New Observations and Studies of Saturn's Long-Lived North Polar Spot

Abstract We report on a new series of ground-based CCD observations at visual wavelengths, covering a period of 1255 days between May 1992 and November 1995, of the longest-lived asymmetric feature known in Saturn's atmosphere: the north polar spot (NPS). This completes our previous analysis of this feature during the period 1990–1991 (A. Sanchez-Lavega, J. Lecacheux, F. Colas, and P. Lagues, 1993, Science 260, 329–332). Longitude measurements of the NPS indicate an averaged longitudinal drift of −0.030 deg/day for the whole period 1990–1995 corresponding to a zonal velocity of 0.11 msec −1 . These data, when combined with previous and new measurements of the NPS position on Voyager 1 and 2 images obtained in 1980 and 1981, indicate a long-term drift in longitude of the NPS with a constant angular acceleration of 1.1 × 10 −5 deg/(day) 2 . High-resolution Voyager 2 violet, blue, green, and orange images were used to measure the size and reflectivity of the NPS. Its structure is characterized by a bright elliptical core surrounded by a dark ring and a large uniform area. The contrast between all these features changes appreciably from violet to orange: the spot is dark in violet but bright in orange relative to its surroundings. The spot is embedded within a region seeded by a “field of bright clouds” with characteristic size 1000 km reminiscent of a cellular convection pattern. The NPS's east–west apparent size is shorter at violet–blue (about 7000 km as limited by a dark ring at these wavelengths) than at green–orange (about 11,000 km corresponding to the large uniform area). Green processed images show apparent spiral patterns within the NPS consistent with anticyclonic vorticity. The results of ground-based photometry of the north polar region (NPR) and the NPS in the red methane absorption bands and their adjacent continuum are consistent with a radiative transfer model of the cloud vertical structure consisting of a clear gas layer, a haze layer, and a semi-infinite cloud. In the context of this model the NPS cloud tops are slightly higher than neighboring clouds reaching a pressure level of 45 mbar. Calculations of the seasonal insolation at the north pole, together with a simple linear radiative response of the atmosphere to this heating at different altitudes, suggest temperature changes at the level of the NPS cloud tops which should influence the NPS dynamics. Because of the long lifetime of the NPS, and because its motions did not vary appreciably during the long observing period, we suggest that the main properties and dynamics of the NPS are insensitive to the external solar forcing.

[1]  Paul G. Richards,et al.  Seismological evidence for differential rotation of the Earth's inner core , 1996, Nature.

[2]  Agustin Sanchez-Lavega,et al.  Large-Scale Storms in Saturn's Atmosphere During 1994 , 1996, Science.

[3]  B. Conrath,et al.  COMPARISON OF THE STRUCTURE AND DYNAMICS OF JUPITER'S GREAT RED SPOT BETWEEN THE VOYAGER 1 AND 2 ENCOUNTERS , 1996 .

[4]  Denis Grodent,et al.  Simultaneous observations of the Saturnian aurora and polar haze with the HST/FOC , 1995 .

[5]  B. Sandel,et al.  Dark Auroral Oval on Saturn Discovered in Hubble Space Telescope Ultraviolet Images , 1995, Science.

[6]  J. Lecacheux,et al.  Photometry of Saturn's 1990 Equatorial Disturbance , 1994 .

[7]  M. Tomasko,et al.  Saturn's Upper Atmospheric Hazes Observed by the Hubble Space Telescope , 1993 .

[8]  J. Lecacheux,et al.  Temporal behavior of cloud morphologies and motions in Saturn's atmosphere , 1993 .

[9]  S. Limaye,et al.  Dynamics of Neptune's Major Cloud Features , 1993 .

[10]  C. Barnet,et al.  The Drift of Saturn's North Polar Spot Observed by the Hubble Space Telescope , 1993, Science.

[11]  J. Lecacheux,et al.  Ground-Based Observations of Saturn's North Polar Spot and Hexagon , 1993, Science.

[12]  M. Tomasko,et al.  Saturn's upper troposphere 1986–1989 , 1992 .

[13]  D. Godfrey.  The Rotation Period of Saturn's Polar Hexagon , 1990, Science.

[14]  D. Gezari,et al.  New features in Saturn's atmosphere revealed by high-resolution thermal infrared images , 1989, Nature.

[15]  J. A. Quesada,et al.  A survey of Saturn's northern hemisphere from 1979 to 1987 , 1988 .

[16]  D. Godfrey,et al.  A hexagonal feature around Saturn's north pole , 1988 .

[17]  M. Allison,et al.  A Wave Dynamical Interpretation of Saturn's Polar Hexagon , 1987, Science.

[18]  E. Battaner,et al.  Long-term changes in Saturn's atmospheric belts and zones , 1986 .

[19]  M. Tomasko,et al.  Observations of the limb darkening of jupiter at ultraviolet wavelengths and constraints on the properties and distribution of stratospheric aerosols , 1986 .

[20]  B. Bézard,et al.  A seasonal climate model of the atmospheres of the giant planets at the voyager encounter time: I. Saturn's stratosphere , 1985 .

[21]  Makiko Sato,et al.  Photometry and polarimetry of Saturn at 2640 and 7500 Å , 1983 .

[22]  W. Kurth,et al.  Saturn as a radio source , 1983 .

[23]  E. V. Hemelrijck.  The oblateness effect on the solar radiation incident at the top of the atmospheres of the outer planets , 1982 .

[24]  Barney J. Conrath,et al.  Thermal Structure of Saturn from Voyager Infrared Measurements. , 1982 .

[25]  J. L. Mitchell,et al.  A New Look at the Saturn System: The Voyager 2 Images , 1982, Science.

[26]  M. L. Kaiser,et al.  Voyager measurement of the rotation period of Saturn's magnetic field , 1981 .

[27]  R. L. Duncombe,et al.  Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites , 1980 .

[28]  R. Cess,et al.  A Saturnian stratospheric seasonal climate model , 1979 .

[29]  A. Brinkman,et al.  The effect of the ring system on the solar radiation reaching the top of Saturn's atmosphere: Direct radiation , 1979 .

[30]  I. Nolt,et al.  Spatially resolved infrared observations of Saturn II. The temperature enhancement at the South pole of Saturn , 1978 .

[31]  V. G. Teifel.  Limb darkening and properties of the polar regions of Jupiter , 1978 .

[32]  C. Barnet.  Saturn's Seasonal Winds and Temperature: the Effect of the Ring System on the Troposphere and Stratosphere , 1990 .

[33]  F. Flasar.  Temporal Behavior of Jupiter's Meteorology , 1989 .