F. M. Flasar

Initial results from radio occultation measurements with Mars Global Surveyor

A series of radio occultation experiments conducted with Mars Global Surveyor in early 1998 has yielded 88 vertical profiles of the neutral atmosphere. The measurements cover latitudes of 29°N to 64°S and local times from 0600 through midnight to 1800 during early summer in the southern hemisphere (Ls = 264°–308°). Retrieved profiles of pressure and temperature versus radius and geopotential extend from the surface to the 10-Pa pressure level. Near-surface uncertainties in temperature and pressure are about 1 K and 2 Pa, respectively, far smaller than in previous radio occultation measurements at Mars. The profiles resolve the radiative-convective boundary layer adjacent to the surface and also reveal gravity waves, particularly at northern and equatorial latitudes, which appear to be breaking in some cases. Distinctive meridional gradients of pressure and temperature indicate the presence of a low-altitude westerly jet at latitudes of 15°–30°S at southern summer solstice. This jet appears in predictions of general circulation models in connection with the strong, seasonal, cross-equatorial Hadley circulation. The pressure gradient at ∼2 km altitude implies a wind speed of 33 m s−1, stronger than predicted, which may help explain the occurrence of numerous local dust storms within this latitude band in late southern spring. These measurements also characterize the response of the atmosphere to stationary thermal forcing at midsouthern latitudes, where high terrain south of Tharsis and low terrain in Hellas Planitia produce large, zonal temperature variations in the lowest scale height above the surface. Pressure measured at constant geopotential decreases at an average rate of 0.13% per degree Ls, due primarily to condensation of CO2 at the North Pole.

Infrared Observations of the Saturnian System from Voyager 2

During the passage of Voyager 2 through the Saturn system, infrared spectral and radiometric data were obtained for Saturn, Titan, Enceladus, Tethys, Iapetus, and the rings. Combined Voyager 1 and Voyager 2 observations of temperatures in the upper troposphere of Saturn indicate a seasonal asymmetry between the northern and southern hemispheres, with superposed small-scale meridional gradients. Comparison of high spatial resolution data from the two hemispheres poleward of 60� latitude suggests an approximate symmetry in the small-scale structure, consistent with the extension of a symmetric system of zonal jets into the polar regions. Longitudinal variations of 1 to 2 K are observed. Disk- averaged infrared spectra of Titan show little change over the 9-month interval between Voyager encounters. By combining Voyager 2 temperature measurements with ground-based geometric albedo determinations, phase integrals of 0.91 � 0.13 and 0.89 � 0.09 were derived for Tethys and Enceladus, respectively. The subsolar point temperature of dark material on Iapetus must exceed 110 K. Temperatures (and infrared optical depths) for the A and C rings and for the Cassini division are 69 � 1 K (0.40 � 0.05), 85 � 1 K (0.10 � 0.03), and 85 � 2 K (0.07 � 0.04), respectively.

Infrared observations of the neptunian system.

The infrared interferometer spectrometer on Voyager 2 obtained thermal emission spectra of Neptune with a spectral resolution of 4.3 cm-1. Measurements of reflected solar radiation were also obtained with a broadband radiometer sensitive in the visible and near infrared. Analysis of the strong C2H2 emission feature at 729 cm-1 suggests an acetylene mole fraction in the range between 9 x 10-8 and 9 x 10-7. Vertical temperature profiles were derived between 30 and 1000 millibars at 70� and 42�S and 30�N. Temperature maps of the planet between 80�S and 30�N were obtained for two atmospheric layers, one in the lower stratosphere between 30 and 120 millibars and the other in the troposphere between 300 and 1000 millibars. Zonal mean temperatures obtained from these maps and from latitude scans indicate a relatively warm pole and equator with cooler mid-latitudes. This is qualitatively similar to the behavior found on Uranus even though the obliquities and internal heat fluxes of the two planets are markedly different. Comparison of winds derived from images with the vertical wind shear calculated from the temperature field indicates a general decay of wind speed with height, a phenomenon also observed on the other three giant planets. Strong, wavelike longitudinal thermal structure is found, some of which appears to be associated with the Great Dark Spot. An intense, localizd cold region is seen in the lower stratosphere, which does not appear to be correlated with any visible feature. A preliminary estimate of the effective temperature of the planet yields a value of 59.3 � 1.0 kelvins. Measurements of Triton provide an estimate of the daytime surface temperature of 38+3-4 kelvins.

TITAN'S SURFACE BRIGHTNESS TEMPERATURES

Radiance from the surface of Titan can be detected from space through a spectral window of low opacity in the thermal infrared at 19 μm (530 cm–1). By combining Composite Infrared Spectrometer observations from Cassinis first four years, we have mapped the latitude distribution of zonally averaged surface brightness temperatures. The measurements are corrected for atmospheric opacity as derived from the dependence of radiance on the emission angle. At equatorial latitudes near the Huygens landing site, the surface brightness temperature is found to be 93.7 ± 0.6 K, in excellent agreement with the in situ measurement. Temperature decreases toward the poles, reaching 90.5 ± 0.8 K at 87°N and 91.7 ± 0.7 K at 88°S. The meridional distribution of temperature has a maximum near 10°S, consistent with Titans late northern winter.

Oceans on titan

If global oceans of methane exist on Titan, the atmosphere above them must be within 2 percent of saturation. The two Voyager radio occultation soundings, made at low latitudes, probably occurred over land, since they imply a relative humidity ≲ 70 percent near the surface. Oceans might exist at other low-latitude locations if the zonal wind velocities in the lowest 3 kilometers are ≤ 4 centimeters per second.

Jupiter's ionosphere: Results from the First Galileo Radio Occultation Experiment

The Galileo spacecraft passed behind Jupiter on December 8, 1995, allowing the first radio occultation measurements of its ionospheric structure in 16 years. At ingress (24°S, 68°W), the principal peak of electron density is located at an altitude of 900 km above the 1-bar pressure level, with a peak density of 105 cm−3 and a thickness of ∼200 km. At egress (43°S, 28°W), the main peak is centered near 2000 km altitude, with a peak density of 2×104 cm−3 and a thickness of ∼1000 km. Two thin layers, possibly forced by upwardly propagating gravity waves, appear at lower altitudes in the ingress profile. This is the first in a two-year series of observations that should help to resolve long-standing questions about Jupiters ionosphere.

Temperature and Composition of Saturn's Polar Hot Spots and Hexagon

Saturns poles exhibit an unexpected symmetry in hot, cyclonic polar vortices, despite huge seasonal differences in solar flux. The cores of both vortices are depleted in phosphine gas, probably resulting from subsidence of air into the troposphere. The warm cores are present throughout the upper troposphere and stratosphere at both poles. The thermal structure associated with the marked hexagonal polar jet at 77°N has been observed for the first time. Both the warm cyclonic belt at 79°N and the cold anticyclonic zone at 75°N exhibit the hexagonal structure.

Infrared Observations of the Uranian System

The infrared interferometer spectrometer (IRIS) on Voyager 2 recorded thermal emission spectra of Uranus between 200 and 400 cm-1 and of Miranda and Ariel between 200 and 500 cm-1 with a spectral resolution of 4.3 cm-1. Reflected solar radiation was also measured with a single-channel radiometer sensitive in the visible and near infrared. By combining IRIS spectra with radio science results, a mole fraction for atmospheric helium of 0.15 � 0.05 (mass fraction, 0.26 � 0.08) is found. Vertical temperature profiles between 60 and 900 millibars were derived from average polar and equatorial spectra. Temperatures averaged over a layer between 400 to 900 millibars show nearly identical values at the poles and near the equator but are 1 or 2 degrees lower at mid-latitudes in both hemispheres. The cooler zone in the southern hemisphere appears darker in reflected sunlight than the adjacent areas. An upper limit for the effective temperature of Uranus is 59.4 kelvins. Temperatures of Miranda and Ariel at the subsolar point are 86 � 1 and 84 � 1 kelvins, respectively, implying Bond albedos of 0.24 � 0.06 and 0.31 � 0.06, respectively. Estimates of phase integrals suggest that these satellites have unusual surface microstructure.

Titan's stratospheric temperatures - A case for dynamical inertia

Temperatures between the 1- and 0.4-mbar barometric pressure levels of Titans atmospere have been retrieved from Voyager IRIS spectral radiances in the λ4-band of CH4. They exhibit a hemispheric asymmetry, being 4–8° K warmeraat 55° S than at 55° N. At the season of the Voyager observations (northern spring equinox), one would have expected a symmetric distribution if the opacity for solar and infrared radiation were uniformly distributed with latitude, because the radiative time constant is so short. Instead, the temperature field appears to lag the solar heating by nearly a quarter of a Titan year. The possibility of an asymmetry in the meridional distribution of opacity about the equator cannot be discounted, but an alternate explanation follows from the need for angular momentum transport concurrent with seasonally varying temperatures in Titans stratosphere, so that the cyclostrophic thermal wind relation between zonal winds and temperatures can be maintained. A simple, zonally symmetric model predicts that the required transport can be affected by a circulation with upwelling at northern latitudes and subsidence at midsouthern latitudes at the time of the Voyager encounter. The adiabatic heating and cooling associated with these motions can produce the observed asymmetry in temperature. In effect, the circulation causes the temperature field to lag the radiative equilibrium field. The dynamical time scale for this lag is the radiative relaxation time multiplied by the Richardson number, and is comparable to a season in Titans stratosphere.

Galileo radio occultation measurements of Io's ionosphere and plasma wake

Six radio occultation experiments were conducted with the Galileo orbiter in 1997, yielding detailed measurements of the distribution and motion of plasma surrounding Io. This distribution has two components. One is highly asymmetric, consisting of a wake or tail that appears only on the downstream side and extends to distances as large as 10 Io radii. The other resembles a bound ionosphere and is present within a few hundred kilometers of Ios surface throughout the upstream and downstream hemispheres. Motion of plasma within the wake was measured through cross correlation of data acquired simultaneously at two widely separated terrestrial antennas. Plasma near Ios equatorial plane is moving away from Io in the downstream direction. Its speed increases from 30 km s−1 at a distance of 3 Io radii from the center of Io to 57 km s−1 at 7 Io radii. The latter corresponds to corotation with Jupiters magnetic field, which suggests that bulk plasma motion rather than wave motion is being observed. Results for the bound ionosphere include vertical profiles of electron density at 10 locations near Ios terminator. The ionosphere is substantial, with the peak density exceeding 50,000 cm−3 at 9 out of 10 locations and reaching a maximum of 277,000 cm−3. The peak density varies systematically with Io longitude, with maxima near the center of the hemispheres facing toward (0°W) and away from (180°W) Jupiter and minima near the center of the downstream (90°W) and upstream (270°W) hemispheres. This pattern may be related to the Alfvenic current system induced by Ios motion through magnetospheric plasma. The vertical extent of the bound ionosphere increases from ∼200 km near the center of the upstream hemisphere to ∼400 km near the boundary between the leading and trailing hemispheres. There is a close resemblance between one ionospheric profile and a Chapman layer, and the topside scale height implies a plasma temperature of 202±14 K if Na+ is the principal ion. Two intense volcanic hot spots, Kanehekili and 9606A, may be influencing the atmospheric structure at this location.