THE GREAT EXTENT OF TYRRHENA PATERA, MARS: MAPPING MTM QUADRANGLE -20257 IN WESTERN HESPERIA PLANUM USING MULTIPLE DATA SETS

Crater size-frequency distributions calculated for the base of the Hesperian Epoch on Mars were collected from Hesperia Planum as mapped at 1:15,00,000. Tyrrhena Patera (22°S, 253.5°W), is a broad, low relief, central-vent volcano located on the western edge of Hesperia Planum. Detailed mapping of Tyrrhena Patera reveals that much of what had been identified as Hesperia Planum are actually Tyrrhena Patera shield materials, most likely emplaced during the Upper Noachian. Thus, existing crater statistics for Hesperia Planum, and the base of the Hesperian Epoch globally on Mars, may be erroneous. We are mapping MTM quadrangles -20257 and -15257 to determine the precise nature of the boundary between Western Hesperia Planum, Tyrrhena Patera, and the surrounding Noachian-aged highlands.

Results from previous mapping of the Tyrrhena Patera summit region using Viking Orbiter (VO) images indicated that the volcano is composed of 5 principle units: 3 shield material units, rille-floor material, and channel-fill material. Close examination of high-resolution (40-60 m/pixel) VO and MOC images do not unequivocally reveal the presence of channel-fill material—interpreted to be an erosional deposit in the interiors of broad, flat-floored channels that dissect the Tyrrhena Patera shield materials—in adjacent regions. However, channel floors in THEMIS daytime infrared images are locally characterized by high daytime temperatures relative to the surrounding landscape, which is consistent with the presence of fine-grained, channel-fill material.

The position of the contact between adjacent plains materials and channel-fill material is cannot be traced westward from the Tyrrhena Patera summit region using VO images, in spite of the presence of dissected shield materials in the area. MOLA data are useful in interpreting contacts between these units. In addition, MOLA data reveal subtle features, such as channels and ridges, which cannot be resolved in VO images.