Ted A. Maxwell

Geomorphic evolution of the Martian highlands through ancient fluvial processes

Craters in the Martian highlands are preserved in various stages of degradation. As a result of an erosional process active from the Middle Noachian (4.40–3.92 b.y.) through the Hesperian (3.55–1.8 b.y.), ejecta associated with fresh impact craters became etched, hummocky, and dissected by runoff channels. With time, interior gullies became deeply incised and ejecta deposits were entirely removed. Infilling of the craters followed until, in some instances, the craters were completely buried. Only fluvial processes explain these morphologic variations, the size range of affected craters, and the size-frequency distribution curves associated with these crater populations. Based on the number of superposed fresh impact craters, fluvial processes affecting the highlands ceased entirely by the end of the Hesperian. No correlation between cessation of degradation and latitude exists. However, a strong correlation exists between cessation of degradation and elevation. Degradation ended at higher elevations (e.g., 3–4 km; N [5]=∼200, Late Noachian) before lower elevations (e.g., 1–2 km; N[5]=∼180, Early Hesperian), suggesting that cessation was coupled to desiccation of the volatile reservoir and degassing of a 5–20 bar primordial atmosphere. Volatiles released to the surface by runoff channel formation and seepage may have been part of a complex hydrologic cycle that included periodic, heavy amounts of precipitation. Rainfall was principally responsible for degrading the highlands, eroding impact craters, and redistributing sediments. Rainfall also recharged the highland aquifers, allowing sapping and seepage to continue for hundreds of millions of years. As the primordial atmosphere was lost, cloud condensation, and thus rainfall and aquifer recharge, occurred at progressively lower elevations. Based on estimates on the amount of material removed and duration of degradation, denudation rates averaged 0.0001–0.005 mm/yr. These rates are equivalent to those in terrestrial periglacial environments.

Crater morphometry and modification in the Sinus Sabaeus and Margaritifer Sinus regions of Mars

Degraded craters in the southern highlands are indicative of an early martian climate much different than the present. Using a photoclinometric model, analyses of degraded crater morphometry have revealed the stages of crater modification and, for the first time, allow a quantitative assessment of the amount of material eroded in the highlands. Central peaks of fresh craters are removed early by degradational processes. The sharp rims of fresh craters also become rounded while the interior slopes become shallower. Continued degradation causes the crater rim to lower, and infilling produces a broad, flat crater floor. Contrary to earlier observations, the degree of rim modification does not appear to be dependent on the presence of ancient valley networks. During degradation, the diameter of the impact craters also increases due to backwasting. A simple algebraic model balancing the measured amount of infilling with that eroded from the interior slopes suggests that the crater diameters were enlarged by 7 to 10% initially, agreeing with prior observations. These models suggest that larger diameter (i.e., 50 km) craters were enlarged a greater amount than smaller diameter craters, which is opposite to what should be observed. To explain this discrepancy, a ∼10 m thick deposit, presumably aeolian in origin, must have been emplaced within the crater interiors following cessation of the degradational process. By the terminal stage of degradation, crater diameters appear to have been enlarged by 30%. In addition, a deposit ∼60 m average thickness must have been emplaced within these rimless craters to explain the discrepancy in crater enlargement. Because this deposit is contained only within the highly eroded, rimless craters, this material most likely originated from erosion of the surrounding terrain. The measured crater morphometry has allowed us to develop equations describing the amount of material eroded at any given stage of degradation. Applying these equations to craters within the Margaritifer Sinus and Sinus Sabaeus region indicates that an equivalent of ∼200 m of highland material was eroded and redistributed within the study area. Depending upon model chronology, degradation operated for either 400 or 600 million years, suggesting that erosion rates were on the order of ∼0.0003 to 0.0005 mm/yr. These erosion rates are equivalent to those determined for terrestrial periglacial environments. Two-dimensional simulations of some possible degradational processes suggest that fluvial erosion and deposition combined with diffusional creep come closest to producing equivalent degrees of modification through the range of crater diameters investigated in this study (20 to 50 km). However, these processes are inefficient at producing the amount of crater enlargement observed, suggesting that crater interior slopes may have also been undermined by sapping. These results imply that geologic processes related to precipitation dominated the early martian environment. Our working hypothesis is that this precipitation was due to the presence of a primordial atmosphere which condensed and collapsed (i.e., precipitated) into the martian regolith; a process which ceased during the late Hesperian/early Amazonian (3.5 to 1.8 Ga).

Distribution, Morphology, and Origin of Ridges and Arches in Mare Serenitatis

Lunar mare ridges and arches in Mare Serenitatis were mapped to understand better their mode of formation. Mapping of these features indicates that several pre-mare impacts in the Serenitatis area may be responsible for the localization of the circular ridge systems and that the subsurface, pre-mare topography is more complex than previously recognized. Apollo Lunar Sounder cross sections of ridge systems in southern Serenitatis indicate 50 to 100 m of local relief on these features. Ridges in the southwestern part of the basin mark the boundary of a bench 200 m above the local mare level. As reflected in their orientation, arches and ridges are possibly controlled both by rings of pre-mare basins resulting from impacts and by a more widespread global stress system. Small-scale features of ridges, such as medial lineations and lobate margins, do not conclusively define the origin of the ridges. However, estimates of crustal shortening from Lunar Sounder data and the coincidence of the major ridge system with the Serenitatis mascon suggest that ridges and arches were formed by gravitational readjustments of the mare fill along four probable impact structures and along a north-trending fracture pattern.

An igneous origin for features of a candidate crater-lake system in western Memnonia, Mars

The association of channels, inner terraces, and delta-like features with Martian impact craters has previously been interpreted as evidence in favor of the past existence of crater lakes on Mars. However, examination of a candidate crater-lake system in western Memnonia suggests instead that its features may have formed through igneous processes involving the flow and ponding of lava. Accumulations of material in craters and other topographic lows throughout much of the study region have characteristics consistent with those of volcanic deposits, and terraces found along the inner flanks of some of these craters are interpreted as having formed through drainage or subsidence of volcanic materials. Channels previously identified as inlets and outlets of the crater-lake system are interpreted instead as volcanic rilles. These results challenge previous interpretations of terrace and channel features in the study region and suggest that candidate crater lakes located elsewhere should be reexamined.

Crosscutting relations and relative ages of ridges and faults in the Tharsis region of Mars

Abstract Observations of ridge-fault crosscutting relationships on the ridged plains units surrounding the Tharsis region of Mars have led to the development of a classification scheme involving three distinct types of intersections. Ridges crosscut by faults are designated Type C and account for 81% of the observed intersections. Ridges terminated at one end by a fault (Type T), as well as those superposed on grabens (Type S), are less numerous. Interpretation of the morphology of these intersections and the angles of intersection between ridges and faults with radial trends to major topographic features in the Tharsis region have led to the following conclusions: (1) the major ridge forming events in the Tharsis region were roughly coincident with, and in some cases possibly prior to, the extensional events that produced the faulting of the Tempe and Mareotis regions, the Coprates and Memnonia regions, and the rifting of Valles Marinrris; (2) the compressional events that formed most of the ridges are restricted in time both by the irrelationship to regional extensional events and by the age of the units on which they formed. The suggestion that compressional ridges are a result of a single long term viscoelastic response of the lithosphere to loading of the crust is not supported by this study. A model involving one or more isostatically compensated uplifts and subsequent relaxation of the crust after the emplacement of the ridged plains volcanic units is favored.

An Acheulian site near Bir Kiseiba in the Darb el Arba'in Desert, Egypt

A small concentration of Acheulian cleavers and handaxes within the driest region on Earth adds to the increasing evidence that the eastern Sahara was considerably more verdant during the Middle Pleistocene than it is today. The similarities to stone artifact assemblages of Acheulian sites in sub-Saharan Africa and in the Levant support the evidence for the movement of hominids, utilizing the Kombewa lithic technology, between Africa and the Middle East during the Middle Pleistocene.

Large-Scale, Low-Amplitude Bedforms (Chevrons) in the Selima Sand Sheet, Egypt

Landsat images of the Selima sand sheet in southwestern Egypt display alternating light and dark chevron-shaped patterns that occur downwind from low scarps and major dune fields. Images acquired between 1972 and 1988 indicate that these features move as discrete bedforms at a rate of up to 500 meters per year. Extremely long-wavelength (130 to 1200 meters), low-amplitude (10 to 30 centimeters) bedforms were measured in the field; the light chevrons seen in the orbital data may be thin accumulations of active sand sheet deposits in the lee of these bedforms. Dark chevrons contain an admixture of coarse-granule lag deposits that are continually winnowed by aeolian erosion on the windward sides of the large bedforms. Sediment transport budgets derived from orbital and field analyses suggest net movement of up to 83,000 cubic meters per year for a single light chevron; such measurements can be used as a check on similar calculations from dunes and other smaller scale features to determine sand transport budgets for large areas of the eastern Sahara.

Age relations of Martian highland drainage basins

Dendritic valley patterns in the equatorial highlands of Mars show evidence of internal drainage into restricted basins, which are interpreted to be floored with sedimentary fill. Based on crater frequency characteristics of six areas of enclosed basins, the origin of these intercrater plains fill units ranges from middle to late Noachian. In contrast, the age of modification of the same plains units derived from the frequency of fresh craters occupies a relatively narrow range centered on the Noachian/Hesperian boundary. In half the areas studied the timing of highlands and plains crater modification is consistent with a sedimentary origin for basin fill materials. The other plains units most likely consist of interlayered sedimentary and volcanic materials. Relations between the age of stability of these internally drained highland units and their elevation are not as distinct as prior studies suggested; a trend of decreasing age with decreasing elevation for the plains materials is not matched by similarly derived ages of the dissected highlands. Remapping and age dating of the dissected highlands and associated basins suggest that volcanic plains may be more extensive than those used in past models for magma and volatile evolution, and support local volcanism rather than a global-scale magmatic head model for highlands plains formation.

Temporal changes in the Cerberus region of Mars: Mariner 9 and Viking comparisons

Abstract As in seen from comparisons of Mariner 9 images obtained in 1972 and Viking Orbiter 1 images obtained in 1978, several changes have occurred in the Cerberus region of Mars. Changes in the boundary of the low albedo feature resulted in an increase of the total area of Cerberus by slightly more than 1%, although the southwestern boundary had shifted as much as 90 km. Relative darkening of Cerberus has resulted in a more uniform tone, and is accompanied by the disappearance of dark filamentary markings. Although several bright streaks within Cerberus changed in length, neither lengthening nor shortening of the streaks predominated. However, changes in streak direction indicate a clockwise rotation of mean streak azimuth between 1972 and 1978. These changes in the outline and appearance of Cerberus can best be explained by eolian redistribution and removal of bright material during major dust storms. Volcanic flow fronts which show through the albedo feature indicate that the contrast between Cerberusand the surrounding light plains is not due to a difference in lithology, but to the distribution of surficial deposits. Because of local topographic influences on the regional atmospheric circulation patterns, it is probable that Cerberus will retain a similar appearance and location.

Evidence for Pleistocene lakes in the Tushka region, south Egypt

Space Shuttle Radar Topography Mission (SRTM) data have revealed new details on the extent and geomorphic relations of paleodrainage in southern Egypt. Following a period of late Tertiary drainage from the Red Sea Hills south through Wadi Qena and west across the Tushka region, the Nile River as we now know it established its connections with Central Africa and the Mediterranean in the middle Pleistocene (oxygen isotope stage, OIS 7 to OIS 5). SRTM topography reveals a lake level at ∼247 m that is coincident with the elevation of middle Pleistocene fish fossils 400 km west of the Nile, and with the termination of shallow runoff channels in northern Sudan that were active during the middle Pleistocene and Holocene pluvial periods. An additional lake level at ∼190 m is based on the current elevation at Wadi Tushka, and is consistent with Paleolithic sites at Bir Kiseiba followed by Neolithic sites at lower topographic levels. Overflow of the Nile through Wadi Tushka during the wetter north African climate of the middle Pleistocene, coupled with limited local rainfall, was the likely source of water for these lakes.