Accretion of Low-albedo Dust by the Outer Satellites of the Gas Giants
The origin and nature of the low-albedo hemisphere of Iapetus represents one of the most intriguing problems in planetary science. Observations of the satellite have shown that the two hemispheres of the satellite are wholly different. The trailing hemisphere has an albedo, composition, and morphology typical of the other icy Saturnian satellites, while the surface of the leading side is composed of an extraordinarily low-albedo, reddish material that includes organic material and carbon. The study of pathological objects such as Iapetus offers clues to general alteration processes that might act throughout the outer Solar System. Bell et al. (1985, Icarus 61, 192) proposed just that by suggesting that Callisto was the Iapetus of the Jovian system in a less extreme manifestation. Unlike the other Galilean satellites, Callisto is darker on its leading side (except right at opposition), suggesting it may have swept up low-albedo dust in a process similar to that occurring on Iapetus. Our recent work seeking hemispheric diversity on the Uranian satellites also shows that the outermost satellites (Titania and Oberon) tend to be darker and redder on the leading side. Our hypothesis is that all of the outermost major satellites of Jupiter, Saturn, and Uranus accrete reddish, low-albedo dust from the small, captured, outer retrograde satellites of these planets. The dust is kicked off by meteoritic impacts and spirals in by Poynting-Robertson drag to impact the leading sides of the outer larger satellites, including Iapetus, Callisto, Titania, and Oberon. Work funded by NASA.
On The Energy Per Ion Pair In The Upper Atmospheres of Planets
The mean energy expended in a collision by an electron in atmospheric gazes is a useful parameter for fast aeronomy computations. Its inverse represents the overall efficiency of a particle or an electromagnetic radiation in ionising a gas or a mixture of gas and is thus characteristic of the species considered. Following a method proposed by Rees (1963), the ion and electron production height profiles can be calculated to derive the luminosity height profiles without having to solve a kinetic transport equation. Although computers are nowadays much faster than some decades ago when the energy per electron-ion pair was first computed, transport codes are still sparse especially when dealing with comparative planetology. Therefore, recent works still use the Rees method. The value of 35 eV has often been used although many authors have shown that it depends on the energy of the precipitated particle and on the atmospheric composition. In the present paper, we use a kinetic transport code adapted to Mars (Simon et al., 2008), Venus (Gronoff et al., 2008), Earth (Lilensten et al., 2002), Titan (Lilensten et al., 2005) and Jupiter to compute the energy per pair. We show that this parameter depends on the planet and propose different average values for each of them.
Pickup ion Phase Space Distributions at the Moon and Titan
The composition and structure of neutral exospheres imbedded in moving plasmas can be determined by measurements of the phase space distributions of pickup ions borne from the exospheres (1). An analytic model describing such relationships between neutral exospheres and their pickup ion progeny was recently developed for a one dimensional atmosphere imbedded in uniformly flowing background plasma with uniform magnetic fields (2). It was derived from the Vlasov equation with an ion source that explicitly accounts for the velocity and spatial variation of the exosphere source gases. The model was applied to the terrestrial moon and the Saturnian moon, Titan, interacting with the solar wind and Saturn's rotating magnetosphere, respectively. This work extends the model to include exosphere source gases in three dimensions (3D model) while retaining the uniform flow approximation. Consequently, more realistic comparisons between model and observations will be possible. One feature appearing in both models is the clear connection between the ratio of an ion gyroradius to an exosphere scale height: 1) when the ratio is less than one, the interaction is fluid like with all orbit phases of pickup ion cycloidal motion present at an observation point; 2) when the ratio is greater than one, the pickup ions appear as beams, where the phase space distribution peaks over a small velocity range at an observation site. The pickup ion model with a 3D exosphere not only yields more reliable ion velocity distributions, it also has the advantage of being able to describe the ion phase space distributions many gyroradii downstream from the exosphere source. This 3D model is applied to describe pickup ion phase space distributions borne from known neutral exosphere sources at the moon and Titan. Other ion exospheres such as Europa's are also considered. (1) R. E. Hartle and G. E. Thomas, J. Geophys. Res., 79, 1519, 1974. (2) R. E. Hartle and E. C. Sittler, J. Geophys. Res., 112, A07104, doi:10.1029/2006JA012157, 2007.
Simulating the 3-D Structure of Titan's Upper Atmosphere
We present results from the 3-D Titan Global Ionosphere-Thermosphere Model (Bell et al , PSS, in review). We show comparisons between simulated N2, CH4, and H2 density fields and the in-situ data from the Cassini Ion Neutral Mass Spectrometer (INMS). We describe the temperature and wind fields consistent with these density calculations. Variations with local time, longitude, and latitude will be addressed. Potential plasma heating sources can be estimated using the 1-D model of De La Haye et al [2007, 2008] and the impacts on the thermosphere of Titan can be assessed in a global sense in Titan-GITM. Lastly, we will place these findings within the context of recent work in modeling the 2-D structure of Titan's upper atmosphere (Mueller-Wodarg et al ).
Titan Coupled Surface/Atmosphere Retrievals
Titan's thick haze obscures its surface at visible wavelengths and hinders surface photometric studies in the near-infrared. The large vertical extent of the haze produces two effects which require radiative transfer analysis beyond the capability of plane-parallel multi-scatter models. Haze aerosols extend to altitudes above 500 km and require a spherical-shell RT algorithm close to the limb or terminator. Even near nadir viewing, horizontal scattering at spatial scales less than a few hundred km requires a code capable of simulating the adjacency effect. The adjacency effect will reduce contrast more for small spatial scales than for large spatial scales, and the amount of contrast reduction depends on many factors (haze optical thickness, vertical distribution, single scattering albedo, scattering geometry, spatial scale). Titan's haze is strongly forward scattering even near 1-µm wavelength and many RT codes do a poor job. Fortunately the problem is more tractable at longer wavelengths. We show how data from the Cassini VIMS and ISS instruments can be used to understand surface contrast and atmospheric haze properties.
Titan's surface from Cassini SAR Principal Component Analysis (PCA) from orthorectified and non-orthorectified image pairs
The objective of this study is to analyze the suitability of principal component analysis (PCA) of Cassini SAR images to enhance interpretability of surface features for geological mapping on Titan. With repeated flybys around Titan, several areas are now covered by overlapping flybys giving the opportunity to map the surface with different incidence angle and at times opposite look direction. We extract principal components (PCs) from overlapping Cassini SAR scenes, acquired in like- and opposite-look direction using both orthorectified (ortho) image pairs, where the distortion due to topographical variations in the surface are removed using digital topographic models (DTMs), and also using non-orthorectified (non-ortho) images pairs. The use of PCA minimizes the data redundancy inherent in the Cassini SAR scenes and creates component images that are characterized by a linear combination of the input data. The feature-oriented principal components selection (FPCS) method has been applied to examine the PCA eigenvector loadings and to understand which principal component images concentrate and enhance information directly related to the backscattering response of specific target area such the dunes area in T8/T21 and T8/T41 overlap. The application of the method to the selected areas shows that the non-ortho PC2 image, the second derived principal component image, optimizes the topographic perception inherent in the radar images and retains information related to radar backscattering response to surface characteristics providing an excellent base for mapping. More importantly, an interesting finding has emerged about the ortho-image PC2 as it seems to define systematic patterns in which the dunes, especially in some areas, are darker. Considering that the ortho-images have been corrected for the mean Titan backscatter law this means the dunes might follow a different law. We have seen differences between dune- nondune areas so far, but we will also be using the technique to look for differences in scattering law behavior for other types of surfaces. We are investigating and testing different backscattering law scenario along with incidence angles changes across the images through the systematic variation observed in the PC2 ortho-images. The stereo coverage on Titan is mainly from pairs of images, but there are a few areas with up to 4 overlapping images. Applying PCA to these areas might show something beyond the simple sum and difference from the use of only 2 input images, and might help in a better understanding of the backscattering law scenario.
Future Exploration of Titan and Enceladus
The future exploration of Titan and Enceladus has become very important for the planetary community. The study conducted last year of the Titan Saturn System Mission (TSSM) led to an announcement in which ESA and NASA prioritized future OPF missions, stating that TSSM is planned after EJSM (for details see http://www.lpi.usra.edu/opag/). TSSM consists of a TSSM Orbiter that would carry two in situ elements: the Titan Montgolfiere hot air balloon and the Titan Lake Lander. The mission could launch in the 2023-2025 timeframe on a trajectory to arrive ~9 years later for a 4-year mission in the Saturn system. Soon after arrival at Saturn, the montgolfiere would be delivered to Titan to begin its mission of airborne, scientific observations of Titan from an altitude of about 10 km. The montgolfiere would have a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) power system and would be designed to last at least 6-12 months in Titan's atmosphere. With the predicted winds and weather, that would be sufficient to circumnavigate the globe! On a subsequent fly-by, the TSSM orbiter would release the Lake Lander on a trajectory toward Titan for a targeted entry. It would descend through the atmosphere making scientific measurements, much like Huygens did, and then land and float on one of Titan's seas. This would be its oceanographic phase, making a physical and chemical assessment of the sea. The Lake Lander would operate 8-10 hours until its batteries become depleted. Following the delivery of the in situ elements, the TSSM orbiter would explore the Saturn system via a 2-year tour that includes in situ sampling of Enceladus' plumes as well as Titan flybys. After the Saturn system tour, the TSSM orbiter would enter orbit around Titan for a global survey phase. Synergistic and coordinated observations would be carried out between the TSSM orbiter and the in situ elements. The scientific requirements were developed by the international TSSM Joint Science Definition Team (JSDT). The orbiter was NASA's responsibility while the in situ elements were designed by ESA. The engineering and flight operations aspects of TSSM were developed in a collaborative study, conducted by NASA and ESA engineering teams working on both sides of the Atlantic. This work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The European part was conducted in ESA within the Cosmic Vision 1 plan. Copyright 2008 California Institute of Technology. Government sponsorship acknowledged.
Exploration of amino acid biomarkers in polar ice with the Mars Organic Analyzer
A portable microfabricated capillary electrophoresis (CE) system named the Mars Organic Analyzer (MOA) has
been developed to analyze fluorescently-labeled biomarkers including amino acids, amines, nucleobases,
and amino sugars with the goal of life detection on Mars (1,2). This technology has also been shown to be
effective in screening the formation of biogenic amines during fermentation (3). The MOA is a part of the Urey
instrument package that has been selected for the 2016 European ExoMars mission by ESA. The identification
of recent gully erosion sites, observations of ice on and beneath the surface of Mars, and the discovery of large
reservoirs of sub-surface ice on Mars point to water-ice as an important target for astrobiological analyses. In
addition, the ice samples on the Moon, Mercury, Europa and Enceladus are of interest due to the possibility that
they may contain information on biogenic material relevant to the evolution of life. We explore here the use of
the MOA instrument for the analysis of amino acids in polar ice samples. The amino acids valine,
alanine/serine, glycine, glutamic acid, and aspartic acid were found in the parts-per-billion range from
Greenland ice-core samples. Chiral analysis of these samples yielded D/L ratios of 0.51/0.09 for
alanine/serine and 0.14/0.06 for aspartic acid. Individual amino acids in the parts-per-trillion range were found
in Antarctic ice samples collected from the surface of a meteorite collection area. The distinct amino acid and
amine content of these samples indicates that further biomarker characterization of ice samples as a function
of sampling location, depth, and structural features will be highly informative. The rapid sensitive analysis
capabilities demonstrated here establish the feasibility of using the MOA to analyze the biomarker content of
ice samples in planetary exploration. 1. Skelley, A. M.; Scherer, J. R.; Aubrey, A. D.; Grover, W. H.; Ivester, R. H.
C., Ehrenfreund, P.; Grunthaner, F. J.; Bada, J. L.; Mathies, R. A. PNAS, 2005, 192, 1041. 2. Skelley, A. M.,
Cleaves, H. J., Jayarajah, C. N., Bada, J. L. and Mathies, R. A., Astrobiology 2006, 6, 824. 3. Jayarajah, C.N.,
Skelley, A.M., Fortner, A.D., and Mathies, R.A., Anal. Chem. 2007, 79, 21, 8162.