After a schematic representation of the IRIS experiment, objectives of the program are outlined. Specifically data cover: (1) determination of energy balance for Jupiter and Saturn, (2) determination of methane, ammonia, hydrogen and water vapor quantities, (3) three dimensional temperature fields, (4) hydrogen-helium ratios and isotopic ratios of H/C, C/N, and C/O

Burke, T. E.

English

NASA Technical Reports Server

N75 17251
Thomas E. Burke
THE MARINER JUPITER/SATURN
IRIS EXPERIMENT
The principal investigator of the infrared spectroscopy and radiometry investiga-
tion is Dr. R. A. Hanel of Goddard Space Flight Center. Unfortunately, he was not
able to be here today.
Figure 1 is a schematic representation of the Jovian spectrum from the near
ultraviolet to the thermal infrared regions. The spectrum for Saturn should not
be markedly different except, of course, for the radiance levels. The incident
solar energy is shown by the solid line at the right of the diagram. Some of the
solar energy is absorbed (the dashed line), and the remainder is reflected or scat-
tered. The energy that is absorbed is reemitted in the thermal infrared, together
with any energy from internal sources.
The IRIS instrument (an acronym for infrared interferometer spectrometer)
consists of a coaxial interferometer and radiometer. The bandpass of the inter-
ferometer extends from 200 cm"1 to 3300 cm"1 (50 to 3.3 pirn); the bandpass of the
radiometer extends from 5000 cm"1 to 33000 cm-' (2.0 to 0.3 /tm). These band-
passes are indicated by the solid horizontal lines near the top of figure 1. We are
considering the possibility of extending the high-frequency cutoff of the inter-
ferometer out to 4000 cm-1 (2.5 /urn).
The optical layout of the MJS IRIS is depicted in figure 2. An f/8 Cassegrain
telescope provides large energy throughout and a narrow field of view (0.25°).
The diameter of the primary mirror is 51 cm. A dichroic element is used to sepa-
rate the energy for the radiometer from that for the interferometer. Near infrared
and visible energy ( > 5000 cm"1) is passed by the dichroic element to a focusing
mirror and to the radiometer detector. Near and thermal infrared radiation
(< 4000 cm"1) is reflected, collimated, and passed to the interferometer. The
radiating surface and the various blankets and baffles are also depicted.
The optical layout of the Michelson interferometer that was used on the Nimbus
3 and 4 spacecraft is depicted in figure 3. The layout of the MJS interferometer
Jet Propulsion Laboratory.
147
https://ntrs.nasa.gov/search.jsp?R=19750009179 2020-03-22T22:46:24+00:00Z
300 1000 3000
Wave number, cm
III 1100 i ooo
30 I 0.310 3
Wavelength, him
FIGURE 1. — Energy balance of Jupiter. The spectral ranges of the interferometer and radiometer are
superposed on the thermal emission spectrum and upon the reflected solar spectrum.
148
FIGURE 2. — Optical schematic diagram of the MJS IRIS.
The Rings of Saturn
Image motion compensator
& calibrator mirror
Parallel
Linear
motion
motor
Moving
mirror
Thermistor
bolometer
-Beamsplitter*
compensating plate
Mirror
FIGURE 3. — Optical schematic diagram of the Michelson interferometer.
will not be substantially different: the image motion compensator and calibrator
mirror will be replaced with the Cassegrain telescope; the entrance window/filter
and the beamsplitter compensator will be deleted; and the thermistor bolometer
will be replaced by a detector with lower noise equivalent power—either a thermo-
pile or a pyroelectric detector. Note that the interferometer has a coaxial inter-
ferometer using the 585.25-nm neon line for wavelength calibration.
Figure 4 indicates the approximate magnitude of the signals expected at Jupiter
and Saturn. The thermal emission curves are blackbody curves for various tem-
peratures (130 K for Jupiter and 96 K for Saturn). The reflected solar radiation
curves are based on an albedo of 0.5. The noise equivalent radiance (NER)—the
level of radiance for which the signal-to-noise ratio is unity—is indicated on the
figure. One can infer the signal-to-noise ratio as a function of frequency from the
expected signals and the NER: near the peaks of the blackbody curves the signal-
to-noise ratio is in excess of 1000, and near the crossover between emitted and
reflected energy the signal-to-noise ratio is not significantly greater than unity.
However, I would emphasize that the instrument has a spectral resolution of 4.5
cm~'. The NER can be lowered by degrading the spectral resolution.
I would like to briefly review some of the objectives of the IRIS investigation.
Measurements of reflected solar radiation and thermally emitted radiation over
a wide range of latitudes and phase angles will permit determination of the energy
MJS IRIS Experiment 149
Ill
100 100(1
Wa\r rmriilirr. i in
1000(1
FIGURE 4. — Radiance vs frequency for Jupiter and Saturn. The
noise-equivalent-radiance (NER) for the MJS-IRIS (Mariner
77) is shown.
balance of Jupiter and Saturn (i.e., the existence of internal heat sources and their
magnitude).
The IRIS will provide information about the quantities (or upper limits) of
methane, ammonia, hydrogen, and water vapor. The high spectral resolution and
low NER of the IRIS will permit a search for a variety of minor constituents,
including the so-called prebiotic molecules. The IRIS will provide information
about the hydrogen-helium ratio as well as about isotopic ratios, H/C, C/N, and
C/O.
The IRIS will permit determination of the three-dimensional temperature
fields (latitude, phase angle, and height) of the planets, Titan, and perhaps other
satellites. The three-dimensional temperature fields will be used in investigations
of atmospheric dynamics.
150 The Rings of Saturn
The IRIS will provide information concerning the presence of water ice, sili-
cates, and chondritic materials in solid bodies and in aerosols. Since these items
are particularly germane to the ring investigations, I would like to show some of
the relevant Mariner Mars 1971 results.
Figure 5 depicts the average of approximately 2000 spectra that were acquired
during the Martian dust storm. One can readily see the wing of the 15 /nm CC>2
absorption band, several other CO? features, HzO lines, and a very broad absorp-
tion feature between 800 and 1400 cm"1. This latter feature is due to the silicates
that were present in the atmosphere. The position of the transmission minimum,
characteristic of the silicate content of the material, permits one to distinguish
between the so-called acidic and basic silicates. A synthetic spectrum for a
Martian atmosphere consisting of only CO2 and H2O is superposed on the same
figure.
During the latter part of the mission, a spectrum was acquired over a cloud-free
region of Arcadia; shortly thereafter, another spectrum was acquired over a cloud-
covered region of the Tharsis ridge. These spectra are shown in figure 6, displaced
for clarity. R. Curran of Goddard Space Flight Center has calculated the absorp-
tion characteristics of a water-ice cloud from the complex refractive index of
water ice and an assumed particle size distribution. One can readily see the good
agreement. The discontinuity in the calculated spectrum is caused by the use of
two sources of refractive index data.
The previous remarks of C. Lillie and B. Smith are also applicable to the IRIS
investigation. The IRIS has a circular field of view (0.25° full cone) that is the same
as the narrow field of view of the photopolarimeter and about one-half the size of
the proposed long focal length imaging system. At closest approach, the 0.25°
field of view will permit a number of observations that are completely within the
extent of the A and the B rings.
2HO
270
260
2.10
Synthetic
'i*if-jnr^r*-~~^^f^
\\veragr
CO-,
100(1 I 100
\\ ;m- Number, cm
1800
FIGURE S. —Mariner 9 spectra showing broad silicate feature between 800-1200 cm~' and atmospheric
COi and HiO features.
MJS IRIS Experiment 151
H.>() «•<• i-louds over I harsis nape
200 400 600 800 1000 1200
Wavrnumbrr, cm
FIGURE 6. — Experimental and theoretical H2O ice cloud
spectra.
The IRIS will be used to measure the equilibrium temperature of ring particles
prior to their shadowing by Saturn and to measure their minimum temperatures
just prior to exiting Saturn's shadow. From these measurements we hope to deter-
mine the thermal inertia of the ring particles and, perhaps, to infer their size and
density. The particle sizes for which we expect this sort of measurement to be
applicable run from several millimeters to the order of a centimeter.
DISCUSSION
James Pollack Could IRIS distinguish between centimeter-size particles and
meter-size particles?
Thomas Burke I don't believe that we would be able to see an appreciable temp-
erature difference for particles substantially larger than a centimeter.
Hugh Kieffer I would maybe push that to 10 cm if the inertia were appropriate,
but beyond 10 cm there is just too much inertia to see an appreciable tempera-
ture change. You said you planned to look at the temperature just before
entrance to and exit from occultation. I presume your plans would include
trying to sweep along the entire shadowed part of the ring?
Burke Such a sweep would be preferable.
Kieffer If there is any hope of separating out the inhomogeneous case —that is,
the thin water frost on top or something else —then you have to look at the knee
of the curve (see contribution by Kieffer) after the particles enter the shadow.
Hopefully that would be part of the plan.
152 The Rings of Saturn
Pollack Have you given any thought to trying to measure the temperature when
looking on the sunlit side, as contrasted with looking at the shadowed side?
Burke Yes, we have. This is an observation we hope to make.
I would like to mention one other ring experiment that we are considering.
We would like to use the radiometer to determine the optical thickness of the
rings when the spacecraft passes behind the rings.
Kieffer Your instrument has very good spectral resolution. Has any thought
been given as to what you might infer based on the departure from a single black-
body curve for the ring particles?
Burke I don't think that has been considered at this point.
MJS IRIS Experiment 153


The Mariner Jupiter/Saturn IRIS experiment

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