IN-SITU ANALYSIS OF EUROPA ICES

BY MELTING PROBES

J. Biele, S. Ulamec, DLR (German J. Biele, S. Ulamec, DLR (German Aerospace Center) RBAerospace Center) RB--MUSC, MUSC, Linder Linder HHööhehe, 51147 Cologne, , 51147 Cologne, Germany. Germany. Jens.Biele@dlr.deJens.Biele@dlr.de

Purpose and Scope•• How best to access (forHow best to access (for sampling/analysis) the sampling/analysis) the

material embedded in or underneath icy layers on material embedded in or underneath icy layers on the surface of. e.g. the surface of. e.g. EuropaEuropa ??

•• One possible technique for ices: penetrate by One possible technique for ices: penetrate by melting, with small probes which do not require a melting, with small probes which do not require a heavy and complicated drilling equipment heavy and complicated drilling equipment

•• In principle, even tens of km could be penetrated In principle, even tens of km could be penetrated with given time and energywith given time and energy

•• Would allow inWould allow in--situ analysis.situ analysis.•• ““Shallow meltingShallow melting”” (~meters) vaporize (~meters) vaporize

ice/volatiles, to be analysed ice/volatiles, to be analysed egeg with GCMSwith GCMS

Melting Probes for PlanetaryExploration

•• HeritageHeritage: : terrestrialterrestrialapplicationsapplications(polar (polar iceice sheetssheets))

•• Europa Europa IcyIcy ShellShell•• Mars Polar Mars Polar CapsCaps•• otherother

icyicy SatellitesSatellites ......

Challenges for PlanetaryApplications

•• MassMass and Power and Power requirementsrequirements•• Lander and Lander and DeploymentDeployment devicedevice requiredrequired•• VacuumVacuum (( sublimationsublimation insteadinstead meltingmelting?)?)•• VeryVery lowlow ambient ambient temperaturestemperatures (( power)power)•• LowerLower gravitygravity (no (no principalprincipal issueissue))•• Communications (Communications (difficultdifficult throughthrough dirty/saltydirty/salty iceice))•• InIn--situsitu instrumentationinstrumentation

Efficiency (mass, energy)•• Energy Energy byby meltingmelting (300 MJ/m(300 MJ/m3)3) higherhigher thanthan

cuttingcutting iceice ((drillingdrilling ~1~1--20 MJ/m20 MJ/m33)), )), butbut thisthis doesdoesnot include transmission losses and the energy for not include transmission losses and the energy for compacting the cuttings and transportation to and compacting the cuttings and transportation to and discharge at the surface discharge at the surface –– increases rapidly with increases rapidly with depth!depth!

•• Mass of drill for planetary Landers: ~4 kg to reach Mass of drill for planetary Landers: ~4 kg to reach 20 cm (Philae), ~20 kg to reach 2m (20 cm (Philae), ~20 kg to reach 2m (ExoMarsExoMars))

•• Melting probes in ice more efficient for depths Melting probes in ice more efficient for depths > few dm, certainly > 2 m> few dm, certainly > 2 m

Principles of melting through ice

2r

2rl

Conclusion: small cross-section A!

To proceed by l, at least the energy

ΔW = Alρ [cp(tF - t) + Lv]

must be expedited. If the heating power is P, then the melting velocity is

ν = lP / ΔW = P / Aρ [cp(tF - t) + Lv]

where

A: cross sectionl: lengthcp: specific heat capacity of ice (2 kJ / kgK)ρ: density of ice (920 kg / m3)LV: heat of fusion/sublimation

of ice (334 kJ / kg ... 2800 kJ / kg)tF: melting/sublimation temperaturet: ice temperature

A = r2π

10-3

10-2

10-1

100

101

10-4

10-3

10-2

10-1

100

101

10

v (m/h)

P (k

W)

27050

270

250

200150

100

50

Required heating power as a function of melt velocity, without (solid lines) and with conductive losses (broken lines). L=1 m, R=5 cm

10-3

10-2

10-1

100

101

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

v (m/h)

Effi

cien

cy E

270

250 200

150

100

50

Efficiency

Ice thermophysical properties

0 50 100 150 200 2500

500

1000

1500

2000

2500

T (K)

Cp J

/(kgK

)

H2O specific heat capacity

50 100 150 200 2500

20

40

60

80

100

120

T/K

λ / W

/m/K

Thermal conductivity of polycrist. ice Ih

0 50 100 150 200 250 3002640

2660

2680

2700

2720

2740

2760

2780

2800

2820

2840

T/K

L s in k

J/kg

Sublimation enthalpy of water ice, Feistel 2006

0 50 100 150 200 250 300916

918

920

922

924

926

928

930

932

934

T/K

ρ / k

g m

- 3

Density of ice Ih after Feistel(2006)

Ices other than water CO2 CO CH4 H2O Tf, melting temperature (~1 bar) (K)

216.6 68.1 90.7 273.1

cp, specific heat capacity (kJ/kgK) 1.38 1.90 2.35 1.13 L, phase transition enthalpy (kJ/kg)

573.3 (sublimation)

29.9 58.6 333.4

ρ, density (kg/m3) 1540 920 500 920 Penetration velocity relative to water ice, Tice=1/2 Tf, losses regarded as equal

0.40 5.1 5.4 1

Experience with melting probes, typical construction•• AWI (AlfredAWI (Alfred--WegenerWegener--Institute for Polar and Marine Institute for Polar and Marine

Research, Bremerhaven, Germany): Achieved easily 250m Research, Bremerhaven, Germany): Achieved easily 250m in in antarcticantarctic shelf ice with a 10cm diameter, 180 cm long shelf ice with a 10cm diameter, 180 cm long melting probe, 1000 W electrical heater externally fed melting probe, 1000 W electrical heater externally fed (1000 V lines with telemetry multiplexed into the supply), (1000 V lines with telemetry multiplexed into the supply), internal umbilical spool. internal umbilical spool.

•• 1 m/h melting velocity. Important lesson learnt: 1 m/h melting velocity. Important lesson learnt: mechanism for steering (keeping vertical) important! mechanism for steering (keeping vertical) important!

•• A 1000 m project failed because the umbilical spool A 1000 m project failed because the umbilical spool procucedprocuced a short due to overheatinga short due to overheating

•• Experiments at DLR in air (Experiments at DLR in air (--3030°°C) and vacuum (100 K)C) and vacuum (100 K)

Setup of Test ProbePower and Pt100

Hook with tether

120 mm

525

mm

605 mm

Heating power:200 – 600 W

Vacuum Facilities

Experimental Setup

Ice

Ln2

Vacuum Chamber

MeltingProbe

Camera

Motor

Power and Pt100

Hook with tether

120 mm

525

mm

605 mm

THEORY and PRAXIS: Conclusions from experiments•• Melting probe Melting probe conceptconcept worksworks in in vacuumvacuum; ;

sublimationsublimation oror meltingmelting dependingdepending on on locallocalpressurepressure, , porosityporosity, , closureclosure of of channelchannel

•• Not just Not just frontfront--spheresphere, , butbut wholewhole probe probe needsneeds to to bebe heatedheated ifif iceice veryvery coldcold

•• PracticalPractical minimumminimum power power levellevel existsexists•• Melting Melting channelchannel closescloses veryvery rapidlyrapidly (liquid (liquid

waterwater in in cavitycavity))

Obstacles, steering …

•• KeepingKeeping probe probe verticalvertical: : solutionssolutions existexist ((tethertetherclutchclutch))

•• DangerDanger: : accumulationaccumulation of of dustdust in front of in front of meltmelt headhead(( molemole mechanismmechanism [PLUTO], [PLUTO], addadd. . drillheaddrillhead ororwaterwater jetjet [NASA [NASA CryobotCryobot]) ]) -- notnot reallyreally maturemature

•• DangerDanger: : obstaclesobstacles leadingleading to to tilttilt (( steeringsteering bybydiffdiff. . HeatingHeating, , mechanismsmechanisms, , ……) ) -- all all notnot provenproven

Proposed Instrumentation

•• Control sensorsControl sensors (temperatures, system attitude, (temperatures, system attitude, signal strength for signal strength for commscomms, etc.), etc.)

•• Habitat sensorsHabitat sensors (conductivity, pH, t, electro(conductivity, pH, t, electro--chemical spectra)chemical spectra)

•• Optical sensorsOptical sensors (Refractive index sensor, (Refractive index sensor, AttenuatedAttenuated Total Total ReflectionReflection spectrometerspectrometer (ATR),(ATR),Cameras, UV Cameras, UV SpectroSpectro--fluorometersfluorometers, Raman , Raman spectrometers, Dissolved Oxygen spectrometers, Dissolved Oxygen chromophorechromophoresystems)systems)

•• Mass spectrometerMass spectrometer (MS) /(MS) /chromotographicchromotographic (C ) (C ) input systemsinput systems

Radioactive heating

•• For planetary missions due to the high For planetary missions due to the high energy demand, only radioactive heating energy demand, only radioactive heating seems to be a feasible solutionseems to be a feasible solution

•• Traditional RHU technology is based on Traditional RHU technology is based on 238238PuPu

•• In case of Antarctica, In case of Antarctica, 4545Ca seems to be an Ca seems to be an attractive alternativeattractive alternative

Communications•• For depths to about 1km, tether based For depths to about 1km, tether based

power/power/commscomms between probe and surface may be between probe and surface may be considered 1 considered 1

•• For greater depths one possibility are ice For greater depths one possibility are ice transceivers (microwave repeaters, 2 cm x 10 cm, transceivers (microwave repeaters, 2 cm x 10 cm, 120 120 mWmW transmit signal power, to relay 10 Kb/s transmit signal power, to relay 10 Kb/s over several 100 m in ice with 13 over several 100 m in ice with 13 ppmppm salt salt impurities). impurities).

•• Alternative (power with Alternative (power with RTHs/RHUsRTHs/RHUs not an not an issue): long wave technology, antenna coils issue): long wave technology, antenna coils instead of long instead of long λλ/4 wires/4 wires

Short range melting probe for a Europa Lander

•• While the longWhile the long--term goal is to penetrate thick ice term goal is to penetrate thick ice crusts and explore the ocean beneath, in the short crusts and explore the ocean beneath, in the short run (e.g., to equip a first run (e.g., to equip a first EuropaEuropa Lander) a simple Lander) a simple melting probe to access the uppermost meters melting probe to access the uppermost meters ofof EuropaEuropa’’ss crusts (where radiation levels are crusts (where radiation levels are already low enough to permit the long term already low enough to permit the long term survival of organic matter) appears to be feasible.survival of organic matter) appears to be feasible.

•• Variants with radioisotope and electrical heating Variants with radioisotope and electrical heating and both sampling and inand both sampling and in--situ probes are possiblesitu probes are possible

Ultimatively…

Reference:

•• StephanStephan Ulamec, JensUlamec, Jens Biele, Biele, OliverOliver Funke and MarcFunke and Marc Engelhardt: Engelhardt: Access to Access to glacialglacial and and subglacialsubglacialenvironmentsenvironments in in thethe Solar System Solar System bybymeltingmelting probe technology, in Life in probe technology, in Life in Extreme Extreme EnvironmentsEnvironments, Springer 2007, , Springer 2007, pp. 1pp. 1--2424