David A. Bahr

Sputtering of water ice

We present results of a range of experiments of sputtering of water ice together with a guide to the literature. We studied how sputtering depends on the projectile energy and fluence, ice growth temperature, irradiation temperature and external electric fields. We observed luminescence from the decay of H(2p) atoms sputtered by heavy ion impact, but not bulk ice luminescence. Radiolyzed ice does not sputter under 3.7 eV laser irradiation.

Radiolysis of water ice in the outer solar system: Sputtering and trapping of radiation products

We performed quantitative laboratory radiolysis experiments on cubic water ice between 40 and 120 K, with 200 keV protons. We measured sputtering of atoms and molecules and the trapping of radiolytic molecular species. The experiments were done at fluences corresponding to exposure of the surface of the Jovian icy satellites to their radiation environment up to thousands of years. During irradiation, O 2 molecules are ejected from the ice at a rate that grows roughly exponentially with temperature; this behavior is the main reason for the temperature dependence of the total sputtering yield. O 2 trapped in the ice is thermally released from the ice upon warming; the desorbed flux starts at the irradiation temperature and increases strongly above 120 K. Several peaks in the desorption spectrum, which depend on irradiation temperature, point to a complex distribution of trapping sites in the ice matrix. The yield of O 2 produced by the 200 keV protons and trapped in the ice is more than 2 orders of magnitude smaller than used in recent models of Ganymede. We also found small amounts of trapped H 2 O 2 that desorb readily above 160 K.

Laboratory studies of the optical properties and stability of oxygen on Ganymede

We present measurements of the optical properties of solid O2 and O2:H2O films and on the stability of oxygen in water ice at different temperatures. The experiments are meant to test hypotheses for the location of condensed molecular oxygen in Ganymede. We find that oxygen is not retained in water ice at Ganymedes recorded surface temperatures and analyze current questions associated with the existence of oxygen in this Jovian moon. The results suggest that solid oxygen and possibly other condensed gases exist on cold patches on the heterogeneous Ganymedes surface.

Solid-state ozone synthesis by energetic ions

Abstract We have synthesized ozone by irradiating thin solid films of oxygen and oxygen–water mixtures with 100 keV protons, motivated by recent reports of condensed O3 on icy satellites in the outer Solar system. We measured the depth of the Hartley absorption band in the ultraviolet by reflectance spectroscopy and used it to quantify the column density of ozone. We analyzed the results using a three-component (O, O2 and O3) model that successfully explains the fluence dependence of ozone production.

PHOTODESORPTION OF SOLID CO2 BY LYα

We measured desorption of atoms and molecules from films of solid carbon dioxide in an ultrahigh vacuum from 6 to 60 K under irradiation with Lyα (121.6 nm, 10.2 eV) photons, an important process in the balance between gas phase and condensed molecules in the interstellar medium. The measurements use microgravimetry and mass spectrometry during irradiation and temperature programmed desorption after irradiation. At low photon fluences, the desorption flux consists mainly of O atoms and, after ∼10 17 photons cm −2 , it is dominated by CO with smaller amount of O2, C, and CO2, with the presence of O2 indicating solid-state chemical reactions. At high fluences (up to 10 18 photons cm −2 ), the desorption yields saturate at values much higher than in previous studies. The yields (molecules/photon), derived assuming stoichiometric desorption, reach 0.014 at 6 K, growing to ∼0.2 at 50 and 60 K. Warming the films during irradiation gives rise to pressure spikes that suggest desorption of trapped species in pores or at defects, possibly assisted by radical-induced reactions. Such an effect could be significant for radiation-processed CO2-coated interstellar grains that are heated by, i.e., cosmic ray impacts or grain‐grain collisions. We discuss the experiments considering photochemical mechanisms and compare them to the results of ion irradiation.

Ion beam induced chemistry: the case of ozone synthesis and its influence on the sputtering of solid oxygen

Abstract To understand chemical effects caused by ion beams in solids we study the simplest case, the formation of ozone in solid oxygen. We irradiate thin O 2 films with 100 keV protons and study the fluence dependence of the sputtering of solid O 2 with a microbalance and a mass spectrometer. The formation of ozone in the film after irradiation is identified by optical reflectance spectroscopy and thermal desorption. We discuss the processes involved and the implications of the fluence dependence of the ozone concentration upon the sputtering yield of oxygen.

THE EFFECTS OF CRACKING ON THE SURFACE POTENTIAL OF ICY GRAINS IN SATURN’S E-RING: LABORATORY STUDIES

Within Saturns E-ring, dust grains are coated by water vapor co-released with ice grains from the geyser-like eruptions of Enceladus. These ice-coated grains have intrinsic surface potential and interact synergistically with the ions and electrons of Saturns magnetospheric plasmas. We perform laboratory experiments to investigate the effects of water-ice growth on the surface potential, using amorphous solid water (ASW) films. We estimate the growth of the surface potential to be ~−2.5 mV (Earth) yr−1 and −112 mV yr−1 for E-ring grains at ~4.5R s and 3.95R s outside Enceladuss plume, respectively. In addition, our measurements show that the linear relationship between the surface potential and the film thickness, as described in previous studies, has an upper limit, where the film spontaneously cracks above a porosity-dependent critical thickness. Heating of the cracked films with (and without) deposited charge shows that significant positive (and negative) surface potentials are retained at temperatures above 110 K, contrary to the minimal values (roughly zero) for thin, transparent ASW films. The significant surface potentials observed on micron-scale cracked ice films after thermal cycling, −(5–20) V, are consistent with Cassini measurements, which indicate a negative charge of up to −5 V for E-ring dust particles at ~5R s. Therefore, the native grain surface potential resulting from water-vapor coating must be included in modeling studies of interactions between E-ring icy surfaces and Saturns magnetospheric plasma.