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Channels in Martian Valley Networks
The highland valley networks are perhaps the most compelling evidence for widespread runoff on
Mars more than 3.5 billion years ago, but until recently we had no means of estimating the size
of the rivers that carved these valleys. New higher-resolution orbital imaging has revealed at
least 21 late-stage channels within valley networks, which we use to estimate river discharges
and determine water sources. 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 valley networks, 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 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
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