Lunar and Planetary Science XXX 1417.pdf

FLOW RATES AND DURATION WITHIN KASEI VALLES, . R. M. Williams1 and R. J. Phillips2, 1,2Department of Earth and Planetary Science, Washington University, St. Louis, MO 63130, 1becky @wurtzite.wustl.edu [email protected].

Introduction and Background sidewalls, a relatively constant valley width from While alternate hypotheses for outflow channel source to outlet, and the development of tributaries formation have been proposed, it is generally accepted along regional fracture patterns are features consistent that these features formed as a result of catastrophic with formation by slow sapping and not rapid slump fluvial activity with peak discharges of 109–1010 m3/s failure. Therefore, the Nilus Mensae channel network [1]. Recently acquired Mars Observer Laser Altimeter may provide a chronometer, albeit crude, for the Kasei (MOLA) elevation data from the Mars Global Sur- Valles system indicating that the entire system experi- veyor (MGS) provides insight into the scope of events enced fluvial activity over a significant period of time. that formed the largest outflow channel system on A series of benches are evident in topographic Mars, Kasei Valles. Morphological relationships profiles for both channels. Although some of the within the system combined with hydraulic calcula- benches may be the result of mass wasting, supple- tions suggest a gradual formation history comprised of mental evidence suggests that several benches are flu- relatively modest discharge (8 x 104 - 2 x 107 m3/s) vial terraces. In Northern Kasei Vallis, two pairs of flood events. benches have comparable elevations on either side of Floodwaters which carved Kasei Valles are be- the channel (figure 1a). These may be paired fluvial lieved to have originated from ponded water in Echus terraces that formed due to differential erosion or , a linear depression located just north of pulses of entrenchment within the fluvial system. In Valles Marneris. The broad outflow channel trends Southern Kasei Vallis, benches on the northern bank northward until ~20° N latitude where the channel detected in MOLA track 233 can be correlated with splits into two narrow, eastward flowing channels: a features in high resolution Viking images and are northern and southern route. These channels recom- traceable along channel for over 40 km (figure 1b). If bine at approximately 63° W longitude, and the sys- these benches are fluvial terraces, their location on the tem debouches into . outside bend of a meander implies they are erosional Channel Geometry and Chronology rock-cut terraces formed by a stream cutting laterally MOLA acquires measurements of topography and and simultaneously downcutting slowly. surface reflectivity that have a maximum vertical Hydraulic Calculations resolution of ~30 cm and along-track spatial resolu- The revised channel geometry revealed by MOLA tion of ~300 m [2]. During the first science phasing has significant implications for hydrologic calcula- orbits (March 26, 1998 - April 30, 1998), three tions for . Unambiguous high water MOLA topographic profiles crossing the Kasei Valles marks have not been identified for martian outflow region reveal new detail of the outflow channel ge- channels. For the purpose of this study, the elevation ometry not resolvable in Viking images (figure 1). of the lowest bench at each site was used as a proxy Both Northern (NKV) and Southern (SKV) Kasei for water depth, which yields upper-bound values of Valles have narrow, inner channels that range in calculated discharge for the channel in question (blue width between 6 and 8 km. Further, a new channel is line in figure 1). This process yields water depths evident in orbit 214 across the Kasei Valles plain (fig- <300 m, comparable to water depth values used in ure 1c). These narrow, well-defined channels record previous hydrologic calculations for outflow channels periods of flow incisement. [1]. Using this "bank-full" condition and Komar's [3] Superposition relationships indicate a sequence of modification of the Manning equation to account for development for these discreet channels. Southern the lower gravitational acceleration on Mars, maxi- Kasei Valles crosscuts Northern Kasei Vallis where mum discharge estimates were obtained for reaches of the two channels recombine. Thus, the last flood flow Northern and Southern Kasei Vallis over a range of through the southern channel post-dated fluvial activ- Manning roughness values from 0.01 to 0.07 (Table ity within the northern channel. Further, development 1). These discharge values are several orders of mag- of the channel network on Nilus Mensae was between nitude lower than estimates made for Northern Kasei floods within Northern and Southern Kasei Valles. Valles (> 1 x 109 m3/s) by Robinson and Tanaka [4]. This channel network exhibits morphological charac- The principal reason for this is the significantly re- teristics of terrestrial channels formed by sapping due duced width of the channels. Estimated peak dis- to seepage erosion. Theater-shaped heads, broad, charges associated with large terrestrial floods, such flattened U-shaped profiles with high and steep valley as the catastrophic draining of Lake or Lunar and Planetary Science XXX 1417.pdf

FLOW RATES AND DURATION WITHIN KASEI VALLES: R. M. Williams and R. J. Phillips

Lake Missoula flood, are comparable to the calculated Table 2: Terrestrial Discharge Rates maximum discharge estimates for Kasei Valles (Table Site Discharge Source 2). Based on the revised channel geometry, there is (107 m3/s) 9 3 no evidence that 10 m /s or greater flows are required Lake Bonneville 0.1 [7] to carve Kasei Valles. Channeled Scablands 2 [8] Implications Ave. Mississippi River 0.002-0.003 [1] The morphological observations and hydraulic cal- culations discussed above are consistent with a grad- Figure 1: MOLA topographic profiles for Kasei Valles. Grey dotted ual formation history of the Kasei Valles system: multiple fluvial events involving relatively modest flows (by previous estimates) operating over a long period of time. If this working model is validated, it suggests that Mars once had an active hydrological cycle capable of recharging the source region for these floods. Parker et al [5] hypothesized that the circum- Chryse outflow channels were active simultaneously during the final episode of flooding and that their combined flow formed a northern plains ocean. By its size, the Kasei Valles system would have been a sig- nificant contributor to the total volume. An accurate volume estimate, based on MOLA data [6], for the younger and smaller proposed martian ocean (Contact 2) has been reported as 1.5 x 107 km3. Using a dis- charge value of 1 X 107 km3/s implies a fill time of 47 years for this smaller ocean. With only part of the Kasei Valles system active at one time and given the unlikely scenario that all outflow channel floods were contemporaneous, an even longer fill time would be required. Standing bodies of water are not currently stable under current martian atmospheric conditions and certainly could not persist for decade time-scales. If a martian ocean did form, a warmer climate would have been required to allow flooding water to accu- mulate to the proposed shorelines. References: [1] Carr M. H. (1996) . [2] , D. E. et al. (1998) Science, 279, 1686-1691. [3] Komar P. D. (1979) Icarus, 37, 156-181. [4] Robinson M. S. and Tanaka K. L. (1990) Geology, 18, 902-905. [5] Parker T. J. et al (1993) JGR, 98, 11,061-11,078. [6] Head, J. W. (1998), GRL, 25, 4401-4404. [7] O’Connor, J. E. (1993), GSA Special Paper 274. [8] Baker, V. R. and Nummedal, D. (1978), line is a liner interpolation of topography for data gaps. Blue line in A) The Channeled Scabland. and B) represents level of lowest bench used in hydraulic calculations. A) Orbit 233 over Northern Kasei Vallis illustrates the narrow, inner Table 1: Calculated Hydrologic Parameters channel and a series of benches. Two sets of these benches may be for Kasei Valles paired fluvial terraces: 1) -1561.90 m and –1557.18 m (solid arrows), Site Slope Depth n V Q 2) -1923.4 m and –1928.9 m (open arrows). B) Orbit 233 over the (m) (m/s) (107 m3/s) inner channel of Southern Kasei Vallis illustrates benches (solid ar- NKV 0.002 92.7 0.01-0.07 51-7 2-0.3 rows) on the northern channel bank. C) Orbit 214 across broad North- SKV 0.0002 22 0.01-0.07 5-1 0.06-0.008 ern Kasei Vallis head. Arrow marks previously unrecognized incised channel on Kasei Valles plain. NKV [4] 0.001 374 0.015 75 230 “ “ “ 0.035 32 100