaperionthatscaleslinearlywiththestoppingpowerof theprojectileSaboveathresholdSt ¼ 4eV8 � 1 .The experiments roughly follow a universal dependence of ion-induced compaction with restricted dose (eV molecule � 1 ). This behavior can be used to extrapolate our results to conditions of the interstellar medium. Relating our results to ionization rates of interstellar H2, we estimate that porous ice mantles on grains in dense molecular clouds are compacted by cosmic rays in � 10Y50 million years. Subject headingg cosmic rays — ISM: molecules — methods: laboratory — radiation mechanisms: nonthermal Online material: color figures

Compaction of microporous amorphous solid water by ion irradiation

We have studied the compaction of vapor-deposited amorphous solid water by energetic ions at 40 K. The porosity was characterized by ultraviolet-visible spectroscopy, infrared spectroscopy, and methane adsorption/desorption. These three techniques provide different and complementary views of the structural changes in ice resulting from irradiation. We find that the decrease in internal surface area of the pores, signaled by infrared absorption by dangling bonds, precedes the decrease in the pore volume during irradiation. Our results imply that impacts from cosmic rays can cause compaction in the icy mantles of the interstellar grains, which can explain the absence of dangling bond features in the infrared spectrum of molecular clouds.

Ozone Synthesis on the Icy Satellites

Condensed O2 and ozone on the surfaces of some icy satellites are thought to originate from the radiolytic decomposition of surface water ice by the impact of energetic magnetospheric ions, but decades of laboratory studies have produced no evidence for ozone from the radiolysis of pure water ice. Here we report for the first time the production of ozone in ice by 100 keV ions. Using a method that departs drastically from those used in all previous experiments, we have simulated more closely conditions on the icy satellites by performing ion irradiation while depositing water concurrently, which takes into account the effects of gravity and surface porosity. This codeposition causes the burial of a high concentration of radiolytic O2 from which ozone is formed. Our results demonstrate that the enhanced trapping of oxygen in surface ices depends on temperature and should vary locally, depending on the rates of irradiation and recondensation. The burial of radiolytic products by redeposition will likely occur in many varied astronomical environments, besides icy satellites.

Enceladus: A Source of Nitrogen and an Explanation for the Water Vapor Plume Observed by Cassini

Recently, the Cassini spacecraft observed an unexpected emission of plumes of water vapor, nitrogen, and icy particles from the southern polar region of Saturns icy moon Enceladus. While these findings support previous ideas of geological activity in this icy moon, there is no experimental evidence explaining how these plumes could be produced at the low (~130-160 K) surface temperatures. Here we show that similar behavior appears when heating water-ammonia ices that have been irradiated with protons that simulate Saturns energetic ion environment. In our experiments, the behavior results from the eruption of high-pressure bubbles of hydrogen and nitrogen molecules that accumulate in the ice due to the radiolytic decomposition of ammonia. The thermal release of nitrogen can explain the intriguing finding of N+ in the inner magnetosphere. Thus, our laboratory simulations indicate that radiation processing of the surface of Enceladus may explain much of the extraordinary phenomena that have been observed by Cassini.

Decomposition of solid amorphous hydrogen peroxide by ion irradiation

We present laboratory studies of the radiolysis of pure (97%) solid H2O2 films by 50 keV H+ at 17 K. Using UV-visible and infrared reflectance spectroscopies, a quartz-crystal microbalance, and a mass spectrometer, we measured the absolute concentrations of the H2O, O2, H2O2, and O3 products as a function of irradiation fluence. Ozone was identified by both UV and infrared spectroscopies and O2 from its forbidden transition in the infrared at 1550 cm(-1). From the measurements we derive radiation yields, which we find to be particularly high for the decomposition of hydrogen peroxide; this can be explained by the occurrence of a chemical chain reaction.

A Model Study of the Thermal Evolution of Astrophysical Ices

We address the question of the evolution of ices that have been exposed to radiation from stellar sources and cosmic rays. We studied in the laboratory the thermal evolution of a model ice sample: a mixture of water, hydrogen peroxide, dioxygen, and ozone produced by irradiating solid H2O2 with 50 keV H+ at 17 K. The changes in composition and release of volatiles during warming to 200 K were monitored by infrared spectroscopy, mass spectrometry, and microbalance techniques. We find evidence for voids in the water component from the infrared bands due to dangling H bonds. The absorption from these bands increases during heating and can be observed at temperatures as high as ~155 K. More O2 is stored in the radiolyzed film than can be retained by codeposition of O2 and H2O. This O2 remains trapped until ~155 K, where it desorbs in an outburst as water ice crystallizes. Warming of the ice also drastically decreases the intrinsic absorbance of O2 by annealing defects in the ice. We also observe loss of O3 in two stages during heating, which correlates with desorption and possibly chemical reactions with radicals stored in the ice, triggered by the temperature increase.

Radiation Effects in Water Ice in the Outer Solar System

Water ice in the outer solar system can either have condensed from the gas phase or have been brought in by colliding bodies, such as interplanetary ice grains to comets. Since icy bodies lack a protective atmosphere, their surface is subject to irradiation by photons, ions and electrons. This chapter discusses how energetic radiation affects the physical and chemical properties of a pure water ice surface, and how the outcome of radiation processes depends on the properties of the surface and on the environment (atmosphere, particle flux and energies).

Radiation chemistry in ammonia-water ices

We studied the effects of 100 keV proton irradiation on films of ammonia-water mixtures between 20 and 120 K. Irradiation destroys ammonia, leading to the formation and trapping of H(2), N(2), NO, and N(2)O, the formation of cavities containing radiolytic gases, and ejection of molecules by sputtering. Using infrared spectroscopy, we show that at all temperatures the destruction of ammonia is substantial, but at higher temperatures (120 K), it is nearly complete (approximately 97% destroyed) after a fluence of 10(16) ions/cm(2). Using mass spectroscopy and microbalance gravimetry, we measure the sputtering yield of our sample and the main components of the sputtered flux. We find that the sputtering yield depends on fluence. At low temperatures, the yield is very low initially and increases quadratically with fluence, while at 120 K the yield is constant and higher initially. The increase in the sputtering yield with fluence is explained by the formation and trapping of the ammonia decay products, N(2) and H(2), which are seen to be ejected from the ice at all temperatures.

IS THE 3.5 μm INFRARED FEATURE ON ENCELADUS DUE TO HYDROGEN PEROXIDE

We use new and previously published measurements from our laboratory to examine the assignment of a 3.5 μm feature in the infrared spectrum of Enceladus. The spectral feature, taken with Cassinis VIMS Spectrometer, has recently been interpreted as an absorption band from hydrogen peroxide on the surface. Such identification is important because it would imply an intense flux of magnetospheric particles, which are thought to be required to produce H2O2 from surface water ice. We compare the position and width of this feature with measurements of infrared spectra of water-hydrogen peroxide mixtures. We conclude that the reported feature from Enceladus does not correspond to hydrogen peroxide.