Topography of the northern hemisphere of Mars from the Mars orbiter laser altimeter

Science

Volumn 279, Issue 5357, 1998, Pages 1686-1692

  Smith, D E ^a   Zuber, M T ^a,b   Frey, H V ^a   Garvin, J B ^a   Head, J W ^c   Muhleman, D O ^d   Pettengill, G H ^b   Phillips, R J ^e   Solomon, S C ^f   Zwally, H J ^a   Banerdt, W B ^g   Duxbury, T C ^g  

^a NASA GODDARD SPACE FLIGHT CENTER   (United States)

^b MASSACHUSETTS INSTITUTE OF TECHNOLOGY   (United States)

^c BROWN UNIVERSITY   (United States)

^d CALIFORNIA INSTITUTE OF TECHNOLOGY   (United States)

^e WASHINGTON UNIVERSITY   (United States)

^f GEOPHYSICAL LABORATORY   (United States)

^g CALIFORNIA INSTITUTE OF TECHNOLOGY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ALTIMETER; MARTIAN SURFACE; SLOPE; SURFACE ROUGHNESS; TOPOGRAPHY;

ARTICLE; ASTRONOMY; PRIORITY JOURNAL; SPACE FLIGHT; SURFACE PROPERTY; TOPOGRAPHY;

FLIGHT EXPERIMENT; LONG DURATION; MARS GLOBAL SURVEYOR PROJECT; UNMANNED;

ATMOSPHERE; EXTRATERRESTRIAL ENVIRONMENT; ICE; MARS; SPACECRAFT;

MARS;

EID: 7144222761     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.279.5357.1686     Document Type: Article

References (53)

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3. Surface reflectivity or albedo at the laser wavelength was determined from the ratio of received to transmitted laser energy. Outgoing pulse energy is sampled with an optical fiber that measures ∼95% of the outgoing pulse cross section. The received energy is derived from the area under the curve representing the best estimate of the returned pulse shape. The accuracy of the reflectivity measurement after calibration is ∼5%.
4. MOLA's return pulse width provides a measure of the footprint-scale slope or rms roughness of the terrain. The instrument detection electronics contain a bank of four parallel bandpass filters with widths of 20, 60, 180, and 540 nsec that correspond to foot-print-scale surface slopes of 1°, 3°, 10°, and 27°. The return pulse triggers the filter that most closely matches the width of the return. Measurement refinement is made using the difference in the times at which the leading and trailing edges of the returned pulse cross the detection threshold. The time difference is translated into an equivalent pulse spread.
5. Ranges were obtained during periapse passes while the MGS spacecraft was at altitudes of ≤786 km bove the martian surface. The maximum range was limited by system hardware. Despite non-optimal operating conditions the instrument obtained valid range measurements for ∼99% of the output pulses. The laser beam divergence of 400 μrad resulted in a surface spot size of 70 to 300 m.
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48. 1/6, where n is the Manning roughness coefficient, here square-root-gravity scaled from an assumed terrestrial value of 0.02. Hydraulic radius is obtained directly from MOLA data, assuming that the water level reached the paired-terraces and attempting to adjust the channel perimeter for post-flow mass wasting. A more difficult problem is estimation of slope. In principle, the apparent slope measured along the groundtrack can be projected into the downstream direction to yield true channel slope. The orbital track makes an angle of 50° with the downstream vector and true slope = apparent slope/ cos(50°). The presence of debris flows on the channel floor could locally modify this slope.
49. 1/2, where h is channel depth and g is gravity] of 2.0. This value indicates supercritical flow.
50. The filter of the Viking Orbiter Camera nearest to the MOLA wavelength is red [0.5 μm to 0.7 μm; M. H. Carr et al., Icarus 16, 17 (1972)]. We have adopted a scaling factor of 0.8 to convert those images to 1.064 μm to be consistent with spectroscopic observations of the average martian surface in visible and infrared wavelengths [L. A. Soderblom, in Mars, H. H. Kieffer, B. M. Jakosky, C. W. Snyder, M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1992), p. 557]. It is not possible to obtain unbiased values of opacity using the above method, because the Viking images are B not themselves corrected for the relevant atmospheric opacity. We used only Viking images that exhibited B little or no obscuration. Such opacities in the Viking images may either darken or brighten them, unlike the effect in MOLA reflectivities, which is pure extinction. We estimated that average Viking images were darkened with an opacity of 0.4 and added this to the MOLA opacities.
51. The filter of the Viking Orbiter Camera nearest to the MOLA wavelength is red [0.5 μm to 0.7 μm; M. H. Carr et al., Icarus 16, 17 (1972)]. We have adopted a scaling factor of 0.8 to convert those images to 1.064 μm to be consistent with spectroscopic observations of the average martian surface in visible and infrared wavelengths [L. A. Soderblom, in Mars, H. H. Kieffer, B. M. Jakosky, C. W. Snyder, M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1992), p. 557]. It is not possible to obtain unbiased values of opacity using the above method, because the Viking images are B not themselves corrected for the relevant atmospheric opacity. We used only Viking images that exhibited B little or no obscuration. Such opacities in the Viking images may either darken or brighten them, unlike the effect in MOLA reflectivities, which is pure extinction. We estimated that average Viking images were darkened with an opacity of 0.4 and added this to the MOLA opacities.
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53. We thank MOLA Instrument Manager R. Follas and the rest of the instrument team, and G. Cunningham, B. McAnally, and the MGS spacecraft and operation teams. We also acknowledge helpful reviews from M. Carr and an anonymous reviewer, and contributions from J. Abshire and J. Smith in instrument calibration and performance assessment, G. Neumann, G. Elman, P. Jester, and J. Schott in altimetry processing, F. Lemoine, D. Rowlands, and S. Fricke in orbit determination, and O. Aharonson, D. Brown, J. Frawley, P. Haggerty, S. Hauk, A. Ivanov, P. McGovern, C. Johnson, S. Pratt, and N. Siebert in analysis. The MOLA investigation is supported by the NASA Mars Global Surveyor Project.