Amazonian Period Research Articles

Heat transfer in volcano–ice interactions on Mars: synthesis of environments and implications for processes and landforms

AbstractWe review new advances in volcano–ice interactions on Mars and focus additional attention on (1) recent analyses of the mechanisms of penetration of the cryosphere by dikes and sills; (2) documentation of the glacial origin of huge fan-shaped deposits on the northwest margins of the Tharis Montes and evidence for abundant volcano–ice interactions during the later Amazonian period of volcanic edifice construction and (3) the circumpolar Hesperian-aged Dorsa Argentea Formation, interpreted as an ice sheet and displaying marginal features (channels, lakes and eskers) indicative of significant melting and interior features interpreted to be due to volcano–ice interactions (e.g. subglacial volcanic edifices, pits, basins, channels and eskers). In this context, we describe and analyse several stages and types of volcano–ice interactions: (1) magmatic interactions with ice-rich parts of the cryosphere; (2) subglacial volcanism represented by intrusion under and into the ice and formation of dikes and moberg-like ridges, intrusion of sills at the glacier–volcano substrate interface and their evolution into subglacial lava flows, formation of subglacial edifices, marginal melting and channels; (3) synglacial (ice contact) volcanism represented by flows banking up against glacier margins, chilling and forming remnant ridges and (4) post-glacial volcanism and interactions with ice deposits.

Modification of impact craters in the northern plains of Mars: Implications for Amazonian climate history

Abstract— We measured the depth, wall steepness, and ejecta roughness and surveyed the wall and floor morphology of all craters 10–25 km in diameter within the typical Vastitas Borealis Formation in the northern lowlands of Mars north of 52°N. Two of the 130 craters have unusually rough ejecta; they are deep, have steep walls, and are apparently the youngest in the population. Icy mantles filling the local subkilometer‐scale topographic lows is the main contribution to ejecta smoothing, which occurs at a time scale on the order of tens of Myr. Wall degradation and crater shallowing generally occur at longer time scales, comparable to the duration of the Amazonian period. Many craters are shallow due to filling of the crater with specific ice‐rich material of uncertain origin. We use our collected data to infer the nature of the past climate back through the Amazonian, a period prior to ˜10–20 Myr ago, when orbital parameter solutions are chaotic and one must rely on geological data to infer climate conditions. We conclude that moderately high obliquity and wide obliquity variations were probable during the last 40–160 Myr. We tentatively conclude that high obliquity peaks (>40–45°) may have occurred episodically through the last 210–430 Myr. A sharp step in the frequency distribution of wall steepness at 20° may indicate a geologically long period prior to that time where obliquity never exceeded 40–45°.

Debris-covered piedmont glaciers along the northwest flank of the Olympus Mons scarp: Evidence for low-latitude ice accumulation during the Late Amazonian of Mars

We use Viking and new MGS and Odyssey data to characterize the lobate deposits superimposed on aureole deposits along the west and northwest flanks of Olympus Mons, Mars. These features have previously been interpreted variously as landslide, pyroclastic, lava flow or glacial features on the basis of Viking images. The advent of multiple high-resolution image and topography data sets from recent spacecraft missions allow us to revisit these features and assess their origins. On the basis of these new data, we interpret these features as glacial deposits and the remnants of cold-based debris-covered glaciers that underwent multiple episodes of advance and retreat, occasionally interacting with extrusive volcanism from higher on the slopes of Olympus Mons. We subdivide the deposits into fifteen distinctive lobes. Typical lobes begin at a theater-like alcove in the escarpment at the base of Olympus Mons, interpreted to be former ice-accumulation zones, and extend outward as a tongue-shaped or fan-shaped deposit. The surface of a typical lobe contains (moving outward from the basal escarpment): a chaotic facies of disorganized hillocks, interpreted as sublimation till in the accumulation zone; arcuate-ridged facies characterized by regular, subparallel ridges and interpreted as the ridges of surface debris formed by the flow of underlying ice; and marginal ridges interpreted as local terminal moraines. Several lobes also contain a hummocky facies toward their margins that is interpreted as a distinctive type of sublimation till shaped by structural dislocations and preferential loss of ice. Blocky units are found extending from the escarpment onto several lobes; these units are interpreted as evidence of lava–ice interaction and imply that ice was present at a time of eruptive volcanic activity higher on the slopes of Olympus Mons. Other than minor channel-like features in association with lava–ice interactions, we find no evidence for the flow of liquid water in association with these lobate features that might suggest: (1) near-surface groundwater as a source for ice in the alcoves in the lobe source region at the base of the scarp, or (2) basal melting and drainage emanating from the lobes that might indicate wet-based glacial conditions. Instead, the array of features is consistent with cold-based glacial processes. The glacial interpretations outlined here are consistent with recent geological evidence for low-latitude ice-rich features at similar positions on the Tharsis Montes as well as with orbital dynamic and climate models indicating extensive snow and ice accumulation associated with episodes of increased obliquity during the Late Amazonian period of the history of Mars.

An Early, Muddy Mars Just Right for Life

MEETING: AMERICAN GEOPHYSICAL UNIONSAN FRANCISCO, CALIFORNIA--Almost 12,000 earth and planetary scientists (a new record) of every stripe met here 5 to 9 December to discuss how clays fit in the geologic history of Mars. [(read more.)][1] [1]: http://sciencemag.org/cgi/content/full/310/5756/1898b

Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for Late Amazonian obliquity-driven climate change

Understanding spin orbital parameter-driven climate change on Mars prior to ~ 20 Ma ago requires geological evidence because numerical solutions for that period are chaotic and non-unique. We show geological evidence that lineated valley fill at low midlatitudes in the northern hemisphere of Mars (~ 37.58 N) originated through regional snow and ice accumulation and underwent glacial-like flow. Breached upland craters and theater-headed valleys reveal features typical of erosion in association with terrestrial glaciers. Parallel, converging and chevron-like lineations in potentially ice-rich deposits on valley floors indicate that flow occurred through constrictions and converged from different directions at different velocities. Together, these Martian deposits and erosional landforms resemble those of intermontaine glacial systems on Earth, particularly in their major morphology, topographic shape, planform and detailed surface features. An inferred Late Amazonian age, combined with predictions of climate models, suggest that the obliquity of Mars exceeded a mean of 458 for a sustained period. During this time, significant transfer of ice occurred from icerich regions (e.g., the poles) to mid-latitudes, causing prolonged snow and ice accumulation there and forming an extensive system of valley glaciers. D 2005 Elsevier B.V. All rights reserved.

Complex evolution of paleolacustrine systems on Mars: An example from the Holden crater

The ∼150 km wide Holden crater lies in an area characterized by high density of valley networks implying conditions conducive to forming of water‐related environments. We undertook geological mapping and a stratigraphic survey in order to probe the evolution of water‐related landforms and their paleoenvironmental implications. Our investigations lead us to propose that the Holden area was subjected to a “wet” lacustrine phase of Hesperian age and an “icy” phase during the Amazonian. Deltaic, coastal, and lacustrine environments occurred during the “wet” phase, some displaying a cyclic depositional pattern presumably related to autogenic processes. Water was delivered to the basin by the Uzboi Vallis and by surface runoff channels from a series of drainage basins along the crater walls. Fan delta geometries and coastal onlap enabled estimation of major water levels. Two levels of major stand of the water have been recognized, possibly reflecting allogenic controls. Geologic units related to this “wet” lacustrine phase were subsequently eroded by glacial abrasion and plucking and were disconformably overlain by glacial deposits of Amazonian age, defining an “icy” phase. These features are consistent with a warm‐based glacier entering the Holden crater through the wide Uzboi Vallis to form a proglacial lake in the central part of the crater. Changes in sedimentary units reflect changes of depositional environments probably connected with climatic variation.

Outflow channel sources, reactivation, and chaos formation, Xanthe Terra, Mars

The undulating, warped, and densely fractured surfaces of highland regions east of Valles Marineris (located north of the eastern Aureum Chaos, east of the Hydraotes Chaos, and south of the Hydaspis Chaos) resulted from extensional surface warping related to ground subsidence, caused when pressurized water confined in subterranean caverns was released to the surface. Water emanations formed crater lakes and resulted in channeling episodes involved in the excavation of Ares, Tiu, and Simud Valles of the eastern part of the circum-Chryse outflow channel system. Progressive surface subsidence and associated reduction of the subsurface cavernous volume, and/or episodes of magmatic-driven activity, led to increases of the hydrostatic pressure, resulting in reactivation of both catastrophic and non-catastrophic outflow activity. Ancient cratered highland and basin materials that underwent large-scale subsidence grade into densely fractured terrains. Collapse of rock materials in these regions resulted in the formation of chaotic terrains, which occur in and near the headwaters of the eastern circum-Chryse outflow channels. The deepest chaotic terrain in the Hydaspis Chaos region resulted from the collapse of pre-existing outflow channel floors. The release of volatiles and related collapse may have included water emanations not necessarily linked to catastrophic outflow. Basal warming related to dike intrusions, thermokarst activity involving wet sediments and/or dissected ice-enriched country rock, permafrost exposed to the atmosphere by extensional tectonism and channel incision, and/or the injection of water into porous floor material, may have enhanced outflow channel floor instability and subsequent collapse. In addition to the possible genetic linkage to outflow channel development dating back to at least the Late Noachian, clear disruption of impact craters with pristine ejecta blankets and rims, as well as preservation of fine tectonic fabrics, suggest that plateau subsidence and chaos formation may have continued well into the Amazonian Period. The geologic and paleohydrologic histories presented here have important implications, as new mechanisms for outflow channel formation and other fluvial activity are described, and new reactivation mechanisms are proposed for the origin of chaotic terrain as contributors to flooding. Detailed geomorphic analysis indicates that subterranean caverns may have been exposed during chaos formation, and thus chaotic terrains mark prime locations for future geologic, hydrologic, and possible astrobiologic exploration.

Evolution of the Tharsis region of Mars: insights from magnetic field observations

Mars Global Surveyor (MGS) observations of crustal magnetic fields over Tharsis provide new constraints on models for the thermal and magmatic evolution of this region. We analyze the distribution of magnetic field anomalies over Tharsis surface units of Noachian, Hesperian and Amazonian age. These data suggest that early Noachian crust underlies the Tharsis province, and formed contemporaneously with the existence of a martian dynamo. This crust either pre-dates the formation of Tharsis, or formed during the earlier phases of Tharsis volcanism. The preservation of strong magnetic field anomalies over some of the earliest Noachian and topographically high units, together with the observation of magnetic field anomalies over Hesperian- and Amazonian-age surface units, indicate that a large fraction of the magnetized crust has remained cool (below the blocking temperature of the magnetic carrier) throughout the construction of Tharsis. Moreover, the distributions of magnetic anomaly amplitudes over Noachian, Hesperian, and Amazonian surface units suggest that the youngest units overlie sites of prolonged intrusion and have undergone a greater extent of thermal demagnetization. The absence of magnetic anomalies around the Tharsis Montes and Olympus Mons argues for strong, localized heating, as would be expected at volcanic centers. We show that end-member models for progressive thermal demagnetization of a Noachian magnetized crustal layer are consistent with the anomaly amplitude distributions. We integrate the magnetic field observations with constraints from tectonics, gravity, and topography, and present a revised scenario for the evolution of the Tharsis region.

Geology and insolation-driven climatic history of Amazonian north polar materials on Mars

Mariner 9 and Viking spacecraft images revealed that the polar regions of Mars, like those of Earth, record the planet's climate history. However, fundamental uncertainties regarding the materials, features, ages and processes constituting the geologic record remained. Recently acquired Mars Orbiter Laser Altimeter data and Mars Orbiter Camera high-resolution images from the Mars Global Surveyor spacecraft and moderately high-resolution Thermal Emission Imaging System visible images from the Mars Odyssey spacecraft permit more comprehensive geologic and climatic analyses. Here I map and show the history of geologic materials and features in the north polar region that span the Amazonian period (approximately 3.0 Gyr ago to present). Erosion and redeposition of putative circumpolar mud volcano deposits (formed by eruption of liquefied, fine-grained material) led to the formation of an Early Amazonian polar plateau consisting of dark layered materials. Crater ejecta superposed on pedestals indicate that a thin mantle was present during most of the Amazonian, suggesting generally higher obliquity and insolation conditions at the poles than at present. Brighter polar layered deposits rest unconformably on the dark layers and formed mainly during lower obliquity over the past 4-5 Myr (ref. 20). Finally, the uppermost layers post-date the latest downtrend in obliquity <20,000 years ago.

Glacial deposits on the rim of a Hesperian‐Amazonian outflow channel source trough: Mangala Valles, Mars

Mangala Valles, located in tropical regions of Mars, is interpreted to have formed by dike‐induced cracking of the cryosphere to produce a linear graben and catastrophic release of groundwater sequestered under hydrostatic pressure. Outpouring of water and downcutting led to erosion and widening of the graben to produce a trough. Patterns of ridges and lobes along the outer rims of the trough are interpreted to have formed primarily by local accumulations of snow and ice on the graben rim (derived from exposed ponded groundwater in the graben), and glacial‐like outward flow of ice lobes resulting in development of moraines and tills along the north and south rims of the trough. These observations and interpretations support the concept that the climate of Mars was a hyper‐arid cold polar‐like desert similar to today in the Late Hesperian‐Early Amazonian period.

Ice‐driven creep on Martian debris slopes

Accumulations of rocky debris at the base of bedrock escarpments on Mars have mean inclinations of ∼20°, well below the angle of repose (∼35°). These inclinations decrease with increasing latitude, suggesting a climatic influence. We present evidence that these low inclinations are the result of gravitational creep driven by repeated deposition and sublimation of ground ice. We estimate the rates of bedrock erosion and ice‐driven creep on Mars and use a simple model to show that the probable time scale for debris slope development is between 20 Myr and 2 Gyr. Our results suggest that water ice has shaped Martian landscapes throughout the late Amazonian period.

Resurfacing history of the northern plains of Mars based on geologic mapping of Mars Global Surveyor data

Geologic mapping of the northern plains of Mars, based on Mars Orbiter Laser Altimeter topography and Viking and Mars Orbiter Camera images, reveals new insights into geologic processes and events in this region during the Hesperian and Amazonian Periods. We propose four successive stages of lowland resurfacing likely related to the activity of near‐surface volatiles commencing at the highland‐lowland boundary (HLB) and progressing to lower topographic levels as follows (highest elevations indicated): Stage 1, upper boundary plains, Early Hesperian, &lt;−2.0 to −2.9 km; Stage 2, lower boundary plains and outflow channel dissection, Late Hesperian, &lt;−2.7 to −4.0 km; Stage 3, Vastitas Borealis Formation (VBF) surface, Late Hesperian to Early Amazonian, &lt;−3.1 to −4.1 km; and Stage 4, local chaos zones, Early Amazonian, &lt;−3.8 to −5.0 km. At Acidalia Mensa, Stage 2 and 3 levels may be lower (&lt;−4.4 and −4.8 km, respectively). Contractional ridges form the dominant structure in the plains and developed from near the end of the Early Hesperian to the Early Amazonian. Geomorphic evidence for a northern‐plains‐filling ocean during Stage 2 is absent because one did not form or its evidence was destroyed by Stage 3 resurfacing. Remnants of possible Amazonian dust mantles occur on top of the VBF. The north polar layered deposits appear to be made up of an up to kilometer‐thick lower sequence of sandy layers Early to Middle Amazonian in age overlain by Late Amazonian ice‐rich dust layers; both units appear to have outliers, suggesting that they once were more extensive.

Degassing of volatiles in the geological history of Mars

Obtuve una estimación de la masa total de volátiles desgasificados en Marte y la distribución de esta desgasificación en el tiempo. La cantidad de volátiles desgasificados en toda la historia de Marte es del orden de 3.73x1020 kg, es decir, 16 000 veces la atmósfera actual marciana. El grueso del desgasamiento ocurrió en todo el periodo Noachiano y en el Hesperiano temprano. En las últimas cinco épocas, del Hesperiano temprano al Amazoniano tardío, el desgasamiento ha sido monótonamente decreciente. La cantidad de agua líquida en la superficie en el pasado ha sido pequeña o inexistente, por lo tanto la existencia de vida en Marte es dudosa.

Amazonis Planitia: The role of geologically recent volcanism and sedimentation in the formation of the smoothest plains on Mars

Amazonis Planitia, located between the two main volcanic provinces on Mars (Tharsis and Elysium), is characterized by extremely smooth topography at several scale lengths, as smooth as oceanic abyssal plains topography on Earth. We use Mars Global Surveyor (MGS) data (primarily very high resolution Mars Orbiter Laser Altimeter (MOLA) topography and derivative slope maps, gradient maps, and detrended maps) to examine the surface morphology of Amazonis Planitia and the stratigraphic relationships among previously mapped and newly defined units. These new data reveal the presence of a 1300 km diameter Noachian impact basin in northwest Amazonis Planitia and an extensive Late Hesperian lava flow unit that appears to have originated from the Olympus Mons source area prior to aureole formation. The presence of this previously unrecognized flow unit strongly suggests that Olympus Mons activity dates back to at least the Hesperian, as did activity on the Tharsis Montes. Emplacement of this ∼100 meter thick flow unit formed a barrier along the northern margin of Amazonis Planitia which had a profound influence on the subsequent geologic history of the region. Formation of Olympus Mons aureole deposits created an eastern topographic barrier, and subsequent Tharsis Montes lava flows entered the basin from the south, flowing around the aureole. These three barriers (degraded Noachian crater rim, proto‐Olympus Mons flow unit, and Olympus Mons aureole) caused subsequent lava flows and outflow channel effluents, primarily from the Elysium region to the west, to pond on the floor of Amazonis Planitia, preferentially smoothing the terrain there. Mars Orbiter Camera (MOC) images substantiate that at least two very fluid lava flows alternated with fluvial episodes from Elysium Planitia, flowing through Marte Valles onto the floor of the Amazonis Planitia basin. Within Amazonis Planitia, MOC images show flow‐like textures heavily mantled by sediments, and radar data reveal the presence of rough lava flow surfaces underlying the sedimentary debris. These data thus suggest that the unique smoothness of Amazonis Planitia is the result of deposition of thin fluid lava flows and fluvial sediments in an enclosed basin. Crater counts suggest that the most recent resurfacing may have occurred in the latest Amazonian Period, in the last 1% of the history of Mars. In light of its unique history, it is somewhat ironic to note that Amazonis Planitia was originally thought to be a typical young Martian surface and therefore used to name the Amazonian era.

Evidence of liquid water in recent debris avalanche on Mars

The identification of landforms associated to liquid water on Mars is a major goal for the understanding of the geologic and climatic evolution of this planet. In the region of Reull Vallis, near Hellas basin, lobate debris aprons are recent landforms due to the creep of water ice mixed with rocks. Among these landforms, one unusually long debris apron has a tongue‐shape and gross morphology that indicates a different mechanism of deposition. This study demonstrates, using topographic data from Viking DEM and MOLA (Mars Observer Laser Altimeter) that the geometric parameters of this feature support the presence of liquid water during its flow. This landform is therefore comparable in size to the most developed wet debris avalanches on Earth. Its late Amazonian age questions the presence of near‐surface liquid water in the recent geological past of Mars.

Comparison of the North and South Polar Caps of Mars: New Observations from MOLA Data and Discussion of Some Outstanding Questions

New high-resolution data from the Mars Orbiter Laser Altimeter (MOLA) have provided detailed topographic maps for the north and south polar regions. These new data allow one to compare the overall topography and geologic histories of the two polar regions and to highlight some specific outstanding questions in Mars polar studies, following earlier comparisons using Viking and Mars Global Surveyor data. The new data show that the centers of symmetry of the polar cap deposits in map view (which include both the layered terrain and residual ice) are offset from the current rotational pole in antipodal directions. Offset in the north appears to be due to retreat of the polar materials from predominantly one direction (180° W). Lines of evidence for movement and melting in different forms (e.g., lobes in young craters, kettle-like depressions, candidate residual mantles overlying polar layered terrain, and possible eskers) have been seen at both poles. The exact timing and causes of movement and melting is yet unknown. Differences in underlying topography (large, low, flat depression in the north; broad, cratered high and edge of a large impact basin in the south) may influence the accumulation, flow, and movement of polar material and the storage and movement of meltwater. The small number of superposed craters has been interpreted to indicate a Late Amazonian age for both caps, with the southern cap being somewhat older (7–15×10 6 years) than the northern cap (<100×10 3 years). The Late Amazonian-aged caps are surrounded and underlain by Hesperian-aged material, indicating an apparent hiatus almost 3-byr in duration. This apparent hiatus in the geologic record from the Late Hesperian to Late Amazonian at both poles may be accounted for in one of three ways. (1) Polar caps are recent events in the history of Mars: This scenario requires that conditions in the Late Amazonian changed to produce environments favorable for cap formation late in the history of Mars, or that polar wander brought the caps to their present position very recently. (2) The polar caps are oscillating: In this scenario, caps are periodically deposited and then retreat and disappear, possibly due to extremes in the obliquity cycle. In this model, the present caps are the latest example of cap deposition and represent the type of deposit that has come and gone numerous times in the Amazonian. (3) The present caps are old, but have been renewed: In this scenario, the present caps have been in their current position for much of the Amazonian, but some process (e.g., melting, flow) is periodically destroying preexisting craters to produce cap surfaces that appear very young in terms of crater retention ages. Documenting the nature of the emplacement history of the present polar deposits and distinguishing among these disparate scenarios for their history are among major challenges facing Mars polar studies in the coming years.

Latent outflow activity for western Tharsis, Mars: Significant flood record exposed

Observations permitted by the newly acquired Mars Observer Laser Altimeter data have revealed a system of gigantic valleys northwest of the huge Martian shield volcano, Arsia Mons, in the western hemisphere of Mars (northwestern slope valleys (NSVs)). These features, which generally correspond spatially to gravity lows, are obscured by veneers of materials including volcanic lava flows, air fall deposits, and eolian materials. Geologic investigations of the Tharsis region suggest that the system of gigantic valleys predates the construction of Arsia Mons and its extensive associated lava flows of mainly late Hesperian and Amazonian age and coincides stratigraphically with the early development of the outflow channels that debouch into Chryse Planitia. Similar to the previously identified outflow channels, which issued tremendous volumes of water into topographic lows such as Chryse Planitia, the NSVs potentially represent flooding of immense magnitude and, as such, a source of water for a northern plains ocean.

System of Gigantic Valleys northwest of Tharsis, Mars: Latent catastrophic flooding, northwest watershed, and implications for Northern Plains Ocean

Mars Orbiter Laser Altimeter (MOLA) reveals a system of gigantic valleys to the northwest of the huge martian shield volcano, Arsia Mons, in the western hemisphere of Mars. These newly identified northwestern slope valleys (NSVs) potentially signify previously undocumented martian catastrophic floods and may corroborate the northern ocean hypotheses. These features, which generally correspond spatially to gravity lows, were previously obscured in Mariner and Viking Orbiter imagery by veneers of materials, including volcanic lava flows and air fall deposits. Geologic investigations of the Tharsis region suggest that the NSVs were mainly carved prior to the construction of Arsia Mons and its associated Late Hesperian and Amazonian age lava flows, concurrent with the early development of the outflow channels that debouch into Chryse Planitia.

96/01917 Operation of blast furnaces with shaped cokes

We have remapped the geology of the north polar plateau on Mars, Planum Boreum, and the surrounding plains of Vastitas Borealis using altimetry and image data along with thematic maps resulting from observations made by the Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter spacecraft. New and revised geographic and geologic terminologies assist with effectively discussing the various features of this region. We identify 7 geologic units making up Planum Boreum and at least 3 for the circumpolar plains, which collectively span the entire Amazonian Period. The Planum Boreum units resolve at least 6 distinct depositional and 5 erosional episodes. The first major stage of activity includes the Early Amazonian (∼3 to 1 Ga) deposition (and subsequent erosion) of the thick (locally exceeding 1000 m) and evenly-layered Rupes Tenuis unit (Abrt), which ultimately formed approximately half of the base of Planum Boreum. As previously suggested, this unit may be sourced by materials derived from the nearby Scandia region, and we interpret that it may correlate with the deposits that regionally underlie pedestal craters in the surrounding lowland plains. The second major episode of activity during the Middle to Late Amazonian (∼<1 Ga) began with a section of dark, sand-rich and light-toned ice-rich irregularly-bedded sequences (Planum Boreum cavi unit, Abbc) along with deposition of evenly-bedded light-toned ice- and moderate-toned dust-rich layers (Planum Boreum 1 unit, Abb1). These units have transgressive and gradational stratigraphic relationships. Materials in Olympia Planum underlying the dunes of Olympia Undae are interpreted to consist mostly of the Planum Boreum cavi unit (Abbc). Planum Boreum materials were then deeply eroded to form spiral troughs, Chasma Boreale, and marginal scarps that define the major aspects of the polar plateau's current regional topography. Locally- to regionally-extensive (though vertically minor) episodes of deposition of evenly-bedded, light- and dark-toned layered materials and subsequent erosion of these materials persisted throughout the Late Amazonian. Sand saltation, including dune migration, is likely to account for much of the erosion of Planum Boreum, particularly at its margin, alluding to the lengthy sedimentological history of the circum-polar dune fields. Such erosion has been controlled largely by topographic effects on wind patterns and the variable resistance to erosion of materials (fresh and altered) and physiographic features. Some present-day dune fields may be hundreds of kilometers removed from possible sources along the margins of Planum Boreum, and dark materials, comprised of sand sheets, extend even farther downwind. These deposits also attest to the lengthy period of erosion following emplacement of the Planum Boreum 1 unit. We find no evidence for extensive glacial flow, topographic relaxation, or basal melting of Planum Boreum materials. However, minor development of normal faults and wrinkle ridges may suggest differential compaction of materials across buried scarps. Timing relations are poorly-defined mostly because resurfacing and other uncertainties prohibit precise determinations of surface impact crater densities. The majority of the stratigraphic record may predate the recent (<20 Ma) part of the orbitally-driven climate record that can be reliably calculated. Given the strong stratigraphic but loose temporal constraints of the north polar geologic record, a comparison of north and south polar stratigraphy permits a speculative scenario in which major Amazonian depositional and erosional episodes driven by global climate activity is plausible.

Geologic evolution of the east rim of the Hellas basin Mars

Geomorphic analyses and crater statistics indicate the distribution of materials on the east rim (27.5°–42.5° S, 260°–275° W) of the Hellas basin and constrain the timing, nature, and interactions of geologic processes which have modified the Hellas region. The discernible geologic history of the area began with the formation of the impact basin and the emplacement of highlands that became heavily cratered during the Noachian Period. The tectonic setting of the region was established at this time, and basin structures promoted and influenced subsequent volcanic, tectonic, and fluvial activity. Degradation of the highland units by fluvial, mass-wasting, and aeolian processes began in the Noachian and continued into the Hesperian and Amazonian Periods. Sedimentary plains covered low-lying regions of the highlands; their pitted surfaces in some areas suggest subsidence and the presence of volatiles. Emplacement of lobate debris aprons, shed from highland peaks and possibly facilitated by ice creep, occurred in the Amazonian Period. The volcanic history of the east rim of the Hellas basin began with the formation of the highland paterae. The low, dissected shields of Hadriaca and Tyrrhena Paterae formed in the Late Noachian or Early Hesperian Epochs and are older than or coeval with the ridged plains of Hesperia Planum. Volcanic activity continued throughout the Hesperian Period with the emplacement of the caldera-filling materials at Hadriaca Patera and the lava flows of the flank flow unit of Tyrrhena Patera. Outflow channels and related channeled plains of the Hesperian and Amazonian Periods suggest an extensive and complex history of volatile-driven erosion and sedimentary deposition. Dao and Harmahkis Valles are interpreted to be collapsed regions of the older plains; a combination of surface and subsurface flow has removed materials with deposition on the floor of the Hellas basin. The geologic evolution of the region, determined from the observed surface morphology and stratigraphic relationships and refined by crater statistics, indicates that a combination of diverse geologic processes acted in the degradation of the Hellas basin. The effects of volcanism and volatile-driven processes were prominent throughout the Hesperian and well into the Amazonian Periods. The proposed transition from explosive to effusive volcanism in the Hellas region is supported by the relative ages indicated by crater statistics. The heat released by volcanism may have resulted in the melting of ground ice and contributed to the development of the channeled plains and outflow channels. The presence of wrinkle ridges on units that are inferred to be relatively young from their crater size-frequency distributions suggests deformation in the Late Hesperian or Early Amazonian Epoch.