Lunar and Planetary Science XXIX 1673.pdf

THE SHAPE OF THE NORTHERN HEMISPHERE OF MARS FROM THE MGS LASER ALTIMETER. M.T. Zuber1,2, D.E. Smith2, J.W. Head3, S.C. Solomon4, G.A. Neumann1, W.B. Banerdt5, 1MIT, Cambridge, MA 02139, 2NASA/Goddard Space Flight Center, Greenbelt, MD 20771, 3Brown University, Providence, RI 02912; 4Carnegie Institution of Washington, Washing- ton, DC 20015, 5Jet Propulsion Laboratory, Pasadena, CA 91109; zuber@tharsis.gsfc.nasa.gov.

Introduction. During the Fall of 1997 the laser altimeter hemisphere was the site of a vast ancient ocean [12]. The new [1] on Mars Global Surveyor [2] acquired 18 profiles across topography shows comparable flatness to the Earth’s oceanic the northern hemisphere of Mars. These tracks provided over abyssal plains, and also indicates that the volume that would 200,000 measurements of the radius of the planet between have been available for an ocean is significant and could easily latitudes 80°N and 12°S, with a precision at the meter level accommodate amounts of water previously estimated for early and overall accuracy ~50 to 100 m, limited by knowledge of Mars [13-15]. If the water was ~1 km deep the volume sug- the spacecraft orbit [3]. These profiles were distributed ap- gested by the MOLA data would be ~15x106 km3. If com- proximately uniformly in longitude (~20° or 1200 km separa- pletely transported to the polar regions it would provide tion at the equator) and constrain the planet’s long wavelength enough ice for 2 polar caps, each 8 km thick and 10° in radius. shape as well as provide high spatial resolution (~300 m) pro- Tharsis. Pass 24 crossed the western flank of Tharsis, files of selected areas. which rises to nearly 15 km. Pass 33, shown in Fig. 2, crossed Radius and flattening. Planetary radius measurements, Alba Patera, whose sloping summit almost parallels the re- plotted in Fig. 1, reveal a planet with a low and flat region at gional terrain, and then Arsia Mons. The rim of Arsia is the high latitudes and higher cratered terrain nearer the equator [4]. highest point on Mars, at about 17 km, so far measured by (Note that MOLA data are referenced to the average value of MOLA, and on this pass MOLA detected a total change in the geoid radius at the equator rather than the traditional 6.1 topography of over 22 km between 80°N and 10°S. On aver- mbar surface.) The MOLA observations have provided a new age, however, the total change in elevation between the polar estimate for the mean equatorial radius of 3396.0 ± 0.3 km and terrain at 80°N and the equator is about 4 km. the north polar radius of 3374.1 ± 0.5 km, and collectively The northward dipping Alba summit approximately par- imply a flattening of the northern hemisphere of 1/(155.1 ± allels (to ~0.1°) the slope of the surrounding terrain north and 4.3). Allowing for the 3.1 km offset of the center of mass south of the edifice, which form part of the Tharsis rise. The from the center of figure along the polar axis [5], we can expect observed coincidence of slopes could be interpreted to mean the flattening of the full planet to decrease by ~15%, corre- that the Alba construct formed prior to the long wavelength sponding to a planetary estimate of around 1/178. This is regional topography, and was tilted northward in association significantly less than earlier values of 1/154.4 [6] and the with the formation of the Tharsis rise. Removing the regional ellipsoidal value of 1/166.53 obtained from a re-analysis of the Tharsis slope [16] flattens the Alba summit and makes the Viking and Mariner occultation data [5]. However, it is larger north and south flank slopes of the shield more comparable. If than the flattening of 1/192 used in making maps [7]. The this line of reasoning holds it would suggest that the regional importance of a smaller flattening for the planet, if ultimately topography of Tharsis in this area is primarily a consequence verified by southern hemisphere observations when MGS of uplift of pre-existing terrain by isostatic or dynamical proc- achieves its polar, circular mapping orbit, is that the it implies esses [17], and/or perhaps intrusive volcanism [18] similar to that Mars has less global-scale topography than previously that now proposed for the Hawaiian swell [19]. The relative thought. ages of certain Alba flows [20] dictate that surface volcanism Northern hemisphere. The most surprising topographic [21] must also have contributed to the regional elevation. The observation so far is the smoothness and flatness of the north- role of each process to shaping current Tharsis topography ern latitudes. Extending over all longitudes between latitudes will require analysis of more spatially distributed topographic 80°N and 55°N there are almost no topographic relief changes profiles as they become available, combined with study of except for occasional craters. The elevation is about -4 km and geological and surface geochemical data. Note that in contrast level except in the region of the Tharsis rise where the surface to Alba, the summit of Arsia Mons is regionally flat, indicating rises at slopes of 0.1°-0.2° toward the south. In some regions that the main edifice formed subsequent to that of regional the smoothness and flatness extends from the polar terrain Tharsis topography. down to the equator. The lowest point on Mars so far ob- References: [1.] M. T. Zuber et al. (1992) J. Geophys. Res., served is the base of the crater Mie (58°N) at nearly -6 km. 97, 7781-7797. [2.] A. A. Albee et al. (1998) Science, , in The origin of the flatness of the northern hemisphere is press. [3.] D. E. Smith et al. (1998) Lunar Planet. Sci. Conf., presently unknown. Various studies to explain the previously XXIX, submitted. [4.] D. E. Smith et al. (1998) Science, in recognized low elevation of the north relative to the south have press. [5.] D. E. Smith, M. T. Zuber (1996) Science, 271, suggested possible thinning of Mars’ lithosphere by regionally 184-188. [6.] B. G. Bills, A. J. Ferrari (1978) J. Geophys. vigorous mantle convection [e.g. 8, 9] or a large impact or im- Res., 83, 3497-3508. [7.] S. S. C. Wu (1991) . (U.S. Geol. pacts [e.g. 10, 11]. It has also been suggested that the northern Survey Map I-2160, . [8.] R. E. Lingenfelter, G. Schubert Lunar and Planetary Science XXIX 1673.pdf

MARS SHAPE: M.T. Zuber et al.

(1973) Moon, 7, 172-180. [9.] D. U. Wise et al. (1979) Icarus, C. W. Snyder, M. S. Matthews, Eds. (Univ. Ariz. Press, Tuc- 35, 456-472. [10.] D. E. Wilhelms, S. W. Squyres (1984) Na- son) p. 249-297. [18.] R. J. Phillips et al. (1990) J. Geophys. ture, 309, 138-140. [11.] G. E. McGill (1989) J. Geophys. Res., 95, 5089-5100. [19.] J. Phipps Morgan et al. (1995) J. Res., 94, 2753-2759. [12.] V. R. Baker et al. (1991) Nature, Geophys. Res., 100, 8045-8062. [20.] K. L. Tanaka et al. 352, 589-594. [13.] M. H. Carr et al. (1977) J. Geophys. Res., (1992) in Mars H. H. Kieffer, B. M. Jakosky, C. W. Snyder, 82, 4055-4065. [14.] R. Greeley (1987) Science, 236, 1653- M. S. Matthews, Eds. (Univ. Ariz. Press, Tucson) p. 345- 1654. [15.] V. R. Baker et al. (1992) in Mars H. H. Kieffer, B. 382. [21.] S. C. Solomon, J. W. Head (1982) J. Geophys. Res., M. Jakosky, C. W. Snyder, M. S. Matthews, Eds. (Univ. 82, 9755-9774. Ariz. Press, Tucson) p. 493-522. [26.] M. T. Zuber, D. E. Smith (1997) J. Geophys. Res., 102, 28,673-28,685. [17.] W. . B. Banerdt et al. (1992) in Mars H. H. Kieffer, B. M. Jakosky,

3420000

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Figure 1. MOLA northern hemisphere radii.

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Figure 2. Pass 33 with and without the regional Tharsis slope.