Thermal segregation of water ice on the Galilean satellites

ICARUS 69, 297--313

(1987)

Thermal Segregation of Water Ice on the Galilean Satellites J O H N R. SPENCER Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721 Received January 2, 1986; revised September 20, 1986 Consideration of the thermal sublimation of ice on the Galilean satellites suggests that dirty-ice surfaces are susceptible to a process of cold-trapping of water in local bright patches and its preferential removal from dark areas. The result may be very rapid (decade time scale) segregation of the surface into bright icy regions and regions covered by dark ice-free lag deposits. Ion sputtering and micrometeorite bombardment are probably insufficient to prevent this process at low latitudes on Ganymede and Callisto. Sputtering on Europa may prevent segregation, especially on the trailing side. Segregated regions must be mostly smaller than the kilometer resolution of the Voyager images, but larger than centimeter size. © 1987AcademicPress, Inc.

INTRODUCTION

Although it has been known for many years that water ice is present on the surfaces of the three outer Galilean satellites of Jupiter (Europa, Ganymede, and Callisto), the abundance and physical distribution of the ice on each body have yet to be determined with any certainty (see Pilcher et al. (1972), Pollack et al. (1978), and Clark (1980) for various approaches to the problem). A particularly important unknown is the degree of mixing of the ice and whatever darker nonicy component is present on the surfaces. If the ice is mixed with the nonicy material, its temperature at the subsolar point may be several tens of degrees warmer than if it is concentrated in discrete relatively pure (and therefore high-albedo) patches. Its vapor pressure will be higher by several orders of magnitude, greatly increasing thermal ice migration rates. The potential for developing an atmosphere of 02 or H20 will also be greater if the ice is warmer (Kumar and Hunten, 1982), and ion sputtering yields, which are temperature dependent (Brown et al., 1982), will be affected. In addition, the more intimately the ice and nonice components are mixed, the less nonice is required on the surface to explain

the satellite albedos, which in the case of Ganymede and especially Callisto are much lower than expected for pure water ice. For Callisto, the deduced weight percentage of ice in the surface layers varies from as high as 90% on the homogeneous model (Clark, 1980) to as little as 6% if the ice is segregated (Lebofsky, 1977; this paper), and thus very different constraints are placed on models of the satellite's surface evolution. It has been argued, for instance by Squyres (1980), that the great geological age of the surface units on at least Ganymede and Callisto implies that impact gardening will have thoroughly mixed the ice and nonice components of the surface. Clark's (1980) interpretation of the near-infrared reflectance spectra of the three objects appears to support the view of a homogeneous surface, with a (minor) nonice component intimately mixed with the ice. However, Seiveka and Johnson (1982) and Spencer and Maloney (1984) argue for segregated, bright ice on Callisto in order to explain the lack of massive poleward ice migration on that object. Purves and Pilcher (1980) and Shaya and Pilcher (1984) implicitly assume a segregated-ice surface in their models of ice migration on Ganymede and the other satellites.

297 0019-1035/87 $3.00 Copyright © 1987by Academic Press, Inc. All rights of reproduction in any form reserved.

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composed of a mixture of ice and a darkening contaminant, presumed for present purposes to be initially in the form of small embedded particles, well mixed with the ice, as might result from a fresh meteorite impact. Variations in surface albedo occur • L a w H I O Vlpar Pralaure. ~ ' H i g h H~O vlDar Preaaura'.. . ~ . _. .. . • Law SublimatiOn Rltl t."..Hlgh Subltlltlon R a t a :" o n tne s c a l e s seen in me voyager images " . . i . i . . i I [:.:::!iil/:i~.)ii)i"f:ii'i.:ii:.:.:;::-i:!i.)"i( i and probably on smaller scales too, due initially to internal geological processes and the effects of meteorite bombardment. ® Regions that are darker will be warmer, 't 1" 1" and the ice within them should have a F~esh 1 y - o I p O 1 1 t e a | greater sublimation rate, than nearby clean lc.e . " I . . . .0?.tg.lnm. 1..!u.r~a.c.ii . . . . brighter regions. Because sublimating wa" ". ter molecules on the Galilean satellites d l p O l l i...'.-~.!.-.~' " j u m p " tens of kilometers before reimpacting the surface (Purves and Pilcher, 1980), any imbalance in sublimation rates in regions less than tens of kilometers apart will FI~. 1. Cartoon illustrating the model for surface segregation described in this paper. An initially result in a transfer of ice from the dark reslightly inhomogeneous surface (A) transfers ice from gions to brighter ones. ff addition of ice dark regions to light regions because of the imbalance brightens the bright regions and loss of ice in sublimation rates. The process continues until the darkens the dark regions, the transfer of ice development of a lag deposit (B) cuts off sublimation will be enhanced by positive feedback and from the darker regions. will only be halted when almost all ice has been lost from the surface of the dark reIs it possible to maintain segregated sur- gions. The result will be very efficient segfaces on the icy Galilean satellites in t h e regation of an initially fairly homogeneous face of the homogenizing effects of meteor- surface into a patchwork of bright icy and ite gardening? One possible way, suggested dark ice-free areas. by Spencer and Maloney (1984), is a positive feedback mechanism in which regions that are ice rich will tend to be brighter and Time Scale for Segregation hence cooler than their surroundings and The process of ice segregation is halted will thus act as cold traps for the accumula- when the darker surface regions become tion of more ice. A similar process for trap- covered in a lag deposit of nonice particles ping of SO2 on Io has been proposed by that cut off further sublimation. The segreFanale et al. (1982). This paper describes gation time scale ts is thus given by an attempt to quantify the cold-trapping ts = x/S, (1) model in order to determine the possible importance of the mechanism and also considers competing surface redistribution pro- where x is the thickness of ice containing enough nonice particles to cover the surcesses. face (i.e., unit total projected area of nonice THE SEGREGATION MECHANISM particles per unit area), and Sn is the diurThe principles of the proposed segrega- nally averaged net rate of surface lowering tion model are simple and are illustrated in due to sublimation. If the volume fraction Fig. 1. The icy Galilean satellites (Europa, of nonice particles is fp and their radius is Ganymede, and CaUisto) have surfaces rp, then by simple geometry, assuming upward Sub l treat t o n l" I" I" ~x ~x ,qx .... . . ... • .~.......-:......-......:.....-....-........: . . . c~"h~c, • . .1::;:;::.!:;Ol~kXc:';':'."::'.".": . . - L,, raw,,,tu,a " • 1:.'..' mgh r,,,nt,;,,".'.:..,

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These equations assume a solid surface, without pore space, which is not realistic. However, if the surface is porous, with a pore space fraction of p, then x is increased by a factor of 1/p, but the surface recession rate Sn is increased by the same factor relative to the solid-ice recession rate. t~ is therefore independent of p. The sublimation rate can be determined as follows. Assuming instantaneous equilibrium with incident sunlight and unit emissivity, surface temperature T is obtained from orT 4 =

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where or is the Stefan-Boltzmann radiation constant, A is the bolometric albedo, K is the solar constant at 1AU, D is the distance from the Sun in AU, and i is the local solar incidence angle, a function of latitude and time of day. The amount of heat removed from the surface due to the latent heat of sublimating ice is too small to affect surface temperatures below about 180°K (Lebofsky, 1975). Water vapor pressure over ice as a function of T is obtained from In Pvap = --4.77 X 1011/RT + 28.9 (4) where Pvap is the vapor pressure and R is the gas constant (all units are erg c.g.s.), which is a fit to experimental data in the range 132-153°K (Bryson et al., 1974). The instantaneous rate of ice surface lowering due to upward sublimation, s, is then obtained from the vapor pressure using s = PvapX/(M/2~rRT)/p

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where M is the molecular weight of water and p is the density of ice (taken as 0.92 g cm-3; see the remarks above about surface porosity). The water sublimation rate is thus obtained as a function of time of day, and diurnal averaging over i gives the mean sublimation rate S.

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S is merely the upward sublimation rate. The net rate Sn will be reduced by the influx of molecules onto the region in question from its surroundings. The influx rate will be the areal mean sublimation rate of ice over a surrounding region tens of kilometers across, weighted by the Maxwell/ Boltzmann distribution of molecular jump distances. However, the variation of ice vapor pressure with temperature is so extreme that, for instance, a change in albedo from 0.4 to 0.5 reduces the ice sublimation rate by a factor of almost 6 at the equator on the Galilean satellites (from Eqs. (3), (4), and (5) and Fig. 5). If the region being considered is significantly darker and warmer than its surroundings (and it will become darker as segregation progresses), its own sublimation rate will be enough greater than the average influx that Sn will be comparable to S. Use of S instead of S, should give the segregation time scale to an order of magnitude. The albedo A of the sublimating surface will decrease as segregation progresses, which will reduce the time scale ts but further complicate its determination. However, assuming a constant A should allow order-of-magnitude time scales to be determined. Figure 2 shows the segregation time scale ts as a function of albedo and latitude at Jupiter's distance from the Sun (D -- 5.2 AU). Nonice particle size rp and volume fractionfp were held constant at 0.1 mm and 0.01, respectively, giving x a value of 13 mm. From Eqs. (1) and (2), ts is proportional to rp and inversely proportional to fp, allowing the calculation of T from Fig. 2 for other values of these parameters. For dark or moderate alhedo ice at low latitudes, ts is remarkably small, indicating surface segregation on time scales of years or decades. Given the assumptions of the model, which are discussed in more detail below, equatorial regions on the icy Galllean satellites are very unstable with respect to this process. Segregation is dramatically slower at high latitudes because

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FIG. 2. Logt0 segregation time scale Ts in years, as defined in the text, c o n t o u r e d as a function of latitude and a s s u m e d ice albedo on the Galilean satellites. The nonice particles are a s s u m e d to have a grain size of 0.1 m m and a v o l u m e fractional a b u n d a n c e of 0.01. Note: c o n t o u r s are h a n d - d r a w n from tabulated data, and their locations are not exact, t h o u g h errors are smaller than the c o n t o u r separation.

of the extreme temperature dependence of ice sublimation rates. On the Saturnian satellites (D= 9.5 AU), assuming the same particle properties as for Jupiter, ts has a value of 108 years for 0.4 albedo equatorial ice, and at Uranus (D -19.2 AU) ts is nearly 10 ]4 years even for 0.0 albedo ice at the equator. Clearly the proposed model for ice behavior is not relevant to the Uranian satellites, and its importance at Saturn is doubtful. The rest of this paper is concerned only with the Galilean satellites, where thermal segregation, on the basis of the calculated time scales, may be a very important process. It should be noted that the ice redistribution discussed here, which is in response to temperature variations, is quite different from the ice grain growth mechanism proposed by Clark et al. (1983). Their model assumes an isothermal surface, the ice redistribution to larger grains being powered by the increase in surface binding energy with the radius of curvature of the grain surface. Grain growth by this process is very slow; their Fig. 3 shows that at 150°K a 0.1-mm grain takes about 50 Myr to dou-

ble in size. However, the same grain has a one-way sublimation rate of 0.25 mm year-J (from Eqs. (4) and (5) above). Temperature gradients within a regolith are inevitable, and if, for instance, the grain's neighbors are at 149°K, with a sublimation rate of 0.19 mm year -], the grain will sublimate at a rate of 0.06 mm year- l and disappear entirely in a year! Smaller temperature contrasts will give sublimation (or growth) rates that are smaller, but contrasts small enough to allow isothermal grain growth to be important seem very unlikely on the Galilean satellites. The redistribution of ice due to temperature gradients within the regolith may, in contrast, be a major influence on regolith structure, though it is not considered further in this paper. Smoluchowski and McWilliam (1984) have considered the problem but only in terms of pore migration in solid ice.

Scale o f Segregation The model assumes that the segregating regions are larger than a few centimeters, so that lateral conduction of heat between the regions during a day is unimportant, and smaller than tens of kilometers, so that each can receive a full flux of sublimated water molecules from the other. Within these very generous limits of scale, the physics of the problem is the same. However, the fact that the Voyager images do not show drastic segregation down to the limit of resolution (typically 1 or 2 kilometers) means that any segregation is at scales smaller than this. There is an exception at high latitudes on Callisto, where topographically associated segregation, probably thermally controlled, is visible in the Voyager 1 images on a scale of tens of kilometers (Spencer and Maloney, 1984; Fig. 3). This example is discussed shortly. ASSUMPTIONS

The assumptions implicit in the above discussion of the segregation mechanism and the derivation of time scale are now listed and discussed explicitly.

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50% FIG. 3. Probable thermally segregated surface ice at high latitudes on Callisto, seen by Voyager 1 (FDS 16424.48). Bright patches of ice are visible on cooler north-facing slopes, resulting in an apparent reversal in illumination direction (arrow shows the true direction of solar illumination). Latitude/ longitude grid from Davis and Katayama (1981) . From Spencer and Maloney (1984).

(1) Dirty-ice surfaces darken as they lose ice by sublimation. The dark particles released by the sublimating ice would be expected to accumulate on the surface and thereby darken it, eventually forming an ice-free lag deposit. However, it is possible that formation of a lag deposit is prevented because the dark particles, rather than remaining on the surface, sink beneath it. Dark particles on the surface may warm the ice immediately around them, causing local sublimation around the particle and eventually creating a hole that it can sink into. Darkening of the surface and attendant positive feedback might thus be prevented. There is evidence from Viking observations that a phenomenon of this type occurs on the Martian CO2 polar caps (Paige, 1985). Such particle sinking has not been observed for dirty-water-ice mixtures in the laboratory, despite attempts to provoke it (by N. G. Purves and C. B. Pilcher; R. N. Clark, personal communication) but cannot be ruled out as a possibility. Another way of violating assumption (I) may be if dark particles embedded in the ice

brighten as they lose their ice coating, due to an enhanced refractive index contrast at their surfaces. Sublimation from a dark dirty-ice surface could thus brighten it somewhat. However, the surface can only be brightened to a maximum albedo equal to that of the dry grains themselves. As most common nonice materials in the solar system have lower albedos than most icy surfaces, it is unlikely that this brightening effect is sufficient to prevent segregation. (2) Dirty-ice surfaces get brighter as ice is deposited on them. This is generally the case experimentally (see, e.g., Fig. 16 of Clark (1981)), but laboratory deposition rates are much faster than the millimeter per year rates relevant to the Galilean satellites. It is possible that ice depositing on an already-icy surface at low rates forms a surface "glaze" that lowers, rather than raises, the albedo. G. T. Sill (personal communication) notes that when he has deposited water ice from vapor onto a smooth surface at about 130°K, it is initially smooth and glassy but becomes bright after it has reached a few microns thickness. The

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lower the deposition rate the darker the ice at a given thickness, and actual deposition rates will be a couple of orders of magnitude slower than Sill has observed in the laboratory. The effect on albedo of very slow ice deposition onto an already rough surface has not been investigated, though nucleation of discrete grains seems more likely than on a smooth surface. Bombardment by heavy ions is known to rapidly brighten glassy ice surfaces (Brown et al., 1978) but results in the "welding" of finegrained frost (R. N. Clark and M. L. Nelson, personal communication) and thus is also an important factor. More experimental work is clearly needed in this area. The segregation model I describe here requires that the albedo increase as ice is deposited and will not work if a dark glaze forms on the brighter regions and makes them as dark as the rest of the surface. There is evidence from the appearance of the surface of Callisto in particular that supports assumptions (1) and (2). The evidence comes from Callistoan high latitudes (Fig. 3), where bright deposits are visible on the north-facing slopes of craters and southfacing slopes are darker than their surroundings (Spencer and Maloney, 1984). If one accepts the simplest explanation of the topographically controlled albedo patterns, they imply formation of a dark lag deposit (or something equivalent) on the warmer south-facing slopes and brightening of north-facing slopes due to ice deposition. The explanation may be more complicated than this, as a small region near Gilpul Catena shows bright east-facing, rather than north-facing, slopes, but the northward orientation is much more widespread, supporting a thermal origin for the albedo patterns. (3) Daytime surface temperatures of darker ice are not dramatically less than predicted on the basis o f equilibrium with sunlight. Daytime infrared effective temperatures will be comparable to equilibrium temperatures due to the low surface thermal inertias inferred from ground-based

thermal infrared observations of the Galilean satellites (Morrison and Cruikshank, 1973). Bright material, contributing little to the daytime thermal flux, may have high thermal inertia (see the Thermal Infrared Evidence section), but it is the temperature of darker ice that controls segregation. Assuming unit emissivity for the moment, the actual surface kinetic temperature of the ice might be different from the infrared effective temperature because thermal emission comes not from the surface but from a finite depth. However, the discrepancy is probably small because of the high opacity of ice to thermal infrared radiation. At 40/.tm, near the peak in the blackbody curve for daytime ice temperatures, the absorption coefficient for water ice is 1040 cm -~ (derived from Warren, 1984), so unit optical depth occurs in a thickness of about 9/xm, and thermal emission will originate preferentially from around this depth into the surface. The kinetic temperature at 18 /~m depth must therefore be comparable to the observed effective temperature of the thermal radiation. Even given the low thermal conductivities of the surfaces of the Galilean satellites (e.g., 40 erg sec -l °K cm -~ for Ganymede; Hansen, 1973), there must be less than a degree or so temperature drop between 9/zm depth and the actual surface or the conducted heat flux toward the surface would exceed the total radiated heat flux. Therefore the kinetic temperature of the very surface of the ice, which determines sublimation rates, is probably very close to the observed effective temperature of the surface. This conclusion is independent of the details of where in the regolith the sunlight is absorbed. Another related way that surface ice might be colder than the effective temperature of the thermal radiation would be if the radiation came mostly from the embedded dark particles rather than the ice itself. However, because of the ice opacity, only particles within tens of microns of the surface can radiate to space. The great trans-

THERMAL ICE SEGREGATION parency of ice in the visible means that most of the incoming solar radiation will be absorbed much more deeply than this by particles that can only lose their absorbed heat by conduction to the surrounding ice. The ice itself must eventually lose their heat by radiation and must thus emit the bulk of the radiated flux. The ice must therefore be as warm as the effective temperature of the radiation. Nonunit emissivity would increase kinetic temperature compared to infrared effective temperature, though low values are unlikely due to the above-mentioned high thermal infrared opacity of ice and the particulate nature of the surfaces. Apart from these probably small effects due to the nonzero infrared transparency of the ice, the temperature of thermal emission should provide a good guide to the expected sublimation rate. Shoemaker et al. (1982) suggested that surface roughness could dramatically reduce sublimation rates (sublimating molecules could be blocked by surface projections), but this seems unlikely, as the projections will be sublimating themselves. Regardless of the roughness of the surface, if an infrared photon can escape to space from a surface region at a particular temperature, a sublimating water molecule should be able to do the same. (4) Dark regions are warmer at the surf a c e during the day than bright regions. This will be true unless dark regions have significantly higher thermal inertia or emissivity than bright regions. However, as discussed above, the thermal inertia of the dark regions cannot be very high (because of the overall low thermal inertia), and the emissivity of icy regions is probably close to unity. It is conceivable that small albedo variations are prevented from growing because correlated small variations in thermal inertia or emissivity prevent the expected temperature contrasts, but there is no obvious reason to expect such a correlation. Voyager thermal infrared observations of Ganymede show the expected correlation between regional albedo and daytime

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brightness temperature (Spencer, 1983), which provides some confidence that the same may be true on local scales. (5) Other processes do not redistribute the ice more rapidly than thermal sublimation. This possibility is considered at some length in the next section. The conclusion is that thermal sublimation is so rapid that neither micrometeorite bombardment nor ion sputtering can compete with it at low latitudes on Ganymede, Callisto, and perhaps Europa. At high latitudes and on Europa sputtering or perhaps micrometeorites might possibly prevent segregation. Assumptions (1) thru (5) thus appear to be at least probably valid, though not proved. If the assumptions are correct, segregation by thermal sublimation, at the rapid rates shown in Fig. 2, is a very probable process on the icy Galilean satellites, especially at low latitudes on Ganymede and Callisto. Indirect observational evidence that segregation has indeed occurred on Ganymede and Callisto is discussed later in this paper. COMPETING ICE TRANSPORTPROCESSES Other surface transport processes may compete with thermally controlled ice segregation, tending to remix the segregated materials. Leaving aside internally driven geological activity, which at least on Ganymede and Callisto is not currently important, the important competing processes, micrometeorite bombardment and ion sputtering, are discussed below. Micrometeorite B o m b a r d m e n t

The micrometeorite environment of the Galilean satellites is still poorly known. Pioneer 10 detected a flux of particles in the region of Ganymede and Callisto that was enhanced over the fairly constant values recorded since leaving Earth by a factor of about 100 (Humes et al., 1974). Recently, Zook et al. (1985) have successfully modeled the Pioneer detections on the assumption that the particles originate on the inner small Jovian moons and then move out-

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FIG. 4. Comparison of micrometeorite gardening rates on the Moon with ice sublimation rates on the Galilean satellites. The point "A12" shows the depth of the regolith at the Apollo 12 landing site (Nakamura et al., 1975). The lines "Gault model" and "Arnold model" show the lunar regolith mixing time as a function of depth as determined from expected meteorite fluxes by Gault et al. (1974) and Arnold (1975, Fig. 7). The point "Apollo microcraters" shows the surface exposure times of typical 100- to 500-/xm lunar soil grains, based on microcrater counts (Poupeau et al., 1975). "Durand model" shows the surface residence time for 1- to 50-/zm grains, derived from a Monte Carlo model tied to observed cosmic ray exposure ages (Durand et al., 1975). The arrow shows the expected 100-fold increase in gardening rates on the Galllean satellites relative to the Moon in the depth range of tens of microns (Humes et al., 1974). The diagonal lines show, for comparison, the diurnally averaged sublimation rates for equatorial ice with the given albedo at 5.2 AU, assuming instantaneous equilibrium with sunlight.

ward, in prograde orbits, under the influence of plasma drag. These are particles capable of penetrating the 25-/xm-thick steel of the Pioneer detector and are thus responsible for regolith gardening to depths of tens of microns. Figure 4 compares rates of regolith gardening on the Earth's Moon as a function of depth, according to numerical models and observational constraints from Apollo, with expected diurnally averaged ice sublima-

tion rates on the Galilean satellites for various ice albedos. Also shown is the expected 100-fold enhancement of gardening rates on the Galilean satellites compared to the Moon, at least in the depth range of a few tens of microns, suggested by the Pioneer results. In this depth range it appears the surface may be turned over in 50 years or so, an equivalent rate of a few × 10-3 mm year -~. For greater depths it is likely that the population of gardening micrometeorites is dominated by gravitationally focused interplanetary dust, which will be enhanced over interplanetary values by a factor of 10 or less (Humes, 1980) for reasonable sizefrequency distributions, giving consequently lower gardening rates. If the model of Zook et al. (1985) is correct and the particles are in near-circular prograde orbits, impact fluxes should be similar on both leading and trailing satellite hemispheres. It should be noted, however, that an alternative model in which the Pioneer dust is derived from outside the Jovian system and impacts Europa and Ganymede primarily on the trailing hemisphere (but Callisto mostly on the leading hemisphere) has been developed by Hill and Mendis (e.g., 1981), and also claims to explain the Pioneer data. Figure 4 shows that, even including the likely enhancement of gardening rates near Jupiter when compared to the Moon, equatorial water ice with a bolometric albedo of less than about 0.5 will sublimate at a rate faster than the rate of impact gardening. Ice with an albedo of 0.2 that is surrounded by brighter material and is thus sublimating water at a rate comparable to the one-way rate shown in Fig. 4 will lose 0.1 mm of ice to its surroundings in a few months, while micrometeorite gardening would take decades to redistribute this thickness of material. Segregation is controlled by sublimation of the ice in the warmer, darker, surface regions, so unless m o s t of the satellite surface has a bolometric albedo higher than about 0.5, as is probably the case on Europa (bolometric albedo is estimated to

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FIG. 5. Ice sublimation rates on the Galilean satellites as a function of ice albedo and latitude compared to sputtering and micrometeorite gardening rates. The contours show log~0 of the one-way ice sublimation rate (in mm year-l), assuming instantaneous equilibrium with sunlight and a heliocentric distance of 5.2 AU. The contour value for which one-way sublimation equals the expected micrometeorite gardening rate (from Fig. 3) is shown as a dashed line. The symbols labeled "Callisto," "Ganymede," and "Europa" show the expected sputtering rates on the three bodies taken from Tables I and II and Appendix A. The tick mark on the top of each symbol identifies the contour for the sputtering rate for low-energy ions. The three tick marks below show rates for high-energy ions, assuming a plasma composition of S, O, or H, in order of decreasing rate. Note: contours are hand-drawn from tabulated data, and their locations are not exact, though errors are smaller than the contour separation.

be in the range 0.48-0.76; Buratti and Veverka, 1983), impact gardening will not affect segregation in equatorial latitudes. At higher latitudes, of course, sublimation drops precipitously. Figure 5 shows one-way sublimation rates as a function of ice albedo and latitude, as well as the estimated rate of impact gardening derived from Fig. 4. Gardening rates exceed sublimation for dark ice poleward of about 60° lat. Even a vertical impact gardening rate in excess of sublimation may be unable to prevent segregation, however. This is because sublimation, with mean molecular jump distances of tens of kilometers, is much more effective at lateral transport than impact processes, where most of the material lands

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within a crater radius of the crater rim. Impacts can only rehomogenize the surface laterally over length scales comparable to the size of the largest ejecta blankets that cover most of the surface in the time interval being considered. This distance will be, to an order of magnitude, ten times the depth of regolith mixing (because ejecta blanket diameter is typically twice crater diameter, and the diameter is typically about five times the crater depth for small craters; e.g., St/Sffler et al., 1975). If 103 years of impact gardening mix the top 1 mm of the surface, lateral mixing will only have occurred up to centimeter length scales. Even if a darker region of the surface is sublimating ice at a net rate much smaller than the rate of vertical mixing by impacts, significant ice depletion of that region can still occur in 103 years provided the region is larger than a few centimeters, and complete segregation can eventually follow. The main effect of the impact gardening will be to slow down the rate of ice depletion and lag deposit formation in the darker regions by increasing the thickness of surface material from which ice must be removed. Another effect may be to increase the minimum allowable size for the segregation patches. There will probably be some sublimation rate below which impact gardening will prevent segregation from occurring at all. Using the arguments of the previous paragraph, one sees that this minimum rate will be much lower than the gardening rate, but detailed quantification of the effect of impact mixing in these circumstances will require sophisticated computer modeling. It is concluded that impact gardening by micrometeorites is unlikely to affect thermal ice segregation except possibly on the very bright surface of Europa and at high latitudes on Ganymede and Callisto. Even in these places the reasoning outlined above suggests that impacts will probably merely slow the segregation process rather than prevent it. However, in areas where sublimation rates are much lower than gar-

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dening rates, segregation may be prevented.

Ion Sputtering Unless the icy satellites have substantial magnetospheres or atmospheres (Wolff and Mendis, 1983), their surfaces will be bombarded by the abundant ions in Jupiter's magnetosphere, resulting in sputtering of surface ice (e.g., Brown et al., 1978), which will act to counter the thermal segregation process. Unlike micrometeorite bombardment, sputtering is a very efficient means of lateral transport of ice. Molecules typically leave the surface with an appreciable fraction of escape velocity and travel hundreds of kilometers before reimpacting (Seiveka and Johnson, 1982). There is also some net erosion of the surface, as a fraction of the sputtered H20 molecules are dissociated by the incoming ion (Brown et al., 1982). If sputtering rates are greater than thermal ice sublimation rates, segregation will thus be prevented, as ice distribution will be rehomogenized on all scales up to and greater than the largest scales at which thermal segregation can occur. The Appendix discusses the likely rates and geometry of sputtering on the three icy satellites, and the rates, from Tables I and II, are compared to the sublimation rates in Fig. 5. Sputtering is most intense on Europa, and if the high energy plasma is dominated by heavy ions erosion rates on the trailing hemisphere may approach sublimation rates for ice with Europa's bolometric albedo of around 0.62 (Buratti and Veverka, 1983). Moderate enhancement of sputtering over estimated rates or reduction in sublimation due for instance to small but finite thermal inertia might be sufficient to prevent segregation here. Sputtering rates on Europa's leading hemispheres are probably less but are hard to calculate. On Ganymede and Callisto, because of the lower ion fluxes and lower albedos, it is very unlikely that sputtering can prevent segregation except at high latitudes.

DISCUSSION

Spectroscopic Evidence The reflectance spectra of the icy Galilean satellites are discussed in a companion paper (Spencer, 1987a). It is concluded that the spectra of Ganymede and Callisto are probably at least consistent with segregated surfaces, contrary to the interpretations of Clark (1980). A match can be made with around 50% areal coverage of bright ice on Ganymede and 10% on Callisto, with the rest of the surface made up of dark material spectrally similar to carbonaceous chondrites. The depths of all observed water-ice absorption features can be accounted for reasonably well by such models. Europa's spectrum, however, can probably only be explained by a homogeneous icy surface, largely because of the extreme darkness in the 3-/xm region.

Indirect Evidence for Segregation Indirect evidence for the segregation of ice on Ganymede and Callisto is provided by the preservation of large albedo contrasts at low latitudes on both objects. Bright ray craters occur in close proximity to much darker terrain on both objects, and on Ganymede bright grooved terrain occurs next to darker cratered terrain with no gradation in albedo between them at the kilometer resolution of the Voyager images, ff the surface is mostly homogeneous, with the albedo contrasts being the result of slightly different concentrations of a minor dark contaminant in the ice, or a grain-size contrast, the ice in the darker regions must be significantly warmer and have a higher sublimation rate than in the neighboring bright areas. For instance, the diurnally averaged sublimation rate of low thermal inertia ice in a bright equatorial ray crater on Ganymede, with a bolometric albedo of 0.5, for instance, is 0.014 mm year -l (Eqs. (3), (4), and (5) and Fig. 5). The equivalent rate for 0.25 albedo cratered terrain is 0.62 mm

THERMAL ICE SEGREGATION year -1. If these regions are within a few tens of kilometers of each other, there must be a rapid transfer of ice to the bright region at a rate much faster than can be counteracted by impact gardening or sputtering, as discussed in the Competing Ice Transport Processes section. Given that thermal sublimation is the dominant ice transport mechanism, stability is only possible if ice regions close enough together to "communicate" by sublimation have diurnally averaged sublimation rates that are very similar. The only obvious ways of achieving such a balance are discussed below: (1) Dark ice has a high enough thermal inertia to reduce its maximum daytime temperature to a value similar to that for bright ice. This is unlikely because, on Ganymede, we can observe the temperature variations associated with the visible albedo patterns. Voyager IRIS observations of Ganymede show that the bright ejecta of the ray crater Osiris is some 15°K colder than its darker surroundings in the midafternoon, and dark cratered terrain is warmer than brighter grooved terrain (Spencer, 1983). And again, ground-based thermal infrared observations (Morrison and Cruikshank, 1973) indicate low global thermal inertias for the surfaces of the Galilean satellites, constraining to small values the local thermal inertias for the dark material that contributes most of the daytime flux. (2) Even if dark areas have high brightness temperatures, their actual ice surface temperatures are much lower, so sublimation rates are comparable to the bright regions. This is unlikely for the reasons given in the discussion of point (3) in the Assumptions section. (3) All the ice has a similar albedo, regardless o f the large-scale (Voyager image resolution) albedo o f the surface. In this case, the ice is segregated and is everywhere at least as bright as the large-scale albedo of the bright ray craters, and the observed albedo variations on Ganymede and

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Callisto result from varying fractional coverage of ice, on a scale too small to be seen in the images. The existence of a plausible mechanism for producing such segregation, as described in this paper, helps make this alternative the most attractive. The above arguments for segregation based on the evident stability of albedo contrasts on Ganymede and Callisto cannot be applied to Europa, where local albedo variations are much more subdued. The Voyager images show a generally bright surface punctuated by dark (mostly linear) markings. Such a surface could be stable against the transfer of ice from the dark to the bright areas if the bright areas are fairly pure ice and the dark markings contain segregated dark regions and ice at the same albedo as the bright areas. A disk-integrated reflectance spectrum, dominated by the light from the bright areas, would show an unsegregated surface, as is observed (Spencer, 1987a). The problem with Europa is then to explain why its surface has not been subject to segregation. Europa is darker than several Saturnian satellites at visible wavelengths and is much redder than water ice, and if this is a result of significant amounts of dark impurities in its surface ice the current model might be expected to operate. There is a possibility, as discussed previously, that sputtering will dominate thermal sublimation on Europa's trailing side and thus prevent segregation, though it is less clear whether the same is possible on the leading hemisphere. Micrometeorite gardening is also possibly sufficient to keep the ice mixed. Alternatively, assumption (1) or (2) may be violated on Europa (e.g., dark particles might sink in warmer surface regions, or ice deposition might glaze and thus darken the surface). There is no a priori reason to expect such different behavior for dirty ice on Europa, except to note that its surface is anomalous in many respects, especially in its reflectance spectrum and photometric behavior (Buratti and Veverka, 1983). It is even possible that ac-

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JOHN R. SPENCER

tive resurfacing preempts segregation (Cook et al., 1983). A similar piece of indirect evidence for segregated ice on Ganymede and Callisto is the apparent lack of massive poleward migration of ice on either body. Ganymede's polar caps are extremely thin, as geological albedo boundaries can be seen through them, and Callisto has none at all. Purves and Pilcher (1980) and Shaya and Pilcher (1984), assuming segregated ice with an albedo of 0.6, inferred migration of tens of meters of ice from low to midlatitudes on both Ganymede and Callisto over the age of the solar system. Unsegregated, warm, ice would suffer much more drastic migration, amounting to redistribution of kilometer thicknesses on Callisto (Spencer and Maloney, 1984). The current segregation model provides a way of avoiding this problem by allowing the ice even on Callisto to be very bright. A high thermal inertia for the ice (as may be consistent with the thermal infrared data; see below) could reduce migration rates still further by reducing daytime temperatures. High thermal inertia ice is not possible if the surfaces are homogeneous because of the observed low thermal inertia of the surface as a whole.

Thermal Infrared Evidence The thermal emission from the Galilean satellites provides an additional probe of surface characteristics. Analysis of the Voyager IRIS observations of the thermal radiation from Ganymede and Callisto reveals very different variations in spectrum slope with solar incidence angle for the two objects (Spencer, 1985, 1987b). Initial analysis suggests the explanation that Callisto has a rougher surface than Ganymede and that Ganymede has two surface components with similar areal coverage but very different albedos and thermal inertias. Callisto could also have two surface components provided that the brighter component occupies only a small fraction of the surface. Europa data is of much poorer quality

but is probably at least consistent with a relatively homogeneous surface. The two-component model for Ganymede's surface also provides an alternative to the two-layer explanation of the observed cooling behavior during eclipse by Jupiter (Morrison and Cruikshank, 1973), provided that the bright component has a higher thermal inertia than the dark component. Such an explanation of the cooling behavior was first proposed by G. H. Reike and R. R. Howell (personal communication) and has been verified by Spencer (1985, 1986). Qualitatively, the initial rapid temperature drop is interpreted as being due to the cooling of the low thermal inertia dark component, which contributes most of the daytime thermal emission, and the subsequent slower drop in flux is due to the cooling of the higher thermal inertia bright component. These interpretations are preliminary and will be checked by further analysis but currently appear to support the idea of surface ice segregation on Ganymede and to be consistent with similar segregation on Callisto.

Implications Cold surface ice implies a lower atmospheric column density of water vapor. Yung and McElroy (1977), assuming an ice albedo of 0.32 for Ganymede (i.e., a homogeneous surface), obtained a surface pressure of 02, produced by UV photolysis of water vapor, of about 1/zbar. However, an atmosphere of this density was ruled out by Voyager l's ultraviolet spectrometry (UVS) observation of a stellar occultation by Ganymede, which gave an upper limit for oxygen partial pressure of 10-5 ~bar (Broadfoot et al., 1981). Kumar and Hunten (1982) show that variations in water vapor abundance can dramatically affect the development of an Oz atmosphere. A reduction in water vapor abundance of only a factor of 10 would put the atmosphere in a "low state" with an

THERMAL ICE SEGREGATION oxygen pressure below the Voyager limit. Segregated ice with an albedo of 0.6 would have a subsolar vapor pressure more than 200 times less than ice with 0.32 albedo (Eqs. (3) and (4)), thereby explaining the lack of a microbar oxygen atmosphere on Ganymede. If, as the current work suggests, Callistoan ice is also segregated, no microbar atmosphere is expected on Callisto either, although there are no good constraints from Voyager observations. An unfortunate implication of the current model is that surface ice abundances probably do not provide a good quantitative indication of subsurface abundances. Much of the observed nonicy surface component may be in the form of thin lag deposits overlying more ice-rich material. Radar observations (Ostro, 1982) should provide a better indication of the subsurface composition. It is therefore comforting that the radar albedos, which probably correlate positively with ice abundance, show the same trend (Europa > Ganymede > Callisto) as the spectroscopically inferred surface ice abundances.

Potential for Galileo The Galileo mission will enormously improve our knowledge of the behavior of ice on the Galilean satellites. Imaging at very high resolution may detect and characterize ice segregation directly if it exists and determine its dependence on latitude and terrain type. The NIMS (mapping spectrometer) instrument will also be able to study latitudinal variations in ice distribution on each icy satellite and will have sufficient resolution, for instance, to obtain separate spectra of the bright and dark segregated patches visible in Fig. 3 at high latitudes on Callisto, thus possibly obtaining a " p u r e " spectrum of the dark, nonicy surface component. Characterization of the latitudinal variations in ice distribution should allow identification of the dominant redistribution processes and their effects. The UV stellar occultation technique, used by Voyager to rule out a microbar ox-

309

ygen atmosphere on Ganymede, is sensitive enough for direct detection of the water vapor in equilibrium with warm ice on the Galilean satellites (Spencer, 1982). Stellar occultations observed with the Galileo UVS instrument will thus be useful indirect probes of surface ice temperature and mobility. In addition, the Galileo plasma instruments and dust detector will greatly improve our understanding of the rates and nature of competing ice transport processes. CONCLUSIONS

The main conclusion of this paper is that thermal segregation of water ice on the icy Galilean satellites is a potentially powerful process that may dominate the appearance of the surfaces at subkilometer length scales. Ganymede and Callisto, geologically dead at the scale of the Voyager images, may show features at small scales that reflect equilibrium with a process that operates on a time scale of decades or less. The competing ice redistribution processes of ion sputtering and micrometeorite bombardment are unlikely to prevent segregation except possibly in polar regions. Evidence for segregation includes direct imaging by Voyager at Callistoan high latitudes, the apparent lack of the kilometers of poleward ice migration that would be expected on Ganymede or Callisto if the ice were unsegregated and warm, the preservation of local albedo contrasts at low latitudes on Ganymede and Callisto, and for Ganymede, the pattern of its thermal emission. The reflectance spectra of Ganymede and Callisto may be consistent with segregated surfaces, though Europa's spectrum probably indicates a homogeneous surface.

APPENDIX CALCULATION OF ION SPUTTERING RATES

The Galilean satellites are immersed in Jupiter's large and dynamic magneto-

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JOHN R. SPENCER

sphere, which contains ions derived variously from the solar wind, the Jovian atmosphere, and the surfaces of the satellites themselves. The major heavy ions (Z > 2) are oxygen and sulfur ions derived from the surface of Io. Outside the Io torus, the plasma contains a thermalized population with a kT of 100s of eV, detected by the Voyager PLS experiment (Bagenal and Sullivan, 1981), and a much hotter component with comparable number density and an equivalent kT of 10s of keV, measured by the LECP instrument (McNutt et al., 1981; Krimigis et al., 1981). There is also a highenergy tail with energies in the MeV range. The hot and cold populations differ in composition, satellite impact geometry, and sputtering mechanism, and must be

considered separately. Tables I and II summarize current knowledge of the important characteristics of each component and estimate the ice erosion rates on the surfaces of the icy Galilean satellites due to each. An ice density of 0.92 is assumed to allow comparison with the sublimation rate calculations that make the same assumption. There are numerous uncertainties inherent in these estimates, including the following: (1) The spatial and temporal variability of the plasma is poorly known, as Voyager only provided localized "snapshots." The tabulated densities and fluxes refer to the "plasma sheet" near the Jovian equatorial plane, and densities outside this sheet are reduced by about an order of magnitude (e.g., McNutt et al., 1981). Callisto, and

TABLE 1 ESTIMATION OF SPUTTERING RATES DUE TO L o w - E N E R G Y PLASMA (VOYAGER P L S )

Europa (9.40Rj) Ion species Mass density (amu cm-3)" Composition b Number density (ions cm 3) Corotational energy (eV ion t), Thermal kT (eV) ° Corotational flux (trailing hemisphere; 106 ions cm -2 sec ~t Thermal flux (isotropic; 106 ions cm 2 sec 1) Yield (H20 molecules per ion at corotational energy) ~ H20 yield due to ion corotational flux (106 molecules cm -2 sec 1) H20 yield due to corotational flux from all ion species (106 molecules cm 2 see-i) H20 ice erosion rate (10 -6 mm year -I)

H

O

Ganymede (15.0Rj) S

H

30.9

1260

160

2560

632

130 106

270

61

30

21

6.1

3.0

8

16

0.4

8

2200

4300

430

39d

11

1300

13

O

S

15

5120

192

Generally approx. 100eV, with a few colder regions 270 27 54 54 3.0

6500

66

H

Typically 2 0 % H , 4 0 % 0 , 40%Swheremeasurable 30.9 1.55 3.09 3.09 0.155

39.5

0.3 d

S

150

1500

15.5

O

Callisto (26.3Rj)

0.309 3080

0.309 6160

5.9

5.9

0.61

0.30

0.21

16

0.5

8

16

860

1.5

47

94

2.1

140

1.4

McNutt et al. (1981). b Bagenal and Sullivan (1981). c Wolff and Mendis (1983). d Yield for thermal energy used (as higher than the corotational energy in this case). "Johnson et al. (1984), assuming the yield for S ions is twice that for O ions with the same velocity (R. E. Johnson, personal communication).

311

THERMAL ICE SEGREGATION TABLE II ESTIMATION OF SPUTTERING RATES DUE TO HIGH-ENERGY PLASMA (VOYAGER L E C P ) Europa (9.40Rj) Assumed composition N u m b e r density (ions cm 3)a Corotational energy (eV ion-I) b Minimum energy detectable by Voyager 2 (E~; keV) ~ Corotational flux (trailing hemisphere; 10~ ions cm -2 sec -l) " T h e r m a l " flux (isotropic; kT = El; (106 ions cm -2 sec -~) Yield (H~O molecules per ion with E = E 0 ~ H 2 0 yield due to " t h e r m a l " flux (106 molecules cm -2 sec -I) H20 ice erosion rate (10 6 mm year -~)

H

O

Ganymede (15.0Rj) S

H

3.0

20

40

39.5

0.15

O 0.7

Callisto (26.3Rj)

S 1.4

H 0.02

O 0.1

S 0.2

632

1,260

160

2560

5120

192

3080

6160

28

66

100

28

66

100

28

66

100

27

180

350

2.6

12

24

0.39

1.9

3.9

200

500

880

9.8

18

31

1.3

2.5

4.4

2

30

40

2

30

40

2

30

40

400

15,000

35,000

20

540

1200

2.6

75

180

150

360

4.0

0.21

5.5

12

0.027

0.77

1.8

Krimigis et al. (1981). E~ is used as the energy for all detected ions because of the observed steepness of the energy distribution. An u n k n o w n number of ions lie below the detection threshold of the instrument. b Wolff and Mendis (1983). c Johnson et al. (1984), assuming the yield for S ions is twice that for O ions with the same velocity (R. E. Johnson, personal communication).

possibly Ganymede, spend some of their time outside the sheet. (2) The energy range between a few keV and several 10s ofkeV per ion was not measured by Voyager and ion densities and compositions in this range are not known. The sputtering from ions in this range is not included in the numbers given. Even the width of the unmeasured energy range is not known, as the lower energy limit of the LECP instrument depends on the (unknown) ion composition (see point (3) below). (3) The composition of the ions in the 100-keV range, which may be the most important sputterers, is essentially unknown. Table II therefore sets limits by calculating rates assuming the plasma is composed entirely of H, O, or S ions. The analysis is complicated by the fact that the Voyager LECP instrument, which measures the ions of this energy, responds differently to ions of different composition (Krimigis et al.,

1981) so that both number densities and energies are a function of assumed composition. It can be seen from Table II that the sputtering rate depends drastically on the assumed plasma composition. For both the high- and low-energy ions, fluxes due to both the thermal motions of the ions and their bulk corotation with Jupiter are tabulated in order to indicate the anisotropy of the flux on the satellite surfaces. For the low-energy ions the corotational flux is much greater than the thermal flux, indicating that bulk motion dominates and almost all the plasma impacts the trailing hemispheres of the satellites. The tabulated sputtering rates thus refer to the center of the trailing hemisphere. In the case of the high-energy ions the corotational flux, though always less than the "thermal" flux, is comparable if the plasma is composed mostly of heavy ions, indicating considerable concentration of the flux of even the energetic ions on the

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JOHN R. SPENCER

trailing hemispheres. Calculation of net fluxes as a function of position on satellite surfaces under these conditions is complex and is not attempted here. The tabulated sputtering rate is that due to the "thermal" flux only, and actual rates will be greater in the center of the trailing hemisphere and less in the center of the leading hemisphere. It appears from comparison of Tables I and II that the sputtering rates due to the two plasma components are comparable, depending on the composition of the highenergy component. The values for Europa are in broad agreement with those given in Seiveka and Johnson (1982). ACKNOWLEDGMENTS This work has benefited from consultation with a large fraction of the authors cited in the reference list. Discussions with R. E. Johnson, D. J. Stevenson, R. N. Clark and R. H. Brown (the latter two as reviewers) have been especially helpful and stimulating. The work was supported by NASA Grant NSG-7114 and by the Air Force Office of Scientific Research. REFERENCES ARNOLD, J. R. (1975). Monte Carlo simulation of turnover processes in the lunar regolith. Proc. Lunar Sci. Conf. 6th, 2375-2395. BAGENAL, F., AND J. D. SULLIVAN (1981). Direct plasma measurements in the Io torus and inner magnetosphere of Jupiter. J. Geophys. Res. 86, 84478466. BROADFOOT, A. L., B. R. SANDEL, D. E. SHEMANSKY, J. C. McCONNELL, G. R. SMITH, J. B. HOLBERG, S. K. ATREYA, T. M. DONAHUE, D. F. STROBEE, AND J. L. BERTAUX (1981). Overview of the Voyager ultraviolet spectrometry results through Jupiter encounter. J. Geophys. Res. 86, 8259-8284. BROWN, W. L., L. J. LANZEROTTI, AND R. E. JOHNSON 0982). Fast ion bombardment of ices and its astrophysical implications. Science 218, 525-531. BROWN, W. L., L. J. LANZEROTTI, J. M. POATE, AND W. i . AUGUSTYNIAK (1978). Sputtering of ice by MeV light ions. Phys. Rev. Lett. 40, 1027-1030. BRYSON, C. E., V. CAZCARRA, AND L. L. LEVENSON (1974). Sublimation rates and vapor pressures of H20, CO2, N:O, and Xe. J. Chem. Eng. Data 19, 107-110. BURATTI, B., AND J. VEVERKA(1983). Voyager photometry of Europa. Icarus 55, 93-110. CLARK,R. N. (1980). Ganymede, Europa, Callisto and

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