Peter G. Ford

Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars

The Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor spacecraft, has measured the topography, surface roughness, and 1.064-μm reflectivity of Mars and the heights of volatile and dust clouds. This paper discusses the function of the MOLA instrument and the acquisition, processing, and correction of observations to produce global data sets. The altimeter measurements have been converted to both gridded and spherical harmonic models for the topography and shape of Mars that have vertical and radial accuracies of ~1 m with respect to the planets center of mass. The current global topographic grid has a resolution of 1/64° in latitude × 1/32° in longitude (1 × 2 km^2 at the equator). Reconstruction of the locations of incident laser pulses on the Martian surface appears to be at the 100-m spatial accuracy level and results in 2 orders of magnitude improvement in the global geodetic grid of Mars. Global maps of optical pulse width indicative of 100-m-scale surface roughness and 1.064-μm reflectivity with an accuracy of 5% have also been obtained.

Advanced CCD imaging spectrometer (ACIS) instrument on the Chandra X-ray Observatory

The ACIS instrument has been operating for three years in orbit, producing high quality scientific data on a wide variety of X-ray emitting astronomical objects. Except for a brief period at the very beginning of the mission when the CCDs were exposed to the radiation environment of the Outer van Allen Belts which resulted in substantial radiation damage to the front illuminated CCDs, the instrument has operated nearly flawlessly. The following report presents a description of the instrument, the current status of the instrument calibration and a few highlights of the scientific results obtained from the Guaranteed Observer Time.

Venus topography and kilometer-scale slopes

During the first 8 months of the Magellan mission, the radar altimeter has made some three million measurements of the surface of Venus covering the latitude range from 85°N to 80°S. Methods involving range correlation, Doppler filtering, multi-burst summation, and range migration are used to focus the observations and to achieve high surface resolution. Results are presented as maps of the global distribution of topography, meter-scale roughness, and power reflection coefficient. The results are similar to those reported in previous experiments (surface heights exhibit a unimodal distribution with more than 80% of the surface lying within 1 km of the 6051.84-km mean radius) but the higher resolution of the Magellan altimeter has disclosed several surprisingly steep features, e.g. the southwest face of the Maxwell Montes, the southern face of the Danu Montes, and the chasmata to the east of Thetis Regio, where average kilometer-scale slopes of greater than 30° are not uncommon. This conclusion is corroborated by close inspection of synthetic aperture radar imagery.

Arecibo radar mapping of the lunar poles : A search for ice deposits

The Arecibo 12.6-centimeter wavelength radar system was used to image the polar regions of the moon at a resolution of 125 meters in a search for ice deposits in areas of possible permanent shadow from the sun. No areas greater than 1 square kilometer were found with high radar backscatter cross sections and high circular polarization ratios, properties suggestive of the presence of ice. A number of areas smaller than 1 square kilometer were found with these properties, but optical images from spacecraft missions have shown some of these features to be in sunlight. Arecibo radar images of Sinus Iridum at latitude 47°N also showed a number of small features with similar properties. The coincidence of some of these features with the radar-facing slopes of craters and their presence in sunlit areas suggests that very rough surfaces rather than ice deposits are responsible for their unusual radar properties.

Magellan: radar performance and data products.

The Magellan Venus orbiter carries only one scientific instrument: a 12.6-centimeter wavelength radar system shared among three data-taking modes. The synthetic-aperture mode images radar echoes from the Venus surface at a resolution of between 120 and 300 meters, depending on spacecraft altitude. In the altimetric mode, relative height measurement accuracies may approach 5 meters, depending on the terrains roughness, although orbital uncertainties place a floor of about 50 meters on the absolute uncertainty. In areas of extremely rough topography, accuracy is limited by the inherent line-of-sight radar resolution of about 88 meters. The maximum elevation observed to date, corresponding to a planetary radius of 6062 kilometers, lies within Maxwell Mons. When used as a thermal emission radiometer, the system can determine surface emissivities to an absolute accuracy of about 0.02. Mosaicked and archival digital data products will be released in compact disk (CDROM) format.

Venus surface radiothermal emission as observed by Magellan

The Magellan radiometer experiment has observed the 12.6-cm-wavelength radio emissivity of more than 91% of the Venus surface during the first 8 months of its operation. The global mean value of emissivity seen using horizontal linear polarization is 0.845, a value that corresponds to a dielectric permittivity of between 4.0 and 4.5, depending on the surface roughness. These values are consistent with the dry basaltic minerals thought to compose the bulk of the Venus surface. The 2° beam width of the Magellan high-gain antenna yields surface resolutions that vary from 15 by 23 km at periapsis (10°N latitude) to about 85 km at the north pole; measurements of emissivity carry an absolute error of about 0.02 and can distinguish local variations as small as 0.005. The observations have confirmed earlier findings that a few regions on Venus, primarily located at high elevations, possess unexpectedly low values of radiothermal emissivity, occasionally reaching as low as 0.3. Some members of each of five classes of features are found to display anomalously low values of emissivity: highlands, volcanoes, novae, ridges, and impact craters. Two possible explanations for these low values have been advanced: (1) emission from a highly reflective single interface between the atmosphere and a surface material having a bulk dielectric permittivity of order 80; or (2) emission from the surface of a low-loss medium having a more usual permittivity (of order 5) but which contains many voids permitting efficient internal multiple scattering. Distinguishing between these hypotheses is difficult with the data presently in hand.

Features on Venus generated by plate boundary processes

Various observations suggest that there are processes on Venus that produce features similar to those associated with plate boundaries on Earth. Synthetic aperture radar images of Venus, taken with a radar whose wavelength is 12.6 cm, are compared with GLORIA images of active plate boundaries, obtained with a sound source whose wavelength is 23 cm. Features similar to transform faults and to abyssal hills on slow and fast spreading ridges can be recognized within the Artemis region of Venus but are not clearly visible elsewhere. The composition of the basalts measured by the Venera 13 and 14 and the Vega 2 spacecraft corresponds to that expected from adiabatic decompression, like that which occurs beneath spreading ridges on Earth. Structures that resemble trenches are widespread on Venus and show the same curvature and asymmetry as they do on Earth. These observations suggest that the same simple geophysical models that have been so successfully used to understand the tectonics of Earth can also be applied to Venus.

Pancakelike domes on Venus

The shape of seven large domes on the plains of Venus, with volumes between 100 and 1000 km3, is compared with that of an axisymmetric gravity current spreading over a rigid horizontal surface. Both the altimetric profiles and the horizontal projection of the line of intersection of domes on the synthetic aperture radar images agree well with the theoretical similarity solution for a Newtonian fluid but not with the shape calculated for a rigid-plastic rheology nor with that for a static model with a strong skin. As a viscous current spreads, it generates an isotropic strain rate tensor whose magnitude is independent of radius. Such a flow can account for the randomly oriented cracks that are uniformly distributed on the surface of the domes. The stress induced by the flow in the plains material below is obtained and is probably large enough to produce the short radial cracks in the surface of the plains beyond the domes. The viscosity of the domes can be estimated from their thermal time constants if spreading is possible only when the fluid is hot and lies between 1014 and 1017 Pa s. Laboratory experiments show that such viscosities correspond to temperatures of 610° to 700°C in dry rhyolitic magmas. These temperatures agree with laboratory measurements of the solidus temperature of wet rhyolite. These results show that the development of the domes can be understood using simple fluid dynamical ideas and that the magmas involved can be produced by wet melting at depths below 10 km, followed by eruption and degassing.

Winter clouds over the North Martian Polar Cap

The Mars Orbiter Laser Altimeter (MOLA) experiment, carried on the Mars Global Surveyor spacecraft, has observed echoes from cloud tops on roughly 2.5% of the total data taken above 70°N over the northern winter Martian polar hood from March to June 1998. Sloping wavefronts are commonly seen at latitudes above 70°N, implying the presence of propagating buoyancy waves. Since these wavefronts frequently extend from the surface up to 10 km at a time when CO2 is known to be condensing on the polar surface, it seems likely that the laser-scattering particles consist primarily of CO2 ice, and that the near-surface temperature lapse rate is 0.85 K km−1, set by the vapor pressure of dry ice. From the observed wavelengths, where available, we have calculated the corresponding phase velocities; some of these waves appear to be correlated with surface discontinuities, and may represent lee waves.

Venus: global surface radar reflectivity.

Observations of the surface of Venus, carried out by the Pioneer Venus radar mapper at a wavelength of 17 centimeters, reveal a global mean reflectivity at normal incidence of 0.13 � 0.03. Over the surface, variations from a low of 0.03 � 0.01 to a high of 0.4 � 0.1 are found, with Theia Mons, previously identified as possibly volcanic, showing a value of 0.28 � 0.07. Regions of high reflectivity may consist of rocks with substantial inclusions of highly conductive sulfides.