High Channels on Mars Indicate Hesperian Recharge at Low Latitudes. N. M. Coleman 1

A special class of Martian outflow channels discharged from enormous fracture zones. These channels were sourced by gro undwater, n ot surface water, and when observed on high-standing plateaus they provide paleo-indicators of c limax groundwater levels. We identify two outflow channels of Hesperian age that issued from a 750-km-long fault zone extending from Candor Chasma to Ganges Chasma. One channel source stands ~2600 m above the datum, too high to be explained by discharge from a global aquifer. The indicated gr oundwater levels require regional sources of recharge and provide evidence that a hi gh-standing, ice-covered lake probably existed in eastern Candor Chasma. Introduction: The transition from the Noachian to the Hesperian era represents a significant shift in surface conditions on Mars. The crust formed during the Noachian, and the planet was heavily bombarded by planetesimals. It is likely that the atmosphere was thickest and the water inventory highest during this time because isotopic evidence shows that the planet has lost large fractions of water and other volatile inventories over geologic time (1). Extensive valley networks developed in the cratered highlands (2) and lakes possibly formed in many craters (3). Noachian erosion rates are estimated to have been ~1000 times greater than during later eras (4). By Hesperian time, which began 3.5-4.3 Gyr ago (4), the atmosphere had thinned and a thick, planet-wide cryosphere may have evolved (5). Immense shield volcanos began forming in Tharsis, and flood basalts inundated larg e areas. The Hesperian was also the peak of outflow channel activity (6), when enormous channels were carved and large water bodies may have collected in the northern lowlands (7,8). But how did water accumu- late in the floodwater source areas? Polar Recharge: Extensive groundwater re- charge is inconsistent with the idea of a thick planet- wide cryosphere on Mars, which would inhibit the migration of water from the surface to underground aquifers. To resolve this dilemma, it has been sug- gested (8,9) that Hesperian recharge occurred at the base of the polar layered deposits and migrated to low latitudes in a globally connected aquifer system. Carr (10) tested this polar rech arge model using the MOLA database of surface elevations obtained by Mars Global Surveyor. He identified 1500 m as a soft upper limit for the elevation at which recharge can efficiently occur by me lting beneath the south polar layered terrains. Carr (10) concluded that major discharges of groundwater onto the surface at eleva- tions >1500 m are unlikely to have the South Pole as a source. Examples of high-standing water-carved features include valleys on the flanks of Alba Patera and Ceraunius Tholus. These valleys could readily be explained by local processes, such as eros ion by discharges from hydrothermal springs (2). Outflow Channels Near Ganges Chas ma: We have studied two specific channels that reveal new insights about Hesperian paleohydrology. They pro- vide evidence that regional groundwater rechar ge occurred at low latitudes during one or more intervals of Hesperian time. Elaver Vallis begins at the eastern rim of a lar ge, 80-km-wide Noachian crater located south of Ganges Chasma (Figure 1A). A pit more than 5000 m deep resides inside the southern part of this crater. We interpret this pit as being undermined and excavated by the eruption of confined grou ndwater, forming a Hesperia n lake in t he surrounding crater. The crater floor is exc eptionally flat and smooth. A profile 40 km long from west to east across the center of the crater shows that elevation varies less than ~30 m (11). This smooth crater floor morphology is consis- tent with lacustrine deposition. Lake levels rose so high that the eastern crater wall was breached, leading to catastrophic release of ponded waters and the carving of the Elaver Vallis channel complex. The flooding apparently occurred during the mid- to late- Hesperian because the channel eroded lower Hesperian strata (unit Hpl 3 ) of the Plateau Sequence (12). West of Elaver Vallis is Al legheny Vallis (name provisionally approved by t he IAU), a channel that emerges from an elongated pit (Figure 1, A to C). This channel was nearly imperceptible in Viking imagery, but MOLA data confirm that it is a continuous valley 250 km long that terminates at t he western rim of Ganges Chasma. At first glance, the channel looks like a typical valley network, but in fact it is a true outflow channel. The waters that carved Allegh eny Vallis erupted from the surface at an elevation of ~2600 m, far above the 1500 m threshold (10) for contributions from polar basal recharge. Could the channel elevation