Lander Cameras Research Articles

Chemical analyses of martian soil and rocks obtained by the Pathfinder Alpha Proton X-ray spectrometer

The US Mars Pathfinder spacecraft, which landed on the red planet on the 4th of July 1997, carried an Alpha Proton X-ray Spectrometer (APXS) that obtained the chemical composition of martian soil and rocks. The principles of the APXS operation are based on three interactions of alpha particles with matter: Rutherford alpha backscattering; (α, p) nuclear reactions; and X-ray generation by charged particles and X-ray excitation. The APXS, as was implemented on the Pathfinder mission, uses for all three modes of operation a monoenergetic beam of alpha particles from about 40 mCi of 244Cm radioisotope. It employs Si charged particle detectors for alpha and proton modes and a specially designed silicon PIN detector for its X-ray mode that does not require cooling for its operation. The APXS can detect all of the elements (except H and He) present above a few tenths of a percent for all major elements and several hundred ppm for many minor and trace elements. The APXS on Pathfinder was transported to various locations on the martian surface by the Sojourner rover which enabled it to analyze multiple soil and rock samples selected by the science team from the lander camera images. The APXS performed excellently under the adverse martian environment conditions and provided important information about the chemical composition of the martian soil and rocks. All of the analyzed rocks at the Pathfinder site were found to have high concentrations of silica, sulfur and iron, and low in magnesium, similar to those of the terrestrial basaltic andesites and definitely different from the SNC meteorites that are believed to have originated from Mars. All of the soil samples analyzed by the APXS have similar composition and are very close to the soil analyses obtained by the two Viking missions. The information derived from the Pathfinder APXS has significant implications about the origin and evolution of planet Mars.

The color of Mars: Spectrophotometric measurements at the Pathfinder landing site

We calculate the color of the Martian sky and surface directly using the absolute calibration of the Mars Pathfinder (MPF) lander camera, which was observed to be stable during the mission. The measured colors of the Martian sky and surface at the Pathfinder site are identical to the Viking sites, i.e., a predominantly yellowish brown color with only subtle variations. These colors are distributed continuously and fall into five overlapping groups with distinct average colors and unique spatial characteristics: shadowed soil, soil, soil/rock mixtures, rock, and sky. We report that the primary difference between the sky color and the color of the rocks is due to a difference in brightness. Measurements of the sky color show that the sky reddens away from the Sun and toward the horizon and that the sky color varies with time of day and is reddest at local noon. We present a true color picture of the Martian surface and color enhancement techniques that increase image saturation, maximize color discriminability while preserving hue, and eliminate brightness variations while preserving the chromaticity of the scene. Although Mars has long been called the “red” planet, quantitative measurements of the surface color from telescopic and surface observations indicate a light to moderate yellowish brown color. The Pathfinder camera measurements presented here support the claim that the red planet is not red but indeed yellowish brown.

Preliminary results on photometric properties of materials at the Sagan Memorial Station, Mars

Reflectance measurements of selected rocks and soils over a wide range of illumination geometries obtained by the Imager for Mars Pathfinder (IMP) camera provide constraints on interpretations of the physical and mineralogical nature of geologic materials at the landing site. The data sets consist of (1) three small “photometric spot” subframed scenes, covering phase angles from 20° to 150°; (2) two image strips composed of three subframed images each, located along the antisunrise and antisunset lines (photometric equator), covering phase angles from ∼0° to 155°; and (3) full‐image scenes of the rock “Yogi,” covering phase angles from 48° to 100°. Phase functions extracted from calibrated data exhibit a dominantly backscattering photometric function, consistent with the results from the Viking lander cameras. However, forward scattering behavior does appear at phase angles >140°, particularly for the darker gray rock surfaces. Preliminary efforts using a Hapke scattering model are useful in comparing surface properties of different rock and soil types but are not well constrained, possibly due to the incomplete phase angle availability, uncertainties related to the photometric function of the calibration targets, and/or the competing effects of diffuse and direct lighting. Preliminary interpretations of the derived Hapke parameters suggest that (1) red rocks can be modeled as a mixture of gray rocks with a coating of bright and dark soil or dust, and (2) gray rocks have macroscopically smoother surfaces composed of microscopically homogeneous, clear materials with little internal scattering, which may imply a glass‐like or varnished surface.

Instruments for the Magnetic Properties Experiments on Mars Pathfinder

The paper gives a description of two instruments, the Magnet Array (MA) and the Tip Plate Magnet (TPM), that are part of the Magnetic Properties Experiments on board the U.S. Mars Pathfinder spacecraft, which was launched 4 December 1996, and is expected to land on Mars 4 July 1997. Both instruments consist of permanent magnets of varying strength that are designed to capture airborne dust. The instruments will be imaged by the Lander camera and the pictures transmitted to Earth. The pictures are the data on which conclusions about the magnetic properties of the Martian dust will be based. As an addition to the description of the construction of the instruments, we give a brief discussion of the background for the interpretation of the results of the Magnetic Properties experiment.

The Pathfinder Microrover

An exciting scientific component of the Pathfinder mission is the rover, which will act as a mini‐field geologist by providing us with access to samples for chemical analyses and close‐up images of the Martian surface, performing active experiments to modify the surface and study the results, and exploring the landing site area. Structures, textures, and fabrics revealed in the rover camera images will provide a bounty of geologic information, which can be put in the larger context of geologic features seen in lander camera images. Rover technology experiments will improve our understanding of “soil” properties (grain size, bulk density, friction angle, cohesion, and compressibility) by contributing information not provided by the other Pathfinder instruments. Unplanned opportunities and activities are also likely to provide valuable science information, such as examination of any slumped or eroded soil that has been piled up by the rover wheels, and examination of overturned rocks. Using its ability to actively explore and experiment, the rover will fill in key gaps in our understanding of the landing site.

The imager for Mars Pathfinder experiment

The imager for Mars Pathfinder (IMP), a stereoscopic, multispectral camera, is described in terms of its capabilities for studying the Martian environment. The camera's two eyes, separated by 15.0 cm, provide the camera with range‐finding ability. Each eye illuminates half of a single CCD detector with a field of view of 14.4×14.0° and has 12 selectable filters. The ƒ/18 optics have a large depth of field, and no focussing mechanism is required; a mechanical shutter is avoided by using the frame transfer capability of the 512×512 CCD. The resolving power of the camera, 0.98 mrad/pixel, is approximately the same as the Viking Lander cameras; however, the signal‐to‐noise ratio for IMP greatly exceeds Viking, approaching 350. This feature along with the stable calibration of the filters between 440 and 1000 nm distinguishes IMP from Viking. Specially designed targets are positioned on the Lander; they provide information on the magnetic properties of wind‐blown dust, measure the wind vectors, and provide radiometric standard reflectors for calibration. Also, eight low‐transmission filters are included for imaging the Sun directly at multiple wavelengths, giving IMP the ability to measure dust opacity and potentially the water vapor content. Several experiments beyond the requisite color panorama are described in detail: contour mapping of the local terrain, multispectral imaging of the surrounding rock and soil to study local mineralogy, viewing of three wind socks, measuring atmospheric opacity and water vapor content, and estimating the magnetic properties of wind‐blown dust. This paper is intended to serve as a guide to understanding the scientific integrity of the IMP data that will be returned from Mars starting on July 4, 1997.

Diurnal variations in optical depth at Mars

Viking lander camera images of the Sun were used to compute atmospheric optical depth at the two sites over a period of 1 1 3 Martian years. The complete set of 1044 optical depth determinations is presented, with error estimates. Optical depths in the morning ( AM) are generally larger than in the afternoon ( PM). The AM-PM differences are ascribed to condensation of water into atmospheric ice aerosols at night and their evaporation in midday. A smoothed time series of these differences shows several seasonal peaks. These are simulated using a one-dimensional radiative-convective model that predicts Martian atmospheric temperature profiles. A calculation combining these profiles with water vapor measurement from the Mars atmospheric water detector (MAWD, on the Viking orbiters)is used to predict when diurnal variations of water condensation should occur. The model reproduces a majority of the observed peaks and shows the factors influencing the process. Diurnal variation of condensation is shown to peak when the latitude and season combine to warm the atmosphere to the optimum temperature, cool enough to condense vapor at night, and warm enough to cause evaporation at midday. The diurnal variation is enhanced by increased water vapor and is sometimes enhanced, sometimes diminished, by enhanced dust loading, depending on the other conditions. Often the model predicts condensation only at altitudes of 25 km or more, while at other times the condensation reaches ground level. Agreement between model and observations is also evident on a time scale of hours, when the data are available at more than two times in a single day.

On The spectral reflectance properties of materials exposed at the Viking landing sites

Bidirectional reflectances, calibrated to within 10% uncertainties, for blue (0.40–0.52 μm), green (0.50–0.59 μm), and red (0.60–0.74 μm) channels were determined for 31 block and soil exposures imaged by the Viking Lander Cameras. Reflectances were corrected for atmospheric attenuation by using optical depth measurements obtained by the Lander Cameras and for skylight illumination by subtracting the brightness values of adjacent shadowed areas. Reflectances for two facets on each of two large blocks were computed for a common geometry by determining facet orientations from high resolution Lander stereo images and by fitting the Hapke‐Irvine photometric function to reflectances estimated for several geometries. Results indicate that individual facets on the same block have different optical properties. The darkest, grayest block facets have properties similar to laboratory reflectance spectra of mafic rocks that are thinly coated with ferric iron‐rich palagonite. Most block surfaces have reflectances consistent with thin to optically thick covers of palagonitic material. Limited exposures of soil have spectra that are also similar to palagonitic material. The majority of the soils sampled and a few block surfaces have spectra with steeper slopes and greater spectral curvatures than the palagonite analog, consistent with greater degree of ferric iron crystallinity. The soils could be an intimate mixture of palagonitic material and other materials not seen in end member form, but, if so, these other materials would need an even greater slope and curvature than observed soils. The differences in reflectance properties for common block surfaces compared to common soils imply that local weathering of blocks has not contributed significantly to the soil exposed at the landing sites.

Spectral properties (0.40 to 0.75 microns) of soils exposed at the Viking 1 landing site

The bidirectional reflectance and photometric function (Hapke, 1981) was determined for seven patches of soil located near the Viking Lander 1 spacecraft. The soil photometric function is strongly backscattering and has a prominent opposition effect such that the ratio of reflectances at 1° and 10° phase angles averages 1.25, 1.24, and 1.19 in blue, green, and red wavelengths, respectively. The reflectance of the soil also exhibits a wavelength dependence as a function of phase angle. For instance, red/blue ratios can vary by up to 33% as the phase angle increases from 10° to 75°. Estimates of soil reflectance at a 5° phase angle averaged over the blue and green passbands of the Viking Lander cameras are 0.11 and 0.17, respectively, while estimates of soil reflectance averaged over the red channel range from 0.30 to 0.39. There is little need to call upon mineralogical variations to explain the subtle color variations exposed at the landing sites. Rather, brightness and color variations within the soil can be correlated with particle size, with finer‐grained soil being brighter and redder than coarser‐grained soil. When compared to earth‐based reflectance data, the soil at the Viking 1 site is most like Martian bright areas. Such a result is consistent with the Lander soil being part of a globally homogenized soil unit.

The Martian twilight

The changing sky brightness during the Martian twilight as measured by the Viking lander cameras is shown to be consistent with data obtained from sky brightness measurements. An exponential distribution of dust with a scale height of 10 km, equal to the atmospheric scale height, is consistent with the shape of the light curve. Multiple scattering resulting from the forward scattering peak of large particles makes a major contribution to the intensity of the twilight. The spectral distribution of light in the twilight sky may require slightly different optical properties for the scattering particles at high levels from those of the aerosol at lower levels.

Properties and effects of dust particles suspended in the Martian atmosphere

Direct measurements of the optical depth above the two Viking landers are reported for a period covering the summer, fall, and winter seasons in the northern hemisphere, a time period during which two global dust storms occurred. The optical depth had a value of about 1 just before the onset of each storm; it increased very rapidly, on a time scale of a few days, to peak values of about 3 and 6 with the arrival of the first and second storms, respectively; and it steadily decreased shortly thereafter (≳ few days to few weeks) for both storms, with the decay occurring more rapidly during the initial period of decay. We have also carried out further analyses of observations of the sky brightness made with the lander cameras during the summer season to obtain improved estimates of other dust particle parameters, including the cross section weighted mean particle radius, several shape factors, and the imaginary indices of refraction. These results have been used to define the radiative properties of the suspended dust particles at solar wavelengths. Particle properties inferred by Toon et al. (1977) from their study of the Mariner 9 Iris data were employed to define the radiative properties of the dust at thermal wavelengths. In an effort to understand the effect of dust content on atmospheric temperatures and winds, we have incorporated the derived radiative properties of the dust into a 1 D radiative convective model. When only radiation and thermal convection are taken into account, these calculations fail to reproduce the temperature structure determined during the descent of the landers to the surface. However, satisfactory agreement is achieved when allowance is made for the effects of vertical motions induced by large scale atmospheric dynamics. The sign and magnitude of the required vertical velocities are crudely consistent with those found by Pollack et al. (1976a) in their general circulation calculations. The diurnal temperature variations predicted by the 1 D calculations for the observed optical depths are also in crude agreement with values inferred from orbiter and lander measurements. The 1 D model predicts that the diurnal temperature change and daily mean temperature, averaged over the entire atmospheric vertical column, steadily increase as the optical depth of the dust increases to a value of several, and then subsequently change little. Thus, for small and moderate values of optical depth, there is a positive feedback between atmospheric dust content and both tidal and seasonal winds, but little feedback occurs for very large values of optical depth. A negative feedback exists between dust content and thermally driven topographic winds. The occurrence of these feedback effects are supported by several Viking observations. It is suggested that they play a role in the generation of certain types of local dust storms, in the development of local dust storms into global ones, and in the decay of global storms. At present, dust is being preferentially deposited in the north polar region due to scavanging of dust by CO2 ice precipitation. Using the results of this paper and those of Pollack et al. (1977), we obtain an estimate of the sedimentation rate of dust and water ice in the polar regions. These results imply a time scale of about 105 years to form individual laminae in the polar laminated terrain, a result consistent with astronomical theories of their origin. Analogous calculations imply that equatorial and mid‐latitude regions are being eroded at a rate of about 7 m per million years.

Color and feature changes at Mars Viking lander site

Analysis of three component color pictures taken by the Viking lander camera on Mars has established color differences for the background material, the rocks and spots on the rocks. Changes in the location of greenish rock patches and ground patterns have been observed over time. A combination of wind movement of dust and dirt dropped by sampler arm operations could have produced the slight changes in pattern and position. However, the observed patches, patterns and changes could also be attributable to biological activity. Analysis of six component color data on the same scene confirms the observations including the greenish color of the rock patches.

Multicolor Observations of Phobos with the Viking Lander Cameras: Evidence for a Carbonaceous Chondritic Composition

The reflectivity of Phobos has been determined in the spectral region from 0.4 to 1.1 micrometers from images taken with a Viking lander camera. The reflectivity curve is flat in this spectral interval and the geometric albedo equals 0.05 +/- 0.01. These results, together with Phobos's reflectivity spectrum in the ultraviolet, are compared with laboratory spectra of carbonaceous chondrites and basalts. The spectra of carbonaceous chondrites are consistent with the observations, whereas the basalt spectra are not. These findings raise the possibility that Phobos may be a captured object rather than a natural satellite of Mars.

Calibration and performance of the Viking Lander cameras

The paper discusses Viking lander cameras which have an angular resolution of 0.12 deg, and (for broadband imaging) a resolution of 0.04 deg, used for the acquisition of data in six spectral bands for color and near-infrared imaging. Attention is given to photogrammetric calibration techniques used in the determination of spatial and spectral brightness variations from image data. The effects of sampling on the achievable photogrammetric precision are described along with techniques for preflight spectral calibrations and the corrections required for degradation in the infrared response of the detectors. The effects of the known internal reflections on the qualitative images (such as the appearance of artifact clouds) are presented, noting their effects on skyline radiometry. The qualitative and quantitative effects of the signal quantization are briefly reviewed.

Spectrophotometric and color estimates of the Viking Lander sites

The spectral radiance and color of the Martian sky and soil and the spectral reflectance of soil features are estimated from six-channel (0.4-1.0 micron) spectral data obtained with the Viking lander cameras. Images taken near local noon from the two landers reveal a sky that is brighter near the horizon than the soil but with a similar spectral radiance shape and color. The scenes are predominantly moderate yellowish brown in color with only subtle variations except for some dark grey rocks. Most spectral reflectance estimates are similar: they rise rapidly with increasing wavelength between 0.4 and 0.8 micron and with only a few exceptions exhibit a pronounced minimum centered about 0.93 micron. These characteristics are consistent with an abundance of Fe(3+)-rich weathering products, notably nontronite. However, the delineation of the number and abundances of total mineral phases requires further analyses and laboratory comparisons. Reflectance estimates for rocks have not been repeatable, probably because most rocks have irregular pitted surfaces that introduce significant shadowing components.

Lander imaging as a detector of life on Mars

Biological goals were among the important science objectives of the Viking lander camera. The camera performance characteristics relevant to these goals are discussed. They include the ability to observe (1) morphological detail, (2) color and reflectance spectra, and (3) motion and change. The scenes obtained by the cameras were scrutinized in many ways: monoscopically, stereoscopically, in color, and by computerized differencing of camera events. At the lander sites and during the times that observations were carried out on the surface of Mars, no evidence, direct or indirect, has been obtained for macroscopic biology on Mars. No obvious examples of geometric distortion that might have been motion induced have been observed. Using the repeated line scanning mode of the camera has revealed no changes or motion suggesting life. These negative results may be due to limitations in sampling, in camera design, or in our understanding of Martian biology, but they are certainly consistent with the hypothesis that macroscopic life is absent on Mars.

Particle motion on Mars inferred from the Viking Lander cameras

The cameras of the Viking landers have uncovered several lines of evidence for fine particle mobility on the Martian surface, including particulate drifts, rock-associated raised streaks, and probable ventifacts. Inferred peak wind directions in both Chryse and Utopia are roughly the same and are consistent with peak winds inferred by orbiter photography. A 24° systematic offset between the direction of rock-associated streaks in the Viking 1 landing site and Mariner 9 and Viking observations of crater-associated streaks is consistent in both sign and magnitude with a Coriolis acceleration of particles entrained by high-velocity winds in the course of the production of crater-associated streaks. If a significant fraction of the impact energy upon collision goes into deformation, strain, and rupture, there should be a preferential destruction of the most easily saltated grains, which are here called kamikaze particles, and a depletion of 150-μm-diameter grains. Observations of fine particulates dumped on the VL-1 grid indicate that major saltation events occurred between sols 96 and 207 and were caused by winds of >50 m s−1, normalized to the top of the velocity boundary layer. This is the first observation of saltation on another planet and a rough confirmation of the usual Bagnold saltation theory applied to another planet.

Quasi-microscope concept for planetary missions

Viking lander cameras have returned stereo and multispectral views of the Martian surface with a resolution that approaches 2 mm/lp in the near field. A two-orders-of-magnitude increase in resolution could be obtained for collected surface samples by augmenting these cameras with auxiliary optics that would neither impose special camera design requirements nor limit the cameras field of view of the terrain. Quasi-microscope images would provide valuable data on the physical and chemical characteristics of planetary regoliths.

Mirages on Mars

The possibility of observing mirages on Mars from the Viking lander cameras is examined. A simple model for the production of both inferior and superior mirages is developed. Assuming the atmospheric index of refraction to be a linear function of density (i.e., temperature), ray curvatures are calculated through layers of large, expected thermal gradient. Assuming the Martian morning inversions of Gierasch and Goody (1968), calculations of ray curvature show the superior mirage to be an unlikely occurrence on Mars since the downward curvature of the ray through the inversion layer is less than the downward curvature of the planet. In order to examine the nature of inferior mirages we select a reasonable expression for temperature profile in the surface layer fitted to the midafternoon, midlatitude summer results of Gierasch and Goody. Integration of the expression for ray curvature yields a relation for the minimum distance between the lander cameras and an inferior mirage as a function of the surface superadiabatic lapse rate. Such calculations indicate that the Viking lander cameras will record inferior mirages at horizontal distances of a kilometer or so from the lander. Given the appearance of an inferior mirage at a measured minimum distance from the observer it should be a simple matter to calculate the corresponding mean temperature lapse rate at the surface.

The Environs of Viking 2 Lander

Forty-six days after Viking 1 landed, Viking 2 landed in Utopia Planitia, about 6500 kilometers away from the landing site of Viking 1. Images show that in the immediate vicinity of the Viking 2 landing site the surface is covered with rocks, some of which are partially buried, and fine-grained materials. The surface sampler, the lander cameras, engineering sensors, and some data from the other lander experiments were used to investigate the properties of the surface. Lander 2 has a more homogeneous surface, more coarse-grained material, an extensive crust, small rocks or clods which seem to be difficult to collect, and more extensive erosion by the retro-engine exhaust gases than lander 1. A report on the physical properties of the martian surface based on data obtained through sol 58 on Viking 2 and a brief description of activities on Viking 1 after sol 36 are given.