Paleolakes and impact basins in southern Arabia Terra, including Meridiani Planum: Implications for the formation of hematite deposits on Mars

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. E12, 8075, doi: /2002je001993, 2003 Paleolakes and impact basins in southern Arabia Terra, including Meridiani Planum: Implications for the formation of hematite deposits on Mars Horton E. Newsom, 1 Charles A. Barber, 1 Trent M. Hare, 2 Rachel T. Schelble 1,3 Van A. Sutherland, 1 and William C. Feldman 4 Received 16 October 2002; revised 8 July 2003; accepted 7 August 2003; published 4 November [1] The hematite deposit in Meridiani Planum was selected for a Mars Exploration Rover (MER) landing site because water could be involved in the formation of hematite, and water is a key ingredient in the search for life. Our discovery of a chain of paleolake basins and channels along the southern margin of the hematite deposits in Meridiani Planum with the presence of the strongest hematite signature adjacent to a paleolake basin, supports the possible role of water in the formation of the hematite and the deposition of other layered materials in the region. The hematite may have formed by direct precipitation from lake water, as coatings precipitated from groundwater, or by oxidation of preexisting iron oxide minerals. The paleolake basins were fed by an extensive channel system, originating from an area larger than Texas and located south of the Schiaparelli impact basin. On the basis of stratigraphic relationships, the formation of channels in the region occurred over much of Mars history, from before the layered materials in Meridiani Planum were deposited until recently. The location of the paleolake basins and channels is connected with the impact cratering history of the region. The earliest structure identified in this study is an ancient circular multiringed basin ( km diameter) that underlies the entire Meridiani Planum region. The MER landing site is located on the buried northern rim of a later 150 km diameter crater. This crater is partially filled with layered deposits that contained a paleolake in its southern portion. INDEX TERMS: 6225 Planetology: Solar System Objects: Mars; 5415 Planetology: Solid Surface Planets: Erosion and weathering; 5470 Planetology: Solid Surface Planets: Surface materials and properties; 5407 Planetology: Solid Surface Planets: Atmospheres evolution; 1860 Hydrology: Runoff and streamflow; KEYWORDS: Mars, hematite, flubial channels, lacustrine basins, impact basins, Mars Exploration Rovers Citation: Newsom, H. E., C. A. Barber, T. M. Hare, R. T. Schelble, V. A. Sutherland, and W. C. Feldman, Paleolakes and impact basins in southern Arabia Terra, including Meridiani Planum: Implications for the formation of hematite deposits on Mars, J. Geophys. Res., 108(E12), 8075, doi: /2002je001993, Introduction 1 Institute of Meteoritics and Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA. 2 U.S. Geological Survey, Flagstaff, Arizona, USA. 3 Now at Department of Earth Sciences, University of Southern California, Los Angeles, California, USA. 4 Los Alamos National Laboratory, Los Alamos, New Mexico, USA. Copyright 2003 by the American Geophysical Union /03/2002JE [2] Christensen et al. [2000] reported the discovery of a hematite-rich deposit (10 15% hematite) on the surface of the smooth deposits in southern Arabia Terra now designated Meridiani Planum, utilizing data from the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor spacecraft (Figure 1a). Christensen et al. [2001] identified the material as gray crystalline hematite with a grain size ranging from 10 microns up to hundreds of microns. The hematite discovery led to intense interest in this region from an astrobiological standpoint and its selection as a prime landing site, first for the cancelled 2001 Mars lander and recently for the 2003 Mars Exploration Rover (MER) missions. The geological setting of the hematite deposit is important because hematite formation may require water which could involve environments suitable for preserving evidence of past life on Mars. New data from the neutron and gamma ray detectors on the Mars Odyssey spacecraft are presented that also indicates the presence of bound-water equivalent hydrogen in this region (Figure 1b). The data indicates the presence of enhanced water in the upper meter of the surface of some of the basins [Feldman et al., 2002, 2003], and supports the existence of the fluvial and lacustrine systems identified in this paper. [3] Bacterial remains may have been preserved if organisms were present during or subsequent to hematite formation in aqueous or hydrothermal environments in Meridiani Planum. Preservation of microbial communities by hematite has been documented by a number of studies focusing on terrestrial Banded Iron Formation (BIF) deposits in the 2.0 Ga Gunflint Iron Formation [Barghoorn and Tyler, ROV 16-1

2 ROV 16-2 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS

3 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS ROV 16-3 Figure 1. (continued) 1965; Tazaki et al., 1992; Allen et al., 2001]. Coccoidal, filamentous, sheath-like bacterial remnants, as well as fossil extracellular polymeric substances (EPS), have been identified in these studies. Kirkland et al. [2002, 2003] identified terrestrial ferricrete and other smooth hematite coated samples that also match the spectral signatures of the Martian hematite material. Microbial remains and abundant EPS in the ferricrete deposits from Shark Bay, Australia suggest a possible link between the formation of the hematite deposits on Mars and microbial activity [Westall and Kirkland, 2002]. [4] The Meridiani Planum area (Figure 1a) is located on a relatively level area of the ancient cratered terrain that interrupts the regional slope from the Schiaparelli basin and the highlands to the east down to the Iani Chaos. From the Iani Chaos, the terrain continues to slope north down to the major outflow channels in Xanthe Terra, including Ares Vallis, the landing site of the Pathfinder mission. The smooth deposits in this area were originally identified as sedimentary by Edgett and Parker [1997], suggesting formation of the hematite and layered materials in a lacustrine environment. The history of the drainage in the Uzboi- Ladon-Margaritifer mesoscale outflow system is described by Grant and Parker [2002]. Following the release of Mars Orbiter Laser Altimeter (MOLA) data the apparent lack of a major topographic depression containing the hematite-bearing deposits led to alternative hypotheses for the origin of the hematite deposits, including volcanism, hydrothermal transport and precipitation [e.g., Christensen et al., 2001]. Detailed crater counts by Lane et al. [2001, 2003] of Meridiani Planum provide evidence that the original surface containing the hematite is probably early Noachian, >3.8 Gy ago. However, the flattening of the mid-sized (1 m 63 m diameter) crater density curve suggests extensive erosion and/or deposition during most of Mars history. The small population of fresh craters <90 m diameter indicates a cessation of crater degradation in the last 10 My. [5] This paper describes the evidence for fluvial and lacustrine activity in the area surrounding the hematite deposits in the southern Arabia Terra region including Meridiani Planum. Utilizing images and topographic data we have mapped a system of channels and topographic depressions interpreted to be paleolake basins along the southern margin of the hematite deposit. The channels that may have supplied water to this system originated in a drainage area extending to the west and south of the Schiaparelli crater. The location of the channels and basins seems to be substantially controlled by the early cratering history of the region. We identified a series of troughs and elevated rings (Figure 1c) associated with a huge circular multiringed impact structure underlying the region [Newsom et al., 2003a, 2003b; Frey, 2003]. We show that this structure, centered to the north east of the hematite deposits, created topography favorable for the formation of paleolakes and shallow aquifers. The channel and paleolake system originated very early in Mars history on the basis of evidence such as the presence of large superimposed craters and the exhumation of channels from under layered deposits. However, there is evidence for more recent channel formation, including the presence of eroded tributary channels superimposed on the relatively fresh ejecta blanket from a 20 km diameter crater. 2. Large Early Multiringed Impact Basin in Southern Arabia Terra [6] A circular feature that appears to be an annular trough of a large impact structure was noticed by a student (R.T.S.) Figure 1. (opposite) (a) MOLA topography of the study area in southern Arabia Terra, including Meridani Planum (Lat 7 N to 5.6 S, Long E to 5.5 E). The area of paleolake basins is outlined and the various basin barriers are labeled as discussed in the text. The elevation scale is in meters. The inset shows the drainage area feeding the southern basins. The locations of channels on the western margins of the hematite are indicated with small red arrows. (b) Epithermal neutron data in counts per second from the Mars Odyssey Neutron Spectrometer for the hematite locality. The decreased epithermal neutron flux indicated by the blue color is thought to be due to the presence of hydrogen, possibly in the form of bound water in palagonite or smectites, and/or oxyhydroxides [Feldman et al., 2002]. The approximate area of the paleolake basins south of the hematite zone and the lowland area east of the hematite zone are outlined in red. Also shown is the outline of the 800 km diameter ring of the underlying multiringed impact structure. (c) Features of an early multiringed impact basin underlying the Meridiani Planum region are outlined. These features include an inner rim (diameter of 380 ± 60 km), and an annular trough, whose outer edge has a diameter of km. An even larger diameter annular trough (1600 km diameter) may be present further to the south. The elevation scale is in meters.

4 ROV 16-4 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS Figure 2. Detail of the southern paleolake basins and channels in the study area. The locations of high-resolution images Figures 3 and 4 are shown. Also shown is the approximate location of the prime Meridiani Planum landing site ellipse for the 2003 MER mission. Elevation scale same as Figure 1c. assisting with this study (Figure 1c). The possible significance of this feature for the origin of the hematite was discussed at the fourth MER landing site conference in January 2003 [Newsom et al., 2003a]. Evidence for a circular structure in this area has been noticed before but not extensively described in the literature (H. V. Frey, Goddard Space Flight Center, personnel communication, 2003; M. D. Lane, Planetary Science Institute, personnel communication, 2003). Currently several lines of evidence suggest that the topographic underpinning for southern Arabia Terra was created by this very ancient large impact structure [Newsom et al., 2003a; Frey, 2003]. The topographic traces of this feature can be seen as a central basin defined by a roughly circular pattern of elevated terrain that is surrounded by an annular trough with a maximum diameter of 800 km. This trough includes the lowland area east of the hematite deposit and the chain of basins along the southern margin of the hematite (Figures 1a and 2). On the basis of detrended MOLA data an additional outer trough may be present at a radius of 800 km. The outer trough appears to localize channels in the vicinity of the crater Mädler. The annular trough and central depression are comparable to the features of the Cassini impact basin, which are easily visible on global MOLA topographic maps. An additional piece of evidence that supports the presence of an ancient large impact basin is the presence of a strong negative magnetic anomaly in the center of the basin [Acuña et al., 1999]. 3. Distribution of Current Channel Networks, Drainages, and Basins [7] Channel systems leading to the southern boundary of the hematite deposit can be observed originating from a large highlands region that extends to the south-east of the hematite regions for hundreds of kilometers (Figure 1a, inset). The origin of the water that formed these channel systems is uncertain. The widely distributed source regions, valley morphometry, and the lack of chaos areas, and lack of evidence for hydrothermal processes upstream support an origin from rain, snowmelt, and near-surface aquifers [e.g., Grant and Parker, 2002; Craddock and Howard, 2002]. [8] The channels originating from the south appear on Viking images to terminate near the boundary with the hematite deposit. On the basis of the Viking data, Edgett and Parker [1997] identified this boundary as the edge of an ocean or large body of water. Early in our study another student (C.A.B.) noticed that these highly visible channels flowed into a system of paleolakes and channels along this boundary. We have used high-resolution Mars Orbiter Camera (MOC) and Thermal Emission Infrared Spectrometer (THEMIS) images and the MOLA topography to map and confirm the existence of these features. The general location of the southern boundary basins and channels is controlled by the location of the first annular trough of the early multiringed impact basin. This channel system currently appears to feed westward into the unnamed 150 km diameter crater and then further east toward Iani Chaos. The drainage area extends to the south of Schiaparelli basin and includes Evros Vallis (Figure 1a inset). The drainage area abuts the Flaugergues Drainage Basin that drains into Schiaparelli [Kramer et al., 2003]. The nexus of the drainage systems in this area is the crater Mädler. Recent THEMIS images (I and V , not shown) show evidence that water was supplied to Mädler from a channel system (not mapped in this study) that extends south to at least the eastern rim of Newcomb crater, and probably included water from the Evros valley system. From Mädler the water flowed north into the system leading to the southern boundary of Meridiani Planum. However, there may have been another exit from the west side of Mädler leading to the channels further south of the hematite area. Our qualitative observations of the large abundance of channels in the drainage area (Figure 1a) feeding the channels on the southern boundary of Meridiani Planum are supported by the results of a quantitative study of impact crater floor morphologies by Forsberg-Taylor and Howard [2003], who detailed the unique abundance of smooth-floored craters in the same area. A more detailed history of the fluvial systems and the source or sources of water in this area will be possible when complete THEMIS imagery becomes available. [9] The total area of the drainage region shown in the inset in Figure 1a is 580,000 km 2, approximately the size of Texas, and does not include the possible drainage area between the craters Mädler and Newcomb. The area for the northwestern portion of the drainage area closest to the hematite area is 200,000 km 2, about the size of Kansas. The area for the Evros Vallis portion is 380,000 km 2. There is also a large system of early highland valleys (not mapped in detail for this study) leading from the Schiaparelli rim area into the lowland region east of the hematite area. If flooded, this lowland would currently drain to the north. [10] The following sections describe the paleolake basins and connecting channel systems surrounding the hematite areas Southern Hematite Margin [11] Fluvial and lacustrine environments along the southern boundary of the hematite deposit are currently con-

5 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS ROV 16-5 Figure 3. Evidence from MOC images of erosional and depositional features due to fluid flow on the floor of the 150 km diameter crater. All scale bars are one kilometer in length. Arrows show the direction of flow consistent with the MOLA topography. a., b. Floor of the channel showing eroded craters and lineated terrain similar to terrain on the floor of Gusev Crater (Figure 5). c. Aligned mesas probably due to fluvial erosion and deposition on the south west floor of the 150 km crater where the flow direction is north east to south west. d. Part of a very fresh tributary channel leading from the layered materials into the main channel in the 150 km diameter crater. A twin tipped arrow denotes the breadth of the channel, which is eroded into ejecta from a 20 km diameter crater (Figure 7). MOC images (a. M , b. M , c. M , d. M ) NASA JPL/MSSS. strained by topographic barriers that serve to define three linked basins, including the southern part of the unnamed 150 km crater (Figures 1a, 3a 3d, and 4a). These basins are progressively lower from east to west, and evidence exists for fluid flow linking the basins through channels that have partially breached the barriers. [12] The western basin of the southern hematite margin is located within the 150 km diameter crater where a channel leads south around the lobe of hematite-rich material that covers the northern portion of the crater. High-resolution MOC images of the floor of the channel (Figures 3a 3c) show characteristic evidence of fluvial erosion of ancient cratered terrain, very similar to images from the floor of Gusev crater (e.g., Figure 5). THEMIS images (Figure 6) beautifully show the location of the channel in the 150 km diameter crater leading up to the exit where a double crater has breached the rim of the 150 km diameter crater. Possible evidence for episodic large floods includes the wide multiple channels (Figure 6) at the north end of the double crater, and the lack of small sinuous channels in the three main basins. Evidence of recent fluid flow into the western basin includes the erosion of ejecta blanket material (Figures 3d and 4a) from a 20 km diameter crater superimposed on the hematite deposit (Figure 7). The western basin is constrained by a barrier (A, Lat S, Long E, 1760 m elevation) where a channel leads to a smaller double crater (19 km diameter) superimposed on the rim of the 150 km diameter crater. The area of the western paleolake basin is approximately 9000 km 2, with an average depth of 40 m and a volume of approximately 1100 km 3. [13] The central basin on the southern margin of the hematite is constrained by a barrier (B, Figure 1a, Lat S, Long E, 1620 m elevation), which was originally formed by the rim of the l50 km diameter crater to the west. The current area of the central basin is somewhat less than 2000 km 2, with an average depth of 40 m and a volume of approximately 80 km 3. At the point where several channels flow into this central basin from the south, evidence exists for possible shorelines. The elevation of the possible shorelines are consistent with the elevation of the barrier (B, Figure 1a) that constrains the height of this paleolake. However, exposure of layered deposits instead of shorelines cannot be ruled out (Figure 4b). [14] In the eastern basin of the southern hematite margin, water flowing in from the south is constrained by a barrier (labeled C Figure 1a, Lat S, Long E, 1450 m elevation) that is formed by a relatively young crater (15 km diameter) to the south, and a still younger crater with a distinct ejecta pedestal (6 km in diameter) to the north. There is evidence for the formation of a channel between these two craters (Figure 4c). Evidence for possible shorelines or eroded layered deposits is present on the floor of the basin (Figures 4d and 4e). The elevation of the shorelines in Figure 4e ( 1495 m) is consistent with the elevation of the barrier C (Figure 1a) that constrains this possible paleolake. Figure 4f shows eroded deposits where a major channel enters the basin. The current area of the eastern basin is about 20,000 km 2 with an average depth of about 100 m and a volume of approximately 2000 km 3. The shoreline elevation for this eastern lake may have been lower earlier in Martian history prior to the formation of the barriers due to the 15 km diameter crater and then the 6 km diameter crater. For example, some channels entering this basin (Figure 2) are eroded down to an elevation of 1615 m, the same elevation as the next barrier further down-stream (B, Figure 1a, Lat S, Long E, 1620 m) Eastern Hematite Margin [15] A broad lowland area is present along the eastern boundary of the hematite deposit. This lowland is approximately 80 km wide and extends for 450 kilometers to the north. There are at least two areas where water in this lowland may have escaped to the west (D, Figure 1, Lat N, Long. 1.60E, elevation 1220 m and E, Figure 1a, Lat N, Long.1.97 E, elevation 1235 m). This eastern lowland has a heavily cratered topography with an extremely gentle gradient ( 30 m over 450 km, slope <0.01%). This lowland area appears to be fed by channels (not mapped) from the vicinity of Schiaparelli basin and the older basin just to the south of Schiaparelli. A smaller basin separates the

6 ROV 16-6 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS lowland area from the chain of basins along the southern boundary of the hematite which is constrained by barriers on the west and east (F, Figure 1a, Lat S, Long E, elevation 1220 m, and G, Figure 1a, Lat S, Long E, elevation 1260 m). The eastern barrier includes an impact crater (9 km diameter) that may have recently created a higher dam between the two drainages. These barriers prevent any recent flow of water eastward or westward between the eastern hematite margin lowland area and the southern hematite margin basin system Northern and Western Hematite Margin [16] The northern boundary of the hematite deposit is characterized by a poorly integrated system of basins and channels that drains westward, and is located on top of the layered materials of Meridiani Planum. These channels are at a lower elevation than the potential sources of surface or groundwater to the east. The hematite-rich surface along the northern margin of the hematite area is patchy in appearance, possibly forming remnants of an earlier more extensive deposit that was removed due to fluvial erosion [Christensen et al., 2001]. However, the poorly defined channels here may reflect the possibility that channel formation in this area occurred during the deposition of the layered materials (e.g., Figure 8), and the channels are partly buried by the latest episode of accumulation. [17] Along the northwestern boundary of the hematite deposit, a well defined channel system has eroded the edge of layered deposits covered by the hematite material (Figure 1a). This channel system is fed from the east and northeast, including the lowland region, and by the series of interconnected basins along the northern boundary of the hematite deposit described in the previous paragraph. 4. Regional Geologic History [18] The complex geologic history of southern Arabia Terra including Meridiani Planum resulted from processes over a time period greater than 4 billion years [Lane et al., 2001, 2003]. Crust formation was followed by the formation of an ancient multiringed impact basin greater than 800 km in diameter (Figure 1c). The subsequent formation of layered deposits throughout southern Arabia Terra occurred over an extended period of time [Malin and Edgett, 2000; Edgett and Malin, 2002; Chapman and Tanaka, 2002; Hynek et al., 2002]. The nature of these layered deposits is not clear but may include basin ejecta, air-fall dust, volcaniclastic, and sedimentary deposits, with the hematite-bearing deposits representing one of the highest Figure 4. (opposite) Additional evidence from MOC images for fluvial and lacustrine features in the channel and paleolake basin system. All scale bars are one kilometer in length. a. Tributary channels are shown leading down from the layered materials into the 150 km diameter crater, just north of Figure 3d, and the corresponding MOLA topographic profile. The channels have eroded the ejecta from a 20 km diameter crater (Figure 7). The deeper fresher looking channel has no apparent impact craters on the surface. b. Possible shorelines in the central basin. c. Barrier C damming the eastern paleolake basin is breached by the channel in this image whose breadth is denoted with a twin tipped arrow. The channel lies between the rim of an older 15 km diameter crater to the south and the pedestal ejecta of a younger 6 km diameter crater to the north. d. Possible shorelines at 1600 m elevation on the south shore of the eastern paleolake basin. e. An embayment of the northern edge of the eastern basin into eroded layered material. An outlier of possibly reworked hematite on the floor of the embayment exhibits possible shorelines. f. Deposits eroded into dune forms on the floor of the eastern basin where a major channel enters the basin. MOC images (a. M , b. mosaic of AB107704, M , c. M , d. M , e. M , f. M ) NASA JPL/MSSS.

7 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS ROV 16-7 filling the crater is approximately 4000 km 3 on the basis of a thickness ranging from 200 m up to 500 m in the northern portion of the 150 km diameter crater. This volume of material could be supplied by the uniform erosion of 4 m over the mapped drainage area in Figure 1a, which is much less than the 200 m of erosion suggested by Craddock and Howard [2002] for the Sinus Sabaeus and Margaritifer Sinus regions. [20] The layered deposits in Meridiani Planum that are the hardest to explain by sedimentation are those highest in elevation. Some of the areas containing the hematite signature are 150 m above the elevation of the present barrier constraining the large eastern basin on the southern margin of the hematite region. Evidence from the topographic profiles across the breaches in the barriers suggests that the elevation of the barriers may have been as much as 50 m higher prior to erosion. A 50 m increase in water level in the southern basins would flood a larger portion of the hematite-rich area, but some areas would still be 100 m above the Figure 5. This distinctively eroded terrain in the main channel on the floor of Gusev crater is similar to terrain on the floors of the paleolake basins bordering the hematite deposits (e.g., Figures 3a 3c). Water flowed from the southeast to the northwest in this location just north of the entrance of Ma dim Vallis into Gusev basin. MOC image M NASA JPL/MSSS. layers. Preliminary studies of the deposits include work by Edgett [2002], Chapman and Tanaka [2002], Hynek et al. [2002], and Edgett and Malin [2002]. The complex layered materials include multiple light and dark units with different thermal inertias, suggesting different compositions or degree of lithification. [19] A sedimentary origin of some of the layered materials in Meridiani Planum is likely because of the presence of topographic barriers and the extensive erosion in the upstream region. The distinctive presence of smooth-floored impact craters documented quantitatively by Forsberg- Taylor and Howard [2003] in the source region for the channels, and the evidence for channel systems being exhumed from under the ancient layered deposits, is consistent with substantial erosion and fluvial transport and deposition of sediment. The most likely candidate for a sedimentary deposit is the lobe of material that fills the northern part of the 150 km diameter crater. Also, the layered materials just to the north of the 150 km diameter crater rim could easily be sedimentary, as this region is dammed by several large craters (21 km, 19 km, and 31 km in diameter) now largely buried but still visible on the western edge of the area covered by the hematite deposits. Some layering in these deposits is visible along the western edge where it appears to be eroded (e.g., MOC E , not shown). The volume of the partly layered material Figure 6. THEMIS day (a) and night (b) infrared images of the southwestern quadrant of the 150 km diameter crater. The outlet of the western paleolake basin is through a smaller double crater that breaches the rim of the basin. The floor of the double crater contains an outlier of hematite that may be remobilized. The day IR image (a) shows the outflow from the double crater through several parallel eroded channels (parallel to the arrow), while the night IR image (b) shows a distinct channel eroded into layered material on the floor of the 150 km crater leading to the double crater. The channel is to the left of and parallel to the arrow on that image. THEMIS images (day IR I and night IR I ) NASA JPL/Arizona State University.

8 ROV 16-8 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS shorelines of the basins, requiring a higher barrier than is readily apparent in the available images. Perhaps very early geomorphic barriers associated with the multiringed impact basin, and now eroded, could have led to a sedimentary origin for much of the layered materials in Meridiani Planum. Although this is very speculative, there is also evidence that layered materials were more widespread to the west and north in the past [e.g., Malin and Edgett, 2001; Edgett and Malin, 2002]. [21] Another geological process that could raise the water level in the basins is the formation of ice jams or dams. Ice jams are responsible for the flooding of major rivers in northern Canada and Siberia on a regular basis [Beltaos and Prowse, 2001]. On the earth ice dammed lakes are caused by the presence of continental ice sheets, continental shelf ice sheets, and ice jams on rivers. The flat nature of the Meridiani Planum area would be conducive to ice dams blocking the narrow outlets of the basins leading to repeated flooding of the areas with the strongest hematite signatures. Although we have not identified evidence for such dams in the high-resolution imagery currently available, small ice dams or jams could provide topographic barriers with little residual geomorphic evidence. The low gravity of Mars could allow for relatively tall ice dams to cause flooding without the need for massive ice caps or glaciers that would have left more distinctive landforms. [22] The formation of the current system of basins and fluvial channels occurred over a long period of time that included the deposition of the layered materials. For example, channels being exhumed from beneath the layered materials can be seen on the northern edge of Meridiani Planum (Figure 8). Extensive denudation has been observed throughout this southern Arabia Terra region [Hynek and Phillips, 2001]. Substantial resurfacing events extended into the Hesperian, and chaos formation extended into the Amazonian and possibly into the recent history [Grant and Parker, 2002]. Cratering events and ice dams could have temporarily formed or increased the water level of paleolakes. For example, at the present time the channels that extend into the eastern and central basins are incised to elevations well below the current elevation of the barriers (Figure 2). This implies that earlier in Martian history the water level was at a lower base level before the late cratering events created new barriers at higher elevations (e.g., barriers C and G, Figure 2). [23] Large-scale episodic floods due to erosional breakouts from lakes or the failure of ice dams may have been a major contributor to the formation of the main channel systems. The evidence for large-scale floods includes broad or multiple outflow channels from the basins along the southern margin of Meridiani Planum. The presence of ice is consistent with another feature of the channel system in this area of Mars, the broad highly incised channel systems. Figure 7. (opposite) THEMIS infrared (a) and visible (b) images of the south east portion of the 150 km diameter crater and the 20 km diameter crater that has excavated the layered deposits filling the larger crater. The main channel from the basins to the east leads south west (open white arrows). The 20 km diameter crater has a multiple layer sinuous rampart ejecta blanket with a prominent sinuous inner layer (black arrows) and a less well defined sinuous outer ejecta layer (white arrows) that overlaps the floor of the 150 km diameter crater on the south east side. Tributary channels (e.g., open black arrows) eroding this outer ejecta layer originates at the base of the sinuous inner ejecta blanket. The locations of higher resolution MOC images (Figures 3d and 4a) of the channels are shown. THEMIS images (day IR I , I , and visible V ) NASA JPL/Arizona State University.

9 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS ROV 16-9 Figure 8. Contact between layered materials representing a hematite outlier with the floor of the northern basin (Figure 1a) in the center of the multiringed impact structure (THEMIS visible image V ). Erosion of the layered material is exhuming buried channels (white arrows) on the floor of the basin. THEMIS image NASA JPL/Arizona State University. The channel systems are deeply incised where they enter into the middle and eastern basins (e.g., Figure 2). River-ice break-up events on the Earth leading to ice scour can be extremely erosive, even more so than high-velocity water scour, and may be responsible for the most severe erosion in northern rivers [Milburn and Prowse, 2000]. This process could enhance erosion of some channels on Mars. [24] The relationship between the hematite and the southern channel system has been used by Hynek et al. [2002] to argue for a young age for the hematite deposits, in contrast to the crater counting results of Lane et al. [2001, 2003]. In particular, Hynek et al. [2002] point to the relationship between the hematite deposits and the underlying material in Figure 4e to suggest that the hematite overlies the channel system and is therefore very young. We agree that there is evidence for channel formation before the deposition of the layered deposits (e.g., Figure 8), but the detailed relationships still support an early origin for the hematite deposits along the southern boundary. The image in question (Figure 4e) shows a deposit on the floor of an embayment of the eastern basin. This deposit contains one of the southern-most outliers of hematite and is located at one of the lowest elevations for hematite outside of the 150 km crater. The lineations parallel to the edge of the hematitebearing deposits are interpreted by Hynek et al. [2002] to represent original depositional layering. An alternative interpretation of Figure 4e, however, is that the southernmost hematite-bearing materials lap onto the cratered and dissected terrain and represent reworked materials on the floor of an inlet from the large eastern basin (Figure 1a). A similar patch of reworked hematite-bearing material is present on the floor of the double crater that breaches the 150 km diameter crater and which represents the outlet for the western basin in the large crater (Figure 6). In addition, there is evidence that much of the erosion of the layered materials occurred before the recent resurfacing episode [Lane et al., 2001, 2003]. The channel on the southern floor of the 150 km diameter channel revealed in the THEMIS data (Figure 6) is parallel to the edge of the layered material to the north, consistent with formation of this channel after deposition of the layered materials. In addition, Edgett and Malin [2002] show that the layered deposits include interbedded impact craters and unconformities, consistent with an old age. If the hematite is a young volcanic deposit on top of an old layered sequence it would be very hard to explain the exact correspondence between the hematite and the older layered deposits as mapped by Hynek et al. [2002] (except for the material easily explained as re-worked hematite in the channels). We therefore conclude that the observed stratigraphic relationships (e.g., in Figures 4d and 8) are more consistent with an ancient origin for the hematite deposit and a close association with the underlying layered deposits, even if the hematite layer is of volcanic origin. 5. Distribution of Hematite-Rich Surface Materials and Hydrogen-Rich Materials in the Greater Meridiani Planum Region [25] The scale and character of the hematite-rich surface is unique. The only other sizable occurrence is in the Aram basin with a few outcrops in Vallis Marineris [Christensen et al., 2001]. The crater counts of abundant degraded craters observed in the high-resolution MOC images suggests the preservation of an ancient terrain (4 GY ago) that has been buried and exhumed. However, the area is extremely smooth and was resurfaced or exhumed in the last 10 million years or so on the basis of the abundance of the freshest craters [Lane et al., 2001, 2003]. Therefore the possibility of more recent surficial processes contributing to the hematite signature or affecting a thin surface veneer cannot be ruled out. [26] The recently released data from the Mars Odyssey Gamma Ray experiment, in particular the neutron spectrometer subsystem [Feldman et al., 2002, 2003; Mitrofanov et al., 2002], provides additional confirmation of the possible role of water in this area of Mars. The presence of a deficit in epithermal neutrons in this equatorial region is thought to represent evidence for the presence of hydrated minerals such as clays in the uppermost regolith. Support for the presence of bound water in the Martian regolith also comes from infrared spectroscopy and data from Viking, and Pathfinder [Houck et al., 1973; Anderson and Tice, 1979; Foley et al., 2001]. The lack of a general explanation for the large neutron deficit localized in Terra Sabaeus and eastern Arabia Terra and its antipodal region suggests that it is premature to place too much importance on the neutron data as evidence of fluvial or lacustrine activity. However, the correlations of the neutron signatures with geologic features could be significant and should be explored in the future. Enhanced neutron absorption (Figure 1b) is present in the eastern lowland area, and possibly in the areas of the eastern basin (which is bordered on the north by the strongest hematite signature) and the basin to the north of the hematite area. The actual resolution or pixel size of the neutron data (somewhat better than 400 km due to atmospheric focusing) is greater than the pixel size in Figure 1b (119 km), but the low actual resolution will only serve to lower the contrast if a strong signature is present in a small area. The neutron data and hydrogen gamma ray data for these areas are consistent with a hydrogen-rich layer buried beneath a cm thick hydrogen-poor layer [Boynton et al., 2002]. The H 2 O- equivalent hydrogen content in the Meridiani Planum region indicated by the neutron data is 6.5 ± 1.0 wt% [Feldman et al., 2003]. The source of the hydrous mineral signature

10 ROV NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS could be lacustrine sediments associated with the basins. The lacustrine deposits could include, for example, alteration phases produced by impact cratering [Newsom, 1980; Newsom et al., 2001] or volcanic hydrothermal processes [Gulick, 1998]. The water indicated by the neutron signature could have been sequestered in this material at any time during or following the formation of the deposits. 6. Evidence for Recent Fluvial and Lacustrine Activity [27] Evidence for recent climate change and fluvial activity on Mars during the Amazonian is slowly accumulating [Mustard et al., 2001; Cabrol et al., 2001]. However, only limited evidence suggests that relatively recent activity has occurred in the Meridiani area fluvial systems. Several high-resolution MOC images show fresh looking tributary channels (Figures 3d and 4a) that drain from the layered crater fill into the main channel on the right side of Figure 7b. This main channel is located between the wall of the 150 km diameter crater on the south east and a scarp that probably represents the layered crater fill on the north west. The main channel eventually leads down to the floor of the unnamed 150 km diameter crater. As shown in Figure 7 these channels eroded the outer sinuous rampart ejecta blanket from the relatively fresh 20 km diameter crater. The channels appear to originate from below the upper sinuous ejecta blanket of the 20 km diameter crater, and may reflect episodic flow of groundwater from springs draining the layered materials. A distinctive example in Figure 4a shows two separate channels, also located on the ejecta blanket of the 20 km diameter crater, where the shallower one is clearly older and more heavily cratered, while the deeper one is essentially free of fresh craters. The main channel in this region also appears to have a very low abundance of fresh craters. [28] A recent resurfacing or exhumation event by nonaqueous processes effecting the surface of the hematite deposits has been proposed by Lane et al. [2001, 2003], and dated as young as a few million years. The low abundance of fresh craters in the tributary channels is qualitatively similar to the fresh crater abundance on the surface of the hematite-rich area described by Lane et al. [2001, 2003], suggesting that substantial flows may have occurred since that resurfacing event. In addition, the distinctive channel features in Figure 3d are unlikely to have survived the resurfacing and/or exhumation event by processes such as aeolian stripping if they were produced early in Mars history. [29] Shorelines are notoriously difficult to identify and can be easily confused with exposures of flat layered deposits. Beginning in the eastern basin, several images (Figures 4d and 4e) may show evidence for possible shorelines that look distinctly different from the exposures of layered terrain. These shorelines are approximately at the elevation of barrier C that dams this basin ( 1450 m). Barrier C is apparently breached by a channel eroding the ejecta blanket of the crater that partially makes up the barrier C (Figure 4c). Additional shorelines may be present on the floor of the central basin (Figure 4b). These shorelines may represent a small residual lake in this basin because of their low elevation ( 1650 m) relative to barrier B( 1550 m). Further investigation of the topography and elevation of possible shorelines is needed to establish their true identity. 7. Origin of the Hematite-Rich Deposits [30] Christensen et al. [2000] proposed a number of mechanisms for the formation of the hematite deposits on Mars that include: 1. direct precipitation from oxygenated, iron-rich water in lakes, 2. precipitation from groundwater due to leaching of iron-rich sediments, 3. surface weathering and formation of Fe-rich coatings, 4. precipitation from hydrothermal fluids, and 5. thermal oxidation of magnetiterich lavas. Four of the five proposed processes involve water. The geological constraints from this study support the involvement of low temperature aqueous processes. The occurrences of hematite in Aram basin and in Vallis Marineris could also be consistent with an origin for hematite that involves the nearby presence of surface water or groundwater. Detailed data about the mineralogy of the hematite from the MER rover experiments will be needed for a more definitive answer. [31] Each mechanism can be considered in light of the evidence presented above: [32] 1) Direct deposition of the hematite associated with the paleolake basins is certainly plausible. The strongest hematite signature is located on the northern shore of the eastern basin at an elevation of 1350 m to 1450 m (Figure 1a). This elevation is just above the current elevation of barrier C at about 1450 m. Deposition of the hematite as a precipitate could have occurred in a paleolake prior to erosion of the barrier to the present level. The formation of ice jams or dams could also raise the lake level. [33] 2) Precipitation of hematite from groundwater under ambient conditions may also be consistent with the low topographic gradients in the region that are conducive to the formation and recharge of shallow aquifers. Evidence for groundwater in the region is consistent with the existence of the eastern portion of Iani chaos located 300 km west of the hematite region. This chaotic terrain may have formed due to groundwater flowing through the highlands mega regolith from the vicinity of the Margaritifer Basin as late as mid- Hesperian times [Grant and Parker, 2002]. In the same fashion the eastern portion of the Iani chaos could have been formed by groundwater from the vicinity of the paleolakes in the hematite region. The elevation of the paleolakes surrounding the hematite area ranges from 1700 to 1500 m, while the elevation of the Iani chaos region is between 2000 to 4000 m. [34] 3) On the basis of the proximity of the basins to the hematite areas, repeated wetting and leaching to form surface coatings in a nearshore environment is also a viable hypothesis for the formation of the hematite deposits. The TES thermal infrared signature of the hematite site is similar to the signature of modern ferricrete deposits in a dry desert environment inland from Shark Bay in Australia, according to data from Kirkland et al. [2002] and Westall and Kirkland [2002]. The ferricrete is reported to form from groundwater leaching of iron-rich minerals under reducing acidic conditions in the soil followed by precipitation of colloidal oxy-hydroxides near the surface under more oxidizing and less acidic conditions. This process could have

11 NEWSOM ET AL.: SOUTHERN ARABIA TERRA, MARS ROV occurred on Mars in the presence of groundwater from shallow aquifers that apparently existed in the Meridiani Planum region. The leaching process may proceed more rapidly if the underlying layered deposits are porous sedimentary fill or air-fall deposits. The position of the strongest hematite signature just above the paleolake level may have prevented the hematite from being eroded or buried by material from late depositional events. The Aram basin hematite deposits are also located on a slightly elevated area within that basin. [35] 4) Hydrothermal processes are less likely to be responsible for the formation of the hematite because of the lack of heat sources. Even though a high temperature hydrothermal origin for the hematite occurrence in Aram basin due to an impact has been suggested [e.g., Catling and Moore, 2000], the formation of the layered deposits in Meridiani Planum occurred long after the formation of the large multiringed impact structure described in this study. In the case of Meridiani Planum the deposition of the layered materials and the formation of the hematite would have to occur within a few million years of the multiringed impact basin formation while the thermal effects of the impact were still present [e.g., Newsom et al., 2001]. Unfortunately, there is strong evidence for an extended period of deposition between the impact and the hematite formation that includes the channels being exhumed in the center of the multiringed impact basin (Figure 8) and evidence for unconformities and interbedded impact craters in the layered materials observed by Edgett and Malin [2002]. One alternative that cannot yet be completely ruled out is the transport and deposition of hydrothermally altered material containing hematite as a surficial sedimentary deposit on Meridiani Planum. [36] Formation of the hematite by hydrothermal processes associated with a young hot volcanic ash deposit was suggested by Hynek et al. [2002]. Although geological relationships do not support a young age for the hematite deposit, an early volcanic deposit could have been subject to hydrothermal processes given the availability of water in the region. Evidence for volcanic constructs in the region is limited, but the lack of volcanic constructs may not be surprising given the probable ancient age of the hematite deposits and the evidence for extensive erosion in the area. Two areas with possible eroded volcanic structures include a highly eroded and cratered massif south of Schiaparelli basin (THEMIS day IR I , I , Latitude 9.46 N, Longitude 16.4 E) and a small symmetrical cone to the north west of the hematite region (THEMIS image V , Latitude 8.58 N, Longitude E). [37] 5) Thermal oxidation of magnetite-rich lavas is not very likely for several reasons. The layered materials containing the hematite do not appear to consist of lava flows, and this theory does not explain the correlation between the basins and the strongest hematite signature. 8. Conclusions [38] This study documents the important role of water in the geologic evolution of the Meridiani Planum region and the origin of the hematite deposits. Evidence has been uncovered for the presence of an extensive series of palelakes and channels surrounding the hematite deposits. The shallow topographic gradient in the region and presence of paleolake basins also supports the existence of an extensive regional aquifer. Observations from MOC, THEMIS, and MOLA data suggest that most of the aqueous activity in the region occurred early in Mars history, but some activity extended into recent history. These paleolakes were fed by a system of channels covering a large area of Mars south of the Schiaparelli basin. Evidence for recent fluvial activity includes the apparent reworking of the hematite in at least two areas and channel formation on relatively recent ejecta deposits. Epithermal neutron flux depletion in at least one of the basins is consistent with the presence of enhanced concentrations of water trapped in hydrous minerals in these areas [e.g., Feldman et al., 2002, 2003] and could also reflect recent fluvial activity. [39] The unique nature of the Meridiani Planum region may be connected with an ancient (>4.0 Gy) multiringed impact structure underlying the region. This structure extends from a center on the northern edge of Meridiani Planum at least 800 km to the south. The formation of the early large multiringed impact basin and associated circumferential troughs created a series of topographic barriers and low areas that localized the drainage from a large area of Mars and resulted in the formation of paleolake basins in the vicinity of Meridiani Planum. The evidence for topographic barriers associated with the multiringed impact structure suggests that at least some of the ancient layered materials that make up Meridiani Planum could be sedimentary in origin. [40] Water from the extensive fluvial and lacustrine system was probably connected with the deposition of the layered materials in the Meridiani Planum region and the formation of the hematite. The presence of the strongest hematite signature adjacent to a paleolake basin supports the role of water in the formation of the hematite. Water could have been responsible for the formation of the hematite in several ways, including deposition in lakes, and precipitation as surface coatings due to wetting by groundwater. Hydrothermal alteration associated with an early hot volcanic ash layer is less likely, but cannot be ruled out. However, formation of the hematite due to hydrothermal processes associated with the large multiringed impact structure is unlikely based on stratigraphic constraints. [41] The evidence for a strong association between nearby water and the hematite deposits, coupled with the potential for finding life preserved by hematite, continues to suggest the importance of this area for exploration. Results from the MER rover on the mineralogy of the hematite deposits will certainly help us understand the role of water in this interesting location on Mars. [42] Acknowledgments. We thank the MOC, TES, MOLA, and THEMIS teams for the exciting data used in this study. We also appreciated comments from Larry Crumpler and careful reviews from Nathalie Cabrol and an anonymous reviewer. This work was partially funded by NASA Planetary Geology and Geophysics grants NAG and NAG (H.E. Newsom P.I.), and the Institute of Meteoritics. References Acuña, M. H., et al., Global distribution of crustal magnetization discovered by the Mars Global Surveyor MAG/ER experiment, Science, 284, , Allen, C. C., F. Westall, and R. T. Schelble, Importance of a Martian hematite site for astrobiology, Astrobiology, 1, , Anderson, D. M., and A. R. Tice, Analysis of water in the Martian regolith, J. Mol. Evol., 14(1 3), 33 38, 1979.