Richard A. Simpson

The Clementine Mission to the Moon: Scientific Overview

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.

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.

Particle size distributions in Saturn's rings from Voyager 1 radio occultation

Abstract Observations of microwave opacity τ[λ] and near forward scatter from Saturns rings at wavelengths λ of 3.6 and 13 cm from the Voyager 1 ring occultation experiment contain information regarding ring particle sizes in the range of about a = 0.01 to 15 m radius. The opacity measurements τ[3.6] and τ[13] are sufficient to constrain the scale factor n(a0) and index q of a power law incremental size distribution n(a) = n(a0)[a0/a]q, assuming known minimum and maximum sizes and a many-particle-thick model. The families of such distributions are highly convergent in the centimeter-size range. Forward scatter at 3.6 cm can be used to solve for a general distribution over the radius range 1 ⪷ a ⪷ 15 m by integral inversion and inverse scattering methods, again assuming a many-particle-thick slab-type radiative transfer model. Distributions n(a) valid over 0.01 ⪷ a ⪷ 15 m are obtained by combining the results from the two types of measurements above. Mass distributions may be computed directly from n(a). Such distributions, partly measured and partly synthesized, have been obtained for four features in the ring system centered at 1.35, 1.51, 2.01, and 2.12 Saturn radii (Rs). The size and mass distributions both cut off sharply at a ≅ 4–5 m; the mass distribution peaks over the narrow size range 3 ⪷ a ⪷ 4 m for all four locations. No single power law distribution is consistent with the data over the entire interval 0.01 ⪷ a ⪷ 5 m , although a power law-type model is consistent with the data over a limited size range of 0.01 ⪷ a ⪷ 1 m , where the indices q = 3.4 and 3.3 are obtained from the slab model for the features located at 1.51 and 2.01 Rs. The fractional contribution of the suprameter particles to the microwave opacity in each feature appears to be about 1 3 , 1 3 , 2 3 , and 1 , respectively, with the fraction at 2.12 Rs being the least certain. The cumulative surface mass per unit area obtained for the classical slab model is approximately 11, 16, 41, and 132 g/cm2 for the four features, respectively, if the particles are solid H2O ice. Both the fractional opacity and the mass density estimates represent upper bounds implied by the assumption of a uniformly mixed set of particles in a many-particle-thick vertical profile; lower estimates would result if the rings were assumed to be nearly a monolayer or if the vertical distribution of particles were size dependent.

Radio science observations with Mars Global Surveyor: Orbit insertion through one Mars year in mapping orbit

Mars Global Surveyor (MGS) radio science comprises studies of the atmosphere and gravity of the planet. Perturbations of the 3.6-cm λ radio path by the atmosphere during periods of atmospheric occultation provide the vertical temperature-pressure structure T[p(r)] to accuracies at the surface of ΔT ≈ 0.4 K and Δp ≈ 2 Pa, and ∼10 K and ∼0.6 Pa at altitudes of 40–50 km; the error in radius is Δr ≈ 1 m at all levels. Accurate knowledge of the radius permits fixing of the T-p structure to the geopotential and use of the gradient wind equation to calculate components of the wind. Systematic sampling of the atmosphere in combination with the accuracy of the MGS radio system supports studies of the large-scale dynamics of the atmosphere, including seasonal variations of the atmospheric fields and embedded waves such as Kelvin and Rossby waves. Terminator region ionospheric electron density profiles are obtained successfully much of the time but on occasion are undetectable with the MGS system. Two-way radio tracking of MGS with uncertainties in the line-of-sight velocity of several to tens of μm s−1 and less supports solution for spherical harmonic models of the gravity field of order and degree in the range of 75×75, although the degree and order of meaningful terms is limited by the ∼400 km spacecraft altitude to ∼62×62, corresponding to a resolution of a few degrees of arc on the surface. This resolution of gravity is sufficient to support geophysical studies of the planets interior structure and history. Additional radio science investigations include the search for gravitational radiation and observation of very low grazing angle forward scattering by the surface of Mars.

Voyager 2 radio science observations of the Uranian system Atmosphere, rings, and satellites

Voyager 2 radio occultation measurements of the Uranian atmosphere were obtained between 2 and 7 degrees south latitude. Initial atmospheric temperature profiles extend from pressures of 10 to 900 millibars over a height range of about 100 kilometers. Comparison of radio and infrared results yields mole fractions near the tropopause of 0.85 and 0.15 � 0.05 for molecular hydrogen and helium, respectively, if no other components are present; for this composition the tropopause is at about 52 kelvins and 110 millibars. Distinctive features in the signal intensity measurements for pressures above 900 millibars strongly favor model atmospheres that include a cloud deck of methane ice. Modeling of the intensity measurements for the cloud region and below indicates that the cloud base is near 1,300 millibars and 81 kelvins and yields an initial methane mole fraction of about 0.02 for the deep atmosphere. Scintillations in signal intensity indicate small-scale stucture throughout the stratosphere and upper troposphere. As judged from data obtained during occultation ingress, the ionosphere consists of a multilayer structure that includes two distinct layers at 2,000 and 3,500 kilometers above the 100-millibar level and an extended topside that may reach altitudes of 10,000 kilometers or more. Occultation measurements of the nine previously known rings at wavelengths of 3.6 and 13 centimeters show characteristic values of optical depth between about 0.8 and 8; the maxim value occurs in the outer region of the ∈ ring, near its periapsis. Forward-scattered signals from this ring have properties that differ from those of any of Saturns rings, and they are inconsistent with a discrete scattering object or local (three-dimensional) assemblies of orbiting objects. These signals suggest a new kdnd of planetary ring feature characterized by highly ordered cylindrical substructures of radial scale on the order of meters and azimuthal scale of kilometers or more. From radio data alone the mass of the Uranian system is GMsys = 5,794,547– 60 cubic kilometers per square second; from a combination of radio and optical navigation data the mass of Uranus alone is GMu = 5,793,939� 60 cubic kilometers per square second. From all available Voyager data, induding imaging radii, the mean uncompressed density of the five major satellites is 1.40� 0.07 grams per cubic centimeter; this value is consistent with a solar mix of material and apparently rules out a cometary origin of the satellites.

The structure of Venus’ middle atmosphere and ionosphere

The atmosphere and ionosphere of Venus have been studied in the past by spacecraft with remote sensing or in situ techniques. These early missions, however, have left us with questions about, for example, the atmospheric structure in the transition region from the upper troposphere to the lower mesosphere (50–90 km) and the remarkably variable structure of the ionosphere. Observations become increasingly difficult within and below the global cloud deck (<50 km altitude), where strong absorption greatly limits the available investigative spectrum to a few infrared windows and the radio range. Here we report radio-sounding results from the first Venus Express Radio Science (VeRa) occultation season. We determine the fine structure in temperatures at upper cloud-deck altitudes, detect a distinct day–night temperature difference in the southern middle atmosphere, and track day-to-day changes in Venus’ ionosphere.

The microwave opacity of Saturn's rings at wavelengths of 3.6 and 13 cm from Voyager 1 radio occultation

Abstract Radio occultation observations of Saturns rings with Voyager 1 provided independent measurements of complex (amplitude and phase) microwave extinction and near-forward scattering cross section of the rings at wavelengths (λ) of 3.6 and 13 cm. The ring opening was 5.9°. The normal microwave opacities, τ[3.6] and τ[13], provide a measure of the total cross-sectional area of particles larger than about 1 and 4 cm radius, respectively. Ring C exhibits gently undulating (∼ 1000 km) structure of normal opacity τ[3.6] ≲ 0.25 except for several narrow imbedded ringlets of less than about 100 km width and τ[3.6] ∼ 0.5 to 1.0. The normalized differential opacity Δτ/τ[3.6], where Δτ = τ[3.6] − τ[13], is about 0.3 over most of ring C, indicating a substantial fraction of centimeter-size particles. Some narrow imbedded ringlets show marked increases in Δτ/τ[3.6] near their edges, implying an enhancement in the relative population of centimeter-size and smaller particles at those locations. In the Cassini division, several sharply defined gaps separate regions of opacity τ ∼ 0.08 and τ ∼ 0.25; the opacity in the Cassini Division appears to be nearly independent of λ. The boundary features at the outer edges of ring C and the Cassini Division are remarkably similar in width and opacity profile, suggesting a similar dynamical control. Ring A appears to be nearly homogeneous over much of its width with 0.6 τ[3.6] ⪞ 1.2 . The differential opacity for the inner one-fourth of ring B is Δτ/τ[3.6] ∼ 0.15. There are no gaps in ring B exceeding about 2 km in width. Ring F was observed at 3.6 cm as a single ringlet of radial width ⪝ 2 km , but was not detected in 13 cm data.

RADIO SCIENCE INVESTIGATIONS WITH MARS OBSERVER

Mars Observer radio science investigations focus on two major areas of study: the gravity field and the atmosphere of Mars. Measurement accuracies expressed as an equivalent spacecraft velocity are expected to be of the order of 100 μm/s (for both types of investigations) from use of an improved radio transponder for two-way spacecraft tracking and a highly stable on-board oscillator for atmospheric occultation measurements. Planned gravity investigations include a combination of classical and modern elements. A spherical harmonic (or equivalent) field model of degree and order in the range 30–50 will be obtained, while interpretation will be in terms of internal stress and density models for the planet, using the topography to be obtained from the Mars Observer laser altimeter. Atmospheric investigations will emphasize precision measurement of the thermal structure and dynamics in the polar regions, which are regularly accessible as a result of the highly inclined orbit. Studies based on the measurements will include polar processes, cycling of the atmosphere between the poles, traveling baroclinic disturbances, small-scale waves and turbulence, the planetary boundary layer, and (possibly) the variability and altitude of the ionosphere. As the radio occultation is insensitive to dust in the atmosphere per se and measures only the resulting change in thermal structure, it is expected that the radio technique can contribute to understanding of dust storm phenomena. Mutual observations of the atmosphere by means of radio occultation and by the pressure modulator infrared radiometer and the thermal emission spectrometer are expected to strengthen the reliability and accuracy of all three investigations.

Reanalysis of Clementine bistatic radar data from the lunar South Pole

On April 9, 1994, the Clementine spacecraft high-gain antenna was aimed toward the Moons surface, and the resulting 13-cm wavelength radio echoes were received on Earth. Using these data, we have found that the lunar surface generally follows a Lambertian bistatic scattering function σ0 = KD cosθi cosθs with KD∼0.003 for the opposite (expected) sense of circular polarization and KD∼0.001 for the same (unexpected) sense. But there are important deviations (of up to 50% in some parts of the echo spectrum) from this simple form. Based on an earlier analysis of these same data, Nozette et al. [1996] claimed detection of an enhancement in echoes with the same sense circular polarization from regions near the South Pole in a near-backscatter geometry. Such behavior would be consistent with presence of perhaps large quantities of water ice near the pole. We have been unable to reproduce that result. Although we find weak suggestions of enhanced echoes at the time of South Pole backscatter, similar features are present at earlier and later times, adjacent frequencies, and in the opposite circular polarization. If enhanced backscatter is present, it is not unique to the South Pole; if not unique to the pole, then ice appears less likely as an explanation for the enhancement.

Scattering properties of the Venusian surface: Preliminary results from Magellan

Radar backscatter functions for incidence angles 0≤ϕ≤4°–10° have been derived from Magellan altimetry radar echoes. The procedure includes constrained solution of a system of simultaneous equations for which the echo spectrum and echo time profile are inputs. An initially practical and workable set of constraints has been applied; optimization and improved results are expected as the analysis matures. The scattering functions yield information on small scale surface structure (tens of centimeters to tens of meters) but averaged over hundreds of square kilometers. RMS surface slopes derived from fits of analytic functions to the results have been converted to map form and show patterns similar to those reported using other techniques. A scattering law of exponential form matches the data better than either the Hagfors or Gaussian form in most areas, but the Hagfors function is generally best at the smoothest sites. Limited study of radar image data indicates that average backscatter cross section, and possibly its derivative with incidence angle, can be derived at oblique angles (17°≤ϕ≤45°). The altimetry results in combination with those derived from the synthetic aperture radar will strongly constrain the form of over the range 0≤ϕ≤45°. Offsets of the echo peak in altimetry spectra from those expected of nadir echoes are surprisingly common and are loosely correlated with Venus topography; to date no specific cause for this phenomenon has been identified.