In 1965, the orbiter Mariner 4 became the first spacecraft to transmit close-range images of Mars (Shaw, 2000). Some of these images showed an area at the border of the Elysium and Amazonis Planitias (25(N, 185(W) (Mathews, 2000), and included an enigmatic, elongate, depression subsequently named Orcus Patera. Because of Mariner 4's limited imaging capabilities, the complexity of this unique feature was not fully appreciated. The research objective of this paper is to determine whether Orcus Patera is a volcanic caldera, impact crater, or a hybrid of both.
The Mars Global Surveyor (MGS) spacecraft (Albee et al., 1998), equipped with the Mars Orbiter Laser Altimeter (MOLA) (Zuber et al., 1992; Smith et al., 1998, 1999) and the Mars Orbiter Camera (MOC) (Malin et al., 1992; Malin and Edgett, 1999) has been providing higher-resolution data. MOLA topography data, in particular, allow a precise comparison of morphological parameters that will help constrain whether Orcus Patera is an oblique impact crater, a volcanic caldera, or a combination of both. These parameters include depth, diameter, flank and cavity slopes, and ellipticity. We here offer a re-evaluation of Orcus Patera on the basis of MOLA data and dimensional analyses of other elliptical structures on the Moon, Earth, and Mars.
The areas of Orcus Patera found from the best-fit ellipse and best-fit polygon differ by a factor of 12.85 %. This percentage, though seemingly large, is an indication of the irregular shape of Orcus Patera.
Summary of Geometric Parameters for Orcus Patera and Schiller
|Minor Axis (km)||129.0||65.0|
|Major Axis (km)||383.0||180.0|
|Area of best fit polygon||33,900||-------|
* Minor and major axes of Schiller provided by Bottke (2000).
MOLA topography of the cavity floor displayed interior deposits that were more complex than we expected from Viking images. An S-shaped ridge system (with about 150 m of relief) runs from the deep northern area of Orcus Patera to the shallower southern area. The lists flank and cavity-wall slope measurements found using the ejecta topographic fit equation (ETF). The parameters of ETF include k, where the ETF line crosses the elevation axis; b, which represents the best fit curvature; and R2, which is how well the ETF fits the actual flank. The averageb value of the ETF of the major and minor axes is -0.30. This average excludes the southern major axis b value of 0.06 since the flank, for the most part, is not para bolic. The major and minor axes have an average inner cavity wall slope of 6.4 °. The southern cavity-wall slope is the steepest at 9.6 ° .
Flank and Cavity Wall Slopes
Orcus Patera is an irregular, large elliptical feature with limited evidence of an ejecta blanket or U-shaped basin that would favor an origin as an impact crater. The strongest arguments suggestive of impact origins are in cavity wall and flank slopes. The odd shape, age relations and flat-floor topography fit better into a model for a volcanic caldera. We conclude that the current evidence favors a volcanic origin for Orcus Patera. We hope to re-evaluate is conclusion when new MOC images and MOLA data become available.