Astrochemistry: Discovery of Novel Forms of Water in Uranus and Neptune

1. Astrochemistry: Discovery of Novel Forms of Water in Uranus and Neptune Nir Goldman Lawrence Livermore National Laboratory March 8th, 2006 This work was performed under the auspices of the U. S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. Experiments: Alex Goncharov, Jonathan Crowhurst, Joe Zaug Theory: Chris Mundy, Will Kuo, Larry Fried (PI)

2. Uranus Neptune Uranus and Neptune have similar properties

3. Interior “hot ice”: • 56% H2O • 36% CH4 • 8% NH3 • T > 1000 K, P > 100 GPa • (1 Gigapascal ≈ 104 atmospheres) Voyager II data provides indirect insight into planetary interiors • Voyager II spacecraft data shows Uranus and Neptune have strong magnetic fields • Due to unique forms of water in interior? Novel lattice phases? • Estimate of interior composition is based on the density profile, and assumed chemistry and Equation of State models Equation of State models provide P-T profiles, and possible states of water

4. H2O dissociation could yield high magnetic field • Conductivity of matter inside planet crudely characterizes the flow that produces the planetary magnetic field • Water inside Uranus and Neptune could have high conductivity • Maybe caused by complete molecular ionization: H2O 2H+ + O2- • Exotic phase: Superionic water? Predictions about planetary interiors rely upon accurate Equation of State modeling (EOS)

5. Equation of state models yield very diverse results for H2O at extreme conditions • EOS models relate pressure and temperature to chemical composition • Accuracy is heavily dependent on initial guesses of chemical products • Requires inputs from theory and experiment Great need for description of interior chemistry in order to derive models consistent with observational data

6. P, T, time - diagram Uncharted territory: (P >10 Kbar, T > 1000 K, t < 1 ms) Chemistry under extreme thermodynamic conditions is not well understood • Major Issues: • Rapid bond dissociation • molecular to non-molecular transition • Covalent vs. ionic bonding??? • Novel states of matter: • Metallization of H2, N2 • Chau et al., PRL (2003) • Galli et al., Nature (2005) Experimental/theoretical challenges involve attaining/modeling this extreme P-T regime

7. Gas guns and Diamond Anvil Cells are used to achieve extreme conditions Diamond Anvil Cell Gas gun Description Advantages Disadvantages We have to rely on computations to determine the atomic structure and dynamics

8. Molecular Dynamics simulations (MD) can provide key answers • Calculate molecular trajectories via Newtonian mechanics: • MD recreates system on computer as close to nature as possible • Underlying physics is very simple. However: • Computationally, MD can be very difficult • Real challenge is in coming up with decent Potential Energy Surface [model; V(rN)] Tools from Statistical Mechanics allow us to connect simulation to experiments

9. Example of MD – simulation of ambient liquid water • Historically, MD simulations could not accurately depict bond breaking • Ab initio modeling = explicit modeling of electronic ground state required • Computers were not fast enough for high levels of theory Faster computers and more efficient theory will allow issue of superionic water to be resolved for the first time

10. Ab initio MD provides structural and dynamic info about “extreme water” • Car-Parrinello Molecular Dynamics (CPMD) ab initio MD software • Explicit modeling of electrons and nuclei (few empirical equations) • Density Functional Theory (DFT) based MD, using a plane-wave basis set • Use larger system size and much larger basis set: • 54 H2O, 120 Ry (vs. 32 H2O, 70 Ry) We will provide chemical insight into experiments on hot, compressed water

11. CPMD computational details • CPMD is about 150,000 lines of F90. • The computational engine is the 3-D FFT parallelized using both MPI and OpenMP directives to take advantage of SMP nodes. • CPMD achieves 65% parallel speed up for 1,920 CPUs (960 nodes) • LLNL’s Thunder: • Linux cluster, Itanium2 processors (1.4 GHz) • 1024 nodes, 4 procs/node • Peak performance: 22.9 TFlops/s • Currently #11 on Top500 list • (#1 once upon a time)

12. Even small systems (100’s of atoms) require LLNL’s supercomputers • 2.0 g/cc, 34 GPa, 2000K: • Real space mesh = 126 processors needed • UV (Power5): 16,000 Hours • Thunder (Itanium2/Linux): 32,000 • MCR (Xeon/Linux): 40,000 • Need 6 – 8 densities for each temperature • 500,000+ CPU hours total An entire supercomputer is filled with many smaller jobs instead of a single gigantic one

13. Does superionic water exist and what is it exactly? • Validate theory via experiments: • Calculate diffusion constants of oxygen and hydrogen and vibrational spectra • Create a simple chemical picture of superionic water: • Observe structure via radial distribution functions • Calculate species concentrations and lifetimes

14. Our starting point: calculated H2O phase diagram • Constant pressure-temperature simulations with Carr-Parrinello molecular dynamics (CPMD) • Small system size: 32 H2O • P=30-300 GPa, T= 300-7000K • “Superionic” phase has oxygen bcc sublattice, mobile protons • DH (2000K, 30 GPa) = 1.8 x 10-3 cm2/s • Uranus, Neptune: 56% H2O, 36% CH4, 8% NH3 • “hot ice” mixture contributes to magnetic field measurements by Voyager 2 spacecraft • Due to high ionic conductivity from completely ionized H2O Cavazzoni, et al., Science, 283, 44, 1999.

15. Cavazzoni, et al., Science, 283, 44, 1999. Structure of Superionic water • Superionic Solids: exhibit exceptionally high ionic conductivity • “partial melting” – one ion diffuses through crystalline lattice of remaining types • some famous examples: PbF2, AgI. • Originally thought to be uncommon in hydrogen-bonding compounds

16. Chau et al., JCP, 114, 1361 (2001) 2. DISSOCIATION: OH- + H3O+ • Experimental results show high pressure yields high ionic conductivity • Due to H2O 2H+ + O2- Somewhat contradictory pieces of data about superionic water 1. HIGH IONIC CONDUCTIVITY Schwegler et al., Phys. Rev. Lett., 87, 265501 (2000) vs. 23 GPa; 1390 K • CPMD results with 54 H2O do not show mobile protons or oxygen lattice. • Absence of lattice confirmed by X-ray data: Frank et al., Geochim. et Cosmochim. Acta, 68, 2781, 2003.

17. Water at High Pressure and Temperature • Our simulations – much bigger than before. • - 1000K – 2000K • - 1.5 g/cc to 3.0 g/cc • - Pressures from 15 to 115 GPa Compressing a “liquid” configuration Heating an “Ice VII” configuration Cavazzoni et al., Science, 283, 44, 1999. • We have determined a more accurate phase boundary of superionic water • We have devised a simple chemical picture for this phase. • Fundamental question: • -How can we define a molecule at these conditions?

18. = Oxygen Water at room temperature and pressure = Hydrogen 3.0 g/cc, 115 GPa, 2000K Superionic network solid with transient bonds 2.0 g/cc, 34 GPa, 2000K Liquid of transient molecules Simulations show dramatic changes in the structure of water with increasing pressure

19. The diffusion constant is calculated from the Einstein-Smoluchowski relation

20. Compressing the liquid Heating ice VII Oxygen freezing point Perform many simulations over several isotherms Compressing the liquid Heating ice VII Oxygen and hydrogen diffuse on two different time scales 2000 K Hydrogen Oxygen We determine the superionic phase boundary from the oxygen freezing point as a function of temperature

21. Laser Heating Probe Gasket Sample Al2O3 plates Coupler Abrupt changes in the Vibrational spectra allow us to determine phase boundaries Melting curve at high pressure and temperature was determined via the changing phonon mode Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug, PRL, 2005

22. Static experiments CPMD simulations Density of states Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug, PRL, 2005 Experiment and simulations show weakening of the O-H bond in liquid water

23. We have redefined the phase diagram of water at extreme conditions Phase diagram of water • Melting line is in agreement with externally heated DAC data • Triple point at 47 GPa and 1020 K, significantly higher than Parrinello (25 GPa) • Transition to a superionic phase is inferred from a combination of experiments and simulations Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug, PRL, 2005 Goldman, Fried, Kuo, Mundy PRL (2005) Simulations can also provide a chemical picture of superionic water

24. Statistical Mechanical Analysis • Validate theory via experiments: • Calculate diffusion constants of oxygen and hydrogen • Create a simple chemical picture of superionic water: • Focus on results at 2000 K (particularly unique) • Observe structure via radial distribution functions • Calculate species concentrations and lifetimes

25. Radial distribution function (RDF) yields structure, g(R) Probability of finding any two particles in the config. (r1,r2) Investigate pairs of OO, OH, HH

26. Oxygen-Oxygen structure (RDF), 2000 K Average out vibrations: bcc lattice, like ice VII, ice X 3.0 g/cc, 115 GPa 2.6 g/cc, 75 GPa 2.0 g/cc, 34 GPa

27. 1.30 Å 1.70 Å Oxygen-Hydrogen RDF Intra-molecular Inter-molecular

28. ra=1.70 Å ra=1.30 Å We use the ROH free energy surface to define molecules 2000 K 115 GPa The O-H free energy barrier decreases dramatically with pressure. 75 GPa 34 GPa

29. We have a simple picture for proton mobility 1-D free energy surface shows pronounced drop in dissociation barrier

30. ra=1.70 Å ra=1.30 Å We use the ROH free energy surface to define molecules 2000 K 115 GPa 75 GPa 34 GPa

31. Almost entirely H2O Superionic Concentrations and lifetimes at 2000 K(<10 fs = non-molecular) H2O, H3O+, OH- • 34 GPa (2.0 g/cc): • H2O lifetime = ~40 fs • H3O+, OH- = < 10 fs “polymer” • 75 GPa (2.6 g/cc): • All species lifetimes = 10 fs or less Neutral and ionic (H2O)2 – (H2O)6 The “polymer” species consists of very short-lived networks of bonds

32. Network solid is partially covalent at 95 – 115 GPa • Non-molecular (based on lifetimes) • At 2000K, 115 GPa, 50% covalent bonding • Tetrahedrally coordinated oxygen • Analog to ice X: symmetric H-bonding Goldman, et al., Phys. Rev. Lett., 94, 217801 (2005).

33. Ice X: 100-1000 K, • 100’s of GPa • H bisects O—O axis Network solid is partially covalent at 95 – 115 GPa Goldman, et al., Phys. Rev. Lett., 94, 217801 (2005).

34. 3. 2. 1. • Liquid, highly reactive H2O (34 – 58 GPa) 2. Superionic phase with asymmetric H-bonding (75 GPa) 3. Superionic phase with symmetric H-bonding (95 – 115 GPa)

35. Discussion • Hydrogen diffusion rates can be extremely rapid over disordered, mobile oxygen phase • Superionic phase occurs at higher pressure than previously predicted • 75 GPa at 2000K (Cavazzoni et al.: 30 GPa) • Superionic water is best understood as transient partially covalent bonds which form networks • Ensemble of transition states • Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug,Phys. Rev. Lett., 94, 125508 (2005). • Goldman, Fried, Kuo, Mundy, Phys. Rev. Lett., 94, 217801 (2005).

36. Discussion • Planetary implications • High water ionic conductivity can happen in absence of superionic phase • Water could be the source of the large magnetic fields in Uranus and Neptune • How does water behave in presence of CH4, NH3? • What simple rules govern superionic behavior? • Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug,Phys. Rev. Lett., 94, 125508 (2005). • Goldman, Fried, Kuo, Mundy, Phys. Rev. Lett., 94, 217801 (2005). Field of Extreme Chemistry has many exciting research opportunities

37. Prediction of Superionic Hydrogen Fluoride (HF) • At 66 GPa and 900 K, have stable F bcc lattice • symmetric H-bonding • Model superionic system: more easily achievable with Diamond Anvil Cell • Further study will allow us to develop simple rules for this system Possible superionic hydrogen diffusion mechanism 1. 2. 3.

38. = H2O = H3O+ and OH- = H+ and O2- = other At 9 km/s, water forms an ionic liquid Shocked molecular simulations of soft condensed matter Advances in tera-scale computing and a novel Multi-scale simulation technique allow for accurate shock simulations for the first time We observe graphite forming diamond at shock velocities of 12 km/s Novel phases and reaction pathways can be elucidated through our simulations

39. Acknowledgments • Larry Fried • Experiments: Alex Goncharov, Jonathan Crowhurst and Joe Zaug • Chris Mundy and Will Kuo

40. Molecular simulation is the foundation for understanding extreme chemistry Models of Uranus and Neptune rely on Equation of State predictions EOS models require inputs from experiment and theory Molecular simulation is needed in order to provide simple chemical pictures for experiments We have used experiments and theory to resolve controversy regarding superionic water

41. Experiments have difficulty describing chemical composition Nitrogen has metallized: Chau et al., PRL (2003) Atomic nitrogen: Radousky et al., PRL (1986) What is made when we shock N2 ?Atoms ? Chains ? Metal ? We have to rely on computations to determine the atomic structure and dynamics

42. 1000 K 1200 K 1500 K 2000 K Diffusion constants, 1000 – 2000K D ~ 10-4 cm2/s D ~ 10-5cm2/s to zero Hydrogen Oxygen We determine the superionic phase boundary from the oxygen freezing point as a function of temperature

43. Molecular Non-molecular H2O lifetimes, 1200 – 2000K 1200 K 1500 K 2000 K = onset of superionic phase Non-molecular: lifetime of all species is less than 10 fs (one O-H vibrational period) Molecular to non-nolecular transition occurs at densities greater than superionic transition (2nd phase transition)

44. 1.63 Å 1.85 Å Hydrogen-Hydrogen RDF

45. “Hot ice” interior contains small molecules at extremely high pressures and temperatures • Gravitational moments and atmospheric composition could provide insight into chemistry of physics of the interior • Data provides constraints for equation of state of candidate materials Uranus and its moons, from Voyager II Water at high P-T conditions of the interior could have unique chemistry which affect planetary processes

46. Diffusion constant and vibrational spectral results • Superionic diffusion of hydrogens occurs in presence of disordered oxygen phase • At 2000K, oxygen freezing occurs at ca. 2.6 g/cc (75 GPa) • Experimental Raman spectra validate theory • Diamond Anvil Cell experiments (DAC) are currently technologically limited • Limits: P < 50 GPa, T < 1500 K • Missing interesting features along 2000K isotherm

47. Structural analysis • O-O exhibits stable bcc lattice at higher densities • Confirms earlier thoughts about superionic water • H-H and O-H shows structure as well (ice X-like) • Lattices are very transient (< 10 fs lifetimes) • Shift in first minimum in g(ROH)

48. Future Work – Shocked Materials • High P-T conditions can be achieved experimentally by shocking materials • Presents very difficult simulation challenges • High level of theory required to accurately model chemical bond dissociation • Traditionally, shocked simulations require very large system sizes • Subsequently, we must use very low levels of theory (no ab initio MD)

49. Future Work – Predicting new Hydrogen-bonding Superionic solids • H-bond symmetrization is a unique phase of superionic H2O • Cannot be observed with current Diamond Anvil Cell technology • Halogen hydrides show promise as model systems (HF, HCl, HBr) • Evidence of non-superionic but symmetric H-bonding at lower P and T

50. Advances in theory and tera-scale computing allow for “ab initio” simulations Overlaps between the timescales of molecular simulations and experiments are becoming possible.