Engineering topics
Publications and source records attributed to Tomasko, M. G..
The Huygens Probe descended through Titan s atmosphere on January 14, 2005. The Descent Imager/Spectral Radiometer (DISR) instrument made optical measurements which constrain the nature and vertical distribution and of the aerosols in the atmosphere.
The Cassini-Huygens mission was launched on October 15, 1997, and arrived in Orbit around Saturn in July, 2004. The Huygens Probe was released from the Cassini Orbiter on December 24, 2004 and entered Titan s atmosphere on January 14, 2005. Here we give the first results from the Descent Imager/Spectral Radiometer (DISR) instrument aboard the Huygens Probe during its descent into the atmosphere of Titan. Measurements were made by several different optical systems and sensors.
The Galileo probe net flux radiometer (NFR) measured radiation fluxes in Jupiter's atmosphere from about 0.44 to 14 bars, using five spectral channels to separate solar and thermal components. Onboard calibration results confirm that the NFR responded to radiation approximately as expected. NFR channels also responded to a superimposed thermal perturbation, which can be approximately removed using blind channel measurements and physical constraints. Evidence for the expected NH3 cloud was seen in the spectral character of spin-induced modulations of the direct solar beam signals. These results are consistent with an overlying cloud of small NH3 ice particles (0.5-0.75 microns in radius) of optical depth 1.5-2 at 0.5 microns. Such a cloud would have so little effect on thermal fluxes that NFR thermal channels provide no additional constraints on its properties. However, evidence for heating near 0.45 bar in the NFR thermal channels would seem to require either an additional opacity source beyond this small-particle cloud, implying a heterogeneous cloud structure to avoid conflicts with solar modulation results, or a change in temperature lapse rate just above the probe measurements. The large thermal flux levels imply water vapor mixing ratios that are only 6% of solar at 10 bars, but possibly increasing with depth, and significantly subsaturated ammonia at pressures less than 3 bars. If deep NH3 mixing ratios at the probe entry site are 3-4 times ground-based inferences, as suggested by probe radio signal attenuation, then only half as much water is needed to match NFR observations. No evidence of a water cloud was seen near the 5-bar level. The 5-microns thermal channel detected the presumed NH4SH cloud base near 1.35 bars. Effects of this cloud were also seen in the solar channel upflux measurements but not in the solar net fluxes, implying that the cloud is a conservative scatterer of sunlight. The minor thermal signature of this cloud is compatible with particle radii near 3 gm, but it cannot rule out smaller particles. Deeper than about 3 bars, solar channels indicate unexpectedly large absorption of sunlight at wavelengths longer than 0.6 microns, which might be due to unaccounted-for absorption by NH3 between 0.65 and 1.5 microns.
The Galileo probe net flux radiometer (NFR) measured radiation fluxes in Jupiter's atmosphere from about 0.44 to 14 bars, using five spectral channels to separate solar and thermal components. Onboard calibration results confirm that the NFR responded to radiation approximately as expected. NFR channels also responded to a superimposed thermal perturbation, which can be approximately removed using blind channel measurements and physical constraints. Evidence for the expected NH3 cloud was seen in the spectral character of spin-induced modulations of the direct solar beam signals. These results are consistent with an overlying cloud of small NH3 ice particles (0.5-0.75 microns in radius) of optical depth 1.5-2 at 0.5 microns. Such a cloud would have so little effect on thermal fluxes that NFR thermal channels provide no additional constraints on its properties. However, evidence for heating near 0.45 bar in the NFR thermal channels would seem to require either an additional opacity source beyond this small-particle cloud, implying a heterogeneous-cloud structure to avoid conflicts with solar modulation results, or a change in temperature lapse rate just above the probe measurements. The large thermal flux levels imply water vapor mixing ratios that are only 6% of solar at 10 bars, but possibly increasing with depth, and significantly subsaturated ammonia at pressures less than 3 bars. If deep NH3 mixing ratios at the probe entry site are 3-4 times ground-based inferences, as suggested by probe radio signal attenuation, then only half as much water is needed to match NFR observations. No evidence of a water cloud was seen near the 5-bar level. The 5 microns thermal channel detected the presumed NH4SH cloud base near 1.35 bars. Effects of this cloud were also seen in the solar channel upflux measurements but not in the solar net fluxes, implying that the cloud is a conservative scatterer of sunlight. The minor thermal signature of this cloud is compatible with particle radii near 3 microns, but it cannot rule out smaller particles. Deeper than about 3 bars, solar channels indicate unexpectedly large absorption of sunlight at wavelengths longer than 0.6 microns, which might be due to unaccounted-for absorption by NH3 between 0.65 and 1.5 microns.
Mars pathfinder successfully landed at Ares Vallis on July 4, 1997 and returned 2.3 Gbits of information during 3 months of surface operations of a lander, rover, three science instruments, engineering sensors and technology experiments.
The plan of the NFR (Net Flux of Radiation) team is for the data from the two solar channels (B and E) of NFR to be reduced with the goal of determining the solar heating rate. In order to determine the solar heating rate from the NFR measurements, effects due to the instrument's spatial and spectral response functions, to the temperature variation of the instrument (and associated drift of calibration), to the setting sun, and to the rotation of the probe (initially at a rate comparable to the NFR sampling frequency), all must be well modelled. In the past year, a forward modeling routine was created to simulate NFR data return in the B and E channels. The effects of varying parameters describing the atmospheric model (such as cloud location and thickness) and the descent profile (such as rotation rate) were investigated and an inversion routine was developed. For the forward modeling, existing radiative transfer codes were used to determine intensity fields within the Jovian atmosphere. A routine was developed to determine instantaneous instrument response by integrating the intensity field over the instrument response functions. A second routine was developed to determine the actual output of the NFR by integrating along an arbitrary descent trajectory. Near the top of the atmosphere, the upflux data alone are used to constrain the cloud structure of he atmosphere. To accomplish this, models are used to describe the variation in up flux between consecutive measurements in terms of variations of cloud opacity and variations in known parameters such as the solar zenith angle. This allows us to develop a zero-order model of cloud structure. Lower in the atmosphere, at levels where there is little or no azimuthal structure to the net flux measurements, both the up flux and net flux are used to derive layer transmission and reflection functions, which then determine layer opacity and single scattering albedo. A preliminary analysis of the data began in December 1995. In these data we could see the rapid oscillations expected at the beginning of the data due to probe rotation and the sun passing through the edge of the field of view. In addition, the time when this oscillation stopped was clearly visible. This sets the rough optical depth above the probe at this time.
With the goal of improving our understanding of how small mineral dust particles scatter light at visible and near-infrared wafelengths we measured the scattering phase function and linear polarization of small mineral dust particles over the scattering angle range 15 to 170 at three wavelengths (0.47, 0.652,and 0.937 m).
A comparison of images of Titan obtained by the HST in August, 1990 with Voyager 1 and 2 images respectively obtained 10 and 9 years earlier has indicated a reversal of the seasonal hemispheric brightness asymmetry near 440 and 550 nm wavelengths; the northern hemisphere is in the more recent observations the brighter of the two, by about 10 percent. Titan's albedo pattern is therefore adequately explained by a seasonal model.
Transmission measurements made on near-infrared laboratory methane spectra have previously been fit using a Malkmus band model. The laboratory spectra were obtained in three groups at temperatures averaging 112, 188, and 295 K; band model fitting was done separately for each temperature group. These band model parameters cannot be used directly in scattering atmosphere model computations, so an exponential sum model is being developed which includes pressure and temperature fitting parameters. The goal is to obtain model parameters by least square fits at 10/cm intervals from 3800 to 9100/cm. These results will be useful in the interpretation of current planetary spectra and also NIMS spectra of Jupiter anticipated from the Galileo mission.
A line at 1280 A, due to Raman scattering of solar Lyman alpha (Ly-alpha) in the atmosphere of Uranus, has been detected by the Voyager Ultraviolet Spectrometer. The measured intensity of 40 + or - 20 R implies that 200 R to 500 R of the measured 1500 R Ly-alpha intensity at the subsolar point is due to Rayleigh scattering of the solar line. The presence of Rayleigh and Raman scattering at 1216 A suggests that the Uranian atmosphere is largely devoid of absorbing hydrocarbons above the 0.5 mbar level. The most natural explanation of this depletion is very weak vertical mixing equivalent to an eddy coefficient on the order of 200 sq cm/sec between 0.5 mbar and 100 mbar.
An observational program was established in 1983 to monitor the spatial and temporal variations in the Jovian atmosphere over short and long time scales. The program involves tracking several different longitudes as they rotate around the planet from one limb to another. This tracking experiment was done at many different wavelengths including the 3-0 S(1) and S(0) hydrogen quadrupole lines as well as several broad band methane absorptions. The June 1983 hydrogen quadrupole data was reduced and equivalent widths were measured for approximately 25 east-west positions across the planet at 7 different latitudes for both wavelengths. The data for the South Tropical Zone (20 deg. S) was modeled extensively and the effects of the various model parameters on the value of the calculated equivalent widths of both lines was measured as a longitude rotated from the east (or morning) limb to the west (or evening) limb. The value of the equivalent width is also quite sensitive to the height of the NH3 cloud top and to the value used for the single scattering albedo. A combination of these parameters changing on a diurnal time scale could also explain these observations. This gradual increase from one limb to the other appears in the data for both the North and South Equatorial Belts as well as the equatorial region and the North Tropical Zone. Models that used only normal hydrogen and models that used only equilibrium hydrogen were studied.
The absolute reflectivity of Jupiter has been obtained in 50 A-wide regions centering on 0.221, 0.233, 0.252, and 0.330 microns from three series of IUE satellite spectra taken in November 1979. The data indicate a strong decrease in reflectivity for latitudes greater than about 30 deg, in keeping with Voyager measurements. An additional 24 spectra were also obtained in a west-east series along the equator, as well as near 40 deg N latitude. These data favor models in which the haze particles have effective radii within a factor of 2 of 0.2 microns. Near the equator, the haze aerosols produce much less absorption than near 40 deg N; the aerosol distributions and optical properties derived are noted to be more dependent on the assumed location and reflectivity of the top of the tropospheric cloud.
The determination of the single scattering phase functions of Jupiter's clouds and a thin upper haze by Tomasko et al. was refined and extended to seven latitudes in blue and red light. The phase function is well-constrained by the Pioneer 10 and 11 photometric data sets. Multiple scattering models were computed to match the limb darkening at each latitude at up to 15 phase angles from 12 deg to 151 deg. Ground-based observations were used for absolute calibration and to extend the data to lower phase angles. The phase functions were parameterized using the double Henyey-Greenstein function. The three Henyey-Greenstein parameters and the single scattering albedo were determined using a non-linear least squares method for the haze and the clouds below. The phase functions derived for the northen zone and belt are remarkably similar to the phase functions of the corresponding regions in the south, with most of the differences in brightness of the northern and southern features resulting from minor differences in single scattering albedo. Analysis of the Equatorial Region is complicated by the presence of numerous small features, but the phase function required is generally similar to that seen in the more homogeneous regions. Details of the phase functions of the haze and clouds are presented, and the differences between the cloud phase functions at low and high latitudes in red and blue light are discussed.
An assessment is made of the development status of concepts for cloud and aerosol compositions, vertical and horizontal distributions, and microphysical properties, in the Jovian upper troposphere and stratosphere. Attention is given to several key photochemical species' relationships to aerosol formation as well as their transport process implications, treating photochemistry in the context of comparative planetology and noting differences and similarities among the outer planet atmospheres; since this approach emphasizes observational data, a variegated assortment of ground-based and spacecraft observations is assembled. Current views on the tropospheric distribution of clouds are challenged, and a rationale is presented for alternative accounts.
The paper presents a summary of the data currently available (June 1984) describing the planet-enshrouding particulate matter in the Venus atmosphere. A description and discussion of the state of knowledge of the Venus clouds and hazes precedes the tables and plots. The tabular material includes a precis of upper haze and cloud-top properties, parameters for model-size distributions for particles and particulate layers, and columnar masses and mass loadings.
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The physical properties and spatial distribution of aerosol and cloud particles in Saturn's atmosphere are discussed based on data from remote measurement of scattered solar radiation and thermal radiation emitted from Saturn's atmosphere. A brief overview of the relations between particle properties and the scattered and emitted radiation field is given, and observations of Saturn at wavelengths from the ultraviolet to the thermal infrared which bear on the atmosphere's aerosol and cloud properties are reviewed. The single-scattering properties of the particles deduced from observations are discussed, commenting on the possible composition of the particles. The implications of the observations for the vertical and horizontal distribution of the clouds and aerosols are reviewed, and current information and areas for further research on the clouds and aerosols are summarized.