The Extremes of the Extremes: Extraordinary Floods (Proceedings of a symposium held at Reykjavik, Iceland, July 2000). IAHS Publ. no. 271, 2002.

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Recent extreme floods on Mars DEVON BURR Department of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East Fourth Street, Tucson, Arizona 85721-0077, USA; mail: 1629 East University Boulevard, Tucson, Arizona 85721, USA e-mail: dburr@pirl.lpl.arizona.edu ALFRED McEWEN Lunar and Planetary Laboratory, University of Arizona, 1629 East University Boulevard, Tucson, Arizona 85721, USA

Abstract New high-resolution images of the large (>20 km wide) flood channels near the Cerberus Plains and in Marte Vallis, Mars, reveal stream-lined islands, longitudinal grooving, and terracing, similar to features in the Channeled Scabland (USA). These are the youngest catastrophic flood channels on Mars, formed less than 200–500 × 106 years ago, and fluvial morphologies are preserved on scales of a few metres. Peak discharges are estimated as more than 107 m3 s-1. Our data indicate that the water originated in the region north of the Cerberus Plains, and was probably related to volcanic processes. Key words Mars; Marte Vallis; Cerberus Plains; longitudinal grooving; streamlined forms

INTRODUCTION Understanding the history of water on Mars provides valuable clues to Mars’ geological processes. Huge channels on Mars were attributed to extreme floods on the basis of their resemblance, though at a larger scale, to catastrophic flood features found in the Channeled Scabland, Washington, USA (Baker & Milton, 1974). Discharges for these channels have been calculated using Manning’s equation modified for Martian gravity (Carr, 1979) at 106–109 m3 s-1 (Carr, 1979; Komar, 1979; Komatsu & Baker, 1997; Smith et al., 1998). Almost all of the large channels belong to the Upper Hesperian stratigraphic series, with age estimates ranging from 1.8 to 3.7 ×109 years (Tanaka, 1986). THE YOUNG AGES OF THE CERBERUS PLAINS AND MARTE VALLIS Flood channels have also been recognized in the Cerberus Plains and Marte Vallis (Fig. 1) largely on the basis of the anastomosing albedo pattern visible in low-resolution Viking images (Tanaka, 1986; Plescia, 1993; Scott & Chapman, 1991). Counts of large (>1 km diameter) craters suggest an age of only 200–500 × 106 years for these plains (Plescia, 1993). Recent high-resolution (2–20 m/pixel) images from the Mars Orbital Camera (MOC) (Malin et al., 1998) on the Mars Global Surveyor spacecraft (Albee et al., 1998) have revealed details of large channels that cut these plains (McEwen et al., 1999). Younger lavas, less than 10 × 106 years old (Hartmann & Berman, 2000), embay these channels, so the age of the channels is probably

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between 10 and 500 × 106 years. MOC images show extreme flood morphologies in Marte Vallis, and in areas west and north of the Cerberus Plains (Fig. 1). In addition, the Mars Orbital Laser Altimeter (MOLA) has provided new and very accurate topographic data (Smith et al., 1999; Fig. 1). EVIDENCE FOR FLOODING IN THE CERBERUS REGION Among the morphologies revealed in MOC images are streamlined islands, longitudinal grooving, and terracing. Large streamlined islands occupy many channels (Fig. 2). These features appear very similar to streamlined islands in the Channeled Scabland, and may constitute the single most reliable class of flood evidence because their planform geometric shapes are most immune to aeolian or volcanic modification. Although mass wasting has affected the planform shapes of many of the islands (e.g. possible landslide scarps in Fig. 2), their general outline can still be determined. The terracing around many of the islands suggests that they are relatively unmodified.

Fig. 1 Viking mosaic of the Cerberus region, Mars, overlain by a contour map derived from one-quarter degree binned MOLA data. The region is about 1500 km across, centred at approximately 10°N latitude and 190°W longitude. Contours are in increments of 200 m relative to Mars’ geoid. K locates images in Table 1 from which K values have been calculated; K in middle left corresponds to Fig. 2. Q locates images in Table 2 from which discharges have been calculated.

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Assuming a smooth hydrodynamic planform, we measured the length (l), width (w) and surface area (A) of two streamlined islands in the Cerberus region and Marte Vallis. Assuming a lemniscate geometry, the elongation can be parameterized by K = l2π/4A (Baker, 1979). The results are presented in Table 1. These K values are similar to those calculated for streamlined islands found in the Channeled Scabland, which ranged from 2.9 to 4.3 in the examples provided (Baker, 1978, pp. 88–92). Long linear ridges that parallel the channel extend for at least a few km in several Cerberus region channels (Fig. 2). Longitudinal grooving is also evident in the

Fig. 2 Subset of MOC image M07-00614, showing streamlined island with terracing, and longitudinal grooving in the surrounding channel floor. The image is 3 km wide; north is up.

Table 1 Geometric measurements and K values for streamlined islands in the Cerberus region.

Image and location Length (km) Width (km) Area (km2) K (unitless) SP2-40703, 19°N 175°W 0.9 0.3 0.1 5.9 M07-00614 (Fig. 2), 9°N 204°W 4.3 1.9 3.1 4.7

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Channeled Scabland, associated with erosive flooding over a single basalt surface (Baker, 1978). The Channeled Scabland examples have an average depth of 5 m and an average width (or spacing) of 50 m. In the Cerberus region, groove widths range from about 50 to 100 m. Depths are difficult to determine due to the illumination geometry and infilling by aeolian material or lava, but appear to be less than 10 m. Thus the grooves have a high width to depth ratio similar to those in the Channeled Scabland. Steep multi-layered terracing is visible around streamlined islands and at channel edges (Fig. 2). The heights of the terraces are often taken to indicate a maximum flood height in discharge calculations (e.g. Smith et al., 1998). However, the terraces may be either depositional or erosional and may be strongly influenced by the volcanic stratigraphy, so that terrace height may not bear any strict relationship to flood-water heights. MOLA topography, which shows the terrain sloping down into the Cerberus Plains from all directions except to the northeast along Marte Vallis (Fig. 1), is consistent with our interpretations of channel flow directions from the MOC images. DISCHARGE ESTIMATES Based on these lines of evidence, we conclude that the channels in the Cerberus region and Marte Vallis were formed by extreme floods. To estimate the discharges of these floods, we used Manning’s equation modified for Martian gravity:

)2/1()3/2(51.0 Θ= ARn

Q (1)

where Q is the discharge in cubic metres per second, n is the roughness coefficient, A is the channel cross-sectional area, R is the hydraulic radius, and Θ is the slope (Carr, 1979). We calculated discharges for three channels, two sloping downward into the Cerberus Plains from the west and north, and one in Marte Vallis (Fig. 1; Table 2). We used a Manning’s n of 0.04, after Baker (1982), and determined the slope from one-degree gridded topography maps (Smith et al., 1999). Because the channel is sometimes wider than the MOC image, we determined the width of the channel from Viking context images. In determining the average water depth, we assumed for the purposes of these relative calculations that the flood reached the top of the channel walls. We used the trigonometric relationship between the incident angle for the image and the length of true shadow to determine the height of the channel walls. The results of these calculations are presented in Table 2.

Table 2 Discharge calculations for channels in the Cerberus region.

Image and location Width (km)

Depth (m)

Slope (unitless)

Discharge (m3 s-1)

M02-00581, 9°N 204°W (inflow channel)

25 75 0.0018 1.70 × 107

M03-07216, 16°N 192°W (inflow channel)

20 135 0.0020 3.90 × 107

M02-01585, 12°N 179°W (Marte Vallis)

69 95 0.0009 4.80 × 107

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ORIGIN OF CERBERUS REGION FLOOD WATER These data allow us to assess the origin of the flood water in the Cerberus region. Plescia (1993) suggested the channels were formed by volcanically induced precipitation. However, precipitation should produce dendritic networks, instead of the apparent anastomosing channels, but such networks have not been seen in regional MOC images. Scott & Chapman (1991) hypothesized that the primary source was channels emanating from highland terrain to the south, but MOC images do not show young channels in this region. Tanaka (1986) and Tanaka & Scott (1986) proposed that the source of the water was the “Cerberus Rupes” fractures (Fig. 1). MOC images do show significant channels and flood features in this region (e.g. Fig. 2) and the discharge calculations, which show significant inflow from the north and the west (Table 1), support the hypothesis that the main water source was north of the plains. The origin of the channels in the remnants of the ancient highland crust suggests that the water may have been emplaced in the crust early in Mars’ history. Alternatively, the proximity to the large Elysium volcanic province to the northwest supports the idea that the water was emplaced or replenished by magmatic processes. In either case, the coincidence in space and time of the volcanism and the flooding suggests a genetic relationship between them. The flood-water release could have been triggered by magmatic melting of ground ice (e.g. Squyres et al., 1987; Mackenzie & Nimmo, 1999), by fracturing of subsurface aquifers (Carr, 1979), for instance by magma, or by explosive exsolution of gases in subsurface aquifers due to magmatic heating (V. Baker, R. Strom, personal communications). Acknowledgements Many thanks to Mike Malin, Ken Edgett, and Elsa Jensen of Malin Space Science Systems for collecting the MOC images, to Ross Beyer, Brad Castalia, Peter Lanagan, and Joe Plassmann (University of Arizona) for data assistance, and to Vic Baker (University of Arizona) for discussion. REFERENCES Albee, A. L., Palluconi, F. D. & Arvidson, R. E. (1998) Mars Global Surveyor Mission; overview and status. Science

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