Mass spectroscopy beyond the moon

International Journal of Mass Spectrometry and Ion Physics

Ftcevier Fublishing Company, Amsterdam . Printed in the Netherlands .

337

MASS SPECTROSCOPY BEYOND THE MOON

L. F . HERZOG Nuclide Corporation, State College, Penna . (U.S.A .) (Received March 16th, 1970)

ABSTRACT

Mass spectrometers reveal the elemental, isotopic and molecular composition of solids, liquids, and gases, from major to microtrace constituents . Hence, the use of "robot" mass spectrometer probes in the exploration of the solar system has been proposed . This paper surveys our present knowledge of solar system composition and the kinds of data on various solar system components it is hoped to obtain through the space program, and the areas in which mass spectroscopy can be most useful . Specific applications discussed include : determining planetary atmospheric compositions . classifying extraterrestrial rock, "geological" (radioactive parent/ daughter) age determination, the detection of extraterrestrial "life", determining the compositions of asteroids . comets and interplanetary dust, and the study of the solar wind and its effects . The mass spectrometric instrumentation available is reviewed and a recommendation is made for consideration of the development of two types of systems for space exploration - a low resolution mass spectrometer without pumping, possibly with "hybrid" ToF-coincidence-mass filter capabilities, and a medium resolution system with pumping, probably using a double-focusing ion analyzer with a permanent magnet.

INTRODUCTION

In reviewing the applications of mass spectroscopy to geo- and cosmochemistry', I mentioned some studies planned for the Moon for which the use of mass spectrometers taken there by Astronauts has been recommended by cognizant NASA advisory boards . The purpose of the present paper is to extend this disL. F. Herzog, "Mass Spectroscopy in Geo- and Cosmo-chemistry-An Overview", Int. J. Mass Spectrom . Ion Phys., 4 (1970) 253. Inr_ S. Mass Spectrom . Ion Phys_, 4 (1970) 337-363

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cussion to the proposed exploration of the solar system by "robot mass spectrometer probes" - mass spectrometers analyzing regions of the solar system as yet inaccessible to manned exploration, controlled by men operating from distant bases, usually on Earth, but perhaps from a Lunar station located at some distance from the instrument, or, in the case of Mars and other planets, perhaps from orbiting or passing spaceships, or space stations on one of the planet's moons . First I will attempt to indicate very briefly and therefore incompletely some of the kinds of information about the solar system that one hopes to obtain through the space program, to which mass spectroscopy is applicable . I will then briefly review the instrumentation that is available and will summarize my own views on what instrumental systems recommend themselves for development for these purposes. Some of these subjects have been treated in more detail by the author and his colleagues in certain reports to NASA on studies made under the Space Agency's sponsorship` - 3 , and in a publication presently in press' .

PRESEN'T KNOWLEDGE OF SOLAR 3B-Ml COMPOSITIONS

Up to the present, knowledge of the elemental and nuclidie composition of the solar system has had to be based principally on quantitative analyses of terrestrial rocks occurring at or near the Earth's surface, and of meteorites, suppleTABLE I ELEME^rrALL A8L' . iDANCES IN THE 5OL . S ATMOSPHERE

Arams per atom Si Element

Unsold 1950'

£cans 1966 6

1 2 6 7

H He C N

51,000 10,000 1 .0 2 .1

50,000 2,500-10,000 10 2v

8

0

280

Na Mg Al Si S K Ca Sc

0.1 1 .7 0 .11 1 .000 0.43 O.UWIS 0.087 0-0001

Z

11 12 13 14 16 19 20 21

Atoms per atom Si Unsold Ecam Z

Eienrent

19501

22

Ti

0 .0047

23

V

0 .0006

2_5

24 25

Cr Mn

0.020 0.015

0.1 1 .6 O.t 1 .000 0.4

26 27 28 29

Fe Co Ni Cu

2.7 0_0055 0.047 0.0009

30

Zn

0 .0031

1966 °

0 .07

0_07

' A . UNsoLD, Trans. Int. Astron_ Union- 7 (1950) 460. ° J. W. EvA`es, Encyclopedia of Science and Technology, Vol . 13, The Sun, McGraw-Hill, New York, 1960. Int. J. Mass Speccrom . Ion Phys ., 4 (1970) 337-363

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339

rnented by optical spectrographic, infrared, radar and other telescopic and radiotelescope studies of solar, stellar and planetary atmospheres, comet spectra, etc . ; and influenced by studies of planetary density distributions, and theoretical inferences from nuclear physics data . The presently-accepted tables of "best" "cosmic" relative abundances of nuclides rest primarily upon the solar, terrestrial and meteoritic data. Most of the isotope data as well as much of the element abundance data for rocks and meteorites, especially for trace constituents, has been determined by laboratory mass spectrometers ; hence, extra-terrestrial-probe mass spectroscopy will be an extension of an already well-established analytical technique to a new group of samples . Only very recently has it become possible to make composition determinations directly beyond the Earth : There have been thus far only three Surveyorprogram alpha-scattering method semi-quantitative determinations of a few elements in lunar surface material, and reconnaissance determinations of a few elements in Venus' atmosphere by the Russian Venus-IV probe* . t . The internal composition of the Sun is thought to be better known than that of any planet (including Earth), because the Sun is comprised wholly of gases and ions at high temperatures, and the laws of plasma physics are applicable . Relative abundances of the elements in the outer portion of the Solar atmosphere are derived from adsorption lines and the emission spectra_ Some 66 elements have been detected, the rest being undetectable either because of low abundance, or because their strong-line wavelengths are below 2900 A, where they are absorbed by the terrestrial atmosphere . A recent evaluation of the available data is given as Table 1 . The degree to which spectra of its atmosphere give the composition of the Sun as a whole depends on the extent to which convective mixing takes place - and that is, unfortunately, not known at present . In any case, the accuracy of the optical-spectrographic abundance data should not be overestimated, as these depend upon many assumptions and extrapolations from terrestrial laboratory work . For this reason, compilers of "cosmic" abundance tables have used instead * Later this year, of course, a great leap forward in knowledge of the Moon's composition will take place with the first Apollo lunar landings and the return of the first samples of lunar surface rocks to Earth for analysis . Possibly by the time this article is published, some detailed analyses will already have been reported . Important as this data will be, it must be remembered that it represents after all, only some very small pieces in a very large puzzle, since the Moon is such a small part of the solar system, and its surface is only a very small portion of the Moon, and not necessarily a representative one . Unfortunately, the same limitations apply almost as strongly, as regards the terrestrial data - the Farth is a small object compared to the Sun, or even, the major planets . As to the meteorites, the total amount of material available is limited, the relative abundances of types havingvarious compositions are not accurately known, and their source or sources in the solar system are subjects of speculation . However, since until 1969 these were our only ponderable samples of extraterestrial matter, the meteorite data have played a major role in all attempts to compile tables of a'aundances of the elements in the solar system and the universet This paper was presented in May 1969 prior to the time of the first Lunar landing in July 1969 . Int. J. Mass Spectrom . Ion Phys ., 4 (1970) 337-363

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concentrations of elements measured in meteorites, especially, of the stony meteorites, for all but the gaseous elements . However, the meteorite data and solar atmosphere data, when both are available, usually agree within a factor of ten .

F_& 1 . For legend see p . 341 . Int_ J Moss Speurom_ Ion Phys ., 4 1:1970) 337-363

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341

Fig. 1 . Graphical representation of Suess-Urey data (Table 1) of abundances of the nuclides, log scale (silicon = 6), vs . mass number, A . Even and odd A nuclides shown as separate curves . Neutron excess numbers (1) are given for each point_ The curve without I numbers shows the sum of the isobars for each even-A . WVe have recently come to understand that not "elements" but "nuclides" were formed when the matter of our solar system was synthesized, that is, that for an understanding of the phenomena of nucleogenesis, regularities in isotones (species with identical numbers of neutrons), and isobars (those of the same integral mass) are as important as isotope (equal number of protons) regularities . It is not the intent of this paper to develop a table of abundances or even to comment critically upon those which have been developed, but merely to indicate to non-specialists the magnitude of the task, the inadequacy of presently-available data, and the need for more direct analyses of many more objects . It will be instructive, however, to consider the results of a milestone review of the abundances of the elements, which was published by Hans Suess and Harold Urey in 1956 s . These authors not only collated but also analyzed the available data, selecting and rejecting results, and sometimes "adjusting" the data dramatically when it did not obey certain rules (derived from nuclear data) which they postulated as controlling the relative abundances of the nuclides . In order to use the meteorite data to derive an "average" composition they had to decide what metal phase/silicate/sulfide ratios to use . Abundance ratios even of major elements, e .g. for Fe/Si, and Fe/S can be altered by a factor of ten by different, apparently reasonable assumptions concerning values for these ratios . The element relative abundances Suess and Urey proposed are given in Table 21. By and large, they have stood the test of time, and in the cases where the authors rejected the then-available data, more recent studies by new techniques Irr. S. Mass Specrrom. Ion Phys., 4 (1970) 337-363



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I . F. HERZOG

TABLE 2 "COSMIC" ATOMIC ABUNDANCES OF THE ELEMENTS (SILICON = I X 10 6 ) AFTER SUESS AND UREY' I H 2 He 3 Li 4 Be 5 B 6 C 7 N 8 O 9 F 10 Ne 11 Na 12 NIS 13 At 14 Si IS P 16S 17 CI 18 A 19 K 20 Ca 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn

4 .00x 10 10 3 .08 x 10' 100 20 24 3 .5 x 106 6 .6 x 106 2 .15 x 10' 1600 8 .6 x 10 6 4 .38x 10' 9.12 x 105 9_48 x 104 I .00 x 106 1_00 x 10' 1-00x 105 8850 1 .5 x 105 3160 4.90x 104 28 2440 220 7800 6850 6 .00 x 104 1800 2,74 x 10 4 212 486

31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 37 Rb 38 Sr 39 Y 40 .7r 41 Nb 42 Mo 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 531 54 Se 55 Cs 56 Ba 57 La 58 Cc 59 Pr 60 Nd

11 .4 50 .5 4 .0 67.6 13 .4 51 .3 6 .5 18 .9 8 .9 54 .5 IMO 2.42 1 .49 0.214 0.675 0.26 0-89 0.11 1 .33 0.246 4 .67 0.80 4 .0 0 .456 3 .66 2 .00 2 .26 0.40 1 .44

62 Sm 63 Eu 64 Gd 6.5 Tb 66 Dy 67 Ho 68 Er 697m 70 Yb 71 Lu 72 Hf 73 Ta 74 w 75 Re 760s 77 !r 78 Pt 79 Au 80 Hg 81 Ti 82 Pb 83 Bi 90Th 92 U

0.664 0.187 0.684 0.0956 0.556 0.118 0.316 0.0318 0.220 0.050 0.438 0.065 0.49 0.135 1 .00 0.821 1 .625 0.145 0 .284 0_108 0.47 0.144

' H . A . Sum AND H. C . UREY, Rev . Mod. Phys., 28 (1956) 53 .

such as neutron-activation and stable isotope dilution have confirmed their educated guesses more often than not . The Suess-Urey plot of nuclide abundance is shown in Fig . 1 . In this it will be seen that : 1 . Averaged abundance decreases in a more or less exponential way over ten orders of magnitude of abundance. from hydrogen to about mass 100 (Z = 40), after which it decreases only about an order of magnitude to the heaviest naturally occurring elements . 2. Elements having . even numbers ofprotons in their nuclei (even-Z elements) are generally more abundant than odd-Z elements, the even/odd abundance ratio generally falling between x 5 and x 10 for adjacent elements. 3 . Nuclides whose mass number, A, is a multiple of 4 are very abundant . 4. "Magic number" nuclei - whether "magic' in neutron number or proton number or both-are very significantly more numerous than adjacent, non-magic nuclides. Int. J. Mass Speetrom . Ion Phys., 4 (1970) 337-363



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343

5 . The light elements Li, Be, and B are tremendously depleted (x 10 6 ) compared to a curve drawn through H, He and Ne, Si and Ca ; even C, N and 0 appear to be somewhat depleted . 6 . A group of elements in the region of iron, 53 < A < 63, including Fe, Ni, Cr, Mn and Co, may be enriched by up to 1000 over a smooth curve drawn through lower and higher mass species . There are other perhaps less obvious features to be explained. A goal of cosmochemistry is to construct such graphs for as many solar system objects as possible and, on the basis of a comparison of these, to determine whether it can be shown that all solar system objects were derived from a single nucleogenic event, or if two or more such events are required. The collection of such data should be a major objective of an orderly and scientifically-oriented space exploration program . Our present knowledge of the compositions of the solid objects in the solar system - planets, satellites, comets, and interstellar dust - is very sparse, indeed, almost non-existent, being based primarily on models constructed on the basis of observed densities and albedos, inferences from meteorite compositions and the Earth's density distribution, and the presence or absence of magnetic fields . Therefore the lunar and planetary surface composition determination experiments proposed by NASA are of first-order importance to solar system science .

COMPOSITIONS OF PLANETARY ATMOSPHERES However, determination of planetary atmosphere compositions-about which we know more, but still very little at present-is an easier experiment and will undoubtedly be done first (except in the case of the Moon, where the Astronauts will soon bring back surface samples) . The atmospheres of the Moon, Mars, Mercury and Venus and the intermediate-altitude atmospheres of the major planets could be analyzed using mass spectrometers similar to those used in studies of the upper atmosphere of the Earth. However, the planetary atmosphere analyzers should possess additional capabilities such as better "abundance sensitivity" to make it possible to determine trace constituents near major peaks and also, the capability of making fairly accurate isotope ratio measurements for certain elements such as H, Ar, C, 0, N, and S, as these will be extremely useful in deciding whether the matter of the solar system all came from a single source, or two or more. Tables 3 and 4 summarize what is known and inferred about the atmospheres of Mars and Jupiter at present . These are included only to illustrate how meager present information is . Indeed, it concerns only a few species which have been detected spectroscopically . Whether difficult-to-detect elements like argon are present even as major constituents is not known, and there is at present even a factorof-ten uncertainty in the total pressure on Mars . Venus, where the Russians have now made some measurements using specific Int. J. Mass Spectrum . ton Phys., 4 ;1970) 337-363

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L . F. HERZOG

TABLE 3 POSSIBLE COMPOSITION

OF THE

MARTIAN ATMOSPHERE Upper limit of (m-atm)

Gas

Volume %

abundance

CO, N, Ar NO, CO, H,S H2O 0, N,O, NO2, N204 NH, CH4 0, S02 Kr

97- (99-20)d 3a (1=I0)d 0 .03° (0-30)d

55 (35-85)b ND° NW ND° < lO' g/cmb < O .7b < 2 .Ob < 0 .2b < 0 .1b <5x10 -4b < 3 x 10 -5 b ND`

9

(frost) < 0.5' 9

9 9 9

a Estimate of F. S . Joa.*3oN, Science, 150 (1965) 1445 . b F . S . JoHNsoN, in R . WAINERDI (Editor),

Analytical Chemistry in Space Exploration, Pcrgamon, Oxford, 1970, Chap- 3 . ' Not detectable specroscopically from Earth because of adsorption by terrestrial atmosphere_ d Estimates byothersTABLE 4 THE APPROXIMATE COMPOSITION Abundance

Gas

Sptnrad-

H2 He

60 36 <1 <3 <1 <1 -

N12, 02. Ne CH, NH, Ar HD Other

OF JL-PFIER'S

(volume %) Opikb 2.3 97_2 0.39 0.063 0 .003 0.042 -

VISIBLE ATMOSPHERE, ABOVE

THE CLOUD

DECK

Estim.. abundance (nn-arm) Spinrad' 27,000 16 ND' 7 F50 0.2 < 500d < 50d

* H_ SPINRAD and L . TRAKroN, Icarus, 2 (1963) 19 . b E . J. OPtx, Icarus, 1 (1962) 200. ° Not detectable . d T. C_ Owvr- (Pubb Asrron . Sac- Pacific, 75 (1963) 314) also set upper limits on other possible species, by examining infrared spectra, as follows : acetylene (C,H,)< 3 ; ethylene (C,H4 ) < 2 ; ethane (C2 H 6 ) < 4; methylaminc (CH3NH2) <3 ; hydrogen cyanide (HCN) <2 ; silane (SiH4 )< 20 ; methyl deuteride (CH,D)< 20 ; deutericm hydride (HD) < 500 (meter-atmospheres) . detectors for a few atmospheric species, is a special case both because the surface pressure is 10 to 100 times that on Earth, and because the surface temperature is probably above 300 °C . There is also the possibility that on Venus a probe might land in a liquid, not on solid rock . For the major planets, such as Jupiter, temperatures may be so low that encountering liquid or solid phases of Earth-STP gases (e .g., methane, ammonia, nitrogen, hydrogen, helium), as well as water, is to be expected . The much higher lower-atmosphere pressures and high gravities on these planets also present special problems, perhaps beyond the capabilities of present technology to solve . Int. J_ Mass Spectrom. Ion F'*ys., 4 (1970) 337-363

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345

CLASSIFYING EXTRATERRESTRIAL ROCK in deriving from the chemical analysis of a rock the type of rock from which it comes, and thereafter to make deductions concerning the rock's origin and history, the amount of petrological information that can be inferred will depend on the completeness of the chemical analysis of the "unknown", on its accuracy, and on the representativeness of the sample . It will also depend on the quality of the data on rocks and meteorites to which it is compared . In this area there are many gaps at present, and we should now be devoting more effort to filling these . At present there are very few spark-source mass spectra of rocks, since, until the mid-1960's there were not even any mass spectrographs in terrestrial geochemical laboratories with which to perform such analyses . The presently available data came almost entirely from other techniques, and the only mass spectral data were obtained with laboratory instruments weighing many tons, not with "flyable" apparatus . Recognition of rock type is the primary objective . The first important distinction one needs to make is between igneous rock and meteorite. Next comes the assignment of the rock to a specific family of rocks (or meteorites) -for example, basalt vs . granite, chondrite vs . achondrite. In making these distinctions on the basis of composition quite accurate analyses are required . The alpha-probe analyses of the lunar surface for example (at least, on the basis of error limits released up to now) are not good enough to rule out certain rock types with high probability . A possibly superior basis for such distinctions when an analytical instrument of limited capabilities must necessarily be used, is the use of ratios of certain key elements, rather than abundances per se . A study some years ago 6 suggested that perhaps even as few as two or three element ratios (such as Fe-Mg-Al, or K-Ca) would suffice for this classification of a rock, if it is in fact one of the common terrestrial types_ Fig . 2 shows the small overlap of the fields of these ratios . However, it should be borne in mind that in fact almost any kind of known igneous rock or meteorite might be encountered, and that rocks with no terrestrial equivalents conceivably occur. It is possible, too, that if the location of sampling is not closely controlled, the sample analyzed could come from an outcrop which is not at all typical and "average" surface material . It is important, therefore, that instruments usedfor robotprobe analysis should be versatile, and not limited in analysis capabilities to specific narrow analytical paths in the expectation of finding only one or another of a very few "expected" types of rock . The development of mass spectrometric and other new methods for the accurate determination of trace elements has opened up additional possibilities for making rock identifications . Nickel, for example, occurs only in part-per-million amounts in most igneous rocks, but in nearly all types of meteorites it is present in much greater quantities . Determination of the concentration of this element- is therefore almost sufficient alone to prove whether the sample is from material of Inn . !. Mass Spectrom. Ion Phys., 4 (1970) 337-363

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L. F. HERZOG

Fig. 2. Ternary variation diagram, system 56Fe- "Mg-27 A1 ; showing compositional ranges for granites, tektites, basalts, b-sahic achondrites, chondrites, and chondritic achondrites . meteoritic composition . Good data on key trace elements would check conclusions reached on the basis of major element determinations, and makes possible finer distinctions as to rock type . For this reason the concentrations of as many elements as possible should be measured . Many deductions and inferences, some of them quite powerful, become possible once the mineralogy of a rock has been determined . For example, if sedimentary rocks are found on the now-arid surface of a planet, the prior existence of a hydrosphere might 1r- inferred . Surface pressures on Mars, Mercury and some of the larger satellites of Jupiter and Saturn are believed to be in the 0,1 to 100 torr (millibar) region and gravities are likewise less than on Earth, so for studying surface rocks and soils on these objects, techniques which can be made to work reliably on Earth should be more than adequate, when temperature differences are compensated . Assuring that the samples analyzed give the composition of the greatest possible area will require either that the analyzer be moved according to a previously designed sampling plan (preferably utilizing feedback information from camples already run), or that samples from many locations can be brought to a stationary spectrometer . Less pumping capacity will be needed than on Earth wherever surface pressures are less on these objects, and the frigid climates of some of them provide a means of pumping that may eliminate the need to bring along a PUMP. Techniques developed for prospecting on the Moon in post-Apollo studies will be applicable to studies of other planetary satellites with atmospheres so tenuous that no separate vacuum system for the analyzer need be brought along . Int. I. Mass Spectrom . Ian P/rys ., 4 (1970) 337-363



347

MASS SPECTROSCOPY BEYOND THE MOON

Age determination The radioactive elements and their daughters are especially important . For these, isotopic composition as well as elemental concentration should be determined, for age determination purposes . The more important "parents" are isotopes of U, Th, K, Rb, and Re ; the "daughters" include Pb, Sr, AT, Ca, and Os . Knowledge of the concentrations of the radioactive species will also give useful information on intraplanetary heat production rates . Space does not permit mere than a superficial treatment of the methods or problems of age determination by radioactivity, in this paper, but Table 5 will at least give an idea of the difficulty of the task of determining the radioactive species and their daughters in terms of their low concentrations in the samples of interest . Daughter concentrations are in genera' less than those of parent isotopes . TABLE 5 RADIOACTIVE ELEMENT CONCENTRATIONS (WEIGHT PER CENT) a Element

Chondrites

Basals

Granites

U

Ix10 -6 2 x 10 -6 0A85 5x10 -4 I x 10 -4

5 xIO - 5 3 x 10 -4 0 .5 2x10 -3 2x10 -4

4x10 -4 2 x to- 3 4_0 2X10-2 6x10 -4

Th K Rb Sm

1 OK =0 .0119°/,ofK B 7 Rb = 28 % of Rb i 47 Sm= 15 % of Sm ' H_ E . SuESS AND H . C. URn, Rev. Mod. Phys.,

28

(1956) 53 .

Isotope fractionation by geological and life processes On Earth, it has been shown that measurements of the isotope ratios of H, C, 0, N, S, Si, and othe, elements, in rocks can provide much information on geological processes and rock history . For example, igneous carbonate can be distinguished from marine carbonate (e.g. limestone), and organic :arbon from inorganic, in many cases . The potential applications of stable isotope geochemistry to deciphering solar system history are similar to those that can be made for the Earth .

EXTRATERRESTRIAL "LIFE" DETECTION

The identification of "organic" compounds and/or their degradation products in extraterrestrial matter would have important consequences, bearing as it would on the questions, where, when, and how did life evolve in the solar system? For this reason, developing means to detect such materials, if present, is one major Inn J_ Mass Spectrom_ Ion Phys. . 4 (1970) 337-363

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L . F . HERZOC

goal of any space program . A number of types of apparatus have been proposed for this . Among them perhaps the mDst complex is the one shown (in laboratory breadboard) in Fig. 3 . This is the combination pyrolysis-gas chromatograph-mass spectrometer under development at the Jet Propulsion Laboratory . Pasadena, Calif. At present it remains to reduce this already small instrument down to Marsprobe size and weight requirements . For planning purposes the operational and

Fig . 3 . "Breadboard" Pv; GC, 1+s (pyrolysis,'gas chromatograph,'mass spectrometer) at the Jet Propulsion Laboratory, a proposed "robot" analytical system for identifying any organic compounds present in the Martian soil . physical specifications proposed by NASA Langley Research Center' for the Mars experiment are as follows : Step heat a 15 to 150 mg soil sample which has been powdered to 50 mesh or finer, in 100 `C increments, from ambient to 500 `C, in a chamber having a maximum free volume (with sample in place) of 100 ttl_ Analyze the volatile products at each temperature step before proceeding to the next higher step. Detect volatile constituents of mass 12 through 300, with unit resolution at 771/C 180, dynamic range of i0' or better 10' . and a minimum sensitivity of 0 .1 p .p.m . by weight of the original soil sample . Analyze up to six samples during a 90 day lifetime on the surface of :Mars . Inr. J. Mass Soectrom . Ion Phys_, 4 (1970) 337-363

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Physical requirements and constraints include, weight : 18 lbs__ power consumption : 50 watt-hours per analysis : size : minimal : temperature - 18' to +37 `C : shock : 40 g for 100 msec, acceleration : 25 g . Dry heat sterilization is required - 6 cycles of 92 hours each at 135 - C . The lander may be tilted as much as 20' from vertical . The anticipated data transmission capability available for the instrument is 10' bits per analysis . Dr. Gelpi later describes a laboratory investigation of meteorites that may have been associated with material originating in living organisms . This important and interesting area of investigation rests upon a very important terrestrial application of high resolution mass spectroscopy combined with gas chromatography which is . itself. very new . This discipline has been named paleo-organic geochemistry . and is the study of organic residues in ancient rocks_ It began as recently as the early 1950's with P . Abelson's discovery that there were such residues even in rocks of billion-year-plus Precambrian age . Shortly thereafter_ Nagy and Meinschein extracted from an unusual (but not rare) class of meteorite, carbonaceous chondritc. which contains up to 7 °„ of carbonaceous material plus more than 10 water and considerable amounts of the soluble salts of NH, . K . Mg . and Na . with SO, and Cl as the anions, material which displayed patterns of molecular fragmentation similar to those observed in the fossil terrestrial organic .natter . These and subsequent related studies form the background for the present research of Gelpi and Oro and several other groups now active, and these studies are themselves the foundation upon which the extra-terrestrial life-detection experiments are being built .

COMPOSITIONS OF ASTEROIDS, COMETS . AND INTERPLANETARY DUST

Geochemists hope somewhere to encounter a sample of "primordial" solar system matter in original condition . The Earth, of course. contains none, all of its crust having been differentiated and reworked by geological processes_ Meteorites, the Moon, and the planets also seem to have come from or be already-differentiated bodies . But the small moons of planets, the asteroids and the comets remain as places to search for such material . The asteroids (or planetoids) are irregularly shaped objects having major dimensions of up to about 800 km . Most of them are found between 2 and 3 .5 Earth distances from the Sun . Over 1600 have been catalogued, and they have been grouped into 20-plus "families' - by similarity of orbit . A number pass very close to Earth . For example Eros, a 20-km object, comes within 23 million kilometers and a close pass, affording an opportunity for study, will come in 1975 . Though there are probably more than 50,000 asteroids, the total mass involved cannot exceed about 1J3)3000 of that of Earth . Origin theories originally centered on disruption of a planet occupying the Mars-Jupiter "gap" but have later included the breaking Int- J_ Mass Spectrom. Ion Phrs., 4 (1970) 337-363



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away of a moon of Jupiter, or imperfect accretion during planet formation, or (for the more eccentric) that they may be nuclei of "old" comets . Some students of the subject have proposed that the very many asteroids present today have resulted from periodic collisions between a few relatively large, spherical "primordial" objects . Anders (for example) calculated that energetic collisions should occur about every 10,000 years on the average, so that the Asteroids might be a periodically-replenished source of meteorites, since any debris which comes within about twice the Earth's orbit will eventually be captured by Earth . The larger asteroids may also, perhaps, have collected appreciable amounts of interplanetary dust in their travels through the solar system . Comets are another possible repository of "primordial" matter. Comets are divided into two geaeral classes-long period (parabolic) and short period (periodic)_ Studies of the approximately one hundred comets with periods less than 100 years indicate with good certainty that they were placed in these orbits by gravitational encounc:,rs %with Jupiter. The orb ; s of the long-period comets are very different . They are distributed essentially without regard to the ecliptic, approaching the perihelion from all directions . Their major axes are nearly infinite and so are their computed periods . Many of them have paths so long that perturbations of orbit by nearby stars can occur. It is at present generally believed that there is no case in which a hyperbolic entering trajectory for a comet has been proven, and thus that very probably none of them represent samples of material from outside the solar system . On the other hand, since most of their lifetimes are spent in regions of space where the temperature is within a few degrees of absolute zero and solar wind effects are negligible, it is thought that the periodic comets may more closely represent "primordial" solar system material with respect to volatile constituents than does any other class of object . Their spectra seem consistent with this . Because they are of low density (as low as 0 .05), and fragile, and produce dust and gas, a structural model has evolved which depicts comets as consisting of a matrix of various `ices" in which grains of "dust" composed of the less volatile elements are imbedded . In fact, since water is the least volatile of the molecules observed in cometary spectra, in the selective volatilization of the surface of a comet, a water-ice coating that insulated the material inside would be developed, and further loss of material from below could occur only after this ice had been evaporated . Wood' has discussed the problem of rendezvousing with or "landing" on a long-period comet . and finds it formidable, especially because the notice we get of the approach of such a comet is always very short_ He calculates that only perhaps one or two per decade could be reached with vehicles of presently-contemplatad thrust, Interplanetary dust is believed to consist principally of material liberated from comets by the Sun during the close-approach portions of their orbits . The enerInt_ J. Moss S_nemom_ Ion Phys., 4 (1970) 337-363

MASS SPECTROSCOPY BEYOND THE MOON

351

getics are such that the production rate increases steeply, perhaps by the fourth power or more, as the distance to the Sun decreases . Unfortunately, only grains smaller than about 5 microns can survive transit through the terrestrial atmosphere . To collect interplanetary dust above the atmosphere by rocket or satellite is not a trivial chore, because the relative velocity of dust and Earth is approximately 300 km/sec so that particles will impact with about 100 times the kinetic energy necessary to vaporize silicates . But this relative velocity "problem" can be made an advantage if one sets out to perform the analysis using a mass spectrometer appropriate to the purpose : the impact "flash" vaporization of a dust particle puts it into the proper state for mass analysis, provided only that the instrument can collect all species essentially simultaneously (i.e. is a mass spectrograph or perhaps a fast time-of-flight instrument) . The analysis of such dust in cometary tails and in various regions of the solar system by mass spectroscopy would be an interesting experiment, and one which is, in the author's view, well within the present state of technology, and which might produce information of high "cost effectiveness", especially if the instrument were programmed also to analyze the neutral and ionized gas molecules or atoms it encountered in its flight. THE SOLAR "WIND"

It goes without saying that it would be of great interest to be able to compare the compositions of the planets, satellites, comets, etc ., when known, with that of the Sun's composition if this could be determined with equal accuracy . Since the temperature of the surface of the sun is about 6000 `C, a direct measurement of its composition by mass spectroscopy to improve on the optical spectral data would seem to be impossible . But this may not necessarily-- be true . The solar wind consists of very energetic ions streaming from the Sun . These are predominantly protons with about 10 % helium, but heavier nuclides have been detected in cosmic-ray plates. The solar wind is mostly deflected past the Earth by the Earth's magnetic field . However, recent space probe experiments indicate that the Moon is not similarly protected, and that the solar wind may actually strike the lunar atmosphere and surface and that the interplanetary electric field may reach the surface also . If both these statements are true, they provide an adequate explanation of how an atmosphere of noble gases and other species too massive to have been lost over 5 billion years by thermal ejection, could nevertheless have been lost front the Moon, if it existed at any time. But further, if the Solar Wind does bombard the Moon's surface, the region of the Moon may be a convenient location from which to study its composition . However, the solar wind will play a complex role on the Moon_ First, it will add material. Second, it will eject atoms and ions from surface material by ion bombardment, and the gaseous components liberated (notably, oxygen from silicate int. J. Mass Spectroin . Ion Pilys., 4 (1970) 337-363

35 2

L . F . HERZOG

rock), may remain in the atmosphere for a long time . Third, atmospheric constituents will be ionized and/or ejected by energetic incoming solar wind ions . It might in fact be the case that the lunar atmosphere consists essentially entirely of the steady state resultant of these processes, if there is now no volcanic activity or eutgassing. If the solar wind is actually a hydrodynamic flow of matter outward from the surface of the Sun as a whole rather than merely a phenomenon of the Sun's exosphere, then (as Gold pointed out recently') "mass spectrography of the solar wind would give us the entire solar surface composition in fine detail. That would represent a fantastic improvement in our knowledge ." Performing this experiment will require use of art instrument capable of resolving highly ionized species differing in rife by a few tenths of a percent, an instrument more like one suitable for analyzing surface rock and soil than the simple low-mass analyzer usually proposed for atmosphere analysis_ The Moon is a suitable platform for such an experiment provided that the instrument can be shielded from the secondary effects of the Wind mentioned above . If this is not possible, a lunar orbiter or a manned spacecraft in Moon-Earth transit might provide a better location . The critical info :nation sought is the concentrations of heavy elements in the solar wind . If then: are present, some believe it will clinch the argument that the wind is a hydradynamic flow, in which case, by measuring their relative abundances one will also be determining those of the Sun's surface. A first step is a search for solar wind noble gases trapped in lunar surface samples, when these are available for study in terrestrial laboratories equipped with suitable mass spectrometers .

SPACE :MASS SPECtRO METER

It is not to be expected that any single type of mass spectrometer will be best for all space applications or even that any single type could be used effectively for all the kinds of studies listed above . However, the applications presently under censideratton do fall more or less into two categories, in terms of instrumental requirements . These can perhaps be described as follows : 1 . A low resolution mass spectrometer, without pumping This instrument would be used primarily to analyze rarefied atmospheres in regions where no pumping system, or only slight pumping, is required . While it does not require a resolution of over 100, it must have very high sensitivity and should be able to detect all constituents of mass 1-50 (or better, 1-150), having partial pressures in the range of say, 10 -4 to 10 -12 torr, and to produce a spectrum in 1-10 minutes or less, for in-flight use . The abundance sensitivity should be above 1000 (or better, 10,000) for adjacent masses at mass 40 to make possible the determination of minor species near major peaks . The spectrometer Int . I Mess Spectrorn. Ion Phys., 4 (1970) 337-363

MASS SPECTROSCOPY BEYOND THE MOON

353

should be capable of analyzing any ion-atmosphere present . For studies of neutral species, development of more efficient means of ionization could extend the range of usefulness of such an instrument dramatically . This instrument could also be used effectively for the analysis of denser atmospheres with the addition of a pumping system adequate in capacity for the particular atmosphere to be studied . When modified by adding appropriate means of vaporizing rock and soil, the low-resolution mass spectrometer could also be used for some of the "prospecting" applications discussed earlier, such as selecting lunar samples for return to Earth, as well as for the asteroid and comet studies . The interplanetary dust study requires no vaporizer but ideally requires simultaneous detection of all species .

2. A medium-resolution analyzer for inorganic solids and "organic" gases, liquids and solids In this case the resolution should be at least 500, or higher if possible within weight( power constraints, and for many of the contemplated studies the abundance sensitivity needs to be high enough that a part per billion (1/10 9 ) species next to a 0 .1 % species can be determined with a signal/noise ratio of at least 2,/1, to above mass 100. The mass range should be at least 1-600, stability must be high, and background very low, so that data can be accumulated over periods of days or more, as is required for certain of the studies listed (and which also may be dictated by power limitations for lop .-duration missions) . At least two means of ionization will be required - one for ionizing gases and the other for vaporizing and ionizing solids . Depending on what type of ionizer is used, a double-focusing analyzer may be required . Different "tuning" of these basic instruments would of course be required for each application . Better performance for particular studies could doubtless be obtained by designing specific instruments for each and every purpose, but for the present it seems to the writer that it is more reasonable to concentrate on the perfecting of a small number of basic analyzers which can be adapted to various studies by smali modifications or the addition of modular components. Preferably, all space mass spectrometers should include provision for analyzing one or more calibration samples . These can also be used to determine the peak shapes actually being given by the instrument when in use at site, and to assist in spectrum deconvolution, correction for misalignment, and data processing by computer methods. The interpretation of spectra from space will be greatly facilitated if a program is also funded for performing in laboratories on Earth, using instruments exactly like the instruments) sent on mission(s), analyses of samples like those believed to be most probably present of the object being explored_ Obviously this should be done before the mission as a means of "peaking-up" the analyzer's performance Eat also, after the remote analysis has been performed, attempts should Int. J. Mass Spectrum . Ion Phys ., 4 (1970) 337-363

FOR USE IN

SPACE

w

a

w w y w

0

w

A

N

a

0 x

J

m

- ------------

Excellent

Double-focusing

+ No velocity focusing .

Good

lair

Coincidence

Sector-Held magi -Jc

Fair

V . good

Good

Pulsed Top

Monopole

Quadrupole

Poor/fair (depending on shielding) Pour/fair (depending on shielding)

Excellent

V, good (no field)

V . good (no field)

V . good (no field)

V . good (no field)

leakage

Poor/fair

Poor/fair

Good

Good

Fall-

Poor/fnir

Resistance to meets shock

SI'EC'rROMErafS

Magnetic field

rtESEARCII MASS

Fair'"

Fair"

Fair*

Good

Good

eV

fons with energy scatter of seoeral

Reselaleg power

ANALYZER SYSTEMS PROPOSED

0

Top

OF SIX

Ahillty to analyze

Type

COMPARISONS

TAULE 6

a

G

4

Fair

Fair

Good

V, good

Good/v. good

Good

unfnf;rtarl:atiwr)

assuming

Weight (Auci . circuitry-

Use of permanent magnet for wide mass range easier than for sector . '

Less circuitry if uses permancnt magnet

Maxi ion current is limited to law value, but very sensitive at low pressures .

On Moon, resolution can be increased by physically separating source and detector .

Like quadrupole ; but capsWe of sitill) uneous collec • (ion .

Not yet fully developed as n reliable means of quantitu • five analysis,

Comments

.T



MASS SPECTROSCOPY BEYOND THE MOON

355

be made to match the spectrum received with one given by a synthesized sample, analyzed by the Earth-based spectrometer . What 'ypes of instrument meet these design requirements best? Certain types of mass analyzer should, I think . he eliminated from consideration for missions in the near future on grounds such as inadequate present experience, inadequate resolving power, inadequate sensitivity and excessive complexity . Analyzers. The analyzers which I believe should be considered are compared in Table 6 . I personally rank the Time-of-Flight analyzers first for the service characterized as "low resolution gas analysis"_ The advantages of the Top include the following : spectra can be obtained at rates up to many thousand per second ; the mass scale calibration (time) is infallible ; the analyzer is light, simple, relatively insensitive to misalignment, and reliable ; and its acceptance angle can be made very large to enhance sensitivity . Finally, a prototype meeting weight requirements is already available . Fig . 4 shows a spectrum of residual gas at 7 x 10 -t ° torn total pressure that was obtained with a prototype lunar-atmosphere analysis Top instrument having a weight of less than 5 lbs . 3

Fig. 4 . Residual gas spectrum of prototype TOF lunar atmosphere analyzer, at a totall pressure of 7 x 10 -10 to-- Inset shows a nitrogen peak at a sensitivity of 2)'. 10 -11 A/in . of chart (peak approximately 6 in . tall on original) . Peaks shown are mle 2, 12, 13, 14, 15, 16, 17, 15, 20, 26, 27, 28, 29, 40, 44 . Gating pulse : 50 nsec .

Direction-focusing magnetic analyzers also deserve strong consideration . A small magnetic analyzer of high performance is shown in Fig . 5 . In spite of its I in. radius it achieves a resolution above 50 even with an ion acceleration as low as 10 V . However, a magnetic instrument of such performance cannot be as light as a Tov instrument. Int. J. Mass Spectram . Ion Phys., 4 (1970) 337-363

356

L . F . HERZOG

Fig- 5 . A small high-performance magnetic mass spectrometer (Nuclide 1-90-MS) . ton acceleration, 10 eV, electromagnet scanning : radius I in. Resolution about 50 . (The flanges, cooling water leads . etc ., shown on the analyzer are required for use in a vacuum chamber and can be eliminated for space applications) For the "solids" prospecting applications, if one is forced to use a means to vaporize and ionize the rock samples that produces ions having a range in energies which the TOE spectrometer or direction-focusing magnetic type cannot handle without an unacceptable loss of resolution, one must turn to another type of analyzer_ In this case one may as well use the "medium" resolution analyzer, or a somewhat degraded vc rsion of it . The most suitable mass spectroscope presently available for the "medium resolution work seems to me to be the double-focusing electrostatic-magnetic type. A spectrograph would be preferable to a spectrometer if the weight-budget permitted_ The double-focusing magnetic analyzer of the type having separate electrostatic and magnetic fields has the advantage of being well Ant_ . Mass Spectroin . Ion Phys_, 4 (1970) 337-363



MASS SPECTROSCOPY BEYOND THE MOON

357

studied, relatively rugged, and demonstrably capable of good performance in flight situations . The double-focusing feature makes it feasible to employ the several traditional means of ionization that produce ions having a relatively large energy range. In my view, the mass filters (monopole and quadrupole) must at present be downrated for space use for several reasons . The most important is that we have not yet learned how to achieve with them a degree of spectrum reproducibility comparable to that given by magnetic analyzers of similar weight . Also, they are unusually sensitive to particulate (dust) contamination, as the "dusty" spectrum of Fig. 6 shows . Further, it has not been demonstrated that the mass filters can 26

40

[co

"Breadboard - lunar exploration monopole mass spectrometer spectrum of perfluorokerosene at 3 x 10 -5 tort . Note' ghost" peaks near, e.g., masses 100, 93, 69 ; these are due to particulate (dust) contamination of the monopole analyzer's electrodes . Fig . 6 .

actually provide additional sensitivity, for a given weight and/or power, at the resolutions and abundance sensitivities needed to study trace constituents at above mass 100 . Nor can they perform simultaneous measurement of many species over a broad range of masses, as can a double-focusing spectrograph . For these reasons I would presently select a spectrograph for these studies . ion detectors. Desirable features in an ion detector, in addition to light weight, low power consumption, and ruggedness, include the ability to measure very small ion currents in radiation-rich environments, and freedom from the need to operate at artificially reduced temperatures or in a magnetic field . Detectors Int. J. Mass Spectrom . Ion Phys., 4 (1970) 337-363

Good/fair

Medium

Medium

V, good None

Medium

Medium/good

V . good

Good

Transmissiondynode E . M .

Scintillationphoto nultipiier detector

Faraday cage

Photoplate

* Not a disadvantage if a long collection time is possible, ** But detects many species simultaneously .

V . good

Good

Good

Tubular E. M .

Medium/good

Poor to medium, depending on dynode construction

V . good

Focused - medium Unfocused - good

Good

Ruggedness

SPpe'raoMareas

Good

Power Consumption (irrd , amplifier)

FOR USIi IN SI'ACII ItESI AIICU MASS

Electron multiplier (conventional, separutc dynodes)

iTCCTORS

Weight

[IF ION O

Tyre

COMPARISONS

TABLE 7

Fair**

Poor*

V . good

V . good

Fair

Focused - v . good Unfocused - good

Sensitit'lly (to me loll)

Only suitable if vehicle is returned to Earth or near-Earth base .

Scaled multiplier feature Is not significant advantage for lunar application .

Suitable for detecting many masses simultaneously . Not yet thoroughly investigated,

Output may be limited by film resistance. Gain much lower than conventional types . Could be very rugged,

Unfocused (e.g., venahm . blind typr) dynodes may be preferable in space analyzer because ufgnment Is less critical,

Comments

t4~1 oe



MASS SPECTROSCOPY BEYOND THE MOON

359

which (in my own opinion) are well enough developed to be considered for use in space-analyzers designed today are listed in Table 7 with some comments and comparisons. The tubular multipliers show great promise. For the simultaneous collection of several ion beams, several multipliers (of any type), operated from a single power supply could be used . Where the attendant loss in instantaneous sensitivity can be compensated by accumulating data over a long tirr_e, the use of one or more Faraday cup detectors is preferable from the standpoint of simplicity and reliability. However, the electron multiplier and scintillation detector can be used for ion counting, while a Faraday cup cannot . When one studies the problem of transmitting data from deep it the Solar System (i.e . Saturn and beyond), at first glance, the photoplate recommends itself, since it is, in fact, an integrating detector which, in suitably designed instruments, can be used to record simultaneously every species present throughout a broad mass range, such as masses 10-400_ Furthermore, a permanent record is produced in a relatively compact space . However, technical inferiorities of photographic compared to electrical detection are numerous . Photoplate response is massdependent and also varies with energy and molecular composition . Under the best of circumstances, the response is only linear over a limited range of line intensity, perhaps one decade . To extend this, one must use a set of exposures of varying lengths . There are also practical drawbacks. Plates must be introduced into the analyzer in such a way as to be coincident with the ion optic focal plane usually within approximately 0.001 inch . There must also be means of "racking" the photoplates up and down so that many exposures can be made on one plate and there must also be means of interchanging them . After a plate has been exposed, it has to be processed before the results are available . In addition, a photoplate can itself be a serious source of background gas contamination . New and difficult problems arise when one considers using photoplates on long space missions . Response is temperature-sensitive ; cosmic rays expose plates, emulsions deteriorate in vacuum, etc. Still, in the judgment of at least one group of experts who considered the deep-solar system data-retrieval problem (the 1965 NASA Woods Hole Study Group' °) even consideration of in-flight film processing is justifiable because ". . . the total amount of information that can be stored on a photoplate is so great that in any case where a return flight can be contemplated, this method of storage is likely to be much superior to any telemetry that could be achieved with the same payload . In-flight film processing provides the additional advantage of accepting a very high data rate at the planet, which can be read out at an arbitrarily low rate later ; and the information so returned to the vicinity of the Earth could either be delivered in the form of a package that is re-entered or could be read out by some mechanism . Finally, total data storage by means of film recording can eliminate storage as a bottleneck in any data return system ." Int. .1. Mass Speetram . Ion Phys., 4 (1970) 337-363

y w w

J O N

Good

Poor

Fair

lon bomb, (sputtering), W,o ./W, F . B, Ionizer

R,f. spark

Arc source

Medium

Poor (mainly electronics)

Medium

Poor (needs mirror and automatic drive)

Medium

Good

nrlniaturi:rrtlmd

IVeiyhr (assuming

of

Absence of

Not properly studied

Good/excellent

Medium/good (if neutrals analyzed)

Medium/good

Good

Good

Fair

Very poor

Good

Good

Good

Good

rf : . ellscrhniuation interference ----------- ------ ---

element

Absence

" Energy scatter Is small but not negligible (energies up to a few cV) .

Good

Good

Electron bomb, ionizer/ solar image vaporizer

Electron bomb . lon ;zer/ laser vaporizer

Medium/poor

Electron bomb, ionizer/ electron bomb . vaporizer

N~

w

corrsrnnptlon

S'ousce type

Power

COMPARISONS OF ION SOURCn SYS'rUMS PROPOSED FOR USE IN SI'ACII RhAEAItCYI MASS SPECYEROS1Rt'ERS

TABLE 8

0 a ro

e

M



lot) energy

Medium

Poor (but usable in DF instrument)

Medium

Good*

Medium/good

Good'

spread

May need substantial gas supply .

Standard laboratory method,

Needs gas supply for bombard mg ions,

Complex, bulky,

Needs further investigation its analysis tool,

Recent work suggests good chance of reducing power consumption .

Co mnetus



MASS SPECTROSCOPY BEYOND THE MOON

361

Ionizers

Table 8 lists a number of types of ionizers that might be considered for space use . Studying naturally-occurring ions of course requires no ionizer, but for studying neutral gaseous species (or volatilized solids) a sensitive electron bombardment (En) ionizer, of well-conceived design, that produces essentially mono-energetic ions, seems the most prudent choice . This might be a "classical" EB source modified only by the use of an "unbreakable" wolfram mesh filament ; or, perhaps, the ionizing electrons could be produced by a beta source, or the combination of a beta source and an electron multiplier, to save on power . Also, adding quadrupole lenses to a space analyzer should be straightforward and might make possible a tenfold increase in sensitivity without adverse side effects . The source developed to analyze gases could also be used to ionize the neutral species produced by the means chosen to vaporize solid samples, such as planetary rock . In this case the analyzer would not need to have the energy-refocusing capability, ae obvious advantage . However, all of the means of vaporizing solid samples that presently seem worth considering for use in planetary/lunar/natural satellite,% asteroid surface analyzers, do produce ions having a markedly larger spread in energy than does electron bombardment, and will require a double-focusing mass spectrometer if used . The best present possibilities for vaporizing solid samples seem to me to be laser beams, ion bombardment, electron bombardment, and solar heating. It is presently difficult to make a choice between laser vaporization and ion bombardment, as neither means has as yet received adequate study as an analytical tool. If a pulsed source is used (for example, with a TOF mass spectrometer), the laser might be preferred while, conversely, if the analyzer operates continuously, the "sputtering" source may be more compatible. Fig. 7 shows a spectrum of a copper sample obtained with a relatively simple ion-bombardment instrument using only

0

r 65

.: 6U

U. a t. F. - 9 5756 5554 53

c. So

Fig. 7 . Spectrum of neutral particles sputtered from Cu target . Gas background spectrum suppressed by use of synchronous detector system; sputtered ion spectrum suppressed by maintaining target at negative potential with respect to second ion source . Mass spectrometer used has 6 in . radius, 60' deflection magnetic direction-focusing ion analyzer (Nuclide 6-60-DE) without energy focusing . Mr. J. Mass Spectrum. Ion Phys., 4 (1970) 337-363

362

L . F_ AESZOG

a single focus ."ng analyzer, by rejecting the energetic secondary ions and then ionizing and studying the neutrals . This technique shows promise . Two recommended mass spectrometer systems Onthe basis of these considerations it seems to me that the full development of at least two different types of space-probe mass spectroscopes should be undertaken . One of these should be a double focusing magnetic spectrograph . The primary version of this should have a large enough analyzer to be useful for the experiments outlined above requiring medium resolution . Use of a permanent magnet and a fixed accelerating voltage offers the advantage of simplicity, and (when a successful electrical analogue of a photoplate has been developed) will offer high sensitivity as well (since many species can be detected simultaneously with a spectrograph) . Until such a detector is available . one can compensate by providing a relatively small . variable accelerating voltage to be used to scan the spectrum past two or more stationary detectors at different radii, each of which would be either a Faraday cage (for stationary site, long time-per-analysis applications) or a multiplier (when greater speed is required) . For the second instrument, I think a "hybrid" analyzer capable of being used principally in a TOF mode, but also capable of being converted to coincidence-TOF or monopole operation, is worth considering . Inclusion of the mass filter capability seems to me to offer a means of multiplying the effectiveness of a single basic apparatus, and also of minimizing eventual cost, if it is granted that eventually it will be desirable to have available both a TOF principle spectrometer and a "mass filter" of some type. If funding or other considerations dictate that only one type of instrument should be developed, with perhaps different sizes being used for different purposes, I feel the choice must be restricted to a double-focusing spectrometer-spectrograph . However, there is a significant advantage in developing at least two instruments of different types, since it would then be possible to have data obtained by two rather independent methods to intercompare . For the same reason, combination instrument, such as the pyrolysisigas chromatograph/mass spectrometer for "life" detection that can also give readouts from the individual instruments run separately will be exceptionally useful in the exploration of the solar system . This ends this review of the possible applications of "robot' mass spectroscopes to the exploration of the solar system, and of types of components appropriate for such uses . Its brevity was dictated by space considerations, the intent being merely to provide a survey of this developing field . A more detailed discussion is, however, available in an article presently in press 11 . Geochem :sts and cosmochemists, planetary physicists, astronomers, nuclear physicists, atmospheric scientists, and indeed the entire scientific community and many laymen, look forward to the results of these contemplated experiments with great anticipation and, I believe, all share the hope that the United States governInt . L Mass Spectrom . Ion Phys., 4 (1970) 337-363

MASS SPECTROSCOPY BEYOND THE MOON

363

ment will continue to provide the funding of an orderly program of such experimentation a reasonable priority in the national budget, even in these days when there are so many other pressing national demands for funds ; since, for man to continue to grow in understanding of his place in nature, gaining this additional knowledge of the solar system, which is now available to us at a reasonable cost, is vitally important .

REFERENCES I L_ F . HERZOG, "A Mass Spectrometer System for Lunar Surface Analysis", Final Report,

NASW 1062, 42 pp . (Dec . 1965) . 2 B . R . F. K.ENDALL, L . F. HERZOG, P . J . WVYLLIE, D- S . EDMONDS AND C . BALER, "Analysis of the Lunar Surface and Atmosphere by .Mass Spectroscopy", NASA CR-56553, 151 pp- (1964) . 3 L . F. HERZOG AND S. R . GavczvK, "Further Development ofa TOF Mass Spectrometer System for the Analysis of the Lunar Atmosphere", NASIJ--1_86 Final Report, 33 pp . (1968) . 4 L- F . HERZOG, "Determi:: :eg the Composition and History of the Solar System", Ch . 1 of Anal. Chem. in Space (R_ WAINERDi, Ed .), Pergamon Press (1970)5 H . E . SuESS .e''D H. C. UREY,"Abundarces or the Elements", Rec . Mod.Phvsics, 19 (1956)53-74, 6 W . G . DEUSER, "The Distinction of Rock Types on the Basis of their Mass Spectra, with Special Reference to Lunar Surface Applications", NASA CR-310, 16 pp . (1965) . 7 NASA Langley Research Center, Statement of [fork L17-9845-"Development of an LLstrutnent for the Analysis of Atmosphere and Organic Surface Composition for Planetary Erptoration" (13 March 1969)8 J . WOOD, "Comets and Interplanetary DA at, is Space Research' in Ref. 10 below, pp . 117-23 (1966) . 9 T. GOLD, "Comment on Paper by A- C W_ Cameron", p . 248 in "Tie Origin and Erolution of the Atmospheres and Oceans (P. J . ER.ANCAzio AND A . G . 1W . CAMERON, Eds .), J . Wiley and Sons, New York (1964) . 10 Space Science Board, H . H . HEss, Chmp ., "Space Research-Directions for the Future", NASNRC Pub. 1403, pp . 61-2 (NASA) (1966) . 11 L. F . HERZOG, " :lass Spectroscopy in Solar System Exploration', Ch .4of Analytical Chemistry in Space (R . WAINERDI, Ed .), Pergamon Press (1970) . Int . J. Mass Spectrom_ fan Pitys ., 4 (1970) 337-363