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Proposed in situ secondary ion mass spectrometry on Mars
Planet. Spcwe .%i.. Vol. 43. Nos. I/?. pp. 129~-137. 1Y45 Copyright T: I995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-~-0633:95 $9.50 + 0.00
Pergamon
0032-0633(94)0022-X
Proposed in situ secondary ion mass spectrometry on Mars R. L. Inglebert,’ B. Klossa,’ J. C. Lorin’ and R. Thomas’ ‘GREMI. Universiti d’Orleans. 45067 Orleans, France ‘Laboratoire de Physique des Solides, Universite Paris-&d, 91405 Orsay, France ‘Laboratoire de Physique et Chimie de I’Environnement, CNRS. 45071 Orleans. France Received 29 March 1994 ; revised 24 October and 28 November 1994 ; accepted 28 November 1994
Abstract. Secondary ion mass spectrometry
is a powerful analytical tool, which has the potentiality, through molecular ion emission, of detecting minor phases, as well as the unique capability of directly measuring isotope abundances in mineral or organic phases without any prior physical, chemical or thermal processing. Applied to the in situ analysis of the Martian regolith, it can provide evidence of the presence of carbonates and, by inference (if carbonates constitute significant deposits), of past liquid water-a necessary condition for the development of life. In addition, oxygen isotopic composition of carbonates preserves a record of the temperature at which this phase precipitated and may therefore help decipher the past climatology of Mars. Detection of a carbon isotopic composition shift between carbonates and organic matter (on Earth, the result of a kinetic fractionation effect during photosynthesis) would provide a definite clue regarding the existence of a past biochemical activity on Mars.
Introduction The search for evidence that life originated on Mars is still a primary reason for further scientific exploration of the planet. One of the conditions generally agreed upon for the development of life is the presence of liquid water for which there seems to be ample geomorphological evidence. One way of achieving sufficiently high temperatures for water to be liquid on Mars is to have an initially dense CO, atmosphere that promotes a greenhouse effect (Moroz and Muhkin, 1977). Such a dense CO, atmosphere is indeed expected from early degassing of the planet (ref. in Kahn, 1985). Substantial carbonate deposits in the then existing bodies of water, making up the most reasonable reservoir for the excess CO?, should now be present in the Martian regolith. So, detection of
these carbonates, beside providing strong and independent evidence of past liquid water. would greatly contribute to our understanding of the past and present interaction of the atmosphere and surface on Mars. Moreover, if feasible. their chemical and isotopic characterization would provide us with some knowledge of the past climate of the planet and its momentary or episodic suitability for sustaining life. Temperature at the time carbonates precipitated can indeed be monitored by the measurement of oxygen isotope abundances in this mineral phase. allowing evaluation of the local ecological stability, which is critical for life to develop. An even more direct test of the eventuality of a past biochemical activity on Mars can be made by measuring the difference in the carbon isotopic composition of organic matter and carbonates. It is known indeed on Earth that significant isotope fractionation favoring the light isotope is induced by the metabolism of autotrophic organisms that fix atmospheric co,. All these data can. in principle. be provided by one technique: secondary ion mass spectrometry (SIMS). Because of its multifaceted potentialities-sensitivity, ability to detect geochemically important light elements, and unique capability in measuring stable isotopes on single mineral grains-SIMS may indeed. in the context of a future exploration and irzsitu analysis of the surface of Mars, yield valuable chemical and isotopic data upon mineral constituents of the soil and outcropping rocks as well as information on the composition of surface layers of mineral grains. The present study. a short account 01 which has been given elsewhere (Inglebert et al., 1994). has been carried out as a preliminary test of the method for detecting and analyzing biogenic elements as well as characterizing their compounds in the context of a planetary regolith.
Experimental The principle of SIMS rests upon the fact that impact of a heavy ion on a surface at energies of a few keV results
R. L. Inglebert
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et al. : Proposed
CI situ secondary
ion mass spectrometry
on Mars
Table 1. Experimental conditions relevant to the three instruments, sector, quadrupole and time-of-flight analyzers. Nature, current and beam size of primary ions, mode of sample preparation (polished section or grain powder dispersed upon an aluminum substrate), acceleration voltage and selected energy band of secondary ions, mass resolution (FWHM) as well as order of magnitude of Ouseful ion yield in the sputtering of silicates and carbonates are listed
Analyzer
Source
Primary current (nA)
Sector Quad
Cs+ Kr’
0.05-O. 1 400
ToF
69Ga+
0.5
Beam size (pm) 0.5 (rastered 400
50-100
30)
Sample
Voltage
Energy band
section powder/Al
4500 v 2040 V
20 eV IOeV
powder/Al
4000 v
extended
in the ejection of surface material with typical yields of one to several atoms per incident ion. A fraction of this sputtered flux is ionized during the ejection process and may be analyzed in a mass spectrometer. Biogenic elements such as carbon, oxygen, nitrogen are best analyzed as secondary negative ions because of the high electronic affinity of atomic species such as C (1.263 eV), 0 (1.46 eV) or molecular species such as CN (3.82 eV). Therefore, the work reported here has concentrated on the study of negative secondary ions emitted from pure phases expected to be present in the Martian regolith, such as silicates, carbonates and carbon compounds, using three different types of instruments : (i) a double focusing magnetic mass spectrometer (Cameca) with a mass resolving power adequate to resolve interfering molecular ions in the mass range of interest and a charge compensation system for the analysis of electrically insulating materials ; (ii) a quadrupole mass spectrometer (Balzers) ; (iii) a timeof-flight spectrometer. The latter instruments, small, lightweight and with limited power consumption may be adapted to comply with the requirements of a space mission for ill situ analysis of planetary surfaces (a noteworthy attempt has been made to analyze the surface of Phobos, one of the moons of Mars, with quads (Balebanov et al., 1988)). Relevant analytical conditions for the three instruments are summarized in Table 1. Two types of sample preparation have been used in the course of this work: polished mineral sections with vapordeposited metal coating and mineral grain powders dispersed upon a polished metallic plate (average grain size : 10 pm ; coverage : 15-30%). Minimization of the charging-up of insulating solids in the extraction of negative secondary ions (and especially when using reactive primary ions that promote the production of negative secondaries) is a key condition for reproducible and meaningful SIMS analysis. This has been achieved in the case of the magnetic sector analyzer by flooding the sample surface with low-energy electrons (Slodzian, 1987). In order to alleviate charging-up with the quad and time-offlight analyzers, samples were crushed down to a fine grain powder before being deposited on a metallic holder.
Results and discussion Elementary
analysis
Low wzuss resolution. One interesting feature of SIMS is its capability of phase detection and characterization. In
Mass resolution M/AM = 8800 AM = 0.5 M/AM = 1070 (m = 29)
O- ion yield 1-3 x IO-’ 5 x 10-n IO-’
order to evaluate this capability in a geological context, negative secondary ion mass spectra have been obtained with a magnetic sector mass spectrometer under Cs+ bombardment at comparatively low mass resolution (M/AM = 300) on different mineral or organic phases expected to be present in planetary regoliths. As shown in Fig. I, the spectra obtained on a silicate (olivine from the Eagle Station pallasite (Buseck. 1977)), a carbonate (Iceland spar) and organics (biological tissue) are characteristic enough to be used to identify these materials. A closer look at Fig. 1 reveals that, besides the O- secondary ion emission that these phases have in common, silicates show peaks at masses 28 and 60 corresponding to Si- and SiO, ions, carbonates, peaks at masses 12 and 60 corresponding to C- and CO, ions (Benninghoven, 1969) and organic compounds, peaks corresponding to CN- as well as to C-, C;, . , C; ions, the latter with relative abundances showing the same type of parity effects as observed for cluster ions emitted from pure carbon (Blaise, 1978). An interesting feature is that nominal masses 12, 16, 24 and 28 amu are virtually free from any molecular interferences and may therefore safely be used to derive unadulterated information from spectra recorded at low mass resolution. A peak at mass 28 will then indicate a silica bearing mineral as will a peak at mass 12, in the absence of carbon contamination of the sample surface, identify a carbon bearing phase. Among carbon compounds, discrimination between organics and carbonates can readily be made because of the widely contrasted values of their respective O-/C- and CT/Cratios. O-/C- = 0.3 in organic matter compared to 200 in carbonates ; C;/C- = 1.5 in organic matter compared to 0.05 in carbonates. Another useful criterion is the distinctly higher ion yield of C- in organic matter and allotropic varieties of carbon than in carbonates. The C- yield in the latter phases is found to be 70 times smaller than the O- yield. It follows from these observations that identification of carbonates in a mixture of carbon-bearing phases and silicates is feasible simply by inspection of low resolution negative secondary ion mass spectra. Furthermore, abundance of carbonates can be quantitatively evaluated by measuring the secondary “C- ion intensity corresponding to a given Cs’ primary ion bombarding density. provided the background level of “C- from the spectrometer residual vacuum is known. Table 2 shows the “C- ion intensity from carbonate samples measured with sector mass spectrometry and experimental conditions as listed in Table 1. It is clear, in this case, that the high “C-
R. L. Inglebert ct trl. : Proposed in situ secondary ion mass spectrometry on Mars 400 .
$ 300 Y .e 200 z a, c 100.
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OH
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Si
i02H
0 30
20
I
400
60
50
40
02
80
90
100
110
80
90
100
110
100
110
02H
O OH
300
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fz -
70
Fe02H
Calcite
e
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Fe02
,. 10
g
131
100,
MgO
0 C 1.CH 10
In. 20
cap
-- \ :.- ’ 1
C2 -- CN
40
30
50
r
Ca02
60
Ca02H
70
Organic Matter
1.. 60
C6C6H I. 70
.
C7
80
C8
._ 90
Mass number Fig. 1. Comparison of low resolution mass spectra (linear scale) of silicates. carbonates and organics showing the possibility of phase diagnosis with SIMS. Silicates are characterized by peaks at masses 28 and 60, corresponding to Si- and SiOi ions. whereas carbonates exhibit peaks at masses I2 and 60 corresponding to C and CO, ions. The intense emission of negative secondary ions of carbon and carbon compounds in the sputtering of organic compounds (biological tissue) is highly characteristic ; it contrasts with the low ion yield of carbon in carbonates. 160- count rates : 20 and IO Mc SK’ for olivine and calcite. respectively. C; count rate : 1.4 MC s-’ for organic matter. Sector analyzer
background intensity (430 c s-‘) is the limiting factor for carbonate detection. Even so, low mass resolution SIMS makes it possible to detect carbonates at a level (_ 150 ppm) far below the upper limit of l-3 wt% presently set
to the abundance of carbonates in the Martian regolith by remote spectroscopy (see, e.g. Blaney and McCord. 1989). The negative secondary ion mass spectrum clearly reflects, for a given mineral phase, its specific chemical
composition. This is illustrated in Fig. 2 where are displayed mass spectra obtained at low resolution on two carbonates : calcite (CaCO,) and magnesi te ( MgCO,). The rise of the peak at mass 40 amu (corresponding to MgO- ions) with increasing magnesium content is manifest. demonstrating that qualitative chemical analysis is possible. Quantitative chemical analysis with SIMS is well known to be complicated by matrix effects. At low mass resolution it will suffer from a further complication caused
Table 2. Carbonate analyzer. minimum seen that chemical
detection limits at both low and high mass resolution using a magnetic sector The reported detection limits have been calculated assuming counting times of 100 s. the detectable signal being chosen so as to exceed the background noise level by 4 0. It can be a better detection limit can be achieved at high mass resolution because of the distinctly smaller background at mass 60
Mass resolution Low High
Selected mass
Raw signal (kc ss’)
Bkgd (c s-‘)
I? 60
51 67
430 50
Detection level (ppm) 150 50
R. L. Inglebert et al. : Proposed in situ secondary ion mass spectrometry on Mars
132
300 .
Y p
200
ii &
100.
Magnesite
4
400 r s
0
OH
MgO
02
MgOH
02H C CH
0
‘-
‘*. 20
10
400
C2 CN ---
Mg03 30
40
50
60
90
100
110
90
100
110
Calcite
4b
0
80
70
02
OH
g 300 Y .$ 200 c” E 100
02H
CaOH CaO
Mg C CH
0 10
,
C2 20
CN
. . 1,. 30
co3 Ca02
,I 40
50
60
70
I
Ca02H 80
Mass number Fig. 2. Low resolution mass spectra (linear scale) of magnesite and calcite. Superimposed on the pattern of C and 0 (atomic and molecular) negative ion emission typical of carbonates, specific molecular combinations of magnesium and calcium permit determination of the carbonate composition in the calcite group. The peak at mass 40 is essentially constituted by MgO- ions with a minor contribution of CzOp ions. while both CaO- and MgO; ions contribute to the peak at mass 56. ‘(‘O- count rate: 10 MC s-‘. Sector analyzer
by mass interferences, since molecular combinations of the form MH-, MO-, MO;, MO,, etc., may interfere on the mass spectrum with the analytical ions. Negative secondary ion spectra obtained on calcite with the three different types of analyzers-sector, quad and time-of-flight-are compared in Fig. 3. In spite of the different analytical conditions employed (sample preparation, nature of primary ions-Cs+ is known to appreciably raise the negative secondary ion yield (Krohn, 1962)-solid angle of collection and energy bandwidth of secondary ions, etc.) a feature common to all these spectra is the presence of characteristic peaks corresponding to O-, 0; and CO; ions emitted from the target material. In addition, positive identification has been made on mass spectra obtained on magnesite with a quadrupole analyzer of MgO- and MgO; ions. It may be of interest to note at this point that good agreement between spectra obtained with a quadrupole analyzer on the one hand and with a sector analyzer on the other hand, is also observed in the case of positive secondary ion emission (Thomas et al., 1990). A point of concern, however, is the presence on the mass spectra obtained with both quad and timeof-flight analyzers and displayed in Fig. 3, of abundant contaminant ions, such as F-, Cl-, CN- and CNOions. This is due in part to the peculiarities of the sample preparation, since the sample is deposited as a fine powder upon a metal plate: contaminants can, in principle, be introduced either at different steps of the plate polishing procedure or during the grinding down process. Contamination is also due to interaction with the spectrometer residual vacuum, and this is especially true of the time-of-
flight analyzer since analysis in this case is confined to the uppermost atomic layer of the sample. As shown in Table 3, which gives the intensity levels of contaminant ions observed in the analysis of Eagle Station olivine with a magnetic sector analyzer, sector analyzers are not immune to this type of problem. While reducing the contaminant level is non-trivial, identification of contaminants originating from the residual vacuum is easily made by studying the relationship between secondary ion intensities and bombarding density (Fig. 4). High mass resolution. Clearly, high mass resolution SIMS presents, for chemical analysis, great superiorities over low mass resolution SIMS. In particular, it affords the means of definitively fingerprinting specific mineral phases by identifying characteristic molecular ions which would otherwise be masked, because of their low abundance, by other more intense neighboring atomic or molecular species. As shown, for instance, on the high mass resolution (M/AM = 8800 [FWHM]) spectra in Fig. 5, carbonates may be detected through CO, molecular ion emission even if they are present only in minor amounts in the analyzed material. In these conditions, the detection level (SO ppm) of this phase is significantly lowet than it is (150 ppm) at low mass resolution (Table 2). Another interest of high mass resolution SIMS is the approach it offers to quantitative chemical analysis, as illustrated in Fig. 6 where the intensity of MgO- ions (separated from interfering C20p ions) has been plotted versus the carbonate content. Also, high mass resolution allows trace element analysis to be carried out at ppm or sub-ppm levels, as for iron in calcite which can be moni-
er ul. : Proposed
R. L. Inglebert
20 F z
OH
.z? 10' 2 B -c5 CCH
133
Quad
C2HCN
CI
C2
-. 20
10
0
co3
I 50
40
30
60
70
80
90
100
110
70
80
90
100
110
ToF
A
A
250 . 200
on Mars
F
OJL-
2
ion mass spectrometry
A
A 0
15
in situ secondary
OH
50
60
Mass number Fig. 3. Comparison of mass spectra (linear scale) obtained on a pure calcite sample with magnetic sector. quadrupole, and time-of-flight analyzers. Contrasted ‘hOm count rates of 60 kc s-’ and IO MC s ’ for quad and sector analyzers, respectively, are due to the exaltation, in the latter case, of negative secondary ion emission by use of Cs+ primary ions. All three spectra show a characteristic CO, peak. Classical contamination ions, and among them. F-, Cl-. CN- and CNO- ions, are conspicuously present on spectra obtained with the quad and time-of-flight analyzers. This is due in part to sample preparation and in part to interaction with the residual vacuum
tored either as Fe- at mass 56 amu or as FeO- at mass 71 amu. Finally. high mass resolution provides the knowledge of the systematics of secondary ion production that we need to extract quantitative information from low mass resolution spectra. As an example. data, obtained at high mass resolution, on the relative abundance of molecular ions of type MO,; emitted from inorganic materials (Fig. 7) help to deconvolute low mass resolution spectra by peak-stripping procedures (Hinthorne and Conrad. 1975).
Table 3. Levels of contaminant
ions (kc SK’) as evaluated from the sputtering of a pure olivine crystal. Contaminants may have been introduced at different steps of the sample polishing and metal-coating procedure or, more likely, originate from the spectrometer residual vacuum (5 x lo-‘” Torr) as it has been established for fluorine. Sector analyzer ,ZC
C;
‘9F-
‘SC] -
0.4
0.7
I I.6
6.7
CNO0.3
Isotopic anal>lsis One of the main interests of high mass resolution in this context is the possibility it affords of measuring isotope abundances in selected mineral phases, the measurement of isotopic abundances of both oxygen and carbon on Mars being of prime importance. Precision and accuracy presently obtained in SIMS determination of the ‘“O/‘hO ratio with magnetic sector mass spectrometry is such that the uncertainty on the determination of 6”O is smaller or equal to 5 x lo-“. This precision makes it presently possible to follow temperature changes recorded in diagenetic quartz overgrowths (Arbey et al., 1993) and, as illustrated in Fig. 8, should make it an appropriate tool in paleotemperature studies. In the same way as it has permitted, through the work of John Imbrie and others (Martinson et al., 1987), to decipher the Earth past climatology, one could expect that the analysis of the oxygen isotope composition of carbonates would help us unravel the time evolution of the climate of the red planet, taking into account possible variations in the isotopic composition of the reservoir(s). Such variations are indeed expected to
R. L. Inglebert et al. : Proposed in situ secondary ion mass spectrometry on Mars
-11
-10.0
-lo,6
-10.4
-10,2
-10
0
log primary ion intensity (A)
Mg/(Mg+Ca)
Principle of identification of contaminants originating from the residual vacuum. Identification is made simply by varying the flux of primary ions and monitoring the corresponding change in the flux of secondary ions. The intensity of negative ions of constitutive elements or molecules such as Si- or MgOin the olivine of Eagle Station varies in direct proportion to the primary beam density whereas the intensity of negative secondary fluorine ions is found to be independent of it, marking the element as a contaminant. Magnetic sector. Residual vacuum : 5 x lo-” Torr Fig.
4.
occur as the result of isotope fractionation processes operating on the Martian surface (Jakosky, 1991). It is well known that kinetic isotope fractionations induced by the metabolism of autotrophic organisms result in a characteristic carbon isotopic shift between
I
0.5 (atom/atom)
Fig. 6. Principle of determination of the composition of carbonates in the calcite-magnesite series from measurement at high mass resolution of the secondary MgO- ion intensities. Note that the MgO- ion intensities can also be inferred from data at low mass resolution such as reported in Fig. 3, the contribution of C,O- ions (500 c so’) becoming significant only if the carbonate magnesium content is smaller than 1%0 by weight. 160- intensity : 10 MC SK’. Sector analyzer
carbonates and organic sedimentary matter. This has been used by Schidlowski (1988) as an indicator of early biochemical activity on Earth (Fig. 9). McKay et al. (1992) persuasively argues that a similar approach might fruitfully be used to trace a fossil biochemical activity on Mars. Precision of SIMS determination of 13C/‘*C ratio depends
Olivine
55.87
55.95
59.90
56.02
59.98
60.05
Calcite
j_ 55.87
55.95
56.02
59.90
A...d
59.98
60.05
Fig. 5. High resolution mass spectra (log scale) at masses 56 and 60 recorded on Eagle Station olivine and calcite samples. At the resolution employed (M/AM = 8800, FWHM), molecular CO; ions are well separated from CaO-, SiO; and C; ions, and allow, in principle, detection of carbonates down to a concentration level of 50 ppm (by weight). Besides permitting interference-free chemical analysis, high resolution permits detection and concentration measurement of trace elements, as shown by the well defined 60Ni- peak (10 c s-‘) observed on the spectrum of the Eagle Station olivine which corresponds actually to a nickel content of only 40 ppm (Reed er al.. 1979). Magnetic sector mass spectrometer, cesium primary ions, beam current : 5 x lo-” A
R. L. lnglebert
in situ secondary
et (II. : Proposed
ion mass spectrometry
on Mars
135
50
40
0 3o m z 20 10 0 1
-20
3
2
0
20
40
60
60
Temp. “C
Number of oxygen atoms
Fig. 7. Plot of the relative intensities of MO,; ions emitted under Cs+ bombardment from silicate and carbonate matrices. M stands for metal (Si or Ca). The rapid drop in intensity of cluster
ions with increasing number of oxygen atoms lessens to some extent the severity of the problem of molecular interferences in SIMS. Knowledge, which can be gained only at high mass resolution, of the systematics of production of aggregate ions is necessary for proper deconvolution of low mass resolution spectra
upon the ion yield of the target material, which is higher in organic matter than in carbonates. The resulting precision presently attainable in the laboratory on the determination by this method of the “life” isotopic shift is 3%0 (1 a), sufficient to detect the eventual isotopic signature of biochemical activity on Mars, if the size of the effect is comparable to that observed in isua metasediments where the first records of primeval life have been found. The possibility of detecting with SIMS the isotopic signature of a past biochemical activity would be presently restricted to work performed in the laboratory on Martian samples brought back to Earth and is not yet within the
Fig. 8. Temperature
dependence of oxygen isotope fractionation between calcite and water according to experimental data compiled by Friedman and O’Neil (1977). Uncertainty of 0.5%0 in SIMS determination of 6’80/‘60 results in an uncertainty of 7°C on the paleotemperature determination
capabilities of instruments considered for on-site analysis of the Martian regolith. However, the performances of time-of-flight analyzers are rapidly improving. Mass resolution of modern instruments permits clear-cut separation of interfering molecular ions (Fig. 10). Instruments with mass resolving powers of 5000 (Niehuis, 1990) or even higher (Schueler et al.. 1992) are under development, and significant progress has recently been made in the measurement of isotopic abundance with time-offlight analyzers (Zscheeg et al., 1992) in the perspective of space research. Time will tell if inherent difficulties such as dead-time effects (Stephan et al., 1991), large duty cycle and contamination problems can find adequate solutions and if time-of-flight is the dedicated tool for isotopic analysis in the framework of in .ritu planetary (or cometary) missions.
0
Francevillian
I 4
100
Archaean
I
I 3
I
Proterozoic
1
1 2 AGE (10’
Series
I
1 Phanerozoic 1 I
1
0
Yr)
Fig. 9. Signature of biochemical activity is manifest in the isotopic composition of carbon. As a result of biological activity, the ‘3C/‘zC value is indeed significantly depressed in sedimentary organic matter on Earth. Present state of the art makes such a shift detectable with SIMS, at least in the laboratory. Differences in uncertainties (1 0) on the determination of the “C/“C ratio in carbonates and organic matter primarily reflect the large differences in secondary C- ion yields in the two materials (adapted from Schidlowski, 1988)
R. L. Inglebert ef al. : Proposed in situ secondary ion mass spectrometry on Mars
136 Time-of-Flight Analyzer Matrix: Aluminum
desired level of mass resolution and effectiveness in data throughput that will help us usefully constrain the possibility of past biochemical activity on other planetary bodies than the Earth.
C2H5t
Ackrlo~rledger?lents. The authors express their gratitude to Professor Georges Slodzian on whose instrument part of the experimental work has been carried out as well as to Drs Fritz Btihler and Franz Kruger for thorough and most helpful reviews. They warmly thank MS Sidonie Foadey for editorial handling. Meteorite samples (Eagle Station olivine) and carbonate samples (Iceland spar ; Eugyii magnesite; dolomite) have kindly been supplied by Drs C. Perron and F. Cesbron. respectively. Work supported in part by CNES.
,‘? $j 300 .
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29
28,9
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References Arbey, F., Klossa, B., Lorin, J. C. and Steinberg, M., In situ
measurement of S”O of diagenetic quartz overgrowths by SIMS. Terra notu 5, 367, 1993.
Magnetic Sector Analyzer Matrix: Olivine
Balebanov, B. M., Begbin, C., Belousov, K. G., Bujor, M., Evlanov, E. N., Hamelin, M., Inal-lpa, A., Khromov, V. I., Kohnev, V. A., Langevin, Y., Lespagnol, J., Managadse, G. G., Martinson, A. A., Michau, J. L., Minkala, J. L., Pelinen, R., Podkolzin, S. N., Pomathiod, L., Riedler, W., Sagdeev, R. Z., Schwingenschuh, K., Steller, M., Thomas, R. and Zubkov, B. V., Presentation of the DION experiment: preliminary
M/AM(50%)= 8800
-
0 28,9
29
29,l
Mass number
Fig. 10. Although
their performances on this ground are generally inferior to those of sector analyzers, mass resolutions presently obtainable in the low mass range with time-of-flight analyzers allow separation of interfering secondary ions and make them, in principle, proper for isotope analysis
Conclusion Performances of modern sector instruments make them indeed proper for high mass resolution analysis of samples brought back from planetary (or cometary) missions, allowing full chemical and isotopical characterization of small mineral grains. Precision and accuracy attainable by such instruments will make them able, in the near future, to detect anomalies in the 6”O and 6180 of extraterrestrial matter at the lop4 level, permitting identification of the SNC meteorite source material. Such instruments are unfortunately too heavy to be flown on planetary (or cometary) surfaces. Quad analyzers appear to be presently well suited for in situ qualitative chemical analysis on planetary surfaces, permitting identification of major carbon-bearing phases. Their mass resolving power is however limited. As regards time-of-flight analyzers, their development will hopefully bring them to the
results of SIMS method on mineralogical samples, in Phohos. Proc. Int. Workshop, pp. 24.5-260. Space Res. Institute, U.S.S.R. Academy of Sci., Moscow, 1988. Benninghoven, A., Die emission negativer sekundarionen von verbindungen mit komplexen anionen. Z. Natw/ixwh. 24a, 859861, 1969. Blaney, D. L. and McCord, T. B., An observational search for carbonates on Mars. J. geophvs. Res. 94B, 10.159-10.166, 1989. Blaise, G., Fundamental aspects of ion microanalysis. in Materiul Characterization Using Ion Beams (edited by J. P. Thomas and A. Cachard). pp. 143-248. Plenum Press, New York. Buseck, P. R., Pallasite meteorites-mineralogy. petrology and geochemistry. Geochim. Cosmochim. Acta 41,71 I-740. 1971. Friedman, 1. and O’Neil, J. R., Compilation of stable isotope fractionation factors of geochemical interest, in Datu qf’Geochemistry (edited by M. Fleischer), 6th Edn. Chapter KK, U.S. Geol. Surv. Prof. Paper 440-KK, I2 pp. U.S.G.S.. Washington, 1977. Hinthorne, J. R. and Conrad, R. L., The application of peak stripping to problems in ion probe microanalysis. Proc. Atm. Conj: Microbeam Anal. S’oc., Vol. IO, pp. 74A-74D. 1975. Inglebert, R. L., Klossa, B., Lorin, J. C. and Thomas, R., SIMS on board the Martian rover, in SIMS IX (edited by A.
Benninghoven, Y. Nihei, R. Shimizu and H. W. Werner). pp. 9388941. Wiley. New York. Jakosky, B. M., Mars volatile evolution : evidence from stable isotopes. Icurus 94, 143 1, 199 1. Kahn, R., The evolution of CO-, on Mars. Icurus 62, 1755190, 1985. Krohn, V. E., Emission of negative ions from metal surfaces bombarded by positive cesium ions. J. Appl. Phys. 33,35233525, 1962. Martinson, D. G., Pisias, N. G., Shays, J. D., Imbrie, J., Moore, T. C. and Shackleton, J., Age-dating and the orbital theory
of the ice ages : development of a high resolution 0 to 300.000year chronostratigraphy. Quaternary Res. 27, l-29, 1987. McKay, C. P., Mancinelli, R. L., Stoker, C. R. and Wharton, R. A., The possibility of life on Mars during a water-rich past.
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c~ rll. : Proposed
in situ secondary
ion mass spectrometry
in Mcrrs (edited by H. H. Kieffer, B. M. Jakosky, C. W. Snyder and M. S. Matthews). pp. 12341245. The University of Arizona Press, Tucson, 1992. Moroz, V. I. and Muhkin, L. M., Early evolutionary stages in the atmosphere and climate of the terrestrial planets. Cosmic, Res. 15, 769979 1, 1977. Niehuis, E., Exact mass determination using TOF-SIMS, in SIMS VII (edited by A. Benninghoven, C. A. Evans, K. D. McKeegan. H. A. Storms and H. W. Werner), pp. 2999304. Wiley. New York, 1990. Reed, S. J. B., Scott, E. R. D. and Long, J. V. P., Ion microprobe analysis of olivine in pallasite meteorites for nickel. Earth Phnet.
Sd. Lett. 43, S--l?. 1979.
Schidlowski, M., A 3.800-million-year isotopic record of life from carbon in sedimentary rocks. Nuture 333, 313-318, 1988. Schueler, B., Odom, R. W. and Chakel, J. A., Surface contamination analysis of semiconductors by time-of-flight
on Mars
137
SIMS, in SIMS VIII (edited by A. Benninghoven, K. T. F. Janssen, J. Tfimpner and H. W. Werner), pp. X3-284. Wiley, New York, 1992. Slodzian, G., An electron mirror configuration for regulating the potential on insulators in ion emission microscopy. Opfik 77, 148-155, 1987. Stephan, T., Bischoff, A., Cramer, H. G. and Zehnpfenning, J., ToF-SIMS. application in meteorite research. first results. Meteoritics
26, 397. 1991.
Thomas, R., Bujor, M., Lorin, J. C., Gao, Y. and Inglehert, R. L., Laboratory simulation of a quadrupolar SIMS analysis performed during a space mission. in SIMS 171 (edited by A. Benninghoven. C. A. Evans. K. D. McKeegan, H. A. Storms and H. W. Werner). pp. 3933396. Wiley. New York. 1990. Zscheeg, H., Kissel, J., Natour, G. H. and Vollmer, E., COMA-an advanced space experiment for in situ analysis of cometary matter. Avtrophg. Space Sci. 195, 447 46 I. 1993.
Pergamon
0032-0633(94)0022-X
Proposed in situ secondary ion mass spectrometry on Mars R. L. Inglebert,’ B. Klossa,’ J. C. Lorin’ and R. Thomas’ ‘GREMI. Universiti d’Orleans. 45067 Orleans, France ‘Laboratoire de Physique des Solides, Universite Paris-&d, 91405 Orsay, France ‘Laboratoire de Physique et Chimie de I’Environnement, CNRS. 45071 Orleans. France Received 29 March 1994 ; revised 24 October and 28 November 1994 ; accepted 28 November 1994
Abstract. Secondary ion mass spectrometry
is a powerful analytical tool, which has the potentiality, through molecular ion emission, of detecting minor phases, as well as the unique capability of directly measuring isotope abundances in mineral or organic phases without any prior physical, chemical or thermal processing. Applied to the in situ analysis of the Martian regolith, it can provide evidence of the presence of carbonates and, by inference (if carbonates constitute significant deposits), of past liquid water-a necessary condition for the development of life. In addition, oxygen isotopic composition of carbonates preserves a record of the temperature at which this phase precipitated and may therefore help decipher the past climatology of Mars. Detection of a carbon isotopic composition shift between carbonates and organic matter (on Earth, the result of a kinetic fractionation effect during photosynthesis) would provide a definite clue regarding the existence of a past biochemical activity on Mars.
Introduction The search for evidence that life originated on Mars is still a primary reason for further scientific exploration of the planet. One of the conditions generally agreed upon for the development of life is the presence of liquid water for which there seems to be ample geomorphological evidence. One way of achieving sufficiently high temperatures for water to be liquid on Mars is to have an initially dense CO, atmosphere that promotes a greenhouse effect (Moroz and Muhkin, 1977). Such a dense CO, atmosphere is indeed expected from early degassing of the planet (ref. in Kahn, 1985). Substantial carbonate deposits in the then existing bodies of water, making up the most reasonable reservoir for the excess CO?, should now be present in the Martian regolith. So, detection of
these carbonates, beside providing strong and independent evidence of past liquid water. would greatly contribute to our understanding of the past and present interaction of the atmosphere and surface on Mars. Moreover, if feasible. their chemical and isotopic characterization would provide us with some knowledge of the past climate of the planet and its momentary or episodic suitability for sustaining life. Temperature at the time carbonates precipitated can indeed be monitored by the measurement of oxygen isotope abundances in this mineral phase. allowing evaluation of the local ecological stability, which is critical for life to develop. An even more direct test of the eventuality of a past biochemical activity on Mars can be made by measuring the difference in the carbon isotopic composition of organic matter and carbonates. It is known indeed on Earth that significant isotope fractionation favoring the light isotope is induced by the metabolism of autotrophic organisms that fix atmospheric co,. All these data can. in principle. be provided by one technique: secondary ion mass spectrometry (SIMS). Because of its multifaceted potentialities-sensitivity, ability to detect geochemically important light elements, and unique capability in measuring stable isotopes on single mineral grains-SIMS may indeed. in the context of a future exploration and irzsitu analysis of the surface of Mars, yield valuable chemical and isotopic data upon mineral constituents of the soil and outcropping rocks as well as information on the composition of surface layers of mineral grains. The present study. a short account 01 which has been given elsewhere (Inglebert et al., 1994). has been carried out as a preliminary test of the method for detecting and analyzing biogenic elements as well as characterizing their compounds in the context of a planetary regolith.
Experimental The principle of SIMS rests upon the fact that impact of a heavy ion on a surface at energies of a few keV results
R. L. Inglebert
130
et al. : Proposed
CI situ secondary
ion mass spectrometry
on Mars
Table 1. Experimental conditions relevant to the three instruments, sector, quadrupole and time-of-flight analyzers. Nature, current and beam size of primary ions, mode of sample preparation (polished section or grain powder dispersed upon an aluminum substrate), acceleration voltage and selected energy band of secondary ions, mass resolution (FWHM) as well as order of magnitude of Ouseful ion yield in the sputtering of silicates and carbonates are listed
Analyzer
Source
Primary current (nA)
Sector Quad
Cs+ Kr’
0.05-O. 1 400
ToF
69Ga+
0.5
Beam size (pm) 0.5 (rastered 400
50-100
30)
Sample
Voltage
Energy band
section powder/Al
4500 v 2040 V
20 eV IOeV
powder/Al
4000 v
extended
in the ejection of surface material with typical yields of one to several atoms per incident ion. A fraction of this sputtered flux is ionized during the ejection process and may be analyzed in a mass spectrometer. Biogenic elements such as carbon, oxygen, nitrogen are best analyzed as secondary negative ions because of the high electronic affinity of atomic species such as C (1.263 eV), 0 (1.46 eV) or molecular species such as CN (3.82 eV). Therefore, the work reported here has concentrated on the study of negative secondary ions emitted from pure phases expected to be present in the Martian regolith, such as silicates, carbonates and carbon compounds, using three different types of instruments : (i) a double focusing magnetic mass spectrometer (Cameca) with a mass resolving power adequate to resolve interfering molecular ions in the mass range of interest and a charge compensation system for the analysis of electrically insulating materials ; (ii) a quadrupole mass spectrometer (Balzers) ; (iii) a timeof-flight spectrometer. The latter instruments, small, lightweight and with limited power consumption may be adapted to comply with the requirements of a space mission for ill situ analysis of planetary surfaces (a noteworthy attempt has been made to analyze the surface of Phobos, one of the moons of Mars, with quads (Balebanov et al., 1988)). Relevant analytical conditions for the three instruments are summarized in Table 1. Two types of sample preparation have been used in the course of this work: polished mineral sections with vapordeposited metal coating and mineral grain powders dispersed upon a polished metallic plate (average grain size : 10 pm ; coverage : 15-30%). Minimization of the charging-up of insulating solids in the extraction of negative secondary ions (and especially when using reactive primary ions that promote the production of negative secondaries) is a key condition for reproducible and meaningful SIMS analysis. This has been achieved in the case of the magnetic sector analyzer by flooding the sample surface with low-energy electrons (Slodzian, 1987). In order to alleviate charging-up with the quad and time-offlight analyzers, samples were crushed down to a fine grain powder before being deposited on a metallic holder.
Results and discussion Elementary
analysis
Low wzuss resolution. One interesting feature of SIMS is its capability of phase detection and characterization. In
Mass resolution M/AM = 8800 AM = 0.5 M/AM = 1070 (m = 29)
O- ion yield 1-3 x IO-’ 5 x 10-n IO-’
order to evaluate this capability in a geological context, negative secondary ion mass spectra have been obtained with a magnetic sector mass spectrometer under Cs+ bombardment at comparatively low mass resolution (M/AM = 300) on different mineral or organic phases expected to be present in planetary regoliths. As shown in Fig. I, the spectra obtained on a silicate (olivine from the Eagle Station pallasite (Buseck. 1977)), a carbonate (Iceland spar) and organics (biological tissue) are characteristic enough to be used to identify these materials. A closer look at Fig. 1 reveals that, besides the O- secondary ion emission that these phases have in common, silicates show peaks at masses 28 and 60 corresponding to Si- and SiO, ions, carbonates, peaks at masses 12 and 60 corresponding to C- and CO, ions (Benninghoven, 1969) and organic compounds, peaks corresponding to CN- as well as to C-, C;, . , C; ions, the latter with relative abundances showing the same type of parity effects as observed for cluster ions emitted from pure carbon (Blaise, 1978). An interesting feature is that nominal masses 12, 16, 24 and 28 amu are virtually free from any molecular interferences and may therefore safely be used to derive unadulterated information from spectra recorded at low mass resolution. A peak at mass 28 will then indicate a silica bearing mineral as will a peak at mass 12, in the absence of carbon contamination of the sample surface, identify a carbon bearing phase. Among carbon compounds, discrimination between organics and carbonates can readily be made because of the widely contrasted values of their respective O-/C- and CT/Cratios. O-/C- = 0.3 in organic matter compared to 200 in carbonates ; C;/C- = 1.5 in organic matter compared to 0.05 in carbonates. Another useful criterion is the distinctly higher ion yield of C- in organic matter and allotropic varieties of carbon than in carbonates. The C- yield in the latter phases is found to be 70 times smaller than the O- yield. It follows from these observations that identification of carbonates in a mixture of carbon-bearing phases and silicates is feasible simply by inspection of low resolution negative secondary ion mass spectra. Furthermore, abundance of carbonates can be quantitatively evaluated by measuring the secondary “C- ion intensity corresponding to a given Cs’ primary ion bombarding density. provided the background level of “C- from the spectrometer residual vacuum is known. Table 2 shows the “C- ion intensity from carbonate samples measured with sector mass spectrometry and experimental conditions as listed in Table 1. It is clear, in this case, that the high “C-
R. L. Inglebert ct trl. : Proposed in situ secondary ion mass spectrometry on Mars 400 .
$ 300 Y .e 200 z a, c 100.
Olivine
w 0
OH
02
Si02 Si03
Si
i02H
0 30
20
I
400
60
50
40
02
80
90
100
110
80
90
100
110
100
110
02H
O OH
300
CaOH
fz -
70
Fe02H
Calcite
e
Y .g?200 $f
Fe02
,. 10
g
131
100,
MgO
0 C 1.CH 10
In. 20
cap
-- \ :.- ’ 1
C2 -- CN
40
30
50
r
Ca02
60
Ca02H
70
Organic Matter
1.. 60
C6C6H I. 70
.
C7
80
C8
._ 90
Mass number Fig. 1. Comparison of low resolution mass spectra (linear scale) of silicates. carbonates and organics showing the possibility of phase diagnosis with SIMS. Silicates are characterized by peaks at masses 28 and 60, corresponding to Si- and SiOi ions. whereas carbonates exhibit peaks at masses I2 and 60 corresponding to C and CO, ions. The intense emission of negative secondary ions of carbon and carbon compounds in the sputtering of organic compounds (biological tissue) is highly characteristic ; it contrasts with the low ion yield of carbon in carbonates. 160- count rates : 20 and IO Mc SK’ for olivine and calcite. respectively. C; count rate : 1.4 MC s-’ for organic matter. Sector analyzer
background intensity (430 c s-‘) is the limiting factor for carbonate detection. Even so, low mass resolution SIMS makes it possible to detect carbonates at a level (_ 150 ppm) far below the upper limit of l-3 wt% presently set
to the abundance of carbonates in the Martian regolith by remote spectroscopy (see, e.g. Blaney and McCord. 1989). The negative secondary ion mass spectrum clearly reflects, for a given mineral phase, its specific chemical
composition. This is illustrated in Fig. 2 where are displayed mass spectra obtained at low resolution on two carbonates : calcite (CaCO,) and magnesi te ( MgCO,). The rise of the peak at mass 40 amu (corresponding to MgO- ions) with increasing magnesium content is manifest. demonstrating that qualitative chemical analysis is possible. Quantitative chemical analysis with SIMS is well known to be complicated by matrix effects. At low mass resolution it will suffer from a further complication caused
Table 2. Carbonate analyzer. minimum seen that chemical
detection limits at both low and high mass resolution using a magnetic sector The reported detection limits have been calculated assuming counting times of 100 s. the detectable signal being chosen so as to exceed the background noise level by 4 0. It can be a better detection limit can be achieved at high mass resolution because of the distinctly smaller background at mass 60
Mass resolution Low High
Selected mass
Raw signal (kc ss’)
Bkgd (c s-‘)
I? 60
51 67
430 50
Detection level (ppm) 150 50
R. L. Inglebert et al. : Proposed in situ secondary ion mass spectrometry on Mars
132
300 .
Y p
200
ii &
100.
Magnesite
4
400 r s
0
OH
MgO
02
MgOH
02H C CH
0
‘-
‘*. 20
10
400
C2 CN ---
Mg03 30
40
50
60
90
100
110
90
100
110
Calcite
4b
0
80
70
02
OH
g 300 Y .$ 200 c” E 100
02H
CaOH CaO
Mg C CH
0 10
,
C2 20
CN
. . 1,. 30
co3 Ca02
,I 40
50
60
70
I
Ca02H 80
Mass number Fig. 2. Low resolution mass spectra (linear scale) of magnesite and calcite. Superimposed on the pattern of C and 0 (atomic and molecular) negative ion emission typical of carbonates, specific molecular combinations of magnesium and calcium permit determination of the carbonate composition in the calcite group. The peak at mass 40 is essentially constituted by MgO- ions with a minor contribution of CzOp ions. while both CaO- and MgO; ions contribute to the peak at mass 56. ‘(‘O- count rate: 10 MC s-‘. Sector analyzer
by mass interferences, since molecular combinations of the form MH-, MO-, MO;, MO,, etc., may interfere on the mass spectrum with the analytical ions. Negative secondary ion spectra obtained on calcite with the three different types of analyzers-sector, quad and time-of-flight-are compared in Fig. 3. In spite of the different analytical conditions employed (sample preparation, nature of primary ions-Cs+ is known to appreciably raise the negative secondary ion yield (Krohn, 1962)-solid angle of collection and energy bandwidth of secondary ions, etc.) a feature common to all these spectra is the presence of characteristic peaks corresponding to O-, 0; and CO; ions emitted from the target material. In addition, positive identification has been made on mass spectra obtained on magnesite with a quadrupole analyzer of MgO- and MgO; ions. It may be of interest to note at this point that good agreement between spectra obtained with a quadrupole analyzer on the one hand and with a sector analyzer on the other hand, is also observed in the case of positive secondary ion emission (Thomas et al., 1990). A point of concern, however, is the presence on the mass spectra obtained with both quad and timeof-flight analyzers and displayed in Fig. 3, of abundant contaminant ions, such as F-, Cl-, CN- and CNOions. This is due in part to the peculiarities of the sample preparation, since the sample is deposited as a fine powder upon a metal plate: contaminants can, in principle, be introduced either at different steps of the plate polishing procedure or during the grinding down process. Contamination is also due to interaction with the spectrometer residual vacuum, and this is especially true of the time-of-
flight analyzer since analysis in this case is confined to the uppermost atomic layer of the sample. As shown in Table 3, which gives the intensity levels of contaminant ions observed in the analysis of Eagle Station olivine with a magnetic sector analyzer, sector analyzers are not immune to this type of problem. While reducing the contaminant level is non-trivial, identification of contaminants originating from the residual vacuum is easily made by studying the relationship between secondary ion intensities and bombarding density (Fig. 4). High mass resolution. Clearly, high mass resolution SIMS presents, for chemical analysis, great superiorities over low mass resolution SIMS. In particular, it affords the means of definitively fingerprinting specific mineral phases by identifying characteristic molecular ions which would otherwise be masked, because of their low abundance, by other more intense neighboring atomic or molecular species. As shown, for instance, on the high mass resolution (M/AM = 8800 [FWHM]) spectra in Fig. 5, carbonates may be detected through CO, molecular ion emission even if they are present only in minor amounts in the analyzed material. In these conditions, the detection level (SO ppm) of this phase is significantly lowet than it is (150 ppm) at low mass resolution (Table 2). Another interest of high mass resolution SIMS is the approach it offers to quantitative chemical analysis, as illustrated in Fig. 6 where the intensity of MgO- ions (separated from interfering C20p ions) has been plotted versus the carbonate content. Also, high mass resolution allows trace element analysis to be carried out at ppm or sub-ppm levels, as for iron in calcite which can be moni-
er ul. : Proposed
R. L. Inglebert
20 F z
OH
.z? 10' 2 B -c5 CCH
133
Quad
C2HCN
CI
C2
-. 20
10
0
co3
I 50
40
30
60
70
80
90
100
110
70
80
90
100
110
ToF
A
A
250 . 200
on Mars
F
OJL-
2
ion mass spectrometry
A
A 0
15
in situ secondary
OH
50
60
Mass number Fig. 3. Comparison of mass spectra (linear scale) obtained on a pure calcite sample with magnetic sector. quadrupole, and time-of-flight analyzers. Contrasted ‘hOm count rates of 60 kc s-’ and IO MC s ’ for quad and sector analyzers, respectively, are due to the exaltation, in the latter case, of negative secondary ion emission by use of Cs+ primary ions. All three spectra show a characteristic CO, peak. Classical contamination ions, and among them. F-, Cl-. CN- and CNO- ions, are conspicuously present on spectra obtained with the quad and time-of-flight analyzers. This is due in part to sample preparation and in part to interaction with the residual vacuum
tored either as Fe- at mass 56 amu or as FeO- at mass 71 amu. Finally. high mass resolution provides the knowledge of the systematics of secondary ion production that we need to extract quantitative information from low mass resolution spectra. As an example. data, obtained at high mass resolution, on the relative abundance of molecular ions of type MO,; emitted from inorganic materials (Fig. 7) help to deconvolute low mass resolution spectra by peak-stripping procedures (Hinthorne and Conrad. 1975).
Table 3. Levels of contaminant
ions (kc SK’) as evaluated from the sputtering of a pure olivine crystal. Contaminants may have been introduced at different steps of the sample polishing and metal-coating procedure or, more likely, originate from the spectrometer residual vacuum (5 x lo-‘” Torr) as it has been established for fluorine. Sector analyzer ,ZC
C;
‘9F-
‘SC] -
0.4
0.7
I I.6
6.7
CNO0.3
Isotopic anal>lsis One of the main interests of high mass resolution in this context is the possibility it affords of measuring isotope abundances in selected mineral phases, the measurement of isotopic abundances of both oxygen and carbon on Mars being of prime importance. Precision and accuracy presently obtained in SIMS determination of the ‘“O/‘hO ratio with magnetic sector mass spectrometry is such that the uncertainty on the determination of 6”O is smaller or equal to 5 x lo-“. This precision makes it presently possible to follow temperature changes recorded in diagenetic quartz overgrowths (Arbey et al., 1993) and, as illustrated in Fig. 8, should make it an appropriate tool in paleotemperature studies. In the same way as it has permitted, through the work of John Imbrie and others (Martinson et al., 1987), to decipher the Earth past climatology, one could expect that the analysis of the oxygen isotope composition of carbonates would help us unravel the time evolution of the climate of the red planet, taking into account possible variations in the isotopic composition of the reservoir(s). Such variations are indeed expected to
R. L. Inglebert et al. : Proposed in situ secondary ion mass spectrometry on Mars
-11
-10.0
-lo,6
-10.4
-10,2
-10
0
log primary ion intensity (A)
Mg/(Mg+Ca)
Principle of identification of contaminants originating from the residual vacuum. Identification is made simply by varying the flux of primary ions and monitoring the corresponding change in the flux of secondary ions. The intensity of negative ions of constitutive elements or molecules such as Si- or MgOin the olivine of Eagle Station varies in direct proportion to the primary beam density whereas the intensity of negative secondary fluorine ions is found to be independent of it, marking the element as a contaminant. Magnetic sector. Residual vacuum : 5 x lo-” Torr Fig.
4.
occur as the result of isotope fractionation processes operating on the Martian surface (Jakosky, 1991). It is well known that kinetic isotope fractionations induced by the metabolism of autotrophic organisms result in a characteristic carbon isotopic shift between
I
0.5 (atom/atom)
Fig. 6. Principle of determination of the composition of carbonates in the calcite-magnesite series from measurement at high mass resolution of the secondary MgO- ion intensities. Note that the MgO- ion intensities can also be inferred from data at low mass resolution such as reported in Fig. 3, the contribution of C,O- ions (500 c so’) becoming significant only if the carbonate magnesium content is smaller than 1%0 by weight. 160- intensity : 10 MC SK’. Sector analyzer
carbonates and organic sedimentary matter. This has been used by Schidlowski (1988) as an indicator of early biochemical activity on Earth (Fig. 9). McKay et al. (1992) persuasively argues that a similar approach might fruitfully be used to trace a fossil biochemical activity on Mars. Precision of SIMS determination of 13C/‘*C ratio depends
Olivine
55.87
55.95
59.90
56.02
59.98
60.05
Calcite
j_ 55.87
55.95
56.02
59.90
A...d
59.98
60.05
Fig. 5. High resolution mass spectra (log scale) at masses 56 and 60 recorded on Eagle Station olivine and calcite samples. At the resolution employed (M/AM = 8800, FWHM), molecular CO; ions are well separated from CaO-, SiO; and C; ions, and allow, in principle, detection of carbonates down to a concentration level of 50 ppm (by weight). Besides permitting interference-free chemical analysis, high resolution permits detection and concentration measurement of trace elements, as shown by the well defined 60Ni- peak (10 c s-‘) observed on the spectrum of the Eagle Station olivine which corresponds actually to a nickel content of only 40 ppm (Reed er al.. 1979). Magnetic sector mass spectrometer, cesium primary ions, beam current : 5 x lo-” A
R. L. lnglebert
in situ secondary
et (II. : Proposed
ion mass spectrometry
on Mars
135
50
40
0 3o m z 20 10 0 1
-20
3
2
0
20
40
60
60
Temp. “C
Number of oxygen atoms
Fig. 7. Plot of the relative intensities of MO,; ions emitted under Cs+ bombardment from silicate and carbonate matrices. M stands for metal (Si or Ca). The rapid drop in intensity of cluster
ions with increasing number of oxygen atoms lessens to some extent the severity of the problem of molecular interferences in SIMS. Knowledge, which can be gained only at high mass resolution, of the systematics of production of aggregate ions is necessary for proper deconvolution of low mass resolution spectra
upon the ion yield of the target material, which is higher in organic matter than in carbonates. The resulting precision presently attainable in the laboratory on the determination by this method of the “life” isotopic shift is 3%0 (1 a), sufficient to detect the eventual isotopic signature of biochemical activity on Mars, if the size of the effect is comparable to that observed in isua metasediments where the first records of primeval life have been found. The possibility of detecting with SIMS the isotopic signature of a past biochemical activity would be presently restricted to work performed in the laboratory on Martian samples brought back to Earth and is not yet within the
Fig. 8. Temperature
dependence of oxygen isotope fractionation between calcite and water according to experimental data compiled by Friedman and O’Neil (1977). Uncertainty of 0.5%0 in SIMS determination of 6’80/‘60 results in an uncertainty of 7°C on the paleotemperature determination
capabilities of instruments considered for on-site analysis of the Martian regolith. However, the performances of time-of-flight analyzers are rapidly improving. Mass resolution of modern instruments permits clear-cut separation of interfering molecular ions (Fig. 10). Instruments with mass resolving powers of 5000 (Niehuis, 1990) or even higher (Schueler et al.. 1992) are under development, and significant progress has recently been made in the measurement of isotopic abundance with time-offlight analyzers (Zscheeg et al., 1992) in the perspective of space research. Time will tell if inherent difficulties such as dead-time effects (Stephan et al., 1991), large duty cycle and contamination problems can find adequate solutions and if time-of-flight is the dedicated tool for isotopic analysis in the framework of in .ritu planetary (or cometary) missions.
0
Francevillian
I 4
100
Archaean
I
I 3
I
Proterozoic
1
1 2 AGE (10’
Series
I
1 Phanerozoic 1 I
1
0
Yr)
Fig. 9. Signature of biochemical activity is manifest in the isotopic composition of carbon. As a result of biological activity, the ‘3C/‘zC value is indeed significantly depressed in sedimentary organic matter on Earth. Present state of the art makes such a shift detectable with SIMS, at least in the laboratory. Differences in uncertainties (1 0) on the determination of the “C/“C ratio in carbonates and organic matter primarily reflect the large differences in secondary C- ion yields in the two materials (adapted from Schidlowski, 1988)
R. L. Inglebert ef al. : Proposed in situ secondary ion mass spectrometry on Mars
136 Time-of-Flight Analyzer Matrix: Aluminum
desired level of mass resolution and effectiveness in data throughput that will help us usefully constrain the possibility of past biochemical activity on other planetary bodies than the Earth.
C2H5t
Ackrlo~rledger?lents. The authors express their gratitude to Professor Georges Slodzian on whose instrument part of the experimental work has been carried out as well as to Drs Fritz Btihler and Franz Kruger for thorough and most helpful reviews. They warmly thank MS Sidonie Foadey for editorial handling. Meteorite samples (Eagle Station olivine) and carbonate samples (Iceland spar ; Eugyii magnesite; dolomite) have kindly been supplied by Drs C. Perron and F. Cesbron. respectively. Work supported in part by CNES.
,‘? $j 300 .
E
M/AM(SO%)= 1070 _
29
28,9
29.1
Mass number
References Arbey, F., Klossa, B., Lorin, J. C. and Steinberg, M., In situ
measurement of S”O of diagenetic quartz overgrowths by SIMS. Terra notu 5, 367, 1993.
Magnetic Sector Analyzer Matrix: Olivine
Balebanov, B. M., Begbin, C., Belousov, K. G., Bujor, M., Evlanov, E. N., Hamelin, M., Inal-lpa, A., Khromov, V. I., Kohnev, V. A., Langevin, Y., Lespagnol, J., Managadse, G. G., Martinson, A. A., Michau, J. L., Minkala, J. L., Pelinen, R., Podkolzin, S. N., Pomathiod, L., Riedler, W., Sagdeev, R. Z., Schwingenschuh, K., Steller, M., Thomas, R. and Zubkov, B. V., Presentation of the DION experiment: preliminary
M/AM(50%)= 8800
-
0 28,9
29
29,l
Mass number
Fig. 10. Although
their performances on this ground are generally inferior to those of sector analyzers, mass resolutions presently obtainable in the low mass range with time-of-flight analyzers allow separation of interfering secondary ions and make them, in principle, proper for isotope analysis
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