Showing papers by "Robert A. West published in 2010"

Detection of visible lightning on Saturn

Abstract: [1] Until now, evidence for lightning on Saturn has been indirect – through radio emissions and cloud morphology. Here we report the first visible detection of lightning, on the night side on August 17, 2009 at −36.4° ± 0.1° planetocentric latitude and 10.6° ± 0.9° west longitude. No other locations produced lightning detectable by either imaging or radio. The lightning images are consistent with a single cloud flashing once per minute. The visible energy of a single flash is comparable to that on Earth and Jupiter, and ranges up to 1.7 × 109 Joules. The diameter of the lightning flashes is ∼200 km, which suggests the lightning is 125–250 km below cloud tops. This depth is above the base of the liquid H2O-NH3 cloud and may be either in the NH4SH cloud or in the H2O ice cloud. Saturn's lower internal heat transport and likely 5–10 fold enrichment of water largely explain the lower occurrence rate of moist convection on Saturn relative to Jupiter.

Iapetus: unique surface properties and a global color dichotomy from Cassini imaging.

Abstract: Since 2004, Saturn’s moon Iapetus has been observed repeatedly with the Imaging Science Subsystem of the Cassini spacecraft. The images show numerous impact craters down to the resolution limit of ~10 meters per pixel. Small, bright craters within the dark hemisphere indicate a dark blanket thickness on the order of meters or less. Dark, equator-facing and bright, poleward-facing crater walls suggest temperature-driven water-ice sublimation as the process responsible for local albedo patterns. Imaging data also reveal a global color dichotomy, wherein both dark and bright materials on the leading side have a substantially redder color than the respective trailing-side materials. This global pattern indicates an exogenic origin for the redder leading-side parts and suggests that the global color dichotomy initiated the thermal formation of the global albedo dichotomy.

Saturn's emitted power

Abstract: Long-term (2004–2009) on-orbit observations by Cassini Composite Infrared Spectrometer are analyzed to precisely measure Saturn's emitted power and its meridional distribution. Our evaluations suggest that the average global emitted power is 4.952 ± 0.035 W m^(−2) during the period of 2004–2009. The corresponding effective temperature is 96.67 ± 0.17 K. The emitted power is 16.6% higher in the Southern Hemisphere than in the Northern Hemisphere. From 2005 to 2009, the global mean emitted power and effective temperature decreased by ~2% and ~0.5%, respectively. Our study further reveals the interannual variability of emitted power and effective temperature between the epoch of Voyager (~1 Saturn year ago) and the current epoch of Cassini, suggesting changes in the cloud opacity from year to year on Saturn. The seasonal and interannual variability of emitted power implies that the energy balance and internal heat are also varying.

Linear polarization and albedo reconnaissance for regolith texture on sat

Abstract: Introduction: Before lunar samples were returned to Earth, it was shown that linear polarization curves and albedo measurements of the moon and asteroids could be used to predict the regolith surface texture of these bodies [1]. The prediction that lunar mare regolith is texturally similar to finely pulverized lava basalts was confirmed by returned lunar samples [2]. Our ultimate objective is to use Cassini Imaging Science Subsystem (ISS) Narrow Angle Camera (NAC) images of icy Saturnian satellites – acquired through broadband color and polarization filters – to measure the linear polarizations and corresponding albedos of icy terrains and to characterize implied surface textural properties. In this preliminary study, we demonstrate the sensitivity of ISS NAC polarization images for the analysis of icy satellite surfaces by testing if these images can detect Umov’s law, a wellknown inverse relationship between the linear-polarization and albedo of regolith-covered surfaces [3]. Iapetus is our test satellite because its terrain provides a broad range of albedos over which we expect linear polarization to systematically vary. In particular, we will use Cassini Regio’s stark contrast with Iapetus’ remaining brighter surface to demonstrate Umov’s law. Our hypothesis is that the linear polarization of all materials on Iapetus will vary inversely and monotonically with their corresponding albedo. Method: Each ISS NAC polarization observation is a three-image sequence obtained for 0, 60, and 90 degree orientations of the electric field vector, respectively. The polarization filters are used in combination with broadband color filters from a tandem filter wheel, so that changes in polarization with wavelength can be studied. Each image sequence is obtained at approximately constant phase angle. First, the images are radio-metrically calibrated so that pixel DN value represents radiance factors. Camera pointing information is updated using control points and limb locations on the satellite’s surfaces. The separate images are spatially co-registered and combined using an algorithm that produces a linear polarization image and a corresponding total intensity (i.e. non-polarized) image [4]. To produce albedo images, the total intensity image must be corrected for the gradient in shading across the visible disk due to gradual systematic gradations in the incidence and emission angles. We model the photometric gradient across the disk with fits of the Minnaert photometric function: ) ( cos