The surface reduction of lunar fines, 14163,111

The Surface Reduction of Lunar Fines, 14163,111 Exposure of Lunar fines (14163,111) to atomic hydrogen resulted in a surface reduction, the final surface density of the added hydrogen being 2 hydrogen atoms/100 ~.~. Exposure to oxygen at 25°C produced a partial reoxidation of the surface while nitrogen had no effect. I t is postulated that the solar wind will completely reduce materials on the lunar surface but that exposure to a terrestrial or similar atmosphere will result in rapid partial reoxidation. Between the high-energy protons in the solar wind and the trapped hydrogen found in lunar soil samples there exists the possibility of reduction by either atomic or molecular hydrogen. We have previously shown that molecular hydrogen can have little if any effect. Thus, while it will physisorb at - 196°C, even this is already absent at - 7 8 ° C and 0°C (1). While it is possible that dissociative chemisorption of molecular hydrogen could occur below the midday lunar equatorial temperature of I30°C (2), it must be a slow process. Because of this we have turned our attention to the possibility of reduction through the more reactive atomic hydrogen. While there is evidence from the bulk composition of lunar materials of such a reduction, it would seem that the exposed surfaces of these materials should exist in a more or less fully reduced state (2). Assuming that this is in fact the case, the question arises: "Would exposure to a terrestrial atmosphere in any way alter this state, in particular would such an exposure, however brief, bring about a reoxidation ?" Since the lunar atmosphere is of the order of 10-12 Tort (3, 4), there would appear to be little hope of obtaining a completely uncontaminated sample. Exposure to the lunar module exhaust, to the atmosphere leaking from the astronaut selecting the sample, to outgassing of the sample container and to nitrogen (including trace impurities) for shipment; all lead to some surface contamination. In addition further accidental and designed exposure to a terrestrial atmosphere has occurred with many samples. At best, we can hope to reduce such exposure to an unavoidable minimum. From the point of view of a surface chemist we must conclude that all samples returned have suffered at least some exposure. On the basis of the above observations, most returned samples would appear suitable candidates for our investigation. For our initial study however, we selected a sample of the bulk fines returned from the Apollo 14 mission (14163) which suffered extensive accidental exposure in that it was found to be at one atmosphere pressure when the sample container was opened at the Lunar Receiving Laboratory in Houston (5). More-

over the oxygen-rich nature of the atmosphere suggested that the leakage had occurred in the lunar or command module during the return to Earth. Clearly if any sample was likely to reveal a partially or completely oxidized surface through exposure, this would be the one. The precise sample chosen for this study was a 0.7-g portion of a 5-g sample 14163,111, designated fine fines ( < 1 mm particle diameter). In fact, particle size was much smaller than this, being closer to 10/~m, although the surface area was one of the smallest so far reported (0.21 m~/g) 1 perhaps because of the high glass content of the sample. (See Table I.) Unfortunately we have no precise information on the immediate surface composition of this sample. Electrolytic hydrogen was purchased from the Linde Division of Union Carbide Corp. and was further purified by passing it through a catalytic hydrogen purifier (for O, removal) and subsequently through a glass bead, liquid-nitrogen trap to remove condensables. Oxygen and nitrogen were also supplied by Linde and were passed through the liquid nitrogen trap before storage and use. The gas adsorption chamber was of all glass construction and approximately 150 ml in volume (Fig. 1). The lunar sample was placed on the floor of this chamber, above which was mounted the tungsten filament approximately 1 cm away. A 6-V dc source was connected to this filament through a variable resistance, and an ammeter, and after considerable experimentation, a current of 1.5 A was found to be the minimum value needed to produce a reasonable rate of thermal dissociation of hydrogen molecules. Through the use of a calibrated optical pyrometer these conditions were related to a temperature of 1250°C at the center of the tungsten filament. The sample temperature was read with a chromel-alumel thermocouple imbedded directly in the sample. Temperature could be read to the nearest 0.01°C. Finally the gas pressure was read with a Pirani gauge tube and controller. These were, respectively, a GP-003 tube and GP-310 controller supplied by Bendix Scientific. The sensing coils

650 Journal of Colloid and Interface Science, Vol. 42, No. 3, March 1973

Copyright; ~ 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.

NOTES

651

TABLE l COMPOSI'.rlON 01," APOLLO I4 BULK FINES 14163 Modal composition %

<37 #m 37-63 #m

Feldspar Olivine and Pyroxine

3 20

7 43

Lithic Glass Spheres and Droplets

-72 1

-48 2

Other

Mineral and elemental composition in ppm except where stated

$10~¢ AlcOa~ MgO ~ FeO ~ CaO ~ TiO~"

4

46 17 11.5 10 11 2.0

La Sr Ba

38 210 720

Sc V Yb

23 52 25

Co Cu

40 16

Y Zr

200 860

Na20~

0.40

Li

20

Th

14

K20 ~ MnO ~ Cr203 ~

0.50 0.19 0.21

Ni Nb Rb

340 48 13

U Cb

4 70-150

Values in weight %. b The precise figure is dependent on the shipping container. were housed in a nickel-coated copper tube connected directly to the apparatus by a metal-to-glass seal. Before the introduction of any sample, the filament was outgassed using a 2.0 A current until no further evolution of gas was noticed. Hydrogen was then admitted in the working pressure range (1-50 reTort) and the filament cycled on and off several times to minimize any subsequent production of extraneous gases such as carbon monoxide. The Lunar sample was then placed in the chamber and heated to 150°C ± 2°C for 6 hr at which point no further evolution of gas could be detected. The filament was then outgassed at the operating current of 1.5 A. This time it was not turned on continuously but rather in bursts of 2-rain duration to negate any possibility of vacuum deposi-

,,/p u m p i n g

tion of tungsten on the sample. This procedure was carried out until Ilo further evolution of gas could be detected. Hydrogen was introduced to a pressure, approximately 1.5 X 10.3 Torr, and the pressure and temperature read. Since the sample chamber volume was previously determined by gas expansion using a mercury-calibrated volume, the totM gas dose could be calculated. The filament was then turned on and a pressure observed and monitored (Fig. 2). After 2 rain tile pressure stabilized and the filament was turned off. While the filament was on, the sample temperature rose from 25°C to 60°C. Because of this, it was necessary to wait 4-6 min for the sample to return to room temperature in order to read the pressure. The amount

rncmifold

( P

Calibolred ~ voiume

Pironi gouge

%,

""

/

e

*x Thermocouple

Tungsten

fi{omenl

Fro. 1, Atomic hydrogen adsorption cell. For details see text. do~rnaI of Colloid and Inlerface Science, Vol 42, No. 3, March 1975

652

NOTES

7

6 Pressure (io~r x 10-3)

X I ~

off

on

f

(2rid dose) 2n6 ~ s e

g "

I

o,,

o,

]

L

It.

off

on off

3

l

~

100

300

I

I

I

500

700

900

] 1300

II00

2

t (seconds)

Fro. 2. Cell pressure as a function of time and tungsten filament activation. Left hand ordinate: Pressure in Torr X 10-~ for the first dose of hydrogen. Right hand ordinate: Pressure in Torr X 10-~ for the second dose of hydrogen. Abscissa: Time in seconds. The terms "on" and "off" refer to the activation of the tungsten filament. of gas adsorbed was then calculated frmn the difference in the "before" and after pressures and from a number of such calculations the entire adsorption isotherm was generated. Blank determinations were run under similar pressure and temperature conditions and the slight amount of adsorption observed subtracted from the adsorption data at each pressure. Subsequently, either oxygen or nitrogen was added to the system at a pressure of approximately 7 X l0 -* Torr for a specified time and then pumped out. The sample was then heated and outgassed at 150°C for 30 rain while the filament was outgassed by 2-rain

on-off cycles. The sample was then rechecked for hydrogen atom uptake by the above described procedure. The effect of outgassing the sample alone was determined by completely saturating the sample with atomic hydrogen, performing the outgassing, then measuring any subsequent hydrogen uptake. The rate of adsorption of atomic hydrogen for the first two doses is shown in Fig. 2. The initial H2 pressures were 2.95 and 4.90 mTorr and the final pressures 1.52 and 1.94 mTorr, respectively. The hydrogen atom uptake diminished with successive "on" cycles and only a small amount of atomic hydrogen was taken up

}4°F 28 × 24 *u

2o o 12

.g t, 8

°~

0

I

[

;

2

4

6

~ B

I0

I

J-

I

J-

T

i2

],4

16

18

20

~ 22

t

l

.J

24

26

20

P (lorr x 10 -3)

FIG. 3. Ordinate: Amount of hydrogen adsorbed (expressed in moles of t-Is X 10-8/g of lunar sample). Abscissa: Pressure of hydrogen in Torr )< 10-~. Journal of Colloid and Interface Science, Vol. 42,

N o . 3, M a r c h

1973

NOTES when the filament was switched on for the third time. Because of the relatively slow response of the Pirani gauge no analysis of the rate of adsorption was attempted with this data. The complete adsorption isotherm for atomic hydrogen (reported as Ho) is shown in Fig. 3. It is essentially linear up to the final adsorption value. At complete adsorption, the coverage represents 52 ~2 per H atom. The results of various gas exposure treatments and outgassing or bakeout are shown in Table II. The sample was exposed to 7 X 10-a Torr of oxygen for periods of 3 and 18 hr and to 7 X 10-a Torr of nitrogen for a period of 12 hr. In addition the effect of pumping on a room-temperature sample at 10-6 Tort for 12 hr and the effect of a 150°C bakeout procedure on the surface state of reduction were determined. In each case, the exposure experiments were followed by an outgassing. Thus the hydrogen atom adsorption reported has been corrected by the amount normally lost on outgassing (4.2 X 10-~ mole). The amounts retained after outgassing would thus correspond to the minimum residual chemisorbed hydrogen under maximumtemperature lunar conditions. Our results clearly show that 14163,111 has suffered a certain amount of oxidation and that exposure to atomic hydrogen will reduce the surface of lunar materials. Subsequent exposure to oxygen at room temperature is sufficient to bring about a reoxidation, particularly when that exposure is prolonged. However, there is in fact, little difference between the 3 and the 18-hr oxygen room-temperature exposure effects, and experiments presently under way on other samples suggest that exposures of less than ten minutes can bring about a significant partial surface oxidation. The facts t h a t after prolonged oxygen exposure the oxidation is only partial and that 150°C outgassing can only remove a small part of the hydrogen, indicate there exists a range of bonding strengths for the chemisorbed hydrogen. That the hydrogen is chemisorbed and not physisorbed is indicated by the failure to remove hydrogen on outgassing at 25°C and 10-6 Torr for some 17 hr, the ability to remove only a small amount on outgassing at 150°C and the absence of any effect with nitrogen. From the results described above, and from further experiments presently under way, we conclude the exposed surfaces of uncontaminated lunar materials should be fully reduced by the solar wind. This would mean, not only that the surfaces of these complex silicated materials would exist in their lowest oxidation state, but that other trace dements (see Table I) would also be reduced. Thus, particularly at the surface, we would expect solar wind-cabron interactions to produce a wide range of hydrocarbons (6-9). There is also no doubt that even a brief exposure to a terrestrial or similar atmosphere will result in a fairly rapid partial oxidation of typical lunar materials. Work is presently

653 TABLE II Sample treatment:

Amount of hydrogen (as H~.) readsorbed after treatment (mole X 10-9

Initial, 150°C bakeout Overnight pumping at 10-~ Torr at 25°C 3 hr, O2 exposure at 25°C 18 hr, O2 exposure at 25°C 12 hr, N.~ exposure at 25°C

24.0 See text 11.4 13.9 No effect

under way to extend this study to other samples and if possible to establish the nature of the reduced and oxidized surfaces. ACKNOWLEDGMENTS This research was supported by the National Aeronautics and Space Agency, Grant No. N.G.R. 33-183004. We wish to express our thanks to Miss Lynn Cope for typing this manuscript. REFEREN CI-S I. CADENtlEAD, D. A., WAOXER, N. J., JONES, B. R., A~D STETTEG J. R., "Proc. Third Lunar Science Conference," Vol. 3, pp. 2243 2257. M.I.T. Press, Cambridge, Mass. (Geockent. Cosmockim. Acla Suppl. No. 3) (1972), 2. CooK, M. A., J. Colloid intetJace Sci. 38, 12 (1972). 3. REED, G. W., GOLEB, J. A., AND JOVANOVIC, S., Science 172, 258 (1971). 4. JoHxsox, 1". S., in Lunar Science III, (C. Watkins, ed.), Lunar Science Institute, Houston, Texas (1972). pp. 436-438. 5. N.A.S.A. Report "Fines Samples Collected by Apollo 14," June (1971). 6. CADOGEN, P. H., EGLINGTON, G., MAXWELL, J. R., AND P1LLINGEE, C. T., Nalure (London) 231, 29 (197l). 7. CADOOEX, P. H., EGLINGTON, G., I"IRTIb J. N. M., MAXWELL, J. R., MAYS, B. J., AND PILLINGER, C. T., "Proc. Third Lunar Science Conference," Vol. 2, pp. 2069 2090. M.I.T. Press, Cambridge, Mass. (1972). 8. PILLINGER, C. T., CADOGEN, P. H., EGLINGTON, G., MAXWZLL, J. R., MA':S, B. J., GRAt,'T, W. A., AND NOBES, 3I. J., Nalm.e (London) 235, 108 (1972). 9. R,~H~vmxL D. W., K~A~R, K., AXD MOSEG H. C., Trans. Faraday Soc. 67, 2333 (1971). D. A. CAOE~,'n~AD B. R. JoxEs

Deparlmenl of Chemistry Slate University of New York at Buffalo Buffalo, New York 74214 Received November 20, 1972; accepled December 8, 1972

Journal of Colloid and Interface Science, Vol. 42, No. 3, March 1973