Martin G. Tomasko

Rain, winds and haze during the Huygens probe's descent to Titan's surface

The irreversible conversion of methane into higher hydrocarbons in Titans stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titans atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titans atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.

Properties of dust in the Martian atmosphere from the Imager on Mars Pathfinder

The Imager for Mars Pathfinder (IMP) returned sequences of images of the Martian sky characterizing the size distribution, optical constants, and nature of the aerosols suspended in the atmosphere of Mars. These sequences were executed when the solar elevation angle was approximately 15° and consisted of images near the elevation of the Sun, spanning a range in azimuth from about 4° to 180° from the Sun. Images were obtained at four wavelengths from 444 to 965 nm. From one sequence of observations, results are shown from a comparison of absolute photometry of the Martian sky with multiple scattering models. Results include the following. (1) The geometric cross-section-weighted mean particle radius is 1.6 ± 0.15 μm almost independent of the assumed width (variance) of the size distribution. (2) The imaginary refractive index shows a steep increase with wavelength from 670 nm to shorter wavelengths, and a shallow increase toward longer wavelengths, consistent with the reflection spectrum observed by IMP for Martian soil. (3) For each assumed variance, two parameters governing the slope and curvature of the portion of the phase function due to internally transmitted light are found uniquely as functions of wavelength. (4) The variance of the gamma size distribution is difficult to constrain from these observations alone. The shape of the single scattering phase functions derived from the IMP observations is compared to laboratory measurements of powder samples. One sample of irregular particles has a single scattering phase function quite similar to that derived for Mars. Overall, the results for the mean cross-section-weighted size and imaginary refractive index as a function of wavelength are in remarkably good agreement with the revised analysis by Pollack et al. [1995] of the observations made by the Viking lander 20 years earlier.

Clouds, aerosols, and photochemistry in the Jovian atmosphere

Abstract In this paper we review current ideas about the composition, horizontal and vertical distribution, and microphysical properties of clouds and aerosols in Jupiters upper troposphere and stratosphere. We also discuss several key photochemical species, their relation to aerosol formation, and their implications for transport processes. We treat photochemistry in the context of comparative planetology and point out important similarities and differences among the outer planet atmospheres. Our approach emphasizes observational data of relevance to cloud properties, and to this end we assemble a wide assortment of ground-based and spacecraft observations. We challenge some widely held views about the distribution of clouds in the troposphere and present a rationale for alternative interpretations.

Imaging photopolarimeter on pioneer saturn.

An imaging photopolarimeter aboard Pioneer 11, including a 2.5-centimeter telescope, was used for 2 weeks continuously in August and September 1979 for imaging, photometry, and polarimetry observations of Saturn, its rings, and Titan. A new ring of optical depth < 2 x 10–3 was discovered at 2.33 Saturn radii and is provisionally named the F ring; it is separated from the A ring by the provisionally named Pioneer division. A division between the B and C rings, a gap near the center of the Cassini division, and detail in the A, B, and C rings have been seen; the nomenclature of divisions and gaps is redefined. The width of the Encke gap is 876 � 35 kilometers. The intensity profile and colors are given for the light transmitted by the rings. A mean particle size ≲ 15 meters is indicated; this estimate is model-dependent. The D ring was not seen in any viewing geometry and its existence is doubtful. A satellite, 1979 S 1, was found at 2.53 � 0.01 Saturn radii; the same object was observed ∼ 16 hours later by other experiments on Pioneer 11. The equatorial radius of Saturn is 60,000 � 500 kilometers, and the ratio of the polar to the equatorial radius is 0.912 � 0.006. A sample of polarimetric data is compared with models of the vertical structure of Saturns atmosphere. The variation of the polarization from the center of the disk to the limb in blue light at 88� phase indicates that the density of cloud particles decreases as a function of altitude with a scale height about one-fourth that of the gas. The pressure level at which an optical depth of 1 is reached in the clouds depends on the single-scattering polarizing properties of the clouds; a value similar to that found for the Jovian clouds yields an optical depth of 1 at about 750 millibars.

Photometry and polarimetry of Jupiter at large phase angles: I. Analysis of imaging data of a prominent belt and a zone from pioneer 10

Limb-darkening curves are derived from Pioneer 10 imaging data for Jupiters STrZ (−18 to −21° latitude) and SEBn (−5 to −8° latitude) in red and blue light at phase angles of 12, 23, 34, 109, 120, 127, and 150°. Inhomogeneous scattering models are computed and compared with the data to constrain the vertical structure and the single-scattering phase functions of the belt and the zone in each color. The very high brightness observed at a 150° phase angle seems to require the presence of at lleast a thin layer of reasonably bright and strongly forward-scattering haze particles at pressure levelsof about 100 mbar or less above both belts and zones. Marginally successful models have been constructed in which a moderate optical thickness (τ ≥ 0.5) of haze particles was uniformly distributed in the upper 25 km-amagats of H2. Excellent fits to the data were obtained with models having a thin (optical depths of a few tenths) haze conentraated above most of the gas. Following recent spectrospcopicanalyses, we have placed the main “cloud” layer or layers beneath about 25 km-amagats of H2, although successful fits to our continuum data probably could be achieved also if the clouds were permitted to extend all the way up to the thin haze layer. Similarly, below the haze level our data cannot distinguish between models having two clouds separated by a clear space as suggested by R. E. Danielson and M. G. Tomasko and models with a single extensive diffuse cloud having an H2 abundance of a few kilometer-amagats per scattering mean free path as described by W. D. Cochran. In either case, the relative brightness of the planet at each phase angle primarily serves to constrain the single-scattering phase functions of the Jovian clouds at the corresponding scattering angles. The clouds in these models are characterized by single-scattering phase functions having strong forward peaks and modest backward-scattering peaks, indicating cloud particles with dimensions larger than about 0.6 μm. In our models, a lower single-scattering albedo of the cloud particles in the belt relative to the zone accounts for the contrast between these regions. If an increased abundance of absorbing dust above uniformly bright clouds is used to explain the contrast between belts and zones at visible wavelengths, the limb darkening is steeper than that observed for the SEBn in blue light at small phase angles. The phase integral for the planet calculated for either the belt or the zone model in either color lies in the range 1.2 to 1.3. If a value of 1.25 is used with D.J. Taylors bolometric geometric albedo of 0.28, the planet emits 2.25 or 1.7 times the energy it absorbs from the Sun if it effective temperature is 134 or 125°K, respectively—roughly as expected from current theories of the cooling of Jupiters interior.

Overview of the Mars Pathfinder Mission: Launch through landing, surface operations, data sets, and science results

Mars Pathfinder successfully landed at Ares Vallis on July 4, 1997, deployed and navigated a small rover about 100 m clockwise around the lander, and collected data from three science instruments and ten technology experiments. The mission operated for three months and returned 2.3 Gbits of data, including over 16,500 lander and 550 rover images, 16 chemical analyses of rocks and soil, and 8.5 million individual temperature, pressure and wind measurements. Path-finder is the best known location on Mars, having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high-resolution orbiter images and in inertial space from two-way ranging and Doppler tracking. Tracking of the lander has fixed the spin pole of Mars, determined the precession rate since Viking 20 years ago, and indicates a polar moment of inertia, which constrains a central metallic core to be between 1300 and ∼2000 km in radius. Dark rocks appear to be high in silica and geochemically similar to anorogenic andesites; lighter rocks are richer in sulfur and lower in silica, consistent with being coated with various amounts of dust. Rover and lander images show rocks with a variety of morphologies, fabrics and textures, suggesting a variety of rock types are present. Rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates (although other explanations are also possible), which almost definitely require liquid water to form and a warmer and wetter past. Air-borne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral, interpreted to be most likely maghemite; explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains. Remote sensing data at a scale of a kilometer or greater and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods, which are relatively dust free. The surface appears to have changed little since it formed billions of years ago, with the exception that eolian activity may have deflated the surface by ∼3–7 cm, sculpted wind tails, collected sand into dunes, and eroded ventifacts (fluted and grooved rocks). Pathfinder found a dusty lower atmosphere, early morning water ice clouds, and morning near-surface air temperatures that changed abruptly with time and height. Small scale vortices, interpreted to be dust devils, were observed repeatedly in the afternoon by the meteorology instruments and have been imaged.

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 cameras 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.

Photometry and polarimetry of Titan: Pioneer 11 observations and their implications for aerosol properties

The preliminary measurements by Pioneer 11 of the limb darkening and polarization of Titan at red and blue wavelenghts (M. G. Tomasko, 1980, J. Geophys. Res., 85, 5937–5942) are refined and the measurements of the brightness of the integrated disk at phase angles from 22 to 96° are reduced. At 28° phase, Titans reflectivity in blue light at southern latitudes is as much as 25% greater than that at northern latitudes, comparable to the values observed by Voyager 1 (L. A. Sromovsky et al., 1981, Nature (London), 292, 698–702). In red light the reflectivity is constant to within a few percent for latitudes between 40°S and 60°N. Titans phase coefficient between 22 and 96° phase angle averages about 0.014 magnitudes/degree in both colors—a value considerably greater than that observed at smaller phase from the Earth. Comparisons of the data with vertically homogeneous multiple-scattering models indicate that the single-scattering phase functions of the aerosols in both colors are rather flat at scattering angles between 80 and 150° with a small peak at larger scattering (i.e., small phase) angles. The models indicate that the phase integral, q, for Titan in both red and blue light is about 1.66 ± 0.1. Together with Younkins value for the bolometric geometric albedo scaled to a radius of 2825 km, this implies an effective temperature in equilibrium with sunlight of 84 ± 2°K, in agreement with recent thermal measurements. The single-scattering polarizations produced by the particles at 90° scattering angle are quite large, >85% in blue light and >95% in red. A vertically homogeneous model in which the particles are assumed to scatter as spheres cannot simultaneously match the polarization observations in both colors for any refractive index. However, the observed polarizations are most sensitive to the particle properties near optical depth 12 in each color, and so models based on single scattering by spheres can be successful over a range of refractive indices if the size of the particles increases with depth and if the cross section of the particles increases sufficiently rapidly with decreasing wavelenght. For example, with nr = 1.70, the polarization (and the photometry) are reproduced reasonably well in both colors when the area-weighted average radous of the particles, α, is given by α = (0.117 μm)(τred/0.5)0.217. While this model does not reproduce the large increase in brightness from 129 to 160° phase observed by Voyager 1, the observed increase is determined by the properties of the particles in the top few hundredths of an optical depth. Thus the addition of a very thin layer of forward-scattering aerosols on top of the above model offers one way of satisfying both the Pioneer 11 and Voyager 1 observations. Of course, other models, using bimodal size distributions or scattering by nonspherical particles, may also be capable of reproducing these data.

Laboratory measurements of mineral dust scattering phase function and linear polarization

With the goal of improving our understanding of how small mineral dust particles scatter light at visible and near-infrared wavelengths, we measured the scattering phase function and linear polarization of small mineral dust particles over the scattering angle range 15°–170° at three wavelengths (0.47, 0.652, and 0.937 μm). Particle samples were obtained from Duke Scientific Corp., and include aluminum oxide, silicon carbide, aluminum silicate, antimony oxide, calcium carbonate, and cerium oxide. Particle equivalent-sphere radii range from a few tenths of a micron to about 10 μm. The particles were injected into a laboratory chamber, where they scattered light as they fell through the air. They were collected on a scanning electron micrograph (SEM) substrate. Particle shapes and sizes were then measured from the SEM images. We compare measured phase functions with those calculated for spheroids with a distribution of axial ratios and sizes, random orientation, and refractive index 1.53+0.008i [Mishchenko et al., this issue]. Two of the samples (one of which has a refractive index close to that used in theoretical computations) produced scattering phase functions that were quite similar to those for spheroids. Two samples produced phase functions whose variation between 15° and 170° was much less than that for the spheroids or for the other samples. We suspect this difference may be due to the very high refractive index of those particles, although differences in particle microstructure may also be important. Two samples produced positive linear polarization which had a single broad maximum near 100° scattering angle, and a magnitude greater than 40% at some wavelengths. Two samples had generally positive linear polarization but a more complicated structure, and two samples produced mostly negative polarization whose amplitude was small. We do not have numerical results for the appropriate refractive index and size parameter with which to compare the polarization measurements. We hope the questions raised by this work will stimulate additional effort to develop and test numerical codes for scattering by nonspherical particles.

Photometry and polarimetry of Jupiter at large phase angles. II: polarimetry of the South Tropical Zone, South Equatorial Belt, and the polar regions from the Pioneer 10 and 11 missions

Abstract The imaging photopolarimeter (IPP) experiment on the Pioneer 10 and 11 missions to Jupiter measured the intensity and linear polarization of red and blue sunlight reflected from the planet over a range of phase angles inaccessible from the Earth. We give an overview of the polarization data obtained in the two Jupiter encounters at phase angles from 43° to 117° and briefly describe the photometry data from the Pioneer 11 encounter at phase angles between 34° and 80° which partially fill a gap in the phase coverage from Pioneer 10 ( M. G. Tomasko, R. A. West, and N. D. Castillo, 1978, Icarus 33, 558–592 ). The polarimetry and photometry of the South Tropical Zone (STrZ), the north component of the South Equatorial Belt (SEBn), and a north-south cut extending to the south pole are given in detailed tables. Comparison of the data to multiple-scattering models yields several details of the distribution and single-scattering properties of the clouds and aerosols on Jupiter. The observed polarization in blue light at latitudes less than about 40° shows only small variations between belts and zones. Simple models indicate that the tops of the belt and zone clouds are reached at nearly the same pressure level of about 320 mb and that the polarization differences are a result of the lower cloud albedo in the belt. The optical thickness of the belt as well as the zone clouds at this level must be at least 1.5 to prevent the polarization produced by underlying gas from being seen in the data. The polarization rises abruptly toward the limb and terminator in red light, indicating a haze of positively polarizing particles with an optical thickness of a few tenths at a pressure level of about 120 mb. The polarization in both colors increases abruptly from latitudes north of 40°N and south of 48°S to values as high as 60% at high latitudes. This effect is not due to a longer slant path but must be due to a large increase in the optical thickness of the polarizing haze at high latitudes. There is some indication that the size of the haze aerosols grows with increasing latitude as well. The photometry data indicate little change in the brightness of planetary features in the year between the two Pioneer encounters. Photometric models that fit the Pioneer 10 data fit the Pioneer 11 data remarkably well with essentially the same phase functions. Using a two-cloud model, we find that our models best fit the limb darkening at 12° phase when the belt absorbers are evenly distributed in both the clouds. There is no evidence for rainbow-like bumps on the single-scattering phase functions in the range of scattering angles from 120° to 140° as might result from scattering by spherical particles.