The shape of mars and the topographic signature of the hemispheric dichotomy

Science

Volumn 271, Issue 5246, 1996, Pages 184-188

  Smith, David E ^a   Zuber, Maria T ^a,b  

^a NASA GODDARD SPACE FLIGHT CENTER   (United States)

^b MASSACHUSETTS INSTITUTE OF TECHNOLOGY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ELLIPSE; ELLIPSOID; HEMISPHERIC DICHOTOMY; MARINER 9; PLANET SHAPE; PLANETARY SHAPE; TOPOGRAPHIC SIGNATURE; VIKING ORBITER;

MARS;

EID: 0029730547     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.271.5246.184     Document Type: Article

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38. -3/2,whereP and Tare atmospheric pressure and temperature, respectively This expression (provided by D Hinson) assumes a constant atmospheric temperature and is estimated to be valid to ∼10% for small refraction angles The largest refraction corrections occurred at high latitudes, where the spacecraft were typically more distant from Mars. Refraction corrections to the radius were typically on the order of 1 to 2 km, but the largest was 12 km. The single largest source of error is the orbit at ∼400 m; the next largest is atmospheric refraction at ∼300 m, followed by occultation timing and planetary ephemens errors. The total root sum square error of a typical occultation is estimated to be a little over 500 m. The error model tor the atmospheric refraction was ∼10% of the total correction.
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47. The primary source of error for the radar data is the planetary ephemeris (42); this error, combined with a lesser error induced by solar plasma effects, indicates a vertical accuracy of ≃300 m for these measurements, which is comparable to that of the occultation data. However, the radar measurement is an average for an area of several square kilometers, whereas the occultation measurement is a point location. Thus, a systematic difference between the radii derived from the two data sets might be expected. In a comparison of the topography derived from the radar data with that derived from the occultation data, a systematic difference in radius was detected, with a mean of 730 ± 173 m; the radar-derived radii were smaller than the occultation radii. The sign of this difference is consistent with the radar data, providing a measure of the planetary radius to the floors of valleys, and with the occultation data, providing a measure of the planetary radius to the higher topographic locations, such as mountains.
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62. We thank S Asmar and M Connolly for "rediscovering" the Viking occultation data in an unmarked file cabinet at the Jet Propulsion Laboratory, G Lindal for providing the refraction angles used to correct the Viking data D Hinson for helpful discussions regarding atmospheric corrections to the occultation data, D. Rowlands for software support in the processing of occultations, S Fncke for orbital computations, J Finnochiaro for assistance with atmospheric refraction corrections, R Jurgens and M Slade for radar data, F. Lemoine for assembling geord values used to determine geopotential topography, G Neumann for assistance with Fig. 1, S Solomon for discussions about interpretation of the results, and N Sleep and an anonymous referee for helpful reviews. The Mariner 9 data (A Kliore, principal investigator) were obtained from the National Space Science Data Center at the Goddard Space Flight Center Supported by the NASA Planetary Geology and Geophysics Program and the NASA Mars Global Surveyor Project.