MR54A-01 INVITED
Cryovolcanism on Titan and Enceladus
Volcanism on an Earth-like rocky planet is the result of partial melting during its thermal evolution. It provides unique information on the internal composition, dynamics and evolution of the planet. For example, the volcanism at mid-ocean ridges and hot spots on Earth is the surface evidence of mantle convection. The partial melt that occurs at depth (a few tens of kilometers) is lighter than the surrounding mantle and the overpressure leads to extrusions of the melt which eventually carries some mantle rocks to the surface. Because the mantle rock is formed of different minerals with different melting temperatures, the partial melting of the mantle leads to the formation of an oceanic crust that has, to first order, the composition of the mineral with the lowest melting temperature. Similarly, cryovolcanism on the icy moons of the outer planets provides unique information on their internal structure and dynamics. The observation of an active plume on the small moon Enceladus is a major discovery of the Cassini mission. The heat necessary to melt and vaporize the icy compounds is provided by the strong tides during Enceladus' eccentric orbit around Saturn. Different models have been invoked in order to explain the 4-8 GW of thermal emission at the South Pole area. Shear heating along the "tiger stripes" has been proposed to explain the energy released in the South Pole area and the cryovolcanism along the faults [Nimmo et al., Nature, 2007]. The presence of a liquid layer between the ice crust and the silicate core also leads to a large production of tidal heating in the ice crust [Tobie et al., Icarus, 2008]. This model also explains the long-lived presence of a liquid layer at depth. The plume is composed of 91% of H2O, 3% of CO2, 4% of N2 , and 2% CH4 [Waite et al., Science, 2008]. This composition suggests interaction between silicates and water with formation of methane by serpentinization of silicates accompanied by the formation of methane at the expense of CO or CO2 [Matson et al., Icarus, 2007]. On Titan, the SAR (Synthetic Aperture Radar) and the VIMS (Visual and Infrared Mapping Spectrometer) images suggest that several morphological features could be formed by cryovolcanic activity [Sotin et al., Nature, 2005; Barnes et al., GRL, 2006; Lopes et al., Icarus, 2007]. Such volcanism would explain the recent release of methane and its presence in the atmosphere where its lifetime is a few tens of millions of years. In order to link thermal evolution models and cryovolcanic models, it is necessary to have laboratory data that describe the melting temperature of different kinds of ices including clathrate hydrates containing ammonia, methane, nitrogen and other volatiles. Since the melting temperature of ammonia is small compared to that of water ice, it is a good candidate for explaining flow features seen on Titan's surface. However, the presence of an ocean at depth makes unlikely the presence of ammonia in the icy crust. More sophisticated models must be found with species such as methane and CO2. This work has been carried out at the JPL, Caltech, under contract with NASA.
MR54A-02
New Laboratory-Based Attenuation Measurements on Ice to Support Tidal Heating Models
The response of icy satellite materials to tidal stress has important consequences on their geophysical, geological, and dynamical evolution. The major issue with modeling the tidal response of these objects is that the viscoelastic properties of planetary material are not constrained by laboratory measurements for the relevant frequency range 10e-7 to 10e-5 Hz. While the Maxwell model is usually applied in icy satellite tidal modeling, laboratory measurements for the Earth's mantle have shown that this model is not applicable at forcing frequencies away from the Maxwell frequency. Alternative models (e.g., Andrade, Cole) based on measurements on silicates or terrestrial ice sheets may be better suited to describe ice attenuation, but they have not been introduced in planetary science studies, in part because laboratory measurements are necessary in order to warrant their extrapolation to conditions applicable to icy satellites. The reason why the laboratory data needed for modeling tidal processes at icy satellites are missing is that it is a challenge to achieve measurements at the low stress, low frequencies, and cryogenic conditions relevant to these objects. In the JPL Ice Physical Properties Laboratory an Instron compression system has been implemented with the capability to measure the phase lag between strain and stress, i.e., the internal friction, of an icy sample at frequencies as low as Enceladus' tidal forcing frequency, temperatures as low as 90 K, and cyclic peak stress lower than 0.1 MPa, characteristic of tidal stress at Enceladus or Europa. We will present the first measurements obtained with this system on monocrystalline ice in the frequency range 6x10e-6 to 10e-2 Hz and temperature range 233 – 253 K. We observed a change in frequency-dependence of the friction coefficient at a frequency about the inverse of the Maxwell time. While the Andrade model can fit the phase lags measured over the entire frequency range, it fails to reproduce the effective moduli measured at frequencies higher than 10e-5 Hz. On the other hand, the model developed by Cole (Philos. Mag. A, 72, 231–248, 1995) can account for both the phase lag and effective moduli data, but we had to determine two different sets of parameters in order to characterize the ice viscoelasticity at frequencies higher than the inverse of the Maxwell time, and lower than this reference. We will also present preliminary measurements on polycrystalline ice. Acknowledgements: This work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Copyright 2008 California Institute of Technology. Government sponsorship acknowledged. Part of this work was also carried out in the Mars and Ice Simulation Laboratory at Caltech.
MR54A-03
Influence of Strain History and Strain Amplitude on the Internal Friction of Ice-I and Two- Phase Ice/Salt Hydrate Aggregates
The dissipation of tidal energy is considered a significant source of heating within many of the icy moons of Jupiter and Saturn. Our experimental study is designed to measure directly the dynamic responses of ice and other cryominerals in periodic loading and so identify and characterize microstructural parameters that influence the magnitude and mechanism(s) of anelastic dissipation. Laboratory-prepared polycrystalline ice and eutectic aggregates of ice/magnesium sulfate hydrate have been tested through a series of low- temperature/ambient-pressure, compression-compression cyclic loading experiments. Temperatures and frequencies were chosen to approach planetary conditions: T = 200 - 260K and f = 0.1 - 0.001 Hz, respectively. In addition to an applied mean stress ~1 MPa, the tests employed a sinusoidally varying stress sufficient to create elastic strains of magnitude ~5x10-6 to 3x10-5. Sample synthesis allowed control of grain-size. Fine-grained ice samples demonstrate a power law relationship between attenuation (QE-1) and frequency, i.e. QE-1 ∝ fm, with m approximately 0.3. Amplitude-dependent attenuation was observed in these samples, such that a 1x10-5 increase in strain amplitude resulted in an increase of QE-1 by 0.25. Measurements of the ice/magnesium sulfate hydrate samples show that the aggregates are more attenuating than pure ice at all frequencies: heterophase boundaries are so identified as significant sites for mechanical dissipation. Additionally, plastic extrusion was used to endow some samples with bulk fabric prior to dynamic testing, allowing exploration of the effect of strain history on attenuation. We will report the dependence of internal friction and dynamic Young's modulus on temperature, composition, strain amplitude, accumulated strain, and grain-size.
MR54A-04
Phase behaviour and thermoelastic properties of ammonia hydrate and ice polymorphs from 0 - 2 GPa
Ammonia remains amongst the most plausible planetary "antifreeze" agents, and its physical properties in hydrate compounds under the appropriate conditions (roughly 0 - 5 GPa, 100 - 300 K) must be known in order for it to be accommodated in planetary models. The pressure melting curve, and the expected polymorphism of the stoichiometric ammonia hydrates have implications for the internal structure of large icy moons like Titan, leading to phase layering and the possible persistence of deep subsurface oceans, the latter being sites of high astrobiological potential. Aqueous ammonia is also a candidate substance involved in cryomagmatism on Titan, and again the melting behaviour, and densities of liquids and solids, in the ammonia-water system must be known to model properly the partial melting and propagation of magma. We describe the results of a series of powder neutron diffraction experiments over the range 0 – 2.0 GPa, 150 - 280 K which were carried out with the objective of determining the phase behaviour and thermoelastic properties of ammonia dihydrate. In addition to the low-pressure cubic crystalline phase, ADH I, we have identified two closely related monoclinic polymorphs of ammonia dihydrate (ADH IIa and IIb) in the range 0.45 - 0.60 GPa (at 175 K), and have determined that this phase dissociates to a mixture of ammonia monohydrate phase II and ice II when warmed to ~190 K, which in turn melts at a binary eutectic at ~196 K; AMH II has a large (Z = 16) orthorhombic unit cell. Above 0.60 GPa, an orthorhombic polymorph of ammonia dihydrate, which we have referred to previously as ADH IV, persists to pressures > 3 GPa, and appears to be the liquidus phase over this whole pressure range. We have observed this phase co- existing with both ice II and ice VI. Here we describe the most plausible synthesis of the high-pressure phase diagram which explains our observations, and provide measurements of the densities, thermal expansion, bulk moduli, and crystal growth kinetics of the high-pressure ammonia dihydrate, ammonia monohydrate and ice polymorphs obtained from our data.
MR54A-05
Formic Acid and Monmorillonite Clay in an Early Earth Environment (<10 GPa, <1000K)
Ambient temperature experiments have shown that silicate minerals such as Na-montmorillonite clay ((Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2 · nH2O) can act as templates for the creation of ordered organic macro-molecules, a mechanism promoted for the origin of replication and chirality of prebiotic molecules. As montmorillonite clay is a weathering product of mafic and intermediate rocks, it was present early in Earth's history, along with organic material delivered by meteorites and interplanetary dust particles, and as such both materials would have been subject to high pressures and temperatures due to burial, subduction or impacts. Infrared spectroscopy of the reaction between montmorillonite and formic acid (HCOOH) in the diamond anvil cell at pressures up to 10 GPa has shown the irreversible formation of peaks not seen in either material. Larger volumes of reacted sample for quantitative analytical study can be created in the piston-cylinder press.
MR54A-06 INVITED
Water, Shallow and Deep, and the Emergence of a Deep Biosphere
In the early Earth, water was a rare commodity except next to the surface where intense bombardment was making life preservation difficult. Hydration of the Earth's interior, which was strongly depleted even in elements much less volatile than water, such as alkaline elements, was postponed until the onset of plate tectonics. It is now well understood that reaction between water and the ferrous components of hot mafic minerals and melts was a major source of hydrogen that early life could eventually use to sustain metabolism and evolve. Due to its rather strong gravity field, the Earth lacked the feldspathic crust present on smaller bodies such as the Moon. On the Moon, such a conductive boundary layer slowed down solidification of the magma ocean. On the Earth, crust was only preserved because surface water reacted with hot and molten silicates to produce buoyant hydrous minerals (serpentine, talc, amphibole), the leftovers of hydrogen generation. Such a hydrous layer was probably very thin (a few km) and vulnerable to impacts. At the surface of a quiescent planet, hydrogen and its byproducts methane and ammonia were quickly removed upon equilibration with H2O, CO2, and N2, gases that also represented a permanent threat to early organisms. The efficiency of the hydrogen factory and the preservation of a deep biosphere were noticeably increased by meteoritic bombardment. Even medium-sized projectiles raise pressure up to a few tens of GPa. Under these conditions, serpentine and the hydrous mafic minerals are transformed into high-pressure hydrous phases such as superhydrous phase B and DHMS D. Buried high-pressure polymorphs of serpentine produced by impacts would rapidly turn unstable and become a significant source of water, and therefore of hydrogen, which may have been critical in the emergence of a deep biosphere.
MR54A-07 INVITED
Abiotic Organic Chemistry of the Terrestrial Deep Subsurface: Isotopic Constraints on Hydrocarbon Formation
In serpentinized terrains in both marine and terrestrial subsurface, recent attention has focused on H2
and hydrocarbon gases – on their potential production by abiogenic processes of water-rock interaction; the
possibility of their use by deep microbial communities as substrates for life; and on the relevance of such
subsurface analogs for the origin of life on earth or elsewhere in the solar system.
In deep subsurface
Precambrian Shield rocks in South Africa, Canada and Finland, H2, methane and higher hydrocarbon
gases have been identified at depths of 1-4 km. While some sites are dominated by gases produced by
microbial methanogenesis, the deepest, most ancient fracture waters with residence times on the order of
tens of millions of years contain hydrocarbon gases with a pattern of carbon isotope depletion in 13C
and hydrogen isotope enrichment in 2H between methane and ethane consistent with abiogenic
polymerization1. More recently, the carbon and hydrogen isotope variation between the higher
hydrocarbon homologues have also been demonstrated to fit a simple mass balance model consistent with
abiogenic polymerization reactions2.
In this study, a series of experiments were performed by heating
aqueous solutions at 250°C and 170Mpa under reducing conditions using powdered native Fe as a
source of H2 and catalyst, and CO as a carbon source in a flexible cell hydrothermal apparatus.
Experiments resulted in rapid generation of methane and higher hydrocarbon products typical of Fischer-
Tropsch abiotic organic synthesis. These gases were analyzed for carbon and hydrogen isotopes to verify
the polymerization model. Unlike the field samples, the experiments showed a carbon isotope enrichment
between methane and ethane – suggesting that the extent of fractionation in the first, most highly
fractionating step may vary as a function of different reaction mechanisms or parameters such as catalysts or
conversion ratios. For the higher hydrocarbons however, carbon isotope values were completely consistent
with the abiogenic polymerization model, as for the field samples. It appears that the rapid rate of chain
polymerization is such that any net isotopic fractionation associated with subsequent carbon addition steps is
negligible and suggests that carbon isotope values for the higher alkane gases may be predicted
independent of the fractionation associated with the first step.
1.Sherwood Lollar et al. (2002) Nature
416:522-524.
2.Sherwood Lollar et al. (2008) GCA doi:10.1016/j.gca.2008.07.004
MR54A-08 INVITED
Experimental Survey of Microbial Survival at High Pressure
The magnitude and onset of lethal pressure effects varies widely even among closely related organisms. This variability complicates the prediction of a species' piezotolerance based on cellular physiology and native stress resistance. In this study several non-piezophilic species were cultured at optimal conditions to both mid log and stationary phases, exposed to elevated pressure for ten minutes, and plated upon return to ambient conditions to determine survival via colony count. The archaeal halophile Halobacterium strain NRC-1 exhibited almost full survival up to pressures of 400 MPa. Model organism Escherichia coli was used to establish a baseline for bacterial organisms but also displayed a bifurcated pressure response, with pressure-sensitive and -tolerant substrains residing within a single population . Pressure exposure proved slightly more lethal to the bacterial halophile Chromohalobacter salexigens than for E. coli up to a critical point of 300 MPa beyond which modest increases in pressure (~ 25 MPa) decreased survival by orders of magnitude. These survival data combined with a comparison of cellular physiology and native stress resistance provide some insight into which aspects of cellular function contribute to high pressure survival.