JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, E12S21, doi:10.1029/2005JE002420, 2005

Major episodes of the hydrologic history in the region of Hesperia Planum, 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. Neukum4 Received 15 February 2005; revised 11 August 2005; accepted 15 August 2005; published 15 December 2005.

[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 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, 0.45–1.5 Â 106 km3, erosion from the Hesperia depression before the lava eruption possibly resulted in thick, 0.5–1.5 km, deposits in . (3) Measurements of the flooded craters within Hesperia Planum provide the estimates of the thickness of lavas there, about 250–500 m. The final volume of lavas within Hesperia Planum (0.4–0.7 Â 106 km3) 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 0.04 Â 106 km3 of material (about 4.5–8.9% 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 , 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 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. (2005), Major episodes of the hydrologic history in the region of Hesperia Planum, Mars, J. Geophys. Res., 110, E12S21, doi:10.1029/2005JE002420.

1. Introduction Planum, which is covered by vast wrinkle ridged plains, is less cratered [Scott and Carr, 1978; Tanaka, 1986], [2] The elevated volcanic plateau of Hesperia Planum smoother, and appears to be within a broad and shallow [Carr et al., 1977; and Spudis, 1981], which is topographic depression. about 1300 by 1700 km across and has the area about 1.5 Â [3] The region was extensively studied since the 106 km2, is situated in the northeastern portion of the broad 9 mission, in particular, through geological mapping [Potter, rim of the Hellas impact basin. The heavily cratered 1976; King, 1978]. On the basis of Viking data, series of Noachian terrains [e.g., Greeley and Guest, 1987] form regional (1:15,000,000 scale) [Greeley and Guest, 1987] the majority of the rim while the surface of Hesperia and detailed local (1:500,000 to 1:1,000,000 scale) geolog- ical maps were compiled for Hesperia Planum and the 1Laboratory of Comparative Planetology, Vernadsky Institute of surrounding uplands [Gregg et al., 1998; Price, 1998; Mest Geochemistry and Analytical Chemistry, RAS, Moscow, Russia. 2Astronomy Division, Department of Physical Sciences, University of and Crown, 2002a, 2002b]. The mapping results and more Oulu, Oulu, Finland. topical studies of specific features [Malin,1976;Pieri, 3International Research School of Planetary Sciences, Universita’ 1976, 1980; Greeley and Spudis, 1981; Greeley and d’Annunzio, Pescara, . 4 Crown, 1990; Crown and Greeley, 1993; Maxwell and Institut fu¨r Geologische Wissenschaften, Department of Earth Craddock, 1995; Gregg et al., 1998] have shown that Sciences, Freie Universita¨t Berlin, Berlin, Germany. Hesperia Planum and the surrounding uplands host an array Copyright 2005 by the American Geophysical Union. of volcanic landforms such as volcanic plains and two low 0148-0227/05/2005JE002420$09.00 volcanic centers, Hadriaca and Tyrrhena Paterae [Greeley

E12S21 1of28 E12S21 IVANOV ET AL.: HYDROLOGIC HISTORY, MARS E12S21 and Spudis, 1981; Greeley and Crown, 1990; Crown and located in the northwest-central portion of Hesperia Planum Greeley, 1993; Gregg et al., 1998], and fluvial structures and is about 1.5 km higher than the surrounding terrain; such as valley networks and large outflow channels (2) the provisionally named Morpheos basin, an elongated [Masursky et al., 1977; Carr and Clow, 1981; Mars topographic low stretched in west-east direction in the Channel Working Group, 1983; Carr, 1995, 1996; Tanaka southeastern portion of Hesperia Planum between about and Leonard, 1995; Scott et al., 1995; Carr and Chuang, 35–40S and 225–240W; (3) a relatively narrow (200 km 1997; Mest and Crown, 2001]. across), southwest-trending depression, informally named [4] The valley networks occur within the cratered uplands the Southwestern trough, in the southwestern corner of on both sides of Hesperia Planum and are thought to be Hesperia Planum. Noachian in age [Malin, 1976; Pieri, 1976; Carr and Clow, [7] In order to assess the topographic configuration of 1981; Scott et al., 1995; Carr, 1996]. Vast wrinkle ridged Hesperia Planum relative to the background of the Hellas plains covering the surface of Hesperia define the base of rim, we have constructed a circum-Hellas topographic the Period of the Martian stratigraphy [Tanaka, profile along a circle centered at 41S, 68E with a radius 1986; Tanaka et al., 1992]. The plains most likely have 2300 km. The profile crosses the central portion of volcanic origin and have been interpreted to consist of Hesperia Planum in general direction from the to relatively thin lava flows [Greeley and Spudis, 1981; northwest (Figure 1a) and consists of 144 points that are Greeley and Guest, 1987]. Tyrrhena Patera shows evidence 2.5 apart. Each point corresponds to the average elevation of a long eruption history including late-stage effusive in a 1 Â 1 box; the data were collected from the MOLA and early explosive episodes [Greeley and Crown, 1990; gridded topography map with the resolution 1/64 degree. Crown et al., 1992; Crown and Greeley, 1993; Gregg et al., The profile shows that the elevations along the Hellas rim 1998]. Materials of the ridged plains embay shield members broadly follow a sine-like line that is lower within Prom- of Tyrrhena Patera and may have been erupted from local, ethei Terra east of the Hellas floor. The Noachian-age presently buried fissures [Leonard and Tanaka, 2001]. terrains of Noachis, Tyrrhena, and Promethei Terrae are Activity at the of Tyrrhena Patera probably extended characterized by significant topographic variations (up to into the late Hesperian to Early [Crown et al., 1.5 km), whereas the surface of Hesperia Planum is much 1992; Gregg et al., 1998]. Large outflow channels (Dao, smoother except for the topographic peak of Tyrrhena Niger, Harmakhis, and Reull Valles), which are among the Patera (Figure 1b). As a whole, Hesperia Planum is on a most spectacular fluvial features of Hesperia Planum, were broad regional slope between Tyrrhena and Promethei apparently formed during the Hesperian [Malin, 1976; Terrae. Masursky et al., 1977; Greeley and Guest, 1987] and [8] The segment of the profile corresponding to Hesperia possibly into the Amazonian [Scott et al., 1995]. Abundant Planum is shown in Figure 2. The mean elevation within debris aprons and flow-like features, the formation of which Hesperia along the profile (excluding Tyrrhena Patera) is likely requires the presence of ground ice [Squyres, 1979; about 1100 m and the mean elevations within Promethei and Squyres and Carr, 1986; Carr, 1996], represent the latest Tyrrhena Terrae are about 1300 m and about 2000 m, (Amazonian) structures related to release of volatiles (water) respectively. Thus the area of Hesperia Planum represents [Crown et al., 2002, 2003; Berman et al., 2003]. a distinct topographic low bordered on both sides by [5] Thus both volcanic and fluvial processes have punc- elevated Noachian terrains. tuated the history of Hesperia Planum during a large time [9] The surface of Hesperia Planum, which is slightly span from the Noachian to Amazonian Periods. The possi- tilted to the south at the mean regional slope about 0.03, ble interactions of these processes likely represent the main consists of two parts separated by a break of slope that theme of the evolution of Hesperia and possibly the history occur in the profile at about 32S, 247W. The first, which of deposition from this particular region to the Hellas basin is essentially horizontal and hosts Tyrrhena Patera, charac- as well. The main goal of out study is to suggest a coherent terizes the northwestern portion of Hesperia Planum. The scenario of hydrologic history of Hesperia Planum based on Morpheos trough, which is about 750–800 m deeper than existing and newly acquired data sets. In this paper we both the rest of Hesperia to the northwest and the cratered outline the most important features in the region of Hesperia uplands of Terra Promethei to the south (Figure 2), charac- Planum and correlate temporally the processes that led to terizes the second part. their formation using all available images and topographic [10] Although the circum-Hellas profile displays the data such as the MOLA-gridded topography (both 64 and general topographic characteristics of the region of Hesperia 128 px/deg resolution), Viking MDIMs, Mars Observer Planum, it crosses boundary of Hesperia only at two points. Camera (MOC), Mars Odyssey THEMIS, and Mars Express In order to document the topographic variations of the High Resolution Stereo Color camera (HRSC). contact that separates Hesperia from the surrounding uplands in more systematic way, we have collected the 2. Topographic Configuration of Hesperia topographic data around the entire boundary of the Planum, which is shown in the geological map of the eastern Planum equatorial region of Mars [Greeley and Guest, 1987] [6] The distribution of the regional-scale topographic (Figure 3a). The data were obtained from the MOLA features along the rim of Hellas basin suggests that Hesperia gridded topography map (resolution 1/64 degrees) in pairs Planum is within a broad and shallow depression. The of points near the boundary (one point is in Hesperia and majority of the flat surface of the depression appears to be another is in the uplands). The size of each point was about at approximately same elevation except for three distinct 1 Â 1 and we were careful not to include in the area of regional-scale features (Figure 1a): (1) Tyrrhena Patera, measurements distinct local topographic features such as

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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 boundary along the southern and western edges of Hesperia, Hesperia (Figure 3b, Table 1). the differences in elevation between the surface of the plains [11] The data show that almost everywhere the surface of and the uplands are greater (up to about 450 m) and reach Hesperia Planum is clearly lower than the surrounding about 800 m along Morpheos Rupes (Table 1, Figures 3a uplands. The edge of Terra Tyrrhena to the east of and 3b). In many places, a distinct scarp separates Hesperia is only slightly higher than the surface of the plains Hesperia Planum from Tyrrhena and Promethei Terrae. (200 m, Table 1) and there is typically no scarp separating [12] The southwestern corner of Hesperia Planum (the these two topographic provinces. For the major part of the Southwestern trough) has very specific topographic charac-

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

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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 Â 1 degree box. of the topographic configuration of impact craters on Mars floor may have been as heavily cratered as the surrounding that are resulted in a robust morphometric correlation upland terrains (e.g., has similar size-frequency distribution among the crater diameter, depth, and rim height [Garvin of impact craters). On the other hand, the ‘‘basement’’ of et al., 2000]. We have conducted a regional survey of the Hesperia may have been resurfaced and some portion of flooded craters within Hesperia Planum and found 43 such impact craters there were destroyed prior to the emplace- features (Table 2, Figures 5 and 7) ranging in diameter from ment of the lava plains. In this case, the size-frequency 6.5 to 63 km and predominantly occurring in the central and distribution of craters may be similar to the distribution of southern parts of Hesperia Planum. The areal distribution of craters on Hesperian units elsewhere on Mars. In order to the craters probably reflects both the initial distribution of test these possibilities, we compared the size-frequency the craters and variation in the thickness of the lava fill. The distribution of the flooded craters in Hesperia with the mean rim height of the flooded craters is estimated from the distribution of craters in a typical Noachian terrain (part crater diameters to be about 325 ± 73 m (1 s standard of Terra Tyrrhena to the west of Hesperia Planum) and in deviation), the median height is about 320 m, and the classical Hesperian-age volcanic provinces, Syrtis Major maximum height is about 495 m (Table 3, Figure 8). On (unit Hs, the Syrtis Major formation) and Lunae Planum the basis of these values and the area of Hesperia Planum (unit Hr, the Ridged Plains material). The areas of the crater (about 1.5 Â 106 km2), the volume of the plains is estimated counting varied from 1.5 Â 106 km2 in Terra Tyrrhena to to be about 0.4 to about 0.7 Â 106 km3 (Table 3). The larger 0.7–0.8 Â 106 km2 in the Hesperian-aged regions (Figures value derived from the maximum diameter of the flooded 9a and 9b). To count craters in these regions, we used the craters probably overestimates the volume or, at least, catalogue of Martian impact craters compiled by N. Barlow represents the upper limit of the volume. (2000, 2003) (available online at ftp://ftpflag.wr.usgs.gov/ [15] The flooded craters may also reflect the morphology dist/pigpen/mars/crater_consortium and http://webgis.wr. of the floor of Hesperia Planum before emplacement of the usgs.gov/mars.htm). In Terra Tyrrhena, craters were counted vast plains unit [Goudy and Gregg, 2002]. For example, the in the area between 0–30S and 275–300W (total area is

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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 Â 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.

1.5 Â 106 km2). This territory is completely within the 65–75N. In both Syrtis Major and Lunae Planum, the ancient Noachian terrains. In Syrtis major, the area of crater subareas were chosen to include territories of ridged plains counting consists of three subareas (total area is 0.77 Â and to avoid areas of the cratered uplands. Results of the 106 km2): (1) 5–15N, 280–285W,(2)0–15N, 285–295, crater count are summarized in Table 4. and (3) 15–20N, 290–295W. In Lunae Planum, the area [16] As it was expected, the Terra Tyrrhena plot shows the of crater counting consists of two subareas (total area is highest crater density and the curves for the Hesperian-aged 0.69 Â 106 km2): (1) 5–15N, 60–65W and (2) 0–15N, regions are similar to one another and lie significantly lower

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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 Â 1 degree box.

(Figures 9a and 9b). The size-frequency distribution of the example, the depression of Hesperia Planum existed as a flooded craters of Hesperia Planum is significantly different natural large-scale topographic variations along the rim of from that in Terra Tyrrhena and closely mimics the distri- the Hellas basin during Noachian (Figure 10a), then the bution in both Syrtis Major and Lunae Planum (Figure 9a), minimum thickness of material that must have been corresponding to Hesperian-aged distributions. The curve for the exposed craters in Hesperia Planum (Figure 9b), Table 1. Mean Elevation Within Uplands and Hesperia Planum although showing slightly higher density of smaller craters Along the Hesperia Boundarya (<30 km) compared to these for Syrtis Major and Lunae Uplands, Mean Hesperia, Mean Mean Planum, is significantly lower than the curve for Terra Elevation, m Elevation, m Difference, m Tyrrhena. We also compared the combined population of Terra Tyrrhena, East of Hesperia the flooded and exposed craters in Hesperia Planum with 1552 ± 493 1370 ± 371 182 that of the Noachian cratered uplands and the Hesperian- aged lava plains. The curve for the combined population is Morpheos Rupes, Southwest of Hesperia slightly shifted toward the higher crater density but is 1591 ± 255 775 ± 123 816 significantly different (within the one s limits) from the Promethei Terra, South of Hesperia distribution of craters within the Hesperian terrains. 1081 ± 413 629 ± 241 453 [17] The crater statistics of the flooded craters within Hesperia Planum strongly suggest that the ancient Hesperia SW Trough (Noachian-type) population of impact craters in this area 495 ± 574 À1195 ± 991 1690 was largely erased before emplacement of the plains fill. Tyrrhena Terra, West of Hesperia The formation of the regional-scale depression that hosts 1647 ± 610 1212 ± 461 436 Hesperia Planum may have existed either during or follow- aVariations in elevation are given as one-sigma. Total mean difference in ing the Noachian Period (Figures 10a and 10b). If, for elevation weighted by boundary length is 500 m.

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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 23.2S, 240.7W.

removed from this area to erase the old craters can be Table 1) and the thickness of the infilling plains-forming estimated by the rim height of the larger impact craters that materials within the Planum (from 250 to 500 m, characterize the surface of the Noachian terrains around Table 3). These numbers give the range of the depression Hesperia. We assume that the similar size-frequency distri- depth from 750 to 1000 m, and thus the maximum total bution of craters, which characterize the Noachian terrains volume of material missed within the Hesperia area is exposed in Terra Tyrrhena, existed in Hesperia Planum. The estimated to be 1.1–1.5 Â 106 km3. 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]. 4. Fluvial Features in and Near Hesperia Planum Importantly, the rim height of the larger craters on Mars [18] There are two classes of fluvial features in the (>100 km) is less dependent on the crater diameter and a region of Hesperia Planum and its surroundings: (1) small rim height of 300 m closely approximates the rim height of valley networks [Carr and Chuang, 1997] and (2) large craters ranging in diameter from 100 to 1000 km [Garvin outflow channels [Baker et al., 1992]. Each class has et al., 2000]. Consequently, the volume estimated for a distinct stratigraphic positions and apparently reflects spe- 300-m-thick layer of Noachian materials removed from the cific episodes of the hydrologic history and the release of 6 3 floor of Hesperia Planum is about 0.45 Â 10 km .Onthe volatiles. other hand, if the depression of Hesperia Planum formed after Noachian, the volume of the removed material would 4.1. Small Valley Networks be significantly larger (Figure 10b). In this scenario, the [19] Small valley networks are abundant within the total depth of the pre-plains depression can be estimated as Noachian terrains on both sides of Hesperia Planum and a sum of the mean topographic difference between the in the area adjacent to Morpheos Rupes (Figure 11) surface of Hesperia and surrounding uplands (500 m, [Craddock and Maxwell, 1993; Maxwell and Craddock,

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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 27.3S, 249.5W, 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 23.8S, 263.2W, 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 23.5S, 239.4W, 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 38.4S, 232.0W, 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, [20] The small valleys dissect the surface of the uplands 2001; Mest and Crown, 2001]. In the units making up the (Figure 12) and thus postdate the heavily cratered Noachian surface of the Noachian terrains to the west and east of terrains. At the contact with the Hesperia plains, the valleys Hesperia planum, Npl1 (cratered unit) and Npld (dissected are usually abruptly terminated. The best available images unit) [Greeley and Guest, 1987], the density of the valley (17 m/px resolution) show no accumulation of materials that networks reaches the maximum on Mars. For example, the may have been removed from the uplands and deposited on unit Npld (Terra Tyrrhena west of Hesperia) has the largest the surface of the plains (Figure 13). The valleys also do not density of the networks, about 0.007 kmÀ1 [Carr and cut the surface of the plains within Hesperia (Figure 13). Chuang, 1997]. In the recent studies (based on new topo- This indicates that formation of the valleys and associated graphic data) the density of the drainage systems within the deposition occurred prior to the emplacement of vast plains cratered uplands surrounding Hesperia Planum were found in Hesperia. Thus the material unit Hr represents the upper to be even higher, reaching the average 0.067 kmÀ1 [Mest stratigraphic limit for the formation of the valleys in the et al., 2002; Mest and Crown, 2004]. The local to regional vicinity of Hesperia Planum and the volatile-rich effluents topographic gradient governs the orientation of the small of the valleys were stored on the original floor of the large valleys within the cratered uplands and in the vicinity of topographic basin of Hesperia. Hesperia Planum (Figure 12). In areas where the small valleys are near the contact of the uplands with Hesperia 4.2. Outflow Channels Planum, they follow the regional topographic slope toward [21] The region between Hesperia Planum and Hellas Hesperia (Figures 11 and 12). basin is one of the four main areas where the large outflow

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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., the trough are marked by curvilinear graben arranged en 1995; Carr, 1996; Crown et al., 2004, 2005; Bleamaster echelon (B in Figure 15). Interconnected wrinkle ridges (C and Crown, 2004]. There are five outflow channels in this in Figure 15) deform the morphologically smooth surface region. Three of them, Dao, Niger, and Harmakhis Valles within and outside the trough. These morphological features cut plains materials [e.g., Price, 1998] within the South- collectively suggest that subsidence of the original surface western trough that connects Hesperia Planum with Hellas of the ridged plains (unit Hr) took place within the upper basin. The fourth channel, Reull Vallis, as it is shown in the portion of Niger, probably due to removal of material by geologic map of the eastern equatorial region of Mars subsurface flow [Crown and Mest, 1997, 2001]. [Greeley and Guest, 1987] begins at western edge of the [24] In contrast to all other channels (compare to Figures 14 Morpheos basin and runs from east to west across the and 16), the source area of Reull Vallis (segment 2 of Reull northern portion of Promethei Terra. The fifth channel Vallis [Mest and Crown, 2001]) is indistinct (Figure 17). The begins within the southeastern portion of Hesperia Planum channel starts as a full-sized topographic and morphologic about 32S, 246.5W, runs southward, and disappears at the feature at the western edge of the topographic low of the northern edge of Morpheos basin at about 35S, 246W (see Morpheos basin (at about 37.5S, 247W). At its very Figures 1a and 3a). In the work by Mest and Crown [2001] beginning, Reull Vallis appears to breach a topographic this channel is considered as the upper segment of Reull barrier of the ridged plains near the southern edge of Hesperia Vallis. Planum (Figure 17). From this point [Mest and Crown, [22] Dao, Niger, and Harmakhis Valles begin in distinct 2001], the channel runs as a steep-sided canyon through the closed depressions sharply outlined by high steep walls cratered uplands of the northern edge of Promethei Terra in (Figure 14). The edges of the depressions are scalloped general western direction and disappears at about 40S, suggesting collapse of the walls. The channels of Dao and 264W near the source area of . A debris Harmakhis Valles breach the depressions and run as steep- apron covers the lower reach of Reull Vallis where it may sided canyons toward the floor of the Hellas basin where have been connected with Harmakhis Vallis [Crown et al., they disappear, leaving little evidence for deposits at their 1992]. apparent mouths [Crown et al., 1992]. In contrast to Dao [25] Because all outflow channels cut into the lava plains and Harmakhis Valles, a set of broad and shallow troughs of Hesperia Planum, they postdate the emplacement of the connecting relatively small circular and elongated depres- plains. Thus outflow channel formation records large epi- sions is visible at the uppermost reaches of Niger Vallis. sodes of groundwater discharge following the formation of Dao and Niger Valles merge at about 37S, 270W and the lava plains in Hesperia. Characteristics of the source of continue as a single channel toward the Hellas basin. the outflow channels in the Hesperia Planum region sug- [23] The high-resolution HRSC image (orbit 38, 12.5 m/ gests that Dao, Niger, Harmakhis Valles, and the channel in px) shows details of the morphology of the upper portion of the southern portion of Hesperia are due to release of Niger Vallis (Figure 15). The uppermost reach of Niger volatiles from the subsurface sources, which are covered forms a broad, shallow trough outlined by gently sloping by a composite layer of Hesperian ridged plains. In contrast, walls (A in Figure 15). The breaks of slopes at the edges of ReullVallismayhaveformedbysurfacerun-offthat

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Table 2. Flooded Craters in Hesperia Planum Viking Image Western Eastern Crater Rim Number Mars Chart Image Resolution, km/px Latitude Lon. Lon. Diameter, km Height, m 1 MC-22_NW 378x30 0.239 À9.5 256.4 103.6 31.1 346.5 2 MC-22_NW 378x31 0.245 À15 254 106 20.2 278.1 3 MC-22_SW 365s74 0.222 À18.5 248.6 111.4 53.8 457.9 4 MC-22_SW 365s74 0.222 À19.7 249.3 110.7 15.4 241.9 5 MC-22_SW 097a27 0.233 À23.8 263.2 96.8 21.7 288.1 6 MC-22_SW 365s40 0.234 À26 254.6 105.4 26.1 316.8 7 MC-22_SW 365s37 0.234 À26.9 249.3 110.7 24.6 307.4 8 MC-22_SW 365s40 0.234 À26.9 256.6 103.4 25.6 313.6 9 MC-22_SW 365s37 0.234 À27.3 249.5 110.5 24.2 304.6 10 MC-22_SW 365s37 0.234 À27.3 248 112 15.0 238.8 11 MC-22_SW 365s38 0.234 À29.3 261.9 98.1 41.4 400.7 12 MC-22_SW 365s38 0.234 À29.5 255.8 104.2 31.4 347.9 13 MC-22_SE 365s73 0.224 À16.8 245.1 114.9 17.8 260.6 14 MC-22_SE 365s72 0.221 À18.9 247.1 112.9 26.1 316.8 15 MC-22_SE 365s71 0.223 À19.6 244.1 115.9 21.1 284.2 16 MC-22_SE 365s69 0.223 À19.7 242.7 117.3 14.5 234.7 17 MC-22_SE 365s67 0.222 À20.8 240.4 119.6 36.2 374.3 18 MC-22_SE 365s67 0.222 À22.2 240.2 119.8 27.2 323.2 19 MC-22_SE 365s67 0.222 À23 239.9 120.1 19.9 275.6 20 MC-22_SE 365s67 0.222 À23.5 239.4 120.6 21.6 287.6 21 MC-22_SE 365s67 0.222 À24 240.4 119.6 45.3 419.5 22 MC-22_SE 365s68 0.22 À25.1 242.7 117.3 37.0 378.3 23 MC-22_SE 365s65 0.222 À25.5 238.7 121.3 36.8 377.3 24 MC-22_SE 365s65 0.222 À25.6 239.8 120.2 18.5 265.6 25 MC-22_SE 365s65 0.222 À25.6 239.6 120.4 12.1 213.7 26 MC-22_SE 365s68 0.22 À25.9 244.3 115.7 30.7 344.1 27 MC-22_SE 365s66 0.22 À27.2 245.5 114.5 28.4 330.7 28 MC-22_SE 365s66 0.22 À27.5 244.6 115.4 28.5 331.5 29 MC-22_SE 365s65 0.222 À27.7 239.6 120.4 33.4 359.0 30 MC-22_SE 365s62 0.22 À29.7 239.8 120.2 15.7 244.2 31 MC-22_SE 365s62 0.22 À29.8 238.7 121.3 6.5 155.8 32 MC-22_SE 365s33 0.234 À29.8 244.6 115.4 12.4 216.7 33 MC-28_NE 553a48 0.187 À31.1 255.3 104.7 47.6 430.3 34 MC-28_NE 553a48 0.187 À31.4 254.2 105.8 28.6 331.6 35 MC-28_NE 365s32 0.233 À34.2 248 112 25.3 311.8 36 MC-28_NE 365s32 0.233 À34.5 247.5 112.5 62.8 495.6 37 MC-29_NW 365s37 0.234 À30.6 244.9 115.1 22.3 292.3 38 MC-29_NW 518a30 0.164 À35.5 240.8 119.2 55.7 466.2 39 MC-29_NW 518a30 0.164 À35.9 241.8 118.2 45.3 419.4 40 MC-29_NW 365s51 0.232 À36.4 228.9 131.1 21.4 286.0 41 MC-29_NW 365s54 0.228 À37.1 235.2 124.8 25.7 314.0 42 MC-29_NW 518a26 0.164 À37.9 247 113 34.4 364.4 43 MC-29_NW 365s52 0.229 À38.4 232 128 45.5 420.5 possibly was localized within the topographic depression of meters wide) ridges that are curved in the apparent direction Morpheos basin. of the lobe movement (Figure 19). Generally, the surface of the lobes displays narrow V-shaped ridges previously de- scribed as rock glaciers mantled by dust or other debris 5. Viscous Flows [Baker, 2003], which requires the existence of nearly pure [26] Viscous flow-like materials were interpreted as ice- near-surface ice [Baker, 2003]. related features [Squyres, 1979; Squyres et al., 1987], which [27] Less conspicuous flow features were recently dis- could represent the presence of water in Hesperia Planum. covered by inspection of a large number of MOC images As such, these flows would provide insight into both [Mustard et al., 2001; Milliken et al., 2003]. An example of climate and volcanically induced local to regional temper- such a flow, which was earlier included in the debris aprons ature variations. The most spectacular flow-like features are database by Pierce and Crown [2003], is displayed in lobate aprons at the base of some isolated upland massifs the high-resolution HRSC image (orbit 248, 12.5 m/px, [Squyres, 1979; Crown et al., 1992; Squyres and Carr, Figure 20a) that shows a relatively thick deposit spreading 1986; Berman et al., 2003; Crown et al., 2002, 2003; Pierce across the floors of two neighboring craters through and Crown, 2003] in the northern part of Terra Promethei breached rims. A distinct set of narrow (tens to a few (Figure 18). Most aprons are characterized by lobate fronts hundred meters wide) ridges indicates flow direction from and convex upward surfaces (Figure 18). At Viking resolu- the topographically higher smaller crater to the larger crater tion, the surface of the aprons appears homogenous and where the lobate flow front displays a set of fine-scale morphologically smooth, but at higher resolution (MOC, curved ridges, similar to the typical lobate debris aprons HRSC), individual lobes within the aprons suggest multiple elsewhere. events [Pierce and Crown, 2003] (Figure 19). The lobe [28] The ridges parallel the flow front and form a com- surfaces display sets of nested narrow (a few hundred plex pattern of nested structures resembling a moire-like

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Figure 8. The size-frequency distribution of the rim heights of flooded craters within Hesperia Planum. Total Figure 7. Areal distribution of flooded impact craters number of craters is 43; mean height is about 325 m. within Hesperia Planum. Map is in sinusoidal projection, which is extended for 30 from the center point (25S, upper into the lower crater. There are two possible alter- 252.5W). natives for the formation of the deposit: (1) a lava flow and (2) a glacial/fluidized flow. The absence of volcanic centers pattern in the central portion of the deposit. Within the near the deposit source appears to rule out a volcanic origin. upper crater, the deposit is characterized by two systems of On the other hand, the presence of lobate aprons, which ridges. The ridges in the central portion are sub-parallel to likely were formed by flow of ice-saturated material each other and oriented toward the breach. The spacing of [Squyres, 1979] in the region near the deposit, favors a the ridges is narrowing toward the breach. In the northern glacial origin. portion of the crater, the ridges are oriented at high angles to [30] The important characteristic of the aprons and flow- and appear to be cut by the ridges in the center of the crater like deposits is that they most likely formed due to ice- (Figure 20a). MOC image M23-00829 (5.58 m/px) facilitated movement (flow/creep) and/or partial melt of shows the details of the deposit within the upper crater interstitial ice. The map of the areal distribution of these (Figure 20b). The central part of the deposit within the features (Figure 21) based on Viking and available HRSC crater is characterized by narrow arcuate ridges that are images shows that they are concentrated in the northern convex to the northwest toward the breach. The arcuate portion of Promethei Terra on both sides of Reull Vallis (see ridges are bound by systems of larger and straighter ridges also the map of debris aprons in the paper by Pierce and that are gently curved to the central area. The parallel ridges Crown [2003]). The aprons and flows are absent within the in the northern part of the crater bend near the juncture of main portion of Hesperia Planum, within the Southwestern the central and outer parts of the flow and eventually trough, and in the cratered uplands to the south of about become parallel to the ridges in the central portion of the 47S. On the right side of Reull Vallis, many aprons and flow. flows are in close spatial association with the outflow [29] The morphology of the deposit is consistent with the channel. A large debris apron covers the lowermost part explanation that it was formed by viscous flow from the 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 Table 3. Estimates of the Volume of Plains Fill in Hesperia material with an extensively corrugated surface enters the Planum source depression of Harmakhis Vallis from the north and Thickness Volume a b 6 3 fills a significant portion of it (Figure 22a). This deposit Parameter of Plains, m of Plains, 10 km begins as a landslide-like structure from the scarp on the Mean thickness 325 0.49 southern part of plains that were interpreted as ice-rich Mean thickness À 1s 252 0.38 material modified by deflation [Leonard and Tanaka, 2001; Mean thickness + 1s 398 0.60 Median thickness 317 0.47 Price, 1998; Crown et al., 1992; Bleamaster and Crown, Maximum thickness 496 0.74 2004]. The flow likely consists of sediments and interstitial aThickness of the plains is adopted to be equal to the rim height. ice and probably formed due to partial melting of ice, which bThe area of Hesperia Planum is about 1.5 Â 106 km2. produced sediments-laden flows that were subsequently

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

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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 basin of Hesperia and the giant impact basin of Hellas. superposed on the original floor of the channels (Figure 23). Following the regional topography, the surface of Hesperia In contrast to the outflow channels within the Southwestern Planum slopes generally to the south toward Hellas Planitia. trough, the area of Reull Vallis displays little (if any) On the average, the major portion of Hesperia is about evidence for flows from the subsurface. All flow-like 500 m lower than the surface of adjacent cratered deposits, which occur near Reull Vallis, appear to have uplands. When compared to the two larger basins, there subaerial or near-surface sources (Figure 22). are two smaller, yet significant, topographic features in Hesperia Planum that played an important role of the hydrologic history of the region. 6. Discussion [35] The first feature is the Southwestern trough that [34] A rich array of volcanic and fluvial landforms connects the main portion of Hesperia Planum with the characterizes Hesperia Planum and the surrounding uplands. basin of Hellas Planitia. The trough is a relatively narrow The interaction of fluvial and volcanic processes is thus the (about 200 km across) feature the surface of which is up to main theme of the geologic history of this region. The 3 km lower than the surface of the uplands and may represent history is highly influenced by both the large topographic 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 106 km2 Region Area, 106 km2 Total Number of Craters N(16) N(5) Terra Tyrrhena 1.50 821 180 514 Syrtis Major 0.77 152 28 169 Lunae Planum 0.69 150 35 171 Hesperia Planum (exposed craters) 1.17 322 55 240 Hesperia Planum (flooded craters) 1.17 43 24 N/A Hesperia Planum (exposed+flooded) 1.17 333a 69 249 aThe 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.

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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 the Morpheos basin may have served as a transient storage Hesperia into the vast lowland of Hellas Planitia. The area for the effluents from the northern channel and later the outflow channels of Dao, Niger, and Harmakhis Valles discharge of the basin led to the formation of Reull Vallis represent the most prominent episodes of fluvial activity. [Kostama et al., 2004]. [36] The second area corresponds to the topographic low [37] Hydrologic activity in the area of Hesperia Planum of the Morpheos basin in the southeastern portion of and its surroundings is recorded during the Noachian Period Hesperia Planum. On the basis of morphologic and topo- through the Amazonian Period [Masursky et al., 1977; Mars graphic evidence, a prominent outflow channel, which Channel Working Group, 1983; Carr, 1995]. The hydro- begins well within Hesperia, disappears near the northern logic history can be divided into three episodes, each of edge of the Morpheos basin. Some authors [Crown and which is characterized by its own specific structures. One of Mest, 1997; Mest and Crown, 2001] consider this channel the first recognizable episodes in the Hesperia Planum area as the uppermost segment of Reull Vallis and propose that is the formation of the small valley networks that dissect the Reull Vallis ‘‘was at one time continuous, as there are no surrounding uplands [Pieri, 1980; Baker and Partridge, other obvious source regions in the area of segment 2’’ 1986; Carr, 1995; Mest and Crown, 2001]. Many valley [Mest and Crown, 2001]; the segment 2 corresponds to the networks are oriented toward Hesperia Planum and termi- upper part of Reull Vallis as it is shown in the geologic map nated at its contact with the uplands (Figure 11). Detail of the eastern equatorial region of Mars [Greeley and Guest, inspection of the valley terminus areas shows that the lava 1987]. The source region of Reull Vallis is indeed indistinct, plains of Hesperia Planum embay and bury the valleys as it initiates as a full-size feature near the westernmost tip (Figure 12). Thus formation of the lava plains establishes of the Morpheos basin. Thus a plausible hypothesis is that the upper stratigraphic limit for the period of the valley

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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 378 Â 23, resolution is 241 m/px, the center of the image is at about 17.9S, 264.2W. (b) Northwestern side of Hesperia. Fragment of Viking image 378 Â 26, resolution is 236 m/px, the center of the image is at about 12.0S, 261.3W. (c) Eastern side of Hesperia. Fragment of Viking image 629a03, resolution is 247 m/px, the center of the image is at about 14.7S, 243.2W. (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 36.5S, 227.3W. 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 [Christensen et al., 2001]. Groundwater sapping requires Planum [Scott et al., 1995; Carr, 1995]. The relationships of either significant accumulations of ground ice in the sub- embayment also suggest that the valleys may have contin- surface to maintain sapping for significant time or some sort ued into Hesperia Planum stored their effluents on the floor of recharge of the source areas, or both. Because many of the Planum. valley networks begin in areas of small subsurface volume [38] Although the sources and modes of origin of the such as rims of impact craters or isolated massifs of the valley networks are disputable [e.g., Scott et al., 1995; Carr, uplands, large accumulations of ground ice/water are less 1996], there is a consensus that these features formed by likely in these areas. The effective recharge of the sources running water due to surface runoff [Craddock and requires the water cycle through the atmosphere and meets Howard, 2002; Hynek and Phillips, 2001; Mangold et al., the difficulties of the precipitation/runoff hypothesis. The 2004], groundwater sapping [Pieri, 1980; Baker, 1990], or third mode of origin due to basal melting effectively avoids by basal malting below thick snow/ice deposits [Carr and the difficulties of the other two hypotheses and, although it Head, 2003]. The first mode of formation requires an is model dependent, offers a plausible explanation for the atmospheric water cycle and, as a consequence, surface mode of origin of the small valley networks. conditions significantly different from those observed today [39] This hypothesis requires a pack of snow and ice as [Baker, 1990]. This is poorly consistent with the apparent thick as several hundred meters up to a few kilometers for absence of weathering products on the surface of Mars the initiation of basal melting [Carr and Head, 2003]. For

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500 m, with volumes of about 0.4 to about 0.7 Â 106 km3 (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 progres- sive 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 250–300 m probably corre- sponds 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]. Figure 12. The surface of Terra Tyrrhena to the west of [43] The size frequency distribution of the flooded craters Hesperia Planum is dissected by numerous small valley in Hesperia coincides with the distribution of craters on the networks. The valleys appear to follow the regional-scale surface of the major occurrences of the Hesperian ridged slopes and are terminated either at the edges of local plains within the Hesperia Planum itself, Syrtis Major, and depressions (black and white arrows, center and right side Lunae Planum (Figure 9a). This suggests that the surface in of the image) or at the contact of the uplands with lava Hesperia before emplacement of lava plains is best charac- plains in Hesperia Planum (HP, black arrows at the bottom terized by the size-frequency distribution of impact craters of the image). Fragment of Viking image 625a31, resolution of these Hesperian surfaces. Thus the crater retention ages is 241 m/px, the center of the image is at about 25.0S, of both the actual surface of Hesperia Planum and the 266.9W. 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 the small valley networks to form in this scenario, a large lavas. The apparent Hesperian age of the original floor of amount of ice is expected to have accumulated in Hesperia Hesperia Planum strongly suggests that this area underwent Planum and the surrounding uplands (where the valley an episode of massive removal of materials that erased the networks density is the highest [Carr and Chuang, 1997; ancient (Noachian) crater record. Magmatically induced Mest et al., 2002; Mest and Crown, 2004]) (Figure 24a). erosion of the volatile-saturated regolith (for example, due Regardless of the specific mode of origin of the small to glaciation in Hesperia [Kargel and Strom, 1992]) in the valleys, their formation was the first recognizable and, beginning of widespread volcanism (Figure 24b) in Hes- apparently, the only episode of inflow and accumulation peria Planum [Tanaka et al., 2002] may explain such an of volatiles on the original floor of Hesperia Planum. The event. later episodes of hydrologic activity are related only to [44] There are two possible end-member scenarios to outflows of fluidized materials from the area of Hesperia. explain the massive erosion for the Hesperia Planum region. [40] One of the major stages in the geologic history of In the first, denudation of material created the total volume of Hesperia Planum was the emplacement of vast volcanic the broad topographic depression of Hesperia (Figure 10b). plains (material unit Hr) [Greeley and Guest, 1987; Scott In this case, the total volume of removed material is and Tanaka, 1986; Tanaka, 1986; Tanaka et al., 1992] that estimated to be about 1.5 Â 106 km3 at most. Alternatively, have covered about 1.5 Â 106 km2 of the broad topographic the broad topographic low existed during the Noachian, and depression of Hesperia. Within the plains, there is the erosion removed a layer that was as thick as the highest evidence for impact craters there flooded by lavas. If one rims of the Noachian craters (about 300 m, Figure 10a). In assumes that these craters formed on the original floor of this case, the total volume of material removed from Hesperia prior emplacement of the plains-forming lavas then Hesperia is minimal and estimated to be about 0.45 Â the flooded crates: (1) provide the mean to estimate the 106 km3. The latter case is supported by the geomorphic thickness and the volume of the lava fill and (2) characterize expression of the valley networks, which indicates drainage the size-frequency distribution of craters on the surface toward Hesperia (Figure 11). Some portion of the eroded before lava emplacement. material probably was deposited within the neighboring [41] On the basis of the diameters of the flooded craters lowland of Hellas Planitia. If all material from the area of we estimate that the thickness of the lava fill within Hesperia would be deposited there, it would produce a layer Hesperia Planum varies in the ranges from about 250 to about 0.5–1.5 km thick (the lower value appears to be more

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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-01047003. Resolution is 17 m/px, the center of the image is at about 34.2S, 65.1E. plausible). The massive erosion of material continued the evidence for the mechanism triggered catastrophic outflow, hydrologic history of Hesperia Planum and was apparently the spatial association of the Dao-Niger system with the first episode of release of volatiles that have been stored Hadriaca Patera suggests that late volcanic activity at the in this region during the Noachian (Figure 24b). The patera played an important role in mobilization and discharge Hesperian lava plains were superposed on the eroded of water [Squyres et al., 1987; Crown et al., 1992; Crown surface that lost a significant part of its ancient crater record and Greeley, 1993; Mest and Crown, 2001]. The role of (Figure 24c). volcanism is less clear for the formation of Harmakhis Vallis [45] The next major episode of hydrological history was and the channel within Hesperia, as there is no evidence for again related with the water release from the area oh Hesperia volcanic centers near their source. Although sill and/or dike Planum following the emplacement of the lava plains and intrusions from either Hadriaca or Tyrrhena Paterae (or both) formation of the centralized volcanoes such as Hadriaca cannot be ruled out as possible triggers, morphological and Patera. During this episode, the large outflow channels topographic evidence for such an interaction is not found formed (Figure 24d) through the catastrophic discharge of yet. Reull Vallis is different from other outflow channels of water [e.g., Baker et al., 1992]. The source areas for the Dao- the region because it may have formed due to catastrophic Niger system, Harmakhis Vallis, and the channel in south- discharge of water stored within the Morpheos basin eastern portion of Hesperia Planum appear to occur in the [Kostama et al., 2004] and volcanism probably did not subsurface beneath the lava plains. Although there is a little play an important role in the formation of this channel.

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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 orig- inal 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 epi- sodes of massive erosion of material from the area of Figure 14. The source area of (canyon-like Hesperia Planum before emplacement of lava plains and feature to the left) is in an elongated flat-floored depression by the outflow channels after the formation of the plateau. the scalloped edges of which cut the southeastern flanks of The absence of the subsurface flows in the region of Reull Hadriaca Patera (left side of the image). Within the channel Vallis may be because Reull runs through the area to the of Niger Vallis (canyon-like feature to the right), numerous south of Hesperia Planum within the Noachian cratered fragments of disrupted lava plateau are seen. Fragment of uplands in northern portion of Promethei Terra. This region Viking image 329s29, resolution is 234 m/px, the center of is separated from the depression of Hesperia by a topo- the image is at about 35.4S, 267.2W. graphic barrier and a significant subsurface reservoir of volatiles may not have been formed there. The abundant [46] The volume of material removed from the outflow occurrence of surface flows around Reull Vallis may be channels in the Hesperia area is estimated to be about 0.04 Â 106 km3 [Rogeiro et al., 2003] or about 4.5–8.9% 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 Figure 15. High-resolution HRSC image (orbit 38, around massifs of the cratered uplands. In many places resolution 12.5 m/px) showing details of the morphology within Dao-Niger and Harmakhis Valles the plateau breakup within the upper portion of Niger Vallis (see text for details).

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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 re- accumulated 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 0.45–1.5 Â 106 km3.Ifallthis material were deposited in Hellas Planitia, it would produce a deposit about 0.5–1.5 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 32S, 246.5W), 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 Figure 17. The head region of Reull Vallis (beginning of shows the NW portion of Morpheos basin (bottom of the segment 2 of Reull Vallis, according to Mest and Crown image) and a broad topographic trough (arrows) that [2001]). The outflow channel starts as a full-sized feature continues the general strike of the channel to the north (center of the image, about 37S, 247W, westernmost tip of from the point (A) where it starts as a distinct morphologic Morpheos basin) without any evidence for a distinct source feature. area. Fragment of the USGS MDIM-2.1, resolution is about 231 m/px at the equator.

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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 0.04 Â 106 km3 [Rogeiro et al., 2003] or about 4.5–8.9% 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 concen- trated in the Southwestern trough (Dao, Niger, and Harma- khis 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 Figure 18. Example of a debris apron around an isolated several hundred meters. This value closely corresponds to massif of the cratered uplands on the northern bank of Reull the visible thickness of blocks of the lava plateau that is Vallis. The similar debris aprons are typical features within breaking up in some places within the large outflow the broad region around Reull Vallis. Fragment of Viking channels. The estimated volume of the lava plains emplaced image 97a62, resolution is 215 m/px, the center of the in Hesperia is thus about 0.4–0.7 Â 106 km3. By both the image is at about 41.7S, 257.6W. 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 Figure 20a. This high-resolution HRSC image (orbit 248, one of the debris aprons. The apron consists of several resolution 12.5 m/px) shows details of the morphology of individual lobes on the surface of which sets of narrow a flow-like deposit within two neighboring craters. Black nested ridges are seen. The ridges are convex downward in lines in the right part of the image indicate position of the the direction of flow. MOC image M23-00829.

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Figure 20b. MOC image M23-00829 (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.

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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.

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Figure 22a. A flow-like deposit at the western edge of Figure 23. Example of a viscous subsurface flow within the Promethei Terra. The deposit spreads on the surface and flows canyon of Niger Vallis (see text for explanation). Fragment of into the head depression of Harmakhis Vallis. Fragment of a a HRSC image (orbit 528, resolution is 25 m/px). HRSC image (orbit 38, resolution is 12.5 m/px).

Figure 22b. Example of the viscous flow-like deposit on the northern bank of Reull Vallis. The flow (arrows) enters an 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).

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

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Table 5. Lava Plateau of Hesperia Planum, Mars, in Comparison to Some Terrestrial Large Igneous Provinces Generalized From Coffin and Eldholm [1994] Province Area, 106 km2 Volume, 106 km3 Age Range, Ma Ontong Java 4.9 36–76 117.7–118.2 121–124 Kerguelen 2.3 15–24 109.5–114 North Atlantic volcanic province >1.3 6.6 54.5–57.5 Deccan 0.8–1.8 8.2 64.5–65.5 65–69 Columbia River basalts 0.16 1.3 6–17.5 15.7–17.2 Hesperia Planum, Mars 1.5 0.4–0.7 ?

Figure 25. Correlation chart of major events of the geologic history of Hesperia Planum.

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[52] Volcanism within Hesperia Planum probably played Coffin, M. F., and O. Eldholm (1994), Large igneous provinces: Crustal the major role in mobilization and release of volatiles stored characteristics, dimensions, and external consequences, Rev. Geophys., 32, 1–36. in the reservoir of Hesperia. It appears to be very plausible Condie, K. C. (2001), Mantle Plumes and Their Record in Earth History, that the volcanic activity induced the main episode of 306 pp., Cambridge Univ. Press, New York. erosion in Hesperia Planum [Tanaka et al., 2002] and it is Craddock, R. A., and A. D. Howard (2002), The case for rainfall on a warm, wet early Mars, J. Geophys. Res., 107(E11), 5111, doi:10.1029/ also possibly that later magmatism served as a trigger 2001JE001505. mechanism for the outflow channels [Squyres et al., 1987; Craddock, R. A., and T. A. Maxwell (1993), Geomorphic evolution of the Crown and Greeley, 1993]. Martian highlands through ancient fluvial processes, J. Geophys. Res., 98, 3453–3468. Crown, D. A., and R. Greeley (1993), Volcanic geology of Hadriaca Patera 53 Acknowledgments. This research was conducted in cooperation [ ] and eastern Hellas region of Mars, J. Geophys. Res., 98, 3431–3451. with the HRSC Co-Investigator Team, and the supporting efforts of the Crown, D. A., and S. C. Mest (1997), Dao, Harmakhis, and Reull Vallis: HRSC Team members (J. Albertz of TU Berlin, A. T. Basilevsky of The role of outflow channels in the degradation of the circum-Hellas Vernadsky Institute-RAS Moscow, G. Bellucci of CNR Rome, J.-P. Bibring highlands of Mars, Proc. Lunar . Sci. Conf. 28th, 269–270. of CNRS Orsay, M. Buchroithner of TU Dresden, M. H. Carr of USGS Crown, D. A., and S. C. Mest (2001), Circum-Hellas outflow channels: Menlo Park, E. Dorrer of Universita¨t der Bundeswehr Mu¨nchen, T. C. New views from , Proc. Lunar Planet. Sci. Conf. Duxbury of JPL Pasadena, H. Ebner of TU Mu¨nchen, B. H. Foing of 32nd, abstract 1344. ESTEC Noordwijk, R. Greeley of Arizona State University Tempe, Crown, D. A., K. H. Price, and R. Greeley (1992), Geologic evolution of E. Hauber of DLR Berlin, J.W. Head III of Brown University Providence, the east rim of the Hellas basin, Mars, Icarus, 100, 1–25. C. Heipke of Universita¨t Hannover, H. Hoffmann of DLR Berlin, A. Inada Crown, D. A., T. L. Pierce, S. B. Z. McElfresh, and S. C. Mest (2002), of Kobe University and of MPAE Katlenburg-Lindau, W.-H. Ip of NCU Debris aprons in the eastern Hellas region of Mars: Implications for styles Chung-Li, B. A. Ivanov of IDG-RAS Moscow, R. Jaumann of DLR Berlin, and rates of highland degradation, Proc. Lunar Planet. Sci. Conf. 33rd, H. U. Keller of MPAE Katlenburg-Lindau, R. Kirk of USGS Flagstaff, abstract 1642. K. Kraus of TU Wien, P. Kronberg of TU Clausthal, R. Kuzmin of Crown, D. A., S. B. Z. McElfresh, T. L. Pierce, and S. C. Mest (2003), Vernadsky Institute-RAS Moscow, Y. Langevin of CNRS Orsay, K. Lumme Geomorphology of debris aprons in the eastern Hellas Region of Mars, of University of Helsinki, W. Markiewicz of MPAE Katlenburg-Lindau, Proc. Lunar Planet. Sci. Conf. 34th, abstract 1126. P. Masson of OrsayTerre Orsay, H. Mayer of Universita¨t der Bundeswehr Crown, D. A., L. F. Bleamaster, and S. C. Mest (2004), Geologic evolution Mu¨nchen, T. B. McCord of PSI-Nw Winthrop, J.-P. Muller of University of Dao Vallis, Mars, Proc. Lunar Planet. Sci. Conf. 35th, abstract 1185. College London, J. B. Murray of The Open University Milton Keynes, Crown, D. A., L. F. Bleamaster, S. C. Mest, and L. T. Teneva (2005), Styles F. M. Neubauer of Universita¨t Ko¨ln, G. Neukum (Principal Investigator) and timing of volatile-driven activity in the eastern Hellas region of Mars, of FU Berlin, J. Oberst of DLR Berlin, G. G. Ori of IRSPS Pescara, Proc. Lunar Planet. Sci. Conf. 36th, abstract 2097. M. Pa¨tzold of Universita¨t Ko¨ln, P. Pinet of Observatoire de Midi-Pyre’ne’e’s Dohm, J. M., K. L. Tanaka, and T. M. Hare (2001), Geologic map of Toulouse, R. Pischel of DLR Berlin, F. Poulet of CNRS/IAS Orsay, the Thaumasia region of Mars, U.S. Geol. Surv. Geol. Invest. Ser., Map J. Raitala of University of Oulu, G. Schwarz of DLR Wessling, T. Spohn I-2650. of DLR Berlin, and S. W. Squyres of Cornell University Ithaca) are Garvin, J. B., J. J. Frawley, S. E. H. Sakimoto, and C. Schnetzler (2000), highly appreciated. The authors thank David Crown and James Dohm, Global geometric properties of Martian impact craters: An assessment who provided critical and very constructive reviews of the earlier version from Mars Orbiter Laser Altimeter (MOLA) digital elevation models, of the manuscript. Thanks are extended to Ron Greeley, who checked the Proc. Lunar Planet. Sci. Conf. 31st, abstract 1619. paper over and made important language corrections. Part of this research Goudy, C. L., and T. K. P. 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