Engineering topics

Samuelson, Robert E.

Publications and source records attributed to Samuelson, Robert E..

8 records

NASA NTRS2019

Cassini Composite Infrared Spectrometer (CIRS) Observations of Titan 2004-2017

From 2004 to 2017, the Cassini spacecraft orbited Saturn, completing 127 close ﬂybys of its largest moon, Titan. Cassini’s Composite Infrared Spectrometer (CIRS), one of 12 instruments carried on board, proﬁled Titan in the thermal infrared (7–1000 μm) throughout the entire 13 yr mission. CIRS observed on both targeted encounters (ﬂybys) and more distant opportunities, collecting 8.4 million spectra from 837 individual Titan observations over 3633 hr. Observations of multiple types were made throughout the mission, building up a vast mosaic picture of Titan’s atmospheric state across spatial and temporal domains. This paper provides a guide to these observations, describing each type and chronicling its occurrences and global-seasonal coverage. The purpose is to provide a resource for future users of the CIRS data set, as well as those seeking to put existing CIRS publications into the overall context of the mission, and to facilitate future intercomparison of CIRS results with those of other Cassini instruments and ground-based observations.

Nixon, Conor A.↗

NASA NTRS2011

Titan's Aerosol and Stratospheric Ice Opacities Between 18 and 500 Micrometers: Vertical and Spectral Characteristics from Cassini CIRS

Vertical distributions and spectral characteristics of Titan's photochemical aerosol and stratospheric ices are determined between 20 and 560 per centimeter (500-18 micrometers) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15 N, 15 S, and 58 S, where accurate temperature profiles can be independently determined. In addition, estimates of aerosol and ice abundances at 62 N relative to those at 15 S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are approximately 3 times more abundant at 62 N than at 15 S. Generally, nitrile ice clouds (probably HCN and HC3N), as inferred from a composite emission feature at approximately 160 per centimeter, appear to be located over a narrow altitude range in the stratosphere centered at approximately 90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58 S. There is some evidence of a second ice cloud layer at approximately 60 km altitude at 58 S associated with an emission feature at approximately 80 per centimeter. We speculate that the identify of this cloud may be due to C2H6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH4) in the stratosphere of Titan. Unlike the highly restricted range of altitudes (50-100 km) associated with organic condensate clouds, Titan's photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15 N and 58 S latitude. The ratio of aerosol-to-gas scale heights range from 1.3-2.4 at about 160 km to 1.1-1.4 at 300 km, although there is considerable variability with latitude, The aerosol exhibits a very broad emission feature peaking at approximately 140 per centimeter. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics. Finally, volume extinction coefficients N chi EPSILON derived from 15 S CIRS data at a wavelength of lambda = 62.5 micrometers are compared with those derived from the 10 S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 micrometers. This comparison yields volume extinction coefficient ratios N chi EPSILON (1.583 micrometers)/N chi EPSILON (62.5 micrometers) of roughly 70 and 20, respectively, for Titan's aerosol and stratospheric ices, The inferred particle cross-section ratios chi EPSILON(1.583 micrometers)/chi EPSILON (62.5 micrometers) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.

Anderson, Carrie M.↗

NASA NTRS1999

A Heuristic Approach to Examining Volatile Equilibrium at Titan's Surface

R. D. Lorenz, J. I. Lunine, and C. P. McKay have shown in a manuscript accepted for publication that, for a given ethane abundance and surface temperature, the nitrogen and methane abundances in Titan's atmosphere can be calculated, yielding a surface pressure that can be compared with the observed value. This is potentially a very valuable tool for examining the evolution of Titan's climatology. Its validity does depend on two important assumptions, however: 1) that the atmosphere of Titan is in global radiative equilibrium, and 2) that volatiles present are in vapor equilibrium with the surface. The former assumption has been shown to be likely, but the latter has not. Water vapor in the Earth's atmosphere, in fact, is generally not very close to equilibrium in a global sense. In the present work a heuristic approach is used to examine the likelihood that methane vapor is in equilibrium with Titan's surface. Plausible climate scenerios are examined that are consistent with methane vapor abundances derived from Voyager IRIS data. Simple precipitation and surface diffusion models are incorporated into the analysis. It is tentatively inferred that methane may be in surface equilibrium near the poles, but that equilibrium at low latitudes is more difficult to establish.

Samuelson, Robert E.↗

NASA NTRS1997

Gaseous abundances and methane supersaturation in Titan's troposphere

Various properties of Titan's troposphere are inferred from an analysis of Voyager I infrared spectrometer (IRIS) data between 200 and 600/ cm. Two homogeneous spectral averages acquired at widely separated emission angles are chosen for the analysis. Both data sets are associated with northern low latitudes very close to that of the radio science ingress occultation point. Solutions require simultaneous nonlinear least-squares fits to the two IRIS data sets, coupled with iteration of the radio occultation refractivity data. Values and associated 1-sigma uncertainties of several parameters are inferred from our analysis. These include mole fractions for molecular hydrogen (approx. 0.0011), argon (small), and methane near the surface ( approx. 0.057). Solutions are also obtained for the hydrogen parafraction (close to equilibrium, with considerable uncertainty), air temperature near the surface ( approx. 93 K), surface surface temperature discontinuity (approx. 1 K), and maximum degree of methane supersaturation in the upper troposphere (approx. 1.5). Actual values for the above-mentioned parameters depend on the amount of ethane cloud near the tropopause. There is no evidence for methane clouds in the upper troposphere, nor is their presence compatible with large degrees of supersaturation. A wave number dependence for the stratospheric haze opacity is inferred similar to that found for a polymeric residue created in laboratory discharge experiments. This haze appears to be uniformly distributed with latitude between altitudes of 40 and 160 km, provided those nighttime data at southern high latitudes that are discounted.

Samuelson, Robert E.↗

NASA NTRS1992

Infrared properties of Titan's clouds and aerosols

Using Voyager 1 Infrared Interferometer Spectrometer (IRIS) data, the infrared properties of Titan's north polar hood continuum were deduced. Stratospheric opacities were found to increase monotonically with wavenumber between 250 and 600/cm. The increase becomes more extreme with decreasing altitude; this behavior is consistent with a photochemical aerosol overlying layers of condensated organics lower down. Stratospheric optical thicknesses of order 0.1 were found at 600/cm, which became smaller as the wavelength decreased. An aerosol mass mixing ratio approximately 6 x 10 exp -8 and a mass flux approximately 10 exp -14 g/sq cm/s was inferred at 160 km. The column abundance for condensed C4N2 was found to exceed that for the vapor by a factor of 30 or more. An upper limit of 4 x 10 exp -10 was inferred for the mole fraction of gaseous C4N2.

Samuelson, Robert E.↗

NASA NTRS1992

Cassini/CIRS capabilities for aerosol, cloud, and surface measurements

Information that should be revealed by the Cassini Composite Infrared Spectrometer (CIRS) about the aerosol, cloud, and surface properties of Titan are addressed. Limb sounding data will be used to determine aerosol abundances, scale heights, and gradients between 80 and 400 km for various latitudes. Stratospheric condensate cloud top altitudes and column abundances will be inferred as functions of latitude. A search for new species will be conducted. Thermal maps between 500 and 550/cm will be used to investigate tropospheric methane clouds and surface topography; time resolution provided by different orbits will be used to distinguish the two.

Samuelson, Robert E.↗

NASA NTRS1991

Thermal infrared properties of Titan's stratospheric aerosol

The monotonic increase with wavenumber of stratospheric IR opacities in the north polar neighborhood of Titan, in the 250-600/cm bandpass, becomes more extreme with decreasing altitude, consistent with an opacity that is primarily dependent on a high altitude photochemical aerosol, on the one hand, but which is moderated by condensed organics at lower altitudes. Because condensates exhibit a wavenumber dependence for opacity which is the opposite of that required, it is suggested that condensed nitriles may be prominent; either of these condensates may be acceptable for sufficiently large particles. While comparisons with low-latitude data indicate no clear dependence of vertically-integrated opacity on latitude above 80 km, indications of a particle size and/or composition gradient with latitude in the lower stratosphere are noted.

Samuelson, Robert E.↗

NASA NTRS1986

Study of the ammonia ice cloud layer in the north tropical zone of Jupiter from the infrared interferometric experiment on Voyager

An average of 51 Voyager 1 IRIS spectra of Jupiter's North Tropical Zone was analyzed to infer the abundance, vertical extent, and size distribution of the particles making up the ammonia cloud in this region. It is assumed that the cloud base coincides with the level at which 100% saturation of ammonia vapor occurs. The vertical distribution of particulates above this level is determined by assuming a constant total ammonia mixing ratio and adjusting the two phases so that the vapor is saturated throughout the cloud. A constant scaling factor then adjusts the base number density. A radiative transfer program is used that includes the effects of absorption and emission of all relevant gases as well as anisotropic scattering by cloud particles. Mie scattering from a gaussian particle size distribution is assumed. The vertical thermal structure is inferred from a temperature retrieval program that utilizes the collision induced S(0) and S(1) molecular hydrogen lines between 300 and 700.cm, and the 1304.cm methane band.

Shaffer, William A.↗