UPDATED GLOBAL MAP OF MARTIAN VALLEY NETWORKS: IMPLICATIONS FOR HYDROLOGIC PROCESSES. B.M. Hynek1-2, M. Beach1-2, and M.R.T. Hoke1, 1Laboratory for Atmospheric and Space Physics, 2Dept. of Geological Sciences (392 UCB, Univ. of Colorado, Boulder, CO 80309) hynek@lasp.colorado.edu

Introduction: The valley networks of Mars have long been viewed as some of the best evidence that water flowed across the surface under climatic condi-tions that were much different in the past. However, many questions remain, including the timing and mechanisms of valley formation. To address these questions, Carr [1] and Carr and Chuang [2] produced a comprehensive Martian valley network map from Viking-based images covering ±65° of latitude. This product contained over 900 networks with over 8,000 branches that generally appeared immature in plani-metric form and contained large, undissected regions between networks. The authors used these attributes to suggest that only limited surface water was neces-sary for their formation. Mars Global Surveyor data-sets have revealed many valleys that were not evident in the Viking images and correspondingly more mature drainage systems [3]. The Thermal Emission Imaging System (THEMIS) experiment [4] on the Mars Odys-sey spacecraft provides an even higher resolution look at valley networks. In particular, the 100 m resolution daytime IR data is ideal for identification and analysis of valleys, which are generally a few kilometers wide and hundreds of meters deep. Coregistered, ~0.5-km-resolution Mars Orbiter Laser Altimeter (MOLA) data [5] are also useful in mapping and studying the val-leys’ characteristics. Methods: We used the THEMIS daytime IR and MOLA datasets to remap valley networks on a global scale in an effort to better understand their distribution and to test hypotheses regarding their origin (Fig. A-global map). The valleys were manually mapped us-ing similar determining characteristics to those of Carr [1] for ease of comparison (i.e. sublinear, erosional channels that form branching networks and increase in size downstream). We remapped valleys primarily with the THEMIS global daytime IR mosaic, which covers ~90% of the planet, with colorized MOLA data to aid in detection of valleys and determine downhill direction. In areas of poor THEMIS coverage, we substituted Viking images. Our mapping concentrated in the region within ±70° of latitude, since very few valleys have ever been noted at the highest latitudes. However, we conducted a cursory assessment at polar latitudes for completeness. Manual mapping of val-leys is a tedious and somewhat subjective process that can be influenced by albedo variations and image qual-ity. While a number of automated routines to map global valleys have been attempted with MOLA grid-ded data and other products [6], these have even

greater limitations. Particularly, the resolution of MOLA data is too coarse to resolve many of the smaller features. The size of the smallest valley the algorithm can detect is limited by the resolution and completeness of the gridded MOLA data at a given latitude: At the equator, one MOLA pixel is ~500 m on a side. The smallest valley that can be reliably re-solved is ~3-km-wide (6 pixels). However, this as-sumes that there is complete coverage. Due to the or-bit of the Mars Global Surveyor, approximately 60% of the pixels in the MOLA gridded data at the equator are interpolated, which often eliminates the signature of a few-km-wide valley. Indeed, Molloy and Stepin-ski [6] found that their MOLA-based algorithm identi-fied on average 69% of the valleys they could manu-ally identify in THEMIS images. Missing ~1/3 of the valleys, particularly all the smaller ones, would result in an incomplete picture of fluvial processes on Mars. Further, MOLA data have higher spatial resolution and more complete coverage toward the poles and this would bias the valley detection. Results: Drainage Density: As expected, many more val-leys were seen in THEMIS data compared to those detected in Viking images. Figure B shows an exam-ple of valleys mapped by Carr [1] compared to this study. In the original mapping, a few, often uncon-nected, valleys without many tributaries could be seen (Fig. 1a). Figure 1b shows that with higher resolution data, we can now resolve these valleys into a dense, mature drainage network. In this example, Carr mapped 16 valley segments totaling 1,492 km in length; giving a corresponding drainage density of 0.034 km-1. In contrast, our updated mapping shows 462 valley segments totaling 6,031 km in length; giv-ing a corresponding drainage density of 0.14 km-1. In this case, the higher resolution mapping reveals a fac-tor of 4 increase in stream length and drainage density. The characteristics of the originally mapped valleys are consistent with formation by groundwater proc-esses. However, the updated map indicates that sus-tained precipitation and surface runoff were likely re-quired due to the dense drainage, high stream order, and many small tributaries that reach right up to drain-age divides. In total, the updated global map shows ~4 times as many valleys identified, totaling a summed length ~2 times greater than Carr [1] (Table 1). Drainage densi-ties of networks are almost always much higher and

SECOND WORKSHOP ON MARS VALLEY NETWORKS 39

Figure 2. Global comparison of valleys identified by Carr [1] from Viking data (top) and our preliminary updated map using THEMIS data (bottom). In the new map, denser concentrations of valleys are seen almost everywhere and some areas with few identified valleys from Viking data show significant dissection (e.g., south of Hellas Basin; 55°S, 70°E). Colors on the bottom map represent inferred valley ages determined by the youngest terrain unit they incise (red = Noachian, purple = Hesperian, blue = Amazonian) (unit ages from [8-9]). Figure 1 is a higher resolution example comparing the two maps.

a greater complexity of drainage is evident. Carr and Chuang [2] did not identify a single valley greater than 4th order in the Strahler system (higher order equals more maturity and water) [7] yet we have found one 7th order, six 6th order and twenty-five 5th order net-works. The average drainage density on Noachian-age units is nearly 4 times greater than previously deter-mined. Many smaller tributaries are seen that were not evident with Viking data as well as structure within the valleys including braided channels and terraces indica-tive of sustained flow. Age Distribution: Figure 2 shows that in terms of age, roughly 84% of newly mapped valley segments lie entirely within Noachian terrains (>3.7 Ga ago), 10% cross into or are entirely contained within Hespe-rian-age surfaces (3.7-3.0 Ga) and 6% occur on Ama-zonian terrain (<3.0 Ga) (Fig. 2, Table 1). These per-centages are shifted to younger ages relative to those reported from Viking-based mapping [1] but still argue for relatively rapid climate change at the end of the Noachian. This is supported by the rapid drop in drainage densities on Martian volcanoes [10] and other terrains at this time boundary. The valleys on Noa-chian surfaces occur across most of the Southern High-lands although a few regions are still relatively undis-sected (such as west of Hellas basin, Fig. 2). We speculate that in these regions additional resurfacing has obscured the signature of valleys and this is sup-ported by the heavily degraded and infilled craters in

Figure 1. Comparison of valleys mapped by Carr [1] from Viking data (a) to those identifiable in THEMIS data (b) near 3°S, 5°E; both on a THEMIS basemap.

SECOND WORKSHOP ON MARS VALLEY NETWORKS 40

Figure 3. Distribution of ages and elevations of all identified valleys on Mars. Results show that valley formation shifted to more distributed elevations as time progressed.

these regions. A number of the larger valley systems have been age-dated with crater density measurements [11-12]. These authors suggested that most of these valleys formed toward the end of the Noachian or start of Hesperian epoch. In some cases, systems were re-activated after a period of up to 100s of millions of years and this may indicate that the climate was not continuously warm and wet, but instead more episodic [11]. Valleys are now identified in terrains thought to be undissected from previous work (Fig. 2). Particu-larly, many valleys are now recognized south of Hel-las basin, which exist on Hesperian-aged terrain. The remainder of Hesperian-aged valleys are peppered throughout the cratered highlands with concentrations near Hellas Basin and on volcanoes (Fig. 2). Many of these are on the gentle flanks of the volcano Amphi-trites Patera. The Amazonian-aged valleys occur predominantly on the flanks of volcanoes, including the Early Ama-zonian-aged Tyrrhena, Hadriaca, and Alba Paterae, but also on the eastern slopes of Hellas basin. We mapped the valleys and crater-age dated each channel-covered volcano [10]. While pre-Amazonian valleys show strong characteristics of precipitation and surface run-off, most valleys on Amazonian-age volcanoes are more consistent with a hydrothermal origin. This is true of the youngest valleys on Mars, which are iso-lated to the Tharsis volcanoes.

Valley Elevations: Figure 3 shows the relative distribution of Mars valley networks across the three defined major geological epochs. As previously un-derstood, most of the valleys occur on ancient terrains, which are typically high in elevation [1-2]. These val-leys have a rough Gaussian distribution centered around 1500 m (Fig. 3). The younger valleys, while comprising a small percentage of the total, developed roughly evenly across all elevations (Fig. 3). These valleys formed after climatic conditions favored pre-cipitation and stable surface water on a global scale. The younger valleys probably originated from proc-esses such as hydrothermal circulation on volcanoes [10] or from asteroid impacts that could force climate change locally [13]. The somewhat uniform distribu-tion of these valleys across all elevations may support these ideas. Comparison to Other Datasets: The updated global valley network map was compared to other datasets including global databases of fluvial features and mineralogical/compositional data. No strong cor-relations exist between valley density and elemen-tal/mineralogical data such as the K/Th ratio from GRS [14] or water-altered minerals from OMEGA and CRISM [15 and 16, respectively]. Figure 4 shows newly-mapped valley drainage density, mathematically defined as the density of linear features within a set radius of each output raster cell. Overlaid on this map are global locations of putative paleolake and fan-delta deposits [databases from 17-18, respectively]. There is a strong correlation between density of valleys and the identified fluvio-sedimentary deposits. That is, most

Table 1. Comparison of global Martian valley network characteristics identified from Viking imagery and recently-acquired topographic and IR data. Carr 1995 [1] This study # valleys 11,336 40,005 total length (km)

398,935 797,083

highest stream order

4 7

highest net-work drain-age density (length/area)

~0.02 km-1 0.14 km-1

>~3.7 Ga

~3.7-3.0 Ga

<~3.0 Ga

>~3.7 Ga

~3.7-3.0 Ga

<~3.0 Ga age break-

down 90%, 5%, 5%, 84% 10% 6%

Noachian units’ avg. drainage density

0.0041 km-1 0.015 km-1

SECOND WORKSHOP ON MARS VALLEY NETWORKS 41

Figure 4. Valley density on Mars in relation to putative paleolake deposits (blue, [17]) and fan-deltas (red, [18]). There is a strong correlation between thigh drainage density and reported fluvio-sedimentary deposits.

of the suggested paleolake deposits and fan-deltas oc-cur in regions of higher drainage density (Fig. 4). Conclusions: An updated global map of valleys on Mars has been completed and will be released to the community once we have finished cleaning and validating the map. The 100-meter-resolution global THEMIS day IR data, combined with topography, allow an unprecedented look at these features related to surface and near-surface water. Many more valleys are seen globally than previously and drainage density measurements are correspondingly higher. Some val-leys are seen with interior anatomizing channels and terracing. These new results are consistent with a warm and wet early climate that incised the crust. Hesperian and Early Amazonian valleys also show signs of formation by precipitation and surface runoff, although at much decreased levels. The youngest val-leys on the planet are confined to volcanic constructs and most likely had a hydrothermal origin. Areas on Mars that have higher valley density correlate with fluvio-sedimentary deposits that have been noted by a number of workers. This supports the idea that many Mars valleys and associated deposits were generally formed by long-lived surface runoff from precipita-tion. However, recent results show that in some places valley formation may have been limited to short, in-tense episodes. In summary, valley networks still pro-

vide the best evidence for water on the surface of Mars, but the magnitude and duration of precipitation-fed valley formation is only now being constrained. References: [1] Carr, M. H. (1995) JGR, 100, 7479–7507. [2] Carr, M. H. and F. C. Chuang. (1997) JGR, 102, 9145-9152. [3] Hynek, B. M. and R. J. Phil-lips. (2001) Geology 29, 407-410. [4] Christensen, P. R., et al. (2004) THEMIS Public Data Releases, PDS node, ASU, http://themis.asu.edu. [5] Smith, D. E. et al. (2001) JGR, 106, 23689-23722. [6] Molloy, I. and T. F. Stepinski. (2007) Comp. and Geosci., 33, 728-738. [7] Strahler, A. N., (1958) Geol. Soc. Am. Bull., 69, 279-300. [8] Scott, D. H., and K. L. Tanaka (1986) U.S. Geol. Surv. Misc. Invest. Ser., Map I-1802-A. [9] Greeley, R., and J. E. Guest, (1987) U. S., Geol. Surv. Misc. Invest. Ser., Map I-1802–B. [10] Bowen T. A. and Hynek B. M. (2008) LPSC XXXIX, abs. 2393. [11] Hoke, M. R. T. and Hynek B. M. (2008) J. Geophys. Res. (in review). [12] Fassett, C. I. and Head, J. W. (2008) Icarus, 195, doi: 10.1016/j.icarus.2007.12.009. [13] Segura, T. L. et al. (2008) LPSC XXXIX, abs. 1793. [14] Taylor, G. J. et al. (2006) J. Geophys. Res., 111, doi:10.1029/2006JE002676. [15] Bibring, J. P. et al. (2006) Science, 312, 400-404. [16] Mustard, J. F. et al. (2008) Nature, 454, 354-359. [17] Fassett, C. I. and Head, J.W. (2008) Icarus, in press. [18] Di Achille, G. et al. (2008) this conference.

SECOND WORKSHOP ON MARS VALLEY NETWORKS 42