Clues to the formation of spiral structure in M51 from the ages and locations of star clusters

We determine the spatial distributions of star clusters at different ages in the grand-design spiral galaxy M51 using a new catalog based on multi-band images taken with the Hubble Space Telescope (HST). These distributions, when compared with the spiral structure defined by molecular gas, dust, young and old stars, show the following sequence in the inner arms: dense molecular gas (and dust) defines the inner edge of the spiral structure, followed by an overdensity of old stars and then young stellar clusters. The offset between gas and young clusters in the inner arms is consistent with the expectations for a density wave. Clusters as old as a few hundred Myr remain concentrated close to the spiral arms, although the distributions are broader than those for the youngest clusters, which is also consistent with predictions from density wave simulations. The outermost portion of the west arm is different from the rest of the spiral structure in that it contains primarily intermediate-age (approximate to 100-400 Myr) clusters; we believe that this is a "material" arm. We have identified four "feathers," stellar structures beyond the inner arms that have a larger pitch angle than the arms. We do not find age gradients along any of the feathers, but the least coherent feathers appear to have the largest range of cluster ages

Jetting Activity and Thermal Emission across the South Polar Terrain of Enceladus: Observations and Comparisons with Shear-Heating Models

Introduction: Cassini ISS Enceladus images reveal a forest of jets of fine icy particles erupting from the moon?s south polar terrain (SPT) and feeding a giant plume that extends thousands of kilometers into space [1] eventually forming Saturn?s E ring. Cassini infrared spectrometer (CIRS) observations have also shown the SPT to be anomalously warm [2], and the comparison of high resolution images of the SPT with the highest resolution thermal measurements has shown a coincidence between the hottest measured temperatures in the SPT and the ?tiger stripe? fractures which straddle the region [1,2]. The most plausible mechanism for heat production on the fractures is tidally driven frictional heating along faults due to cyclical shearing. The time-averaged absolute shear stress on each fracture segment can be calculated [3] and will be affected by location and orientation, and proportional to the local frictional heat production rate (assuming a constant coefficient of friction). Initial triangulation measurements of the jets seen in ISS images spanning 1.3 years (from late 2005 to early 2007) and taken over a broad range of image scales (1 to 14 km/pixel), phase angles (148? to 178?), and viewing directions with respect to the surface revealed the source locations for the 8 most prominent jets [4]. All 8 fell, within 2? measurement uncertainties, on the fractures, and about half of these sources were found to be coincident, within uncertainties, with the CIRS hot spots initially reported in 2006. Moreover, ISS jet locations were used to successfully predict the discovery of additional hot spots [4]: eg, the prominent hot spot on Baghdad Sulcus near Jet #I. Results: We have taken new, more precise jet positional measurements in the 62 highest resolution images (down to 40 m/pixel spatial scale) taken from late 2005 to early 2011 at phase angles of 130? to 165?. We now find over 80 distinct, measureable jets. Nearly all of them fall on one of the four main fractures, most on Cairo and Baghdad; only a few fall off a main fracture and on a nearby branch. To within measurement errors, all 8 previously measured jets have been recovered, with the notable distinction that at very high spatial scale, most of these 8 resolve into multiple narrower jets. Some jet sources that were prominent earlier in the mission appear to be less so in more recent images, indicating some degree of time variability. The most robust jetting activity still appears to be correlated with the hottest locales on the fractures. We will use this correlation between jetting activity and thermal power to produce a proxy thermal map of the SPT that we will then compare with (i) the best available thermal maps of the south polar terrain obtained by CIRS over the last 6 years and (ii) a map of predicted shear-heating along the south polar fractures

The Jets of Enceladus: Locations, Correlations with Thermal Hot Spots, and Jet Particle Vertical Velocities

High resolution images of Enceladus and its south polar jets taken with the Cassini ISS cameras in the last year have provided an opportunity for detailed study of the jetting phenomenon and its relationship to features and thermal hot spots on the moons south polar terrain. We have identified ~ 30 individual jets in a series of images, ranging from 43 to 100 m per pixel, taken in November 2009. All jets are found to be erupting through `tiger stripe fractures that cross the south polar terrain. The most intense jetting activity generally corresponds to the hottest regions on the fractures. One of the brightest, most prominent jets observed in this image series vents from a region on the Damascus Sulcus fracture that was imaged at 16 m/pixel during Cassinis August 13, 2010 flyby; it is also one of the hottest places found so far on the south polar region. Several jets were selected for dynamical modeling. These were jets whose source regions were on the limb as seen from Cassini, allowing extraction of brightness profiles down to a few hundred meters of the surface. We infer the velocity distribution of the particles as they leave the surface by modeling the integrated brightness vs. altitude. The particles are assumed to follow ballistic trajectories, and their contribution to the brightness in each thin layer is proportional to the time that they spend in the layer. We find slow jets, fast jets, and jets in between. After a rapid ~ 2-km-scale-height decrease near the surface, the most prominent jet (mentioned above) extends with constant integrated brightness to the edge of the image 25 km above the surface; some of the particles in this jet appear to have mean velocities that exceed the 235 m/sec escape speed from Enceladus. Further analysis of higher-altitude images from the November flyby is in progress to verify this result. The integrated brightness of slow jets falls off with a scale height of 5 km or less, implying mean vertical velocities of order 30 m/s or less, much less than either the escape speed or the thermal speed for a temperature of 273 K. From the collimation of the vapor in the jets, the Cassini UVIS team infers vertical velocities of 1000 m/s or more [Hansen et al. (2008) Nature 456, 477-479]. Schmidt et al. [(2008) Nature 451, 685-688] account for the slow particle speeds by invoking collisions with the walls of the vent. Ingersoll and Pankine [(2010) Icarus 206, 594-607] invoke short distances during which the gas velocity is high; the particles dont have time reach escape speed. The third possibility is that the particles are so large that the gas cannot accelerate them to escape speed. This possibility is testable with Cassini ISS high-resolution images, which span phase angles up to 176 degrees and wavelengths from UV to near-IR. Our ultimate goal is to test models of how the jets form. The particles form either by condensing directly from vapor, by spallation from the icy walls of the vent, or by freezing of liquid water droplets. Images collected by Cassini thus far will help us choose among the possibilities