JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. E3, PAGES 5433-5447, MARCH 25, 1995 The circum-Chryse region as a possible example of a hydrologic cycle on Mars: Geologic observations and theoretical evaluation Jeffrey M. Moore, • Gary D. Clow, 2 Wanda L. Davis, • Virginia C. Gulick, 3 DavidR. Jankeri Christopher P. McKay, 3 Carol R. Stoker, 3 Aaron P. Zent I Abstract. The transection and superposition relationships among channels, chaos, surface materials units, andother features in thecircum-Chryse region of Mars wereused to evaluate relativeagerelationships and evolution of flood events. Channels andchaos in contact (with one another) were treated assingle discrete flood-carved systems. Some outflow channel systems formnetworks and areinferred to have been created by multiple flood events. Within some outflow channel networks, several separate individual channel systems can betraced to a specific chaos which acted asflood-source area to that specific flood channel. Individual flood-carved systems were related to widespread materials units or other surface features thatserved as stratigraphic horizons.Chryse outflowchannels areinferred to haveformed overmost of the perceivable history of Mars. Outflow channels are inferred to become younger withincreasing Proximity to theChryse basin.In addition, outflow channels closer to the basin show a greater diversity in age. The relationship of subsequent outflowchannel sources to the sources of earlier floods is inferred to disfavor episodic flooding due to the progressive tapping of a juvenile near- surface water supply. Instead, wepropose the circum-Chryse region as a candidate site of past hydrological recycling. The discharge rates necessary to carve thecircum-Chryse outflow channels would have inevitably formed temporary standing bodies of H20 on the Martian surface where theflood-waters stagnated and pooled (the Chryse basin is topographically enclosed). These observations and inferences have ledus toformulate and evaluate two hypotheses: (1) large amounts of the sublimated H20 off theChryse basin flood lakes precipitated (snowed) onto the flood-source highlands and this H20 was incorporated into the near surface, recharging the H20 sources, making possible subsequent deluges; and (2) ponded flood-water in Chryse basin drained back down an anti basinward dipping subsurface layer accessed along the southern edge of the lake, recharging the flood-source aquifers. H20 not redeposited in the flood-source region was largely lost to the hydrologic cycle.This loss progressively lowered thevitality of thecycle, probably by nowkillingit. Ournumerical evaluations indicate that of thetwohypotheses we formulated, the groundwater seep cycle seems byfarthe more viable. Optimally, ~ 3/4 of the original mass of anice-covered cylindrical lake (albedo 0.5, 1 kmdeep, 100-km radius, draining along itsrim for one quarter of itscircumference into substrata witha permeability of 3000 darcies) can be modeled to have moved underground (on timescales ofthe order of 103 years) before thecompeting mechanisms of sublimation andfreeze downchoked off further water removal. Once underground, this water can travel distances equal tothe separation between Chryse basin and flood-source sites in geologically short (~106 year-scale) times. Conversely, wecalculate that optima!ly only ~40% of the H20 carried from Chryse can condense at the highlands, and most of the precipitate would either collect atthe base of the highlands/lowlands scarp or sublimate at rates greater thanit wouldaccumulate over the flood-source sites. Further observations from forthcoming missions maypermit the determination of which mechanisms mayhave operated to recycle theChryse flood-waters. l Center forMars Exploration and the SETI Institute, NASA Ames Research Center, Moffett Field, California. 9. Branch of Astrogeology, U.S. Geological Survey, Menlo Park, California. 3 Space Science Division, NASA Ames Research Center, Moffett Field, California. 4Department of Geography, Arizona State University, Tempe. 1995by tl• •n Geophysical Uaioa. Introduction There has been a renewed interest in the idea of Martian hydrologic cycleslargely brought on by T. S. Parker,V. R. Baker, and their colleagues, who explored the possibilitythat Mars once had large bodies of standing water and large continental-style glacial ice sheets located in topographically confined lowlands [e.g., Parker et al., 1989; Baker et al., 1991; Kargel and Strom, 1992]. Other studies havepointed to evidence for large lacustrine basins on Mars which are postulated to be very young in the martian stratigraphic sequence[Scott and Chapman,1991; Scott et al., 1992]. 5433