Abstract.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 29øN to 64øS and local times from 0600 through midnight to 1800 during early summer in the southern hemisphere (Ls -264 ø-308 ø). 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 i K and 2 Pa, respectively, far smaller than in previous radio occultation measurements at Mars. The profiles resolve the radiativeconvective 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 15ø-30øS 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 •02 km altitude implies a wind speed of 33 m s -t, 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.
In the course of 71 days in lunar orbit, from 19 February to 3 May 1994, the Clementine spacecraft acquired just under two million digital images of the moon at visible and infrared wavelengths. These data are enabling the global mapping of the rock types of the lunar crust and the first detailed investigation of the geology of the lunar polar regions and the lunar far side. In addition, laser-ranging measurements provided the first view of the global topographic figure of the moon. The topography of many ancient impact basins has been measured, and a global map of the thickness of the lunar crust has been derived from the topography and gravity.
The Voyager 2 encounter with the Neptune system included radio science investigations of the masses and densities of Neptune and Triton, the low-order gravitational harmonics of Neptune, the vertical structures of the atmospheres and ionospheres of Neptune and Triton, the composition of the atmosphere of Neptune, and characteristics of ring material. Demanding experimental requirements were met successfully, and study of the large store of collected data has begun. The initial search of the data revealed no detectable effects of ring material with optical depth tau [unknown] 0.01. Preliminary representative results include the following: 1.0243 x 10(26) and 2.141 x 10(22) kilograms for the masses of Neptune and Triton; 1640 and 2054 kilograms per cubic meter for their respective densities; 1355 +/- 7 kilometers, provisionally, for the radius of Triton; and J(2) = 3411 +/- 10(x 10(-6)) and J(4) = -26(+12)(-20)(x10(-6)) for Neptune's gravity field (J>(2) and J(4) are harmonic coefficients of the gravity field). The equatorial and polar radii of Neptune are 24,764 +/- 20 and 24,340 +/- 30 kllometers, respectively, at the 10(5)-pascal (1 bar) pressure level. Neptune's atmosphere was probed to a pressure level of about 5 x 10(5) pascals, and effects of a methane cloud region and probable ammonia absorption below the cloud are evident in the data. Results for the mixing ratios of helium and ammonia are still being investigated; the methane abundance below the clouds is at least 1 percent by volume. Derived temperature-pressure profiles to 1.2 x 10(5) pascals and 78 kelvins (K) show a lapse rate corresponding to "frozen" equilibrium of the para- and ortho-hydrogen states. Neptune's ionosphere exhibits an extended topside at a temperature of 950 +/- 160 K if H(+) is the dominant ion, and narrow ionization layers of the type previously seen at the other three giant planets. Triton has a dense ionosphere with a peak electron concentration of 46 x 10(9) per cubic meter at an altitude of 340 kilometers measured during occultation egress. Its topside plasma temperature is about 80 +/- 16 K if N(2)(+) is the principal ion. The tenuous neutral atmosphere of Triton produced distinct signatures in the occultation data; however, the accuracy of the measurements is limited by uncertainties in the frequency of the spacecraft reference oscillator. Preliminary values for the surface pressure of 1.6 +/- 0.3 pascals and an equivalent isothermal temperature of 48 +/- 5 K are suggested, on the assumption that molecular nitrogen dominates the atmosphere. The radio data may be showing the effects of a thermal inversion near the surface; this and other evidence imply that the Triton atmosphere is controlled by vapor-pressure equilibrium with surface ices, at a temperature of 38 K and a methane mixing ratio of about 10(-4).
Observations of the icy Galilean satellites, conducted during 1987-1991 with the Arccibo 13-cm system and the Goldstone 3.5-cm system, yield significant improvements in our knowledge of the satellites' radar properties. Hardly any wavelength dependence is seen for either the total power radar albedo •T or the circular polarization ratio bt c . For Europa, Ganymede, and Callisto our 13-cm estimates of mean values and rms dispersions are •T = 2.60 + 0.22, 1.39 + 0.14, and 0.69 + 0.06; and bl.c = 1.53 + 0.03, 1.43 + 0.06, and 1.17 + 0.04. Radar albedo features arc seen on each satellite. Evidence for btc featurcs is lacking, except for indications of a weak hemispheric asymmetry for Callisto. That intersatellite and intrasatellite fractional variations in albedo greatly exceed variations in bl.c is consistent with prcdictions of coherent backscatter theory and implies that albedo might be a crude indicator of relative silicate abundance. The satellites' albedo distributions overlap. The most prominent radar featurcs are tentatively identified with Galileo Regio and the Valhalla basin. Estimates of echo Doppler frcquencies show Callisto to be lagging its ephemeris by 200 + 50 km. INTRODUCTIONThe radar echoes from Europa, Ganymede, and Callisto are extraordinary. It has been known for 15 years that these objects' radar reflectivities dwarf values reported for comets, the Moon, the inner planets, and nonmetallic asteroids. When the radar transmission is circularly polarized, the icy satellites return echoes with the incident handedness preserved, in contrast with the behavior of other targets. At the principal Arecibo wavelength of 13 cm, the circular polarization ratio Rc, of echo power in the same sense of circular polarization as transmitted (the SC sense) to that in the opposite (OC) sense, exceeds unity for each of the icy Galilean satellites but is only ~0.1 for the Moon and less than 0.4 for most other planetary radar targets. The linear polarization ratio (gL = OL/SL) is about one half for all three satellites, again considerably larger than for other targets. The satellites' 13-cm radar albedos increase from Callism to Ganymede to Europa, whose OC radar reflectivity is the same as that of a metal sphere. Observations of Ganymede at Goldstone in 1977 [Goldstein and Green, 1980] indicated that this object's exotic radar behavior is preserved at 3.5 cm. (Articles reporting radar observations of the satellites are listed in Table 1.) Most efforts to understand the satellites' radar signatures have focused on the search for an electromagnetic scattering mechanism capable of yielding strong echoes with Ix c greater than unity. The 1978-1989 literature suggested that the satellites' signatures might be understood as being due to Eshleman, 1986a] and/or mode-decoupled, multiple, total-internal reflection [Goldstein and Green, 1980; Eshleman, 1986b] from subsurface variations in refractive index. Ostro and Shoemaker [1990] approached the problem from a geologic perspective and outlined explanations for the satellites' ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.