ICARUS 128, 171–180 (1997)ARTICLE NO. IS975705

The Ammonia–Water System and the Chemical Differentiationof Icy Satellites

D. L. Hogenboom

Department of Physics, Lafayette College, Easton, Pennsylvania 18042

J. S. Kargel

U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001E-mail: jkargel@flagmail.wr.usgs.gov

G. J. Consolmagno

Vatican Observatory, Specola Vaticana, V-00120, Vatican City State

and

T. C. Holden,1 L. Lee, and M. Buyyounouski

Department of Physics, Lafayette College, Easton, Pennsylvania 18042

Received June 6, 1996; revised January 2, 1997

potentially observable aspects of cryovolcanic morphologies,surface compositions, and interior structures of icyWe report the discovery of the first high-pressure poly-satellites. 1997 Academic Pressmorphs of ammonia hydrates: ammonia monohydrate II and

ammonia dihydrate II. The subsolidus transitions and meltingcurves of these substances are shown by their volume–temperature functions; uncalibrated calorimetry corroborates INTRODUCTIONthese phase changes. From 20 to 300 MPa ammonia dihydrateand ice melt at a eutectic to form water-rich liquids; at lower The behavior of ices and their liquids in icy satellitesand higher pressures, ammonia dihydrate melts incongruently often parallels the physical chemistry of Earth’s silicateto ammonia-rich liquids. The new data are consistent with rocks. For instance, insights into the mechanics of terres-independently known thermodynamic parameters of the am- trial mantle rocks at elevated pressures and temperatures,monia–water system. These results fill in an important region

including the origins of deep earthquakes (Kirby et al.of pressure–temperature space not previously studied; a body1991), have been gained by studying ices; and striking simi-of previous data reported by other investigators covers alarities in the rheologies of ammonia–water and silicatecomplementary region (higher pressures), but in the light ofliquids have prompted an application of geological under-the new data those earlier results now appear to have beenstanding of terrestrial volcanism to cryovolcanism on Tri-misinterpreted. We show that a suitable reinterpretation of

the previous data supports the identification of at least one ton and certain other icy satellites (Kargel et al. 1991,high-pressure polymorph of each compound. The behavior Croft et al. 1995). Our data extend analogies between theof the system H2O–NH3 in many ways follows that of MgO– behavior of ices and magnesium-rich silicate rocks to in-SiO2 , and the roles of ammonia–water in icy satellite evolution clude mixtures of water ice with ammonia hydrates.may parallel those of magnesium silicates in Earth’s structure, Chemical thermodynamic models indicate that ammoniavolcanism, and deep mantle tectonism. Pressure-related ef- hydrates may be an abundant nebular condensate alongfects, including a pressure influence on the ammonia content

with water ice and other volatiles in certain circumstancesof cryomagmas, might be significant in determining some(Lewis 1972, Lunine and Stevenson 1982, Prinn and Fegley1989) and the NH3 can be produced hydrothermally inicy satellites’ cores (Shock and McKinnon 1993). Thermal1 Now at Department of Geophysics, Stanford University, Stanford,

CA 94305. transport models of icy satellites indicate that the ammo-

1710019-1035/97 $25.00

Copyright 1997 by Academic PressAll rights of reproduction in any form reserved.

172 HOGENBOOM ET AL.

nia–water solidus (176.16 K at 0.1 MPa) may have been mercury in the magnet pipe. As the magnet moves verti-cally a transducer generates a dc voltage that varies linearlyattained in icy satellites despite their tight energy budgets

and convecting interiors (Lewis 1971, Consolmagno 1975, with the magnet’s displacement.The pressure vessel is placed in a 500-ml dewar (Fig. 1),Consolmagno and Lewis 1978, Ellsworth and Schubert

1983, Federico and Lanciano 1983, Schubert et al. 1986, which is surrounded by liquid N2 . A heater tape aroundthe pressure vessel allows the sample to be warmed. TheSquyres et al. 1988, McKinnon et al. 1995, Grasset and Sotin

1996). For these reasons H2O–NH3 melting has become a temperature of the sample is measured with an accuracyof 60.01 K by a calibrated silicon diode thermometer,staple of icy satellite geologic interpretations (Stevenson

1982, Squyres et al. 1983, Greenberg et al. 1991, Croft and which is mounted in an armored probe extending into thecenter of the sample capsule. Pressure is measured withSoderblom 1991, 1995, Kargel and Pozio 1996). Further

support for H2O–NH3 cryovolcanism comes from rheologi- bourdon tube gauges to an accuracy of 60.14 MPa in therange from 0 to 200 MPa and to 62 MPa from 200 tocal and other physical chemical studies of this system (Kar-

gel et al. 1991, Kargel 1992). Titan, Triton, and Enceladus 400 MPa.Variation in pressure, temperature, or phase can changemay even now have molten H2O–NH3 ‘‘oceans’’ or partly

melted icy mantles and occasional eruptions (Schubert et the sample’s volume and cause 2MB to be pushed into orforced out of the pressure vessel, resulting in displacementsal. 1986, McKinnon et al. 1995, Grasset and Sotin 1996,

Kargel and Pozio 1996). of the magnet and proportional changes in transducer volt-age. The magnet pipe and transducer have been calibratedWe have examined subsolidus and melting relationships

and phase densities in the system H2O–NH3 at pressures to provide accurate determinations of volume changes.The volume changes of isothermal phase transitions thusfrom atmospheric to 400 MPa, which covers the relevant

pressures in most icy satellites except for the deep interiors can be calculated; however, until we perform the necessarycalibrations to determine empirical equations of state forof the biggest satellites (Kargel 1990). Previous work on

high-pressure melting of H2O–NH3 (Johnson and Nicol the 2MB and Teflon, the absolute density of the samplecannot yet be determined on runs that use the sample1987, Croft et al. 1988, Dalrymple et al. 1988, Cynn et

al. 1989, Hogenboom et al. 1989, 1995a, Hogenboom and capsule. Our older experimental configurations did nothave the sample capsule, and thus prevented our investiga-Kargel 1990, Boone and Nicol 1991, Grasset et al. 1995,

Kargel and Hogenboom 1995) has produced conflicting tion of the subsolidus region; but those configurations didallow precise (1 part in 105) and accurate (1 part in 104)results. In part, the conflicts stem from the difficulty in

working with this system (Van Kasteren 1973, Kargel 1992, determinations of the absolute density of H2O–NH3 liquids(Dalrymple et al. 1988, Hogenboom et al. 1989, Hogen-Yarger et al. 1993), but some divergences may be more

interpretive than real (Kargel and Hogenboom 1995). In boom and Kargel 1990).At the start of a typical 3-day data run, an initial pressurethis paper we (1) present major results of a study of phase

densities and equilibria of H2O–NH3 up to 50% NH3 , (2) was generated, a valve was closed to isolate the system,and cooling was initiated. A computer read and recordedcompare and combine our results with those of other re-

searchers, and (3) explore planetological implications of pressure, temperature, and transducer voltage at fixed timeintervals, usually 1 to 3 min, as the sample cooled fromthe new results.well above the melting point to a minimum of p130 Kover a period of hours, and then began to warm slowlyEXPERIMENTAL(0.003 to 0.03 K per minute during phase transitions). Acombination of liquid N2 cooling and a heater current al-A thorough description of a previous version of our

apparatus and applications to the system H2O–MgSO4 was lowed the operator to control the warming or cooling rateand to reverse transitions while observing a real-time graphpresented previously (Hogenboom et al. 1995b). The appa-

ratus used in our most recent studies is shown schematically of the data.Crystallization or glass formation from supercooled liq-in Fig. 1. About 1 ml of the degassed sample is contained

in a thin-walled capsule of FEP Teflon. The capsule’s walls uid occurred after 8–24 hr at 120–160 K. These reactionsinvariably evolved heat and caused a substantial volumeare about 0.25 mm thick, making it easily deformable by

gentle finger pressure. The filled capsule is weighed and change over the course of a few minutes. We believe thesesignals represented crystallization rather than glass forma-then inserted into the No. 316 stainless-steel pressure ves-

sel, and a chilled pressure-transmitting fluid, 2-methylbu- tion because the reactions evolved considerable latentheat, although we did not obtain X-ray diffraction data thattane (2MB), is added so that it surrounds the capsule and

fills the connecting tubing and expansion pipe. The expan- would prove this. In any event, whenever the supercooledmixture gave indications of such a phase change, the sam-sion pipe and a precision bore steel capillary (the magnet

pipe) form a U-tube with mercury separating the 2MB ple subsequently gave volumetric and enthalpic indicationsof melting at an appropriate higher temperature.from the pump fluid. A small alnico magnet floats on the

AMMONIA–WATER SYSTEM AT HIGH PRESSURES 173

FIG. 1. Instrument schematic.

RESULTS Recrystallization and recooling to 128 K produced no sub-solidus phase change, and renewed warming substantially

The H2O–NH3 system at low pressures exhibits five stoi- retraced the previous warming curve and melting transitionchiometric solids, including pure ammonia ice, water ice, of the solid.and three ammonia hydrates (dihydrate, monohydrate, and At 375 MPa, ammonia monohydrate exhibits a differenthemihydrate). We made a series of roughly isobaric runs behavior (Fig. 2b). The initial solid (ammonia monohy-on NH3–H2O liquids containing 10.0, 20.0, 28.6, 31.5, 32.1, drate I) is less dense than the supercooled liquid, but as itand 50.25% NH3; the latter two correspond almost to am- is warmed the solid undergoes a reproducible, endothermicmonia dihydrate and monohydrate. Results of four runs transition to a denser solid (ammonia monohydrate II),on 32.1 and 50.25% NH3 (at two pressures each) are shown which melts reversibly and reproducibly near 199.7 K.in Fig. 2. (Previous abstracts present additional raw data: Figure 2c shows that, at the comparatively low pressureDalrymple et al. 1988, Hogenboom et al. 1989, Hogenboom of 40.4 MPa, ammonia dihydrate I at its melting pointand Kargel 1990.) [177.21 K, compared with 176.16 at 1 atm (Van Kasteren

During most runs the melting points of ammonia mono- 1973)] is denser than its coexisting liquid, as predictedhydrate and dihydrate were corroborated by thermal rever- (Croft et al. 1988). Above 310 MPa (Fig. 2d) the solidsals and partial recrystallization partway through melting. transformed to the denser ammonia dihydrate II, whichThe melting curves in Fig. 2 are not quite thermally invari- melts incongruently at a peritectic to a more dense liquidant because melting produces a change in sample volume and a more dense solid (presumably ice II), which thenand hence a small pressure change and a shift in the melting melts completely at a slightly higher temperature.point. At low pressures we found that ammonia monohy- Phase diagrams for ammonia dihydrate and ammoniadrate and dihydrate melt at the expected temperatures monohydrate are shown in Figs. 3 and 4. The melting(Hildenbrand and Giaque 1953, Van Kasteren 1973) and curve of ammonia dihydrate I (Fig. 3a) determined fromwith volume changes of the expected magnitudes and sign Lafayette data alone is given by(Bertie and Shehata 1964, Croft et al. 1988).

In the case of ammonia monohydrate I at the low pres- Tm 5 176.29 1 0.0243P 2 7.7583 3 1025P2(60.18 K), (1)sure of 8 MPa (Fig. 2a), the liquid supercooled to p158 Kand then crystallized. The initial solid was slightly more

that for ammonia monohydrate I (Fig. 4a) is given bydense (the thermal expansivity much less) than that of thesupercooled liquid (as predicted, Croft et al. 1988). Melting

Tm 5 194.05 1 0.04326P 2 0.000191974P2

(2)was reversible and occurred at 194.22 6 0.06 K, comparedto 194.15 K at 1 atm (Hildenbrand and Giaque 1953). 1 4.4794 3 1027P3 2 4.3266P4(60.19 K),

174 HOGENBOOM ET AL.

FIG. 2. Experimental runs for the compositions NH3 5 50.25 mass% (a,b) and 32.1% NH3 (c,d). Voltage is almost proportional to samplevolume. Annotations give our interpretations of key events. (a) 50.25% NH3 , 8 MPa. (b) 50.25% NH3 , 375 MPa. (c) 32.1% NH3 , 40.4 MPa. (d)32.1% NH3 , 330 MPa.

and that for ammonia monohydrate II (Fig. 4a) is given by 300 MPa, 177 K (Fig. 3). The comparable triple point ofammonia monohydrate is near 198 K, 330 MPa (Fig. 4).

Tm 5 184.22 1 0.04229P(60.04 K), (3) The relative volume changes during melting of the 32.1%NH3 and 50.25% NH3 solids (Fig. 5) start out at low pres-sure within the ranges allowed by published data; namely,where Tm is in Kelvin and the pressure, P, is in megapascals.

The incongruent melting curve for ammonia dihydrate II/ the solid is slightly denser than the liquid when near theice II 1 liquid shown in Fig. 3b is poorly defined by our melting point (Croft et al. 1988). The data for both sub-data. The melting curve for ammonia dihydrate I gives a stances show that the liquid is more compressible than itsslope dP/dT 5 41 MPa/K at the limit of zero pressure, solid (resulting in concave-down melting curves shown inwhich compares well with the independent calculation of Figs. 3 and 4); volume changes during melting vanish atthe Clapeyron slope, about 160 MPa (for ammonia dihydrate) and 280 MPa (for

ammonia monohydrate) and become negative at higherpressures, thermodynamically consistent with their meltingdP/dT 5 DH/T0 DV 5 38.5 MPa/K, (4)curves (Figs. 3 and 4).

Besides the ammonia monohydrate and dihydrate com-where DH is the enthalpy of fusion of ammonia dihydrate,6999 J/mole (Chan and Giaque 1964); T0 is the melting positions (run with the use of flexible sample capsules),

we also ran samples containing 31.5, 28.6, 20.0, and 10.0%point at 0.1 MPa, 176.16 K (Van Kasteren 1973); and DVis the volume change during melting, determined from our NH3 . These more H2O-rich mixtures were run without

use of a flexible capsule, so we could not investigate thedata to be 1.03 3 1026 m3/mole.Three two-phase coexistence curves for ammonia dihy- subsolidus region, although melting points and liquid den-

sities were obtained.drate (two solids and liquid) intersect at a triple point near

AMMONIA–WATER SYSTEM AT HIGH PRESSURES 175

atm (a peritectic) to about 28.5% NH3 at 200 MPa (aeutectic). Above 200 MPa, the water ice phase changes toice II, and the eutectic composition starts to shift backto more NH3-rich compositions. Above p300 MPa theeutectic seems to have shifted so far that ammonia dihy-drate once again starts to melt incongruently, thus produc-ing liquids with .32.1% NH3 and residual water ice II.Around the same pressure, ammonia dihydrate I under-goes a transition to ammonia dihydrate II.

RECONCILIATION OF UCLA AND LAFAYETTE DATA

The UCLA group (Johnson and Nicol 1987, Cynn et al.1989, Boone and Nicol 1991) could make optical observa-

FIG. 3. Phase diagram of ammonia dihydrate. (a) Phase diagramconstructed from Lafayette data; phase boundaries with high-pressurephases are not well determined and were chosen to be consistent withreinterpretations of UCLA data. The dashed line gives the Clapeyronslope at the limit of zero pressure, dT/dP, of the melting curve based onindependently known thermodynamic data. (b) Our reinterpreted phaseboundaries and data from the Lafayette and UCLA investigations. Mea-surement errors of Lafayette data are smaller than the symbols.

Figure 6 is an interpretive synthesis of our melting-pointdeterminations. The familiar low-pressure H2O–NH3 peri-tectic disappears near 20 MPa due to the pressure-induceddepression of the water ice liquidus and the pressure eleva-tion of the ammonia dihydrate liquidus. Above this pres-

FIG. 4. Phase diagram of ammonia monohydrate based on (a) La-sure, ammonia dihydrate I melts at a eutectic with water fayette data and (b) the combined Lafayette and UCLA data. Measure-ice. The composition of the ammonia dihydrate I melting ment errors of Lafayette data are smaller than the symbols or are shown

by error bars.point shifts steadily with pressure from 32.1% NH3 at 1

176 HOGENBOOM ET AL.

Nicol (1991) of the ammonia dihydrate melting-pressurecurve at the limit of zero pressure (obtained by extrapolat-ing high-pressure data) is inconsistent with known thermo-dynamic parameters of the system (Croft et al. 1988, con-firmed here); this problem cannot be resolved by taking adifferent extrapolation. On the other hand, as noted above,our melting curves for the monohydrate and dihydratephases are thermodynamically consistent with the Cla-peyron slope and indicate, as expected, that the liquidsare more compressible than their solids (Fig. 3a). Besideshaving an unlikely slope at the limit of zero pressure, thenear linearity of the UCLA melting curve (as interpretedby them) for ammonia dihydrate at elevated pressuresimplies unlikely small differences between the liquid’s andsolid’s compressibilities.

The slope at zero pressure of UCLA’s melting curve ofammonia monohydrate (Boone and Nicol 1991), which isalmost the same as the slope determined by us, does notviolate the thermodynamic constraints (Croft et al. 1988);however, the nearly linear UCLA melting curve would im-ply little or no difference in the compressibility of the solidand liquid, whereas the Lafayette data adhere to the generalrule that a liquid is more compressible than its solid.

As noted above, the UCLA and Lafayette data covercomplementary pressure ranges. A thermodynamic andobservational reconciliation of the two laboratories’ datawould present a substantial advance in our knowledge ofthe ammonia–water system. The UCLA group’s recentwork (Cynn et al. 1989, Boone and Nicol 1991) has pro-

FIG. 5. Relative volume changes during melting of (a) the 32.1% duced a large, self-consistent set of observations from nu-NH3 solid (ammonia dihydrate) and (b) the 50.25% NH3 solid (mostly merous experimental runs, which would not be expectedammonia monohydrate). Measurement errors are smaller than the if there remained serious contamination and compositionalsymbols.

uncertainty. Thus, we consider it highly probable that themelting points and phase regions outlined by the UCLAinvestigations are essentially accurate but that they mayhave misidentified some of the solid phases. Specifically,tions of high-pressure assemblages, something we cannot

do. Unlike Lafayette’s capabilities, the UCLA group had our analysis suggests that the UCLA group may have en-countered high-pressure polymorphs of the ammonia hy-no way to measure volumes or an easy means to observe

or reverse phase transitions while they were occurring. The drates without recognizing them.The combined Lafayette and UCLA data (Figs. 3b, 4b)UCLA apparatus is well suited for studies from several

hundred to .5000 MPa (with large uncertainties in pres- can be fit with a high-pressure solid and the familiar low-pressure phase of each ammonia hydrate. The UCLA datasure), whereas the Lafayette apparatus is limited to studies

below 400 MPa (with precise determinations of pressure, could admit but do not seem to require multiple high-pressure phases. In so fitting the data, the reinterpretedtemperature, and volume).

Ideally, the Lafayette and UCLA investigations would UCLA data no longer violate thermodynamic propertiesof the ammonia–water system, and the curvatures of thebe complementary; however, these groups’ latest interpre-

tations of their data are discrepant. The UCLA group pressure-melting curves imply liquids that are more com-pressible than the solids, as desired. We conclude solelydid not see evidence for the existence of high-pressure

polymorphs of ammonia dihydrate and monohydrate, on the basis of Lafayette data that ammonia monohydrateand ammonia dihydrate each possess at least one high-whereas we see very clear and thermodynamically plausi-

ble evidence for them. This discrepancy was disconcerting, pressure polymorph. We conclude that the UCLA resultssupport our interpretation and also point the way to futurebecause the UCLA apparatus provided a capability to

make optical observations of the solid and liquid phases laboratory work that could explore the possibility of addi-tional high-pressure phases.that were encountered. The slope given by Boone and

AMMONIA–WATER SYSTEM AT HIGH PRESSURES 177

FIG. 6. Phase diagram of the ammonia–water system for isobaric slices of 0.1, 100, 200, and 300 MPa. Inset shows details of the systemMgO–SiO2 for pressures of 0.1 MPa and 1.2 GPa. (after Chen and Presnall (1975)). In the inset, Fo 5 forsterite, En 5 protoenstatite, and Si 5

silica solid phase (cristoballite or high-quartz). A transition from incongruent melting to congruent melting occurs near 20 MPa in the systemNH3–H2O and near 200 MPa in MgO–SiO2 .

Other previous data which bear on the existence of high- assumption that the ammonia–water system (plus inertrock) is in some cases a better approximation than a satel-pressure polymorphs of ammonia hydrates include the X-

ray diffraction studies of Durham et al. (1993). In a study lite model made of pure ice and rock. What follows are afew possible implications of our work for processes anddevoted primarily to ammonia–water ice rheology, those

researchers discerned the existence of the familiar low- structures of icy moons; what we state at the outset butignore below is that reality is no doubt even more complex.pressure polymorphs of ammonia mono- and dihydrate at

Our data confirm the result of Croft et al. (1988) that thepressures from 50 to 100 MPa, i.e., well below the subsoli-NH3–H2O peritectic liquid has a density at low pressuresdus transition pressures we have found.(p0.946 g cm23 at 0.1 MPa) that is less than the density ofsolid ammonia dihydrate I (p0.965 g cm23) but greaterDISCUSSIONthan that of water ice I (0.917 g cm23). Hence, NH3–H2O

Density Controls on Icy Satellite Geological and liquid can be extruded from a rock-rich icy mantle andStructural Evolution through a thick ammonia hydrate crust, but it cannot be

extruded readily through a thick mantle or crust of waterThere are some rather strong theoretical hints that icyice I. These density relations extend to 160 MPa, abovesatellites are complex chemically, leading one to wonderwhich the liquid is denser than ammonia dihydrate I upabout the accuracy of existing structural evolution modelsto p300 MPa, where a transition to ammonia dihydrate IIand geological concepts of these objects (Lewis 1972, Prinnoccurs. A eutectic ammonia–water ocean is possible onand Fegley 1989, Kargel 1991, 1992, Shock and McKinnonicy satellites at pressures between 160 and 300 MPa.1993). A satellite made of pure water ice and chemically

The density difference between ammonia dihydrate andinert rock is a very special case. But does this type of icythe eutectic liquid is never very large, and near 160 MPamoon exist at all in this Solar System? Is the simplifiedthe difference vanishes, so that equilibrium crystallizationchemical system explored here, pure ammonia–water, re-of NH3–H2O plutons is favored over fractional crystalliza-ally very much better in describing the geological processes

and interior structures of icy moons? We hesitate to answer tion (see Fig. 16 of Hogenboom et al. 1995b). These smalldensity differences and the low gravity of icy satellitesthese questions, but instead prefer to make the plausible

178 HOGENBOOM ET AL.

suggest that segregation of melts from icy rock-free solids ences in physical properties that could have observablegeological manifestations (Kargel et al. 1991, Kargel 1992).generally should occur at high degrees of partial melting,

thus tending to result in a small number of massive cryovol- The specific volumes of solid and liquid phases in the am-monia–water system are rather similar (Croft et al. 1988,canic flows or intrusions, perhaps as seen on Ariel and

Miranda (Croft and Soderblom 1991, Greenberg et al. this work), so subtle pressure-related composition and den-sity differences could substantially impact the relative1991). The small density differences between ammonia–

water solids and liquids, however, increase the possibility buoyancy of the various phases and may influence crustaland mantle differentiation of ammonia-rich icy satellites.that small amounts of rocky contaminants could alter the

magnitude or sign of density relations and the course of Rheological variations among eutectoid liquids formed atvarious pressures also are small, amounting to about ageological evolution of an icy satellite.

The viscosity of ammonia–water liquids (Kargel et al. factor of 2 or somewhat more if crystallization effects areconsidered (Kargel et al. 1991), but as described below1991) also reduces chances for ammonia–water plutons to

fractionate gravitationally as they solidify and thereby pro- these differences could affect volcanic landforms.duce chemically layered bodies analogous to Earth’s mas-

Rheological Controls of Volcano Morphologysive layered igneous intrusions. Gravitationally drivenchemical stratification by fractional crystallization of am- The correlation of silicate volcano morphologies withmonia–water oceans cannot, however, be ruled out because lava types ranging from basalt (yielding shield volcanoesof the long time available for cooling and crystal settling. for instance) to dacite and rhyolite (yielding bulbous lava

Among the most important unresolved problems in the domes and stratovolcanoes) is well established in geology.geology of the icy moons is the origin of global extensional The principal cause of these morphologic variations is anfeatures on some but not all of these moons; these fractures eight-order-of-magnitude range in viscosity among com-indicate a net increase of surface area of several percent. mon silicate lavas. This rheological control is manifestedWe can now completely rule out earlier speculation (Con- directly in terms of deformation characteristics of lavasolmagno 1985) that these extensional features could be flows and indirectly through influence on the retention ofcaused by the melting and refreezing of NH3-rich liquids; volatiles by magmas and consequent effects on eruptionit is now clear that the volume change is too small to style. Rheological differences caused by more subtle com-account for the inferred expansion. Phase changes of H2O positional variations also may have significant effects onremain as expansion candidates but they raise other prob- volcano and lava flow morphologies; for instance, a factorlems (Consolmagno 1985, Squyres and Croft 1986). Thus, of 2 or 3 difference in the viscosity (and concordant varia-this question remains unresolved. Another possible cause tions in volatile abundances) between alkali basalt andof global volume changes and tectonic stress raised by tholeiite basalt can affect the propensity of lava flows tothe new data is that of subsolidus phase transitions of form lava tubes; to erupt explosively or effusively; and toammonia hydrates. form shield volcanoes, lava plains, or cinder cones. Funda-

The disappearance of the peritectic at 20 MPa is at a mentally, these correlated variations in composition, rheol-pressure reached inside all the principal icy satellites except ogy, and morphology in a given setting (e.g., Hawaii) canMimas, Enceladus, and Miranda (Kargel 1990); however, be ascribed to differences in the pressure and fractionalthe high-pressure phases of the ammonia hydrates occur degree of partial melting in the mantle.only above 300 MPa, i.e., pressures attained in the deep Among icy satellites, Triton is remarkable for its diver-interiors of Ganymede, Callisto, Titan, Triton, and Pluto, sity of cryovolcanic terrains, including one landscape wherebut not in other icy moons (Kargel 1990). If Triton and large volcanic craters and possible collapsed lava tubesPluto have a dense rock/iron core, the icy mantle above are prevalent (Fig. 8 of Kargel 1995), another landscapetheir cores would not include the high-pressure poly- dominated by what appear to be pitted cinder cones ormorphs. It is likely that high-pressure ammonia hydrates small stratovolcanoes (Fig. 2 of Croft et al. 1995), and yetserved important roles in the evolution of Titan’s interior other landscapes marked by huge caldera-like structuresand atmosphere and possibly in the evolution of Ganymede partly filled by smooth plains (Fig. 3 of Croft et al. 1995).and Callisto if they are ammonia-rich. Possible gravita- We cannot determine which of many possible factors aretional layering in both high-pressure water ice and ammo- responsible for the differences among cryovolcanic land-nia hydrate phases should be considered in models of the scapes, but pressure influences on the ammonia–water orgeological evolution of large icy moons and in drawing other cryogenic aqueous systems could be involved.structural interpretations from spacecraft-tracking gravity

Comparison of Melting in the Systems NH3–H2Odata (Schubert et al. 1994).and MgO–SiO2The eutectoid liquids formed by melting at various pres-

sures (e.g., ranging from 32.1% NH3 at 0.1 MPa to 28.6% The NH3–H2O and MgO–SiO2 systems are comparablein many respects. The latter system has a central bearingat 2000 MPa) have small but potentially influential differ-

AMMONIA–WATER SYSTEM AT HIGH PRESSURES 179

sures: Melting curves of ammonia dihydrate and ammonia monohy-on the mantle structures (McKenzie et al. 1983) and maficdrate and a revised high-pressure phase diagram for the water-richsilicate petrology of rocky planets (Bowen and Andersenregion. Proc. Lunar Planet. Sci. Conf. 21, 603–612.1914, Cox et al. 1979), whereas the former is thought to

Bowen, N. L., and O. Anderson 1914. The system MgO–SiO2 . Am. J.pertain to the structure and petrology of some icy satellites. Sci. 4th Ser. 37, 487–500.The similar behavior of these two chemical systems offers Chan, J. P., and W. F. Giaque 1964. The entropy of NH3 ? 2H2O and heatinsights into the nascent field of icy-satellite cryopetrology. capacity from 15 to 300 K. Phys. Chem. 68, 3053–3057 (Erratum, 3912).Water ice in NH3–H2O takes the role of forsterite in repre- Chen, C.-H., and D. C. Presnall 1975. The system Mg2SiO4–SiO2 atsenting the dominant refractory mantle phase; ammonia pressures up to 25 kilobars. Am. Mineral. 60, 398–406.dihydrate takes the role of enstatite; ammonia monohy- Consolmagno, G. J. 1975. Thermal History Models of Icy Satellites, Mas-

ter’s thesis, Massachusetts Institute of Technology.drate takes the role of silica (present only in crustal rocks).Consolmagno, G. J. 1985. Resurfacing Saturn’s satellites: Models of partialBoth systems exhibit high-pressure polymorphs of the prin-

differentiation and expansion. Icarus 64, 401–413.cipal solids and low-pressure incongruent melting and high-Consolmagno, G. J., and J. S. Lewis 1978. The evolution of icy satellitepressure congruent melting of the intermediate composi-

interiors and surfaces. Icarus 34, 280–293.tion solid. In both systems application of pressure causesCox, K. G., J. D. Bell, and R. J. Pankhurst 1979.The Interpretation ofa shift of eutectic melt compositions toward the more re-

Igneous Rocks. George Allen & Unwin, London.fractory phase (Fig. 6).

Croft, S. K., and L. A. Soderblom 1991. Geology of the uranian satellites.The system MgO–SiO2 holds a key to the origin of In Uranus (J. T. Bergstralh, E. D., Miner, and M. S. Matthews, Eds.

Earth’s silica-undersaturated and silica-saturated lavas, pp. 561–628. Univ. of Arizona Press, Tucson.which have fascinated petrologists since Bowen (Bowen Croft, S. K., J. I. Lunine, and J. S. Kargel 1988. Equations of stateand Andersen 1914, Cox et al. 1979, Morse 1981). By exten- of ammonia–water liquid: Derivation and planetological applications.

Icarus 73, 279–293.sion, we predict that NH3-rich icy satellites may have am-Croft, S. K., J. S. Kargel, R. L. Kirk, J. M. Moore, P. M. Schenk, andmonia monohydrate-saturated lavas formed either at high

R. G. Strom 1995. The geology of Triton. In Neptune and Triton. (D. P.pressures (.300 MPa, such as deep in Titan, Kargel 1990)Cruikshank, Ed.). pp. 879–948. Univ. of Arizona Press, Tucson.or very low pressures (,20 MPa, as throughout Enceladus)

Cynn, H.-C., S. Boone, A. Koumvakalis, M. Nicol, and D. J. Stevensonand water-ice-saturated lavas formed at intermediate pres-1989. Phase diagram for ammonia–water mixtures at high pressure:

sures (20–300 MPa, as throughout Triton’s icy mantle). Implications for icy satellites. Proc. Lunar Planet. Sci. Conf. 19,The composition of ammonia–water lava flows sampled 433–441.by future landed spacecraft could indicate the pressure and Dalrymple, W., III, D. L. Hogenboom, and G. J. Consolmagno 1988. The

density of ammonia–water solution to 400 MPa (4 kilobars) (abstract).depth of origin of the cryomagmas much as the degree ofProc. Lunar Planet. Sci. Conf. 19, 241–242.silica saturation of mafic silicate lavas on Earth is used to

Durham, W. B., S. H. Kirby, and L. A. Stern 1993. Flow of ices in theconstrain the depth of partial melting.ammonia–water system, J. Geophys. Res. 98, 17,667–17,682.

Ellsworth, K., and G. Schubert 1983. Saturn’s icy satellites: Thermal andCONCLUSIONSstructural models. Icarus 54, 490–510.

Federico, C., and P. Lanciano 1983. Thermal and structural evolution ofThis work has filled in some additional details of thefour satellites of Saturn. Ann. Geophys. 1, 469–476.melting behavior and phase stabilities in the pure system

Grasset, O., and C. Sotin 1996. The cooling rate of a liquid shell in Titan’sNH3–H2O between 0.1 and 400 MPa. In particular, weinterior. Icarus 123, 101–112.

have documented the existence of one high-pressure solidGrasset, O., S. Beauchesne, and C. Sotin 1995. Investigation of the NH3for ammonia dihydrate and another for ammonia monohy- (15%)–H2O (85%) phase diagram close to the liquids in the range

drate. It remains for future work to extend data to higher 100–1500 MPa using in-situ Raman spectroscopy: Application to Ti-tan’s dynamics. In Solar System Ices: An International Symposium,pressures; to include additional chemical components such27–30 March 1995, Toulouse, France, p. 49 (Abstract).as methane and methanol; to assess in quantitative detail

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