Chryse Planitia, Mars: Topographic configuration, outflow channel continuity and sequence, and tests for hypothesized ancient bodies of water using Mars Orbiter Laser Altimeter (MOLA) data

Many of the largest and most prominent outflow channels on Mars debouch int o Chryse Planitia. Pre-Mars Global Surveyor topographic data show Chryse to be a closed depression almost 2000 kin in diameter. New Mars Orbiter Laser Altimeter (MOLA) data reveal the following: (1)Chryse is not a locally clo sed basin but instead opens into the North Polar basin. (2) The highly dist inctive morphology of the six largest predominantly Hesperian-aged channels (Kasei, Maja, Simud, Tiu, Ares, and Mawrth) disappears into the northern l owlands at average elevations that all occur within less than similar to 17 0 m of a mean elevation of -3742 (SD = 153 m), over a lateral distance in e xcess of 2500 km. (3) The elevations where the distinctive morphology of ea ch channel disappears all fall within similar to 190 m of Contact 2, a boun dary mapped by Parker et nl. [1993] and interpreted to represent an ancient shoreline, and the mean elevation values of Contact 2 and circum-Chryse ch annel termini fall within 18 m of each other. In contrast, the termini of s even later Amazonian-aged channels emerging From Elysium into Utopia Planit ia are spread over a vertical range of >1500 m. (4) Topographic evidence of the continuation of some of the outflow channels can be observed for dista nces of 250-450 km into the North Polar basin, but the morphology is subdue d and distinctly different. (5) The nature of this less distinctive topogra phy and its crosscutting relationships show that Simud acid Tiu are likely to represent the youngest activity (specifically crosscutting Ares Valles). (6) The distinctive change in channel morphology is consistent with rapid loss of energy encountered at base level (subaerial/submarine boundary) and emplacement into a shallow submarine environment. Channel characteristics, lack of distinctive deltas or lobes, and continuation or subdued channel m orphology suggest hyperpychnal flow and the possibility of density/turbidit y currents. Estimates of the volumes of individual channel events are wide- ranging. The minimum volume estimates of Carr [1996] suggest that 46 such e vents would be required to fill the basin to the level of Contact 2 and thu s that the channels may have emptied into an existing standing body of wate r. Volume estimates of Baker ct nl. [1991] assume that single individual ev ents may have filled the basin to the level of Contact 2, thus requiring si gnificant water loss between events and refilling during subsequent events to essentially the same level. In both end-member cases these observations are consistent with the presence of large standing bodies of water in the n orthern lowlands in Hesperian-Early Amazonian times.