P11D-0803
The synthesis and direct detection of O-atoms in water ice
The detection of molecular oxygen on the surfaces, and O-atoms in the tenuous atmospheres of icy outer solar system bodies has resulted in a number of laboratory studies that have investigated how molecular oxygen is formed in radiolyzed ices. Due to the weak band strengths of oxygen's vibronic and electronic transitions, detection of O2 is often performed by a mass spectrometer as the O2 is sputtered, desorbed or sublimed out of the ice. We present a new in situ method for inferring O2 formation via a method in which isotopologues of ozone are detected in experiments where water ice is doped with small quantities of 18O2. The detection of ozone isotopologues containing 16O shows that 16O atoms are produced directly from the radiolysis of water. Based upon our laboratory data, we provide a lower estimate for O2 formation in radiolyzed ices that is in agreement with the estimated O2 abundance on Ganymede.
P11D-0804
Laboratory Studies of Ammonia-Water Ice--Relevance to Outer Solar system Surfaces
Water- and ammonia-ices have been observed or postulated as important components of the icy surfaces of planetary satellites and KBO objects in the outer Solar System. A feature in the near-infrared, near 2.2 microns, is attributed to NH3 in the spectra of Charon, Quaoar, and Miranda, a possible source being cryomagma brought up to cold surfaces where it is quenched. Models used to fit the data include the presence of ammonia hydrates. These observations, along with some evidence for NH3 ice on Enceladus, lead to the prediction that NH3 is one of the more abundant species incorporated into outer Solar System bodies. Significant gaps exist in our knowledge of the spectra of solid water and ammonia mixtures, the formation of NH3-hydrates, and the stability of these ices for relevant physical conditions. The role of thermal processing and exposure to different particle radiation environments has not been investigated. Therefore our laboratory study has examined the influences of composition, formation temperature, thermal- and radiation-processing, on the spectra of both pure NH3 ice and various H2O- NH3 icy mixtures. We have completed low-temperature spectroscopic studies (1 to 20 microns) of H2O -rich ices containing NH3, with an emphasis on features in the near-IR region, which is accessible to ground-based observers. Conditions for the formation and thermal stability of the ammonia hemihydrate and the ammonia monohydrate have been examined. The former undergoes a slow loss of NH3 in a vacuum environment, to give the monohydrate. Additional warming removes the remaining NH3 to give the spectrum of H2O -ice. IR band positions of NH3 in different H2O -ices have been tabulated, and compared to the positions for NH3 hydrates over a large temperature range. We report spectral shifts that depend on both concentration and temperature. The radiation-induced amorphization of hydrates was observed and the radiolytic destruction of NH3 in H2O -ices was measured. We also have tabulated the near-IR spectral position of the ammonium ion which is observed to form in irradiated water and ammonia ices. Implications of these results for the formation, stability, and detection NH3 on Charon and other icy satellite surfaces will be discussed. http://www- 691.gsfc.nasa.gov/cosmic.ice.lab/
P11D-0805
Carbon dioxide segregation in mixed carbon dioxide/water ices
The mid-infrared spectra of mixed vapor deposited ices of CO2 and H2O were studied as a function of both deposition temperature and warming from 15 K to 100 K. The spectra of ices deposited at 15 K show marked changes on warming beginning at 60 K. These changes are consistent with CO2 segregating within the ice matrix into pure CO2 domains. Ices deposited at 60 and 70 K show a greater degree of segregation, as high as 90% for 1:4 CO2:H2O ice mixtures deposited at 70 K. As the ice is warmed above 80 K, preferential sublimation of the segregated CO2 is observed. The kinetics of the segregation process is also examined. The segregation of the CO2 as the ice is warmed corresponds to temperatures at which the structure of the water ice matrix changes from the high density amorphous phase to the low density amorphous phase. We show how these micro-structural changes in the ice have a profound effect on the photochemistry induced by ultraviolet irradiation. These experimental results provide a framework in which observations of CO2 on the icy bodies of the outer solar system can be considered. This research was carried out at the Jet Propulsion Laboratory under a contract with the National Aeronoutics and Space Administration and funded through the Research and Technology Development Program.
P11D-0806
Substrates and the growth of crystals on for icy planetary surfaces
Experiment has shown that particle size is a strong function of growth temperature, and a weaker function of substrate, for deposition of amorphous ice in laboratory conditions. The size of the crystals remains stable over the time scales accessible in the laboratory (short - weeks). Additionally, the temperature regimes for outer planet satellites sufficiently low that ice should deposit in the amorphous form. This is contrary to observation spectral evidence that shows the outer planet satellites to be surfaced with hexagonal ice (which has a higher formation temperature than found on the surfaces of satellites) with reasonably large crystal sizes. This discrepancy between observed crystal form and deposition temperature has been explained variously with evolution of the crystal form from amorphous to hexagonal over geologic time; effects of radiation or impact; and low mobility of water vapor at temperatures extant on the outer planet satellites (< 140 K). Substrate effects may provide an additional mechanism for forcing crystalline deposition of ice at low temperatures. A crystalline ice substrate, once formed, may make deposition of ice in amorphous form energetically unfavorable. This mechanism, if operative, could imply a formation history for the substrate - though the timescales for which this record would be valid will be very difficult to establish experimentally. This work was supported by JPL under contract to NASA
P11D-0807
The perturbation of near IR optical features of water ice induced by frozen acids, brines and organic polymers.
We report the temperature and pH dependencies of the near IR optical signatures of low-temperature ice containing acids, brines or tholins. In the case of flash frozen acids and brines, the 1.5 and 2 micron features shift in wavelength and broaden primarily due to the presence of solvated protons. The role of protons can be seen most dramatically when examining the dissolution of sulfuric acid in ice. Specifically, at temperatures below 135 K, the second pKa of sulfuric acid increases as the temperature decreases due to the increased coupling of the SO42- with the surrounding water molecules. These shifts and broadening are also inherent in the dissolution of MgSO4 or Na2SO4, two candidate materials possibly present in the non-ice regions of Europa. We also compare the optical signatures of organic polymers (tholins) created by discharge devices vs. those created photochemically. The relevance of this work to Titan's surface composition is discussed.