Rossman P. Irwin

An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development

[1] To explain the much higher denudation rates and valley network development on early Mars (>∼3.6 Gyr ago), most investigators have invoked either steady state warm/wet (Earthlike) or cold/dry (modern Mars) end-member paleoclimates. Here we discuss evidence that highland gradation was prolonged, but generally slow and possibly ephemeral during the Noachian Period, and that the immature valley networks entrenched during a brief terminal epoch of more erosive fluvial activity in the late Noachian to early Hesperian. Observational support for this interpretation includes (1) late-stage breaching of some enclosed basins that had previously been extensively modified, but only by internal erosion and deposition; (2) deposition of pristine deltas and fans during a late stage of contributing valley entrenchment; (3) a brief, erosive response to base level decline (which was imparted as fretted terrain developed by a suite of processes unrelated to surface runoff) in fluvial valleys that crosscut the highland-lowland boundary scarp; and (4) width/contributing area relationships of interior channels within valley networks, which record significant late-stage runoff production with no evidence of recovery to lower-flow conditions. This erosion appears to have ended abruptly, as depositional landforms generally were not entrenched with declining base level in crater lakes. A possible planetwide synchronicity and common cause to the late-stage fluvial activity are possible but remain uncertain. This increased activity of valley networks is offered as a possible explanation for diverse features of highland drainage basins, which were previously cited to support competing warm, wet and cold, dry paleoclimate scenarios.

Interior channels in Martian valley networks: Discharge and runoff production

The highland valley networks are perhaps the most compelling evidence for widespread fluvial activity on Mars .3.5 Ga. However, determining the hydrology of these features has been difficult owing to poor image resolution and the lack of available topographic data. New orbital imaging reveals 21 late-stage channels within valley networks, which we use to estimate formative discharges and to evaluate water supply mechanisms. We find that channel width and associated formative discharge are comparable to terrestrial valley networks of similar area and relief. For 15 narrow channels in basin-filling networks, likely episodic runoff production rates up to centimeters per day and first-order formative discharges of ;300‐3000 m 3 /s are similar to terrestrial floods supplied by precipitation. Geothermal melting of ground ice would produce discharges ;100 times smaller per unit area and would require pulsed outbursts to form the channels. In four large valleys with few tributaries, wider channels may represent large subsurface outflows or paleolake overflows, as these four channels originate at breached basin divides and/or near source regions for the catastrophic outflow channels.

HiRISE imaging of impact megabreccia and sub-meter aqueous strata in Holden Crater, Mars

High Resolution Imaging Science Experiment (HiRISE) images of Holden crater, Mars, resolve impact megabreccia unconformably overlain by sediments deposited during two Noachian-age phases of aqueous activity. A lighter-toned lower unit exhibiting phyllosilicates was deposited in a long-lived, quiescent distal alluvial or lacustrine setting. An overlying darker-toned and often blocky upper unit drapes the sequence and was emplaced during later high-magnitude flooding as an impounded Uzboi Vallis lake overtopped the crater rim. The stratigraphy provides the first geologic context for phyllosilicate deposition during persistent wet and perhaps habitable conditions on early Mars.

Fluvial features on Titan: Insights from morphology and modeling

Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft flybys by the Cassini Titan Radar Mapper (RADAR) and in Descent Imager/Spectral Radiometer (DISR) images taken during descent of the Huygens probe to the surface. Interpretations using terrestrial analogs and process mechanics extend our perspective on fluvial geomorphology to another world and offer insight into their formative processes. At the landscape scale, the varied morphologies of Titan’s fluvial networks imply a variety of mechanical controls, including structural influence, on channelized flows. At the reach scale, the various morphologies of individual fluvial features, implying a broad range of fluvial processes, suggest that (paleo-)flows did not occupy the entire observed width of the features. DISR images provide a spatially limited view of uplands dissected by valley networks, also likely formed by overland flows, which are not visible in lower-resolution SAR data. This high-resolution snapshot suggests that some fluvial features observed in SAR data may be river valleys rather than channels, and that uplands elsewhere on Titan may also have fine-scale fluvial dissection that is not resolved in SAR data. Radar-bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fine-scale fluvial dissection. Fluvial deposition is inferred to occur in braided channels, in (paleo)lake basins, and on SAR-dark plains, and DISR images at the surface indicate the presence of fluvial sediment. Flow sufficient to move sediment is inferred from observations and modeling of atmospheric processes, which support the inference from surface morphology of precipitation-fed fluvial processes. With material properties appropriate for Titan, terrestrial hydraulic equations are applicable to flow on Titan for fully turbulent flow and rough boundaries. For low-Reynolds-number flow over smooth boundaries, however, knowledge of fluid kinematic viscosity is necessary. Sediment movement and bed form development should occur at lower bed shear stress on Titan than on Earth. Scaling bedrock erosion, however, is hampered by uncertainties regarding Titan material properties. Overall, observations of Titan point to a world pervasively influenced by fluvial processes, for which appropriate terrestrial analogs and formulations may provide insight.

The relationship between the Southern Oscillation Index and tropical cyclone tracks in the eastern North Pacific

Points of origin and downgradation of tropical cyclones in the eastern North Pacific (ENP) east of 160°W are related to the Southern Oscillation Index (SOI) during the hurricane season. All ENP tropical storms and hurricanes from 1966 through 1997 were grouped into three SOI-based categories: those occurring during strong El Nino events (mean SOI 0.6), or near zero periods (−0.6 < mean SOI < 0.6). During El Nino storm seasons, ENP tropical cyclones originated approximately 5.7° (617 km) west and downgraded 7.5° (780 km) west of the long-term mean longitudes for the positive SOI group. Near zero group storms also followed more northerly tracks than the negative SOI group storms. However, no significant differences in storm track are evident between the positive SOI and near zero groups, and the track length is not significantly different for any storm group.

Aqueous depositional settings in Holden crater, Mars

Holden crater in southwestern Margaritifer Terra likely contained two distinct lakes of differing character and sources during the Late Noachian on Mars. The first lake may have included distal alluvial deposits, was longer-lived, and formed when drainage from within the crater ponded on the crater floor. The second lake was shorter-lived and created when water impounded in Uzboi Vallis breached Holdens rim and rapidly drained into the crater. Holden crater is partially filled by an impressive sequence of more than 150 m of finely bedded, sedimentary deposits that are best exposed in the southwestern portion of the crater but are also observed in the central and eastern portions of the crater. The sedimentary beds comprising the lower and upper units exposed in Holden record two intervals of ponding within the crater. Various processes have been proposed to account for the origin of the lower unit deposits in Holden crater. These processes include one or multiple lacustrine episodes on the crater floor, air-fall or glacial origin. The thin, laterally continuous bedding that is confined below –1960m within the crater is most consistent with a water-lain origin for all three members of the lower unit. While non-aqueous depositional processes for the lower unit appear improbable, distinguishing between a distal alluvial versus lacustrine depositional environment is challenging.