Table Of ContentANALYTICAL
CHEMISTRY IN SPACE
Edited by
RICHARD E. WAINERDI
Professor and Head
Activation Analysis Research Laboratory
College of Engineering
Texas A & M University, U.S.A.
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FOREWORD
THE drama of manned space flight has tended to obscure the underlying scientific potential
of space research. Yet already, at a fraction of the cost of placing a man in orbit, extremely
important results have been obtained by quite simple scientific instruments in space vehicles.
The discovery of the trapped radiation zones, the investigation of the interplanetary plasmas
and magnetic fields, for example, have revolutionized our concept of the environment of
the Earth and interplanetary space.
A start has been made, too, on the analysis of the chemical constitution of space. It is
this analytical chemistry with which this book is concerned, that is the analysis of the
constitution of the particles in the solar wind, of the planetary atmospheres and the surfaces
of the Moon and planets. The extension of terrestrial techniques to space presents formidable
problems. First, there are the severe restraints on the size and weight of the apparatus which
can be carried in space vehicles, then all the problems associated with the operation of the
apparatus in the hostile environment of space whether this be interplanetary space, on the
Moon or on a planetary surface.
Overriding all is the problem of contamination. The experimenter must be certain that
his techniques do not themselves contaminate the specimen which he is analyzing and
thereby yield false information. Even when the operator has firm control over this problem
it is probable that, at least, in the case of the Moon and planets previous terrestrial probes
and rockets will have landed or passed through the atmosphere. The analytical procedures
must evaluate and eliminate these difficulties if the eventual result is to be relevant to the
prime object of the investigation.
The potential dividends are enormous; the prospects of an incontrovertible assessment
of the constitution of the major bodies of the solar system open before us, and hence a firm
understanding of the processes of evolution of planetary systems. The realization of these
prospects depend on the careful and precise execution of the topics discussed in this book.
The publication of this work is a timely and valuable contribution to the subject.
Jodrell Bank SIR BERNARD LOVELL, O.B.E., F.R.S.
vii
CHAPTER 1
DETERMINING THE COMPOSITION
AND HISTORY OF THE SOLAR SYSTEM
WHAT IS KNOWN AT PRESENT AND WHAT WE HOPE TO LEARN FROM SPACE PROBES
L. F. HERZOG
Nuclide Corporation, State College, Pennsylvania
NOTE: This chapter and chapter 4 together comprise a discussion of the possibilities for exploration of the
solar system by mass spectroscopy. The sections, exhibits and references in chapter 4 have therefore been
numbered consecutively following those of chapter 1.
Contents
I. Introduction
II. Compositional Data on Solar System Objects
A. Solar and "Cosmic" Abundances
B. The Earth and the Meteorites
Chemical composition of terrestrial igneous rocks
Chemical composition of meteorites
Major element variations in igneous rocks and meteorites
Trace element variations in igneous rocks and meteorites
Interpretation of a single chemical analysis
Composition of the terrestrial atmosphere
C. The Moon
The surface of the Moon
Physical characteristics
Origin of the lunar surface features
Composition of the lunar surface layer
The alpha-scattering analysis technique
The sampling problem
Determination of the mineralogy and petrology of a sample
on the surface of the Moon (or a planet) from its
chemical analysis
The lunar atmosphere
Inferences from lunar atmosphere constituents
Contamination of the lunar atmosphere by man
Measurement of escape rates
Exhaust-gas spectrum
D. Mars
The atmosphere of Mars
E. Mercury
1
2 L. F. HERZOG
F. Venus
The atmosphere of Venus
G. Jupiter
The atmosphere of Jupiter
H. Saturn
I. The Outer Planets
Uranus
Neptune
Pluto
J. Asteroids, Comets and Interplanetary Dust
Asteroids
Comets
Interplanetary dust
References
COMPOSITION AND HISTORY OF THE SOLAR SYSTEM 3
I. Introduction
Determining the composition of the surface of the Moon is of great practical interest
today because we plan within the next few years to establish manned space stations on the
Moon. The exploration of the rest of the solar system will obviously proceed much more
rapidly if it proves economically feasible to obtain from the Moon materials which can
help sustain life there and also support exploration missions beyond the Earth-Moon
system. Thus, knowledge of where and how water, oxygen, hydrogen, carbon, nitrogen,
hydrocarbons and structural metals, for example, are distributed is of immediate tech-
nological as well as scientific significance. The composition of the rarefied lunar atmosphere
is likewise of interest because it is an environment in which men, instruments and machines
will have to function.
However, scientists are also anxiously awaiting compositional data on the atmosphere
because these could possibly also provide information bearing on the origin and evolution
of our solar system and our galaxy.
Our understanding of solar system history and the relationships of its member objects
is at present in an unsatisfactory state, in a large measure because we have had available
until now reliable analytical data on only a small proportion of the objects in the solar
system, and even these analyses were for the most part representative of only the outer
portions of the objects sampled.
We do have numerous high-quality quantitative analyses of terrestrial rocks of types
exposed at the Earth's surface or reached by mining or drilling, and quantitative analyses of
its atmosphere and hydrosphere. We also have a large number of good analyses of meteorites
of various types, as well as of certain objects of questionable origin, such as the tektites. For
most of these samples, fair to good-quality data on isotopic abundances is now available also.
We have semi-qualitative data on the relative concentrations of some sixty-six elements,
plus a few isotope ratios, in the Sun's atmosphere, obtained by studies of the Sun's optical
spectrum. Until now, these data, supplemented by additional spectrographic infrared, radar,
and other radiotélescope measurements of solar, stellar and planetary atmosphere and
comet spectra—plus the few possible deductions from planetary density distributions—have
been the basis for our ideas of "cosmic" abundances—the relative abundances of the elements
in all matter—and the relationships of various distributions of matter in the Universe.
In attempting to determine average abundances, great difficulty arises in extrapolating
from the available analyses to average compositions for whole objects. Take, for example,
the Earth: it has a radius of 6371 km but our deepest penetration by drilling has been only
about half-way to the base of the so-called "crust", at approximately 30 km. Below this
there is a "mantle" which extends downwards some 2870 km to a boundary with a "core".
There is also a hydrosphere and an atmosphere. The relationships of these parts of the body
are as follows :
Zone Thickness Vol.(1027cm3) Density (g/cm3) Mass(1027 gm) Mass(%)
Atmosphere (300) 0.000,005 0.000,09
Hydrosphere (mean) 4 0 0014 1.03 0.0014 0.024
Crust 30 0.015 2.8 0.043 0.7
Mantle 2870 0.89 4.5 4.06 67.8
Core 3471 0.18 10.7 1.88 31.5
Whole Earth 6371 1.08 5.52 5.98 100.00
4 L. F. HERZOG
The atmosphere and hydrosphere are both relatively homogeneous in composition but
the crust is highly differentiated, and even if we could accurately deduce an average com-
position for it by properly weighting the analyses we have, we would have accurate relative
abundance data for only 0.024% (by mass) of the whole Earth, since the density data, the
heat-flow data and other considerations demand that the mantle and core have much
different compositions from that of the crust, and from each other. In fact while the crust
is largely composed of silicate rocks, the core is thought to consist principally of an iron-
nickel alloy.
Meteorites, under some early theories, were formed by the breaking up of terrestrial-
planet-sized bodies. There are iron meteorites as well as stony meteorites and types con-
taining the three phases, silicate, sulfide, and metal. It was attractive to assume that the
various types were actually samples from different depths within an original Earth-like
body, so that one could infer from actual meteorite compositions, compositions for the
mantle and core of the Earth. However, a difficulty of the same sort as was encountered for
the Earth's crust immediately arises—how to weight the various types of meteorites in con-
structing an average composition for meteoritic material. In 1940 there were over 1200
different meteorites in museums. Some of these had been seen to fall, then recovered, others
had only been found—in some cases by farmers ploughing fields centuries or longer after
the fall. A table of "finds" and "falls" by type shows that there is a strong selection process
controlling what is found—the less like the terrestrial rocks of the region a meteorite looks,
and the more resistant it is to terrestrial erosion processes, the greater the chance it will be
found.
Type of
meteorite Finds Falls Total
Irons 409 66% 29 5% 438
Stony irons 46 7.5% 8 1.5% 54
Stones 165 26.5% 547 93.5% 712
100.0 100.0 1204
So the sample-weighting problem is also difficult if one seeks to determine the average
elemental composition of meteorites—the only samples of extraterrestrial matter which,
until now, could be analyzed directly in the laboratory.
Because the Sun is a gas and follows the laws of magnetohydrodynamics, it may be that
we have better information on its average composition, from the analyses we have of its
atmosphere, than we have of the average compositions of the Earth or meteoritic material.
But even this depends on whether the Sun's matter mixes continuously from surface to core,
or not, which is as yet an unanswered question.
We have had until now only the sparsest information on the compositions of other solar
system objects—for the planets, only the detection of a few atmospheric gases by spectro-
scopy (and inferences of other gases, necessary to give "model planets" having the observed
densities), and some comet and meteor spectra.
Nevertheless, scientists have been making estimates of the average elemental composition
of the Universe for over a century. The major inputs to such tables have been the meteoritic
and terrestrial rock data for the non-volatile elements, and the solar data for the volatiles.
As analytical techniques have improved, these tables of abundance have been refined, and
as the radius of the known Universe has expanded, and compostional data have become
available not only for other stars but also for distant galaxies, it has come to be recognized
COMPOSITION AND HISTORY OF THE SOLAR SYSTEM 5
that the "cosmic" abundance tables based on Sun, Earth and meteorites were possibly
only applicable to a limited region, of unknown extent, in which our solar system is located.
With the development of the mass spectrometer just before 1920, the fact that most
elements were actually composed of two or more types of nuclei containing the same
number of protons but different numbers of neutrons—isotopes—became demonstrable.
It has become apparent through studies made between then and now that, in general, for
samples taken from the Earth and meteorites, isotope ratios show very little variation as a
function of source.
With the development of theoretical nuclear physics it became obvious that at sufficiently
high temperatures and pressures one could form heavy nuclei from a few building-block
particles—neutrons, protons, electrons—either under equilibrium conditions or in a dyna-
mic situation where temperature and density were changing too rapidly for equilibrium to
be attained by the processes permissible at each stage. Theories of nucleogenesis based on
nuclear physics data began to be formulated. The earlier of these such as the "big bang"
theory of Gamow still formed all nuclei in the Universe in one event; however, some who
felt that keeping average density constant in an expanding universe was necessary, for ex-
ample, to keep the gravitational constant constant, soon offered alternative theories involving
continuous creation.
At the same time from work related to hydrogen bomb production there was developed
a highly acceptable theory of energy production in the Sun, based on the "burning" of hydro-
gen to produce helium, with the conversion of the excess mass into energy by E=mc 2.
Several workers took the logical next step and developed predictions of the history of the
Sun (and stars in general) after so much of the available hydrogen has been used up that the
star cannot maintain its former rate of energy production. In this case it contracts, it gets
hotter, and the conversion of helium to heavier species can begin. Fowler (I.A1), for
example, has worked out a comprehensive theory of nucleosynthesis in stars.
It is found that the history and the end products differ depending on the original com-
position—whether pure hydrogen for example or hydrogen containing impurities from an
earlier nucleogenesis. The theory now speaks of first, second, or nth generation stars—and
our Sun may contain third-generation matter. It appears, from the element abundances
and the few available isotopic data for the Sun, that the Sun, Earth and meteorites all are
comprised of matter that formed at the same time, and for the Earth and meteorites, radio-
active decay dating places the time of that event about 5000 million years in the past.
Was all of the material in present solar system objects formed in this one event? Or are
some of these objects captured samples from another system? Do some of them predate
the solar system, as Urey once proposed the Moon might? These are important questions
to cosmology and the space program now offers means of studying them by actually deter-
mining the compositions of other objects in the solar system.
The Moon is the closest and also one of the most important objects for study in the solar
system. It is unique among the natural satellites in its large size relative to its primary,
the Earth, and also in its rock-range density. For a long time there have been theories
under which it formed from the Earth at an early stage, but these present numerous diffi-
culties, so that it has seemed more likely that it did not. Is it a differentiated body like the
Earth? The density data derived from its gravitational interactions with the Earth and Sun
indicate that its density is uniform—there is no core. If it did not differentiate at all, might
it not show at, or near, the surface, its average composition throughout—which com-
position might be that of average solar system material (at least for the involatile elements)?
6 L. F. HERZOG
Of course it has been bombarded by vast numbers of meteorites over the years but these
may remove more material than they add and may cause surface contamination only.
Is there life on Mars? That there may be has been a supportable speculation since the
"canals" were taken to be works of intelligent beings. The Martian environment is severe,
but it appears that at least some of the simpler forms of life found on Earth might indeed
survive there; for this reason we have a sterilization requirement for space probes intended
to terminate on Mars.
Does Jupiter represent a sample of undifferentiated proto-solar system material, including
the volatiles? Are the long-period comets truly a part of the solar system?
These are a few of the important questions which students of the solar system now see a
chance of answering through the new mobility given to man and his instruments by the
space program.
Already the old analytical barrier has been broken. Several semi-quantitative analyses of
eight elements have been made of very small samples of the Moon's surface, by alpha-
probes soft-landed on the Moon aboard U.S. Surveyor-program vehicles. Meanwhile, the
U.S.S.R. has provided some compositional data on Venus by dropping sensors specific
to a few expected constituents into its atmosphere aboard Venus IV. And very soon, perhaps
in mid-1969, astronauts will bring back the first specimens of Moon rock and soil to Earth
for analysis in terrestrial laboratories.
The time therefore seems exceptionally suitable for a review of the present state of our
knowledge about the composition of the various objects comprising the solar system, and
also, of the analytical techniques which we now have at our disposal which might be used
to make compositional determinations in remote, unmanned missions, as well as a part
of the manned exploration program, and for an assessment of the relative capabilities of
each, and their areas of greatest relative utility and cost effectiveness.
In this chapter the writer attempts to review the present compositional data, and to set
down some of the open questions which space-probe exploration may hope to answer. The
emphasis is on the Moon and beyond. Data on the Sun, Earth, and meteorites are included
only as references for the discussions of the other objects, and because of the obvious
importance that comparing extraterrestrial observations against the great body of know-
ledge on these objects we already have will have in formulating improved theories of the
origin and history of the solar system and its member bodies.
In this chapter, mention is made from time to time of analysis techniques already in use
for certain studies that can be considered precursors of space exploration by robot probes,
and in some cases suggestions are made as to instrumentation appropriate for robot
analyzers. The writer trusts that the reader will understand, and excuse the fact that most
of these examples pertain to mass spectroscopy, since this is the writer's field of specialization.
Π. Compositional Data on Solar System Objects
A. Solar and "Cosmic" Abundances
THE SUN
The Sun is the only star on which surface details can be studied by present techniques.
Furthermore, 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.
COMPOSITION AND HISTORY OF THE SOLAR SYSTEM 7
The Sun's mean radius is 7 χ 105 km, its mass approximately 2 χ 1033 grams; it rotates
with a 27-day period, and points near the equator rotate most rapidly. The temperature of
the core, where hydrogen fusion is occurring, is approximately 20 χ 10 6°K, but it decreases
outward with a minimum of approximately 4200° near the surface, increasing again to
1 χ 106°K in the corona. The visible spectrum is approximately that of a black body at
6000°K.
Relative abundances of the elements in the outer portion of the atmosphere are derived
from measurements of solar adsorption lines as well as the emission spectrum. Evans'
(ILAl) abundances are given in Table II.A1 together with Unsold's (II.A2) earlier table of
abundances, which had some rather différent values. Some sixty-six elements have been
detected, the number being limited not solely by low abundance but also because wave-
lengths below 2900 Â are strongly absorbed by the terrestrial atmosphere.
TABLE II.Al. Elemental Abundances in the Solar Atmosphere
Abundance Atoms per atom Si Solar atmosphere
data
Unsold Evans Ζ Ele- Unsold Evans
Ζ Element 1950* 1966t ment
1 Η 51,000 50,000 22 Ti 0.0047
2 He 10,000 2500-10,000
6 C 1.0 10 23 V 0.0006
7 Ν 2.1 20 24 Cr 0.020
8 Ο 280 25 25 Mn 0.015
11 Na 0.1 0.1 26 Fe 2.7 0.07
12 Mg 1.7 1.6 27 Co 0.0055
13 Al 0.11 0.1 28 Ni 0.047
14 Si 1.000 1.000 29 Cu 0.0009
16 S 0.43 0.4
19 Κ 0.0008 30 Zn 0.0031
20 Ca 0.087 0.07
21 Sc 0.0001
* Trans. Intl. Astro. U. 7, 460.
f Encyclopedia of Sei. and Tech., vol. 13, The Sun (McGraw-Hill).
The degree to which these spectra give the composition of the Sun as a whole depends
on the extent to which convective mixing takes place, and the evidence on the extent of
mixing is, unfortunately, not conclusive at present. Furthermore, the abundances derived
from these optical spectra have large limits of error, as the values given are dependent
upon a choice of temperature model, corrections for interferences, and extrapolations from
work with terrestrial laboratory standards, among other factors. For these reasons, it has
been common practice for compilers of "cosmic" abundance tables to use relative abund-
ances measured in, for example, certain types of meteorites and terrestrial rocks for all but
the lighter gaseous elements. The meteorite data do, however, agree with the solar atmos-
phere data fairly well—generally to within a factor of 10.
A discussion of the composition of Milky Way Galaxy stars other than the Sun, or of
other galaxies, is beyond the scope of this book; however, it may be noted that stellar
spectra differ enormously depending on temperature and pressure—for example, hot type
Ο and Β stars have strong Η and He lines and no molecule lines, while cool M and R stars
are rich in molecule lines and weak in H and He. Such differences are primarily not thought
to represent real differences in composition; on the other hand, in cases like the M stars
