Ted Maxwell - Academia.edu (original) (raw)

Papers by Ted Maxwell

Geophysical Research Letters, 1978

Journal of Geophysical Research Atmospheres, Jun 25, 1997

Degraded craters in the southern highlands are indicative of an early martian climate much differ... more 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 ~10m 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 ~60m 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 ~200m 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).

Lunar and Planetary Science Conference, Mar 1, 1983

Journal of Geophysical Research: Planets, 2015

, Mars orbital synthetic aperture radar: Obtaining geologic information from radar polarimetry,

The Selima Sand Sheet occupies more than 120,000 km of the hyperarid, uninhabited Darb el-Arba'in... more The Selima Sand Sheet occupies more than 120,000 km of the hyperarid, uninhabited Darb el-Arba'in Desert centered at the border of Egypt and Sudan at latitude 223 N, and is characterized by a featureless surface of lag granules and "ne sand broken only by widely separated dune "elds and giant ripples of varying height and wavelength. Monitoring of the largest of these chevron-shaped ripples using repeat orbital images and "eld surveys indicates migration rates of 500}1000 m/yr, accompanied by 0}2.0 cm erosion or deposition of the youngest sand sheet stratigraphic units. Beneath this active surface, several developmental stages of sand sheet sediments have undulatory upper contacts and varying degrees of pedogenic alteration. The younger stages retain their horizontal lamination and have cracking patterns indicative of past wetter conditions, while older stages have lost their laminar structure through pedogenesis. Historical remains in the desert as well as C and Uranium-series dating indicate that the younger strata of the sand sheet have a very low accumulation rate, despite the active movement of the surface. The lower strata were extensively modi"ed during mid and late Pleistocene pluvials, resulting in an initial undulatory surface that set the stage for later accumulation of sand sheet. Below these Quaternary sediments lies irregular topography dissected by channels of mid-Tertiary drainage. The Selima Sand Sheet is neither the result of net aggradation nor degradation, but results from inheritance of an initial #uvial landscape increasingly modi"ed during climatic cycles. Wet periods led to local drainage and deposition, while the increasingly severe arid periods of the late Pleistocene and Holocene resulted in deposition of the blanketing bimodal sediments of the sand sheet.

Release of water in a CO2 rich atmosphere by precipitation and channel forming processes has led ... more Release of water in a CO2 rich atmosphere by precipitation and channel forming processes has led to speculation on the creation of Martian carbonate deposits. On Mars water probably was not on the surface long enough to allow eroded material to concentrate, raise the pH, and induce the formation of carbonates. This suggests that the Martian primordial atmosphere could be thinner (approximately 5 bars) and still allow highland degradation to occur over a long period of time (.45 to 1.2 billion years).

The geological, atmospheric, and climatic history of Mars is explored in reviews and reports of r... more The geological, atmospheric, and climatic history of Mars is explored in reviews and reports of recent observational and interpretive investigations. Topics addressed include evidence for a warm wet climate on early Mars, volatiles on Earth and on Mars, COâ adsorption on palagonite and its implications for Martian regolith partitioning, and the effect of spatial resolution on interpretations of Martian subsurface volatiles. Consideration is given to high resolution observations of rampart craters, ring furrows in highland terrains, the interannual variability of the south polar cap, telescopic observations of the north polar cap and circumpolar clouds, and dynamical modeling of a planetary wave polar warming mechanism.

Science, 2002

At 8 to 15 kilometers wide, Ma'adim Vallis is one of the largest valleys in the martian highlands... more At 8 to 15 kilometers wide, Ma'adim Vallis is one of the largest valleys in the martian highlands. Although a groundwater source was previously suggested, the channel originates at a spillway in the divide of a ϳ3,000,000-squarekilometer closed drainage basin. The interior morphology of this source basin, including likely shoreline features following topographic contours, suggests that Ma'adim Vallis was created through catastrophic overflow of a ϳ1,100,000-square-kilometer highland lake. The size, constant levels, and interior morphology of three regional paleolake basins require a warmer paleoclimate and a long-term, recharged, stable highland water table more than ϳ3.5 billion years ago.

Science, 1989

Landsat images of the Selima sand sheet in southwestern Egypt display alternating light and dark ... more 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.

Quaternary Science Reviews, 2001

Palaeogeography, Palaeoclimatology, Palaeoecology, 1995

As documented by radiocarbon dating and geoarchaeological investigations, the now hyperarid north... more As documented by radiocarbon dating and geoarchaeological investigations, the now hyperarid northwestern Sudan and southwestern Egypt experienced a period of greater effective moisture during early and middle Holocene time, about 10-5 ka. We have used the uranium-series technique to date lacustrine carbonates from Bir Tarfawi, Bir Sahara East, Wadi Hussein, Oyo Depression, and the Great Selima Sand Sheet localities. Results indicate five paleolakeforming episodes occurred at about 320-250, 240-190, 155-120, 90-65 and 10-5 ka. Four of these five pluvial episodes may be correlated with major interglacial stages 9, 7, 5e, and 1; the 90-65 ka episode may be correlated with substage 5c or 5a. Our results support the contention that past pluvial episodes in North Africa corresponded to the interglacial periods farther north. Ages of lacustrine carbonates from existing oases and from the sand sheet fail to indicate pluvial conditions between about 60 and 30 ka. Age results and field relationships suggest that the oldest lake-and ground-water-deposited carbonates were much more extensive than those of the younger period, and that carbonate of the latest wet periods were geographically localized within depressions and buried channels.

Journal of Geophysical Research, 1990

The southern cratered highlands of Mars contain a large population of flat-floored, rimless crate... more The southern cratered highlands of Mars contain a large population of flat-floored, rimless craters which have previously been interpreted to have formed by aeolian mantling or flood volcanism. Neither of these geologic processes accurately explains the observed morphology or the crater statistics. Geologic mapping in the Amenthes and Tyrrhena region indicates that craters with this morphology occur on undulating intercrater materials near the dichotomy boundary and on more rugged materials farther into the highlands containing numerous ancient valley networks. Cumulative size-frequency distribution curves indicate ages of N(5) = 790 (early to middle Noachian) and N(5) = 540 (middle Noachian) for the cratered plateau and cratered highland materials, respectively, opposite the observed stratigraphic relations. For crater diameters > 16 km the population of impact craters is consistent with stratigraphy, but the population of smaller craters in the region indicates the importance of resulTacing. Superposed, fresh craters indicate a resulTacing event that ceased at N(5) = 200-250 (late Noachian to early Hesperian). Crater counts divided into 5 ø latitudinal bins show an increase in the number of craters between the 8-and 50-km-diameter range with increasing northerly latitudes, suggesting that the resurfacing was not a single event. Statistical modelling of an erosive event capable of removing the continuous ejecta deposits from the craters, eroding them to reduce the apparent diameter, and simultaneously burying smaller eroded craters explains both the morphology of the flat-floored, rimless craters, and their population distribution. Matching the slope of the modelled curves with that of the cratered highland materials suggests that up to 1400 m of erosion may have taken place in the highlands. Some of this eroded material probably remained locally, burying the smaller craters. Later stripping of this material indicated by exposed layering and inverted topography has exhumed the smaller craters in the cratered plateau material resulting in the apparent age discrepancy determined from crater counts. Up to 1.0 km of material may have been removed by this later erosive event to explain the low density of ancient valley networks in the cratered plateau materials.

Journal of Geophysical Research, 1986

The Tharsis ridge system is roughly circumferential to the regional topographic high of northern ... more The Tharsis ridge system is roughly circumferential to the regional topographic high of northern Syria Planum and the major Tharsis volcanoes. However, many of the ridges have orientations that deviate from the regional trends. Normals to vector means of ridge orientations, calculated in 10 ø boxes of latitude and longitude, show that the Tharsis ridges are not purely circumferential. Intersections of vector mean orientations plotted as great circles on a stereographic net show that the ridges are not concentric to a single point but to three broad zones. These data indicate either that the Tharsis ridge system did not form in response to a single regional compressional event with a single stress center or that the majority of the ridge system did form in a single event but with locally controlled deflections in the generally radially oriented stress field. Superposition relationships and relative ages suggest that the compressional stresses that produced the ridges occurred after the emplacement of the ridged plains volcanic units (early in the Hesperian period) but did not extend beyond the time of emplacement of the Syria Planum Formation units or the basal units of the Tharsis Montes Formation (during the latter half of the Hesperian period). Comparison of topographic data with the locations of ridges demonstrates a good correlation between the topographic edge of the Tharsis rise and outer extent of ridge occurrence. Compressional deformation related to Tharsis is present out to about 5100 km from the regional topographic center located near northern Syria Planum. The innermost extent cannot be determined because the ridged plains units have been buried by more recent volcanic units, although ridge formation appears to have extended farther inward than is presently observed. Models involving a combination of isostatic stresses and stresses resulting from flexural loading appear to explain best the observed tensional features, topography, gravity, and, to a first approximation, the locations and orientations of the compressional ridges.

Journal of Geophysical Research, 1993

Craters in the Martian highlands are preserved in various stages of degradation. As a result of a... more 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 rimoff 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 rimoff 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 ten'estrial periglacial environments. [Soderblom et al., 1973; Mutch et al., 1977, pp. 138-150]. The comprehensive analysis of the cratered highlands presented here suggests that during the Noachian (4.6 to ~3.5 Ga), surface processes and denudation rates on Mars were similar to those presently occurring in periglacial environments on Earth. Cessation of these processes appears to have been coupled to desiccation of the volatile reservoir and degassing of the early planetary atmosphere. 3453 3454 CRADDOCK AND MAXWF_,LL: ANCIENT FLUVI• PROCESSES ON MARS ß o o • Fig. 1. Shaded relief map of the equatorial region of Mars. Areas outlined show the location of the Npl• and Npld materials between 30 ø and-30 ø latitude investigated in this study. Base maps are the 1:15,000,000 Shaded Relief Map of Mars, Eastern and Western Regions [U.S. Geological Survey, 1985]. MORPHOLOGY OF HIG• IMPACT CRATERS A number of investigations have dealt with the general degrade d appearance of craters in the Martian highlands. Based on Mariner spacecraft data, McGill and Wise [1972] and Arvidson [1974] indicated that fresh, bowl-shaped craters grade into flatfloored, rimless craters. Using high-resolution Viking orbiter data, Grant and Schultz [1991a, b] compared styles of crater degradation in southern Ismenius Lacus to the evolution of terrestrial impact craters. These studies are important for determining how highland impact craters became degraded and for identifying the processes that operated to modify them. However, the Mariner-based studies presented broad crater classes without distinguishing smaller, significant features that are visible in Viking orbiter images. Also, highland studies north of 30 ø latitude are biased towards identifying the degradational process as aeolian in nature due to the presence of large airfall (i.e., dust) deposits in the region [Christensen, 1982, 1986; Greeley and Guest, 1987; Schultz and Lutz, 1988; Grizzaffi and Schultz, 1989; Dimitriou, 1990a, b; Grant and Schultz, 1990; Moore, 1990] which were emplaced subsequent to highland degradation and crater modification [Dimitriou, 199Os, b]. Highland crater populations in the equatorial region of Mars show styles of degradation that are consistent with fluvial processes. Fresh impact craters typically have sharply defined raised rims, hummocky floors, and obvious ejecta deposits (Figure 2a). Martian gravity causes some small-scale collapse of the rim to occur on craters with diameters greater than ~5 km, producing a fresh impact crater with a complex morphology [Pike and Davis, 1984; Pike, 1988]. The ejects associated with fresh Martian impact craters either radiates out from the center of impact (similar to most hmar craters), is lobate from the center and is said to be "fluidized" [Mouginis-Mark, 1979], or falls somewhere in between these two types. Fluidized ejects may represent the presence of subsurface volatiles [Carr et al., 1977] or may be the result of atmospheric deceleration of ejected particles [Schultz and Gault, 1979]. Geographic variations in the occurrence of these types of fresh impactscraters have been the subject of numerous CRADDOCK AND MAXWELL: ANCIENT FLUVIAL PROCESSES ON MARS 3455

Journal of Geophysical Research, 2004

1] Ma'adim Vallis, one of the largest valleys in the Martian highlands, appears to have originate... more 1] Ma'adim Vallis, one of the largest valleys in the Martian highlands, appears to have originated by catastrophic overflow of a large paleolake located south of the valley heads. Ma'adim Vallis debouched to Gusev crater, 900 km to the north, the landing site for the Spirit Mars Exploration Rover. Support for the paleolake overflow hypothesis comes from the following characteristics: (1) With a channel width of 3 km at its head, Ma'adim Vallis originates at two (eastern and western) gaps incised into the divide of the $1.1 M km 2 enclosed Eridania head basin, which suggests a lake as the water source.