Experimental results on Experimental results on isotopic fractionation of dusty isotopic fractionation of dusty deuterated water ice during deuterated water ice during

sublimationsublimation

John E. MooresJohn E. MooresP.H. Smith, R.H. Brown, D.S. Lauretta, W.V. P.H. Smith, R.H. Brown, D.S. Lauretta, W.V.

Boynton, and M.J. DrakeBoynton, and M.J. Drake

LPSC, March 14LPSC, March 14thth, 2008, 2008

Importance of FractionationImportance of Fractionation Fractionation is a record of how water has moved Fractionation is a record of how water has moved

between different reservoirsbetween different reservoirs– Generally, the more water has been removed from a Generally, the more water has been removed from a

particular location, the higher the expected Deuterium particular location, the higher the expected Deuterium ConcentrationConcentration

Many important estimates of the age, source Many important estimates of the age, source material and history of water reservoirs in the Solar material and history of water reservoirs in the Solar System are linked to D/H ratiosSystem are linked to D/H ratios– i.e. Comets which may represent early solar system i.e. Comets which may represent early solar system

material and a source for the water of the terrestrial material and a source for the water of the terrestrial planetsplanets

– i.e. Mars, where the history of water speaks to the i.e. Mars, where the history of water speaks to the evolution of the Terrestrial Planetsevolution of the Terrestrial Planets

Complicating factorsComplicating factors – It is important to have some idea of the D/H ratio of the It is important to have some idea of the D/H ratio of the

initial water reservoirinitial water reservoir– It is important to know the mechanism by which the water It is important to know the mechanism by which the water

is movedis moved

Sublimation FractionationSublimation Fractionation Main Process active on these bodies in the geologic Main Process active on these bodies in the geologic

recent past is sublimationrecent past is sublimation– Can sublimation cause an important fractionation in these Can sublimation cause an important fractionation in these

reservoirs?reservoirs?

Two factors affect the surface concentration of Two factors affect the surface concentration of HDO:HDO:– Sublimation Sublimation

removes material from the surface, preferentially Hremoves material from the surface, preferentially H22OO HDO builds up due to KIEHDO builds up due to KIE

– Solid State DiffusionSolid State Diffusion Moves the HDO that builds up away from the surfaceMoves the HDO that builds up away from the surface Attempts to Homogenize the SampleAttempts to Homogenize the Sample

Depending on the conditions of sublimation ice will Depending on the conditions of sublimation ice will fractionate differently.fractionate differently.– i.e. Sublimation into Vacuum vs. into an Enclosed Spacei.e. Sublimation into Vacuum vs. into an Enclosed Space– Whether or not a regolith is presentWhether or not a regolith is present

Three RegimesThree Regimes(solid ice particles)(solid ice particles)

Rayleigh FractionationRayleigh Fractionation (Solid State Diffusion Wins)(Solid State Diffusion Wins)(Well Mixed at all times)(Well Mixed at all times)

Static LatticeStatic Lattice (Sublimation Wins)(Sublimation Wins)(No movement of molecules)(No movement of molecules)

Dynamic LatticeDynamic Lattice (Both Effects are Competitive)(Both Effects are Competitive)(Brown et al. 2008)(Brown et al. 2008)

Experimental ApparatusExperimental Apparatus

Which applies to dusty ice?Which applies to dusty ice?

Cometary Sublimation Cometary Sublimation Fractionation Apparatus: Fractionation Apparatus: medium volume cryostat medium volume cryostat (150cc) (150cc) – Porous ice mixed with dust Porous ice mixed with dust

subliming into vacuumsubliming into vacuum– Basal temperatures ranging Basal temperatures ranging

from 60K to 210Kfrom 60K to 210K– Designed to simulate a Designed to simulate a

cometary environmentcometary environment– 5-10cm thick stack5-10cm thick stack– D/HD/HBULKBULK = 5% = 5%

Mixed Dust and Ice: Mixed Dust and Ice: ObservationsObservations

With 1.5 micron TiOWith 1.5 micron TiO22 at 25wt% at 25wt%

Begin without dustBegin without dust

0 5 10 15 20 25 30 350.015

0.02

0.025

0.03

0.035

0.04

0.045

Time (days)

D/H

0 2 4 6 8 10 12 14 16 180.04

0.045

0.05

0.055

Time (days)

D/H

More realistic Albedo – JSC Mars-1, 1-10 micron particles More realistic Albedo – JSC Mars-1, 1-10 micron particles obtained by crushing and settling in waterobtained by crushing and settling in water

Mixed Dust and Ice: Mixed Dust and Ice: ObservationsObservations

0 1 2 3 4 5 6 70.045

0.05

0.055

Time (days)

D/H

0 2 4 6 8 10 120.04

0.045

0.05

0.055

Time (days)

D/H

0 2 4 6 8 10 12 14 160.04

0.045

0.05

0.055

Time (days)

D/H

0 1 2 3 4 5 6 70.045

0.05

0.055

Time (days)

D/H

0 2 4 6 8 10 120.04

0.045

0.05

0.055

Time (days)

D/H

0 2 4 6 8 10 12 14 160.04

0.045

0.05

0.055

Time (days)

D/H

0 1 2 3 4 5 6 70.045

0.05

0.055

Time (days)

D/H

0 2 4 6 8 10 120.04

0.045

0.05

0.055

Time (days)

D/H

1wt%

3wt%

6wt%

INCREASING DUST CONTENTINCREASING DUST CONTENT

Mixed Dust and Ice: Mixed Dust and Ice: ObservationsObservations

0 2 4 6 8 10 12 14 16 180.02

0.025

0.03

0.035

0.04

0.045

0.05

0.055

Time (days)

D/H

0 5 10 15 20 25 30 350.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

Time (days)

D/H

9wt%

25wt%

How to Explain Sample How to Explain Sample BehaviorBehavior

None of the expected mechanisms producesNone of the expected mechanisms produces the observed profilesthe observed profiles– All mechanisms produce an increasing D/H ratio in the All mechanisms produce an increasing D/H ratio in the

sublimate gas with time.sublimate gas with time.

What phenomena can potentially produce a What phenomena can potentially produce a declining D/H ratio with time?declining D/H ratio with time?– Migration of the heavier isotope within the sampleMigration of the heavier isotope within the sample

Due to the temperature gradient, it is very difficult to get Due to the temperature gradient, it is very difficult to get material to migrate in the sample ~1cm Maximum in material to migrate in the sample ~1cm Maximum in actual samplesactual samples

Migrating material tends to be isotopically lightMigrating material tends to be isotopically light– Gas/Dust and Ice/Dust interactionsGas/Dust and Ice/Dust interactions

Gas/Dust interactions would increase as more dry Gas/Dust interactions would increase as more dry overburden is exposedoverburden is exposed

What does this mean for What does this mean for Sublimating Bodies?Sublimating Bodies?

Comets:Comets:– If there is no circulation within the comet, the nucleus If there is no circulation within the comet, the nucleus

should be up to 2.5 times more enriched in HDO then should be up to 2.5 times more enriched in HDO then the comathe coma

– If there is circulation, the coma may not be If there is circulation, the coma may not be representative of the bulk nucleusrepresentative of the bulk nucleus

Mars:Mars:– Ice laid down by precipitation or condensation can not Ice laid down by precipitation or condensation can not

be considered well-mixedbe considered well-mixed– Current atmospheric inventory is dependant on the last Current atmospheric inventory is dependant on the last

sublimated gassublimated gas– An understanding of the circulation history is required to An understanding of the circulation history is required to

interpret the D/H ratiointerpret the D/H ratio

Experimental ApparatusExperimental Apparatus

Martian Sublimation Martian Sublimation Fractionation Apparatus Fractionation Apparatus (TUAQ apparatus): large (TUAQ apparatus): large volume cryostat (2000cc)volume cryostat (2000cc)– Solid ice overlain by regolith Solid ice overlain by regolith

with different grain sizes, with different grain sizes, sublimating into 6.1mBar of COsublimating into 6.1mBar of CO22

– Basal temperatures ranging Basal temperatures ranging from 175K to 235Kfrom 175K to 235K

– Designed to simulate the Designed to simulate the northern polar cap of Marsnorthern polar cap of Mars

– Relatively thin ice deposit Relatively thin ice deposit (3cm)(3cm)

TUAQ setup: Gas/Dust TUAQ setup: Gas/Dust InteractionsInteractions TYPICAL RUN: confined grain size distribution of JSC-1 dust in overburden (scale in TYPICAL RUN: confined grain size distribution of JSC-1 dust in overburden (scale in

Hours at a basal temp of Hours at a basal temp of -40°C with 1cm of dry overburden of 0.55 to 1mm particles)

0.030

0.035

0.040

0.045

0.050

0 10 20 30 40 50 60 70

Elapsed Time

D/H

For most runs, the change in D/H is broadly consistent with KIE This may not be as clear-cut at lower temperatures and smaller

grain sizes of dust

SummarySummary

Sublimation does cause fractionation when ice is Sublimation does cause fractionation when ice is mixed directly with dustmixed directly with dust– The effect of dust mixed with ice is significant but not The effect of dust mixed with ice is significant but not

well understoodwell understood– Deuterium can be more concentrated in the samples Deuterium can be more concentrated in the samples

then is explainable by KIE for dusty runsthen is explainable by KIE for dusty runs– Interactions with the dust are the likely culpritInteractions with the dust are the likely culprit

Direct Sampling of the solid reservoir is required Direct Sampling of the solid reservoir is required to unambiguously determine the degree of to unambiguously determine the degree of fractionationfractionation– Different histories of circulation between different Different histories of circulation between different

reservoirs can produce different values for the sublimate reservoirs can produce different values for the sublimate gasgas

Thank-youThank-you

This work funded in part by:This work funded in part by:

Lunar and Planetary LaboratoryLunar and Planetary Laboratory

National Aeronautics and Space AdministrationNational Aeronautics and Space Administration

National Science and Engineering Research Council of CanadaNational Science and Engineering Research Council of Canada

Phoenix Mars MissionPhoenix Mars Mission

University of ArizonaUniversity of Arizona