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Mars: Components of infrared spectra and the composition of the dust cloud
ICAHCS
18,
459-469
(1973)
Mars: Components and the Composition GRAHAM
R. HUNT,
Terrestrial
LLOYD
M. LOGAN,
Laboratory, Air Force G. Han-scorn Field, Bedford,
Sciences
L.
of Infrared Spectra of the Dust Cloud
Boceived
February
1, 1972;
AND
JOHN
W. SALISBURY
Cambridge Research Laboratories, Massachus$.s 01730
revised
August
1, 1972
Infrared spectra of Mars are made up of three separate components, each of which may dominate the spectrum under different Martian meteorological and observational conditions. By means of laboratory examples we show that both the shape and spectral contrast of the spectral curves change dramatically, depending on which component is dominant. Each experimental condition has been experienced during either the Mariner 69 or 71 observations. Comparing the preliminary Mariner 71 radiance data with laboratory transmission spectra, we suggest that the clay mineral montmorillonite could be the major component of the Martian dust cloud.
INTRODUCTION
Any remotely sensed infrared spectrum of Mars will be affected by the presence of fine silicate particles in the atmosphere. The composite spectrum will have components arising from one or more of the following : (a) surface emission ; (b) attenuation of surface emission by particle haze transmission ; and (c) cloud emission. In previous papers, we have investigated emission spectra of fine particulate silicates in a space or lunar environment, and have shown that the positions of the emission maxima, like those of the minima, are diagnostic of composition and are strongly affected by the thermal environment of the silicate material (Logan and Hunt, 1970a, b ; and Hunt and Logan, 1972). In this paper, results are presented of an investigation of the three component spectra under simulated Martian conditions and a discussion is given of their relative importjance for the atmospheric conditions that prevailed on ,Mars during t,he Mariner 1969 and 1971 experiments. Preliminary Mariner 197 1 infrared spectra are interpreted in terms of cloud particle composition. Copyright a(> 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
EXPERIMENTAL
PROCEDURES
Transmission measurements. Transmission spectra were recorded first using a Perkin-Elmer Model 521 spectrophotometer and the optics of a multipass absorption cell capable of yielding an equivalent path length of 40m. A [email protected] cell with easily removable ends w‘as constructed such that it could be inserted between the multipassing mirrors. The samples, consisting of particles less than 5p.m in diameter were blown into this cell with a blast of dry nitrogen, while the cell was held vertically with the ends removed. The ends were placed on t’he cell to prevent currents while it was being relocated horizontally in the spectrophotometer light beam. The ends were then immediately removed and spectra were continuously recorded as the particles settled out of suspension, which for the finest. (submicron) particles required more t’han TOmin. _4 second met’hod for measuring t,ransmission spectra of samples of known particle size utilized a Perkin-Elmer 180 spectrophotometer and a mirror sample holder. A layer of fine (t5pm) part’icles
460
GRAHAM
R.
HUNT,
LLOYD
M.
LOGAN,
was deposited on the mirror from an airelutriation column, the mirrored surface then providing a double-pass transmission through this layer. Such simulated “clouds” have t,he advantage of easy characterization and handling, and gave results essentially identical with actual clouds of particles suspended in the multipass cell. Emission measurements. The apparatus used to record emission spectra has been discussed in detail elsewhere (Logan and Hunt, 197Ob). Briefly, the apparatus consists of s circular variable-filter spectrometer equipped with a liquid heliumcooled, copper-doped germanium detector. Emission spectra can be recorded from samples in a pressure environment which can be varied between 760 and 10Wtorr. The sample can be heated either by visible radiation from above or by conduction from below. The sample surface is surrounded by a shield which can be cooled to liquid nitrogen temperature to simulate radiative cooling of the surface layer in a space environment. This equipment allows emission spectra to be recorded over a wide range of conditions which can be varied to simulate different space environments. Reduction of emission data presents a particular problem because an emissivity spectrum records the departure of the sample emission from that of a black body at the same temperature. The problem
FIG. rock
1. Effectix-e types under
emissirity simulated
spectra of lunar
different conditions.
AND
JOHN
W.
SALISBURY
lies in correctly choosing the black body temperature. In producing the spectra presented here the procedure adopted was to elect the lolack body whose energy matched that of the sample at the wavelength of the sample’s maximum emission. This produces an emissivity spectrum only if the emissivity of the sample is unity at, this wavelength, and if the sample is isothermal. Because thermal gradients exist in the sample and because the absolut,e emissivity of silicate rock samples is not, known at the match points used, the spectra produced here are effective emissivities rather than absolute emissivities. RESULTS
AND
DISCUSSION
(a) Surface emission. For a fine particulate soil, compositionally diagnostic information is available in the form of bot,h maxima and minima. The minima are associated with the increase in the absorption coefficient of the materials in the molecular vibration bands, while the maxima are associated with the change in the refractive index of the materials on the short wavelength side of the molecular vibration bands. These maxima are found only in finely particulate samples (
MARS
: COMPONENTS
OF
rounded by a radiation shield cooled to liquid nitrogen temperatures. The samples were composed of particles less than 74-pm in diameter and were deposited in a layer sufficiently deep to achieve optical thickness. For particles smaller than 74-pm in diameter, features in the vibrational band region near 10pm are not well defined, particularly for basic rocks. The most useful spectral features are the more welldefined maxima to shorter wavelength. The position and contrast of these emission maxima are sensitive to changes in particle size, packing density, pressure, and background temperature, which in t,urn determine the thermal distribution within the target. However, for the same set of target conditions, the position and contrast of these features are diagnostic of rock type. The extent to which this is so for minerals and rocks is discussed in detail elsewhere (Logan et al., 1972). Of most importance in a Mars application is the effect of pressure on the appearance of the spectra. Increasing the pressure from the near-vacuum of the Moon to the 5-10 mb appropriate for Mars has two effects upon a spectrum, which are illustrated in Fig. 2. It degrades the contrast of the spectrum and shifts the emission maximum to longer wavelength. Pressure has such a large effect upon these spectra because conduction via the gas becomes an impor-
.../
:
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tant energy-transport mechanism in determining the thermal distribution within the sample. This alters the overall conductivity of the sample, which depends chiefly on radiative transfer in vacuum, and drastically changes the thermal distribution within the surface layer, resulting in a changed spectrum. The explanation for the change in spectral contrast and the emission peak shift with changing thermal regime is complex, and is considered in detail elsewhere (Logan et al., 1972). Briefly, lacking gas conduction in a vacuum, the sample surface becomes cooler than the interior by radiating its heat to the cold background. As a result, the energy emitted by the sample is largely determined by its transmissivity. At Martian surface pressure, gas conduction reduces the efficiency of radiative cooling of the surface zone. Consequently, the sample radiates relatively more energy at wavelengths where the transmissivity is low (in the molecular vibration band region). This both reduces the spectral contrast and distorts the emission peak. In general, acidic rocks retain more spectral contrast and suffer less peak shift than do basic rocks. (b) Transmission effects of haze particles. Figure 3 illustrates the appearance of transmission spectra of *-pm particles suspended in the multipass cell. The transmission measurements record the
‘L.C.
- ._,/. 9
,,,*T / ; 59 iii
-.-.-._
MARS SIMULATION
: ,.i
,...*
z w
‘+LUNAR
9 F,, 8-
SIMULATION
I.,.. . . . . . . . . . ..I”
i
..a*’
,..d,’
..*.”
,.
BROWN RHYOLITE o-74 !A I 7
3
FIG. 2. EfTwtl\-r is typical of rocks.
461
SPECTRA
Prnissirity
sperkra
I 8 WAVELENGTH
of rhyolit,e
I 9 IN MICRONS
under
lurmr
I IO
and
Martian
II
conditions.
l’his
behavior
462
FIG.
Vertical increasing
GRAHAM
R.
HUNT,
LLOYD
M.
LOGAN,
3. Transmission spectra of a cloud of less than 5-pm lines indicate onset of H,O and CO, absorption. upward in arbitrary units.
effect, of scattering out of the beam, as well as t.rue absorption by the particles. Two features in these spectra are of particular interest. First, well-defined transmission minima appear (for quartz and olivine) in the molecular vibration region. As Lyon (1963), for instance, pointed out, the positions and shapes of features in this region are compositionally diagnostic. Second, transmission maxima occur (for olivine and ilmenite) to shorter wavelength, which are related in position to the Christiansen frequency. The transmission maximum for quartz and the minimum for ilmenite are obscured by laboratory a’tmospheric absorption. Obviously, any source behind the Martian atmosphere will be attenuated by this type of spectrum, the extent of such at,tenuation will depend upon both the concentration of particles in t’he atmosphere and the path length through it. (c) Emission from, cloud particles. A precise experimental simulation of emission from a cloud of pa’rticles suspended in an at’mosphere was not, at’tempted. Knowledge suflicient for t,he present discussion was obtained bai recording emission spect)ra from layers of particles support,ed on a mirror. ‘I’he mirror. because of it,s 10%~ emissivitg. was used t,o provide an insignificant source of background energy. Such a simulated “cloud” of part,icles on a mirror
AND
JOHN
particles Ordinate
W.
SALISBURY
of quartz, olivine, and ilmenite. is percentage of transmission
proved to be a valid experimental method for measuring the transmission properties of silicates, so it would appear valid in the case of emission measurements as well. Referring first to emission from an optically thin haze, we show in Fig. 4 that an emission peak occurs in the vibrational band region near 1Opm. For an optically thin layer, it is essentially the inverse of the absorption band, becoming less well defined as the physical (and optical) thickness of the layer is increased. Emission from an optical15 thin haze will take this form, which tends to fill in the transmission feature discussed above. When the particles of such a layer are cold and dispersed relative to the surface particles, their emission contribution is not significant. A contribution from this type of emission should btl apparent only when significant emission from the surface can be excluded, such as when the polar regions form the background. Given an optically thick cloud of pal titles, on the ot,her hand, both the charart’rl, and importance of their emission changerlra~tically. As illustrated in Fig. 1. t,he radiative interaction of an opticall!~ t,hick layer of particles produces a spectrum with a minimum in the vibration band region and an emission peak near thc2
MARS:
COMPONENTS
OF INFRARED
463
SPECTRA
ANORTHOCLASE
6
I 7
I 6
I 9
I IO
I II
WAVELENGTH IN MICRONS
FIG. 4. Effect Htmt and Logan,
of sample 1972.)’
thickness
on
the
effective
Christiansen frequency. Obviously, the appearance of the emission spectrum recorded is governed by many parameters which permit it to assume a range of appearances between these two limits, as illustrated in Fig. 4. These experiments do not simulate the thermal distribution in a layer of particles suspended in an atmosphere. However, the effects of altering the thermal distribution in a sample are known (Logan and Hunt, 1970b), and this information can be used when the thermal distribution in the Martian atmosphere uuder different’ meteorological conditions is evaluated. Kadiative cooling will be an important factor in det,ermining both the thermal distribution and the actual temperatures of the particles suspended in an atmosphere. The thermal distribution produced by radiative transfer alone is very close to the conditions which were operative during the production of the spectra for the lunar analog (Fig. 1). However, it is expected that some -modifica,tion to t)his ext)reme case will occur because of heat transfer between the particles and atmospheric molecules. The effect of t*his modification on an emission spectrum for an optically thick layer of particles is that the spectrum should remain verv similar to the type 17
emissirit,y
spectrum
of anorthoclaso.
(From
shown in Fig. 1, but with a reduction in the spectral contrast. We can say at this point with certainty that the emission spectrum from an optically thick cloud of particles suspended in the atmosphere will show higher contrast than the spectrum of a surface layer in contact with the same atmosphere. APPLICATION
TO MARINER~ESERVATIONS
Two target regimes have been operative on Mars during Mariner spectroscopic observations. During Mariner 1969 measurements, the atmosphere generally contained only a thin haze of suspended particles, while initial observations with Mariner 1971 were made through a major dust storm. In t#he first case, where a t,hin haze was present, spectra recorded while the surface was viewed perpendicularly are dominated by the emission from the surface layer (Fig. 2), very slightly modified by transmission through t,he atmospheric particles (Fig. 3). The contribution due to emission from a t,hin haze of particles away from the poles will be negligible relative t,o background surface emission. In this type of spectrum, therefore, there will be a low cont8rast emission maximum in the Chris-
464
GRAHAM
R.
HUNT,
LLOYD
M.
LOGAN,
tiansen frequencv region w&h, perhaps, a small minimum “in the vibrational band region imposed by transmission through the atmospheric haze layer. The amount of contrast in a spectrum which is dominated by surface emission will depend upon the composition of the surface, displaying at most a 5% departure from unit effective emissivity in the region of the molecular vibration band. On the other hand, when the surface is viewed at a large angle to the normal, involving a long slant path through the atmosphere, the transmission effect can become dominant even for a thin haze. In t’his case the major feature will be an intense minimum in the vibrational band region accompanied by a maximum to shorter wavelengths (Fig. 3), their positions being dependent both on the surface composition and (primarily) on the composition of the haze particles. Such spectra were obtained by the Mariner 1969 infrared experiment, but, unfortunately, these spectra have not yet been published. When Mariner 1971 reached Mars, the planet’s surface was overlain not by a thin haze, but by a dust cloud thick enough to obscure most surface features. For a small slant path the cloud was not optically thick in the infrared, having an absorb-
Wave
FIG.
5. (A)
03) Example (From Hanel
Example of Martian nonpolar of polar thermal emission. The et al., 1972.)
AND
JOHN
W.
SALISBURY
ance estimated to be about 0.5 (Chase et al., 1972). Infrared spectra of Mars obtained under these conditions have been published by Hanel et al. (1972), and are shown in Fig. 5. They attribute the diffuse emission features near 9.3p.m (1075cm-I) and 21.3pm (470cm-‘) to dust in the atmosphere and compare them with absorption spectra of fine dusts recorded in KBr pellets by Lyon (1964). Their preliminary conclusion is that there is generally good agreement with minerals and rocks whose SiO, content is in the intermediate range (55-65%) but’ poor agreement with highly acidic (greater than 65%) as well as basic (45-55%) and ultrabasic materials (less t,han 45%). Simply subtracting the published polar spectrum from the background produces radiance excess maxima at 9.3 and 2 1.3 pm. Direct comparison of the positions of these maxima with those of Lyon’s (1964) absorption data for primary igneous rocks and minerals seems to suggest an acidic composition for the cloud, with the best fit for the positions of these two features being with the fundamental silicon-oxygen stretching and oxygen-silicon-oxygen bending modes of the highest SiO, content material of all, quartz. Closer inspection reveals, however, that both quartz and acidic rock spectra demand the presence of
number
thermal smooth
(cm-‘)
emission compared curve is the composite
to three of two
black black
body body
curves. spectra.
MARS:
COMPONENTS
additional features in the Martian spectrum, which are absent. These additional bands occur near 12.6pm (790cm-‘) (which would only be partially obscured by the CO, features) and weaker features near 25pm (400cm-I). Also, it is difficult to support the concept of an acidic Martian crust on geological grounds. Hanel et aZ.‘s (1972) preliminary identification of intermediate rocks or minerals in the clouds, therefore, implies that the positions of the polar radiance excess maxima cannot be directly correlated with absorption maxima in silicate dusts. Rather, comparisons ought to be made between laboratory absorption spectra and emissivity spectra derived by dividing the Martian radiance excess by the Planck function corresponding to the temperature of the particles. Even then the following assumptions would have to be made : (a) That the laboratory absorption spectrum does measure the profile which is appropriate for cloud emission. Strictly this should be compared to cloud true absorption which has scattering as a complementary process. (b) That the particles are optically thin in the absorption band region. This requires that the majority of the particles be submicrometer in size, which may not be the case. Increasing the size of the particles so that they become optically thick has a dramatic effect on the emission spectrum (see Hunt and Logan, 1972). (c) That the cloud be optically thin in bhe absorption band region. The effect of cloud thickness is shown in Fig. 4 Thus, the wavelength of the radiance maximum is sensitive to many parameters and care must be taken in evaluating the compositional significance of this wavelength. A spectral feature less sensitive to particle size and cloud thickness is the vibrat’ional band arising from transmission through the dust cloud observed in nonpolar spectra. Exact, definition of the transmission band in the nonpolar spectra requires deconvolution of the components arising from the surface emission, dust cloud emission and dust cloud transmission, which is a complex problem and is not
OF
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SPECTRA
465
possible for us to attempt in any precise way from these preliminary data. The major problem in ultimately defining an exact transmission spectrum is adequately accounting for dust emission. The surface emission, which acts as source for the transmission component, should pose no particular problem, as it can be assumed to be a gray body in the absorption band region (see Fig. 2). As the dust, cloud dissipates, additional data should be obtainable from which the transmission spectrum of the dust cloud can be more accurately estimated. As an interim measure, advantage may be taken of the fact that the correction of dust cloud emission is significant only near the center of the transmission band. Consequently, the position of the transmission band can be temporarily defined using the band edges, which are relatively undistorted by cloud emission. If the assumption is made that the surface source emission can be approximated by a 260°K black body, a pseudo-cloud transmission spectrum can be generated by dividing the radiance spectrum recorded in the nonpolar regions by the radiance spectrum of a 260°K black body. Such a spectrum is presented as the solid curve in Fig. 6. The 260°K temperature was chosen to ascribe a reasonable peal& transmittance of -95% to the calculated spectrum. Variation of surface temperature and emissivity within reasonable bounds have a negligible effect on the overall spectrum. This spectrum is, however, still distorted by cloud emission, but a first-order correction for this can be made by subtracting the polar cloud emission excess radiance from the nonpolar radiance spectrum before dividing by the black body profile. The dashed curve in Fig. 6 was generated in this way. We do not presume to define the wavelength of the transmission minimum with such a correction. but rather attempt to display which regions of the spectrum are sensitive to contributions from cloud emission. Comparison of these curves shows that while the center of the band is greatly affected by cloud emission, the band edges are relatively insensitive and ran be used in an initial attempt at
466
GRAHAM
6
7
R.
HUtiT,
LLOYD
WAVELENGTH (MICRONS) 6 9 IO I2 12 15 I5
M.
20
30
LOGAN,
50 W
*
/
AND
JOHN
WAVELENGTH (MICRONS) 7 8 9 IO 12 15
6
I I400 PO0 loo0 600 VMENUMBER (CM-‘)
600
400
i
for the 10%.
rock
spectra,
and
ordinate
30
QUARTZ
LAERADORITE
divisions
obscured
Shaded region of the spectrum is by Martian CO, lines in Mariner 1971
spectra.
Mariner
spectra, reare shown at top. Vertical arrows indicate positions of polar radiance maxima, near which we also expect t’he nonpolar transmission minima.
calculated
1971
20
1!OO
FIG. 6. Transmission spectra of different igneous rock types ground to a pdrticle size less than 5pm and suspended on a mirror. Curves have been separated vertically for clarity. Peak transmission near 8pm is typically about 90% are
SALISBURY
intermediate rocks display multiple bands in the ZO-pm region rather than a single band (see Fig. -6). Also it is difficult to rationalize the Martian reflection bands near 0.95p.m and between 1.4 and 2.2t~m (McCord et al., 1971), and near 3.Opm (Sinton, 1967; Houck et ab., 1973) with intermediate composition for the surface dust layer, because these bands are not typical of the spectra of intermediate rocks. At. first glance, basalt appears to be a good candidate among the primary igneous rocks because of its single band at 21.3pm. Indeed, basaltic rocks may also display near-infrared bands near 0.95pm,
!
I600
W.
as described
nonpolar
in the text,
compositional analysis. We have not included Martian data processed in a similar manner in the 20-p” region, because even the outer envelope of the band is difficult to define in these preliminary spectra. However, from the polar spectrum it is fairly clear that there is a
single feature in this region and its transmission minimum should be located near 21.3/.Lm. The preliminary
conclusion of Hanrl ef Ul. from the polar spectra that the cloud particles are int,cqmedia,t~e ill conposition is consistent’ with t,he indeherminacy of the t’ransmission minima near 9.3 and 21.3ym. Typically, however,
1600
1400 1200 1000 800 WAVENUMBER (CM-’ )
600
400
l’ln. 7. Transmission spectra of major igneous rock-forming minerals recorded and displayed as ill*Fig. 6. Empirical chemical formulae arc : quart~z. SiO z : albite. NaAISi,O, ; lnbradorite, 50 701nok ‘Y0 anortlutc: 50. YOmole ‘!4, alblte, C’aAl,Si,O hedenbergite, CaFe anorthitc, [Si,O,] : olivine. (Ye.&) SiOl (this sample -Fq,Mg,,).
MARS:
COMPONENTS
0F
as a result of which Adams and McCord (1969) have suggested a basaltic composition for the Martian surface. However, the spectral fit of the band envelopes in the mid-infra’red is poor near 9.3 pm. In addition, it is difficult to ascribe the poorly defined bands between 1.4 and 2.2 pm and the band at 3 pm to basaltic rocks.
INFRARED
If we do not restrict ourselves to primary igneous minerals and rocks, (see Fig. 7) then we find that the clay mineral montmorillonite, of general empirical formula (&Ca,Na), , (A1,Mg,Fe),(Si,Al),02,,(OH), .nH,O, exhibits spectroscopic properties which closely match the available Martian data (see Fig. 8). Not only does montmorillonite display two- major bands in the appro-
yAVEL:NGTH 6
I
-I
467
SPECTRA
,JM’C;ONS’
I
15
20
30
MONTMORILLONITE
KAOLINITE
HEMATITE
GOETHITE
CHLORITE
GYPSUM CALCITE
1600
1400 1200 I000 800 WAVENUMBER (CM-‘)
600
flc.. 8. .l‘rarlsrnisslori s~w’ttril of II~~~oI~H~~~~oII(~R~‘~. lninerals productd by \vt:atllering and scdimelltary ~~rwf-‘sse~. recordrd and displayed as in Fip. 6. Empirical chrmical formulae we: montmurillonite, (1Ca,Sa),.,(AI,Ma,~e),(8i,*~)~~~~(~H)~. 1tH,0 ; kaolinite, Al,[Si,O,,](OH)s; chlorite, (Mg,& F~:!1,[(Si.rll),O,,](OH),,: poethite. E’eHO, (this sample is cont,aminated with a small amount of quartz. and the corrected spectrum is dashed as uncertain in this region) ; hematite, Fe,O,; gypsum. C’aSO, .2H,O ; calcite, CaC!O,.
468
GRAHAM
R.
HUNT,
LLOYD
M.
I LOGAN,
priate positions in the mid-infrared, while lacking any intense subsidiary, but it also displays bands near 0.95, 1.4, and 1.9pm in the near-infrared (Hunt and Salisbury, 1970) which do not conflict with the bright area spectral features found by McCord et al. (1971) from ground-based observations. Certainly, the Martian 3.0-pm band first observed by Sinton (1967) is consistent with the abundant presence of such a hydrated mineral as montmorillonite. If our suggestion of montmorillonite is supported by refinement of the preliminary spectroscopic data from Mariner 1971, it has two important implications for Mars. First, the presence of abundant clay minerals indicates deep chemical weathering of Martian surface materials, which would require the presence of liquid water on Mars during some former geological epoch. Interestingly enough, recent calculations by Sagan and Mullen (1972) show that Mars could, indeed, have had a wetter climate in the remote past. Second, although montmorillonite on earth is typically derived from basic igneous eruptive rocks, it may also be derived from more acidic rocks. In either case, however, weathering must take place in a semiarid climate (Loughnan, 1969). Consequently, the presence of montmorillonite suggests the occurrence, but an average low abundance, of liquid water on Mars early in its geological history. Finally, it is of interest to note that the present data exclude the possibility of either limonite or carbonates as major components of the Martian dust cloud, as indicated by their spectra in Fig. 8. The carbonate vibrational band near 7t~rn is, in fact, so strong that as little as 10% carbonate particles in the dust cloud should be readily detectable. Other salts, such as sulfates, on the other hand, could be present in large amount with montmorillonite, without substantially distorting the observable portion of its midinfrared spectrum. A thin ferric oxide stain cm the soil particles, which is commonly associated with montmorillonite, could mcount for the reddish color of the Martian regolith, as suggested by Van
AND
JOHN
W.
SALISBURY
Tassel and Salisbury (1964) and Binder and Cruikshank (1964). ACKNOWLEDGMENTS Many helpful J. P. Dybwad. improving this Clark Chapman, Johnson.
discussions were held with Suggestions were offered for paper by A. G. W. Cameron, Paul Gast, and Torrance
REFERENCES
ADAMS,
J. B., SN~ MCCORD. T. B. (1969). Mars : Interpretation of spectral reflectivity of light and dark regions. J. Geophys. Res. 74,
4851-4856. BINDER, A.B.,
AND CRUIKSHANK, D. P. (1964). Comparison of the infrared spectrum of Mars with spectra of selected terrestrial rocks and minerals. Commun. Lunar Planet. Lab. 2, 193-196. CHASE,S.C.,HATZENBELER, H., KIEFFER,H.H., MINER, E., MuNcR,G., ANDNEUGEBAUER,G. (1972). Infrared radiometry experiment on Mariner 9. Science 175, 308-309. CONEL, J. E. (1969). Infrared emissivities in silicates: Experimental results and a cloudy atmosphere model of spectral emission from condensed particulate mediums. J. Geophys. Res. 74, 1614-1634. HANEL, R. A., CONRATH, B. J., Hovrs, W. A., KUNDE, V. G., LOWMAN, P.D., PEARL, J.C., PRABHAKARA, C., SCHLACHMAN, B., AND LEVIN, G. V. (1972). Infrared spectroscopy experiment on the Mariner 9 mission: Preliminary results. Science 175, 305-308. HOUCK,J.,POLLACK,J. B.,SA~AN,C., SCHAA~K, D., AND DEKKER, J. (1973). “High altitude aircraft infrared spectroscopic evidence for bound water on Mars.” Icarus 18, 470-480. HUNT, G. R., AND LOGAN, L. M. (1972). Variation of single particle mid-infrared emission spectra with particle size. AppZ. Opt. 11, 142-147. HUNT, G. R., AND SALISBURY, J. W. (1970). Visible and near-infrared spectra of minerals and rocks: I. Silicate minerals. Mod. Geol. 1, 283-300. LOGAN, L. M., AND HFNT, G. R. (1970a). emission features
spectra by the
169,865-866. LOCAN.L.M..AXD
Infrared : Enhancement of diagnostic lunar environment. Scierbce
HusT,G.
sion spectra of particulate simulated lunar conditions.
R.(1970b).Emissilicates J. Geophys.
under Res.
75,6539-6548. LOGAN,
L. M.,
HUNT,
G. R.,
SALISBURY,
J. W.,
MARS:
AND BALSAMO,
COMPONENTS
S. R. (1972). Compositional of Christiansen frequency peaks. J. Geophys Res.) LOUGHNAN, F. C. (1969). “Chemical Weathering of the Silicate Minerals.” American Elsevier, New York. LYON, R. J. P. (1963) Evaluation of infrared spectrophotometry for compositional analysis of lunar and planetary soils. Stanford Research Institute Project PHU-3943, Final Report, Part I, Contract NASr 49(04), NASA Tech. Note, TND-1871. LYON, R. J. P (1964). Evaluation of infrared spectrophotometry for compositional analysis of lunar and planetary soils: Rough and implications (Submitted
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SPECTRA
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powdered surfaces. Stanford Research Institute Project No. PSU-3943, Final Report, Part II, Contract No. NASr-49(04). MCCORD, T. B., ELIAS, J. H., AND WESTPHAL, J. A. (1971). Mars: The spectral albedo (0.3-2.5~) of small bright and dark regions. Icarus 14, 245251. SAGAN, C., AND MULLEN, 0. (1972). Earth and Mars: Evolution of atmospheres and surface temperatures. Science 177, 52-56. SINTON, W. M. (1967). On the composition of the Martian surface materials. Icarus 6, 222-226. VAN TASSEL, R. A., AND SALISBURY, J. W. (1964). The composition of the Martian surface. Icarus 3, 264-269.
18,
459-469
(1973)
Mars: Components and the Composition GRAHAM
R. HUNT,
Terrestrial
LLOYD
M. LOGAN,
Laboratory, Air Force G. Han-scorn Field, Bedford,
Sciences
L.
of Infrared Spectra of the Dust Cloud
Boceived
February
1, 1972;
AND
JOHN
W. SALISBURY
Cambridge Research Laboratories, Massachus$.s 01730
revised
August
1, 1972
Infrared spectra of Mars are made up of three separate components, each of which may dominate the spectrum under different Martian meteorological and observational conditions. By means of laboratory examples we show that both the shape and spectral contrast of the spectral curves change dramatically, depending on which component is dominant. Each experimental condition has been experienced during either the Mariner 69 or 71 observations. Comparing the preliminary Mariner 71 radiance data with laboratory transmission spectra, we suggest that the clay mineral montmorillonite could be the major component of the Martian dust cloud.
INTRODUCTION
Any remotely sensed infrared spectrum of Mars will be affected by the presence of fine silicate particles in the atmosphere. The composite spectrum will have components arising from one or more of the following : (a) surface emission ; (b) attenuation of surface emission by particle haze transmission ; and (c) cloud emission. In previous papers, we have investigated emission spectra of fine particulate silicates in a space or lunar environment, and have shown that the positions of the emission maxima, like those of the minima, are diagnostic of composition and are strongly affected by the thermal environment of the silicate material (Logan and Hunt, 1970a, b ; and Hunt and Logan, 1972). In this paper, results are presented of an investigation of the three component spectra under simulated Martian conditions and a discussion is given of their relative importjance for the atmospheric conditions that prevailed on ,Mars during t,he Mariner 1969 and 1971 experiments. Preliminary Mariner 197 1 infrared spectra are interpreted in terms of cloud particle composition. Copyright a(> 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
EXPERIMENTAL
PROCEDURES
Transmission measurements. Transmission spectra were recorded first using a Perkin-Elmer Model 521 spectrophotometer and the optics of a multipass absorption cell capable of yielding an equivalent path length of 40m. A [email protected] cell with easily removable ends w‘as constructed such that it could be inserted between the multipassing mirrors. The samples, consisting of particles less than 5p.m in diameter were blown into this cell with a blast of dry nitrogen, while the cell was held vertically with the ends removed. The ends were placed on t’he cell to prevent currents while it was being relocated horizontally in the spectrophotometer light beam. The ends were then immediately removed and spectra were continuously recorded as the particles settled out of suspension, which for the finest. (submicron) particles required more t’han TOmin. _4 second met’hod for measuring t,ransmission spectra of samples of known particle size utilized a Perkin-Elmer 180 spectrophotometer and a mirror sample holder. A layer of fine (t5pm) part’icles
460
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was deposited on the mirror from an airelutriation column, the mirrored surface then providing a double-pass transmission through this layer. Such simulated “clouds” have t,he advantage of easy characterization and handling, and gave results essentially identical with actual clouds of particles suspended in the multipass cell. Emission measurements. The apparatus used to record emission spectra has been discussed in detail elsewhere (Logan and Hunt, 197Ob). Briefly, the apparatus consists of s circular variable-filter spectrometer equipped with a liquid heliumcooled, copper-doped germanium detector. Emission spectra can be recorded from samples in a pressure environment which can be varied between 760 and 10Wtorr. The sample can be heated either by visible radiation from above or by conduction from below. The sample surface is surrounded by a shield which can be cooled to liquid nitrogen temperature to simulate radiative cooling of the surface layer in a space environment. This equipment allows emission spectra to be recorded over a wide range of conditions which can be varied to simulate different space environments. Reduction of emission data presents a particular problem because an emissivity spectrum records the departure of the sample emission from that of a black body at the same temperature. The problem
FIG. rock
1. Effectix-e types under
emissirity simulated
spectra of lunar
different conditions.
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lies in correctly choosing the black body temperature. In producing the spectra presented here the procedure adopted was to elect the lolack body whose energy matched that of the sample at the wavelength of the sample’s maximum emission. This produces an emissivity spectrum only if the emissivity of the sample is unity at, this wavelength, and if the sample is isothermal. Because thermal gradients exist in the sample and because the absolut,e emissivity of silicate rock samples is not, known at the match points used, the spectra produced here are effective emissivities rather than absolute emissivities. RESULTS
AND
DISCUSSION
(a) Surface emission. For a fine particulate soil, compositionally diagnostic information is available in the form of bot,h maxima and minima. The minima are associated with the increase in the absorption coefficient of the materials in the molecular vibration bands, while the maxima are associated with the change in the refractive index of the materials on the short wavelength side of the molecular vibration bands. These maxima are found only in finely particulate samples (
MARS
: COMPONENTS
OF
rounded by a radiation shield cooled to liquid nitrogen temperatures. The samples were composed of particles less than 74-pm in diameter and were deposited in a layer sufficiently deep to achieve optical thickness. For particles smaller than 74-pm in diameter, features in the vibrational band region near 10pm are not well defined, particularly for basic rocks. The most useful spectral features are the more welldefined maxima to shorter wavelength. The position and contrast of these emission maxima are sensitive to changes in particle size, packing density, pressure, and background temperature, which in t,urn determine the thermal distribution within the target. However, for the same set of target conditions, the position and contrast of these features are diagnostic of rock type. The extent to which this is so for minerals and rocks is discussed in detail elsewhere (Logan et al., 1972). Of most importance in a Mars application is the effect of pressure on the appearance of the spectra. Increasing the pressure from the near-vacuum of the Moon to the 5-10 mb appropriate for Mars has two effects upon a spectrum, which are illustrated in Fig. 2. It degrades the contrast of the spectrum and shifts the emission maximum to longer wavelength. Pressure has such a large effect upon these spectra because conduction via the gas becomes an impor-
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:
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tant energy-transport mechanism in determining the thermal distribution within the sample. This alters the overall conductivity of the sample, which depends chiefly on radiative transfer in vacuum, and drastically changes the thermal distribution within the surface layer, resulting in a changed spectrum. The explanation for the change in spectral contrast and the emission peak shift with changing thermal regime is complex, and is considered in detail elsewhere (Logan et al., 1972). Briefly, lacking gas conduction in a vacuum, the sample surface becomes cooler than the interior by radiating its heat to the cold background. As a result, the energy emitted by the sample is largely determined by its transmissivity. At Martian surface pressure, gas conduction reduces the efficiency of radiative cooling of the surface zone. Consequently, the sample radiates relatively more energy at wavelengths where the transmissivity is low (in the molecular vibration band region). This both reduces the spectral contrast and distorts the emission peak. In general, acidic rocks retain more spectral contrast and suffer less peak shift than do basic rocks. (b) Transmission effects of haze particles. Figure 3 illustrates the appearance of transmission spectra of *-pm particles suspended in the multipass cell. The transmission measurements record the
‘L.C.
- ._,/. 9
,,,*T / ; 59 iii
-.-.-._
MARS SIMULATION
: ,.i
,...*
z w
‘+LUNAR
9 F,, 8-
SIMULATION
I.,.. . . . . . . . . . ..I”
i
..a*’
,..d,’
..*.”
,.
BROWN RHYOLITE o-74 !A I 7
3
FIG. 2. EfTwtl\-r is typical of rocks.
461
SPECTRA
Prnissirity
sperkra
I 8 WAVELENGTH
of rhyolit,e
I 9 IN MICRONS
under
lurmr
I IO
and
Martian
II
conditions.
l’his
behavior
462
FIG.
Vertical increasing
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3. Transmission spectra of a cloud of less than 5-pm lines indicate onset of H,O and CO, absorption. upward in arbitrary units.
effect, of scattering out of the beam, as well as t.rue absorption by the particles. Two features in these spectra are of particular interest. First, well-defined transmission minima appear (for quartz and olivine) in the molecular vibration region. As Lyon (1963), for instance, pointed out, the positions and shapes of features in this region are compositionally diagnostic. Second, transmission maxima occur (for olivine and ilmenite) to shorter wavelength, which are related in position to the Christiansen frequency. The transmission maximum for quartz and the minimum for ilmenite are obscured by laboratory a’tmospheric absorption. Obviously, any source behind the Martian atmosphere will be attenuated by this type of spectrum, the extent of such at,tenuation will depend upon both the concentration of particles in t’he atmosphere and the path length through it. (c) Emission from, cloud particles. A precise experimental simulation of emission from a cloud of pa’rticles suspended in an at’mosphere was not, at’tempted. Knowledge suflicient for t,he present discussion was obtained bai recording emission spect)ra from layers of particles support,ed on a mirror. ‘I’he mirror. because of it,s 10%~ emissivitg. was used t,o provide an insignificant source of background energy. Such a simulated “cloud” of part,icles on a mirror
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particles Ordinate
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of quartz, olivine, and ilmenite. is percentage of transmission
proved to be a valid experimental method for measuring the transmission properties of silicates, so it would appear valid in the case of emission measurements as well. Referring first to emission from an optically thin haze, we show in Fig. 4 that an emission peak occurs in the vibrational band region near 1Opm. For an optically thin layer, it is essentially the inverse of the absorption band, becoming less well defined as the physical (and optical) thickness of the layer is increased. Emission from an optical15 thin haze will take this form, which tends to fill in the transmission feature discussed above. When the particles of such a layer are cold and dispersed relative to the surface particles, their emission contribution is not significant. A contribution from this type of emission should btl apparent only when significant emission from the surface can be excluded, such as when the polar regions form the background. Given an optically thick cloud of pal titles, on the ot,her hand, both the charart’rl, and importance of their emission changerlra~tically. As illustrated in Fig. 1. t,he radiative interaction of an opticall!~ t,hick layer of particles produces a spectrum with a minimum in the vibration band region and an emission peak near thc2
MARS:
COMPONENTS
OF INFRARED
463
SPECTRA
ANORTHOCLASE
6
I 7
I 6
I 9
I IO
I II
WAVELENGTH IN MICRONS
FIG. 4. Effect Htmt and Logan,
of sample 1972.)’
thickness
on
the
effective
Christiansen frequency. Obviously, the appearance of the emission spectrum recorded is governed by many parameters which permit it to assume a range of appearances between these two limits, as illustrated in Fig. 4. These experiments do not simulate the thermal distribution in a layer of particles suspended in an atmosphere. However, the effects of altering the thermal distribution in a sample are known (Logan and Hunt, 1970b), and this information can be used when the thermal distribution in the Martian atmosphere uuder different’ meteorological conditions is evaluated. Kadiative cooling will be an important factor in det,ermining both the thermal distribution and the actual temperatures of the particles suspended in an atmosphere. The thermal distribution produced by radiative transfer alone is very close to the conditions which were operative during the production of the spectra for the lunar analog (Fig. 1). However, it is expected that some -modifica,tion to t)his ext)reme case will occur because of heat transfer between the particles and atmospheric molecules. The effect of t*his modification on an emission spectrum for an optically thick layer of particles is that the spectrum should remain verv similar to the type 17
emissirit,y
spectrum
of anorthoclaso.
(From
shown in Fig. 1, but with a reduction in the spectral contrast. We can say at this point with certainty that the emission spectrum from an optically thick cloud of particles suspended in the atmosphere will show higher contrast than the spectrum of a surface layer in contact with the same atmosphere. APPLICATION
TO MARINER~ESERVATIONS
Two target regimes have been operative on Mars during Mariner spectroscopic observations. During Mariner 1969 measurements, the atmosphere generally contained only a thin haze of suspended particles, while initial observations with Mariner 1971 were made through a major dust storm. In t#he first case, where a t,hin haze was present, spectra recorded while the surface was viewed perpendicularly are dominated by the emission from the surface layer (Fig. 2), very slightly modified by transmission through t,he atmospheric particles (Fig. 3). The contribution due to emission from a t,hin haze of particles away from the poles will be negligible relative t,o background surface emission. In this type of spectrum, therefore, there will be a low cont8rast emission maximum in the Chris-
464
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tiansen frequencv region w&h, perhaps, a small minimum “in the vibrational band region imposed by transmission through the atmospheric haze layer. The amount of contrast in a spectrum which is dominated by surface emission will depend upon the composition of the surface, displaying at most a 5% departure from unit effective emissivity in the region of the molecular vibration band. On the other hand, when the surface is viewed at a large angle to the normal, involving a long slant path through the atmosphere, the transmission effect can become dominant even for a thin haze. In t’his case the major feature will be an intense minimum in the vibrational band region accompanied by a maximum to shorter wavelengths (Fig. 3), their positions being dependent both on the surface composition and (primarily) on the composition of the haze particles. Such spectra were obtained by the Mariner 1969 infrared experiment, but, unfortunately, these spectra have not yet been published. When Mariner 1971 reached Mars, the planet’s surface was overlain not by a thin haze, but by a dust cloud thick enough to obscure most surface features. For a small slant path the cloud was not optically thick in the infrared, having an absorb-
Wave
FIG.
5. (A)
03) Example (From Hanel
Example of Martian nonpolar of polar thermal emission. The et al., 1972.)
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ance estimated to be about 0.5 (Chase et al., 1972). Infrared spectra of Mars obtained under these conditions have been published by Hanel et al. (1972), and are shown in Fig. 5. They attribute the diffuse emission features near 9.3p.m (1075cm-I) and 21.3pm (470cm-‘) to dust in the atmosphere and compare them with absorption spectra of fine dusts recorded in KBr pellets by Lyon (1964). Their preliminary conclusion is that there is generally good agreement with minerals and rocks whose SiO, content is in the intermediate range (55-65%) but’ poor agreement with highly acidic (greater than 65%) as well as basic (45-55%) and ultrabasic materials (less t,han 45%). Simply subtracting the published polar spectrum from the background produces radiance excess maxima at 9.3 and 2 1.3 pm. Direct comparison of the positions of these maxima with those of Lyon’s (1964) absorption data for primary igneous rocks and minerals seems to suggest an acidic composition for the cloud, with the best fit for the positions of these two features being with the fundamental silicon-oxygen stretching and oxygen-silicon-oxygen bending modes of the highest SiO, content material of all, quartz. Closer inspection reveals, however, that both quartz and acidic rock spectra demand the presence of
number
thermal smooth
(cm-‘)
emission compared curve is the composite
to three of two
black black
body body
curves. spectra.
MARS:
COMPONENTS
additional features in the Martian spectrum, which are absent. These additional bands occur near 12.6pm (790cm-‘) (which would only be partially obscured by the CO, features) and weaker features near 25pm (400cm-I). Also, it is difficult to support the concept of an acidic Martian crust on geological grounds. Hanel et aZ.‘s (1972) preliminary identification of intermediate rocks or minerals in the clouds, therefore, implies that the positions of the polar radiance excess maxima cannot be directly correlated with absorption maxima in silicate dusts. Rather, comparisons ought to be made between laboratory absorption spectra and emissivity spectra derived by dividing the Martian radiance excess by the Planck function corresponding to the temperature of the particles. Even then the following assumptions would have to be made : (a) That the laboratory absorption spectrum does measure the profile which is appropriate for cloud emission. Strictly this should be compared to cloud true absorption which has scattering as a complementary process. (b) That the particles are optically thin in the absorption band region. This requires that the majority of the particles be submicrometer in size, which may not be the case. Increasing the size of the particles so that they become optically thick has a dramatic effect on the emission spectrum (see Hunt and Logan, 1972). (c) That the cloud be optically thin in bhe absorption band region. The effect of cloud thickness is shown in Fig. 4 Thus, the wavelength of the radiance maximum is sensitive to many parameters and care must be taken in evaluating the compositional significance of this wavelength. A spectral feature less sensitive to particle size and cloud thickness is the vibrat’ional band arising from transmission through the dust cloud observed in nonpolar spectra. Exact, definition of the transmission band in the nonpolar spectra requires deconvolution of the components arising from the surface emission, dust cloud emission and dust cloud transmission, which is a complex problem and is not
OF
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possible for us to attempt in any precise way from these preliminary data. The major problem in ultimately defining an exact transmission spectrum is adequately accounting for dust emission. The surface emission, which acts as source for the transmission component, should pose no particular problem, as it can be assumed to be a gray body in the absorption band region (see Fig. 2). As the dust, cloud dissipates, additional data should be obtainable from which the transmission spectrum of the dust cloud can be more accurately estimated. As an interim measure, advantage may be taken of the fact that the correction of dust cloud emission is significant only near the center of the transmission band. Consequently, the position of the transmission band can be temporarily defined using the band edges, which are relatively undistorted by cloud emission. If the assumption is made that the surface source emission can be approximated by a 260°K black body, a pseudo-cloud transmission spectrum can be generated by dividing the radiance spectrum recorded in the nonpolar regions by the radiance spectrum of a 260°K black body. Such a spectrum is presented as the solid curve in Fig. 6. The 260°K temperature was chosen to ascribe a reasonable peal& transmittance of -95% to the calculated spectrum. Variation of surface temperature and emissivity within reasonable bounds have a negligible effect on the overall spectrum. This spectrum is, however, still distorted by cloud emission, but a first-order correction for this can be made by subtracting the polar cloud emission excess radiance from the nonpolar radiance spectrum before dividing by the black body profile. The dashed curve in Fig. 6 was generated in this way. We do not presume to define the wavelength of the transmission minimum with such a correction. but rather attempt to display which regions of the spectrum are sensitive to contributions from cloud emission. Comparison of these curves shows that while the center of the band is greatly affected by cloud emission, the band edges are relatively insensitive and ran be used in an initial attempt at
466
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WAVELENGTH (MICRONS) 6 9 IO I2 12 15 I5
M.
20
30
LOGAN,
50 W
*
/
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WAVELENGTH (MICRONS) 7 8 9 IO 12 15
6
I I400 PO0 loo0 600 VMENUMBER (CM-‘)
600
400
i
for the 10%.
rock
spectra,
and
ordinate
30
QUARTZ
LAERADORITE
divisions
obscured
Shaded region of the spectrum is by Martian CO, lines in Mariner 1971
spectra.
Mariner
spectra, reare shown at top. Vertical arrows indicate positions of polar radiance maxima, near which we also expect t’he nonpolar transmission minima.
calculated
1971
20
1!OO
FIG. 6. Transmission spectra of different igneous rock types ground to a pdrticle size less than 5pm and suspended on a mirror. Curves have been separated vertically for clarity. Peak transmission near 8pm is typically about 90% are
SALISBURY
intermediate rocks display multiple bands in the ZO-pm region rather than a single band (see Fig. -6). Also it is difficult to rationalize the Martian reflection bands near 0.95p.m and between 1.4 and 2.2t~m (McCord et al., 1971), and near 3.Opm (Sinton, 1967; Houck et ab., 1973) with intermediate composition for the surface dust layer, because these bands are not typical of the spectra of intermediate rocks. At. first glance, basalt appears to be a good candidate among the primary igneous rocks because of its single band at 21.3pm. Indeed, basaltic rocks may also display near-infrared bands near 0.95pm,
!
I600
W.
as described
nonpolar
in the text,
compositional analysis. We have not included Martian data processed in a similar manner in the 20-p” region, because even the outer envelope of the band is difficult to define in these preliminary spectra. However, from the polar spectrum it is fairly clear that there is a
single feature in this region and its transmission minimum should be located near 21.3/.Lm. The preliminary
conclusion of Hanrl ef Ul. from the polar spectra that the cloud particles are int,cqmedia,t~e ill conposition is consistent’ with t,he indeherminacy of the t’ransmission minima near 9.3 and 21.3ym. Typically, however,
1600
1400 1200 1000 800 WAVENUMBER (CM-’ )
600
400
l’ln. 7. Transmission spectra of major igneous rock-forming minerals recorded and displayed as ill*Fig. 6. Empirical chemical formulae arc : quart~z. SiO z : albite. NaAISi,O, ; lnbradorite, 50 701nok ‘Y0 anortlutc: 50. YOmole ‘!4, alblte, C’aAl,Si,O hedenbergite, CaFe anorthitc, [Si,O,] : olivine. (Ye.&) SiOl (this sample -Fq,Mg,,).
MARS:
COMPONENTS
0F
as a result of which Adams and McCord (1969) have suggested a basaltic composition for the Martian surface. However, the spectral fit of the band envelopes in the mid-infra’red is poor near 9.3 pm. In addition, it is difficult to ascribe the poorly defined bands between 1.4 and 2.2 pm and the band at 3 pm to basaltic rocks.
INFRARED
If we do not restrict ourselves to primary igneous minerals and rocks, (see Fig. 7) then we find that the clay mineral montmorillonite, of general empirical formula (&Ca,Na), , (A1,Mg,Fe),(Si,Al),02,,(OH), .nH,O, exhibits spectroscopic properties which closely match the available Martian data (see Fig. 8). Not only does montmorillonite display two- major bands in the appro-
yAVEL:NGTH 6
I
-I
467
SPECTRA
,JM’C;ONS’
I
15
20
30
MONTMORILLONITE
KAOLINITE
HEMATITE
GOETHITE
CHLORITE
GYPSUM CALCITE
1600
1400 1200 I000 800 WAVENUMBER (CM-‘)
600
flc.. 8. .l‘rarlsrnisslori s~w’ttril of II~~~oI~H~~~~oII(~R~‘~. lninerals productd by \vt:atllering and scdimelltary ~~rwf-‘sse~. recordrd and displayed as in Fip. 6. Empirical chrmical formulae we: montmurillonite, (1Ca,Sa),.,(AI,Ma,~e),(8i,*~)~~~~(~H)~. 1tH,0 ; kaolinite, Al,[Si,O,,](OH)s; chlorite, (Mg,& F~:!1,[(Si.rll),O,,](OH),,: poethite. E’eHO, (this sample is cont,aminated with a small amount of quartz. and the corrected spectrum is dashed as uncertain in this region) ; hematite, Fe,O,; gypsum. C’aSO, .2H,O ; calcite, CaC!O,.
468
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priate positions in the mid-infrared, while lacking any intense subsidiary, but it also displays bands near 0.95, 1.4, and 1.9pm in the near-infrared (Hunt and Salisbury, 1970) which do not conflict with the bright area spectral features found by McCord et al. (1971) from ground-based observations. Certainly, the Martian 3.0-pm band first observed by Sinton (1967) is consistent with the abundant presence of such a hydrated mineral as montmorillonite. If our suggestion of montmorillonite is supported by refinement of the preliminary spectroscopic data from Mariner 1971, it has two important implications for Mars. First, the presence of abundant clay minerals indicates deep chemical weathering of Martian surface materials, which would require the presence of liquid water on Mars during some former geological epoch. Interestingly enough, recent calculations by Sagan and Mullen (1972) show that Mars could, indeed, have had a wetter climate in the remote past. Second, although montmorillonite on earth is typically derived from basic igneous eruptive rocks, it may also be derived from more acidic rocks. In either case, however, weathering must take place in a semiarid climate (Loughnan, 1969). Consequently, the presence of montmorillonite suggests the occurrence, but an average low abundance, of liquid water on Mars early in its geological history. Finally, it is of interest to note that the present data exclude the possibility of either limonite or carbonates as major components of the Martian dust cloud, as indicated by their spectra in Fig. 8. The carbonate vibrational band near 7t~rn is, in fact, so strong that as little as 10% carbonate particles in the dust cloud should be readily detectable. Other salts, such as sulfates, on the other hand, could be present in large amount with montmorillonite, without substantially distorting the observable portion of its midinfrared spectrum. A thin ferric oxide stain cm the soil particles, which is commonly associated with montmorillonite, could mcount for the reddish color of the Martian regolith, as suggested by Van
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SALISBURY
Tassel and Salisbury (1964) and Binder and Cruikshank (1964). ACKNOWLEDGMENTS Many helpful J. P. Dybwad. improving this Clark Chapman, Johnson.
discussions were held with Suggestions were offered for paper by A. G. W. Cameron, Paul Gast, and Torrance
REFERENCES
ADAMS,
J. B., SN~ MCCORD. T. B. (1969). Mars : Interpretation of spectral reflectivity of light and dark regions. J. Geophys. Res. 74,
4851-4856. BINDER, A.B.,
AND CRUIKSHANK, D. P. (1964). Comparison of the infrared spectrum of Mars with spectra of selected terrestrial rocks and minerals. Commun. Lunar Planet. Lab. 2, 193-196. CHASE,S.C.,HATZENBELER, H., KIEFFER,H.H., MINER, E., MuNcR,G., ANDNEUGEBAUER,G. (1972). Infrared radiometry experiment on Mariner 9. Science 175, 308-309. CONEL, J. E. (1969). Infrared emissivities in silicates: Experimental results and a cloudy atmosphere model of spectral emission from condensed particulate mediums. J. Geophys. Res. 74, 1614-1634. HANEL, R. A., CONRATH, B. J., Hovrs, W. A., KUNDE, V. G., LOWMAN, P.D., PEARL, J.C., PRABHAKARA, C., SCHLACHMAN, B., AND LEVIN, G. V. (1972). Infrared spectroscopy experiment on the Mariner 9 mission: Preliminary results. Science 175, 305-308. HOUCK,J.,POLLACK,J. B.,SA~AN,C., SCHAA~K, D., AND DEKKER, J. (1973). “High altitude aircraft infrared spectroscopic evidence for bound water on Mars.” Icarus 18, 470-480. HUNT, G. R., AND LOGAN, L. M. (1972). Variation of single particle mid-infrared emission spectra with particle size. AppZ. Opt. 11, 142-147. HUNT, G. R., AND SALISBURY, J. W. (1970). Visible and near-infrared spectra of minerals and rocks: I. Silicate minerals. Mod. Geol. 1, 283-300. LOGAN, L. M., AND HFNT, G. R. (1970a). emission features
spectra by the
169,865-866. LOCAN.L.M..AXD
Infrared : Enhancement of diagnostic lunar environment. Scierbce
HusT,G.
sion spectra of particulate simulated lunar conditions.
R.(1970b).Emissilicates J. Geophys.
under Res.
75,6539-6548. LOGAN,
L. M.,
HUNT,
G. R.,
SALISBURY,
J. W.,
MARS:
AND BALSAMO,
COMPONENTS
S. R. (1972). Compositional of Christiansen frequency peaks. J. Geophys Res.) LOUGHNAN, F. C. (1969). “Chemical Weathering of the Silicate Minerals.” American Elsevier, New York. LYON, R. J. P. (1963) Evaluation of infrared spectrophotometry for compositional analysis of lunar and planetary soils. Stanford Research Institute Project PHU-3943, Final Report, Part I, Contract NASr 49(04), NASA Tech. Note, TND-1871. LYON, R. J. P (1964). Evaluation of infrared spectrophotometry for compositional analysis of lunar and planetary soils: Rough and implications (Submitted
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powdered surfaces. Stanford Research Institute Project No. PSU-3943, Final Report, Part II, Contract No. NASr-49(04). MCCORD, T. B., ELIAS, J. H., AND WESTPHAL, J. A. (1971). Mars: The spectral albedo (0.3-2.5~) of small bright and dark regions. Icarus 14, 245251. SAGAN, C., AND MULLEN, 0. (1972). Earth and Mars: Evolution of atmospheres and surface temperatures. Science 177, 52-56. SINTON, W. M. (1967). On the composition of the Martian surface materials. Icarus 6, 222-226. VAN TASSEL, R. A., AND SALISBURY, J. W. (1964). The composition of the Martian surface. Icarus 3, 264-269.