Late-Stage Fluvial Processes
Figure 1. Naktong Vallis is an example of a Martian valley network with features consistent of a brief, late-stage epoch of surface runoff.
Why are the ancient Noachian impact craters on Mars so deeply eroded while the contemporary river valley networks remain poorly developed1? We are studying Martian paleolakes and valley networks to answer this question. We find that in addition to the intense crater degradation that occurred during the Noachian Period, a late stage of valley network activity occurred between the Lake Noachian and Early Hesperian Epochs, about 3.7 billion years ago1. The immature fluvial features briefly preserved on Mars are remnants of this brief late stage.
Determining the age of valleys requires studying relationships between the valleys and the landscape around them. The main lines of evidence for an intense late epoch of fluvial erosion include: (1) late-stage overflow of basins, (2) late-stage downcutting of contributing valleys, (3) deposition of pristine deltas and fans of sediment; (4) incision of valleys into the highland/lowland boundary scarp after fretted terrain formed to the north of the Early Hesperian and (4) the widths of channels observed in a number of valley networks, which record late-stage flooding similar to what we commonly see on Earth1.
In addition to studying the channels through their interactions with the surrounding landscape, we studied them on a narrower scale. We examined 21 late-stage channels within the valley networks, eight of which were previously identified and 13 were discovered in our study2. Formative discharges were estimated and water supply mechanisms2 were evaluated for the 21 channels. For the 15 narrow channels found in valleys with broad networks of tributaries, likely episodic runoff up to centimeters per day and first-order formative discharges of ~300-3,000 m3/s are similar to terrestrial floods supplied by precipitation2. Geothermal melting of ground ice would produce discharges ~100 times smaller per unit area and would require pulsed outbursts to form the channels2. Moreover, three of the larger channels had formative discharges of ~3000-7000 m3/s 2; their large valleys with few tributaries and wider channels may represent large subsurface outflows or paleolake overflows, since they originate at breached basin divides and/or near source regions for the catastrophic outflow channels2. The preservation and relatively straight, rather than meandering courses of most channels2 suggest they are immature and underwent an abrupt decline in climate. The fact that few deltas were eroded with falling lake level supports this conclusion.
Our explanation for why the Noachian craters are so heavily degraded, but the valley networks are not equally so is that the valley networks represent a later stage of fluvial activity.
1. Irwin et al. (2005), An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development.
2. Irwin et al. (2005), Interior channels in Martian valley networks: Discharge and runoff production.
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