ISSN 00380946, Solar System Research, 2014, Vol. 48, No. 1, pp. 48–61. © Pleiades Publishing, Inc., 2014. Original Russian Text © V.V. Busarev, 2014, published in Astronomicheskii Vestnik, 2014, Vol. 48, No. 1, pp. 50–63. Characteristic Features in the Spectra of Europa, Ganymede, and Callisto V. V. Busarev Sternberg Astronomical Institute, Universitetskii pr. 13, Moscow, 119992 Russia Received June 7, 2012 Abstract—The results of groundbased spectrophotometry of the icy Galilean satellites of Jupiter—Europa, Ganymede, and Callisto—are discussed. The observations were carried out in the 0.39–0.92 μm range with the use of the CCD spectrometer mounted on the 1.25m telescope of the Crimean laboratory of the Stern berg Astronomical Institute in March 2004. It is noted that the calculated reflectance spectra of the satellites mainly agree with the analogous data of the earlier groundbased observations and investigations in the Voyager and Galileo space missions. The present study was aimed at identifying new weak absorption bands (with the relative intensity of ~3–5%) in the reflectance spectra of these bodies with laboratory measurements (Landau et al., 1962; Ramaprasad et al., 1978; Burns, 1993; Busarev et al., 2008). It has been ascertained that the spectra of all of the considered objects contain weak absorption bands of molecular oxygen adsorbed into water ice, which is apparently caused by the radiative implantation of O+ ions into the surface material of the satellites in the magnetosphere of Jupiter. At the same time, spectral features of iron of different valence (Fe2+ and Fe3+) values typical of hydrated silicates were detected on Ganymede and Callisto, while probable indi cations of methane of presumably endogenous origin, adsorbed into water ice, were found on Europa. The reflectance spectra of the icy Galilean satellites were compared to the reflectance spectra of the asteroids 51 Nemausa (Cclass) and 92 Undina (Xclass). DOI: 10.1134/S003809461401002X INTRODUCTION Since the Galilean satellites were formed together with Jupiter beyond the watericecondensation Among 63 satellites of Jupiter that are known to boundary, their common property is the substantial date, the Galilean satellites—Io, Europa, Ganymede, enrichment with water and other volatiles (СО2, H2S, and Callisto—are the largest and best known. The lat NH3) (see, e.g., Safronov, 1969; Anders and Grevesse, ter three of them are called icy because of the composi 1989; Lissauer, 2005; Lunine, 2006; Kuskov et al., tion of their surface material. An interesting characteris 2009). One of the confirmations of this property is the tic of these satellites is the simultaneous decrease of the composition of the surface material of Europa (more geometric albedo ()ρv and the density ρ with the distance than 90% of water ice), Ganymede (not less than 50% of water ice), and Callisto (less than 10% of water ice) from Jupiter: from pv = 0.68 and ρ = 3.01 for Europa to ρ ρ estimated by their reflectance spectra (McCord et al., pv = 0.44, = 1.94 for Ganymede and to pv = 0.19, = 1997b). The analogous modeling showed that the non 1.83 for Callisto (http://nssdc.gsfc.nasa.gov/plane icy materials of Ganymede and Callisto are similar to tary/factsheet/joviansatfact.html). The presence of carbonaceous chondrites from their spectral charac water ice in these bodies was ascertained even in the teristics, though they may contain more organics and early groundbased spectrophotometric and infrared hydrosilicates like serpentine (Calvin and Clark, (IR) observations (Moroz, 1965; Johnson and 1989). (This circumstance is considered in the present McCord, 1970; 1971; Pilcher et al., 1972). Signs of the paper in more detail.) A significant factor is also a gen presence of internal water oceans heated by tidal dis eral process of maturation of the surface material of turbances from Jupiter, rather than only the ice crust, celestial bodies lacking an atmosphere under space were found during the Voyager (NASA, from the late conditions. By the cratering degree, the surface of 1970s to the mid1980s) and Galileo (NASA, from Europa is youngest, and its age is estimated at about 1995 to 2003) space missions. Specifically, from the 50 Myr (Zahnle et al., 2003). At the same time, the Galileo images taken with a resolution of 54 m, the surface of Ganymede is older than that of Europa, and signs of the global renewal of the surface were discov the surface of Callisto is older that that of Ganymede ered on Europa: the crater density is extremely low for (Passey and Shoemaker, 1982; Greenberg, 2010). It is the body lacking an atmosphere, and “iceberg”type worth noting that, since the meteoroid flux is gravita formations that have recently moved over the surface tionally focused by Jupiter, its density and intensity in (in the geological time scale) are present. the zone of the Jovian satellites can be the same or 48 CHARACTERISTIC FEATURES IN THE SPECTRA OF EUROPA 49 Europa (March 22–26, 2004) 1.6 3 1.4 2 1.2 1.0 1 0.8 0.6 0.4 0.2 Normalized reflectance spectrum 0 0.35 0.45 0.55 0.65 0.75 0.85 0.95 Wavelength, μm Fig. 1. The reflectance spectra of the Jovian satellite Europa (1–3) obtained on March 22–26, 2004. They are smoothed, nor malized (to the value at λ = 0.5503 μm), and vertically shifted for comparison. The recording time and the errors of the reflectance spectra are listed in Table 1. Spectra 1, 2, and 3 correspond to the leading, trailing, and leading sides of the satellite, respectively. even larger than those in the Main asteroid belt favor the transformation of ice to a crystalline state on (Busarev et al., 2007b). the surface of Callisto. Whereas on the surface of Ganymede, both ice forms are quite abundant (Dal Thorough spectral observations showed that water ton et al., 2010). ice is found in crystalline and amorphous forms on the icy Galilean satellites (e.g., Dalton et al., 2010). Crys It follows from the IR characteristics that the non talline ice differs from amorphous one by the presence icy materials of other types are present on the surface of a narrow absorption band at 1.65 µm and the of Europa: sulfates of sodium and magnesium (like µ Fresnel maximum at 3.1 m in its spectrum; the latter MgSO4 and Na2SO4) and their hydrates. Such com is considered to be an indication of the crystalline pounds could appear in an aqueous medium, as well as structure of the material in the upper layer several during the implantation of S+, Na+, and K+ ions trans microns thick (Fink and Larson, 1975; Dalton et al., ported by the magnetosphere of Jupiter to Europa 2010). As it turned out, two competitive processes take from the vicinity of the neighboring Io (McCord et al., place on the surfaces of the icy Galilean satellites; they 1997a; 1997b; 1998; 2010; Carr et al., 1998; Carlson are the crystallization of amorphous ice by heating and et al., 1999; 2005; Orlando et al., 2005; Greenberg, degradation or amorphization of crystalline ice under 2010). Moreover, on these bodies, unusual molecules the influence of intense fluxes of solarwind particles + + + appearing under the constant exposure of ices (mostly and other particles (mainly O , S , and H ) from the Н O and CO ) to the solar ultraviolet radiation and magnetosphere and radiation belts of Jupiter extend 2 2 ing to 50–100 Jovian radii. It is worth stressing that the highenergy fluxes of electrons and ions (see, e.g., particleradiation flux substantially changes with the McCord et al., 1998; Delitsky and Lane, 1998) were distance from Jupiter: it decreases by approximately detected. On these bodies, SO2 (Lane et al., 1981), 300 times between Europa and Callisto (Dalton et al., CO2, H2S, H2O2 (Smythe et al., 1998; Carlson et al., ⋅ 2010). At the same time, though the change of the 1999), and hydrates of sulfuric acid (H2SO4 8H2O, temperature maximum from 132 K on Europa (Spen H2SO4 ⋅ 6.5H2O, and H2SO4 ⋅ 4H2O) (Carlson et al., cer et al., 1999) to 158 K on Callisto (Moor et al., 2005; McCord et al., 2010) were found. The experi 2004) is small, this leads to a difference in the rate of mental studies showed that the influence of fluxes of thermal crystallization of ice up to five orders of mag protons, electrons, and other charged particles on SO2 nitude between the satellites (Dalton et al., 2010). and H2S frozen into water ice under the surface tem Since the surface temperature of Europa is lower (due perature of Europa (86–130 K) leads to radiolysis and to the higher albedo), no crystallization of amorphous 2− − − ice produced under the influence of intense radiation appearance of anions (,SO4 HSO3 , and HSO4 ) and + + flux occurs there. Because of this, the ice surface layer cations (H3O and H5O ) that have specific absorp (at least to a depth of about 1 mm) can be amorphous tion bands in the IR range (Moore et al., 2007). On the on Europa (Dalton et al., 2010). And, on the contrary, surfaces of icy Galilean satellites, the product of radi the more moderate radiation and higher temperatures olysis is also molecular oxygen O2 (sometimes O3) that SOLAR SYSTEM RESEARCH Vol. 48 No. 1 2014 50 BUSAREV Ganymede (March 22–26, 2004) 1.6 1.4 3 1.2 2 1.0 0.8 1 0.6 0.4 0.2 Normalized reflectance spectrum 0 0.35 0.45 0.55 0.65 0.75 0.85 0.95 Wavelength, μm Fig. 2.The reflectance spectra of the Jovian satellite Ganymede (1–3) obtained on March 22–26, 2004.