The Layered Upper Crust of Mars: an Update on Mgs Moc Observations after Two Mars Years in the Mapping Orbit

Introduction: The upper crust of Mars is layered and geologically diverse. Burial and exhumation complicate the use of impact craters to determine the age of materials. Since September 1997, we have employed the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) to test hypotheses regarding the nature of the layered crust [1, 2]. Here we summarize key observations and update earlier findings, based on new MOC images acquired in 2002 and early 2003. North Polar Stratigraphy—A Third Unit: Two distinct stratigraphic units were recognized in the north polar layered materials from MOC images acquired 1999–2001: a lower, dark-toned layered unit and a younger, upper, lighter-toned layered unit [1, 3, 4]. The lower unit is a source for dune-forming sand [3, 4] and erosion of this unit by wind is proposed to cause the undermining that creates north polar arcuate scarps [4]. In a recent paper, we stated that there are no other stratigraphic units [4]. We were wrong. Late 2002 MOC images of the arcuate scarp at the end of Chasma Boreale show that there is a third unit, distinct from the other two, that underlies the dark-toned sand-producing unit (Fig. 1). Now that we know what the unit looks like, we find that it is readily recognized in earlier images. The north polar layers preserve a record of at least three major, distinct changes in sedimentary material and depositional pattern over time. Athabasca Vallis Source is Buried: The chan-neled scabland of Athabasca Vallis and a similar valley system in east Cerberus were proposed to be young features that resulted from discharge of water from the Cerberus Fossae [5]. The main argument against this hypothesis for the fluid source has been evident since the early MGS mission: image SP2-53103 shows a large streamlined " island " in the wake of a crater that was cut by one of the Cerberus Fossae. The trough postdates the crater and the streamlined landform. More importantly, the size and location of the streamlined landform indicates that the responsible fluid had sufficient volume and energy to create the " island " where it is found—thus the source must have been farther away, somewhere further up-slope, where there are no Cerberus Fossae troughs. Recent MOC images (e.g., Fig. 2) show that the scabland landforms emerge, fully-born, from beneath a plain in eastern Cerberus. Either the entire Athabasca Vallis system (and its eastern counterpart) were buried and …