Comment on “Laboratory studies of the optical properties and stability of oxygen on Ganymede” by Raul A. Baragiola and David A. Bahr

Abstract

IntroductionIt has been over 15 years since Brown et at. [1982] showed that the bombardment of the icy satellites by magnetospheric plasma particles would produce significant quantities of 02, including an 02 atmosphere . It is now accepted that radiolysis and, possibly, photolysis of ice [Johnson and Quickenden, 1997] (see appendix below) produce the 02 observed on the icy satellites, but modeling the fate of this 02 is in its infancy. Baragiela and Bahr [1998] (hereafter BB) recently presented experimental data relevant to the two 02 bands observed in the visible by Spencer et al. [1995] and Calvin and Spencer [1998]. These bands are indicative of some form of condensed 02 [Calvin et al., 1996]. Because the 02 band is seen primarily at low latitudes on Ganymede, the stability of "solid" 02 was thought to be a problem. Hence the suggestion of Calvin et al. [1996] and Johnson and Jesser [1997] (see appendix below) that the 02 exists as radiation-induced inclusions (bubbles) in the icy surface. In BB and in an earlier paper [Vidal et al., 1997], they concluded that the 02 is not trapped in inclusions in the ice. Rather, it was suggested that solid 02 exists on Ganymede either in surface regions with very low temperatures or as 02 ice crystals in Ganymede's atmosphere. Before commenting on this, clarification is required.That the observed bands require "solid-state" 02 densities is not in question; this became clear in the comparisons by Calvin et al. [1996] of earlier laboratory data with the observed spectra. Therefore the proposed 02 inclusions were presumed to have such densities. (The word "inclusion" is used here to indicate the trapping of a number of 02 molecules in a volume, rather than a single 02 in a single defect.) With this in mind, two principal issues need to be addressed in order to understand the observed 02 bands. Since nonoxygen neighbors and lattice structure can affect the spectrum of solid 02, what amount and which lattice structure are required to match the band profile? In addition, noting that the ultimate source is likely to be radiolysis, how is the observed 02 formed? Is it formed by condensation after production or in situ in voids in the surface material?Based on the telescopic observations, it might appear that solid 02 is unique to Ganymede's near-equatorial region. Spencer [1998] . This implies a slower diffusion rate than that for voids or defects. Since stable areas of O: at low latitudes exposed to space are unlikely, this issue needs to be studied for nonporous, crystalline overlayers.It is, of course, possible that the O: observed on Ganymede might not be trapped in ice. It could be in another substance, which is also transparent in the visible and forms trapped-gas inclusions, such as certain salts [Karget, 1991] The conclusion in BB that the observed 02 bands are indicative of the presence of condensed 0 2 is based on matching the positions and shapes of the absorption bands of condensed 0 2 at 22 K with the absorption bands observed by Spencer et al. [1995]. Such a...

Energetic Ion and Electron Irradiation of the Icy Galilean Satellites

“…More recent experiments (Vidal et al 1997, Baragiola andBahr 1998) suggest varied but lower O 2 yields. Baragiola et al (1999) report only small upper limits for direct production of trapped O 2 within ice by 60 keV O + 2 (G O 2 < 5 × 10 −4 ) and 100 keV H + (G O 2 < 4 × 10 −5 ), but higher yields are possible (Johnson 1999). For instance, G O 2 ∼ 0.01 is obtained from other cross section data for O 2 sputtering from low energy electrons impacting thin ice films (Brown et al 1982, Sieger et al 1998.…”

“…If so, the penetrating particle fluxes could have been much higher for thousands to millions of years, and a significant O 2 reservoir could then have accumulated in or under the surface. Preferential vapor deposition at mid-latitudes (Purves andPilcher 1980, Shaya andPilcher 1984), followed by thermal annealing (Johnson 1985(Johnson , 1999, may have been sufficient to keep optical pathlengths long in the relatively dark material at low latitudes in long intervals between dipole reversals.…”

Energy, Chemical Disequilibrium, and Geological Constraints on Europa

“…In addition, protons and heavier ions create cascades of secondary electrons. The most energetic electrons will penetrate a few to a few tens of centimeters into the ice (Johnson, 1990;Berger et al, 2005). The most energetic ions (H ϩ ) will only penetrate a few millimeters below the surface (Johnson, 1990 (solar, jovian, particles) and internal (radiogenic, tidal) inputs.…”

“…The most energetic electrons will penetrate a few to a few tens of centimeters into the ice (Johnson, 1990;Berger et al, 2005). The most energetic ions (H ϩ ) will only penetrate a few millimeters below the surface (Johnson, 1990 (solar, jovian, particles) and internal (radiogenic, tidal) inputs. Europa's energetic particle flux is ϳ0.037% that of Earth's average incident solar flux (Cooper et al, 2001).…”

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

“…The hypothesis that 02 forms radiolytically inside the ice and is trapped in bubbles produced by radiation damage is also problematic because the 02 bands are seen mainly in Ganymede's equatorial region, where the energetic particle flux is mostly excluded by its magnetic field. Arguments on the controversy of this topic have been published recently [Johnson, 1999;Baragiola et al, 1999a].…”