JULIANNE: Rychle jsem mrkla na program nedávného meetingu DPS, kde se to mohlo řešit, ale nic moc nového jsem nenašla - podle abstraktů je to pořád celkem nejasné (ony ani ty soli zdaleka nejsou stoprocentní).
TITLE: Photometric Properties of Ceres and the Occator Bright Spots
ABSTRACT BODY:
Abstract (2,250 Maximum Characters): Dawn discovered several extremely bright spots on Ceres, the most prominent of which is located inside the Occator crater that is at least 4-5 times brighter than the average Ceres. Interestingly, these bright spots are located in relatively young craters that are at the longitudes corresponding to the maximum water vapor observed by the Herschel Space Observatory, suggesting possible correlation with water sublimation on Ceres. We used the multi-color imaging data collected by the Dawn Framing Camera to analyze the global photometric properties of Ceres and the bright spots, especially those located inside the Occator crater. Our objectives are to determine the albedo and other light scattering properties of the bright spots on Ceres in the visible wavelengths, in order to characterize their physical properties and find clues about their composition and possible formation mechanisms and the correlation with water sublimation. The overall geometric albedo of Ceres’ global surface is 0.09-0.10, consistent with previous studies. The Hapke roughness parameter is about 20°, close to many other asteroids, rather than 44° as reported earlier. Correspondingly, the phase function of Ceres is less backscattering than previously modeled. In contrast, the geometric albedo of the bright spots inside the Occator crater is 0.4-0.5, and the single scattering albedo is 0.7-0.8, brighter than Vesta’s global albedo but much darker than many icy satellites in the outer solar system. The Hapke roughness of the bright spots is much higher than Ceres average, suggesting relatively loose deposit of materials rather than more coherent or tightly packed materials. The phase function of bright spots material is relatively more forward scattering than average Ceres, possibly correlated to stronger multiple scattering due to high albedo resulting from more transparent materials. The highest resolution images as of late-August 2015 show fine structures within the Occator bright spots. We will also report the results from the albedo distribution within the bright spots.
TITLE: Post-Impact Cryovolcanism and the Bright Spots on Ceres
ABSTRACT BODY:
Abstract (2,250 Maximum Characters): The bright spots on Ceres recently found by NASA’s Dawn mission have defied easy explanation. The spots are young, because the 90 km diameter crater in which they are found generally lacks superposed craters, but the composition is equivocal. The very high albedo contrasting on Ceres’ dark surface suggests ice, but the spots could be salt deposits. In either case, formation via water volcanism is plausible, either as ice or a salty sublimate. These observations pose the problem: How can there be recent cryovolcanic processes on a relatively low mass and density body that thus has a small complement of long lived radiogenic isotopes (the only plausible heat source)? Here, we suggest the spots are a consequence of the impact process. Ceres’ low bulk density indicates abundant water ice, yet spectra indicate a non-volatile surface. Various interior models predict full or partial differentiation, which means that the near surface is structured as a largely icy layer covered by a non-volatile layer of unknown thickness. For a sufficiently thick layer (> 5-10 km), the formation of a 90 km crater will not penetrate this layer; however, the flow field associated with transient crater collapse may extend into the underlying icy material to give final crater morphological characteristics consistent with an icy surface. Deposition of impact heat would primarily be in this non-volatile layer, which would then diffuse away, including some downwards. We have performed finite element simulations of this diffusion, finding that for a non-volatile layer ~10 km thick, this downward diffusion can lead to supra-melting temperatures in the underlying icy material from 0.1 to 10 Myr after the impact, leading to the deposits. This phenomenon might explain why bright spots are not abundant: the crater needs to be the right size relative to layer thickness, the impact would likely need to occur at a high strike angle (for the deepest deposition of impact heat), and any resultant deposits could be ephemeral, consistent with the crater’s apparent very young age. Additionally, no special conditions are needed for the evolution of Ceres; instead, these bright spots result from the most common geological process in the solar system: impact.