Astrochemistry: Discovery of Novel Forms of Water in Uranus and Neptune Nir Goldman Lawrence...

CMSchemistry &materialsscienceCMSchemistry &materialsscience

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

Nir Goldman

Lawrence Livermore National Laboratory

March 8th, 2006This 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 ZaugTheory: Chris Mundy, Will Kuo, Larry Fried (PI)


Uranus and Neptune have similar properties

Mean distance from Sun 2.87x 109 km

(19.19 times Earth’s)

4.50x 109 km

(30.06 times Earth’s)

Equatorial diameter 51,118 km

(4.007 Earths)

49,528 km

(3.883 Earths)

Mass 8.68 x 1025 kg

(14.536 Earths)

10.24 x 1025 kg

(17.147 Earths)

Equatorial gravity 8.69 m/s2 11.15 m/s2

Rotation period 17 h 14 min 24 s 16 h 6 min 36 s

Uranus Neptune


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

• Interior “hot ice”:56% H2O36% CH4

8% NH3

• T > 1000 K, P > 100 GPa(1 Gigapascal ≈ 104 atmospheres)

Equation of State models provide P-T profiles, and possible states of water

Equation of State models provide P-T profiles, and possible states of water


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)Predictions about planetary interiors rely upon

accurate Equation of State modeling (EOS)


Equation of state models yield very diverse results for H2O at extreme conditions

Great need for description of interior chemistry in order to derive models consistent with observational data

Great need for description of interior chemistry in order to derive models consistent with observational data

• 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


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)

P, T, time - diagram

Uncharted territory:(P >10 Kbar, T > 1000 K, t < 1 ms)

Experimental/theoretical challenges involve attaining/modeling this extreme P-T regimeExperimental/theoretical challenges involve attaining/modeling this extreme P-T regime


Gas guns and Diamond Anvil Cells are used to achieve extreme conditions

Gas gun induces shock wave in sample; measure ionic/electronic conductivity

Sample is squeezed in gasket and heated via internal wire; measure vibrational spectra

Can achieve very high pressures and temperatures

Sample is in equilibrium, and is long-lived

•Limited to P-T along shock Hugoniot

•In Equilibrium?

•Can get high P or high T, but hard to get both (P<50 GPa + T<1500K)

Gas gun Diamond Anvil Cell

Description

Advantages

Disadvantages

We have to rely on computations to determine the atomic structure and dynamics



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

Tools from Statistical Mechanics allow us to connect simulation to experiments

i

NN

i

Vf

r

rr


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 timeFaster computers and more efficient theory will allow

issue of superionic water to be resolved for the first time


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

We will provide chemical insight into experiments on hot, compressed water

CMSchemistry &materialsscienceCMSchemistry &materialsscienceCPMD 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)


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

An entire supercomputer is filled with many smaller jobs instead of a single gigantic one


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


Our starting point: calculated H2O phase diagram

Cavazzoni, et al., Science, 283, 44, 1999.

• 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


Structure of Superionic water

Cavazzoni, et al., Science, 283, 44, 1999.

• 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


Somewhat contradictory pieces of data about superionic water

1. HIGH IONIC CONDUCTIVITY 2. DISSOCIATION: OH- + H3O+

Schwegler et al., Phys. Rev. Lett., 87, 265501 (2000)

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.

Chau et al., JCP, 114, 1361 (2001)

•Experimental results show high pressure yields high ionic conductivity•Due to H2O 2H+ + O2-

vs.


Water at High Pressure and Temperature

Compressing a “liquid” configuration

Heating an“Ice VII” configuration

•Our simulations – much bigger than before.

- 1000K – 2000K- 1.5 g/cc to 3.0 g/cc - Pressures from 15 to 115 GPa

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?


Simulations show dramatic changes in the structure of water with increasing pressure

2.0 g/cc, 34 GPa, 2000KLiquid of transient molecules

Water at room temperature and pressure

3.0 g/cc, 115 GPa, 2000KSuperionic network solid with transient bonds

= Oxygen

= Hydrogen


The diffusion constant is calculated from the Einstein-Smoluchowski relation

22 )0()()( ii rrtR

0 ;6 D


3.0x10-4

2.5

2.0

1.5

1.0

0.5

0.0

Dif

fusi

on

co

nst

ant

(D, c

m2 /s

)

3.02.82.62.42.22.0Density (g/cc)

Oxygen and hydrogen diffuse on two different time scales

HydrogenCompressing

the liquid

Compressing the liquid

Heating ice VII

OxygenHeating ice VII

2000 K

Oxygen freezing point

Perform many simulations over

several isotherms

We determine the superionic phase boundary from the oxygen freezing point as a function of temperature



Abrupt changes in the Vibrational spectra allow us to determine phase boundaries

Raman Shift (cm-1)

-1000 0 1000 2000 3000

Ram

an In

ten

sity

(ar

b. u

nit

s)

1200 K

850 K

300 K

28 GPa

Raman Shift (cm-1)

-1000 0 1000 2000 3000

Ram

an In

ten

sity

(ar

b. u

nit

s)

300 K

650 K

800 K

1050 K

1150 K

1300 K

50 GPa(a) (b)H2O O-H stretch

Phonon

Stokes

O-H stretch

Stokes

Phonon

Antistokes

H2O

Antistokes

Probe

Laser Heating

Gasket

Sample

CouplerAl2O3 plates

Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug,PRL, 2005

Melting curve at high pressure and temperature was determined via the changing phonon mode

Melting curve at high pressure and temperature was determined via the changing phonon mode


Experiment and simulations show weakening of the O-H bond in liquid water

Wavenumber (cm-1)

400 800

Inte

nsi

ty (

arb

. un

its)

Raman Shift (cm-1)

400 800 2400 3200

Ram

an In

ten

sity

(ar

b. u

nit

s)

25 GPa, 1100 K

10 GPa, 1200 K

49 GPa, 1200 K

55 GPa, 1400 K

(a) (b)

2400 3200

75 GPa

62 GPa

40 GPa

30 GPa

Raman experiment

Theoretically computed power spectra, 1500 K

O-H stretch

O-H stretch

Phonon

Phonon

Static experiments CPMD simulations

Den

sit

y o

f st

ates

Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug,PRL, 2005


We have redefined the phase diagram of water at extreme conditions

Phase diagram of water

Pressure (GPa)

0 20 40 60 80

Tem

peratu

re (K)

500

1000

1500

2000

ice VII

molecular liquid

dynamicallydisorderedice VII

superionic phase

H2O

Simulations can also provide a chemical picture of superionic water

Simulations can also provide a chemical picture of superionic water

• Melting line is in agreement with externally

heated DAC data

• Transition to a superionic phase is inferred from a

combination of experiments and

simulations

Goldman, Fried, Kuo, Mundy PRL (2005)

Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug, PRL, 2005

• Triple point at 47 GPa and 1020 K, significantly higher

than Parrinello (25 GPa)


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


Radial distribution function (RDF) yields structure, g(R)

Investigate pairs of OO, OH, HH

21

)2(21

)2(

21)2(2

12

r,r1r,r

r,rr

NN

N

PNN

g

Probability of finding any two particles in the config. (r1,r2)


Oxygen-Oxygen structure (RDF), 2000 K

3.0 g/cc, 115 GPa

2.6 g/cc, 75 GPa

2.0 g/cc, 34 GPa

Average out vibrations: bcc lattice, like ice VII, ice X

CMSchemistry &materialsscienceCMSchemistry &materialsscienceOxygen-Hydrogen RDF

5

4

3

2

1

0

g(R

OH)

4321ROH (Å)

115 GPa

75 GPa

34 GPa

1.30 Å 1.70 Å

Intra-molecular Inter-molecular


We use the ROH free energy surface to define molecules

ra=1.70 Å

ra=1.30 Å

)](ln[)( OHB RgTkrW

34 GPa

115 GPa

75 GPa

5

4

3

2

1

0

g(R

OH)

4321ROH (Å)

115 GPa

75 GPa

34 GPa

2000 K

The O-H free energy barrier decreases dramatically with

pressure.

Pressure (GPa)

O-H barrier (kcal/mol)

34 10.7

75 7.5

115 7.5

RT (2000K) 4.0


We have a simple picture for proton mobility

1-D free energy surface shows

pronounced drop in dissociation

barrier


We use the ROH free energy surface to define molecules

ra=1.70 Å

ra=1.30 Å

)](ln[)( OHB RgTkrW

34 GPa

115 GPa

75 GPa

2000 K


Concentrations and lifetimes at 2000 K(<10 fs = non-molecular)

H2O, H3O+, OH-

Neutral and ionic (H2O)2 – (H2O)6

“polymer”

Almost entirely H2O

Superionic

34 GPa (2.0 g/cc):•H2O lifetime = ~40 fs•H3O+, OH- = < 10 fs 75 GPa (2.6 g/cc):

•All species lifetimes = 10 fs or less

The “polymer” species consists of very short-lived networks of bonds

The “polymer” species consists of very short-lived networks of bonds


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).


Network solid is partially covalent at 95 – 115 GPa

• Ice X: 100-1000 K, 100’s of GPa

• H bisects O—O axis

Goldman, et al., Phys. Rev. Lett., 94, 217801 (2005).

3. Superionic phase with symmetric H-bonding (95 – 115 GPa)

1.2.

3.

2. Superionic phase with asymmetric H-bonding (75 GPa)

1. Liquid, highly reactive H2O (34 – 58 GPa)

CMSchemistry &materialsscienceCMSchemistry &materialsscienceDiscussion

• 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).


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?

Field of Extreme Chemistry has many exciting research opportunities

Field of Extreme Chemistry has many exciting research opportunities

•Goncharov, Goldman, Fried, Crowhurst, Mundy, Kuo, Zaug, Phys. Rev. Lett., 94, 125508 (2005).•Goldman, Fried, Kuo, Mundy, Phys. Rev. Lett., 94, 217801 (2005).


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

1. 2. 3.

Possible superionic hydrogen diffusion mechanism


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

Novel phases and reaction pathways can be elucidated through our simulations

At 9 km/s, water forms an ionic liquid

= H2O

= H3O+ and OH-

= H+ and O2-

= other

CMSchemistry &materialsscienceCMSchemistry &materialsscienceAcknowledgments

• Larry Fried

• Experiments: Alex Goncharov, Jonathan Crowhurst and Joe Zaug

• Chris Mundy and Will Kuo


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

We have used experiments and theory to resolve controversy regarding superionic water


Experiments have difficulty describing chemical composition

Atomic nitrogen:Radousky et al., PRL (1986)

Nitrogen has metallized:Chau et al., PRL (2003)



What is made when we shock N2 ? Atoms ? Chains ? Metal ?


4x10-5

3

2

1

0

Dif

fusi

on

co

nst

ant

(D,

cm2/s

)

3.02.82.62.42.22.0Density (g/cc)

Diffusion constants, 1000 – 2000K

Hydrogen Oxygen

D ~ 10-4 cm2/s D ~ 10-5 cm2/s to zero

1000 K1200 K

2000 K

1500 K



3.0x10-4

2.5

2.0

1.5

1.0

0.5

DIf

fusi

on

co

nst

ant

(D,

cm2/s

)

3.02.82.62.42.22.0Density (g/cc)


H2O lifetimes, 1200 – 2000K

Molecular

Non-molecular

Non-molecular:lifetime of all species

is less than 10 fs (one O-H vibrational

period)

2000 K

1200 K1500 K

Molecular to non-nolecular transition occurs at densities greater than superionic transition (2nd

phase transition)

= onset of superionic phase

CMSchemistry &materialsscienceCMSchemistry &materialsscienceHydrogen-Hydrogen RDF

2.5

2.0

1.5

1.0

0.5

0.0

g(R

HH)

4321RHH (Å)

115 GPa

75 GPa

34 GPa

1.63 Å 1.85 Å


“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

Water at high P-T conditions of the interior could have unique chemistry which affect planetary processes

Water at high P-T conditions of the interior could have unique chemistry which affect planetary processes

Uranus and its moons, from Voyager II


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


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)


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)


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


Advances in theory and tera-scale computing allow for “ab initio” simulations

Overlaps between the timescales of molecular simulationsand experiments are becoming possible.

CMSchemistry &materialsscienceCMSchemistry &materialsscienceWater is nonmolecular at 3 g/cc and 2000 K

H2O

OH-

H3O+

Molecular

Non-molecular

•Free H+ and O2- have negligible lifetimes

It forms a covalent transient network structureIt forms a covalent transient network structure

Non-molecular:lifetime of all species

is less than 10 fs (one O-H vibrational

period)


Water phase diagram is largely unchartered at extreme conditions

• Predictions are based on equation of state (EOS) models

• Accuracy is heavily dependent on description of chemical products at those thermodynamic conditions

Astrochemistry: Discovery of Novel Forms of Water in Uranus and Neptune Nir Goldman Lawrence...

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