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Investigation of surface properties of lunar regolith: Part I
Applied Surface Science 253 (2007) 5709–5714 www.elsevier.com/locate/apsusc
Investigation of surface properties of lunar regolith: Part I E. Robens a,*, A. Bischoff b, A. Schreiber c, A. Da˛browski d, K.K. Unger a a
Institut fu¨r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universita¨t, Duesbergweg 10-14, D-55099 Mainz, Germany b Institut fu¨r Planetologie, Universita¨t Mu¨nster, Wilhelm-Klemm-Str. 10, D-48149 Mu¨nster, Germany c POROTEC GmbH, Niederhofheimer Str. 55a, D-65719 Hofheim, Germany d Uniwersytet Marii Curie-Skłodowskiej, Fac.Chimii, pl.M.Curie-Skłodowskiej 2, PL-20-031 Lublin, Poland Available online 5 January 2007
Abstract This paper describes an initial investigation of the surface properties of three lunar soil samples from the Apollo 11, 12 and 16 missions, respectively. We report on density measurements using a helium pycnometer, adsorption isotherms of krypton applied for the determination of specific surface area of the samples and gravimetric measurement of the isotherms of water, heptane and octane. Electron-microscopic photographs are described and discussed. # 2007 Elsevier B.V. All rights reserved. PACS : 96.20. n; 92.30.Dt; 68.43. h Keywords: Moon; Regolith; Surface; Adsorption; Water
1. Introduction
2. Samples
Lunar soil and rock samples of the Apollo and Luna missions 1969–1976 have been already examined in detail [1–5]. Planning of new missions and the establishment of a manned station at the Moon require some additional information. The assumption of an inventory of water ice [6,7] suggests studying the surface properties of regolith in contact with water [8]. We expect that water – if present on the moon – exists exclusively in the form of chemisorbed and vicinal water immersed in the regions protected from sun irradiation. The aim of this investigation is to obtain parameters relevant to the capability of the Moon surface to store water, both the amount and kinetics adsorption/desorption in relation to the environmental conditions. New investigation methods and new evaluation procedures are applied, which had been developed recently by us and elsewhere [9–11].
Looking at the Moon with the naked eye, it can be seen easily that there are bright and dark areas called ‘maria’ on its surface [12]. Telescopic observations showed that the maria are very flat, and quite different from the heavily cratered and mountainous so-called highlands. We have learned that the maria are relatively young areas on the Moon generated after very large impacts penetrated the crust of our Moon and excavated the basins. During later volcanic episodes, liquid magma came to the surface and filled these basins. As this happened in comparatively recent times, the number of impact craters is far fewer than in the highland areas. Impact processes have formed the lunar regolith. Lunar soil is the subcentimeter fraction of the lunar regolith. Detailed studies have shown that five basic particle types make up the lunar soils: mineral fragments, pristine crystalline rock fragments, breccia fragments, glasses of various kinds, and the unique lunar structured particles called agglutinates [1]. The mineral and chemical composition of the lunar soils depends on the mission landing sites [13]. Apollo 11 and Apollo 12 landed well inside mare basalt regions, and consequently, soil samples from these sites have abundant mare-derived basaltic rock clasts and mafic minerals like olivine and pyroxene. On the other hand, Apollo 16 landed in
* Corresponding author. E-mail addresses: [email protected] (E. Robens), [email protected] (A. Bischoff), [email protected] (A. Schreiber), [email protected] (A. Da˛browski), [email protected] (K.K. Unger). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.098
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highland regions. Soils from this mission contain abundant highland-derived components, e.g., lithic fragments of anorthositic rocks, breccias, and anorthitic feldspar. We started to investigate three soil samples from those missions, each of about 3 g.
gravimetric techniques using that DVS apparatus at 24.9 8C. The specific surface area was calculated using the molecular areas of n-heptane and n-octane of 0.57 nm2 and 0.63 nm2, respectively. 4. Results
3. Measuring methods and instruments 4.1. Visual inspection 3.1. Scanning electron microscopy The fine-grained textures of the three lunar soils were studied with a field-emission Scanning Electron Microscope (JEOL JSM-6300F) at the University of Mu¨nster (ICEM, Interdisciplinary Center for Electron Microscopy and Microanalysis) operating at 5 kV. 3.2. Density Helium displacement within a calibrated vessel [14] at 20 8C was applied using a PYCNOMATIC ATC instrument of Thermo Electron S.p.A., Milano, Italy. 3.3. Specific surface area—krypton adsorption
All samples consist of weakly coherent fines. Sample 12001.922 and sample 12001.922 appear light grey and including a few white particles. Sample 10084.2000 had a dark grey colour. 4.2. Scanning electron microscopy The Apollo 16 soil sample 64501 contains abundant clastic grains (Fig. 1a), but also impact melt spherules of various sizes (Fig. 1b). Fragments with internal pores like those found in the Apollo 11 and 12 soils (e.g., Fig. 2a) are either rare or absent. Kempa and Papike [15] found that in the particle size range of 10–90 mm sample 64501 is highly feldspathic having more than 50% plagioclase. In the Apollo 12 sample more small-grained and more molten and porous particles are found compared with
Prior to the adsorption measurements the samples had been degassed at 80 8C for 4 h. This is far below the temperature which occurs on the Moon during its diurnal cycle. Krypton adsorption isotherms were measured stepwise at 77.1 K using a volumetric/manometric apparatus SORPTOMATIC of Thermo Electron S.p.A., Milano, Italy. In the range of about 0.07 < p/p0 < 0.23 the specific surface area was calculated by means of the 2-parameter BET equation using a molecular area of 0.195 nm2. 3.4. Water vapour adsorption Several investigations of lunar soils are reported in literature [1–5] in which microbalances had been used to measure adsorption and desorption isotherms. The authors report on difficulties in calibrating the balances probably due to adsorption of water on various parts of the balance [2,8]. In the present investigation therefore the apparatus was not evacuated and the case containing the sensor system was always protected by flushing it with dry nitrogen. We used a DVS 1/Advantage apparatus of Surface Measurement Systems, Ltd., Wembley, Middlesex, U.K. The apparatus encompasses a Cahn microbalance with a maximum load of 1.5 g. Water vapour pressure is adjusted and varied by means of a flow of nitrogen carrier gas saturated with water vapour. The isotherms were measured stepwise at a temperature of 24.9 8C slightly above ambient to avoid condensation within the apparatus. The specific surface area was calculated using a molecular area of 0.129 nm2. 3.5. n-Heptane and n-octane adsorption n-Heptane and n-octane adsorption and desorption isotherms were measured similarly to those of water by
Fig. 1. (a) (#64501-11) and (b) (#64501-14). Electron micrographs of lunar regolith sample 64501.228 of the Apollo 16 mission from highland regions.
E. Robens et al. / Applied Surface Science 253 (2007) 5709–5714
Fig. 2. (a) (#12001-22) and (b) (#12001-06). Electron micrographs of lunar regolith sample 12001.922 of the Apollo 12 mission from inside of a mare basalt region.
the Apollo 16 soil sample (Fig. 2b). Sample 12001 was previously characterized as a well-gardened soil, random mixture of debris fragments that mostly derive from nearby bedrock and whose particles largely consist of mare basalts or degradation products thereof [16]. The Apollo 11 soil sample (Fig. 3a) has a high abundance of glassy particles or features that indicate melting (Fig. 3b). It was found that the large proportion of the <10 mm material in the soil (14.2%) is consistent with its high maturity [17]. The 10084 soil is very fine-grained with a mean grain size of 51 mm and very poor uniformity. The high concentration of agglutinates (52%) was determined by Simon et al. [18].
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Fig. 3. (a) (#10084-05) and (b) (#10084-09). Electron micrographs of lunar regolith sample 10084.2000 of the Apollo 11 mission from inside of a mare basalt region.
On account of the cleaved and rough particle the Apollo 12 sample seems to have a larger surface than that of Apollo 16. However, the structures resolved in the micrometer range do not contribute much to the specific surface area as determined by gas adsorption. Inspection by means of a light-optical microscope of Apollo 14 fines and bearing in mind observed roughness and internal pore structure Cadenhead et al. [2] calculated a specific surface area of about 0.01 m2 g 1 whereas nitrogen adsorption revealed the values of 0.1– 1.0 m2 g 1. Thus, evaluations of that parameter from the image analysis or particle size distribution seem to be misleading.
Table 1 Density and specific surface area of regolith samples Sample
Density (g cm 3)
Specific surface area from Kr adsorption (m2 g 1)
n-Heptane adsorption (m2 g 1)
10084.2000 Apollo 11, Mare 12001.922 Apollo 12, Mare 64501.228 Apollo 16, Highland
3.10 0.01 2.79 0.01
0.579 0.653
n-Octane adsorption (m2 g 1)
water vapour adsorption (m2 g 1)
0.481 0.432
0.368
0.408 0.540
0.358 0.397
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lines intersecting the ordinate at small positive values. The resulting values of the specific surface area (0.3–0.7 m2 g 1) are within the region of values determined from N2 isotherms by other investigators (0.2–1.5 m2 g 1). 4.5. Heptane and octane vapour adsorption
Fig. 4. Krypton adsorption isotherm at 77.1 K of lunar regolith sample 12001.922. The ordinate represents the adsorbed gas volume at S.T.P. divided by the mass of the degassed sample. The abscissa is the measured pressure above the capillary (Ø10 mm) of the sample vessel related to the saturation pressure of 233 Pa. No correction was made concerning the thermal pressure difference in capillaries.
n-Heptane and n-octane ad- and desorption at ambient temperature proceeded rapidly, which indicates the absence of pores with the entrance width of a few molecular diameters. In addition, fast desorption indicates weak binding. The resulting isotherms correspond to type II of the IUPAC classification with nearly linear slopes within 0 < p/p0 < 0.7 and without hysteresis between adsorption and desorption branches (Fig. 5). This suggests the absence of pores having nanometer widths. The specific surface area determined with those
4.3. Density The He-density values for lunar soils range from 2.3 to >3.2 g cm 3 giving a mean value of 3.1 g cm 3 which has been recommended [1]. The results of our measurements listed are the mean values of repeated measurements. Our results are well within the range of the literature values (Table 1). 4.4. Krypton adsorption Kr adsorption had been measured at 77.1 K and all the resulting isotherms (Fig. 4) correspond to type II of the IUPAC classification [19–21]. BET plots show well defined straight
Fig. 5. n-Heptane adsorption and desorption isotherm at 77.1 K of lunar regolith sample 12001.922. (~) Adsorption and (!) desorption. ma Adsorbed heptane mass related to unit sample mass, p/p0 heptane pressure related to saturation pressure.
Fig. 6. (a) Measurement of the water vapour adsorption and desorption isotherm at 24 8C of lunar regolith sample 12001.922. The lower curve represents the preset relative humidity (%), the upper curve the adsorbed mass (mg). (b) Water adsorption and desorption isotherm at 24.9 8C of lunar regolith sample 12001.922. (~) Adsorption and (!) desorption. ma Adsorbed water mass related to unit sample mass, p/p0 water vapour pressure related to saturation pressure (relative humidity).
E. Robens et al. / Applied Surface Science 253 (2007) 5709–5714
adsorptives is lower than that determined with krypton. This is reasonable because the molecular diameters of n-hydrocarbons are of about 0.6 nm and are larger than that of krypton (diameter 0.36 nm). They cannot penetrate into the small pores which are accessible to Krypton. 4.6. Water vapour adsorption Water vapour adsorption and desorption at ambient temperature also proceeds rapidly (Fig. 6a). A diffusion process through small pores has no effect. The isotherm increases rather slowly from origin (Fig. 6b), consistent with the literature reports, which is typical of material having an intermediate character between hydrophilic and hydrophobic, and it indicates weak binding. Hysteresis loops extending over the whole region of relative pressure were observed which may be due to the fact that in many cases after changing the water vapour pressure a saturation value of the adsorbed mass was not attained. Furthermore, we should also keep in mind that these measurements are near the detection limit of the measuring method and therefore a rigorous interpretation of the results therefore is not possible. Indeed, the investigations reported in literature [1–5] show similar scatter of the results. The specific surface area determined with water vapour adsorption (diameter 0.28 nm) is lower than that determined with krypton (diameter 0.36 nm). The reason for this is unclear and confirms uncertainties in the measurement. 5. Conclusions Commercial density and sorption measuring instruments proved to be suitable for investigating the lunar soil samples. Sensitivity and resolution are comparable to those of instruments used in the investigations reported in literature [22–24]. No significant differences in sorption properties have been detected for the samples of different origin (mare and highland). Lunar regolith can hardly store water due to low specific surface area, little nano-porosity and a surface character lying between hydrophilic and hydrophobic. Structures like permafrost as observed on the Earth [25] and assumed on Mars [26,27] cannot be expected on the Moon, on which the conditions could not allow the existence of liquid water. Therefore, the gravity induced release of liquid water into the deeper regions is impossible. Transport of water under lunar conditions could take place only via the gas phase or in a quasiliquid phase of a few molecular layers [28] at the surface of the soil material. Water-ice is imported to the Moon by comets and cosmic dust particles. Remnants may be stored as thin, incomplete physisorbed or chemisorbed layers at the soil material, and bulk stocks are assumed in craters near the poles inaccessible to the sun radiation. Continuous evaporation takes place everywhere on the Moon surface. With regard to the diurnal temperature cycle evaporation will be stronger near the outer surface and is less obstructed by diffusion through the regolith layer. Therefore, physisorbed water should be found in some depth
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and water-ice – assuming it exists as a bulk phase – may be stored below the regolith covering. Acknowledgement The samples had been kindly placed for our disposal by the NASA Lunar sample curator, Dr. Gary Lofgren, Houston, Texas. References [1] G.H. Heiken, D.T. Vaniman, B.M. French (Eds.), Lunar Sourcebook, Cambride University Press, Cambridge, 1991. [2] D.A. Cadenhead, et al., Some surface characteristics and gas interactions of Apollo 14 fines and rock fragments, in: Proceedings of the Third Lunar Science Conference (Supplement 3, Geochimica et Cosmochimica Acta), M.I.T. Press, 1972, pp. 2243–2257. [3] D.A. Cadenhead, R.S. Mikhail, Water vapour weathering of TaurusLittrow orange soil: a pore-structure analysis, in: Proceedings of the Sixth Lunar Science Conference, 1975, pp. 3317–3331. [4] R.B. Gammage, H.F. Holmes, Blocking of the water-lunar fines reaction by air and water concentration effects, in: Proceedings of the Sixth Lunar Science Conference, 1975, pp. 3305–3316. [5] H.F. Holmes, R.B. Gammage, Interaction of gases with lunar materials: revised results for Apollo 11, in: Proceedings of the Sixth Lunar Science Conference, 1975, pp. 3343–3350. [6] D.R. Williams, Ice on the Moon, NASA, 2005, http://nssdc.gsfc.nasa.gov/ planetary/ice/ice_moon.html. [7] W.C. Feldman, et al., Polar hydrogen deposits on the Moon, 2006, http:// lunar.lanl.gov/pubs/2000/Polar_H_Deposits_on_Moon.pdf#search=%22Water%20moon%20Lucey%22. [8] E.L. Fuller Jr., P.A. Agron, Sorption by low area materials, in: C. Eyraud, M. Escoubes (Eds.), Progress in Vacuum Microbalance Techniques, Heyden, London, 1975, pp. 71–82. [9] P. Staszczuk, D. Sternik, G.W. Chadzynski, Application of quasi-isothermal thermogravimetry and sorptometry for estimation of heterostructure and fractal dimension of high temperature superconductors, J. Therm. Anal. Cal. 71 (1) (2003) 173–182. [10] P. Staszczuk, et al., Studies of heterogeneity properties of selected hightemperature superconductor surfaces, J. Therm. Anal. Cal. 86 (1) (2006) 133–136. [11] V.V. Kutarov, E. Robens, B. Kats, Universal function for the description of multilayer adsorption isotherms, J. Therm. Anal. Cal. 86 (1) (2006) 35–38. [12] J.-L. Josset, B.H. Foing, SPACE-X Space Exploration Institute and ESA SMART-1 Project, 2006. [13] D. Sto¨ffler, A. Bischoff, et al. J. Geophys. Res. 90 (1985) C449–C506. [14] A. Da˛browski, et al., Standardization of methods for characterizing the surface geometry of solids, Particle Particle Sys. Character. 20 (5) (2003) 311–322. [15] M.J. Kempa, J.J. Papike, The Apollo 16 Regolith: Comparative Petrology of the >20 and <20 mm Soil Fractions, In Lunar and Planetary Sciences XI, The Lunar and Planetary Institute, Houston, 1980 , pp. 535–537. [16] J.A. Wood, et al., Mineralogy and Petrology of the Apollo 12 Lunar Sample, SAO Special Report, SAO, #333, 1971. [17] A. Basu, S.J. Wentworth, D.S. McKay, Submillimeter grain-size distribution of Apollo 11 soil 10084, Meteor. Planet. Sci. 36 (2001) 177–181. [18] S.B. Simon, J.J. Papike, J.C. Laul, The lunar regolith: comparative studies of the Apollo and Lunar sites. petrology of soils from Apollo 17, Lunar 16, 20, and 24, in: Proceedings of the Lunar Planet. Sci. Conf. 12B, 1981, pp. 371–388. [19] R.S. Mikhail, E. Robens, Microstructure and Thermal Analysis of Solid Surfaces, Wiley, Chichester, 1983. [20] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders & Porous Solids, Academic Press, San Diego, 1999.
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[21] K.K. Aligizaki, Pore structure of cement-based materials, Modern Concrete Technology, vol. 12, Taylor & Francis, London, 2006. [22] E.L. Fuller Jr., et al., Volumetric and gravimetric methods of determining monolayer capacities, Thermochim. Acta 29 (1979) 315–318. [23] E. Robens, A.W. Czanderna, J.A. Poulis, Surface area and pore size determination on finely divided or porous substances by adsorption measurements: comparison of volumetric and gravimetric methods, Powder Metallur. Inter. 12 (4) (1980) 201–203. [24] J.U. Keller, E. Robens, C. du Fresne von Hohenesche, Thermogravimetric and Sorption Measurement Techniques/Instruments, in: F. Rodrı´guezReinoso, et al. (Eds.), Characterization of Porous Solids VI., Elsevier, Amsterdam, 2002, pp. 387–394.
[25] D.M. Anderson, N.R. Morgenstern, Permafrost Proc., in: Permafrost Second International Conference, National Academy of Science, Washington, DC, 1973. [26] D. Mo¨hlmann, Liquid-like adsorption water in the Martian soil at mid and low latitudes, Icarus 4 (2003). [27] D. Mo¨hlmann, Unfrozen subsurface water on Mars: presence and implications, Int. J. Astrobiol. 2 (3) (2003) 213–216. [28] E. Robens, Some remarks on the interface ice/water, in: Sergijiwka-Odesa, Ukraine, R.L. Yu. Tarasevich, B. Kats, E. Aksenenko (Eds.), Theoretical and Experimental Studies of Interfacial Phenomena and Their Technological Applications: Book of Abstracts, VIII Ukrainian-Polish Symposium, SCSEIO: Odesa, September 19–24, 2004, pp. 274–276.
Investigation of surface properties of lunar regolith: Part I E. Robens a,*, A. Bischoff b, A. Schreiber c, A. Da˛browski d, K.K. Unger a a
Institut fu¨r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universita¨t, Duesbergweg 10-14, D-55099 Mainz, Germany b Institut fu¨r Planetologie, Universita¨t Mu¨nster, Wilhelm-Klemm-Str. 10, D-48149 Mu¨nster, Germany c POROTEC GmbH, Niederhofheimer Str. 55a, D-65719 Hofheim, Germany d Uniwersytet Marii Curie-Skłodowskiej, Fac.Chimii, pl.M.Curie-Skłodowskiej 2, PL-20-031 Lublin, Poland Available online 5 January 2007
Abstract This paper describes an initial investigation of the surface properties of three lunar soil samples from the Apollo 11, 12 and 16 missions, respectively. We report on density measurements using a helium pycnometer, adsorption isotherms of krypton applied for the determination of specific surface area of the samples and gravimetric measurement of the isotherms of water, heptane and octane. Electron-microscopic photographs are described and discussed. # 2007 Elsevier B.V. All rights reserved. PACS : 96.20. n; 92.30.Dt; 68.43. h Keywords: Moon; Regolith; Surface; Adsorption; Water
1. Introduction
2. Samples
Lunar soil and rock samples of the Apollo and Luna missions 1969–1976 have been already examined in detail [1–5]. Planning of new missions and the establishment of a manned station at the Moon require some additional information. The assumption of an inventory of water ice [6,7] suggests studying the surface properties of regolith in contact with water [8]. We expect that water – if present on the moon – exists exclusively in the form of chemisorbed and vicinal water immersed in the regions protected from sun irradiation. The aim of this investigation is to obtain parameters relevant to the capability of the Moon surface to store water, both the amount and kinetics adsorption/desorption in relation to the environmental conditions. New investigation methods and new evaluation procedures are applied, which had been developed recently by us and elsewhere [9–11].
Looking at the Moon with the naked eye, it can be seen easily that there are bright and dark areas called ‘maria’ on its surface [12]. Telescopic observations showed that the maria are very flat, and quite different from the heavily cratered and mountainous so-called highlands. We have learned that the maria are relatively young areas on the Moon generated after very large impacts penetrated the crust of our Moon and excavated the basins. During later volcanic episodes, liquid magma came to the surface and filled these basins. As this happened in comparatively recent times, the number of impact craters is far fewer than in the highland areas. Impact processes have formed the lunar regolith. Lunar soil is the subcentimeter fraction of the lunar regolith. Detailed studies have shown that five basic particle types make up the lunar soils: mineral fragments, pristine crystalline rock fragments, breccia fragments, glasses of various kinds, and the unique lunar structured particles called agglutinates [1]. The mineral and chemical composition of the lunar soils depends on the mission landing sites [13]. Apollo 11 and Apollo 12 landed well inside mare basalt regions, and consequently, soil samples from these sites have abundant mare-derived basaltic rock clasts and mafic minerals like olivine and pyroxene. On the other hand, Apollo 16 landed in
* Corresponding author. E-mail addresses: [email protected] (E. Robens), [email protected] (A. Bischoff), [email protected] (A. Schreiber), [email protected] (A. Da˛browski), [email protected] (K.K. Unger). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.098
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highland regions. Soils from this mission contain abundant highland-derived components, e.g., lithic fragments of anorthositic rocks, breccias, and anorthitic feldspar. We started to investigate three soil samples from those missions, each of about 3 g.
gravimetric techniques using that DVS apparatus at 24.9 8C. The specific surface area was calculated using the molecular areas of n-heptane and n-octane of 0.57 nm2 and 0.63 nm2, respectively. 4. Results
3. Measuring methods and instruments 4.1. Visual inspection 3.1. Scanning electron microscopy The fine-grained textures of the three lunar soils were studied with a field-emission Scanning Electron Microscope (JEOL JSM-6300F) at the University of Mu¨nster (ICEM, Interdisciplinary Center for Electron Microscopy and Microanalysis) operating at 5 kV. 3.2. Density Helium displacement within a calibrated vessel [14] at 20 8C was applied using a PYCNOMATIC ATC instrument of Thermo Electron S.p.A., Milano, Italy. 3.3. Specific surface area—krypton adsorption
All samples consist of weakly coherent fines. Sample 12001.922 and sample 12001.922 appear light grey and including a few white particles. Sample 10084.2000 had a dark grey colour. 4.2. Scanning electron microscopy The Apollo 16 soil sample 64501 contains abundant clastic grains (Fig. 1a), but also impact melt spherules of various sizes (Fig. 1b). Fragments with internal pores like those found in the Apollo 11 and 12 soils (e.g., Fig. 2a) are either rare or absent. Kempa and Papike [15] found that in the particle size range of 10–90 mm sample 64501 is highly feldspathic having more than 50% plagioclase. In the Apollo 12 sample more small-grained and more molten and porous particles are found compared with
Prior to the adsorption measurements the samples had been degassed at 80 8C for 4 h. This is far below the temperature which occurs on the Moon during its diurnal cycle. Krypton adsorption isotherms were measured stepwise at 77.1 K using a volumetric/manometric apparatus SORPTOMATIC of Thermo Electron S.p.A., Milano, Italy. In the range of about 0.07 < p/p0 < 0.23 the specific surface area was calculated by means of the 2-parameter BET equation using a molecular area of 0.195 nm2. 3.4. Water vapour adsorption Several investigations of lunar soils are reported in literature [1–5] in which microbalances had been used to measure adsorption and desorption isotherms. The authors report on difficulties in calibrating the balances probably due to adsorption of water on various parts of the balance [2,8]. In the present investigation therefore the apparatus was not evacuated and the case containing the sensor system was always protected by flushing it with dry nitrogen. We used a DVS 1/Advantage apparatus of Surface Measurement Systems, Ltd., Wembley, Middlesex, U.K. The apparatus encompasses a Cahn microbalance with a maximum load of 1.5 g. Water vapour pressure is adjusted and varied by means of a flow of nitrogen carrier gas saturated with water vapour. The isotherms were measured stepwise at a temperature of 24.9 8C slightly above ambient to avoid condensation within the apparatus. The specific surface area was calculated using a molecular area of 0.129 nm2. 3.5. n-Heptane and n-octane adsorption n-Heptane and n-octane adsorption and desorption isotherms were measured similarly to those of water by
Fig. 1. (a) (#64501-11) and (b) (#64501-14). Electron micrographs of lunar regolith sample 64501.228 of the Apollo 16 mission from highland regions.
E. Robens et al. / Applied Surface Science 253 (2007) 5709–5714
Fig. 2. (a) (#12001-22) and (b) (#12001-06). Electron micrographs of lunar regolith sample 12001.922 of the Apollo 12 mission from inside of a mare basalt region.
the Apollo 16 soil sample (Fig. 2b). Sample 12001 was previously characterized as a well-gardened soil, random mixture of debris fragments that mostly derive from nearby bedrock and whose particles largely consist of mare basalts or degradation products thereof [16]. The Apollo 11 soil sample (Fig. 3a) has a high abundance of glassy particles or features that indicate melting (Fig. 3b). It was found that the large proportion of the <10 mm material in the soil (14.2%) is consistent with its high maturity [17]. The 10084 soil is very fine-grained with a mean grain size of 51 mm and very poor uniformity. The high concentration of agglutinates (52%) was determined by Simon et al. [18].
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Fig. 3. (a) (#10084-05) and (b) (#10084-09). Electron micrographs of lunar regolith sample 10084.2000 of the Apollo 11 mission from inside of a mare basalt region.
On account of the cleaved and rough particle the Apollo 12 sample seems to have a larger surface than that of Apollo 16. However, the structures resolved in the micrometer range do not contribute much to the specific surface area as determined by gas adsorption. Inspection by means of a light-optical microscope of Apollo 14 fines and bearing in mind observed roughness and internal pore structure Cadenhead et al. [2] calculated a specific surface area of about 0.01 m2 g 1 whereas nitrogen adsorption revealed the values of 0.1– 1.0 m2 g 1. Thus, evaluations of that parameter from the image analysis or particle size distribution seem to be misleading.
Table 1 Density and specific surface area of regolith samples Sample
Density (g cm 3)
Specific surface area from Kr adsorption (m2 g 1)
n-Heptane adsorption (m2 g 1)
10084.2000 Apollo 11, Mare 12001.922 Apollo 12, Mare 64501.228 Apollo 16, Highland
3.10 0.01 2.79 0.01
0.579 0.653
n-Octane adsorption (m2 g 1)
water vapour adsorption (m2 g 1)
0.481 0.432
0.368
0.408 0.540
0.358 0.397
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lines intersecting the ordinate at small positive values. The resulting values of the specific surface area (0.3–0.7 m2 g 1) are within the region of values determined from N2 isotherms by other investigators (0.2–1.5 m2 g 1). 4.5. Heptane and octane vapour adsorption
Fig. 4. Krypton adsorption isotherm at 77.1 K of lunar regolith sample 12001.922. The ordinate represents the adsorbed gas volume at S.T.P. divided by the mass of the degassed sample. The abscissa is the measured pressure above the capillary (Ø10 mm) of the sample vessel related to the saturation pressure of 233 Pa. No correction was made concerning the thermal pressure difference in capillaries.
n-Heptane and n-octane ad- and desorption at ambient temperature proceeded rapidly, which indicates the absence of pores with the entrance width of a few molecular diameters. In addition, fast desorption indicates weak binding. The resulting isotherms correspond to type II of the IUPAC classification with nearly linear slopes within 0 < p/p0 < 0.7 and without hysteresis between adsorption and desorption branches (Fig. 5). This suggests the absence of pores having nanometer widths. The specific surface area determined with those
4.3. Density The He-density values for lunar soils range from 2.3 to >3.2 g cm 3 giving a mean value of 3.1 g cm 3 which has been recommended [1]. The results of our measurements listed are the mean values of repeated measurements. Our results are well within the range of the literature values (Table 1). 4.4. Krypton adsorption Kr adsorption had been measured at 77.1 K and all the resulting isotherms (Fig. 4) correspond to type II of the IUPAC classification [19–21]. BET plots show well defined straight
Fig. 5. n-Heptane adsorption and desorption isotherm at 77.1 K of lunar regolith sample 12001.922. (~) Adsorption and (!) desorption. ma Adsorbed heptane mass related to unit sample mass, p/p0 heptane pressure related to saturation pressure.
Fig. 6. (a) Measurement of the water vapour adsorption and desorption isotherm at 24 8C of lunar regolith sample 12001.922. The lower curve represents the preset relative humidity (%), the upper curve the adsorbed mass (mg). (b) Water adsorption and desorption isotherm at 24.9 8C of lunar regolith sample 12001.922. (~) Adsorption and (!) desorption. ma Adsorbed water mass related to unit sample mass, p/p0 water vapour pressure related to saturation pressure (relative humidity).
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adsorptives is lower than that determined with krypton. This is reasonable because the molecular diameters of n-hydrocarbons are of about 0.6 nm and are larger than that of krypton (diameter 0.36 nm). They cannot penetrate into the small pores which are accessible to Krypton. 4.6. Water vapour adsorption Water vapour adsorption and desorption at ambient temperature also proceeds rapidly (Fig. 6a). A diffusion process through small pores has no effect. The isotherm increases rather slowly from origin (Fig. 6b), consistent with the literature reports, which is typical of material having an intermediate character between hydrophilic and hydrophobic, and it indicates weak binding. Hysteresis loops extending over the whole region of relative pressure were observed which may be due to the fact that in many cases after changing the water vapour pressure a saturation value of the adsorbed mass was not attained. Furthermore, we should also keep in mind that these measurements are near the detection limit of the measuring method and therefore a rigorous interpretation of the results therefore is not possible. Indeed, the investigations reported in literature [1–5] show similar scatter of the results. The specific surface area determined with water vapour adsorption (diameter 0.28 nm) is lower than that determined with krypton (diameter 0.36 nm). The reason for this is unclear and confirms uncertainties in the measurement. 5. Conclusions Commercial density and sorption measuring instruments proved to be suitable for investigating the lunar soil samples. Sensitivity and resolution are comparable to those of instruments used in the investigations reported in literature [22–24]. No significant differences in sorption properties have been detected for the samples of different origin (mare and highland). Lunar regolith can hardly store water due to low specific surface area, little nano-porosity and a surface character lying between hydrophilic and hydrophobic. Structures like permafrost as observed on the Earth [25] and assumed on Mars [26,27] cannot be expected on the Moon, on which the conditions could not allow the existence of liquid water. Therefore, the gravity induced release of liquid water into the deeper regions is impossible. Transport of water under lunar conditions could take place only via the gas phase or in a quasiliquid phase of a few molecular layers [28] at the surface of the soil material. Water-ice is imported to the Moon by comets and cosmic dust particles. Remnants may be stored as thin, incomplete physisorbed or chemisorbed layers at the soil material, and bulk stocks are assumed in craters near the poles inaccessible to the sun radiation. Continuous evaporation takes place everywhere on the Moon surface. With regard to the diurnal temperature cycle evaporation will be stronger near the outer surface and is less obstructed by diffusion through the regolith layer. Therefore, physisorbed water should be found in some depth
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and water-ice – assuming it exists as a bulk phase – may be stored below the regolith covering. Acknowledgement The samples had been kindly placed for our disposal by the NASA Lunar sample curator, Dr. Gary Lofgren, Houston, Texas. References [1] G.H. Heiken, D.T. Vaniman, B.M. French (Eds.), Lunar Sourcebook, Cambride University Press, Cambridge, 1991. [2] D.A. Cadenhead, et al., Some surface characteristics and gas interactions of Apollo 14 fines and rock fragments, in: Proceedings of the Third Lunar Science Conference (Supplement 3, Geochimica et Cosmochimica Acta), M.I.T. Press, 1972, pp. 2243–2257. [3] D.A. Cadenhead, R.S. Mikhail, Water vapour weathering of TaurusLittrow orange soil: a pore-structure analysis, in: Proceedings of the Sixth Lunar Science Conference, 1975, pp. 3317–3331. [4] R.B. Gammage, H.F. Holmes, Blocking of the water-lunar fines reaction by air and water concentration effects, in: Proceedings of the Sixth Lunar Science Conference, 1975, pp. 3305–3316. [5] H.F. Holmes, R.B. Gammage, Interaction of gases with lunar materials: revised results for Apollo 11, in: Proceedings of the Sixth Lunar Science Conference, 1975, pp. 3343–3350. [6] D.R. Williams, Ice on the Moon, NASA, 2005, http://nssdc.gsfc.nasa.gov/ planetary/ice/ice_moon.html. [7] W.C. Feldman, et al., Polar hydrogen deposits on the Moon, 2006, http:// lunar.lanl.gov/pubs/2000/Polar_H_Deposits_on_Moon.pdf#search=%22Water%20moon%20Lucey%22. [8] E.L. Fuller Jr., P.A. Agron, Sorption by low area materials, in: C. Eyraud, M. Escoubes (Eds.), Progress in Vacuum Microbalance Techniques, Heyden, London, 1975, pp. 71–82. [9] P. Staszczuk, D. Sternik, G.W. Chadzynski, Application of quasi-isothermal thermogravimetry and sorptometry for estimation of heterostructure and fractal dimension of high temperature superconductors, J. Therm. Anal. Cal. 71 (1) (2003) 173–182. [10] P. Staszczuk, et al., Studies of heterogeneity properties of selected hightemperature superconductor surfaces, J. Therm. Anal. Cal. 86 (1) (2006) 133–136. [11] V.V. Kutarov, E. Robens, B. Kats, Universal function for the description of multilayer adsorption isotherms, J. Therm. Anal. Cal. 86 (1) (2006) 35–38. [12] J.-L. Josset, B.H. Foing, SPACE-X Space Exploration Institute and ESA SMART-1 Project, 2006. [13] D. Sto¨ffler, A. Bischoff, et al. J. Geophys. Res. 90 (1985) C449–C506. [14] A. Da˛browski, et al., Standardization of methods for characterizing the surface geometry of solids, Particle Particle Sys. Character. 20 (5) (2003) 311–322. [15] M.J. Kempa, J.J. Papike, The Apollo 16 Regolith: Comparative Petrology of the >20 and <20 mm Soil Fractions, In Lunar and Planetary Sciences XI, The Lunar and Planetary Institute, Houston, 1980 , pp. 535–537. [16] J.A. Wood, et al., Mineralogy and Petrology of the Apollo 12 Lunar Sample, SAO Special Report, SAO, #333, 1971. [17] A. Basu, S.J. Wentworth, D.S. McKay, Submillimeter grain-size distribution of Apollo 11 soil 10084, Meteor. Planet. Sci. 36 (2001) 177–181. [18] S.B. Simon, J.J. Papike, J.C. Laul, The lunar regolith: comparative studies of the Apollo and Lunar sites. petrology of soils from Apollo 17, Lunar 16, 20, and 24, in: Proceedings of the Lunar Planet. Sci. Conf. 12B, 1981, pp. 371–388. [19] R.S. Mikhail, E. Robens, Microstructure and Thermal Analysis of Solid Surfaces, Wiley, Chichester, 1983. [20] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders & Porous Solids, Academic Press, San Diego, 1999.
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[21] K.K. Aligizaki, Pore structure of cement-based materials, Modern Concrete Technology, vol. 12, Taylor & Francis, London, 2006. [22] E.L. Fuller Jr., et al., Volumetric and gravimetric methods of determining monolayer capacities, Thermochim. Acta 29 (1979) 315–318. [23] E. Robens, A.W. Czanderna, J.A. Poulis, Surface area and pore size determination on finely divided or porous substances by adsorption measurements: comparison of volumetric and gravimetric methods, Powder Metallur. Inter. 12 (4) (1980) 201–203. [24] J.U. Keller, E. Robens, C. du Fresne von Hohenesche, Thermogravimetric and Sorption Measurement Techniques/Instruments, in: F. Rodrı´guezReinoso, et al. (Eds.), Characterization of Porous Solids VI., Elsevier, Amsterdam, 2002, pp. 387–394.
[25] D.M. Anderson, N.R. Morgenstern, Permafrost Proc., in: Permafrost Second International Conference, National Academy of Science, Washington, DC, 1973. [26] D. Mo¨hlmann, Liquid-like adsorption water in the Martian soil at mid and low latitudes, Icarus 4 (2003). [27] D. Mo¨hlmann, Unfrozen subsurface water on Mars: presence and implications, Int. J. Astrobiol. 2 (3) (2003) 213–216. [28] E. Robens, Some remarks on the interface ice/water, in: Sergijiwka-Odesa, Ukraine, R.L. Yu. Tarasevich, B. Kats, E. Aksenenko (Eds.), Theoretical and Experimental Studies of Interfacial Phenomena and Their Technological Applications: Book of Abstracts, VIII Ukrainian-Polish Symposium, SCSEIO: Odesa, September 19–24, 2004, pp. 274–276.