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NASA NTRS1984

Nature of gamma-prime precipitates in rheocast IN-100 alloy

The microstructures of commercial VIM-cast and rheocast (Apelian and Cheng, 1983) IN-100 superalloy are investigated optically and by two-stage replica TEM, with an emphasis on the nature of the gamma-prime (GP) precipitates. Photomicrographs at magnifications of 39, 390, 2145, and 6630 are presented and discussed. VIM-cast IN-100 has dendritic structure, 10-20-micron primary GP particles, and uniformly distributed cuboid GP precipitates of a uniform size (about 0.5 micron). Rheocast IN-100 (stirred 20 min at 350 rpm, 1291 C, and 0.0006 torr) has a nondendritic structure, 20-100-micron primary GP particles, and a smaller volume fraction of cuboid GP precipitates with a bimodal (either 1-1.5 microns or less than 0.5 micron) size distribution. Such distribution should increase fatigue-crack-nucleation resistance.

Surappa, M. K.↗
NASA NTRS1988

Responsivity of silicon photodiodes from 0.5 to 1.1 micron at 77 K

The responsivities of Si, Ge, and InSb photodiodes at 77 K are measured and compared. The results are used to determine the wavelength at which a change should occur in the detector materials used in the focal plan of the Mars Observer Visual and IR Mapping Spectrometer (VIMS). Quantitative responsivity data for Si photodiodes in the band from 0.5 to 1.1 were needed for the VIMS system analysis. The results show that the Si array for the focal plane should have two different antireflection coatings. It is found that the Si and InSb materials have equivalent quantum efficiencies at about 0.9 microns. Because of unknown signal chain complications that could be caused by a multiplexer accessing two different capacitances, the focal plane was designed with a change from Si to InSb at 1 micron.

Staller, C.↗
NASA NTRS1992

VIMS/Cassini mission at Titan: Scientific objectives and observational scenarios

The scientific objectives and observational scenarios of the Cassini/Visual Infrared Mapping Spectrometer (VIMS) Mission at Titan are addressed. The VIMS represents a powerful and effective means to both investigate, in four dimensions (latitude, longitude, altitude, and time), Titan's atmospheric structure and to map the near infrared spectral character of Titan's surface. Its broad spectral coverage from 0.35 to 5.1 micrometers together with its significant spectral resolution allows it to determine minor constituent distributions and cloud optical/microphysical properties from the surface to several hundred km. A promising means of obtaining high vertical resolution stratospheric profiles of hydrocarbons, oxides, and hazes via stellar occultation observations is discussed.

Baines, Kevin H.↗
NASA NTRS1991

Visible and infrared mapping spectrometer for exploration of comets, asteroids, and the Saturnian system of rings and moons

Consideration is given to a visible and infrared mapping spectrometer (VIMS) developed by American, Italian, and French scientists and engineers as a facility instrument for the Comet Rendezvous Asteroid Flyby (CRAF) mission and as a candidate facility instrument for the Cassini mission to explore Saturn and its rings and moons. VIMS is characterized by 0.35-5.2 micron spectral range with a nominal spatial resolution of 0.5 mrad and spectral resolutions of 7 nm at 0.35-1.0 micron wavelength and 16 nm at 1.0-5.2 micron wavelength.

Juergens, David W.↗
NASA NTRS1992

A radiation-hard, low-background multiplexer design for spacecraft imager applications

A possible multiplexer design for the focal plane for the Cassini Visible and Infrared Mapping Spectrometer (VIMS) is reviewed. The instrument's requirements for the multiplexed array are summarized. The VIMS instrument has a modest radiation-hardness requirement due to the trajectory and planetary environments in which the instrument will be required to operate. The total ionizing dose hardness requirement is a few tens of kilorads. A thin-gate oxide of a few hundred angstroms thickness is to be used. Field hardness is to be achieved by guard bands or hardened dielectric isolation. The design is argued to meet the low-noise and radiation-hardness required for imaging at Saturn. The design is versatile enough to provide double-correlated and double-uncorrelated sampling, which is accomplished in the signal processing electronics outside the focal plane.

Staller, Craig↗
NASA NTRS1994

Surface Fatigue Tests Of M50NiL Gears And Bars

Report presents results of tests of steels for use in gears and bearings of advanced aircraft. Spur-gear endurance tests and rolling-element surface fatigue tests performed on gear and bar specimens of M50NiL steel processed by vacuum induction melting and vacuum arc remelting (VIM-VAR). Compares results of tests with similar tests of specimens of VIM-VAR AISI 9310 steel and of AISI 9310 steel subjected to VAR only.

Townsend, Dennis P.↗
NASA NTRS1996

Grant Proposal for the Continuation of the Voyager Interstellar Mission: LECP Investigation

This proposal documents the plans of the Low Energy Charged Particle (LECP) investigation team for participation in NASA's Voyager Interstellar Mission (VIM) as the Voyager 1 and 2 spacecraft explore the outer reaches of the heliosphere and search for the termination shock and the heliopause. The proposal covers the four year period from 1 January 1997 to 31 December 2000. The LECP instruments on Voyager 1 and 2 measure in situ intensities of charged particles with energies from about 30 keV to 100 MeV for ions, and about 20 keV to greater than 10 MeV for electrons. The instruments provide detailed spectral, angular, and compositional information about the particles. Composition is available for greater than 200 keV/nuc using multi-parameter measurements. Angular information is obtained by a mechanically scanned platform that rotates at various commanded rates. Measurements of low energy ion and electron intensities versus time and spatial location within the heliosphere contain an abundance of information regarding various transport and acceleration processes on both local (approx. 1 hr, approx. 0.01 AU) and global (approx. 11 yrs, approx. 100 AU) scales. The LECP instruments provide unique observations of such dynamical processes, and we anticipate that it will return critical information regarding the boundaries of the heliosphere. Several recent and exciting discoveries based on LECP measurements emphasize the important role that low energy charged particle distributions play in physical processes in the interplanetary medium. Yet, at the same time, these discoveries also underscore the fact that our understanding of processes in the outer heliosphere is, in most cases, incomplete, and in others, only rudimentary at best. Among the discoveries referred to above are the following: (1) Shocks: Examination of greater than 30 keV ion intensities have revealed: (a) a total absence of acceleration beyond only -100-200 keV at a strong transient shock in May 1991 at 35 AU, despite an enhanced level of seed particles; (b) a large transient shock in September 1991 of global scale, with intensities of shock-accelerated ions greater than or equal to 30 keV to approx. 30 MeV showing complex, highly energy-dependent spatial evolution, and small-scale (approx. few gyroradii), often anisotropic, micro-structures; (c) recurrent intensity increases in greater than or equal to 30 keV to -few MeV ions, with structures that, in some cases, show no correlation with the associated corotating shock. (2) Superthermal ion pressure: A global merged interaction region with a leading shock, downstream of which the superthermal ion (greater than or equal to 30 keV to approx. 4 MeV) pressure is comparable to that of the thermal plasma, and the total particle pressure yields a plasma beta of order unity. (3) Pickup ions: Measurements of the C/O ratio within transient structures at 35-45 AU showing the first clear evidence that transient shocks can pre-accelerate interstellar pickup ions from approx. 1 keV/nuc to at least 1 MeV/nuc. (4) Seed particles: Injection of ions for acceleration to high energies at the termination shock is unlikely to be a problem, since interplanetary transient and recurrent shocks are continually accelerating ions, of solar wind or interstellar origin, to highly superthermal energies. (5) Precursor electrons: Ambient solar electrons (greater than or equal to few tens of keV) that exist in the outer heliosphere ca form a broad precursor, several days wide, that is upstream of the termination shock and potentially observable a few months prior to the shock crossing. (6) Solar wind velocity at Voyager 1: We can use LECP ion data to obtain the solar wind velocity at Voyager 1, enabling us to provide critical measurement of the plasma flow as we approach and encounter the termination shock and other regions (necessary due to the partial failure of the Voyager 1 PLS experiment). The work of the LECP investigator team during the VIM will include: (1) Continuing operations with regard to the receipt, processing, verification, cataloging, display, and distribution of the data from the LECP instruments on Voyager 1 and 2, (2) Monitoring the health and performance of the LECP instruments, and evaluating and characterizing the response of the LECP instruments to various energetic particle and plasma environments, (3) Participating in, and supporting Voyager Project planning exercises and other coordinated activities relevant to exploration of the outer heliosphere, (4) Developing analysis techniques and operational procedures suitable for searching for and characterizing the boundaries and unique regions of the outher heliosphere, (5) Continuing the preparation of data sets appropriate for submission to the National Space Sciences Data Center (NSSDC) and, where appropriate, the Planetary Data System (PDS), (6) Maintaining direct Web access to online LECP data through the JHU/APL Voyager LECP home page, (7) Performing scientific evaluations of the Voyager 1 and 2 LECP data sets in conjunction with other data sets and other investigators, with particular focus on the outer regions of the heliosphere, and (8) Publishing the results of these evaluations in the scientific literature and presenting the results in scientific conferences.

Krimigis, Stamatios M.↗
NASA NTRS2005

Processing ISS Images of Titan's Surface

One of the primary goals of the Cassini-Huygens mission, in orbit around Saturn since July 2004, is to understand the surface and atmosphere of Titan. Surface investigations are primarily accomplished with RADAR, the Visual and Infrared Mapping Spectrometer (VIMS), and the Imaging Science Subsystem (ISS) [1]. The latter two use methane "windows", regions in Titan's reflectance spectrum where its atmosphere is most transparent, to observe the surface. For VIMS, this produces clear views of the surface near 2 and 5 microns [2]. ISS uses a narrow continuum band filter (CB3) at 938 nanometers. While these methane windows provide our best views of the surface, the images produced are not as crisp as ISS images of satellites like Dione and Iapetus [3] due to the atmosphere. Given a reasonable estimate of contrast (approx.30%), the apparent resolution of features is approximately 5 pixels due to the effects of the atmosphere and the Modulation Transfer Function of the camera [1,4]. The atmospheric haze also reduces contrast, especially with increasing emission angles [5].

Perry, Jason↗
NASA NTRS2005

Photometric and Spectral Study of the Saturnian Satellites

Photometric and spectra analysis of data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS) has yielded intriguing findings regarding the surface properties of several of the icy Saturnian satellites. Spectral cubes were obtained of these satellites with a wavelength distribution in the IR far more extensive than from any previous observations. Disk-integrated solar phase curves were constructed in several key IR wavelengths that are indicative of key properties of the surface of the body, such as macroscopic roughness, fluffiness (or the porosity of the surface), global albedo and scattering properties of surface particles. Polynomial fits to these phase curves indicate a linear albedo trend of the curvature of the phase functions. Rotational phase functions from Enceladus were found to exhibit a double-peaked sinusoidal curve, which shows larger amplitudes for bands corresponding to water ice and a linear amplitude-albedo trend. These functions indicate regions on the surface of the satellite of more recent geologic activity. In addition, recent images of Enceladus show tectonic features and an absence of impact craters on Southern latitudes which could be indicative of a younger surface. Investigations into the properties of these features using VIMS are underway.

Newman, Sarah F.↗
NASA NTRS2012

Hydrocarbons on Saturns Satellites: Relationship to Interstellar Dust and the Solar Nebula

To understand the origin and evolution of our Solar System, and the basic components that led to life on Earth, we study interstellar and planetary spectroscopic signatures. The possible relationship of organic material detected in carbonaceous meteorites, interplanetary dust particles (IDPs), comets and the interstellar medium have been the source of speculation over the years as the composition and processes that governed the early solar nebula have been explored to understand the extent to which primitive material survived or became processed. The Cassini VIMS has provided new data relevant to this problem. Three of Saturn's satellites, Phoebe, Iapetus, and Hyperion, are found to have aromatic and aliphatic hydrocarbons on their surfaces. The aromatic hydrocarbon signature (C-H stretching mode at 3.28 micrometers) is proportionally significantly stronger (relative to the aliphatic bands) than that seen in other Solar System bodies (e.g., comets) and materials (Stardust samples, IDPs, meteorites) and the distinctive sub-features of the 3.4 micrometer aliphatic band (CH2 and CH3 groups) are reminiscent of those widely detected throughout the diffuse ISM. Phoebe may be a captured object that originated in the region beyond the present orbit of Neptune, where the solar nebula contained a large fraction of original interstellar ice and dust that was less processed than material closer to the Sun. Debris from Phoebe now resident on Iapetus and Hyperion, as well as o Phoebe itself, thus presents a unique blend of hydrocarbons, amenable to comparisons with interstellar hydrocarbons and other Solar System materials. The dust ring surrounding Saturn, in which Phoebe is embedded, probably originated from a collision with Phoebe. Dust ring particles are the likely source of the organic-bearing materials, and perhaps the recently identified small particles of Fe detected on Saturn's satellites. Lab measurements of the absolute band strengths of representative aliphatic and aromatic molecules, together with measurements from the VIMS data, allow us to calculate the number of C atoms to find the relative abundances of C atoms in the two kinds of organic molecules. The strength of the prominent aromatic C-H stretch band relative to the aliphatic band complex in Phoebe and Iapetus indicates that the relative abundance of aromatic to aliphatic carbon is very large (greater than 200). In contract, the aromatic band is nearly imperceptible in spectra of interplanetary dust particles (IDP), returned samples from comet 91P/Wild 2, insoluable carbonaceous material in most meteorites, and the diffuse interstellar dust (DISM) (although aromatics are known in all these materials-here we consider only the spectroscopic signature)

Cruikshank, D. P.↗
NASA NTRS2014

Interstellar Organics, the Solar Nebula, and Saturn's Satellite Phoebe

The diffuse interstellar medium inventory of organic material (Pendleton et al. 1994, Pe 2002) was likely incorporated into the molecular cloud in which the solar nebula condensed. This provided the feedstock for the fo planets, and the smaller icy bodies in the region outside Neptune's orbit (transneptunian objects, or TNOs). Saturn's satellites Phoeb open a window to the composition of one class of TNO as revealed by the near-infrared mapping spectrometer (VIMS) on the Cass Phoebe (mean diameter 213 km) is a former TNO now orbiting Saturn. VIMS spectral maps of Phoebe's surface reveal a complex consisting of prominent aromatic (CH) and aliphatic hydrocarbon (CH2, CH3) absorption bands (3.2-3.6 μm). Phoebe is the source encircling Saturn, and from which particles (~5-20 μm size) spiral inward toward Saturn. They encounter Iapetus and Hyperion wh blanket the native H2O ice of those two bodies. Quantitative analysis of the hydrocarbon bands on Iapetus demonstrates that aroma abundant as aliphatic CH2+CH3, significantly exceeding the strength of the aromatic signature in interplanetary dust particles, com carbonaceous meteorites (Cruikshank et al. 2013). A similar excess of aromatics over aliphatics is seen in the qualitative analysis o itself (Dalle Ore et al. 2012). The Iapetus aliphatic hydrocarbons show CH2/CH3 ~4, which is larger than the value found in the di as Phoebe is a primitive body that formed in the outer regions of the solar nebula and has preserved some of the original nebula inv understanding the content and degree of processing of that nebular material. There are other Phoebe-like TNOs that are presently b in the organic spectral region, but JWST will open that possibility for a number of objects. We now need to explore and understand organic-bearing Solar System material to the solar nebula and the inventory of ISM materials incorporated therein.

Interstellar↗
NASA NTRS2014

Interstellar Organics, the Solar Nebula, and Saturn's Satellite Phoebe

The diffuse interstellar medium inventory of organic material (Pendleton et al. 1994, Pendleton & Allamandola 2002) was likely incorporated into the molecular cloud in which the solar nebula condensed. This provided the feedstock for the formation of the Sun, major planets, and the smaller icy bodies in the region outside Neptune's orbit (transneptunian objects, or TNOs). Saturn's satellites Phoebe, Iapetus, and Hyperion open a window to the composition of one class of TNO as revealed by the near-infrared mapping spectrometer (VIMS) on the Cassini spacecraft at Saturn. Phoebe (mean diameter 213 km) is a former TNO now orbiting Saurn. VIMS spaectral maps of PHoebe's surface reveal a complex organic spectral signature consisting of prominent aromatic (CH) and alophatic hydrocarbon (CH2, CH3) absorption bands (3.2-3.6 micrometers). Phoebe is the source of a huge debris ring encircling Saturn, and from which particles (approximately 5-20 micrometer size) spiral inward toward Saturn. They encounter Iapetus and Hperion where they mix with and blanket the native H2O ice of those two bodies. Quantitative analysis of the hydrocarbon bands on Iapetus demonstrates that aromatic CH is approximately 10 times as abundant as aliphatic CH2+CH3, significantly exceeding the strength of the aromatic signature in interplanetary dust particles, comet particles, ad in carbonaceous meteorites (Cruikshank et al. 2013). A similar excess of aromatics over aliphatics is seen in the qualitative analysis of Hyperion and Phoebe itself (Dalle Ore et al. 2012). The Iapetus aliphatic hydrocarbons show CH2/CH3 approximately 4, which is larger than the value found in the diffuse ISM (approximately 2-2.5). In so far as Phoebe is a primitive body that formed in the outer regions of the solar nebula and has preserved some of the original nebula inventory, it can be key to understanding the content and degree of procesing of the nebular material. There are other Phoebe-like TNOs that are presently beyond our ability to study in the organic spectral region, but JWST will open that possibility for a number of objects. We now need to explore and understand the connection of this organic-bearing Solar System material to the solar nebula the the inventory of ISM materials incorporated therein.

Interstellar↗
NASA NTRS2012

Low-Latitude Ethane Rain on Titan

Cassini ISS observed multiple widespread changes in surface brightness in Titan's equatorial regions over the past three years. These brightness variations are attributed to rainfall from cloud systems that appear to form seasonally. Determining the composition of this rainfall is an important step in understanding the "methanological" cycle on Titan. I use data from Cassini VIMS to complete a spectroscopic investigation of multiple rain-wetted areas. I compute "before-and-after" spectral ratios of any areas that show either deposition or evaporation of rain. By comparing these spectral ratios to a model of liquid ethane, I find that the rain is most likely composed of liquid ethane. The spectrum of liquid ethane contains multiple absorption features that fall within the 2-micron and 5-micron spectral windows in Titan's atmosphere. I show that these features are visible in the spectra taken of Titan's surface and that they are characteristically different than those in the spectrum of liquid methane. Furthermore, just as ISS saw the surface brightness reverting to its original state after a period of time, I show that VIMS observations of later flybys show the surface composition in different stages of returning to its initial form.

Cassini↗
NASA NTRS2011

Saturn Ring Data Analysis and Thermal Modeling

CIRS, VIMS, UVIS, and ISS (Cassini's Composite Infrared Specrtometer, Visual and Infrared Mapping Spectrometer, Ultra Violet Imaging Spectrometer and Imaging Science Subsystem, respectively), have each operated in a multidimensional observation space and have acquired scans of the lit and unlit rings at multiple phase angles. To better understand physical and dynamical ring particle parametric dependence, we co-registered profiles from these three instruments, taken at a wide range of wavelengths, from ultraviolet through the thermal infrared, to associate changes in ring particle temperature with changes in observed brightness, specifically with albedos inferred by ISS, UVIS and VIMS. We work in a parameter space where the solar elevation range is constrained to 12 deg - 14 deg and the chosen radial region is the B3 region of the B ring; this region is the most optically thick region in Saturn's rings. From this compilation of multiple wavelength data, we construct and fit phase curves and color ratios using independent dynamical thermal models for ring structure and overplot Saturn, Saturn ring, and Solar spectra. Analysis of phase curve construction and color ratios reveals thermal emission to fall within the extrema of the ISS bandwidth and a geometrical dependence of reddening on phase angle, respectively. Analysis of spectra reveals Cassini CIRS Saturn spectra dominate Cassini CIRS B3 Ring Spectra from 19 to 1000 microns, while Earth-based B Ring Spectrum dominates Earth-based Saturn Spectrum from 0.4 to 4 microns. From our fits we test out dynamical thermal models; from the phase curves we derive ring albedos and non-lambertian properties of the ring particle surfaces; and from the color ratios we examine multiple scattering within the regolith of ring particles.

Saturn's rings↗
NASA NTRS2015

PAHs in the Ices of Saturn's Satellites: Connections to the Solar Nebula and the Interstellar Medium

Aliphatic hydrocarbons and PAHs have been observed in the interstellar medium (e.g., Allamandola et al. 1985, Pendleton et al. 1994, Pendleton & Allamandola 2002, Tielens 2013, Kwok 2008, Chiar & Pendleton 2008) The inventory of organic material in the ISM was likely incorporated into the molecular cloud in which the solar nebula condensed, contributing to the feedstock for the formation of the Sun, major planets, and the smaller icy bodies in the region outside Neptune's orbit (transneptunian objects, or TNOs). Additional organic synthesis occurred in the solar nebula (Ciesla & Sandford 2012). Saturn's satellites Phoebe, Iapetus, and Hyperion open a window to the composition of one class of TNO as revealed by the near-infrared mapping spectrometer (VIMS) on the Cassini spacecraft at Saturn. Phoebe (mean diameter 213 km) is a former TNO now orbiting Saturn (Johnson & Lunine 2005). VIMS spectral maps of Phoebe's surface reveal a complex organic spectral signature consisting of prominent aromatic (CH) and aliphatic hydrocarbon (=CH2, -CH3) absorption bands (3.2-3.6 micrometers). Phoebe is the source of a huge debris ring encircling Saturn, and from which particles ((is) approximately 5-20 micrometers size) spiral inward toward Saturn (Verbiscer et al. 2009). They encounter Iapetus and Hyperion where they mix with and blanket the native H2O ice of those two bodies. Quantitative analysis of the hydrocarbon bands on Iapetus demonstrates that aromatic CH is approximately 10 times as abundant as aliphatic CH2+CH3, significantly exceeding the strength of the aromatic signature in interplanetary dust particles, comet particles, and in carbonaceous meteorites (Cruikshank et al. 2014). A similar excess of aromatics over aliphatics is seen in the qualitative analysis of Hyperion and Phoebe itself (Dalle Ore et al. 2012). The Iapetus aliphatic hydrocarbons show CH2/CH3 (is) approximately 4, which is larger than the value found in the diffuse ISM ((is) approximately 2-2.5). Insofar as Phoebe is a primitive body that formed in the outer regions of the solar nebula and has preserved some of the original nebula inventory, it can be key in understanding the content and degree of processing of that nebular material. A dynamical subset of TNOs define the Kuiper Belt, from which the short-period comets originate. Particles collected from comet 81P/Wild contain PAHs with an interstellar signature of deuterium. By inference, the PAHs contained in Phoebe and now dusted on the surfaces of two other Saturn satellites share that interstellar origin. There are other Phoebe-like TNOs that are presently beyond our ability to study in the organic spectral region, but JWST will open that possibility for a number of objects.

Solar nebula↗
NASA NTRS2014

Amorphous and Crystalline H20 Ice at Rhea's Inktomi Crater

We present the analysis of Cassini spectral data from spectral mapping of Saturnian icy moons Dione and Rhea, to investigate possible effects of impact crater formation on the relative abundances of crystalline and amorphous water ice in the moons' ice crusts. Both moons display morphologically young ray craters as well as older craters. Possible changes in ice properties due to crater formation are conjectured to be more visible in younger craters, and as such Rhea's well imaged ray crater Inktomi is analysed, as are older craters for comparison. We used data from Cassini's Visual and Infrared Mapping Spectrometer (VIMS). For each pixel in the VIMS maps, spectral data were extracted in the near-infrared range (1.75 micrometers less than lambda less than 2.45 micrometers). Analysis was begun by fitting a single Gaussian to the peak in absorption at 2.0 micrometers, which was then subtracted from the data, leaving residuals with a minimum on either side of the original 2.0-micrometers band. The spectra of the individual spatial pixels were then clustered by the differences between these minima, which are sensitive to changes in both ice grain size and crystallinity. This yielded preliminary maps which approximated the physical characteristics of the landscape and were used to identify candidates for further analysis. Spectra were then clustered by the properties of the 1.5-micrometers band, to divide the map into regions based on inferred grain size. For each region, the predicted differences in minima from the Gaussian residuals, over a range of crystallinities, were calculated based on the found grain sizes. This model was used to find the crystallinity of each pixel via grain size and characteristics of the residual function. Preliminary results show a greater degree of crystallization of young crater interiors, particularly in Rhea's ray crater Inktomi, where ice showed crystalline ice abundances between 33 percent and 61 percent. These patterns in ice crystallization are possibly attributable to increased heat generated during crater formation.

Saturn Satellites↗
NASA NTRS2014

Interstellar Organics, the Solar Nebula, and Saturn's Satellite Phoebe

The diffuse interstellar medium inventory of organic material (Pendleton et al. 1994, Pendleton Allamandola 2002) was likely incorporated into the molecular cloud in which the solar nebula condensed. This provided the feedstock for the formation of the Sun, major planets, and the smaller icy bodies in the region outside Neptune's orbit (transneptunian objects, or TNOs). Saturn's satellites Phoebe, Iapetus, and Hyperion open a window to the composition of one class of TNO as revealed by the near-infrared mapping spectrometer (VIMS - Visible and Infrared Mapping Spectrometer) on the Cassini spacecraft at Saturn. Phoebe (mean diameter 213 kilometers) is a former TNO now orbiting Saturn. VIMS spectral maps of Phoebe's surface reveal a complex organic spectral signature consisting of prominent aromatic (CH) and aliphatic hydrocarbon (CH2, CH3) absorption bands (3.2-3.6 meters). Phoebe is the source of a huge debris ring encircling Saturn, and from which particles (5-20 meter size) spiral inward toward Saturn. They encounter Iapetus and Hyperion where they mix with and blanket the native H2O ice of those two bodies. Quantitative analysis of the hydrocarbon bands on Iapetus demonstrates that aromatic CH is 10 times as abundant as aliphatic CH2 plus CH3, significantly exceeding the strength of the aromatic signature in interplanetary dust particles, comet particles, and in carbonaceous meteorites (Cruikshank et al. 2013). A similar excess of aromatics over aliphatics is seen in the qualitative analysis of Hyperion and Phoebe itself (Dalle Ore et al. 2012). The Iapetus aliphatic hydrocarbons show CH2/CH3 4, which is larger than the value found in the diffuse ISM (InterStellar Matter - (2-2.5). Insofar as Phoebe is a primitive body that formed in the outer regions of the solar nebula and has preserved some of the original nebula inventory, it can be key to understanding the content and degree of processing of that nebular material. There are other Phoebe-like TNOs that are presently beyond our ability to study in the organic spectral region, but JWST (James Webb Space Telescope) will open that possibility for a number of objects. We now need to explore and understand the connection of this organic-bearing Solar System material to the solar nebula and the inventory of ISM (Interstellar Medium) materials incorporated therein.

Phoebe↗
NASA NTRSDate not supplied

Surface Temperature and the Compositional Effects of Electron Bombardment on Dione

Saturn’s icy moons are composed of mostly water ice with minor “contaminants,” such as CO2 ice, and a dark component thought to be organics, hematite and/or metallic Fe [1]. The space weathering process of electron bombardment is expected to be particularly important on the surfaces of Saturn’s inner moons (Mimas, Tethys, Dione, and Rhea), as they orbit within the extended plasma sheet of Saturn’s magnetosphere. Terrains exhibiting thermal anomalies (i.e., colder temperatures in the day and warmer temperatures at night than surrounding areas) correspond to regions of high energy electron bombardment [2; 3]. Energetic electrons impact the surfaces, fracturing ice particles into sub-micron sized grains, and this process becomes more effective for increasing particle energies [4]. In addition to alteration of ices, the dark material may also be modified or generated within icy surfaces by interaction with solar UV radiation, cosmic rays, or by particles trapped in Saturn’s magnetic field [5]. CO2 could be sourced from irradiation of dark organic material [7]. Many of these alterations should be observable in Visible and Infrared Mapping Spectrometer (VIMS) spectra [8]. We used surface temperatures derived from Cassini’s Composite Infrared Spectrometer (CIRS) to help isolate areas where space weathering due to electron bombardment is more predominant and investigated the subtle changes in VIMS spectra using machine learning techniques. We present our results here for Dione. On ocean worlds, space weathering products would be present along with any potential indications of life. We will need to understand the manifestation of space weathering in the data to separate the two processes.

Cindy Lee Young↗