Vortrag > Autor > Dokumentname > 09.11.2005 Folie 1 EMC/ERH-properties of thin layers of martian...

Folie 1Vortrag > Autor > Dokumentname > 09.11.2005

EMC/ERH-properties of thin layers of martiananalogue soils – First resultsA. Lorek

German Aerospace Center (DLR)

([email protected] / Fax: +49 30 67055 303 / Phone: +49 30

67055 390)

Vortrag > Autor > Dokumentname > 09.11.2005

Folie 2

Motivation

• [Rivkina et al., 2000] has shown that live its possible below 0°C and at low liquid water contends ( > 2 mono layer of water -- 1 mono layer = 0,3nm thickness)

• Chemical experiments at DLR have shown that the Photo-Fenton-Reaction is possible at low water contents (corresponding to a few - n > 2 – mono layers).

• Liquid water influences physical properties of soils

• Rheology

• Electric- and heat conductivity

• Dielectric and related spectral properties

The water content of soils significantly influences their chemical, physical and biological properties.

gullies in a crater wall(at 168.2°W, 37.4°S) (MOC M15-01466) Photo NASA


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Mars analogue soil

Bentonite (Clay with a high content of montmorillonite)

• Montmorillonite was detected on the basis of spectrometer data from Mars Express [Poulet et al., 2005]

• Is distinguished as a promising habitat for earlier or current live [Chevrier; Mathe, 2006]

• specific surface 50,3 m²/g, absolute density 2,38 g/cm³ (Federal Institute for Materials Research and Testing (BAM))

JSC MARS-1 (Volcanic material from Mauna Kea Vulcan (Hawaii))

• Its spectra is like the Olympus-Amazonis Region of Mars [Allen et al., 1998]

• Specific surface 146 m²/g, absolute density 2,64 g/cm³ (BAM)

The upper millimeter of the martian surface is of particular interest

• It interacts directly with the diurnally varying atmospheric humidity, which can reach saturation during the night and early morning hours

• Adsorption/desorption of water in the soil and freezing of water can be a consequence

• Could contain several mono layers of liquid water


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EMC/ERH

• Water content in the upper soil layers depends on the relative humidity of the atmosphere

• Equilibrium relative humidity (ERH) is related to an equilibrium moisture content (EMC)

• An increase of relative humidity leads to an increase of the soil moisture content

• The properties of the soil (structure, specific surface, free charges, van der Waals-forces) determine the EMC

Tk

EK

eap

0

saturation vapor pressure p0 [Pa]

water activity [aw]

0p

pa

W

%100..% w

ahr

Equilibrium relative humidity

Desorption isothermes from Bentonite and JSC Mars 1 at 25°C

0

1

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30 35 40

Relative humidity [%r.h.]

Wei

gh

t p

er c

ent

[%w

t]

Bentonite JSC Mars 1

Desorption isotherms [BAM]

Equilibrium moisture content %100)(

)(%

soildryweight

waterweightwt


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Theory of the liquid water content in soils (Möhlmann`s formula)

•Möhlmann’s formula establish a relationship between ERH and adsorbed (liquid) water content

•Theory (Möhlmann) based on Hamaker-Theory of van der Waals interactions (planar surfaces) in combination with the saturation water vapor formula

•Hamaker- constant (10-18J to 10-21J ), specific surface and r.h. must be known

L

VdW

VdW NE

am = MH2O

mdry

= SM H2O

A

6 (H2 O/mH2O) kT ln(p/p0)

1/3

3

132

VdW

vdW

d 6

A =

S

F

kTEE( - vdWKea = p /)

kT

E-

w

0

VdW

e = app

Tk

EK

eap

0

Ice

Liquid like water

Mineral surface

d van der Waals force


Folie 6

Atmospheric humidity and soil moisture on Mars

• It is expected that the water content in upper soil is in a range from 2 to 10%wt in mid- and low latitudes

• Relative humidity ranges between 100 %r.h. in night - and early morning hours and near 0%r.h. about noon

• During 100% r.h. the first millimeter can adsorb a lot of mono layers water• Atmospheric humidity at mid- and low latitudes is in the range of

- 80° to -100°C Td (0,12Pa to 0,004 Pa)• There is only a weak increase of soil moisture with decreasing temperature• Sorption isotherm at 20°C corresponds nearly to an isotherm at lower

temperatures Viking 2 at 47.97° N and 225.74° W (Foto NASA 18 Mai 1979)

Feldman, Los Alamos National Laboratory, 2003Desorption isothermes of Montmorillonite at -16°C, 2°C, 20°C u. 30°C based on data from Jähnchen, Bish, Möhlmann, Stach “Investigation of the water sorption properties of Mars-relevant micro- and mesoporous minerals”, Icarus, 2006

Soil moisture content in Montmorillonite at different temperaturesDesorption isothermes

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Relative humidity [%]

So

il m

ois

ture

co

nte

nt

[%w

t]

-16°C 2°C 20°C 30°C


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Dielectric test plate capacitor (10 Hz - 1MHz)

• Measurement system for detection of liquid water content in soils

• Distance between capacitor plates is 1,27mm (0.82mm soil, 0.45mm air gap)

• Gas in- and output• Possible to evacuate the capacitor• Maximum gas flow through the capacitor is 15 NL/h

• Soil adsorb/desorb water from gas flow• Water significantly influence the DK of soil• If formation of ice occurs, the DK decreases to 3,25

at frequencies about 100KHz

CddDK

DKCdDK

KPGas

Gas

Boden

1

2

0

2

4

Soil


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Soil moisture measurement at JSC Mars 1

• Measurements at 25°C and 1013 hPa for frequencies (10Hz, 90Hz, 1KHz, 5KHz, 10KHz, 100 KHz and 1MHz)

• Soil probes were baked out at 105°C over 24h

Results• Considerable and repeatable dependence of the DK on soil moisture content • 3,8 %wt correspond to approximately 1 mono layer of water

10 Hz

90 Hz

1 KHz

5 KHz

10 KHz

100 KHz

1 MHz

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

6,5

7,0

7,5

8,0

8,5

9,0

1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5 9,0 9,5 10,0

Soil moisture content [%wt]

DK

of

JS

C M

ars

1


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Liquid water content in JSC Mars 1 and Bentonitein a temperature range from +25°C to -70°C

• JSC Mars 1 shows indication of freezing at -35°C (9,4 %wt curve)

• 7,6 %wt curve (close of the 9,4 %wt curve) indicates that, down to -35°C, 2 mono layers are liquid

• 6 %wt curve shows no indication of freezingapproximately 1,5 mono layers are liquid down to -70°C

• 9,4%wt curve for Bentonite shows indication of freezing at -15°C

• 7,8%wt curve has a nearly linear decrease over the full temperature range: No freezing is observable

• 4,8 %wt curve shows the same behavior: Approximately 3 mono layers are in the liquid state down to -70°C

2

2,1

2,2

2,3

2,4

2,5

2,6

2,7

2,8

2,9

3

-75 -70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25

Temperature [°C]

DK

(JS

C M

ars

1)

2,0

2,5

3,0

3,5

4,0

4,5

-80 -75 -70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25

Temperatur [°C]

DK

(B

ento

nit

e)


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• Diurnal temperature variation (-70°C to +5°C), Gas flow 15Nl/h (air), constant humidity of -80°C Td, pressure1013 hPa, Time period 28 days (Equilibrium not yet reached)

• Result

• 2,8 %wt soil moisture content at the end of the measurement (0.75 mono layer)

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700

Time [h]

DK

(J

SC

Ma

rs 1

)

-75

-65

-55

-45

-35

-25

-15

-5

5

15

25

Tem

pe

ratu

re [

°C]

DK 1MHz Temp. °C


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Conclusions• Measurements (under normal pressure at JSC Mars 1) show that adsorbed water in the

simulated martian soils is present with contents not less than 3%wt (1 mono layer)

• First Measurements lead to the assumption (JSC Mars 1) that, down to -35°C, more than 2 mono layers (8 %wt) are in a liquid state and more than 1 mono layer is liquid at -70°C

• A liquid water content of 2 mono layer at -35°C would corresponds to a Hamaker-constant of 3,5x10-19J (Möhlmann’s - formula)

• Chemical reactions and live processes can be supported by liquid water below 0°C

Future prospects• Further measurements under normal pressure and at temperatures below 0°C• Measurements under martian conditions (CO2 atmosphere, 600 Pa)• Use of a dielectric measurement system (TDR) to confirm and to extend the given results

Vortrag > Autor > Dokumentname > 09.11.2005 Folie 1 EMC/ERH-properties of thin layers of martian...

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