ESA Science & Technology - Publication Archive
The planet-encircling springtime storm in Saturn's troposphere (December 2010-July 2011) produced dramatic perturbations to stratospheric temperatures, winds and composition at mbar pressures that persisted long after the tropospheric disturbance had abated. Thermal infrared (IR) spectroscopy from the Cassini Composite Infrared Spectrometer (CIRS), supported by ground-based IR imaging from the VISIR instrument on the Very Large Telescope and the MIRSI instrument on NASA's IRTF, is used to track the evolution of a large, hot stratospheric anticyclone between January 2011 and March 2012. The evolutionary sequence can be divided into three phases: (I) the formation and intensification of two distinct warm airmasses near 0.5 mbar between 25 and 35°N (B1 and B2) between January-April 2011, moving westward with different zonal velocities, B1 residing directly above the convective tropospheric storm head; (II) the merging of the warm airmasses to form the large single 'stratospheric beacon' near 40°N (B0) between April and June 2011, disassociated from the storm head and at a higher pressure (2 mbar) than the original beacons, a downward shift of 1.4 scale heights (approximately 85 km) post-merger; and (III) the mature phase characterised by slow cooling (0.11 ± 0.01 K/day) and longitudinal shrinkage of the anticyclone since July 2011. Peak temperatures of 221.6 ± 1.4 K at 2 mbar were measured on May 5th 2011 immediately after the merger, some 80 K warmer than the quiescent surroundings. From July 2011 to the time of writing, B0 remained as a long-lived stable stratospheric phenomenon at 2 mbar, moving west with a near-constant velocity of 2.70 ± 0.04 deg/day (-24.5 ± 0.4 m/s at 40°N relative to System III longitudes). No perturbations to visible clouds and hazes were detected during this period. [Abstract abbreviated due to character limitations.]
While landing on Titan, several instruments onboard Huygens acquired measurements that indicate the probe did not immediately come to rest. Detailed knowledge of the probe's motion can provide insight into the nature of Titan's surface. Combining accelerometer data from the Huygens Atmospheric Structure Instrument (HASI) and the Surface Science Package (SSP) with photometry data from the Descent Imager/Spectral Radiometer (DISR) we develop a quantitative model to describe motion of the probe, and its interaction with the surface. The most likely scenario is the following. Upon impact, Huygens created a 12 cm deep hole in the surface of Titan. It bounced back, out of the hole onto the flat surface, after which it commenced a 30-40 cm long slide in the southward direction. The slide ended with the probe out of balance, tilted in the direction of DISR by around 10°. The probe then wobbled back and forth five times in the north-south direction, during which it probably encountered a 1-2 cm sized pebble. The SSP provides evidence for movement up to 10 s after impact. This scenario puts the following constraints on the physical properties of the surface ... [Abstract abbreviated due to character limitations.]
We have detected in Cassini data the signature of the periodic tidal stresses within Titan driven by the eccentricity (e = 0.028) of its 16-day orbit around Saturn. Precise measurements of the acceleration of the Cassini spacecraft during six close flybys between 2006 and 2011 have revealed that Titan responds to the variable tidal field exerted by Saturn with periodic changes of its quadrupole gravity, at about 4% of the static value. Two independent determinations of the corresponding degree-2 Love number yield k2 = 0.589 ± 0.150 and k2 = 0.637 ± 0.224 (2-sigma). Such a large response to the tidal field requires that Titan's interior is deformable over time scales of the orbital period, in a way that is consistent with a global ocean at depth.
We present in situ Cassini Radio Plasma Wave Science observations in the vicinity of Enceladus and in the E ring of Saturn that indicate the presence of dusty plasma. The four flybys of Enceladus in 2008 revealed the following cold plasma characteristics: (1) there is a large plasma density (both ions and electrons) within the Enceladus plume region, (2) no plasma wake effect behind Enceladus was detected, (3) electron densities are generally much lower than the ion densities in the E ring (ne/ni < 0.5) as well as in the plume (ne/ni < 0.01), and (4) the average bulk ion drift speed is significantly less than the corotation speed and is instead close to the Keplerian speed. These signatures result from half or more of the electrons being attached to dust grains and by the interaction between the surrounding cold plasma and the predominantly negatively charged submicrometer-sized dust grains. The dust and plasma properties estimated from the observations clearly show that the dust-plasma interaction is collective. This strong dust-plasma coupling appears not only in the Enceladus plume but also in the Enceladus torus, typically from about 20 RE (~5000 km) north and about 60 RE (~15,000 km) south of Enceladus. We also suggest that the dust-plasma interaction in the E ring is the cause of the planetary spin-modulated dynamics of Saturn's magnetosphere at large.
Published online on 22 June 2011The discovery of a plume of water vapour and ice particles emerging from warm fractures ('tiger stripes') in Saturn's small, icy moon Enceladus raised the question of whether the plume emerges from a subsurface liquid source or from the decomposition of ice. Previous compositional analyses of particles injected by the plume into Saturn's diffuse E ring have already indicated the presence of liquid water, but the mechanisms driving the plume emission are still debated. Here we report an analysis of the composition of freshly ejected particles close to the sources. Salt-rich ice particles are found to dominate the total mass flux of ejected solids (more than 99 per cent) but they are depleted in the population escaping into Saturn's E ring. Ice grains containing organic compounds are found to be more abundant in dense parts of the plume. Whereas previous Cassini observations were compatible with a variety of plume formation mechanisms, these data eliminate or severely constrain non-liquid models and strongly imply that a salt-water reservoir with a large evaporating surface provides nearly all of the matter in the plume.