The quantitative measurement of atomic and molecular O2 in rarefied atmospheres by mass spectrometers onboard high speed spacecraft is reported. Data are also given on instrument performance in high speed molecular beams and in the fly through mode

Nier, A.

English

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' " N75 20399
DETERMINATION OF THE COMPOSITION OF RARIFIE_ NEUTRAL ATMOSPHERES
BY MASS SPECTROMETERS CARRIED ON HIGH-SPEED SPACECRAFT
Professor A. Nier
University of Minnesota
INTRODUCTION - A. NIER:
As all of you know, mass spectrometers have been used in the laboratory
for many years for analyzing mixtures of gases and it has been possible to
analyze rather complex mixtures if one has calibrations for the individual
gases. There have been dozens of sounding rocket flights which have carried
mass spectrometers to the thermosphere region of our atmosphere, and there
have been at least a half dozen satellites which have carried mass spectro-
meters for making analysis of the neutral atmosphere as well as the ionized
atmosphere. On the Viking mission there will be two mass spectrometers on each
of the landers. One will make measurements in the upper atmosphere and the
other will be o_ the lander itself. The latter will make atmospheric analyses
once the lander touches down on the surface and will also look for organic
compounds and volatiles in the soil. The Pioneer Venus program will have mass
spectrometers both on the entry vehicle and large probe, and on the orbiter.
As can be seen, mass spectrometers are playing an important role in the space
program, and for good reason, especially in those missions where one doesn't
know what is present. Where there are unknown mlxtures,there's probably not a
more versatile tool than a mass spectrometer. One has enormous dynamic range
and can detect very rare constituents in the presence of much more abundant ones.
Unlike many methods which may be sensitive for particular classes of compounds,
the sensitivity is roughly the same for all compounds.
Today I want to talk about the use of mass spectrometers carried on high
speed vehicles through rarified atmospheres. Following what Don Hunten
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mentioned yesterday, if you are going with a probe to a planet surface, one
should take advantage of the opportunity to makemeasurementsas one approaches
the planet. Tying measurementsin the thermosphere to those in the lower
atmosphere provides valuable information concerning atmospheric processes.
Making quantitative measurementswith mass spectrometers carried on high
speed vehicles poses certain problems. I want to discuss solutions to some
which arise as one passes through a rarlfied atmosphere. Other speakers will
discuss measurementsin more dense atmospheres.
THEOPENSOURCEATMOSPHEREEXPLORERMASSSPECTROMETER
In our work we have been using magnetic deflection instruments for per-
forming massanalysis. The ion sources are of our own design and the mass
analyzer employs the familiar Mattauch-Herzog geometry. Figure 8-1 is a schematic
drawing of the instrument we have provided for the Atmosphere Explorer satellites
C, D, and E. lons are produced by an electron beammoving perpendicular to
the figure. It is represented by the black dot between the two bar magnets M
which collimate the beam.
If the instrument moves to the left, the ambient gas entering the instrument
is equivalent to a beamtoward the right as shown. For an earth satellite
such as AtmosphereExplorer-C the beamconsists of a stream of particles having
a unidirectional componentof velocity of 8.5 km/sec to the right and an
omnidirectional componentcorresponding to a Maxwell-Boltzmann distribution
having an average speed of about 1 km/sec. Particles entering the region
between the magnets M will be ionized, somedirectly as they pass through the
electron beam, others after they have struck surfaces and are slowed down. lons
formed are accelerated toward the slit SI, in part due to a repelling field
between grid 3 and the assembly Sh, and in part due to an attracting field
between Sh and the focusing plates Jl and J2"
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In the instrument shown there are two collectors beyond the slits S 3
and $4, making possible the simultaneous collection of ions differing in mass
by a factor of 8. Mass spectra are swept by changing the total accelerating
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potential applied to the ions along with the field in the electric analyzer.
In the case of missions to comets where the atmosphere is extremely tenuous
the two multipliers could be replaced by a channel multiplier array, making
possible the simultaneous collection of many masses. A practical instrument such
as discussed can be built to weigh 6 kg or less and consume under 5 watts of
power.
MASS SPECTROMETER PERFORMANCE IN HIGH SPEED MOLECULAR BEAMS
Last year (thanks to the cooperation of Prof. J. B. French and his
colleagues) we had occasion to test one of our Atmosphere Explorer open source
instruments in the high speed molecular beam facility of the Institute for
Aerospace Studies of the University of Toronto. Figure8-2 is a schematlc view of
the test facility. The high speed beam is produced by the free expansion of a
low molecular weight carrier gas (helium in our case) seeded with a small amount
of argon and CO 2, the gases of interest to us in our tests. The mixture
leaves the heated ceramic tube through a pinhole as shown to the left in the
figure. After passing through a skimming and collimating chamber, the beam
impinges on the mass spectrometer attached to the main chamber as shown.
The response of the instrument to different angles of attack could be
checked by bending the bellows. When the beam flag was rotated into place,
the background in the chamber could be measured. When the stagnation plate was
slid into place, the bellows chamber became an idealized stagnation chamber,
making possible a check of the extent to which the ion source departed from
an idealized closed source.
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beam impinges on the ion source. One sees peaks corresponding to CO 2 and Ar
as well as impurities such as 02 , N 2, H20 and some hydrocarbons. When the beam
flag is _placed in front of the source, one obtains curve B, corresponding to the
background in the chamber due to impurities present in the system as well as
the scattered molecular beam. As can be seen, for beam particles the back-
ground accounts for only about 20 percent of the readings. It is interesting
to note that for 02, H20 and the hydrocarbon impurities the A and B curves coincide,
showing that these gases are due entirely to impurities in the chamber, none
being present in the beam.
From a comparison of the A-B difference and the spectrum obtained when the
stagnation plate was in place while the flag covered the source, it was possible
to show that for Ar and CO 2 the ion source itself behaved as if it were
96 percent stagnated. In other words, the laboratory measurements predicted that
the source as designed, when exposed to an ambient atmosphere of heavy gases,
would give essentially the same readings as an ideally closed source with a
knife-edged orifice. For helium the readings were somewhat lower, showing that
this light gas is not completely accommodated upon collision.
MASS SPECTROMETER PERFORMANCE IN FLY-THROUGH MODE
The availability of the high speed molecular beam made possible tests not
previously undertaken. In particular, if grid 3 and the focusing plates J
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and J2 are tied to the assembly Sh, there is no field drawing ions out of the
region where they are formed, and the instrument is in the retarding potential,
or fly-through mode. In this case, incoming gas molecules which strike the
ion source and are accommodated have only the energy characteristic of the ion
source surface temperature, a few hundredths of an electron volt. On the other
hand, those particles which have not struck surfaces have an energy characteristic
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of their velocity in the beam, about 0.09 eV per atomic mass unit of mass for
a 3.9 km/sec beam. This initial energy is enough to permit them to pass into
the accelerating region after being ionized by the electron beam. In other
words, in the fly-through mode the instrument, when carried on a high speed
spacecraft, has the capabilities of distinguishing between true ambient particles
and ones which have hit the instrument's surfaces and become thermalized or altered
in nature.
Figure 8-4 shows spectra corresponding to those shown in Eigure _3 obtained
when the instrument was in the fly-through mode. It is interesting to note
that except for the 12, 16 and 28 peaks the background curve FB is zero,showing
that the instrument indeed discriminates sharply against particles which do
not have the energy of the beam. The fact that the background at 12, 16 and 28
is not zero comes about because these peaks are fragment ions produced by the
dissociation of the background CO 2. In the ionization process they acquire
kinetic energy. •Hence these fragments are not excluded. The 14 peak is due
entirely to background N 2 in the chamber and as in the case of the CO 2 fragments,
acquires kinetic energy in the dissociation and ionization process. As will
be discussed later, ambiguities due to the energetic fragments can be eliminated.
The beam tests just discussed were conducted in time to include the fly-
through feature in the instrument carried on Atmosphere Explorer-C launched in
December 1973.
APPLICATION OF FLY-THROUGH FEATURE TO ATMOSPHERE EXPLORER ML%SURD_EI_-fS
The determination of the absolute densities of atomic and molecular oxq/gen
by mass spectrometers carried on sounding rockets and satellites has been the
subject of some controversy. In the case of open source instruments carried on
rockets,it was recognized that atomic oxygen was lost by reactions with instrument
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surfaces but the extent of the loss was not clear. In the case of closed
source mass spectrometers carried on satellites,it was found that after several
orbits all of the atomic oxygen was converted to molecular oxygen by reactions
on the walls of the cavity enclosing the source. While this made possible the
quantitative measurement of atomic oxygen at high altitudes where ambient
molecular oxygen was negligible, the method merely gave total oxygen in the
interesting region of the atmosphere where atomic and molecular oxygen have
comparable abundances.
As has already been mentioned, in its normal mode of operation the
Atmosphere Explorer open source mass spectrometer performs essentially as a
closed source instrument. In this mode it gives quantitative values for number
densities of N 2, Ar, He and total oxygen. When switched to the fly-through mode,
it distinguishes between atomic and molecular oxygen, giving absolute number
densities for each. Figure8-5 illustrates the performance in this mode. The
mode is particularly applicable when the spacecraft is spinning at its normal
spin rate of I revolution per 15 seconds and the instrument is set to toggle
back and forth between masses 16 and 32 rather than look at a large number of
masses.
Mul_iplier counts are accumulated for 1/16 sec while the instrument is
set to collect mass 16. It then counts mass 32 ions for 1/16 second, switches
back to mass 16 for 1/16 second, etc. The results are shown for two different
altitudes of orbit 912 as the instrument passes through the forward looking
direction as the spacecraft spins. Particle densities are roughly proportional
to count rates.
The figure illustrates a number of interesting points: (I) the 16 peak
is always greater than the 32 peak at the same altitude, as it indeed should be
in the altitude range shown, (2) in going from 179 to 259 km the 32 peak falls
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off very much faster than the 16 peak, as it should due to the large dif-
ference in scale heights, (3) the background of a few percent of the peak
values near the "forward" direction is due primarily to incomplete rejec-
tion of slow particles. By operating the instrument with low electron ac-
celerating potentials, 25 volts in our case, the production of energetic
fragment ions such as 0 from 02 is reduced. Since they are made from 02 pro-
duced in part by chemistry in the source, they do not contribute to the sharp
peak when the instrument looks forward, and merely add to the background,
(4) while part of the width of the peaks is due to the finite acceptance
angle of the instrument, the largest part is caused by the fact, which was
mentioned earlier, that the "beam" seen by the instrument has a unidirec-
tional component having the spacecraft velocity of 8.5 km/sec and an omni-
directional component of roughly i km/sec average velocity corresponding to
the Maxwell-Boltzmann distribution at the thermospheric temperature of about
900=K. As the spacecraft spins, particles can thus enter over an angle of
a number of degrees. It is interesting to note that at half height the
width of the 16 peak is approximately 2 1/2 wider than the 32 peak, as it
should be because of the difference in average Maxwell velocities of the two
species in the atmosphere. It appears that with proper calibration, the
width of the peaks can be used to deduce in situ atmosphere temperatures.
Figure 8-6 gives the plot of peaks such as shown in Figure 8-5 as a
function of altitude, and from the relative scale heights provides additional
proof that the peaks as read in the fly-through mode are indeed due to the
ambient atmosphere uncontaminated by wall collisions effects. The count rates
are reduced to ambient number densities through laboratory calibrations sup-
plemented by calibrations in orbits where fly-through readings are inter-
spersed with readings in the normal mode.
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FIGURE 8-6
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CONCLUSIONS
In a properly designed open source mass spectrometer one can operate in
both a "normal" mode and in a mode in which particles arising from collisions
with instrument surfaces are excluded. In instruments carried on high speed
spacecraft such as will be sent to the unknown atmospheres of other planets
or comets this feature is of considerable importance in making a distinction
between the true ambient atmosphere and gases which arise as the result of
chemical reactions on instrument surfaces.
An example is given in which atomic and molecular oxygen are distinguished
by the open source mass spectrometer carried on the Atmosphere Explorer-C
satellite.
Vlll-15


Determination of the composition of rarefied neutral atmospheres by mass spectrometers carried on high-speed spacecraft

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