ISSN 0038-0946, Solar System Research, 2018, Vol. 52, No. 4, pp. 301–311. © Pleiades Publishing, Inc., 2018. Original Russian Text © V.V. Busarev, A.M. Tatarnikov, M.A. Burlak, 2018, published in Astronomicheskii Vestnik, 2018, Vol. 52, No. 4, pp. 305–315.
Comparison and Interpretation of Spectral Characteristics of the Leading and Trailing Hemispheres of Europa and Callisto V. V. Busareva, *, A. M. Tatarnikova, and M. A. Burlaka a
Sternberg Astronomical Institute, Lomonosov Moscow State University, (SAI MSU), Moscow, 119992 Russia *e-mail:
[email protected] Received July 12, 2017
Abstract—Europa and Callisto are two “extreme members” in a sequence of the Galilean ice satellites formed at different distances from Jupiter. The difference in their mean density probably reflects the material density gradient that appeared even in the subplanetary disk of Jupiter. At the same time, general peculiarities in the composition of the surfaces of Europa and Callisto apparently characterize the accumulated effect of all subsequent evolutionary processes, including current volcanic activity on the satellite Io and its ionized material transfer in Jovian magnetosphere, as well as chemical reactions taking place under low-temperature (within ~90–130 K) and irradiation conditions. In 2016–2017, we observed the leading and trailing hemispheres of Europa and Callisto in the spectral range of 1.0–2.5 μm at 2-m telescope of Caucasian Mountain Observatory (CMO) of Sternberg Astronomical Institute (SAI) of Moscow State University (MSU). We found that, on a global scale, Europa and Callisto exhibit similar spectral characteristics and, particularly, the maxima in the distributions of sulfuric acid hydrate in the trailing hemispheres of the both moons, which agrees with the data of previous measurements. This can be considered as evidence for general ion implantation on these and other moons in the radiation belts of Jupiter. Moreover, our spectral data suggest that water ice and hydrates (clathrates) of other compounds are dominant or abundant in the leading hemispheres of Europa and Callisto. Specifically, we detected a weak absorption band of CH4 clathrate centered at ~1.67 μm in the reflectance spectra of the leading (the band is more intense) and trailing (the band is less intense) hemispheres of Europa. Weak signs of the same absorption band are also in the reflectance spectra of Callisto measured at its different orientations. Keywords: Galilean ice satellites of Jupiter, reflectance spectra, material composition, sulfuric acid hydrate, methane clathrate DOI: 10.1134/S0038094618030036
INTRODUCTION There are some properties in evolution, dynamics, and structure of Galilean satellites of Jupiter that determine the mean composition of their material and, in part, the spectral characteristics observed. As follows from the models, Europa is stratified into layers of different density, while the interior of Callisto is undifferentiated or differentiated partly (see, e.g., Anderson et al., 1997a, 1997b; Kuskov and Kronrod, 2001; McKinnon and Desai, 2003; Kuskov et al., 2009). Such specific structural properties of Europa and Callisto, which are at different distances from Jupiter (Europa is three times as close as Callisto), might appear due to a large difference between the amounts of the thermal energy released in periodical tidal strains (Greenberg, 1981; Moore and Schubert, 2003; Javier, 2012). This effect is strengthened by gravitational resonance interactions of the first three Galilean satellites (as is known, Io, Europa, and Ganymede move around Jupiter under the relative resonance of 1 : 2 : 4, which is absent in Callisto) (see, e.g.,
Greenberg et al., 1997). The decrease in the mean density of the Galilean satellites shows that the difference in the value of this parameter between Europa and Callisto (ρ = 3.01 and 1.83 g/cm3, respectively (https://nssdc.gsfc.nasa.gov/planetary/factsheet/galileanfact_table.html)), probably appeared even in the protosatellite disk. This should have also led to general differences in the material composition. The main extrinsic peculiarities of the considered Galilean satellites are different ages of their surfaces. The whole surface of Europe is rather young: it was formed due to the relatively recent (in geologic scale) activity of the global subsurface ocean and exhibits a negligible number of impact craters and a high-albedo fissured ice shell. The surface of Callisto is, on the contrary, extremely old: it is strongly cratered and exhibits no signs of internal activity. Occasional manifestations of the internal activity on Europa, such as local ejections, were observed even recently (see, e.g., Sparks et al., 2016). According to the models, more than 90% of craters on the surface of the Galilean sat-
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ellites were formed due to the impacts of nuclei of Jupiter-family comets having been present in Jupiter’s vicinity for a long time (see, e.g., Zahnle et al., 1998). From the sizes and statistics of craters, the age of the surfaces of Europa and Callisto is estimated at ~10 Myr and ~4 Gyr, respectively (Zahnle et al., 1998). Nevertheless, there might be also an ocean on Callisto; however, it is under a thick ice crust (see, e.g., Zimmer et al., 2000; Spohn and Schubert, 2003; Javier, 2012). The presence of salt (electroconductive) water oceans on Europa and Callisto is evidenced by their induced magnetic fields discovered with the Galileo spacecraft (SC) (Carr et al., 1998; Khurana et al., 1998; Zimmer et al., 2000). On this basis, we may suppose that the main composition of the surface of Europa should be close to that of its subglacial ocean. At the same time, the surface material of Callisto is likely to be a mixture of its ancient material and different substances having fallen on the surface for its whole life time. However, there are two more factors that may substantially change the surface composition of the considered Galilean moons of Jupiter. They are the powerful magnetosphere of the planet and the high volcanic activity of its closest moon, Io. Changes in the spectral characteristics of the trailing and leading hemisphere of ice satellites due to the influence of the magnetospheric plasma of giant planets and, particularly, Jupiter were theoretically predicted by R.E. Johnson and confirmed with observations (Johnson et al., 1988; Pospieszalska and Johnson, 1989; Sack et al., 1992). Thus, the ice moons of Jupiter—Europa, Ganymede, and Callisto—are exposed to fluxes of electrons, photons, and ions of the magnetospheric plasma. However, with the course of time, radiolysis (radiation decomposition) of water and carbon dioxide ices on the surfaces and subsequent reactions of chemical synthesis under low surface temperatures, within ~90–130 K (see, e.g., Urquhart and Jakosky, 1996; Spencer et al., 1999; Trumbo et al., 2017), should yield more complex CHO-containing molecules, which may include СН3ОН, H2CO, CH2CO, C3O2, НСООН, СН3СООН, H2CO3, HCOOCH, (СН3)2СО, CH3CH2OH, HOCH2CH2OH, and C3O2, while fragmentation of water products should result in producing О, О2, НО2, Н2О2, and О3 (see, e.g., Delitsky and Lane, 1997, 1998; Materese et al., 2015). However, the issue arises of which of the listed compounds can be accumulated and prevail due to the action of permanent or repetitive physical processes. A large amount of sulfur and its compounds is ejected on to the surface of Io and its vicinity. Moreover, Io is moving near the inner boundary of Jupiter’s magnetosphere, where the magnetic field strength is highest. The material escaping from Io is permanently ionized (see, e.g., Bagenal and Sullivan, 1981; Bagenal, 1994, 1997) and forms a rather massive
plasma torus abundant in Sn+, On+, and Na+ ions, which spreads along a whole orbit of the satellite. As it turned out (Hall et al., 1995; Shematovich and Johnson, 2001), the rarefied exosphere and the plasma torus, where ions of atomic and molecular oxygen are predominant, are also formed around Europa due to the radiation-induced damage and decomposition of water ice and other compounds on its surface. This is consistent with the results of our spectral analysis of Europa in the visible range: they suggest that its surface material is rich in molecular oxygen (Busarev, 2014). Thus, ions of H+, H2+, He+ (a source of the latter is the exosphere of Jupiter), О+, О++, S+ and S++, and Na+ and a small amount of heavier SО+ and SO2+ ions (~1–5%) (see, e.g., Bagenal and Sullivan, 1981; Pospieszalska and Johnson, 1989; Bagenal, 1994; 1997; Bolton et al., 1997; Cooper et al., 2001) are present in Jupiter’s magnetosphere and transferred to all of the satellites present within its boundaries and, primarily, to Europa. Though Callisto moves on the periphery of Jupiter’s magnetosphere, the fluxes of ions also reach it and partially change the composition of its surface material. An important peculiarity of the interaction between the high-energy plasma and Europa in the magnetosphere of Jupiter is the accumulation of hydrates of sulfuric compounds (see, e.g., McCord et al., 1998; Carlson et al., 1999), which mostly takes place in its trailing hemisphere (e.g., Pospieszalska and Johnson, 1989; Cooper et al., 2001; Paranicas et al., 2002; Carlson et al., 2005; Dalton et al., 2013). We measured the near-IR reflectance spectra of Europa and Callisto, which can be used to analyze the composition of the surface material of these bodies in more detail. OBSERVATIONS Spectral observations of Jupiter’s satellites in the near-IR range (from 1.0 to 2.5 μm) were carried out with the ASTRONIRCAM IR camera–spectrograph mounted at the 2.5-m telescope of Caucasian Mountain Observatory of SAI MSU (Nadzhip et al., 2017). The spectrograph operated in the crossed-dispersion mode; the slit was 10'' long and 1.8'' wide. In this mode, the spectrum is registered separately in the ranges of the Y–J and H–K bands (1.0–1.5 and 1.5– 2.5 μm, respectively). If the slit is 1.8'' in width, the spectral resolution for these ranges is 15–20 Å. The spectra were calibrated (corrected for atmospheric absorption) with the use of telluric standards. As such standards, we used the stars of early spectral types with a small number of lines in the near-IR range (the stars of types A–B) or G-type stars (with the energy distribution close to solar). The standard stars were, if possible, observed at air masses close to those of the satellites. Moreover, the sky background was taken into account by measuring its spectrum at a distance of 2'–3' SOLAR SYSTEM RESEARCH
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from the standard star. We used the energy distribution in the spectrum of a standard star according to the data by Pickles (1998). In the paper, the energy distribution is given with a step of 5 Å, which is close to the value of the linear dispersion of our spectra (2.3 Å) and approximately three times better than our spectral resolution (~17 Å) for the slit width used in the observations. The calibration accuracy was estimated from the observations of an additional standard of the G2V spectral type. These observations showed that the errors are less than 10% outside the absorption bands of the atmosphere. Moreover, we used the averaged spectrum of the G2V-type star (Pickles, 1998; http://www.eso.org/sci/facilities/paranal/ decommissioned/isaac/tools/lib.html) to calculate the reflectance spectra of Europa and Callisto from their recorded and corrected for the atmospheric absorption spectra according to the standard technique (Busarev, 1999). The observational parameters of the spectra of Europa and Callisto, as well as some current parameters of the satellites at the mean moments of their observations (for the whole spectral range) are listed in Tables 1 and 2. The reflectance spectra themselves are presented in Figs. 1 and 2. INTERPRETATION OF THE RESULTS AND DISCUSSION Attempts to model the reflectance spectra of different formations with sizes down to several tens of meters on the surface of the Galilean ice satellites of Jupiter under laboratory conditions were made more than once. The size corresponds to the resolution of the Near-Infrared Mapping Spectrometer (NIMS) onboard the Galileo SC (see, e.g., Carlson et al., 1996, 1999; McCord et al., 1998). In these studies, it was found that the closest analogues of the surface material of Europa are samples of sulfur-containing hydrated salts frozen to the temperatures of 150–200 K and exposed to radiation. The examples of such salts are hexahydrite (MgSO4 · 6H2O), bloedite (Na2Mg(SO4)2 · 4H2O), mirabilite (Na2SO4 · 10H2O), and some others (corresponding to different water contents, temperatures, particle sizes, and received radiation doses). The main factors for producing and accumulating sulfuric acid and sulfurous salts in the surface material of Europa are exposure to fluxes of protons, rather than only ions of sulfur and its molecular compounds (see, e.g., Carlson et al., 1999; Dalton et al., 2013), and low-temperature reactions on the surface of the satellite itself (e.g., Loeffler and Hudson, 2013). The possibility for accumulating such sulfurous hydrated salts in the ice substance of Europa and other satellites under exposure to ion fluxes in the magnetosphere of Jupiter is provided by their thermodynamical stability (see, e.g., Zolotov and Shock, 2000; Dalton et al., 2005, 2013); however, the condition is that the rate of their synthesis should be higher than that of radiolysis. Judging by the reflectance SOLAR SYSTEM RESEARCH
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spectra of Europa, which we measured in the near-IR range, such conditions are satisfied in the trailing hemisphere of Europa (Fig. 1, spectrum 1), where the surface material is less damaged by radiation and micrometeorites; however, these conditions are not satisfied in the leading hemisphere (Fig. 1, spectrum 2). The latter is exposed to a micrometeorite flux of a higher velocity (its intrinsic velocity is summed with the orbital velocity of the satellites, which is 13.7 and 8.2 km/s for Europa and Callisto, respectively), and the incident plasma wave has also a higher velocity rather than only a higher density. Detection of plasma particles near the equatorial plane of Jupiter’s magnetosphere by the Galileo SC showed that the velocities of charged particles are mainly in the range of 200– 300 km/s (Kane et al., 1999). At the same time, in the satellites’ vicinity, the effects of screening and induced magnetic field may lead to a decrease in these velocities by a factor of 2–3 (see, e.g., Paranicas et al., 2002). In addition, it is necessary to take into account the difference in the temperature conditions on the surfaces of the Galilean satellites. Due to the substantial difference in albedo, Callisto receives more than 2.5 times the solar energy than Europa (in the V band, the geometric albedo of Europa and Callisto is ρ = 0.67 and 0.17, respectively (http://www.sai.msu.ru/neb/nss/parcohr.htm)). The corresponding difference in the temperature on the satellites may be actually reached in separate surface formations depending on their orientation relative to the incident solar light. However, according to the Voyager measurements, the difference in the mean surface temperature of Europa and Callisto turned out to be substantially smaller: 104 K on Callisto versus 94 K on Europa (Urquhart and Jakosky, 1996), which suggests that there is an internal thermal source in Europa. The main inference, which can be made from the analysis of the spaceborne and model experiments, is that none of the sulfates and other salts used corresponds to the material on Europa in spectral characteristics; this substance is most likely a more complex mixture of such salts (see, e.g., Dalton et al., 2005; Shirley et al., 2010). It is important to note that a common spectral feature of the reflectance spectra of all hydrated compounds of sulfur is their short-wavelength asymmetry (a sharper shape of the short-wavelength wing) of the main absorption bands at 1.5 and 2.0 μm (Carlson et al., 1996, 1999; McCord et al., 1998; Dalton et al., 2005, 2013). One of the closest analogues of the material in the trailing hemisphere of Europa is a mixture of water ice and hydrated sulfuric acid (see, e.g., Carlson et al., 1999; Fig. 3). However, as the modeling of the observational data shows (see, e.g., Hansen and McCord, 2004), ice in the leading hemisphere of Europa is in the main amorphous state due to strong radiation-induced damage and more intense micrometeorite reworking. Together with Jupiter’s magnetosphere, which is an exogenous source of sulfur compounds, an endoge-
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3 × 100
3 × 100
2 × 100
2 × 100
3 × 30
3 × 30
1 × 100
1 × 100
3 × 30
3 × 30
13 21 05
13 26 05
1 27 37
11 32 33
11 32 32
–06 52 37
–07 27 08
5 03 25
4 28 32
4 28 36
DEC. (2000), Deg, arcmin, arcsec
53.15
51.95
44.27
39.55
45.53
z, deg
5.5
8.5
4.1
6.4
6.5
PhA, deg
0.984
0.064
0.932
0.296
0
RRPh
272.53
300.46
297.42
67.56
321.11
λobs, deg
–3.40
–3.37
–1.92
–1.93
–1.93
2.0
0.9
1.5
0.6
2.5
1.0
0.9
0.9
1.9
1.4
HIP 54849, A0, 58
HIP 54849, A0, 47
HIP 48 414, A0, 43
HD 114174, G5, 45
HD 114174, G5, 56
Relative error Standard, (MSE) Spst, zst, ϕobs, deg at 1.1 deg and 2.2 μm, %
R.A. (2000) and DEC. (2000) are the equatorial coordinates of the object at the mean time of the spectrum registration in the range of 1.0–2.5 μm; z is the zenith distance of the satellite; PhA is the phase angle; RRPh is the relative rotation phase of the satellite at the mean time of the spectrum registration (RRPh is assumed to be 0 for the moment of the first spectrum registration); λobs and ϕobs are the longitude and latitude of a center of the observed region on the satellite’s surface at the mean time, which were obtained with the HORIZONS Web-Interface interactive system (https://ssd.jpl.nasa.gov/horizons.cgi); MSE is the mean-squared error; Spst and zst are the spectral type and the zenith distance of the standard star.
Mar. 10, 2017, 00:29 (range YJ) Mar. 10, 2017, 00:35 (range HK)
Feb. 17, 2017, 00:05 (range YJ) Feb. 17, 2017, 00:11 (range HK)
4
5
Feb. 16, 2016, 21:14 ( range YJ) Feb. 16, 2016, 21:16 (range HK)
Feb. 3, 2016, 23:27 ( range YJ) Feb. 3, 2016, 23:30 (range HK)
2
3
Feb. 2, 2016, 22:08 (range YJ) Feb. 2, 2016, 22:26 (range HK)
1
No. Date, UT (mean)
R.A. (2000), Exposure, s hms
Table 1. Observations of Europa (Trot = 3.551d)
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Table 2. Observations of Callisto (Trot = 16.689d)
No.
Date, UT (mean)
1
Feb. 2, 2016, 22:53 (range YJ) Feb. 2, 2016, 22:56 (range HK) Apr. 20, 2017,22:05 (range YJ) Apr. 20, 2017, 22:09 (range HK) Apr. 25, 2017, 22:34 (range YJ) Apr. 25, 2017, 22:50 (range HK)
2
3
Exposure, s
R.A. (2000), hms
DEC. (2000), Deg, arcmin, arcsec
z, deg
PhA, RRPh deg
λobs, deg
ϕobs, deg
3 × 30 3 × 30
1.7 11 32 13
4 30 40
41.0
6.5
0
307.42 –1.89
1 × 100 1 × 100
2.3 1.9
13 03 16 –05 02 23
55.0
2.6
0.542 118.04 –2.82
3 × 100 6 × 100
Relative error Standard, (MSE) Spst, zst, at1.1 deg and 2.2 μm, %
2.9 1.4
13 00 06 –04 42 46
61.0
3.6
0.845 227.27 –2.80
1.8
HD 114174, G5, 56 HIP 50303, A0, 53 HIP 61960, A0, 60
The designations are the same as those in Table 1.
nous (alternative) source is often considered for Europa: this is the material ejection from the subglacial ocean induced by geologic activity (see, e.g., Carlson et al., 1999; Shirley et al., 2010; Dalton et al., 2013; Ligier et al., 2016). Since high correlations between singularity of many morphologic features of Europa and their chemical and mineralogical composition were determined from the spectral data (see, e.g., Dalton et al., 2013; Ligier et al., 2016), the endogenous delivery of sulfur compounds to the surface is probable. The reflectance spectrum of Europa (Fig. 1, curve 1) agrees well with the earlier published data of the same kind, particularly, with the high-resolution spectra of dark linear formations in the trailing hemisphere of the satellite obtained with the NIMS spectrometer onboard the Galileo SC (see, e.g., Carlson et al., 1996; 1999; McCord et al., 1998; Shirley et al., 2010; Dalton et al., 2013). In Fig. 3a (adapted from Carlson et al. (1999)), we present the reflectance spectra for two extreme-in-composition formations on the surface of Europa obtained by the Galileo SC (NIMS): the spectra of water ice (solid curve) and sulfuric acid hydrate (circles). It is clear that the first one (in Fig. 3a) agrees well with spectrum 2 (in Fig. 1), while the second one is in close agreement with spectrum 1 (in Fig. 1). Figure 3b (adapted from Carlson et al. (1999)) shows that the reflectance spectrum of the aforementioned formation on Europa, where sulfuric acid hydrate presumably is predominant (Fig. 3a, circles), is consistent with the laboratory reflectance spectrum of pure sulfuric acid hydrate. Spectrum 2 (Fig. 1) approximately corresponds to the leading hemisphere of Europa. A symmetric shape of the main bands at 1.5 and 2.0 μm is clearly noticeSOLAR SYSTEM RESEARCH
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able, which is typical of water ice or frozen hydrates of other simple compounds (such as methane, ammonia, and some others), whose spectral properties turn out to be close to those of water ice. The main cause of this similarity is the clathrate (cellular) structure of hydrates, where the main (guest) molecule is included in a cell formed by water molecules. Due to the action of rather weak van der Waals forces (Van der Waals, 1956; Englezos, 1993; Langreth et al., 2009), when clathrates (or gas-hydrates) are formed between water and guest molecules, the latter produce almost no effect on the general spectral characteristics of clathrates. By way of illustration, in Figs. 4a–4d (adapted from Smythe (1975)), we compare the positions of close absorption bands in the reflectance spectra of water ice frosts, a mixture of water ice and methane, methane, and methane clathrate (hydrate). In addition, the position of the CH4 absorption band at ~1.67 μm is indicated by vertical lines in the spectra of the analogue compounds under discussion. In Figs. 4a and 4b, there are also insets showing our measurements of the reflectance spectra of Europa and Callisto; the position of the same absorption band is marked with a vertical arrow there. It is worth stressing that, if water ice and methane are mixed, their near-IR absorption bands compensate each other; consequently, it is difficult to identify methane or its clathrate. However, in the interval of 1.66–1.80 μm, there are three sufficiently narrow, though weak, absorption bands of methane clathrate connected with transitions 2v3 (1.670 μm), v2 + v3 + v4 (1.724 μm), and v3 + 2v4 (1.797 μm), the position of the central wavelengths, which was later determined more accurately under helium temperatures (Brunetto et al., 2008; Dartois
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Europa
4.0 Reflectance spectrum
3.5 3.0 2.5 2.0 2
1.5 1.0 0.5
1 0 0.9
1.4
1.9 Wavelength, µm
2.4
Fig. 1. Reflectance spectra of the trailing (1) and leading (2) hemispheres of Europa normalized to 1.5 μm. Spectrum 1 is the average of the original spectra obtained at close relative rotation phases of the satellite (see numbers 1 and 3–5 in Table 1). Spectrum 2 is arbitrarily shifted up for convenient comparison. A vertical arrow at spectrum 2 shows the supposed position of the absorption band of methane clathrate.
Callisto
4.0
Reflectance spectrum
3.5 1
3.0 2.5 2.0
2
1.5 1.0 3
0.5 0 0.9
1.4
1.9 Wavelength, µm
2.4
Fig. 2. Reflectance spectra of Callisto normalized to 1.5 μm (1–3). They are indicative of the range of variations in the spectral characteristics of the satellite in a global scale due to rotation. Spectra 1 and 2 are arbitrarily shifted up for convenient comparison. The parameters of observations are listed in Table 2.
et al., 2010). The first of these absorption bands is seen in the spectra of a mixture of water and methane frosts (Figs. 4a and 4b) and in the spectrum of methane clathrate (Figs. 4c and 4d); this band is marked with a vertical line. Specifically, as is seen in Figs. 4a and 4b, the weak absorption band of pure H2O, which is close to the considered one, is somewhat shifted to the short-wavelength range. In our opinion, it is this absorption band of CH4 clathrate that is also clearly visible in spectrum 2 of the leading hemisphere of Europa that we obtained (marked with a vertical arrow in Fig. 1). The same absorption band (though somewhat weaker) is also in the reflectance spectrum of the
trailing hemisphere of Europa (Fig. 1, spectrum 1). This is consistent with our earlier interpretation of the signs of CH4 (or its clathrate) in the reflectance spectrum of Europa made from the weak absorption band at 0.86 μm (Busarev, 2014). It is necessary to stress that weak and noisier signs of the absorption band of CH4 clathrate at ~1.67 μm are probably also present in all three reflectance spectra of Callisto obtained at its different rotation phases (Fig. 2). In view of these results, the problem of probable origin of CH4 and/or CH4 clathrate on Europa and Callisto arises. Though we admit the possibility of radiation-induced origin of a portion of methane on Europa, we may suppose that SOLAR SYSTEM RESEARCH
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(а)
Reflectance spectrum
1.0
0.8
0.6
0.4 1.5
1.0 0.8
Reflectance spectrum
2.0
2.5
2.0
2.5
Wavelength, µm (b)
0.6
0.4
0.2
0 1.0
1.5 Wavelength, µm
Fig. 3. (a) Reflectance spectra of two extreme-in-composition formations on the surface of Europa obtained with the NIMS spectrometer onboard the Galileo SC: water ice (solid curve) and sulfuric acid hydrate (circles). (b) Comparison of the reflectance spectrum of Europa (shown by circles in panel (a)) to that of a sample of sulfuric acid hydrate H2SO4·8H2O measured in laboratory (T = 140 K, the size of particles is d = 50 μm); the spectra are normalized to 1.1 μm. (The both plots are adapted from the paper by Carlson et al. (1999).)
the presence of CH4 clathrate on Europa is mainly connected with bringing the products of geological processes to the surface or even with some biological activity in the global interior water ocean. Such interpretation agrees with a stronger intensity of the considered absorption band in the leading hemisphere of Europa (Fig. 1). As far as Callisto is concerned, since its surface is very old, CH4 (and/or its clathrate), as well as sulfur compounds, are likely to appear on the surface due to the action of radiative and chemical processes. At the same time, the earlier published results about the presence and distribution of sulfur compounds on Callisto are contradictory. For example, SOLAR SYSTEM RESEARCH
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some papers report that SO2 was detected on this satellite, mainly in the leading hemisphere, by a wide absorption band centered at 0.28 μm in the reflectance spectra obtained from the Voyager data (see, e.g., Noll et al., 1997; Lane and Domingue, 1997). However, the authors of the first paper point to the ambiguity of this identification. According to the laboratory data, the absorption band at 0.28 μm can be induced not only by SO2, but also by hydroxyl (OH), which is an expected product of H2O radiolysis (Johnson and Quickenden, 1997), and by other compounds. Particularly, as the laboratory experiments showed (Sack et al., 1992), in water ice exposed to fluxes of different ions (Ar+, He+, and S+), films absorbing radiation at wavelengths close
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0.58 H2O + 0.15 CH4 1.0
CH4 hydrate
(а)
Europa
(c)
1.0
0.8
0.6
Reflectance spectrum
Reflectance spectrum
0.8 1.4 1.9 Wavelength, µm
0.4
0.2
0 1.0 1.0
0.6
0.4
0.2
1.5
2.0
Callisto
0 1.0
2.5
1.5
2.0
2.5
(b) (d)
0.8
0.6 H2 O 0.4
0.2
0 1.0
1.5
2.0 2.5 Wavelength, µm
Reflectance spectrum
Reflectance spectrum
0.8 1.4 1.9 Wavelength, µm
CH4 0.6
0.4
0.2
0 1.0
1.5 2.0 Wavelength, µm
2.5
Fig. 4. Reflectance spectra of water frost (a), a mixture of frosts of water (0.58H2O) and methane (0.15CH4) (b), methane frosts (c), and methane hydrate (d). For the first two cases, the temperature is 178 K, and the particles are 50–100 μm in size; for the latter two cases, the temperature is 100 K, and the sizes of particles are d < 50 μm. Vertical lines indicate the CH4 absorption band position in the spectra of the considered analog compounds. Inserts in panels (a) and (b) show the reflectance spectra of Europa and Callisto we obtained; the position of the same band is marked with a vertical arrow. The plots are adapted from the paper by Smythe (1975).
to 0.28 μm are formed. Because of this, in our view, it is too early to assert that SO2 was identified in the material of Callisto by the absorption band. The conclusion on the dominant abundance of sulfur dioxide on the leading hemisphere of Callisto (made from the same data of the Voyager SC) (Noll et al., 1997; Lane and Domingue, 1997) also raises doubts. According to
the earlier (Nelson, 1987) and later (Lane and Domingue, 1997) measurements onboard the International Ultraviolet Explorer SC, there are almost no differences between the leading and trailing hemispheres of Callisto in the UV range (at 0.28 μm), while noticeable local (longitudinal) and temporal changes were detected in the course of about 10 years. At the SOLAR SYSTEM RESEARCH
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same time, the results of our observations suggest that the distribution of sulfur compounds on Callisto is not uniform on a global scale (Fig. 2) and has a maximum on the trailing hemisphere. Consequently, we may suppose that a general magnetospheric process exists to deliver such compounds to the Galilean ice moons. However, that action may be affected by variations connected with, for example, volcanic activity on Io. It is interesting that the theoretical modeling of the spectra of Callisto in the range 0.25–4.31 μm showed that the surface material of the satellite is composed of large-grained water ice in the amount of 20–45% by mass (Calvin and Clark, 1991; Roush et al., 1990), which contains interpenetrating mixtures of magnetite and serpentine (Roush et al., 1990) or Fe- and Mgserpentines (Calvin and Clark, 1991). It was concluded that the leading hemisphere of Callisto is covered with ice particles, whose sizes are smaller than those on the trailing hemisphere due to more intense micrometeorite reworking of the material (Calvin and Clark, 1993). This is apparently confirmed by a smoother shape of the absorption bands in the reflectance spectrum of the leading hemisphere of Callisto (Fig. 2, spectrum 2), while the position of these bands (centered at 1.5 and 2.0 μm) indicates, analogously to Europa, the presence of a substantial amount of water ice and/or frozen hydrates of methane and other simple compounds (roughly several tens of percent in volume). Consequently, the general and, probably, main process delivering sulfur compounds to Europa and Callisto may be the exogenous one connected with active volcanism on Io embedded in Jupiter’s magnetosphere. CONCLUSIONS Thus, we have found that the near-IR spectral characteristics of Europa and Callisto are similar on a global scale and, particularly, the maxima of the distribution of sulfuric acid hydrate (or similar products) on the trailing hemispheres of both moons are close. This suggests that there is a process of ion implantation of sulfur compounds on these and other satellites in the radiation belts of Jupiter. We also detected the spectral signs of CH4 clathrate as a weak absorption band centered at ~1.67 μm in the reflectance spectra of the leading and trailing hemispheres of Europa. Moreover, weaker signs of the same absorption band of CH4 clathrate are in all three reflectance spectra of Callisto obtained at its different orientations. However, since this spectral feature is noisier for Callisto, these data should be examined in more detail. Though we admit the possibility of radiation-induced origin of a portion of methane on Europa, we may suppose that the presence of CH4 clathrate on Europa is mainly connected with bringing the products of geological processes to the surface or even with some biological activity in the global interior water ocean. Since the surface of Callisto is very old, methane (or its clathrate), as well as SOLAR SYSTEM RESEARCH
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sulfur compounds, are very likely to appear due to the action of radiative and chemical processes. As the discussion of our results and other studies shows, the spectral characteristics of the Galilean ice satellites of Jupiter are formed under complex physical, chemical, and dynamic conditions. When attempting to identify these or those spectral features of these bodies, one should consider an integral effect of many preceding and current evolutionary factors. Because of this, in parallel with the earlier results of the high-resolution measurements onboard the Voyager and Galileo SC, which were very useful for studying the local peculiar properties of the Galilean moons, the remote investigations of these bodies, which are mainly focused on determining the global characteristics connected with large-scale processes in the magnetic and gravitational fields of Jupiter, remain topical. In this light, it is necessary to model the interaction of the induced magnetic fields of Europa and Callisto with the magnetic field and radiation belts of Jupiter. The range of compounds that may exist on the surface of the Galilean ice satellites of Jupiter under manifold and varying irradiation conditions is wide, which creates additional difficulties in the spectral identification of not only neutral elements or molecules, but also their ionized varieties. Consequently, studies of this kind should be based on the laboratory spectral data of such compounds and on the analysis of the kinetics of possible low-temperature chemical reactions on the surface of these celestial ice bodies and similar objects (see, e.g., Delitsky and Lane, 1997, 1998; Materese et al., 2015). ACKNOWLEDGMENTS The equipment for this work was purchased with the Development Program funds of the Moscow State University. REFERENCES Anderson, J.D., Lau, E.L., Sjogren, W.L., Schubert, G., and Moore, W.B., Europa’s differentiated internal structure: Inferences from two Galileo encounters, Science, 1997a, vol. 276, pp. 1236–1239. Anderson, J.D., Lau, E.L., Sjogren, W.L., Schubert, G., and Moore, W.B., Gravitational evidence for an undifferentiated Callisto, Nature, 1997b, vol. 387, pp. 264– 266. Bagenal, F., Empirical model of the Io plasma torus: Voyager measurements, J. Geophys. Res., 1994, vol. 99, pp. 11043–11062. Bagenal, F., Ionization source near Io from Galileo wake data, Geophys. Res. Lett., 1997, vol. 24, pp. 2111–2114. Bagenal, F. and Sullivan, J.D., Direct plasma measurements in the Io torus and inner magnetosphere of Jupiter, J. Geophys. Res., 1981, vol. 86, pp. 8447–8466. Bolton, S.J., Thorne, R.M., Gurnett, D.A., Kurth, W.S., and Williams, D.J., Enhanced whistler-mode emis-
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Translated by E. Petrova
