JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. E5, PAGES 7781-7797, MAY 25, 1992 The Mars Observer Laser Altimeter Investigation M. T. ZUBER,•,2 D. E. SMITH,• S.C. SOLOMON, 3 D. O. MUHLEMAN,4 J. W. HEAD, 5 J. B. GARVIN, • J. B. ABSHIRE, 1 AND J. L. BUFTON1 The primary objective of the Mars Observer laser altimeter (MOLA) investigation is to determine globally the topographyof Mars at a level suitablefor addressing problems in geologyand geophysics. Secondary objectives are to characterize the 1064-nm wavelength surface reflectivity of Mars to contribute to analyses of global surface mineralogy and seasonal albedo changes, to assist in addressingproblems in atmospheric circulation, and to provide geodetic control and topographic context for the assessment of possiblefuture Mars landing sites. The principal componentsof MOLA are a diode-pumped, neodymium-doped yttrium aluminum garnet laser transmitter that emits 1064-nm wavelength laser pulses, a 0.5-m-diameter telescope, a silicon avalanche photodiode detector, and a time interval unit with 10-nsresolution. MOLA will provide measurementsof the topography of Mars within approximately 160-m footprints and a center-to-center along-track footprint spacing of 300 m along the Mars Observer subspacecraft ground track. The elevation measurements will be quantized with 1.5 m vertical resolution before correction for orbit- and pointing-induced errors. MOLA profiles will be assembledinto a global 0.2ø x 0.2øgrid that will be referenced to Mars' center of mass with an absoluteaccuracy of approximately 30 m. Other data productswill include a global grid of topographic gradients, corrected individual profiles, and a global 0.2ø x 0.2ø grid of 1064-nm surface reflectivity. INTRODUCTION Surface topography is one of the fundamental measure- ments required to understand the structure and evolution of solid planetary bodies. Topographic measurements refer- enced to the planetary center of mass provide a basis for interpreting the gravity field in the context of a planet's internal structure and state of stress. Regional-scale topog- raphy and derived topographic gradients are frequently required in investigationsof tectonic, volcanic, impact, and surface modification processes.Topography also provides a necessaryconstraint for atmospheric circulation and climate models. Because of its general importance to so many geoscience disciplines, global characterization of topogra- phy was advanced as one of the fundamental goals of NASA's Mars Observer (MO) Mission [Mars Observer Science Working Group, 1987]. The topographic objective of the Mars Observer mission will be accomplished using data from the Mars Observer laser altimeter (MOLA). Specific measurementobjectives of the MOLA investigation are (1) to derive a global, geodeti- cally referenced 0.2ø x 0.2ø topographic grid of Mars with a vertical accuracyof better than 30 m suitable for addressing problems in geophysics, geology, and atmospheric circula- tion; (2) to acquire globally distributed topographic profiles of the Martian surface on short baselines (---100 km) with a vertical precision of better than 2 m suitablefor addressing a i Laboratory forTerrestrial Physics, NASAGoddard Space Flight Center, Greenbelt, Maryland. 2Department of Earthand Planetary Sciences, Johns Hopkins University, Baltimore, Maryland. 3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge. 4Division of Geological and Planetary Sciences, California Insti- tute of Technology, Pasadena. 5Department of Geological Sciences, Brown University, Provi- dence, Rhode Island. Copyright 1992 by the American Geophysical Union. Paper number 92JE00341. 0148-0227/92/92JE-00341 $05.00 range of problems in local- and regional-scale geology; and (3) to determine a global 0.2ø x 0.2ø grid of the 1064-nm wavelength surface reflectivity of Mars to a precision of ---20%, suitable for contributing to studies of global surface mineralogy and seasonal albedo variations. Present knowledge of the topography of Mars has been derived from several sources and is of widely varying spatial and vertical resolution. Topography has been determined from infrared [Herr et al., 1970] and ultraviolet [Barth and Hord, 1971; Hord, 1972; Hord et al., 1974] spectroscopy, Earth-based radar observations [Goldstein and Gillmore, 1963; Kotelnikov et al., 1963, 1983; Goldstein, 1965; Dyce et al., 1967; Pettengill et al., 1969, 1971, 1973; Downs et al., 1971, 1973, 1975, 1978, 1982; Fjeldbo et al., 1977; Roth et al., 1980], radio occultation data [Cain et al., 1972; Kliore et al., 1973; Christensen, 1975; Simpson et al., 1977; Lindal et al., 1979] and stereophotogrammetry [Blasius, 1973; Wu et al., 1973, 1984; Wu, 1978, 1979; Blasius and Cutts, 1981]. The Mars Digital Elevation Model [Wu et al., 1986; U.S. Geo- logical Survey, 1989], shown in Figure 1, incorporates the above data types and is characterized by a typical horizontal resolutionof 1/64 ø(---1 km at the equator) and vertical errors that range from 500 m near the equator to over 2.5 km near the poles (e.g., Plate l). The highest-resolution spherical harmonic representationof the topographicfield is of degree and order 16 [Bills and Ferrari, 1978] and has a coarser resolution than current gravity models [Balmino et al., 1982; Smith et al., 1990b]. Much higher resolution topography exists for certain areas from stereophotogrammetric analy- ses, but these data are not referenced to a global datum and are therefore of limited quantitative utility. The Mars Observer mission is expected to improve con- siderably our knowledge of Martian topography. MOLA data should provide a high-integrity, internally consistent long-wavelengthtopographic field appropriate for addressing geophysical and atmospheric circulation problems. In addi- tion, the topography will be used to derive a global geodetic control grid referenced to Mars' center of mass. MOLA should also produce high-resolution short baseline (---100 km) profiles suitable for addressing many geological prob- 7781