Ultraviolet observations of Mars made by the orbiting astronomical observatory

ICARUS

l&489-496

(1973)

Ultraviolet

Observations Astronomical

of Mars Made Observatory

JOHN Planetary

Space

Astronomy

Research Center, Laboratory,

Received

by the

CALDWELL’

Lowell Observatory, Flagstaff, Arizona Department of Astronqmy, University Madison, Wisconsin 53 7 06

February

Orbiting

25,

1972;

revised

July

86001, and of Wisconsin,

31, 1972

Ultraviolet albedos of Mars in the region hh2000-3600&k are discussed. When the reflect.ivity due to the known amount of CO, on Mars is accounted for, the remaining reflectivity may be used to set an upper limit for the surface albedo. The result disagrees qualitatively with published ultraviolet reflectivities of limonite and carbon suboxide. An alternate interpretation of the observations leads to the conclusion that CO, comprises at least 60% of the molecular atmosphere of Mars, assuming the remainder to be argon. A comparison of the OAO results with 1969 Mariner ultraviolet data reveals some important areas of conflict. Attempts to detect Mars at wavelengths less than X2000L& were unsuccessful, with only very high upper limits being set.

1. INTRODUCTION

Ultraviolet albedos in the region Xx20003600A from OAO-2 have been given by Wallace et al. (1972) (hereinafter referred to as “Paper 1”) for Venus, Mars, Jupiter, and Saturn. Using these data, Owen and Sagan (1972) have sought trace absorbing constituents in the atmospheres of the planets. They were able to place upper limits on the abundance of various potential minor constituents of Mars’ atmosphere. The present paper discusses further the implications of the Mars observations. Mars was successfully observed twice by the satellite with a spectrum scanner operating in the range M1800-3600 8. The albedo curves derived from these observations are given in Figs. 1 and 2, which are reproduced from Figs. 3 and 4 of Paper 1, with some modifications. Observational parameters are summarized in Table I. The error bars in the geometric albedo curves show only the uncertainty in estimating the instrumental background. Other possible sources of error in deriving the curves are discussed in Paper 1. 1 Presently atory, Princeton,

at

Princeton New Jersey

University 08540.

Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in sny form reserved.

Observ489

After a preliminary evaluation of the expected Rayleigh scattering from CO, in the Martian atmosphere in section 2, section 3 of the present paper gives a discussion of the continuous albedo curves. In section 4, the high spatial resolution ultraviolet photometry of Mariners 6 and 7 (Hord, 1972) is comp&ed with the OAO observations. Section 5 is a brief summary of unsuccessful attempts to detect Mars at wavelengths below h2000A using other instruments of the Wisconsin Experiment Package aboa,rd the satellite. 2.

RAYLEIGH IN

THE

SCATTERING MARTIAN

FROM

CO,

ATMOSPHERE

From the general run of the albedo curves, it is apparent that much of the reflectivity is due to Rayleigh scattering from the t’hin Martian atmosphere. It will be shown that there is significant, additional reflectivity throughout the wavelength interval of interest, hh2000-3600 A. However, it’ is not possible to separate the t’wo components without ot’her information. Therefore, in order to proceed with the interpretation of the albedo curves, the ultraviolet reflectivity of the known atmos-

490

CALDWELL

Phase

2,000

Angle:

34.5’

3,000 WAVELENGTH

!ii

1. The ultraviolet geometric albedo of Mars at phase angle 34?5, adapted The ground-baaed photometry is from Irvine et al. (1971). The residual albedo between the observed geometric albedo and that calculated for 80matm of COz. represent only the uncertainty in estimating the instrumental background. FIG.

phere of Mars will first be taken into account. There have been two recent determinations of the average CO, abundance on Mars (Belton et al., 1968; Carleton et al., 1969). Both studies placed heavy emphasis on increasing the accuracy over previous observations. Both employed moderate spatial resolution, whereas OAO-2 was capable only of integrated-disk spectrophotometry. The two studies are in good agreement, with a mean CO, abundance determination of 80matm. Neither observation was able to detect abundance variations over the planet’s surface. Subsequent detection of such variation at extremely high spatial resolution is not important here. Since bhe ground-based spectroscopic studies both had spatial resolution superior to that of the satellite, it was decided that 80matm was a reason-

from Paper 1. is the difference The error bars

able CO, abundance on which to base consideration of the present data. Barker (1969) has discussed the seasonal variation in Martian CO, abundance. He finds that the abundance is a strong function of Martian orbital longitude, L,. However, the satellite observations were made only about 8 and 21” later in orbital longitude (although one apparition later) than the ground-based studies of Belton et al. and Carleton et al. From Barker’s Fig. 3.3, it does not appear t’hat such a small difference in longitude is significant, so that Martian seasonal effects can be ignored. To calculate the reflectivity of the CO,, the computing program of Dave and Warten (1968) for the brightness of a Rayleighscattering, plane-parallel, nonabsorbing, finite atmosphere was employed. The spherical surface was divided into planes

491

UVOBSERVATIONSOFMARSBYTHEOAO

Phase

Angle = 26.0’

._ -_ 1.. -‘I . . ...

FIG. Satellite

2. The ultraviolet geometric broadband photometry was

albedo of Mars at phase also available during this TABLE

OBSERVATIONAL

Date (1969) March April

29 22123

Universal time 05” 22. Olh

FOR MARS

Phase angle

4 scans Hroadbnnd photometry and 1 scan

extending _‘)” in latitude and longitude, and the contributions from t)hese sect,ions were intepratcd over the surface following the geometry of Horak ( 19.50). The gcomet,ric albedo 1;~s calculated for a series of phase angles, optical depths, and surface reflect,ivities. Optical depths were calculated from the Kayleigh scwt,tering cross sections of Allen (1963).

adapted

from

Paper

1.

I

PARAMETERS

Obswvat~ions

angle 26V8, observation.

34i5 26”8

DATA

Orbital longitude (Ls) 133” 145”

Longitude of central meridian &CM) ~~ __~~ 130” 156-200”

3.

DISCUSSION OF THE CONTINUOUS XLBEDO OF MARS, hh%OOO-3600!1

Besides the scattering by the known amount of PO,. the uhraviolet reflectivit>y could have contribut,ions from atmospheric aerosols and other gases, as well as the surface. Figures 1 and 2 include t’he calculat,ed “residual albedo,” which is

492

FIG. 3. Photographic The photograph at the taken March 29, 1969, at right is from the files of It is in blue light and was Cerro To1010 Inter-American photos.

composites of Mars near the times of OAO-2 scans. South is at the top. left is from the New Mexico State University Observatory. It was 1137UT (LCM = 227”) and is in ultraviolet light. The photograph at t,he the International Planetary Patrol Program of the Lowell Observatory. taken at 0704UT on April 25, 1969 (LCM = 269”). It was obtained at the Observatory in Chile. The south polar hood is prominent in both

defined as the excess of the observed albedo over the Rayleigh scattering of 80matm of CO,. It is evident that the reflectivity from non-CO, sources is decreasing rapidly toward shorter wavelengths. An upper limit to the surface reflectivity may be calculated by assuming that the entire residual albedo is the result of surface scattering. To calculate this upper limit, two approximations were made: (1) that the surface brightness is uniform over the disk, and (2) that the surface reflects according to Lambert’s law, that is, all radiation is reflected isotropically, independently of the angle of incidence. The first approximation was made necessary by the lack of spatial resolution in these observations. Figure 3 illustrates TABLE

SPECTRALVARIATION

that the disk of Mars is not uniformly bright, at least at about h4000A. The second approximation was imposed by the lack of resolution as well as the limited range of phase angle of observation. Table II summarizes calculations of Lambert reflectivity (ratio of total reflected light to total incident light) at two phase angles, using the data of Figs. 1 and 2. The tabulated Lambert reflectivity is that required to give the observed total geometric albedo (CO, plus surface). Since the computed Lambert reflectivity agrees well, at least over this range of phase angle, it is concluded that this approximation is compatible with the limitations of the data. This approach has some support from the 1969 Mariners 6 and 7 ultraviolet data (Hord, 1972). The Mariner observations II

OFMARS' hr,smo

Wavelength LX)

Optical depth (CO,)

Geometric albedo (3415)

Surface albedo (34’5)

Lambert reflectivity (34”5)

Geometric albedo (26”8)

3820 3215 2800 2550 2300 2100

0.010 0.020 0.036 0.054 0.085 0.120

0.036 0.085 0.038 0.041 0.049 0.066

0.030 0.025 0.020 0.015 0.009 0.012

0.055 0.042 0.037 0.029 0.019 0.025

0.041 0.040 0.041 0.046 0.054 0.073

Surface it1 bedo (268) 0.035 0.028 0.021 0.017 0.009 0.012

Lambert reflectivity (26%) -.___.~~0.058 0.048 0.037 0.030 0.018 0.024

UV

OBSERVATIONS

have spatial resolution of the order of 20 x ZOOkm, at phase angles of 46, 62, and 91”. Hord finds that all of the 400 nonpolar region spectra have the same relative shape. This increases confidence that the integrated-disk spectrophotometry from OAO-2 can be used to derive meaningful, planet-wide average photometric properties. Lambert scattering is also compatible with Mariner data. (In Hord’s notation, k _N 1 in his Eq. 5.) The reflectivity of the surface as calculated here as a function of wavelength disagrees qualitatively with two published models of the Martian surface. Sagan et al. (1965) suggested that limonite, a mixture of various forms of hydrated ferric oxide, reproduced many of the properties of the Martian surface. They based their arguments on properties at longer wavelengths. Their measured ultraviolet reflectivity is constant, or even increasing slightly from h4000 to h20008, contrary to the present result. Sagan (private communication) has stated that limonite exhibits enough variation in ultraviolet reflectivity from one sample to the next that the present results do not rule out limonite as a major constituent on the Martian surface. However, Younkin (1966), Sinton (1967), and O’Leary and Rea (1968) have also argued against the limonite model on other grounds. Plummer and Carson (1969) claimed that “the reflection spectrum of Mars can be well matched from 2000 to 16 OOOH by polymers of carbon suboxide, C,O,.” In a graph they show the reflectivity of a C,O, polymer which best represents Martian properties at longer wavelengths. The reflectivity rises from 0.05 at X38OOA to 0.075 at h3000.& again contrary to the variation exhibited here. Even though various polymers do not have identical ultraviolet, reflectivities, Plummer and Carson are confident enough of the increasing reflectivity of C,O, toward shorter wavelengt,hs to use it, as a criterion for preferring C,02 to limonite as a surface model. If the rise in reflectivity below h3000W is indeed characteristic of carbon suboxide, a’sPlummer and Carson indicate,

OF MARS

BY

THE

OAO

493

then the present results rule out that substance as a major constituent of the surface of Mars. If the amount of CO, used to calculate the Rayleigh scattering were in error, then of course the derived surface reflectivity would also be incorrect. However, for the computed surface reflectivity to agree with the published models, an overestimate of the CO, abundance by a factor of 2 or more would be required ; and this is extremely unlikely. It is also conceivable that an absorbing substance in the atmosphere could be depressing the calculated surface albedo. If this is the case, to make the computed surface reflectivity agree with the models, the absorption would have to be continuously increasing from X4000 to hZOOO& with no prominent line or band structure detectable at 20 d resolution. Also, since the present data span 25 days and the conditions in the Martian atmosphere do not appear to have changed in that time, the lifetime of such an absorber would be comparable to or greater than that interval. The apparent ultraviolet absorption observed by Mariner 9 during the 1971 dust storm on Mars (Barth et al., 1972) is probably not relevant to this data, since the atmospheric conditions were very different between the apparitions. Ahmad and Deutsc@nann (1972) have reviewed lunar ultraviolet measurements in connection with their Celescope observations with OAO-2. It is of interest that the upper limit to the Martian surface albedo calculated here generally agrees with lunar albedo measurements in the spectral region of the present observations. Other observations of solid surfaces in the solar system recently made by the Wisconsin Experiment Package will be discussed in a future paper. It should be noted here that the absolute magnitude of the error bars in Figs. 1 and 2 apply to the calculated residual albedo, so t,hat the apparent upturn at A2100 A may not be real. An upper limit on the abundance of other gaseous constituents of the atmosphere can also be derived from the albedo curves. Since all other plausible candidates, including recently N, (Dalgarno and

CALDWELL

494

McElroy, 1970), have been eliminated as additional major components of Mars’ atmosphere, argon will be used as an example. From the total geometric albedo in Fig. 2 at h23008, where the residual albedo is a minimum, and using the Rayleigh scattering cross sections for argon given by Allen (1963), it may be calculated that there is at most 48 f 7matm of argon on Mars. Unfortunately, this is not an improvement over previous upper limits for the total molecular abunda.nce (e.g., Carleton et ul., 1969). On Earth, argon is more than 30 times more abundant than CO, (Kuiper, 1952). The large discrepancy between the relative abundances of CO, and argon in the atmospheres of the Earth and Mars may be due to the large amount of CO, trapped in the Earth’s crust in the form of carbonates (Rasool and de Bergh, 1970). 4. COMPARISON WITHMARINER ULTRAVIOLETOBSERVATIONS

The continuous albedo curves of the present paper and their interpretation conflict with those of Hord (1972) in his discussion of ultraviolet spectra of nonpolar regions of Mars by Mariners 6 and 7. The sample albedo curve presented by Hord (his Fig. 4), which is representative of all Mariner nonpolar scans, increases by a factor of about two from h3500 to h2600 a : whereas the present albedo curves increase by less than 20% in this interval. As discussed in Paper 1, the OAO albedos are very accurate at these wavelengths, at least in the relative sense, being independent of instrument calibration errors. In order to develop a photometric model of the Martian surface and atmosphere, Hord chooses to normalize his data to Mariner infrared spectrometer pressure data for common regions of observation. This is somewhat analogous to the use of the ground-based determination of the XOmatm (XJ2 abundance here. In neither case is t,he ultraviolet data alone sufficient to exbract physically signikant~ information about, &Mars. In his analysis Hord assumes constant photometric parameters and uniform small

particle content, neglects atmospheric absorption, and uses the single-scattering approximation for small optical depths. He uses data only from a band 100 A wide, centered at X3050a. To test his model, Hord compares his derived pressures with Mariner infrared pressures, Earth-based spectroscopic pressure determinations, and radar altitude measurements. He finds generally good agreement with some notable exceptions. His data do not allow good separation of the phase function and optical depth parameters. In this regard it is very unfortunate that data at shorter wavelengths were not usable. However, Hord deduces that the optical depth at h3050 A is at least twice that of the CO, atmosphere and that the scattering is non-Rayleigh. He therefore concludes that there is a significant mount of small particle scattering in the Martian atmosphere. Hard’s conclusions conflict with the shorter wavelength observations of this paper. He calculates that at X305OA the atmosphere contributes a fraction (O.OSS)/ (0.035) 21 75% of the total brightness, with the surface contributing the remainder. However, this would imply greater albedos than are observed below X3000A. From Figs. 1 and 2, it is apparent that at X2300 A, the total reflectivity does not exceed that due to the CO, atmosphere by more than 25’/, . At X3050 8, therefore, it may be concluded that the Martian surface contributes at least 50% and as much as 65% of the total planetary reflectivity, the exact percentage depending on the abundance of other atmospheric scatters and the wavelength dependence of their scattering cross sections (k4 for molecular scattering, X-’ for particle scattering). Implicit in this estimate, of course, is the assumption of no atmospheric absorption, which, if present, would cause an underestimate of atmospheric abundances. Hord points out t,hat if the sources of error in his analysis are uncorrelated, t’hen all errors should be less than the total rms err& in his pressure normalizat’ion, ~9%. However, it is by no means clear that the sources of error are uncorrelated. For example, if the ultJraviolet ground albedo

UVOBSERVATIONSOFMARSBYTHEOAO

varies with altitude, then his analysis would be invalidated. Ingersoll (1971) has concluded from ground-based polarization studies that the Martian atmosphere is purely molecular and that the surface contributes a fraction of 0.6 of the planetary albedo at h3200& in good agreement with the present paper. A very important Mariner observation (Barth and Hord, 1971) is that the ultraviolet spectrum of the south polar cap is systematically different from the desert regions. The difference is suggestive of absorption by a small amount of ozone above or adsorbed on the polar cap. In Figs. 1 and 2 it is seen that the integrateddisk spectrophotometry of OAO-2 does not reveal any indication of the characteristic ozone absorption, centered at A2550 A. Besides the va,st difference in spatial resolution between *Mariner and OAO-2 observations, there was a significant time difference. Mariner polar cap spectra were obtained in early August 1969, and OAO-2 observations were made in March and April of that year. At the time of the OAO-2 observations, the north polar cap, which was tilted slightly toward the Earth, was decreasing rapidly and covered less than 2% of the visible disk. The south pole was covered by a large polar hood, through which the cap was not visible (see Fig. 3). Therefore, the apparent absence of an ozone absorption feature in the OAO observations does not contradict the Mariner data. 5. BROADBAND FILTER PHOTOMETRY AXD SHORT WAVELENGTHSCANS OFMARS

In addition to the scans discussed above, observations of Nars were also made using four telescopes equipped with broadband filter photometers and a spectrum scanner operating in the wavelength range hh1000-2000 8. Those filters with which Mars was successfully observed essentially tluplicat,ed the spect*ral range of the longer wavelength scanner. hut with greatly reduced resolut,ion, anti art’ therefore not f’urt bet discussed here. An attempt to detect Nars with a broadband filt,er having an effective wavelength 18

495

of approx 1700 A was successful only in establishing an upper limit to the Martian signal. This upper limit was converted to a flux through comparisons of observations of early type stars with model stellar atmosphere calculations. The upper limit was found to correspond to a geometric albedo of 0.05. In view of the strong absorption by CO, at wavelengths less than h2000.& (a hint of which is apparent in Figs. 1 and 2), this albedo is probably too high, and the true Martian signal is likely very much smaller than the upper limit, determined here. Scans of Mars from Xl000 to X2000X were examined for spectral emission features. Sgain, only upper limits could be established, but these proved to be from 50 to 100 times larger than fluxes actually measured by Mariners 6 and 7 (Barth et al., 1971) and therefore do not’ contain any useful information about Mars. k2KNOWLEDGMENTS The author

is grateful

to Mr. Charles Capen of

the Lowell Observatory for discussions on the visible appearance of Mars during the period when these observations were made. He is also grateful to Dr. Blair Savage of the University of Wisconsin for many very helpful discussions throughout the preparation of this paper. Financial support for this r&earch was available from NASA Headquarters under Research Gra,nt NGR-03-003-001. The author also received support from NASA Contract NAS 5-1348 tllrough a research assist’antship with the Spare ,Istronomy Laboratory of the University of LVisconsou.

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KELLY, K. K., ANDERSON, G. P., AND STEWART, A. I. (1971). Mariner 6 and 7 ultraviolet spectrometer experiment : Upper atmosphere data. J. Geophys. Res. 76, 2213. BARTH, C. A., HORD, C. W., STEWART, A. I., AND LANE, A. L. (1972). Mariner 9 ultraviolet spectrometer experiment, : Initial results. Science 175, 309. BELTON, M. J. S., BROADFOOT, A. L., AND HUNTEN, D. M. (1968). Abundance and temperature of CO, on Mars during the 1967 opposition. J. Geophys. Res. 73, 4795. CARLETON, N. P., SHARMA, A., GOODY, R. M., LILLER, W. I~.. AND ROESLER, F. L. (1969). Measurement of the abundance of CO, in the Martian atmosphere. Astrophys. J. 155, 323. I~ALGARNO, A., AND MCELROY, M. B. (1970). Xars : Is nitrogen present? Science 170, 16i. DAVE, J. v., AND WARTEN, R. M. (1968). Program for computing tho Stokes parameters of the radiation emerging from a plane-parallel nonabsorbing Rayleigh atmosphere. IBM (Palo Alto) Scientific Center Report No. 3203248. HORAK, H. (1950). Diffuse reflection by pla,netary atmospheres. Astrophys. J. 112, 449. HORD, C. W. (1972). Mariner 6 and 7 ultraviolet spectrometer experiment : Photometry and topography of Mars. Icarus, 16, 253. INGERSOLL, A. P. (1971). Polarization measurements of Mars and Mercury: Rayloigh scattering in the Martian atmosphere. Astrophys. J. 163,121.

IRVINE, W. M., HIGDON, J. C., AND EERLICH, S. J. (1971). 1m “Planetary Atmospheres” (C.Sagan,T.Owen,andH.Smith,eds.),p. 141, Springer-Vqrlag New York, Inc., New York. KUIPER, G. P. (1952). “The Atmospheres of the Earth and Planets” (G. P. Kuiper, ed.), p. 1, The University of Chicago Press, Chicago, Illinois. O’LEARY, B. T., AND REA, D. G. (1968). The opposition effect of Mars and its implications. Icarus 9, 405. OWEN, T., AND SAGAN, C. (1972). Minor constituents in planetary atmosphere : Ultraviolet spectroscopy from the Orbiting Astronomical Observatory. Icarus 16, 557. PLVMMER, W. T., AXD CARSON. R’. K. (1969). Mars: Is the surface colored by carbon suboxide? Science 166, 1141, RASOOL. S. I., AND DE BERGH, S. C. (1970). Tht, runaway greenhouse and the accumulation of CO, in the Venus atmosphere. Nature (Londov ) 226, 1037. SAGAN,C.,PHANEUF, J.P., AXDIHNAT, M.(1966). Total reflection spectrophotometry and thermogravimetric analysis of simulated Martian surface materials. Icarus 4, 43. SINTON, W. M. (1967). On the composition of Martian surface materials. Icarus 6, 222. WALLACE, L., CALDWELL, J., AND SAVAGE, B.D. (1972). Ultraviolet photometry from thp Orbiting Astronomical Observatory. III. Observations of Venus, Mars, Jupiter, and Saturn longward of 2OOOA. Astrophys. J.,

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A

search features

for on

limonito Mars.