JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. E4, PAGES , APRIL 25, 1999

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. E4, PAGES , APRIL 25, 1999 Optical properties of the Martian aerosols as derived from Imager for Mars Pathfinder midday sky brightness data W. J. M rkiewicz, R. M. S blotny, H. U. Keller, nd N. Thomas Max-Planck-Institut fiir Aeronomie, Katlenburg-Lindau, Germany D. Titov Max-Planck-Institut ffir Aeronomie, Katlenburg-Lindau, Germany Space Research Institute, Russian Academy of Sciences, Moscow P.H. Smith Lunar and Planetary Laboratory, University of Arizona, Tucson Abstract. The Imager for M rs P thfinder (IMP) obtained d t on the midday sky brightness in filters centered t 443.6, 481.0, 670.8, nd nm. Useful d ta sets were returned on sols 27, 40, 56, 65, 68, 74, nd 82. D t from sol 56 were fitted with multiple scattering radiative transfer c lcul tions, to extract the size distribution, optical properties nd shape of the erosols suspended in the tmosphere. The derived effective r dius of the p rticles is bout /- 0.26pm with n effective v ri nce of t ff = / The estimated v lues of the refractive index and shape p mmeters re close to those derived from Viking nd Phobos d t. This in turn implies that dust plays significant nd relatively constant role in the energy budget of the M rti n tmosphere over the l st two dec des. Estimates of the optical depth gree well with those obtained independently from direct IMP imaging of the Sun. The derived single scattering phase function is more compatible with plate (cl y) like p rticles rather th n equ l dimensional p rticles. The presented n lysis ssumes simple single-component dust tmosphere. The d t -model residuals exhibit, lbeit we k, w velength dependence. This dependence c n be interpreted s n indication that during the time the n lyzed images were t ken, the dust p rticle distribution w s bimod l or that the M rti n tmosphere contained second component, possibly submicron ice p rticles, in the erosol's population. 1. Introduction ness are required to separate the atmospheric illumination from the direct one, and hence to understand the There are several good reasons to study the optical mineralogy of the surface rocks and soil. Such accurate properties of the aerosols in the Martian atmosphere. models are only possible if the optical properties of the The dust particles suspended in the atmosphere absorb Martian aerosols are known. solar radiation and play a major role in its energy bud- Past work dedicated to estimates of the optical propget. This, in turn, controls the global circulation and, on a long timescale, the Martian climate [Gierasch and erties of Martian aerosols for wavelengths ranging from ultraviolet to infrared go back to Mariner 9 and con- Goody, 1972; Haberle et al., 1982]. The aerosols also intinued with the analysis of data from the Viking and fluence measurements of the spectrophotometry of the surface. As is now clear from images acquired by the )obos mission s/-a partial summary of these results is given in Table i of the paper by Pollack et al. [1995]. Imager for Pathfinder (IMP) of, for example, the rock Further analysis can be found in the work by Clancy et Yogi, the diffuse illumination by the sky significantly al. [1995], Moroz et al. [1993], Wuttke et al. [1997], and alters the apparent spectrum of the surface [Thomas et el., this issue]. Very accurate models of the sky bright- Petrova al. [1996]. Most recently, Ockert-Bell et el. [1997] used a combination of ground-based and space observations of Mars to extract aerosol properties from Copyright 1999 by the American Geophysical Union. the images of the surface. In spite of this ongoing effort, Paper number 1998JE there are still some variations in the estimates. Some /99/1998JE of these can, of course, be explained by seasonal and 9009

2 9010 MARKIEWICZ ET AL.' OPTICAL PROPERTIES OF THE MARTIAN AEROSOLS longer term variations in the Martian atmosphere, but at least partly they are also the result of the inherent difficulty of solving the inverse problem of constraining the aerosol properties from the measured brightness of the sky. This work continues this effort by analyzing the IMP images of the brightness of the Martian sky at midday acquired from the surface of Mars during the Mars Pathfinder mission. Details of the imp design and its science objectives can be found in the work by Smith et al. [1997a]. The first results were presented by Smith et al. [1997b]. The paper starts with a description of the sequence used to obtain the data. It continues with a presentation of the full data set returned from sol 56 and a procedure used for the geometric calibration needed to correct for the pointing errors. The models for sol 56 with the derived size distribution and optical parameters follow. The paper ends with a discussion of the results, their limitations and prospects for the future. 2. Sequence S0283' "Cross Sky Brightness Measurement" The objective of the IMP imaging sequence S0283 was to image the Martian sky brightness as seen from the surface, along the great circle joining the north and south horizons to the zenith, when the local time was near noon. Although the coverage in scattering angles of this sequence is limited to about 1000, having the Sun near zenith minimizes multiple scattering. This property should help in accuracy of constraining the size distribution and material properties. The shape of the particles can be expected to be less well constrained, as scattering events at angles around 1500 are only present through multiple scattering. A full single-sol session consisted of images in five filters at 12 camera positions. The filters used were L0, R10, L5, L8, and Lll [Smith et al., 1997a]. The corresponding central wavelengths are 443.6,481.0,670.8, 896.1, and nm, respectively. In writing the sequence, care had to be taken to avoid direct imaging of the Sun on any part of the CCD and to minimize pointing inaccuracy. Pointing inaccuracy of about 0.50 Table 1. IMP Data Sets Obtained With the Imaging Sequence S0283 Sol Number of LST Sun elev. [o] Images Start Stop Min Max : : : : : : : : : : : : : : lo... I... I... I ' ' I ' ' I I I S0156-6S , ,' :i..... / ':i ;..... i... i... i,,, i i i North Elevotion [ø] South Elevotion [ø] Figure 1. Complete set of precalibration data from sequence S0283 obtained on sol 56. Curves are labeled by center wavelength of the filter. can result from the backlash of the camera stepper motor bearings [Smith et al., 1997a; Reid et al., this issue]. Timing errors, error in spacecraft orientation, or errors in camera model can all cause an apparent pointing error. Errors in the azimuth direction were minimized by splitting the sequence into two sets, the "north side" and the "south side." Within each of these sets, the movement of the camera was purely in the elevation as seen in the lander coordinate frame. All images used the full 248 pixels available in elevation. The two images taken at camera positions with the lowest elevation (horizon images) had a width of 128 pixels. The width of all other images was set at 8 pixels. All images were centered on the middle column of the corresponding eye section of the CCD. The elevations of the horizon images were chosen in a way that distinctive surface features of the landing site horizon would be visible. The features were "North Knob" in the horizon images of the north set and "Misty Mountain" in the south section. Because of the increased width of these horizon images, a precise determination of the camera position is possible by comparison with other images taken by IMP, e.g., the panoramas of the landing site. The sequence was run on seven sols. Complete data sets, 60 images per sol, could be returned for sols 27, 56, 65, 68, and 74, while for sols 40 and 82, only 42 and 48 images were returned, respectively (Table 1). The full raw data set obtained on sol 56 is shown in Figure 1, as intensity versus. elevation plots. Exposure times were typically of the order of 0.02 through 0.2s depending on the filter being used. The last image of the sequence also returned a null strip for CCD quality assessment. Pixel blocking was set to 2x2, and the compression used was ARIRAT (Joint Photographic Experts Group (JPEG) Discrete Cosine Transform (DCT)). The compression ratio for the horizon images was 6; for all others it was 25.

3 MARKIEWICZ ET AL.' OPTICAL PROPERTIES OF THE MARTIAN AEROSOLS Calibration and Pointing Accuracy The data set was radiometrically calibrated to convert pixel values to physical units using the standard IMP software [Reid et al., this issue]. The transformation matrix used for geometric calibration was tested against images of objects with known azimuth and elevation, e.g., the Sun and the star Arcturus (a B5o). The absolute value of the difference between measured position and the position as given by the Jet Propulsion Laboratory (JPL) mission ephemeris (Solar System Calculator) was between 10 and 30. Several sources for this difference are possible: pointing inaccuracy of the camera, errors in the transformation of the spacecraft clock to universal time used in the ephemeris computations, and inaccuracy of ephemeris data (especially orientation of the lander). Since mission end, all IMP data have been reprocessed several times, eliminating various header entry errors. The most recent reprocessing had to do with changes in the decompression algorithm. The 8 pixel-wide image of sequence S0283 was most strongly affected by this latest change. The data presented here are identified by the VICAR header Scottering ongle [ø] ScoRering ongle [ø] Figure 2. Data from sol 56, filter L5 from the image closesto the Sun a) before geometric calibration, and b) after geometric calibration. investigation of the dependence of several models to derive universal time from the spacecraft clock does not suggest a deviation of more than about 30s, if any, of the true universal time from that given in the image headers for sequence S0283. Varying the position of the Sun within such a small time interval does not change the shape of the curve in Figure 2a. Serious errors in ephemeris computations could still be considered, but these are very unlikely. The lander orientation with respect to local ground is well known; hence errors in transformation from lander frame to surface fixed frame are also negligible. Therefore inaccuracy of camera azimuth and/or elevation values in the lander frame, which were computed from raw motor steps given in the image header, is considered to result in the double-valued behavior shown in Figure 2a. Figure 2b shows the same data as in Figure 2a after 0.60 was subtracted from the elevation in the lander frame derived from the pixel positions prior to the transformation to the surface fixed frame. The S0283 sequence was designed not to change the camera azimuth (in lander frame) within the north and south parts of the sequence. The azimuth pointing accuracy was hence expected to be good. The first image of the north sequence, a horizon image, was taken after the camera motor was driven against a hard stop in azimuth. Throughout the north part of the sequence, only elevation changes were commanded. To move to the south part of the sequence, a single 1800 movement entry SOFTWARE_VERSION_ID=" V " Miscalculations of the correct camera position and/or the position of the Sun have a profound impact on the interpretation of the data of sequence S0283, as can be seen in Figure 2a, which shows part of the radiometrically calibrated data for sol 56 taken through a nm filter as a function of the scattering angle. With the Sun close to zenith, the sky brightness near the Sun should have point symmetry about the Sun position unin azimuth was commanded. Again, the subsequent images were obtained with elevation changes only. The expected accuracy in azimuth was confirmed by comparless clouds are significant. The double-valued behavior ing distinct features, "Misty Mountain" (south horizon) for smallest scattering angles (closest approach to the and "North Knob" (north horizon), in the S0283 hori- Sun) is hence not physical and must be due to an er- zon images with the map of the landing site [Golombek ror in the Sun-camera geometry. Changing universal et al., 1997]. In both cases the error was less than 0.1 ø, time, and hence the Sun position, within reasonable which is within the error of reading the map. Estimatboundaries does not remove this behavior; however, it ing the elevation pointing accuracy in the same way can change the general slope of the curve. A thorough resulted in 0.10 error for the north side and 0.20 for the south. This magnitude of error is negligible for the purpose of the present analysis and well below the ex pected pointing error in any single camera movement of o) about The north part of the sequence started with the horizon image; hence the smaller error in pointing may seem reasonable. The south part, however, finished with the horizon image after moving the camera five times in elevation. Pointing error for each camera movement is random. Overall they could either cancel each other or accumulate. Hence the very small eleva tion error in the position of the Misty Mountain on the S0283 image is almost surprising. Pointing accuracy for the intermediate images is not known. For all the ,..., ,...,... above reasons the position of only the image closest to the Sun (but in all wavelengths) was adjusted in elevation by 0.60 to eliminate the double-valued behavior seen in Figure 2a. The result of applying this correction is shown in Figure 2b. All other images were left uncorrected for any possible pointing errors.

4 9012 MARKIEWICZ ET AL.: OPTICAL PROPERTIES OF THE MARTIAN AEROSOLS 4. Model Atmosphere The sky brightness is the result of the radiative transfer, including surface reflection as the boundary condition, of solar photons in the Martian atmosphere. Within the wavelength range of the IMP instrument, the atmospheric scattering is due only to aerosols. The contribution from Rayleigh scattering by the gas molecules can be estimated as follows. The optical depth due to molecular scattering (CO2) is rg = 0.946PoA4/(157A 2-1) 2 [e.g., Allen, 1963], where P0 is the surface pressure in millibar, and A is wavelength in microns. Hence rg increases rapidly with decreasing wavelength, but within the IMP range, it is at most O(10-3). Even taking account of the significant air-mass for very small elevation angles, the gas will always be optically thin. Hence the single scattering approximation can be used to estimate gas contribution to the total diffuse radiation field. Figure 3 shows the ratio of the expected intensity due to the Rayleigh scattering by CO2 divided by dust contribution, for IMP wavelength range, as a function of the elevation angle. The dust contribution here is also only due to single scattering to maximize the ratio in Figure 3. The model calculations below use full radiative transfer with mul- tiple scattering and surface reflection. From Figure 3, it can be seen that molecular scattering can be neglected within the IMP wavelength range. Judging from past data, the aerosols can be expected to be primarily dust particles, although depending on time of day and season, clouds composed of ice particles and hazes of ice-covered dust can be also expected [Petrova et al., 1996, and references therein]. The differential number density of the dust size distribution n(r) is taken to be the modified gamma function with 7 = 1 [e.g., Hansen and Travis, 1974], n(r) crr ( -3 ")/ " exp [--r/(refzueff)],(1) where r is the particle radius and the effective radius re,, and variance uej,, are the first two moments of n(r). As pointed out by Hansen and Travis [1974], the choice of the type of the distribution is of secondary importance as long as the first two moments are the SalIle. The material properties of the dust particles enter the modeling through the complex index of refraction, hi. The imaginary component was kept as a free parameter, and the real component was kept at a constant value of 1.5. Variations of the real component within the IMP wavelength range should be less than 1% [e.g., Ockert-Bell et al., 1997]. An initial estimate of the imaginary part at IMP wavelengths was obtained from Viking lander and Phobos results (cf. Figure 7). It should be noted that especially the near-ir region was very poorly constrained from these previous data sets. Single scattering properties of the dust particles are computed using the semiempirical theory for scatter " x I i I i i _ Elevation [ø] Figure 3. Upper bound of the ratio of gas scattered to dust scattered radiation within the IMP wavelength ing and absorption by nonspherical particles of Pollack and Cuzzi [1980], which was subsequently modified by Showalter et al. [1992]. This theory requires no fewer than five parameters, x, x2, S, b, and 0min. The theory uses Mie solution for particles with a size parameter x = 2 rr/ < x, where is wavelength, and approximate formulas for irregular particles with For x < x < x2 a weighted mean of the two solutions is used. S is the ratio of irregular particle surface area to that of an equal-volume sphere. The remaining two parameters, b and 0rain describe the forward to backscattered ratio for the transmitted component; b is the slope of the phase function at forwardscatter, defined by -dlnp(o)/doio=o, where 0 is the scatter- ing angle and P(O) is the phase function. Here 0rain is the scattering angle at which the phase function has its minimum. It should be noted that 0min does not need to be less than The atmosphere was assumed to consist of a single layer of dust particles. Within this approximation, the only independent variable (describing the atmosphere) is the optical depth of the layer, r. The value of r was kept as a free parameter and a posteriori compared (see Figure 12, below) to independent estimates obtained frmn direct i nages of the Sun through the solar filters [Smith et al., 1997b]. The surface reflection was calculated using Hapke's bidirectional reflection function [Hapke, 1993] with the parameters fitted to the Viking lander images by Arvidson et al. [1989]. In the future, these parameter should be modified to be appropriate to the Sagan Memorial Station landing site, but for the present case, since the surface boundary condition only enters the problem as a second-order effect, small changes in reflection function should not alter the results presented below. If the parameters derived for the Pathfinder site will turn out to be significantly different from those for the Viking lander site, some average will need to be used, since the surface reflection over scales much larger than the local,

5 MARKIEWICZ ET AL.' OPTICAL PROPERTIES OF THE MARTIAN AEROSOLS 9013 O lo I ' ' ' [ ' ' ' [ ' ' ' I Sol nm _ Doto re,,= 1.60 /,m Model ve,= 0.14,,, i,., i,., i, scottering ongle [o] Figure 4. Data from sol 56, filter L5 and best model ing angle. Apart from the horizon images and several scene contribute to the diffuse sky brightness. For this points at the scattering angle near 120 the accuracy is reason as well a more direct comparison with the Viking about 5%, which is comparable to the relative accuracy results [e.g., Pollack et al., 1995] parameters derived by of the data. Almost all the points are within the es- A rvidson et al. [1989] are used in the present work. timated absolute accuracy of about 10%. There is a The set of parameters described above, consisting of very noticeable difference in the general smoothness of single scattering properties, optical depth, and surface the error for horizon images as compared to all others. photometry, all functions of wavelength, is the necessary This difference is most certainly due to the compresinput to the radiative transfer problem. As mentioned sion algorithm. As discussed above the horizon images above, in this first analysis of the $0283 sequence data, are 128 pixels wide, while all the remaining ones are the dust is assumed to be confined to a single layer only 8 pixels. The general scatter in the nonhorizon of infinite horizontal extent. In these plane-parallel images can be taken as an estimate of error resultant approximations the radiative transfer equations were from compression. The larger scale features in Figure 5 solved using the doubling-adding method treating mul- are likely partly due to errors in Sun-camera geometry tiple scattering exactly [e.g., Hansen and Travis, 1974]. and/or clock, but also possibly due to inhomogeneities The above parameters and methods used for modeling of the Martian sky. were chosen so that a most direct comparison to past The X 2 values at the best fit minimum are shown modeling (cf. Introduction) of the optical properties of in Figures 6, 7, and 8. The minimum was found in the Martian aerosols can be made. the full parameter space and these figures show plane cuts within this space. Figure 6 shows the dependence 5. Results for Sol 56 of X 2 on variations in ref f and lyef f. Note that the The data set obtained on sol 56 has Sun-camera geometry with smallest scattering angle of This set was hence chosen for modeling in the first instance giving the best chance to constrain the size distribution of the aerosols. Also the midday optical depth estimated from independent images of the Sun was relatively high on sol 56, varying between 0.55 and 0.62 as a function of wavelength. During the initial stages of model calculations, all of the above described parameters were varied until a best fit was found. The goodness of the fit was defined by the convergence of the minimum in the squared relative difference between data and model, defined by N - - b, j=l 10 ø were used for calculating X 2. This excludes points near the horizon which might be expected to have significant second-order effects from the curvature of the atmosphere not modeled in the calculations. The relative error definition for X 2 was chosen to give all points equal weight. In fitting the size distribution, an absolute error definition, giving forward scattering stronger weighting, was also tested. This did not alter the results presented below. The first sol 56 data modeled was from filter L5, centered at nm. The data and the best fit are shown in Figure 4. Except for the points near the horizon, the data and model agree to a very high accuracy. The agreement for scattering angles below 20 degrees (and hence the constraint of the size distribution) is especially good. Figure 5 shows the relative difference between the data and the model as a function of scatter- -o o.o Sol56, X=670.8 nm ' ' i ' ' ' i ' ' ' I ' ' ' I ' ' ' I ',,, I,,, I,,. I,.. I... i O0 scottering ongle [ø] where N is the number of the data points in the modeled set. All data points with elevation greater than Figure 5. Figure 4. Relative error in the model fit to data in

6 9014 MARKIEWICZ ET AL.' OPTICAL PROPERTIES OF THE MARTIAN AEROSOLS X2X103, Sol56, X nm......,,,. 0'065 L. '.O 0, 2x103, Sol56, ),=670.8 nm 0'060 I,, - ' ,.....OO....'... contours are labeled with X 2 x 10 a. A minimum is found for re/! = 1.6 tim and v j! = During the Pathfinder, mission a complementary data set to that presented here was obtained. The Martian sky, with the Sun low in the sky (150 to 250 elevation), was imaged at constant elevation and varying azimuth [Smith et el., 1997b]. The initial analysis by Smith et el. [1997b] of this data set obtained r j! = 1.0(+0.3,-0.2) rim. This very small value of r j! was subsequently increased to 1.6 rim [Tomesko, this issue]. Figure 7 shows the dependence of X2 on variations in the imaginary part of the index of refraction ni and the optical depth r. The X 2 ellipses are much more elongated than in Figure 6; nevertheless, a minimum is X2X103 Sol56 2x-6708 nm re, [ m] Figure 8. The X 2as defined by equation (2) for vari- Figure 6. The X 2as defined by equation (2) for varia- ations of the particle shape parameters. Data are from sol 56 and filter L5 at nm. tions in first two moments of the size distribution, re/! and u /j. Data are from sol 56 and filter L5 at rim. present at ni and r = As mentioned above, the optical depth was also estimated independently from images of the Sun. This estimate for sol 56, at local noon and at 670 nm is 0.57, a value which agrees well with the value derived here from sky bright- ness, given variations in r expected from, for example, spatial variations in the sky. Figure 8 shows the behavior of X 2 in the b-0min plane. It can be seen that these parameters are much more poorly constrained, as expected, since shape parameters are generally important at scattering angles of about 1500 These scattering events enter S0283 data only through multiple scattering. A minimum is found at 0min : 1600 and b = The remaining parameters describing the shape of the particles do not affect the model fits significantly the best values found are Xl = 3, x2-5, S = 1.3. All of these values are in very good agreement with those obtained by Polleck et el. [1995]. A complete iteration on all of the free parameters did not alter their values any further ,, i,,, i,,, i,,, i,,, i, E 1000 Sol nm _ ::t. k Doto ref f /.m hn -. Model veff-' 0.28 E n imoginory port of index of refroction Figure 7. The X 2 as defined by equation (2) for variations of the imaginary part of the complex index of refraction and the optical depth. Data are from sol 56 and filter L5 at nm scottering ongle [ø] Figure 9. Data from sol 56, filter L0 and best model

7 _ MARKIEWICZ ET AL' OPTICAL PROPERTIES OF THE MARTIAN AEROSOLS 9015 I ' ' ' [ ' ' ' I ' ' ' I I ' i,,, f i i i iaaa Sol nm O Sol nm _ Data reff= 1.66 /,m Model %,= 0.50 Data ref / m Model Veff lo..., i... i... i... i scattering angle [ø] lo o i!! I,!, I i i i I i,, i,,, i, scattering angle [ø] Figure 10. Data from sol 56, filter R10 and best model Figure 12. Data from sol 56, filter Lll and best model The data and best models for the remaining filters are shown in Figures The models fit the data quite well for nm, while the remaining ones are somewhat poorer. The model for never properly converged in X 2 space. It was found to be impossible to fully constrain the imaginary index of r fraction at this wavelength. The X 2 manifold is extremely fiat near the parameter values for the model in Figure Summary and Discussion The present work adds to a continuing effort of constraining the optical properties of the Martian atmosphere by analyzing the IMP images of the midday sky obtained on sol 56 of the Mars Pathfinder mission. The derived parameters are summarized in Table 2 and in Figures 13, 14, and 15. As shown in Figure 13, the optical depth values r agree very well with those obtained independently from direct images of the Sun through the solar filters [Smith et al., 1997b]. The direct estimates were all obtained within one hour of the local O lo o ' ' ' I I I I ' ß ' I Sol nm _ Data r ff /,m L Model veff = 0.25,. I i I,,, I, I, i I scattering angle [o] Figure 11. Data from sol 56, filter L8 and best model noon and hence within less than an hour of the S0283 data (cf. Table 1). The differences between the values obtained directly and those obtained here are comparable to the variations of r itself as obtained directly. The variation in r, at any particular wavelength in Figure 13, could be interpreted as an estimate of the spatial anisotropy of the Martian atmospheric dust itself. The derived effective radius r /! has a minimum of 1. 5 tim and a maximum of 2 tim. The average estimate over all wavelengths is / tim. The derived effective variance is u /! / The shape parameters do not vary appreciably with wavelength. Figure 14 shows the single scattering phase functions for the five filters corresponding to the best models. The most obvious characteristic of these phase functions is the lack of any backscattering peak. This monotonic decrease in the phase function with the scattering angle, the result of 0mi n > 140 ø, is more compatible with plate (clay) like particles rather then equal dimensional particles [Pollack and Cuzzi, 1980]. As mentioned above, however, the S0283 data have limited sensitivity to the particle shapes. Comparison must be made with the data obtained at other Sun-camera ge- ometries [Tomasko et al., this issue]. The values for the imaginary part of the index of refraction ni are compatible with the previous estimates and, more importantly, provide two additional points within the previously poorly constrained nm region. This is apparent in Figure 15, where ni values from several sources, including this work, are plotted against wave- length. Analysis of the IMP images of the calibration target on the lander, obtained through all 12 geological filters [Thomas et al., this issue], will in the future improve the coverage of this mineralogically important wavelength region even further. It is interesting to note that the effective radius of the dust distribution increases with wavelength (cf. Table 2). The exception to this is the model for the nm filter. This filter is the only one of the five used that was on the right filter wheel of IMP's stereo channels. The

8 9016 MARKIEWICZ ET AL.' OPTICAL PROPERTIES OF THE MARTIAN AEROSOLS Table 2. The Optical Depth and Single Scattering Properties Derived from Sol 56 Data /k pm r rcff b'½ff b 0,,, n Q xt Q c, < cos0 > remaining four filters were all on the left filter wheel. If the wavelength dependence of the effective radius is real, it could be an indication that the single-component dust model used in the present work is not accurate enough. An apparent increase of particle size with wavelength could be produced by the existence of a second component of small particles. The second component could be clouds or hazes composed of water ice particles [cf. Petrova et al., 1996]. If these particles have a narrow size distribution and an effective radius below I / m, their extinction decreases very strongly with increasing wavelength. It may also be possible that the dust particles themselves have a bimodal distribution, one component being particles freshly introduced into the atmosphere and a second component being those that have been suspended in the atmosphere for a long time and sorted by, for example, sedimentation. Adding another aerosol component further increases the number of free parameters in the model. A consistent search through this parameter space is beyond the scope of the present work. Before a systematic search for this model can be attempted, the data from other sols must first be analyzed. Results of this ongoing work will be reported in the future. In conclusion, the present work demonstrates that the data obtained with the IMP imaging sequence S0283 can be used to derive the optical properties of the Martian aerosols. The results are in very good agreement with those obtained from analysis the Viking lander data [Pollack et al., 1995]. They are also similar to the results from the KRFM/Phobos instrument [Moroz et al., 1993], except that during the Phobos mission, optical depth was somewhat smaller. This, in turn, implies that dust plays a significant and relatively constant role in the energy budget of the Martian atmosphere over the last two decades. Further work is required to analyze the data from the remaining sols. Since the atmospheric conditions during the Pathfinder mission were relatively calm, future work should improve the statistical significance of the results presented here. Images of the calibration target as well as the multispectral spot of the sky [Thomas et al., 1998], although of limited Sun-observer geometry, provide data with full IMP resolution in each wavelength. Analysis of these data will further constrain the material properties of the aerosols. The results presented here, although preliminary, have already successfully been used to model the influence of diffuse sky il i... i... i... i... '... i... i... i q- 'r from direct imaging of the Sun 0 'r from models of S0285 data (this work) Sol ' ' I ' ' I I ' ' I ' ' nm nm o + + +o o+ c- O :.oo nm nm nm i... I... I... I... i... i... i X (nm) 0.01 o,,, i 3o ""'"'%' " --' :..'t-: ',, I,, I, I, I,, 60 go scattering angle [ø] Figure 13. Optical depth r as derived from IMP data from sol 56. The values from direct images of the Sun were obtained within one hour of 12'00 LST. Figure 14. Phase functions for single scattering used to calculate best models shown in Figures 4, 9, 10, 11, and 12.

9 MARKIEWICZ ET AL.' OPTICAL PROPERTIES OF THE MARTIAN AEROSOLS O.OLO o.ooo maginary part of index of refraction... i... i... i... i... i... i... A A Viking Lander, Pollack et el., (1995) + Phobos, Petrova et al., (1996) Ockert-Bell et al., (1997) 0 IMP, this work A 0 -F ( A 0 ZS - ) 0 )... i... i... i... i... i... i X [nm] Figure 15. Imaginary part of index of refraction as a function of wavelength in the IMP range from several sources, including this work. lumination on the images of the surface [Thomas et al., this issue]. Acknowledgments. The starting point for the development of the nmnerical codes used in this work was the radiative transfer codes from James Pollack's group at the Ames Research Center. The authors would like to acknowledge this fact, and at the same time thank M. Ockert-Bell for introducing us to these codes during W.J.M.'s visit to Ames Research Center in References Allen, C. W., Astrophysical Quantities, 2nd ed.; University of London, Athlone Press, Arvidson, R. E., E. A. Guinness, M. A. Dale-Bannister, J. Adams, M. Smith, P. R. Christensen, and R. B. Singer, Nature and distribution of surficial deposits in Chryse Planitia and vicinity, Mars, J. Geophys. Res., 9d, , Clancy, R. T., S. W. Lee, G. R. Gladstone, W. W. McMillan, and T. Rousch, A new model for Mars atmospheric dust based upon analysis of ultraviolet through infrared observations from Mariner 9, Viking, and Phobos, J. Geophys. Res., I00, , Gierasch, P. J., and R. M. Goody, The effect of dust on the temperature structure of the Martian atmosphere, J. Geophys. Res., 29, , Golombek, M.P., et al., Overview of the Mars Pathfinder mission and assessment of landing site predictions, Science, 278, , Haberle, R. M., C. B. Leovy, and J. B. Pollack, Some effects of global dust storms on the atmospheric circulation of Mars, icarus, 50, , Hansen, J. E., and L. D. Travis, Light scattering in planetary atmospheres, Space Sci. Rev., 16, , Hapke, B., Theory of reflectance and emittance spectroscopy, in Topics in Remote Sensing, Cambridge Univ. Press, New York, Moroz, V. I., E. V. Petrova, and L. V. Ksanfomality, Spectrophotometry of Mars in the KRFM experiment of the Phobos mission: Some properties of the particles of atmospheric aerosols and the surface, Panet. Space Sci., di, , Ockert-Bell, M. E., J.F. Bell III, J. B. Pollack, C. P. McKay, and F. Forget, Absorption and scattering properties of the Martian dust in the solar wavelengths, J. Geophys. Res., I02, , Petrova, E., H. U. Keller, W. J. Markiewicz, N. Thomas, and M. W. Wuttke, Ice hazes and clouds in the Martian atmosphere as derived from the Phobos/KRFM data, Panet. Space Sci., dd, , Pollack, J. B., and J. N. Cuzzi, Scattering by nonspherical particles of size comparable to wavelength- A new semi-empirical theory and its application to tropospheric aerosols, J. Atmos. Sci., 37, , Pollack, J. B., M. E. Ockert-Bell, and M. K. Shepard, Viking Lander image analysis of Martian atmospheric dust, J. Geophys. Res., 100, , Reid, B. J. et al., IMP image calibration, J. Geophys. Res., this issue. Showalter, M. R., J. B. Pollack, M. E. Ockert, L. R. Doyle, and J. B. Dalton, A photometric study of Saturn's F Ring, Icarus, 100, , Smith, P. H., et al., The imager for Mars Pathfinder experiment, J. Geophys. Res., 102, , 1997a. Smith, P. H., et al., Results from the Mars Pathfinder camera, Science, 278, , 1997b. Thomas, N., W. J. Markiewicz, R. M. Sablotny, M. W. Wuttke, H. U. Keller, J. R. Johnson, R. J. Reid and P. H. Smith, The color of the Martian sky and its influence on the illumination of the Martian surface, J. Geophys. Res., this issue. Tomasko, M. G., L. R. Doose, M. Lemmon, P. H. Smith, and E. Wegryn, Properties of dust in the Martian atmosphere from the Imager on Mars Pathfinder, J. Geophys. Res., this issue. Wuttke, M. W., H. U. Keller, W. J. Markiewicz, E. Petrova, K. Richter, and N. Thomas, Properties of dust in the Mars atmosphere: A revised analysis of Phobos/KRFM data, Panet. Space Sci., d5, , H.U. Keller, W.J. Markiewicz, R.M. Sablotny, D. Titov and N. Thomas, Max-Planck-Institut ffir Aeronomie, Max- Planck-Strasse 2, Katlenburg-Lindau, Germany. (markiewicz@linmpi. mpg.de) P.H. Smith, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ (Received March 19, 1998; revised September 24, 1998; accepted November 11, 1998.)