Magnetospheric considerations for solar system ice state

Abstract

a b s t r a c tThe current lattice configuration of the water ice on the surfaces of the inner satellites of Jupiter and Saturn is likely shaped by many factors. But laboratory experiments have found that energetic proton irradiation can cause a transition in the structure of pure water ice from crystalline to amorphous. It is not known to what extent this process is competitive with other processes in solar system contexts. For example, surface regions that are rich in water ice may be too warm for this effect to be important, even if the energetic proton bombardment rate is very high. In this paper, we make predictions, based on particle flux levels and other considerations, about where in the magnetospheres of Jupiter and Saturn the ∼MeV proton irradiation mechanism should be most relevant. Our results support the conclusions of Hansen and McCord (2004), who related relative level of radiation on the three outer Galilean satellites to the amorphous ice content within the top 1 mm of surface. We argue here that if magnetospheric effects are considered more carefully, the correlation is even more compelling. Crystalline ice is by far the dominant ice state detected on the inner Saturnian satellites and, as we show here, the flux of bombarding energetic protons onto these bodies is much smaller than at the inner Jovian satellites. Therefore, the ice on the Saturnian satellites also corroborates the correlation.

Variability in the Energetic Electron Bombardment of Ganymede

“…However, energetic electrons also amorphize low-temperature ices (Baragiola, 2003;Dubochet & Lepault, 1984;Loeffler et al, 2020), consistent with the observations of the amorphous (i.e., noncrystalline) state of Ganymede's polar surface (e.g., Hansen & McCord, 2004;Paranicas et al, 2018). Considering the dominance in both number and energy flux of energetic electrons into this region compared to ions (see Table 1), the high-latitude bombardment by these energetic electrons is likely a crucial process influencing the ice state of Ganymede's polar caps.…”

The Space Environment of Io and Europa

“…For our purposes this means that while many energetic particles can penetrate Io's atmosphere, these particles have very low flux near Io so are relatively unimportant for the surface. On the other hand, Europa's thin atmosphere does little to protect Europa and the chemistry of exposed icy surfaces are likely altered by space weathering—some combination of delivering exogenic O and S plus radiolysis of surface ices (as reviewed by Paranicas et al, 2009, 2018).…”

Photometric Modeling and VIS‐IR Albedo Maps of Tethys From Cassini‐VIMS

“…For example, the intensity of 1 MeV protons near the inner Saturnian satellites (Paranicas et al, 2012) is several orders of magnitude below the one at Ganymede . Moreover, whereas along the orbits of Janus, Mimas, and Enceladus, the proton fluxes were measured at low levels (macrosignatures), at Tethys (like Dione) no flux decrease effect has been observed, likely because of the faster radial transport there and the rate at which protons re-encounter that moon (see Figure 4 in Paranicas et al, 2018). As discussed in Hendrix et al (2018), while the effect of surface's alteration caused by protons is negligible, the cold plasma embedded in Saturn's magnetosphere is the principal mechanism causing the darkening occurring on Tethys' trailing hemisphere.…”

“…These high-energy electrons drift in a retrograde direction relative to corotation (Howett et al, 2012), and they preferentially impact low latitudes of Tethys' leading hemisphere (Paranicas et al, 2012). Moreover, whereas along the orbits of Janus, Mimas, and Enceladus, the proton fluxes were measured at low levels (macrosignatures), at Tethys (like Dione) no flux decrease effect has been observed, likely because of the faster radial transport there and the rate at which protons re-encounter that moon (see Figure 4 in Paranicas et al, 2018). We note, however, that the energetic protons of Saturn's magnetosphere have very low fluxes along the moon orbits (Kollmann et al, 2013).…”