Major episodes of the hydrologic history in the region of Hesperia Planum, Mars

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2005je002420, 2005 Major episodes of the hydrologic history in the region of Hesperia Planum, Mars M. A. Ivanov, 1,2 J. Korteniemi, 2 V.-P. Kostama, 2 M. Aittola, 2 J. Raitala, 2 M. Glamoclija, 3 L. Marinangeli, 3 and G. Neukum 4 Received 15 February 2005; revised 11 August 2005; accepted 15 August 2005; published 15 December [1] The High Resolution Stereo Color camera (HRSC) data over Hesperia Planum and its surroundings reveal important details of geologic episodes and water-related processes in this region. (1) The Noachian fluvial events of Hesperia Planum depression included accumulation of water and formation of a water/ice reservoir there. Later, the reservoir was depleted in several phases reflecting diminishing amounts of water. Climate changes and/or volcanism were important in these volatile releases. (2) The massive, km 3, erosion from the Hesperia depression before the main lava eruption possibly resulted in thick, km, deposits in Hellas Planitia. (3) Measurements of the flooded craters within Hesperia Planum provide the estimates of the thickness of lavas there, about m. The final volume of lavas within Hesperia Planum ( km 3 ) is comparable with the range of some terrestrial igneous provinces such as Columbia River Basalts. (4) Extended magmatism possibly triggered formation of the outflow channel in a few locations after the lava emplacement. During this episode, about km 3 of material (about % of the volume eroded in the episode of massive erosion) were removed. The thickness of the composite lava layer exposed on the walls of the outflow channels, a few hundreds of meters, corresponds well to the thickness estimates made by the measurements of the flooded craters. (5) Dispersed viscous flows (debris aprons, flow-like deposits) reflect the final fluvial events. Viscous flows from the subsurface sources in the Southwestern trough associate with Dao, Niger, and Harmakhis Valles. These flows represent the final volatile discharge from the Hesperia reservoir that mostly was depleted by the earlier events of massive erosion and formation of the outflow channels. Viscous surface flows are mostly associated with Reull Vallis and probably reflect redistribution of volatiles related to the late episodes of evolution of this outflow channel. Citation: Ivanov, M. A., J. Korteniemi, V.-P. Kostama, M. Aittola, J. Raitala, M. Glamoclija, L. Marinangeli, and G. Neukum (2005), Major episodes of the hydrologic history in the region of Hesperia Planum, Mars, J. Geophys. Res., 110,, doi: /2005je Introduction [2] The elevated volcanic plateau of Hesperia Planum [Carr et al., 1977; Greeley and Spudis, 1981], which is about 1300 by 1700 km across and has the area about km 2, is situated in the northeastern portion of the broad rim of the Hellas impact basin. The heavily cratered Noachian terrains [e.g., Greeley and Guest, 1987] form the majority of the rim while the surface of Hesperia 1 Laboratory of Comparative Planetology, Vernadsky Institute of Geochemistry and Analytical Chemistry, RAS, Moscow, Russia. 2 Astronomy Division, Department of Physical Sciences, University of Oulu, Oulu, Finland. 3 International Research School of Planetary Sciences, Universita d Annunzio, Pescara, Italy. 4 Institut für Geologische Wissenschaften, Department of Earth Sciences, Freie Universität Berlin, Berlin, Germany. Copyright 2005 by the American Geophysical Union /05/2005JE Planum, which is covered by vast wrinkle ridged plains, is less cratered [Scott and Carr, 1978; Tanaka, 1986], smoother, and appears to be within a broad and shallow topographic depression. [3] The region was extensively studied since the Mariner 9 mission, in particular, through geological mapping [Potter, 1976; King, 1978]. On the basis of Viking data, series of regional (1:15,000,000 scale) [Greeley and Guest, 1987] and detailed local (1:500,000 to 1:1,000,000 scale) geological maps were compiled for Hesperia Planum and the surrounding uplands [Gregg et al., 1998; Price, 1998; Mest and Crown, 2002a, 2002b]. The mapping results and more topical studies of specific features [Malin, 1976; Pieri, 1976, 1980; Greeley and Spudis, 1981; Greeley and Crown, 1990; Crown and Greeley, 1993; Maxwell and Craddock, 1995; Gregg et al., 1998] have shown that Hesperia Planum and the surrounding uplands host an array of volcanic landforms such as volcanic plains and two low volcanic centers, Hadriaca and Tyrrhena Paterae [Greeley 1of28

2 and Spudis, 1981; Greeley and Crown, 1990; Crown and Greeley, 1993; Gregg et al., 1998], and fluvial structures such as valley networks and large outflow channels [Masursky et al., 1977; Carr and Clow, 1981; Mars Channel Working Group, 1983; Carr, 1995, 1996; Tanaka and Leonard, 1995; Scott et al., 1995; Carr and Chuang, 1997; Mest and Crown, 2001]. [4] The valley networks occur within the cratered uplands on both sides of Hesperia Planum and are thought to be Noachian in age [Malin, 1976; Pieri, 1976; Carr and Clow, 1981; Scott et al., 1995; Carr, 1996]. Vast wrinkle ridged plains covering the surface of Hesperia define the base of the Hesperian Period of the Martian stratigraphy [Tanaka, 1986; Tanaka et al., 1992]. The plains most likely have volcanic origin and have been interpreted to consist of relatively thin lava flows [Greeley and Spudis, 1981; Greeley and Guest, 1987]. Tyrrhena Patera shows evidence of a very long eruption history including late-stage effusive and early explosive episodes [Greeley and Crown, 1990; Crown et al., 1992; Crown and Greeley, 1993; Gregg et al., 1998]. Materials of the ridged plains embay shield members of Tyrrhena Patera and may have been erupted from local, presently buried fissures [Leonard and Tanaka, 2001]. Activity at the volcano of Tyrrhena Patera probably extended into the late Hesperian to Early Amazonian [Crown et al., 1992; Gregg et al., 1998]. Large outflow channels (Dao, Niger, Harmakhis, and Reull Valles), which are among the most spectacular fluvial features of Hesperia Planum, were apparently formed during the Hesperian [Malin, 1976; Masursky et al., 1977; Greeley and Guest, 1987] and possibly into the Amazonian [Scott et al., 1995]. Abundant debris aprons and flow-like features, the formation of which likely requires the presence of ground ice [Squyres, 1979; Squyres and Carr, 1986; Carr, 1996], represent the latest (Amazonian) structures related to release of volatiles (water) [Crown et al., 2002, 2003; Berman et al., 2003]. [5] Thus both volcanic and fluvial processes have punctuated the history of Hesperia Planum during a large time span from the Noachian to Amazonian Periods. The possible interactions of these processes likely represent the main theme of the evolution of Hesperia and possibly the history of deposition from this particular region to the Hellas basin as well. The main goal of out study is to suggest a coherent scenario of hydrologic history of Hesperia Planum based on existing and newly acquired data sets. In this paper we outline the most important features in the region of Hesperia Planum and correlate temporally the processes that led to their formation using all available images and topographic data such as the MOLA-gridded topography (both 64 and 128 px/deg resolution), Viking MDIMs, Mars Observer Camera (MOC), Mars Odyssey THEMIS, and Mars Express High Resolution Stereo Color camera (HRSC). 2. Topographic Configuration of Hesperia Planum [6] The distribution of the regional-scale topographic features along the rim of Hellas basin suggests that Hesperia Planum is within a broad and shallow depression. The majority of the flat surface of the depression appears to be at approximately same elevation except for three distinct regional-scale features (Figure 1a): (1) Tyrrhena Patera, located in the northwest-central portion of Hesperia Planum and is about 1.5 km higher than the surrounding terrain; (2) the provisionally named Morpheos basin, an elongated topographic low stretched in west-east direction in the southeastern portion of Hesperia Planum between about S and W; (3) a relatively narrow (200 km across), southwest-trending depression, informally named the Southwestern trough, in the southwestern corner of Hesperia Planum. [7] In order to assess the topographic configuration of Hesperia Planum relative to the background of the Hellas rim, we have constructed a circum-hellas topographic profile along a circle centered at 41 S, 68 E with a radius 2300 km. The profile crosses the central portion of Hesperia Planum in general direction from the south to northwest (Figure 1a) and consists of 144 points that are 2.5 apart. Each point corresponds to the average elevation in a 1 1 box; the data were collected from the MOLA gridded topography map with the resolution 1/64 degree. The profile shows that the elevations along the Hellas rim broadly follow a sine-like line that is lower within Promethei Terra east of the Hellas floor. The Noachian-age terrains of Noachis, Tyrrhena, and Promethei Terrae are characterized by significant topographic variations (up to 1.5 km), whereas the surface of Hesperia Planum is much smoother except for the topographic peak of Tyrrhena Patera (Figure 1b). As a whole, Hesperia Planum is on a broad regional slope between Tyrrhena and Promethei Terrae. [8] The segment of the profile corresponding to Hesperia Planum is shown in Figure 2. The mean elevation within Hesperia along the profile (excluding Tyrrhena Patera) is about 1100 m and the mean elevations within Promethei and Tyrrhena Terrae are about 1300 m and about 2000 m, respectively. Thus the area of Hesperia Planum represents a distinct topographic low bordered on both sides by elevated Noachian terrains. [9] The surface of Hesperia Planum, which is slightly tilted to the south at the mean regional slope about 0.03, consists of two parts separated by a break of slope that occur in the profile at about 32 S, 247 W. The first, which is essentially horizontal and hosts Tyrrhena Patera, characterizes the northwestern portion of Hesperia Planum. The Morpheos trough, which is about m deeper than both the rest of Hesperia to the northwest and the cratered uplands of Terra Promethei to the south (Figure 2), characterizes the second part. [10] Although the circum-hellas profile displays the general topographic characteristics of the region of Hesperia Planum, it crosses boundary of Hesperia only at two points. In order to document the topographic variations of the contact that separates Hesperia from the surrounding uplands in more systematic way, we have collected the topographic data around the entire boundary of the Planum, which is shown in the geological map of the eastern equatorial region of Mars [Greeley and Guest, 1987] (Figure 3a). The data were obtained from the MOLA gridded topography map (resolution 1/64 degrees) in pairs of points near the boundary (one point is in Hesperia and another is in the uplands). The size of each point was about 1 1 and we were careful not to include in the area of measurements distinct local topographic features such as 2of28

3 Figure 1a. Regional distribution of major topographic features in the area of Hesperia Planum and its surroundings. Hesperia is within a regional-scale topographic low and is surrounded by elevated cratered uplands. Important topographic features within Hesperia Planum discussed in text such as the Southwestern trough and Morpheos basin are marked. Dots indicate the trace of the circum-hellas topographic profile at 2300 km radius. MOLA topographic map, resolution is 64 px/deg, Mercator projection. large impact craters in the uplands or wrinkle ridges within Hesperia (Figure 3b, Table 1). [11] The data show that almost everywhere the surface of Hesperia Planum is clearly lower than the surrounding uplands. The edge of Terra Tyrrhena to the east of Hesperia is only slightly higher than the surface of the plains (200 m, Table 1) and there is typically no scarp separating these two topographic provinces. For the major part of the boundary along the southern and western edges of Hesperia, the differences in elevation between the surface of the plains and the uplands are greater (up to about 450 m) and reach about 800 m along Morpheos Rupes (Table 1, Figures 3a and 3b). In many places, a distinct scarp separates Hesperia Planum from Tyrrhena and Promethei Terrae. [12] The southwestern corner of Hesperia Planum (the Southwestern trough) has very specific topographic charac- 3of28

4 Figure 1b. The circum-hellas topographic profile at 2300 km radius (thick zigzag-like line) shows the large-scale topographic variations along the broad rim of Hellas basin (thin smooth sinusoidal line). On the background of the rim, the area of Hesperia Planum is a prominent depression with the smooth surface. Topographic data were collected from the MOLA topographic map with the resolution 64 px/deg. Each point of the profile is the average elevation within a 1 1 degree box. teristics. Within this area, the surface of the plains lies significantly lower than the rest of Hesperia, while the elevations within the uplands vary about the same mean level that characterizes the uplands around Hesperia (Figures 3a and 3b). Due to this, the difference in elevation between the Hesperia surface and the bordering uplands reaches its maximum within the trough (the mean, which is about 1700 m, is shown in Table 1, while the maximum, which is about 3000 m, is shown in Figures 3a and 3b). Topographically, the southwestern part of Hesperia represents a relatively narrow (about 200 km wide) trough-like feature that breaches the cratered uplands and runs downward toward the Hellas basin. The large outflow channels, Dao, Niger, and Harmakhis Valles, cut the surface of the plains within the trough. 3. Impact Craters and Volcanic Plains Within Hesperia Planum [13] Hesperia Planum is covered mostly by Hesperian ridged plains (unit Hr) [Greeley and Spudis, 1981; Scott and Tanaka, 1986; Tanaka, 1986; Greeley and Guest, 1987] and is the type locality for this regionally important timestratigraphic unit of Mars [Tanaka, 1986; Tanaka et al., 1992]. In the Thaumasia region, however, wrinkle ridged plains began to form during the Late Noachian [Dohm et al., 2001]. [14] The characteristic features of the surface of Hesperia Planum are numerous wrinkle ridges that form polygonal networks throughout Hesperia Planum. Typically, the ridges are linear structures but sometimes they form unusual circular patterns (Figure 4), which are thought to be formed by the deformation of lava layers over the rims of flooded craters [Scott and Carr, 1978; Chicarro et al., 1985; Watters and Chadwick, 1989]. Such an interpretation is supported by the observation of crater rim crests near the edge of Hesperia Planum (Figure 5). The flooded craters are important features because they can be used to estimate the thickness of the lavas filling Hesperia Planum (Figure 6), assuming that they formed on the floor of the basin of Hesperia before lava emplacement. Buried impact craters were also used to estimate the thickness of plains materials in Hesperia Planum [Goudy and Gregg, 2002]; in their work, an isopach map of the plains was presented and the total thickness of Hesperia deposits estimated to be less than 2 kilometers. The MOLA data allow precise measurements 4of28

5 Figure 2. The section of the circum-hellas topographic profile at 2300 km radius for Hesperia Planum and its vicinity. The surface of Hesperia in its NW portion is approximately at the same elevation except for the topographic peak of Tyrrhena Patera. Area of the Morpheos basin in the SE portion of Hesperia is a distinct depression that is about 800 m lower than the major portion of Hesperia Planum. Topographic data were collected from the MOLA topographic map with the resolution 64 px/deg. Each point of the profile is the average elevation within a 1 1 degree box. of the topographic configuration of impact craters on Mars that are resulted in a robust morphometric correlation among the crater diameter, depth, and rim height [Garvin et al., 2000]. We have conducted a regional survey of the flooded craters within Hesperia Planum and found 43 such features (Table 2, Figures 5 and 7) ranging in diameter from 6.5 to 63 km and predominantly occurring in the central and southern parts of Hesperia Planum. The areal distribution of the craters probably reflects both the initial distribution of the craters and variation in the thickness of the lava fill. The mean rim height of the flooded craters is estimated from the crater diameters to be about 325 ± 73 m (1 s standard deviation), the median height is about 320 m, and the maximum height is about 495 m (Table 3, Figure 8). On the basis of these values and the area of Hesperia Planum (about km 2 ), the volume of the plains is estimated to be about 0.4 to about km 3 (Table 3). The larger value derived from the maximum diameter of the flooded craters probably overestimates the volume or, at least, represents the upper limit of the volume. [15] The flooded craters may also reflect the morphology of the floor of Hesperia Planum before emplacement of the vast plains unit [Goudy and Gregg, 2002]. For example, the floor may have been as heavily cratered as the surrounding upland terrains (e.g., has similar size-frequency distribution of impact craters). On the other hand, the basement of Hesperia may have been resurfaced and some portion of impact craters there were destroyed prior to the emplacement of the lava plains. In this case, the size-frequency distribution of craters may be similar to the distribution of craters on Hesperian units elsewhere on Mars. In order to test these possibilities, we compared the size-frequency distribution of the flooded craters in Hesperia with the distribution of craters in a typical Noachian terrain (part of Terra Tyrrhena to the west of Hesperia Planum) and in classical Hesperian-age volcanic provinces, Syrtis Major (unit Hs, the Syrtis Major formation) and Lunae Planum (unit Hr, the Ridged Plains material). The areas of the crater counting varied from km 2 in Terra Tyrrhena to km 2 in the Hesperian-aged regions (Figures 9a and 9b). To count craters in these regions, we used the catalogue of Martian impact craters compiled by N. Barlow (2000, 2003) (available online at ftp://ftpflag.wr.usgs.gov/ dist/pigpen/mars/crater_consortium and usgs.gov/mars.htm). In Terra Tyrrhena, craters were counted in the area between 0 30 S and W (total area is 5of28

6 Figure 3a. Pairs of points where measurements of elevation of the surface of lava plains in Hesperia Planum and of the surface of the cratered uplands near the contact with lava plains were made. Numbers indicate each fifth pair in the clockwise direction. The size of each point was about 1 1 ; topographic features such as large impact craters and wrinkle ridges were excluded from measurements. MOLA topographic map, resolution is 64 px/deg, Mercator projection km 2 ). This territory is completely within the ancient Noachian terrains. In Syrtis major, the area of crater counting consists of three subareas (total area is km 2 ): (1) 5 15 N, W, (2) 0 15 N, , and (3) N, W. In Lunae Planum, the area of crater counting consists of two subareas (total area is km 2 ): (1) 5 15 N, W and (2) 0 15 N, N. In both Syrtis Major and Lunae Planum, the subareas were chosen to include territories of ridged plains and to avoid areas of the cratered uplands. Results of the crater count are summarized in Table 4. [16] As it was expected, the Terra Tyrrhena plot shows the highest crater density and the curves for the Hesperian-aged regions are similar to one another and lie significantly lower 6of28

7 Figure 3b. Distribution of topography along the boundary of Hesperia Planum in lava plains (open circles) and in the cratered uplands (solid circles). The surface of the lava plains is systematically lower and the largest and consistent differences occur along Morpheos Rupes (northern border of Morpheos basin) and within the Southwestern trough. Topographic data were collected from the MOLA topographic map with the resolution 64 px/deg. Each point of the profile is the average elevation within a 1 1 degree box. (Figures 9a and 9b). The size-frequency distribution of the flooded craters of Hesperia Planum is significantly different from that in Terra Tyrrhena and closely mimics the distribution in both Syrtis Major and Lunae Planum (Figure 9a), corresponding to Hesperian-aged distributions. The curve for the exposed craters in Hesperia Planum (Figure 9b), although showing slightly higher density of smaller craters (<30 km) compared to these for Syrtis Major and Lunae Planum, is significantly lower than the curve for Terra Tyrrhena. We also compared the combined population of the flooded and exposed craters in Hesperia Planum with that of the Noachian cratered uplands and the Hesperianaged lava plains. The curve for the combined population is slightly shifted toward the higher crater density but is significantly different (within the one s limits) from the distribution of craters within the Hesperian terrains. [17] The crater statistics of the flooded craters within Hesperia Planum strongly suggest that the ancient (Noachian-type) population of impact craters in this area was largely erased before emplacement of the plains fill. The formation of the regional-scale depression that hosts Hesperia Planum may have existed either during or following the Noachian Period (Figures 10a and 10b). If, for example, the depression of Hesperia Planum existed as a natural large-scale topographic variations along the rim of the Hellas basin during Noachian (Figure 10a), then the minimum thickness of material that must have been Table 1. Mean Elevation Within Uplands and Hesperia Planum Along the Hesperia Boundary a Uplands, Mean Elevation, m Hesperia, Mean Elevation, m Mean Difference, m Terra Tyrrhena, East of Hesperia 1552 ± ± Morpheos Rupes, Southwest of Hesperia 1591 ± ± Promethei Terra, South of Hesperia 1081 ± ± Hesperia SW Trough 495 ± ± Tyrrhena Terra, West of Hesperia 1647 ± ± a Variations in elevation are given as one-sigma. Total mean difference in elevation weighted by boundary length is 500 m. 7of28

8 Figure 4. The surface of Hesperian lava plains is deformed by networks of linear wrinkle ridges. In places, the wrinkle ridges form an unusual, circular pattern suggesting that the ridges were formed above the rims of flooded impact craters. Fragment of Viking image 365s67, resolution is 222 m/px, the center of the image is at about 23.2 S, W. removed from this area to erase the old craters can be estimated by the rim height of the larger impact craters that characterize the surface of the Noachian terrains around Hesperia. We assume that the similar size-frequency distribution of craters, which characterize the Noachian terrains exposed in Terra Tyrrhena, existed in Hesperia Planum. The largest crater used for determining crater statistics in Terra Tyrrhena is about 170 km. The rim height for such a crater is estimated to be about 300 m [Garvin et al., 2000]. Importantly, the rim height of the larger craters on Mars (>100 km) is less dependent on the crater diameter and a rim height of 300 m closely approximates the rim height of craters ranging in diameter from 100 to 1000 km [Garvin et al., 2000]. Consequently, the volume estimated for a 300-m-thick layer of Noachian materials removed from the floor of Hesperia Planum is about km 3.Onthe other hand, if the depression of Hesperia Planum formed after Noachian, the volume of the removed material would be significantly larger (Figure 10b). In this scenario, the total depth of the pre-plains depression can be estimated as a sum of the mean topographic difference between the surface of Hesperia and surrounding uplands (500 m, Table 1) and the thickness of the infilling plains-forming materials within the Planum (from 250 to 500 m, Table 3). These numbers give the range of the depression depth from 750 to 1000 m, and thus the maximum total volume of material missed within the Hesperia area is estimated to be km Fluvial Features in and Near Hesperia Planum [18] There are two classes of fluvial features in the region of Hesperia Planum and its surroundings: (1) small valley networks [Carr and Chuang, 1997] and (2) large outflow channels [Baker et al., 1992]. Each class has distinct stratigraphic positions and apparently reflects specific episodes of the hydrologic history and the release of volatiles Small Valley Networks [19] Small valley networks are abundant within the Noachian terrains on both sides of Hesperia Planum and in the area adjacent to Morpheos Rupes (Figure 11) [Craddock and Maxwell, 1993; Maxwell and Craddock, 8of28

9 Figure 5. Examples of flooded craters in Hesperia Planum that were included in the crater counting. (a) Fragment of Viking image 365s40, resolution is 234 m/px, the center of the image is at about 27.3 S, W, crater diameter is about 24.3 km. (b) Fragment of Viking image 92a27, resolution is 233 m/px, the center of the image is at about 23.8 S, W, crater diameter is about 21.7 km. (c) Fragment of Viking image 365s67, resolution is 222 m/px, the center of the image is at about 23.5 S, W, crater diameter is about 21.6 km. (d) Fragment of Viking image 365s52, resolution is 229 m/px, the center of the image is at about 38.4 S, W, crater diameter is about 45.5 km. Scale bar is about 15 km in each image. Arrows indicate direction to the north. 1995; Scott et al., 1995; Carr, 1995; Cabrol and Grin, 2001; Mest and Crown, 2001]. In the units making up the surface of the Noachian terrains to the west and east of Hesperia planum, Npl 1 (cratered unit) and Npld (dissected unit) [Greeley and Guest, 1987], the density of the valley networks reaches the maximum on Mars. For example, the unit Npld (Terra Tyrrhena west of Hesperia) has the largest density of the networks, about km 1 [Carr and Chuang, 1997]. In the recent studies (based on new topographic data) the density of the drainage systems within the cratered uplands surrounding Hesperia Planum were found to be even higher, reaching the average km 1 [Mest et al., 2002; Mest and Crown, 2004]. The local to regional topographic gradient governs the orientation of the small valleys within the cratered uplands and in the vicinity of Hesperia Planum (Figure 12). In areas where the small valleys are near the contact of the uplands with Hesperia Planum, they follow the regional topographic slope toward Hesperia (Figures 11 and 12). [20] The small valleys dissect the surface of the uplands (Figure 12) and thus postdate the heavily cratered Noachian terrains. At the contact with the Hesperia plains, the valleys are usually abruptly terminated. The best available images (17 m/px resolution) show no accumulation of materials that may have been removed from the uplands and deposited on the surface of the plains (Figure 13). The valleys also do not cut the surface of the plains within Hesperia (Figure 13). This indicates that formation of the valleys and associated deposition occurred prior to the emplacement of vast plains in Hesperia. Thus the material unit Hr represents the upper stratigraphic limit for the formation of the valleys in the vicinity of Hesperia Planum and the volatile-rich effluents of the valleys were stored on the original floor of the large topographic basin of Hesperia Outflow Channels [21] The region between Hesperia Planum and Hellas basin is one of the four main areas where the large outflow 9of28

10 Figure 6. Diagram illustrating relationships of the crater dimensions [Garvin et al., 2000] and thickness of the lava fill in Hesperia Planum. channels occur [Crown and Mest, 1997, 2001; Scott et al., 1995; Carr, 1996; Crown et al., 2004, 2005; Bleamaster and Crown, 2004]. There are five outflow channels in this region. Three of them, Dao, Niger, and Harmakhis Valles cut plains materials [e.g., Price, 1998] within the Southwestern trough that connects Hesperia Planum with Hellas basin. The fourth channel, Reull Vallis, as it is shown in the geologic map of the eastern equatorial region of Mars [Greeley and Guest, 1987] begins at western edge of the Morpheos basin and runs from east to west across the northern portion of Promethei Terra. The fifth channel begins within the southeastern portion of Hesperia Planum about 32 S, W, runs southward, and disappears at the northern edge of Morpheos basin at about 35 S, 246 W (see Figures 1a and 3a). In the work by Mest and Crown [2001] this channel is considered as the upper segment of Reull Vallis. [22] Dao, Niger, and Harmakhis Valles begin in distinct closed depressions sharply outlined by high steep walls (Figure 14). The edges of the depressions are scalloped suggesting collapse of the walls. The channels of Dao and Harmakhis Valles breach the depressions and run as steepsided canyons toward the floor of the Hellas basin where they disappear, leaving little evidence for deposits at their apparent mouths [Crown et al., 1992]. In contrast to Dao and Harmakhis Valles, a set of broad and shallow troughs connecting relatively small circular and elongated depressions is visible at the uppermost reaches of Niger Vallis. Dao and Niger Valles merge at about 37 S, 270 W and continue as a single channel toward the Hellas basin. [23] The high-resolution HRSC image (orbit 38, 12.5 m/ px) shows details of the morphology of the upper portion of Niger Vallis (Figure 15). The uppermost reach of Niger forms a broad, shallow trough outlined by gently sloping walls (A in Figure 15). The breaks of slopes at the edges of the trough are marked by curvilinear graben arranged en echelon (B in Figure 15). Interconnected wrinkle ridges (C in Figure 15) deform the morphologically smooth surface within and outside the trough. These morphological features collectively suggest that subsidence of the original surface of the ridged plains (unit Hr) took place within the upper portion of Niger, probably due to removal of material by subsurface flow [Crown and Mest, 1997, 2001]. [24] In contrast to all other channels (compare to Figures 14 and 16), the source area of Reull Vallis (segment 2 of Reull Vallis [Mest and Crown, 2001]) is indistinct (Figure 17). The channel starts as a full-sized topographic and morphologic feature at the western edge of the topographic low of the Morpheos basin (at about 37.5 S, 247 W). At its very beginning, Reull Vallis appears to breach a topographic barrier of the ridged plains near the southern edge of Hesperia Planum (Figure 17). From this point [Mest and Crown, 2001], the channel runs as a steep-sided canyon through the cratered uplands of the northern edge of Promethei Terra in general western direction and disappears at about 40 S, 264 W near the source area of Harmakhis Vallis. A debris apron covers the lower reach of Reull Vallis where it may have been connected with Harmakhis Vallis [Crown et al., 1992]. [25] Because all outflow channels cut into the lava plains of Hesperia Planum, they postdate the emplacement of the plains. Thus outflow channel formation records large episodes of groundwater discharge following the formation of the lava plains in Hesperia. Characteristics of the source of the outflow channels in the Hesperia Planum region suggests that Dao, Niger, Harmakhis Valles, and the channel in the southern portion of Hesperia are due to release of volatiles from the subsurface sources, which are covered by a composite layer of Hesperian ridged plains. In contrast, Reull Vallis may have formed by surface run-off that 10 of 28

11 Table 2. Flooded Craters in Hesperia Planum Number Mars Chart Viking Image Image Resolution, km/px Latitude Western Lon. Eastern Lon. Crater Diameter, km Rim Height, m 1 MC-22_NW 378x MC-22_NW 378x MC-22_SW 365s MC-22_SW 365s MC-22_SW 097a MC-22_SW 365s MC-22_SW 365s MC-22_SW 365s MC-22_SW 365s MC-22_SW 365s MC-22_SW 365s MC-22_SW 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-22_SE 365s MC-28_NE 553a MC-28_NE 553a MC-28_NE 365s MC-28_NE 365s MC-29_NW 365s MC-29_NW 518a MC-29_NW 518a MC-29_NW 365s MC-29_NW 365s MC-29_NW 518a MC-29_NW 365s possibly was localized within the topographic depression of Morpheos basin. 5. Viscous Flows [26] Viscous flow-like materials were interpreted as icerelated features [Squyres, 1979; Squyres et al., 1987], which could represent the presence of water in Hesperia Planum. As such, these flows would provide insight into both climate and volcanically induced local to regional temperature variations. The most spectacular flow-like features are lobate aprons at the base of some isolated upland massifs [Squyres, 1979; Crown et al., 1992; Squyres and Carr, 1986; Berman et al., 2003; Crown et al., 2002, 2003; Pierce and Crown, 2003] in the northern part of Terra Promethei (Figure 18). Most aprons are characterized by lobate fronts and convex upward surfaces (Figure 18). At Viking resolution, the surface of the aprons appears homogenous and morphologically smooth, but at higher resolution (MOC, HRSC), individual lobes within the aprons suggest multiple events [Pierce and Crown, 2003] (Figure 19). The lobe surfaces display sets of nested narrow (a few hundred meters wide) ridges that are curved in the apparent direction of the lobe movement (Figure 19). Generally, the surface of the lobes displays narrow V-shaped ridges previously described as rock glaciers mantled by dust or other debris [Baker, 2003], which requires the existence of nearly pure near-surface ice [Baker, 2003]. [27] Less conspicuous flow features were recently discovered by inspection of a large number of MOC images [Mustard et al., 2001; Milliken et al., 2003]. An example of such a flow, which was earlier included in the debris aprons database by Pierce and Crown [2003], is displayed in the high-resolution HRSC image (orbit 248, 12.5 m/px, Figure 20a) that shows a relatively thick deposit spreading across the floors of two neighboring craters through breached rims. A distinct set of narrow (tens to a few hundred meters wide) ridges indicates flow direction from the topographically higher smaller crater to the larger crater where the lobate flow front displays a set of fine-scale curved ridges, similar to the typical lobate debris aprons elsewhere. [28] The ridges parallel the flow front and form a complex pattern of nested structures resembling a moire-like 11 of 28

12 Figure 7. Areal distribution of flooded impact craters within Hesperia Planum. Map is in sinusoidal projection, which is extended for 30 from the center point (25 S, W). pattern in the central portion of the deposit. Within the upper crater, the deposit is characterized by two systems of ridges. The ridges in the central portion are sub-parallel to each other and oriented toward the breach. The spacing of the ridges is narrowing toward the breach. In the northern portion of the crater, the ridges are oriented at high angles to and appear to be cut by the ridges in the center of the crater (Figure 20a). MOC image M (5.58 m/px) shows the details of the deposit within the upper crater (Figure 20b). The central part of the deposit within the crater is characterized by narrow arcuate ridges that are convex to the northwest toward the breach. The arcuate ridges are bound by systems of larger and straighter ridges that are gently curved to the central area. The parallel ridges in the northern part of the crater bend near the juncture of the central and outer parts of the flow and eventually become parallel to the ridges in the central portion of the flow. [29] The morphology of the deposit is consistent with the explanation that it was formed by viscous flow from the Table 3. Estimates of the Volume of Plains Fill in Hesperia Planum Parameter Thickness of Plains, a m Volume of Plains, b 10 6 km 3 Mean thickness Mean thickness 1s Mean thickness + 1s Median thickness Maximum thickness a Thickness of the plains is adopted to be equal to the rim height. b The area of Hesperia Planum is about km 2. Figure 8. The size-frequency distribution of the rim heights of flooded craters within Hesperia Planum. Total number of craters is 43; mean height is about 325 m. upper into the lower crater. There are two possible alternatives for the formation of the deposit: (1) a lava flow and (2) a glacial/fluidized flow. The absence of volcanic centers near the deposit source appears to rule out a volcanic origin. On the other hand, the presence of lobate aprons, which likely were formed by flow of ice-saturated material [Squyres, 1979] in the region near the deposit, favors a glacial origin. [30] The important characteristic of the aprons and flowlike deposits is that they most likely formed due to icefacilitated movement (flow/creep) and/or partial melt of interstitial ice. The map of the areal distribution of these features (Figure 21) based on Viking and available HRSC images shows that they are concentrated in the northern portion of Promethei Terra on both sides of Reull Vallis (see also the map of debris aprons in the paper by Pierce and Crown [2003]). The aprons and flows are absent within the main portion of Hesperia Planum, within the Southwestern trough, and in the cratered uplands to the south of about 47 S. On the right side of Reull Vallis, many aprons and flows are in close spatial association with the outflow channel. A large debris apron covers the lowermost part of Reull Vallis [Crown et al., 1992; Tanaka and Leonard, 1995] and along the lower stretches of the channel there are numerous examples of the flow deposits that spread on the channel banks, enter the channel and partly fill it. Flow material with an extensively corrugated surface enters the source depression of Harmakhis Vallis from the north and fills a significant portion of it (Figure 22a). This deposit begins as a landslide-like structure from the scarp on the southern part of plains that were interpreted as ice-rich material modified by deflation [Leonard and Tanaka, 2001; Price, 1998; Crown et al., 1992; Bleamaster and Crown, 2004]. The flow likely consists of sediments and interstitial ice and probably formed due to partial melting of ice, which produced sediments-laden flows that were subsequently 12 of 28

13 Figure 9a. The size-frequency distribution of flooded craters in Hesperia Planum in comparison with the distributions of exposed craters in typical Noachian (Terra Tyrrhena) and Hesperian (Syrtis Major and Lunae Planum) terrains. The distribution of the flooded craters in Hesperia Planum almost exactly coincides with the Hesperian curves and lies well below the curve for the craters from Terra Tyrrhena. refrozen. In places, the flows continued for many tens of kilometers, fill topographic lows such as impact craters, and finally enter the channel of Reull Vallis where they are spread on its floor (Figure 22b). [31] Viscous flow materials also occur in the Dao-Niger and in Harmakhis Valles regions. These regions, however, display several important differences in the appearance and distribution of flows from the region in the northern portion of Promethei area around Reull Vallis. First, the debris aprons are less abundant within the Southwestern trough where the large outflow channels are concentrated. The aprons are completely absent in the upper portion of the trough north of about 37 S and only a few aprons are seen near the head area of Harmakhis Vallis in close spatial association with shallow fluvial channels. Second, all viscous flows in the Southwestern trough are spatially associated with the large outflow channels and occur both on the walls and floors of the channels. The third and probably the most important characteristic of the flows in this area is that they appear to originate from beneath the ridged plains at a relatively shallow depth. [32] The largest occurrences of these flow types occur within Niger Vallis at its upper and middle stretches (Figure 23). The flow seen in a high-resolution HRSC image (orbit 528, resolution 25 m/px) partly fills the canyon of Niger Vallis and clearly blankets the canyon floor (A in Figure 23). The flow is characterized by relatively narrow (several hundreds of meters wide) ridges that are parallel to each other and the canyon walls (B in Figure 23). These ridges strongly resemble those on the debris aprons and the deposits near Reull Vallis (Figures 19 and 20), and the medial moraines of terrestrial glaciers [e.g., Carr, 1996]. There are several characteristic structures within the Hesperian lava plateau in the vicinity of Niger Vallis that suggest the subsidence and collapse of the surface may be due to removal of supporting material: (1) pits and chains of pits that coalesce and form elongated trough that opens into the canyon (1 in Figure 23), indicating local collapse, (2) chaotic terrain, which comprise rectangular blocks of the plains-forming materials (2 in Figure 23), suggesting plateau breakup, and (3) terraced topography within the main lava plateau near the canyon where individual terraces are separated by relatively low straight or arcuate scarps (3 in Figure 23). The flows that fill the canyon appear to begin at the lower side of the terraces and there is the evidence for the plateau breakup where the flows begin (4 in Figure 23). Such a lowering of the surface suggests broad-scale subsidence resulting from the removal of material. [33] The viscous flows, which originate from the subsurface, are characteristic features of the Dao, Niger, and Harmakhis Valles systems (Figure 21). Within these outflow channels, there is the evidence for both the occurrence of the subsurface flows during the episode of the channel formation (Niger and Harmakhis Valles, Figure 15) and 13 of 28

14 Figure 9b. The same diagram as in the previous figure but for the combined population of the flooded and exposed craters in Hesperia Planum. The addition of the flooded craters to the population of the exposed ones only slightly shifts the curve up, but it is still significantly lower than the Noachian curve. subsequent to the channel formation, as some flows are superposed on the original floor of the channels (Figure 23). In contrast to the outflow channels within the Southwestern trough, the area of Reull Vallis displays little (if any) evidence for flows from the subsurface. All flow-like deposits, which occur near Reull Vallis, appear to have subaerial or near-surface sources (Figure 22). 6. Discussion [34] A rich array of volcanic and fluvial landforms characterizes Hesperia Planum and the surrounding uplands. The interaction of fluvial and volcanic processes is thus the main theme of the geologic history of this region. The history is highly influenced by both the large topographic basin of Hesperia and the giant impact basin of Hellas. Following the regional topography, the surface of Hesperia Planum slopes generally to the south toward Hellas Planitia. On the average, the major portion of Hesperia is about 500 m lower than the surface of adjacent cratered uplands. When compared to the two larger basins, there are two smaller, yet significant, topographic features in Hesperia Planum that played an important role of the hydrologic history of the region. [35] The first feature is the Southwestern trough that connects the main portion of Hesperia Planum with the basin of Hellas Planitia. The trough is a relatively narrow (about 200 km across) feature the surface of which is up to 3 km lower than the surface of the uplands and may represent a primary conduit through which fluidized materials of both Table 4. Results of Crater Counting Within the Noachian and Hesperian Terrains Number of Craters per 10 6 km 2 Region Area, 10 6 km 2 Total Number of Craters N(16) N(5) Terra Tyrrhena Syrtis Major Lunae Planum Hesperia Planum (exposed craters) Hesperia Planum (flooded craters) N/A Hesperia Planum (exposed+flooded) a a The number of craters in the last row is not equal to the sum of the two previous ones because some of the flooded craters were included in the catalogue of the Martian impact craters by N. Barlow. 14 of 28

15 Figure 10. Diagram illustrating two end-member scenarios of possible formation of the broad topographic depression of Hesperia Planum. The topographic profile shows an idealized distribution of topography before the proposed episode of massive erosion in Hesperia. (a) The area of Hesperia Planum had the same average elevation as the Noachian territories on both sides of it. In this case, a larger amount of material should be removed to produce the depression in the area of Hesperia Planum and erase existed craters. (b) A large-scale depression in the area of Hesperia Planum existed before the episode of erosion. In this case, a relatively small amount of material should be eroded to erase the ancient crater record. volcanic and fluvial origin may have flowed from the Hesperia into the vast lowland of Hellas Planitia. The outflow channels of Dao, Niger, and Harmakhis Valles represent the most prominent episodes of fluvial activity. [36] The second area corresponds to the topographic low of the Morpheos basin in the southeastern portion of Hesperia Planum. On the basis of morphologic and topographic evidence, a prominent outflow channel, which begins well within Hesperia, disappears near the northern edge of the Morpheos basin. Some authors [Crown and Mest, 1997; Mest and Crown, 2001] consider this channel as the uppermost segment of Reull Vallis and propose that Reull Vallis was at one time continuous, as there are no other obvious source regions in the area of segment 2 [Mest and Crown, 2001]; the segment 2 corresponds to the upper part of Reull Vallis as it is shown in the geologic map of the eastern equatorial region of Mars [Greeley and Guest, 1987]. The source region of Reull Vallis is indeed indistinct, as it initiates as a full-size feature near the westernmost tip of the Morpheos basin. Thus a plausible hypothesis is that the Morpheos basin may have served as a transient storage area for the effluents from the northern channel and later the discharge of the basin led to the formation of Reull Vallis [Kostama et al., 2004]. [37] Hydrologic activity in the area of Hesperia Planum and its surroundings is recorded during the Noachian Period through the Amazonian Period [Masursky et al., 1977; Mars Channel Working Group, 1983; Carr, 1995]. The hydrologic history can be divided into three episodes, each of which is characterized by its own specific structures. One of the first recognizable episodes in the Hesperia Planum area is the formation of the small valley networks that dissect the surrounding uplands [Pieri, 1980; Baker and Partridge, 1986; Carr, 1995; Mest and Crown, 2001]. Many valley networks are oriented toward Hesperia Planum and terminated at its contact with the uplands (Figure 11). Detail inspection of the valley terminus areas shows that the lava plains of Hesperia Planum embay and bury the valleys (Figure 12). Thus formation of the lava plains establishes the upper stratigraphic limit for the period of the valley 15 of 28

16 Figure 11. Small valley networks flowing into Hesperia Planum occur almost everywhere along the contact of the lava plains with the cratered uplands. (a) Southwestern side of Hesperia. Fragment of Viking image , resolution is 241 m/px, the center of the image is at about 17.9 S, W. (b) Northwestern side of Hesperia. Fragment of Viking image , resolution is 236 m/px, the center of the image is at about 12.0 S, W. (c) Eastern side of Hesperia. Fragment of Viking image 629a03, resolution is 247 m/px, the center of the image is at about 14.7 S, W. (d) Southeastern side of Hesperia at the eastern edge of Morpheos basin. Fragment of Viking image 365s51, resolution is 232 m/px, the center of the image is at about 36.5 S, W. Dashed lines indicate approximate contact of the uplands with lava plains in Hesperia Planum (HP). Scale bar is about 25 km in each image. Arrows indicate direction to the north. networks development within the uplands near Hesperia Planum [Scott et al., 1995; Carr, 1995]. The relationships of embayment also suggest that the valleys may have continued into Hesperia Planum stored their effluents on the floor of the Planum. [38] Although the sources and modes of origin of the valley networks are disputable [e.g., Scott et al., 1995; Carr, 1996], there is a consensus that these features formed by running water due to surface runoff [Craddock and Howard, 2002; Hynek and Phillips, 2001; Mangold et al., 2004], groundwater sapping [Pieri, 1980; Baker, 1990], or by basal malting below thick snow/ice deposits [Carr and Head, 2003]. The first mode of formation requires an atmospheric water cycle and, as a consequence, surface conditions significantly different from those observed today [Baker, 1990]. This is poorly consistent with the apparent absence of weathering products on the surface of Mars [Christensen et al., 2001]. Groundwater sapping requires either significant accumulations of ground ice in the subsurface to maintain sapping for significant time or some sort of recharge of the source areas, or both. Because many valley networks begin in areas of small subsurface volume such as rims of impact craters or isolated massifs of the uplands, large accumulations of ground ice/water are less likely in these areas. The effective recharge of the sources requires the water cycle through the atmosphere and meets the difficulties of the precipitation/runoff hypothesis. The third mode of origin due to basal melting effectively avoids the difficulties of the other two hypotheses and, although it is model dependent, offers a plausible explanation for the mode of origin of the small valley networks. [39] This hypothesis requires a pack of snow and ice as thick as several hundred meters up to a few kilometers for the initiation of basal melting [Carr and Head, 2003]. For 16 of 28

17 Figure 12. The surface of Terra Tyrrhena to the west of Hesperia Planum is dissected by numerous small valley networks. The valleys appear to follow the regional-scale slopes and are terminated either at the edges of local depressions (black and white arrows, center and right side of the image) or at the contact of the uplands with lava plains in Hesperia Planum (HP, black arrows at the bottom of the image). Fragment of Viking image 625a31, resolution is 241 m/px, the center of the image is at about 25.0 S, W. the small valley networks to form in this scenario, a large amount of ice is expected to have accumulated in Hesperia Planum and the surrounding uplands (where the valley networks density is the highest [Carr and Chuang, 1997; Mest et al., 2002; Mest and Crown, 2004]) (Figure 24a). Regardless of the specific mode of origin of the small valleys, their formation was the first recognizable and, apparently, the only episode of inflow and accumulation of volatiles on the original floor of Hesperia Planum. The later episodes of hydrologic activity are related only to outflows of fluidized materials from the area of Hesperia. [40] One of the major stages in the geologic history of Hesperia Planum was the emplacement of vast volcanic plains (material unit Hr) [Greeley and Guest, 1987; Scott and Tanaka, 1986; Tanaka, 1986; Tanaka et al., 1992] that have covered about km 2 of the broad topographic depression of Hesperia. Within the plains, there is the evidence for impact craters there flooded by lavas. If one assumes that these craters formed on the original floor of Hesperia prior emplacement of the plains-forming lavas then the flooded crates: (1) provide the mean to estimate the thickness and the volume of the lava fill and (2) characterize the size-frequency distribution of craters on the surface before lava emplacement. [41] On the basis of the diameters of the flooded craters we estimate that the thickness of the lava fill within Hesperia Planum varies in the ranges from about 250 to 500 m, with volumes of about 0.4 to about km 3 (Table 3). The size-frequency distribution of the flooded craters displays a distinct roll-off of the curve for the smaller craters (Figure 9a), which is interpreted be due to progressive burial by plains-forming lavas. The rim height for the fresh 20-km craters on Mars is about 300 m [Garvin et al., 2000]. This value probably corresponds to the minimum thickness of lavas that can effectively bury the smaller craters. For the larger craters, the size-frequency distribution of which appears to be undisturbed, the thickness of the lava plains should be comparable with the rim heights of these craters. Thus the value about m probably corresponds to the lover thickness limit, whereas the rim height of the largest flooded craters, which can be detected on the surface (about 500 m), may represent the maximum thickness. [42] The estimated dimensions (lateral extent, thickness, and volume) of Hesperia Planum are comparable to those of large igneous provinces (LIP) on Earth (Table 5). An important characteristic of the terrestrial LIPs is their relatively short duration of formation, several millions of years [Coffin and Eldholm, 1994], related to the active phase of melting in the diapir head [e.g., Condie, 2001]. [43] The size frequency distribution of the flooded craters in Hesperia coincides with the distribution of craters on the surface of the major occurrences of the Hesperian ridged plains within the Hesperia Planum itself, Syrtis Major, and Lunae Planum (Figure 9a). This suggests that the surface in Hesperia before emplacement of lava plains is best characterized by the size-frequency distribution of impact craters of these Hesperian surfaces. Thus the crater retention ages of both the actual surface of Hesperia Planum and the surface below the lava plains are indistinguishable, which is consistent with the possibly short time interval when the topographic depression of Hesperia was partly filled by lavas. The apparent Hesperian age of the original floor of Hesperia Planum strongly suggests that this area underwent an episode of massive removal of materials that erased the ancient (Noachian) crater record. Magmatically induced erosion of the volatile-saturated regolith (for example, due to glaciation in Hesperia [Kargel and Strom, 1992]) in the beginning of widespread volcanism (Figure 24b) in Hesperia Planum [Tanaka et al., 2002] may explain such an event. [44] There are two possible end-member scenarios to explain the massive erosion for the Hesperia Planum region. In the first, denudation of material created the total volume of the broad topographic depression of Hesperia (Figure 10b). In this case, the total volume of removed material is estimated to be about km 3 at most. Alternatively, the broad topographic low existed during the Noachian, and erosion removed a layer that was as thick as the highest rims of the Noachian craters (about 300 m, Figure 10a). In this case, the total volume of material removed from Hesperia is minimal and estimated to be about km 3. The latter case is supported by the geomorphic expression of the valley networks, which indicates drainage toward Hesperia (Figure 11). Some portion of the eroded material probably was deposited within the neighboring lowland of Hellas Planitia. If all material from the area of Hesperia would be deposited there, it would produce a layer about km thick (the lower value appears to be more 17 of 28

18 Figure 13. At the contacts of small valleys entering Hesperia Planum there is no evidence either for delta-like or fan-shaped deposits or for the continuation of the valleys on the surface of lava plains. Fragment of THEMIS day-visible image V Resolution is 17 m/px, the center of the image is at about 34.2 S, 65.1 E. plausible). The massive erosion of material continued the hydrologic history of Hesperia Planum and was apparently the first episode of release of volatiles that have been stored in this region during the Noachian (Figure 24b). The Hesperian lava plains were superposed on the eroded surface that lost a significant part of its ancient crater record (Figure 24c). [45] The next major episode of hydrological history was again related with the water release from the area oh Hesperia Planum following the emplacement of the lava plains and formation of the centralized volcanoes such as Hadriaca Patera. During this episode, the large outflow channels formed (Figure 24d) through the catastrophic discharge of water [e.g., Baker et al., 1992]. The source areas for the Dao- Niger system, Harmakhis Vallis, and the channel in southeastern portion of Hesperia Planum appear to occur in the subsurface beneath the lava plains. Although there is a little evidence for the mechanism triggered catastrophic outflow, the spatial association of the Dao-Niger system with Hadriaca Patera suggests that late volcanic activity at the patera played an important role in mobilization and discharge of water [Squyres et al., 1987; Crown et al., 1992; Crown and Greeley, 1993; Mest and Crown, 2001]. The role of volcanism is less clear for the formation of Harmakhis Vallis and the channel within Hesperia, as there is no evidence for volcanic centers near their source. Although sill and/or dike intrusions from either Hadriaca or Tyrrhena Paterae (or both) cannot be ruled out as possible triggers, morphological and topographic evidence for such an interaction is not found yet. Reull Vallis is different from other outflow channels of the region because it may have formed due to catastrophic discharge of water stored within the Morpheos basin [Kostama et al., 2004] and volcanism probably did not play an important role in the formation of this channel. 18 of 28

19 Figure 14. The source area of Dao Vallis (canyon-like feature to the left) is in an elongated flat-floored depression the scalloped edges of which cut the southeastern flanks of Hadriaca Patera (left side of the image). Within the channel of Niger Vallis (canyon-like feature to the right), numerous fragments of disrupted lava plateau are seen. Fragment of Viking image 329s29, resolution is 234 m/px, the center of the image is at about 35.4 S, W. [46] The volume of material removed from the outflow channels in the Hesperia area is estimated to be about km 3 [Rogeiro et al., 2003] or about % of the volume of material eroded from Hesperia Planum before emplacement of the ridged plains. On the basis of these estimates, a significant diminishing of fluvial activity is observed during formation of the outflow channels. [47] Debris aprons and flow-like deposits partly infill outflow channels and apparently correspond to the final stages of fluvial activity in the Hesperia Planum region. The total volume of these flows appears to be significantly smaller compared to the amount of material eroded from the outflow channels. The most important characteristic of the flows is that in different areas they have distinctly different source regions. [48] The flows that occur along the Dao-Niger system and at Harmakhis Vallis originate in the subsurface. At the areas where the subsurface flows begin the volcanic plateau is broken into numerous tilted and displaced blocks that are several hundred meters thick and a few kilometers wide. The surface of the flows is texturally smooth (except for the longitudinal ridges), homogeneous (even at the resolution of MOC images), and displays a few boulders in places. These characteristics of the flows are consistent with those expected for the flows of fine-grained materials such as ice-saturated regolith hypothesized for the debris aprons around massifs of the cratered uplands. In many places within Dao-Niger and Harmakhis Valles the plateau breakup and viscous flows occur together. This suggests that the texture and homogeneity of the flow material are not due to a progressive fragmentation of blocky material of the plateau, but more likely reflect the flow of different material from beneath the lava plains. Thus the subsurface flows reveal two important features of the lava plains and the substratum: (1) visible thickness of the lava plateau closely corresponds to the thickness estimated from the flooded craters and (2) the surface characteristics of the flows strongly suggest that materials of the flows and the lava plains are different in rheologic properties. These features support our assumption that the flooded craters in the area of Hesperia Planum were initially superposed on the original floor before emplacement of volcanic plains. [49] The subsurface viscous flows appear to be absent in the region along Reull Vallis. All occurrences of both the debris aprons and flow-like deposits there seem to have sources localized either on or near the surface. The different position of the source regions of the viscous flows both geographically and relative to the surface suggests the different modes of the flow formation. The subsurface flows are likely representing the final stages of discharge of the reservoir of volatiles under the layer of Hesperian lava plains. The reservoir was largely depleted during the episodes of massive erosion of material from the area of Hesperia Planum before emplacement of lava plains and by the outflow channels after the formation of the plateau. The absence of the subsurface flows in the region of Reull Vallis may be because Reull runs through the area to the south of Hesperia Planum within the Noachian cratered uplands in northern portion of Promethei Terra. This region is separated from the depression of Hesperia by a topographic barrier and a significant subsurface reservoir of volatiles may not have been formed there. The abundant occurrence of surface flows around Reull Vallis may be Figure 15. High-resolution HRSC image (orbit 38, resolution 12.5 m/px) showing details of the morphology within the upper portion of Niger Vallis (see text for details). 19 of 28

20 related to the formation of this outflow channel. Because there is the evidence that Reull formed due to discharge of a transient water reservoir in the Morpheos basin [Kostama et al., 2004] the effluents from the basin could have reaccumulated around Reull and established the source for the debris aprons and flow-like deposits. 7. Conclusions [50] The area of Hesperia Planum and its surroundings displays a rich array of volcanic and fluvial features suggesting that interaction of fluvial and volcanic processes was a major theme of both the geologic history of Hesperia and the depositional history of Hellas Planitia. The analysis of the morphology and stratigraphic relationships of features in and around Hesperia Planum allows establishing a scheme of major episodes of the evolution of this region [Greeley and Crown, 1990; Crown et al., 1992; Price, 1998; Mest and Crown, 2002a, 2002b]. The hydrologic history of Hesperia Planum (Figure 25) appears to begin with the initial accumulation of volatiles (water) within the broad depression of Hesperia and formation of a large reservoir of volatiles there during the Noachian. Later, the reservoir was depleted in three distinctly different modes that reflect diminishing amount of volatiles. (1) An episode of massive areal erosion in Hesperia Planum probably occurred before emplacement of the Hesperian lava plains [Tanaka et al., 2002]. The volume of material eroded during this episode is estimated to be about km 3. If all this material were deposited in Hellas Planitia, it would produce a deposit about km thick. (2) The large outflow Figure 16. The unnamed outflow channel within Hesperia Planum (Mest and Crown [2001] considered this channel as the uppermost portion of Reull Vallis). (a) The USGS MDIM-2.1 (resolution is about 231 m/px at the equator) shows that the channel starts as a full-sized feature (A, about 32 S, W), runs southward, and disappears within a degraded crater (bottom of the image), which is at the northern edge of the depression of Morpheos basin. (b) The MOLA 1/128 topographic map for the same area shows the NW portion of Morpheos basin (bottom of the image) and a broad topographic trough (arrows) that continues the general strike of the channel to the north from the point (A) where it starts as a distinct morphologic feature. Figure 17. The head region of Reull Vallis (beginning of segment 2 of Reull Vallis, according to Mest and Crown [2001]). The outflow channel starts as a full-sized feature (center of the image, about 37 S, 247 W, westernmost tip of Morpheos basin) without any evidence for a distinct source area. Fragment of the USGS MDIM-2.1, resolution is about 231 m/px at the equator. 20 of 28

21 Figure 18. Example of a debris apron around an isolated massif of the cratered uplands on the northern bank of Reull Vallis. The similar debris aprons are typical features within the broad region around Reull Vallis. Fragment of Viking image 97a62, resolution is 215 m/px, the center of the image is at about 41.7 S, W. channels formed after the emplacement of the lava plains. The channels were concentrated in a few places and the volume of material removed is about km 3 [Rogeiro et al., 2003] or about % of the volume of material eroded from the area of Hesperia Planum during the first episode of denudation. (3) Dispersed viscous flows (debris aprons and flow-like deposits) manifest the final episodes of the hydrologic history in the area under study. There are two types of the flows: (a) the subsurface flows that originated from beneath the Hesperian lava plains and occur within the canyons of the outflow channels concentrated in the Southwestern trough (Dao, Niger, and Harmakhis Valles) and (b) the flows from surface sources that occur almost exclusively in the area around Reull Vallis. The subsurface flows probably represent the final episodes of discharge of the reservoir of volatiles in Hesperia Planum below lava plains that were largely depleted by massive erosion and the outflow channels. The flows from the sources on the surface may be due to accumulation of effluents from the Morpheos basin that were drained by Reull Vallis. [51] The thickness of lave plains within Hesperia Planum is estimated by the measurements of flooded craters to be several hundred meters. This value closely corresponds to the visible thickness of blocks of the lava plateau that is breaking up in some places within the large outflow channels. The estimated volume of the lava plains emplaced in Hesperia is thus about km 3. By both the areal extent and the volume of extruded lavas, the volcanic province of Hesperia Planum is well within the range of the terrestrial large igneous provinces. Figure 19. This high-resolution HRSC image (orbit 248, resolution 12.5 m/px) shows details of the morphology of one of the debris aprons. The apron consists of several individual lobes on the surface of which sets of narrow nested ridges are seen. The ridges are convex downward in the direction of flow. Figure 20a. This high-resolution HRSC image (orbit 248, resolution 12.5 m/px) shows details of the morphology of a flow-like deposit within two neighboring craters. Black lines in the right part of the image indicate position of the MOC image M of 28

22 Figure 20b. MOC image M (5.58 m/px, left) shows details of morphology of the flow-like deposit shown in Figure 20a. The sketch map of narrow ridges superposed on the image is shown on the right. 22 of 28

23 Figure 21. Map of areal distribution of viscous flows in the area of Hesperia Planum and its surroundings. Black dots show flows the sources of which are on the surface (debris aprons and flow-like deposits). White dots indicate beginning of the subsurface flows. Thick lines show position of the outflow channels. The map is in sinusoidal projection. 23 of 28

24 Figure 22a. A flow-like deposit at the western edge of Promethei Terra. The deposit spreads on the surface and flows into the head depression of Harmakhis Vallis. Fragment of a HRSC image (orbit 38, resolution is 12.5 m/px). Figure 23. Example of a viscous subsurface flow within the canyon of Niger Vallis (see text for explanation). Fragment of a HRSC image (orbit 528, resolution is 25 m/px). Figure 22b. Example of the viscous flow-like deposit on the northern bank of Reull Vallis. The flow (arrows) enters an impact crater at two inlets (A) and flow out of it through the outlet in the southern rim of the crater (B). The flow enters the canyon of Reull Vallis through a distinct channel (C) and spreads on the floor of the channel (D). Fragment of a HRSC image (orbit 506, resolution is 25 m/px). 24 of 28

25 Figure 24. Block diagrams illustrating proposed major episodes of the geologic history in Hesperia Planum (not to scale; see text for explanation). 25 of 28

26 Table 5. Lava Plateau of Hesperia Planum, Mars, in Comparison to Some Terrestrial Large Igneous Provinces Generalized From Coffin and Eldholm [1994] Province Area, 10 6 km 2 Volume, 10 6 km 3 Age Range, Ma Ontong Java Kerguelen North Atlantic volcanic province > Deccan Columbia River basalts Hesperia Planum, Mars ? Figure 25. Correlation chart of major events of the geologic history of Hesperia Planum. 26 of 28