L. E. Roth

Radar altimetry of South Tharsis, Mars

Abstract Martian altitudes were measured by radar during the oppositions of 1971 and 1963 using the 64-m antenna at Goldstone (California). The resultant topographic profiles substantiate a zonal classification of the volcanic flows blanketing the south flanks of Arsia Mons, and they confirm the existence of a secondary, parasitic shield attached from the SSW to the main Arsia shield. The secondary shield is about 400 km in diameter at its base and at least 4 km high at its center. South of Valles Marineris, the Tharsis plateau is bounded by the approximate longitudes of 80° in the east and 140° in the west. In the Sinai Planum, closely adjacent to Coprates Chasma, another rise has been detected, bounded by longitudes of 55° in the east and 80° in the west. A volcanic shield of diameter 80 km, capped with a 22 km caldera has been identified near the crest of the rise. Topographic highs of about 1 km are associated with heavily faulted tracts such as Claritas Fossae. The distribution and orientation of the lunar-mare-like ridges in Sinai Planum appear to be independent of the regional gradients. Segments of the chaotic terrain at the eastern terminus of Valles Marineris are located as much as 6 km below the level of the surrounding plains.

Radar altimetry of large Martian craters

Abstract Mars topography in the 23 to −22° latitude band has been measured by the Goldstone λ13-cm radar during the 1971–1982 oppositions. A total of 108 scans were produced. Each scan spans several tens of degrees in longitude. The longest scan spans 133° (∼8000 The vertical resolution generally is better than 200 m. With some exceptions, the areal resolution is 0.16° (∼8 km) in longitude and 1.30° (∼80 km) in latitude. On regional scales, elevation differences of as much as 3 km over less than 10° longitude have been observed (Aeolis, Memnonia). Most local relief is provided by structural elements of impact craters. One hundred fifty-two complex, degraded craters, some as small as 25 km in diameter, have been identified in this data set. Repeated scans of the same craters yield nearly identical elevation profiles. Measured crater depths rarely exceed 2.5 km. Depths of the most degraded craters in the sample are a weak function of crater diameter. Depths of the largest craters D>125km show no functional relationship to diameters. Measured rim heights rarely exceed several hundred meters. Structures with no discernible rim (e.g., Ladon) have retained a basin shape. Low exterior relief rather than overall low relief seems to be the distinguishing characteristic of large, degraded craters included in the Goldstone sample. Craters or basins that serve as sinks for major channel systems (e.g., Gusev, Ladon) appear to be as deep as comparable structures not associated with such systems, suggesting that processes other than fluvial sedimentation contributed to crater shallowing.