Meridiani Planum Region Research Articles

Aqueous sulfate contributions in terrestrial basaltic catchments: Implications for understanding sulfate sources and transport in Meridiani Planum, Mars

The Meridiani Planum region on Mars has extensive sulfate-rich sedimentary deposits (∼20 wt% SO42−) that are hypothesized to have formed from regional groundwater upwelling that led to the precipitation of secondary Fe-, Mg-, Ca-sulfate minerals and cementation of basaltic sediments. However, the primary source of sulfur (S) for these abundant secondary sulfate minerals is unclear. Therefore, in this study the contributions of volcanic S via surface water and groundwater were investigated in the terrestrial basaltic analogs of Hawaii and Iceland to determine the importance of active volcanism and climate on S cycling as well as the resulting timescale of aqueous activity in the Meridiani Planum region. SO42− fluxes (contributions) were calculated in metric tons/yr using historical data from online repositories and normalized to the catchment area to determine the SO42− load in metric tons/yr/km2. Our results show that the SO42− load is greatly affected by climate, typically ranging from ∼7.3 to 170 metric tons/yr/km2 under wetter conditions and ∼ 2.6 to 43 metric tons/yr/km2 under dry conditions. Active S degassing and accompanying S-rich mineralization from current hydrothermal activity greatly increased the SO42− loads (∼2.8 to 170 metric tons/yr/km2) compared to non-active catchments (2.6 to 13 metric tons/yr/km2). Younger basaltic bedrock with greater permeability and groundwater-rock interactions was also found to be important, resulting in higher SO42− loads (∼26 to 170 metric tons/yr/km2) compared to older, less permeable catchments (∼2.6 to 12 metric tons/yr/km2). Based on these terrestrial SO42− loads in Hawaii and Iceland, we calculated a range of possible loads and timescales of SO42− transport in Meridiani Planum under variable environmental conditions. Results show that the smallest SO42− loads and longest timescales would occur in Meridiani under dry, non-volcanically active conditions, typically requiring ∼16 to 65 Ma of an active aqueous system, as in the older catchments of Hawaii and Iceland. Conversely, the largest SO42− fluxes and shortest timescales would occur under wet, volcanically active conditions, requiring ∼1.0 to 6.9 Ma, as in the younger catchments of Hawaii and Iceland. Our results suggest that moderately wet conditions with some active hydrothermal S input would be needed to transport and deposit the equivalent mass of SO42− currently present in the sulfate-rich deposits of Meridiani Planum.

A Spectral Comparison of Jarosites Using Techniques Relevant to the Robotic Exploration of Biosignatures on Mars.

The acidic sulfate-rich waters of the Meridiani Planum region were potentially a habitable environment for iron-oxidizing bacteria on ancient Mars. If life existed in this ancient martian environment, jarosite minerals precipitating in these waters may record evidence of this biological activity. Since the Meridiani jarosite is thermodynamically stable at the martian surface, any biosignatures preserved in the jarosites may be readily available for analysis in the current surface sediments during the ongoing robotic exploration of Mars. However, thermal decomposition experiments indicate that organic compound detection of sediments containing jarosite may be challenging when using pyrolysis experiments; the instrument commonly used to assess organic matter in martian samples. So, here, we assess if the biogenicity of the Meridiani-type jarosites can be determined using complimentary spectroscopic techniques also utilized during the robotic exploration of Mars, including the upcoming ExoMars2020 rover mission. An abiotic jarosite, synthesized following established protocols, and a biological jarosite counterpart, derived from a microbial enrichment culture of Rio Tinto river sediments, were used to compare four spectroscopy techniques employed in the robotic exploration of Mars (Raman spectroscopy, mid-infrared (IR) spectroscopy, visible near-infrared reflectance (VNIR) spectroscopy and Mössbauer spectroscopy) to determine if the complimentary information obtained using these instruments can help elucidate the biological influence of Meridiani-type jarosites. Raman spectral differences might be due to the presence of unreacted reagents in the synthetic spectra and not biological contributions. Reflectance (IR/VNIR) spectra might exhibit minor organic absorption contributions, but are observed in both sample spectra, and do not represent a biosignature. Mössbauer spectra show minor differences in fit parameters that are related to crystal morphology and are unrelated to the biological (i.e., organic) component of the system. Results of this study suggest that the identification of biosignatures in Meridiani-type jarosites using the in situ robotic exploration on Mars may be possible but will be challenging. Our work provides additional insight into extraterrestrial biosignature detection and data interpretation for Mars exploration and indicates that sample return missions are likely required to unequivocally resolve the possible biogenicity of the Meridiani sediments or other jarosite-containing sediments.

Valley network morphology in the greater Meridiani Planum region, Mars

ABSTRACTThe Greater Meridiani Planum region on Mars is a key locale for a diverse range of fluvial landforms. Valley networks in this region have a range of geomorphologic styles that include negative relief, positive relief, or some combination of both along their lengths. Using high-resolution ∼5–6 m/pixel orbital images in ArcGIS Desktop software, we mapped previously under-recognized fine-scale valley networks within the Greater Meridiani Planum region and recorded their geomorphic characteristics as feature attributes. The objectives in using the mapped features are to 1) document the full range of valley network morphologic types in the region, 2) document changes in morphologic types both on a regional scale and along the valley network segments, and 3) to use the mapped features along with other geologic information from previous studies to better understand landscape evolution in the Greater Meridiani Planum region.

Young, small-scale surface features in Meridiani Planum, Mars: A possible signature of recent transient liquid and gas emissions

Enigmatic small-scale (<1 m) depositional and erosional features found in basaltic sands partly covering bedrock exposures, imaged at several locations in the equatorial Meridiani Planum region by the Mars Exploration Rover Opportunity, may be evidence of previously unrecognized, geologically young (or even contemporary) Martian surface processes. Leveed fissures appear to have formed by venting from beneath; possible explanations include aeolian blowholes near crater margins, volcanic fumarole activity, or gas/vapour escape resulting from the decomposition of small pockets of ground ice, methane clathrates or hydrated sulphate minerals. Some leveed fissures cross-cut and are therefore younger than aeolian ripples thought to have last been active c. 50,000 years ago. Erosional gutters are sharply defined and fresh-looking, internally terraced, sometimes are deeper near one end, and in one case seem to give way to small depositional fans downslope; they have the appearance of having been formed by liquid flow rather than by wind erosion. There is evidence elsewhere that contemporary ground-ice thaw and consequent transient surface run-off may occur occasionally under present conditions at low, near-equatorial latitudes on Mars; short-lived (even for just a few minutes) meltwater emission and flow at the surface could form gutters before evaporating. Further possibilities are the decomposition of buried pockets of methane clathrates (which theoretical considerations suggest might be present and stable even in equatorial regions) giving rise to both methane gas venting and transient surface water, or the release of liquid brines by decomposition of hydrated magnesium sulphate minerals or deliquescence of perchlorates. Dry granular flow mechanisms proposed as explanations for Recurring Slope Lineae seem inadequate to explain the morphologies of leveed fissures and gutters.

Surface-based 3D measurements of small aeolian bedforms on Mars and implications for estimating ExoMars rover traversability hazards

Recent aeolian bedforms comprising loose sand are common on the martian surface and provide a mobility hazard to Mars rovers. The ExoMars rover will launch in 2020 to one of two candidate sites: Mawrth Vallis or Oxia Planum. Both sites contain numerous aeolian bedforms with simple ripple-like morphologies. The larger examples are ‘Transverse Aeolian Ridges’ (TARs), which stereo imaging analyses have shown to be a few metres high and up to a few tens of metres across. Where they occur, TARs therefore present a serious, but recognized and avoidable, rover mobility hazard. There also exists a population of smaller bedforms of similar morphology, but it is unknown whether these bedforms will be traversable by the ExoMars rover. We informally refer to these bedforms as “mini-TARs”, as they are about an order of magnitude smaller than most TARs observed to date. They are more abundant than TARs in the Oxia Planum site, and can be pervasive in areas. The aim of this paper is to estimate the heights of these features, which are too small to measured using High Resolution Imaging Science Experiment (HiRISE) Digital Elevation Models (DEMs), from orbital data alone. Thereby, we aim to increase our knowledge of the hazards in the proposed ExoMars landing sites. We propose a methodology to infer the height of these mini-TARs based on comparisons with similar features observed by previous Mars rovers. We use rover-based stereo imaging from the NASA Mars Exploration Rover (MER) Opportunity and PRo3D software, a 3D visualisation and analysis tool, to measure the size and height of mini-TARs in the Meridiani Planum region of Mars. These are good analogues for the smaller bedforms at the ExoMars rover candidate landing sites. We show that bedform height scales linearly with length (as measured across the bedform, perpendicular to the crest ridge) with a ratio of about 1:15. We also measured the lengths of many of the smaller aeolian bedforms in the ExoMars rover Oxia Planum candidate landing site, and find that they are similar to those of the Meridiani Planum mini-TARs. Assuming that the Oxia Planum bedforms have the same length/height ratio as the MER Opportunity mini-TARs, we combine these data to provide a probabilistic method of inferring the heights of bedforms at the Oxia Planum site. These data can then be used to explore the likely traversability of this site. For example, our method suggests that most of the bedforms studied in Oxia Planum have ridge crests higher than 15 cm, but lower than 25 cm. Hence, if the tallest bedforms the ExoMars rover will be able to safely cross are only 15 cm high, then the Oxia Planum sites studied here contain mostly impassable bedforms. However, if the rover can safely traverse 25 cm high bedforms, then most bedforms here will be smaller than this threshold. As an additional outcome, our results show that the mini-TARs have length/height ratios similar to TARs in general. Hence, these bedforms could probably be classified simply as “small TARs”, rather than forming a discrete population or sub-type of aeolian bedforms.

Multiple surface wetting events in the greater Meridiani Planum region, Mars: Evidence from valley networks within ancient cratered highlands.

Morphological characterization of valley networks in three exposures of ancient cratered highlands (Nhc1) in the greater Meridiani Planum region yields insight into the Martian aqueous history. From our mapping, key regional differences are apparent in fine-scale valley network attributes including morphologic type, planimetric form, density, and links to candidate paleolakes. This information, combined with crater retention age (inferred exposure age), provides new details on the relative timing and nature of aqueous processes in the region. Newly identified pitted-type valley networks have morphological similarity to terrestrial pitted landforms in an evaporite setting. We interpret the pitted valley networks to reflect late-stage groundwater processes concentrated along the former fluvial conduits. Evidence from this study indicates that localized reactivation of valley networks occurred during or after exhumation of eastern Nhc1 unit.

Ancient Impact and Aqueous Processes at Endeavour Crater, Mars

The rover Opportunity has investigated the rim of Endeavour Crater, a large ancient impact crater on Mars. Basaltic breccias produced by the impact form the rim deposits, with stratigraphy similar to that observed at similar-sized craters on Earth. Highly localized zinc enrichments in some breccia materials suggest hydrothermal alteration of rim deposits. Gypsum-rich veins cut sedimentary rocks adjacent to the crater rim. The gypsum was precipitated from low-temperature aqueous fluids flowing upward from the ancient materials of the rim, leading temporarily to potentially habitable conditions and providing some of the waters involved in formation of the ubiquitous sulfate-rich sandstones of the Meridiani region.

Meteorites at Meridiani Planum provide evidence for significant amounts of surface and near‐surface water on early Mars

Abstract– Six large iron meteorites have been discovered in the Meridiani Planum region of Mars by the Mars Exploration Rover Opportunity in a nearly 25 km‐long traverse. Herein, we review and synthesize the available data to propose that the discovery and characteristics of the six meteorites could be explained as the result of their impact into a soft and wet surface, sometime during the Noachian or the Hesperian, subsequently to be exposed at the Martian surface through differential erosion. As recorded by its sediments and chemical deposits, Meridiani has been interpreted to have undergone a watery past, including a shallow sea, a playa, an environment of fluctuating ground water, and/or an icy landscape. Meteorites could have been encased upon impact and/or subsequently buried, and kept underground for a long time, shielded from the atmosphere. The meteorites apparently underwent significant chemical weathering due to aqueous alteration, as indicated by cavernous features that suggest differential acidic corrosion removing less resistant material and softer inclusions. During the Amazonian, the almost complete disappearance of surface water and desiccation of the landscape, followed by induration of the sediments and subsequent differential erosion and degradation of Meridiani sediments, including at least 10–80 m of deflation in the last 3–3.5 Gy, would have exposed the buried meteorites. We conclude that the iron meteorites support the hypothesis that Mars once had a denser atmosphere and considerable amounts of water and/or water ice at and/or near the surface.

Bounce Rock—A shergottite‐like basalt encountered at Meridiani Planum, Mars

Abstract– The Opportunity rover of the Mars Exploration Rover mission encountered an isolated rock fragment with textural, mineralogical, and chemical properties similar to basaltic shergottites. This finding was confirmed by all rover instruments, and a comprehensive study of these results is reported here. Spectra from the miniature thermal emission spectrometer and the Panoramic Camera reveal a pyroxene‐rich mineralogy, which is also evident in Mössbauer spectra and in normative mineralogy derived from bulk chemistry measured by the alpha particle X‐ray spectrometer. The correspondence of Bounce Rock’s chemical composition with the composition of certain basaltic shergottites, especially Elephant Moraine (EET) 79001 lithology B and Queen Alexandra Range (QUE) 94201, is very close, with only Cl, Fe, and Ti exhibiting deviations. Chemical analyses further demonstrate characteristics typical of Mars such as the Fe/Mn ratio and P concentrations. Possible shock features support the idea that Bounce Rock was ejected from an impact crater, most likely in the Meridiani Planum region. Bopolu crater, 19.3 km in diameter, located 75 km to the southwest could be the source crater. To date, no other rocks of this composition have been encountered by any of the rovers on Mars. The finding of Bounce Rock by the Opportunity rover provides further direct evidence for an origin of basaltic shergottite meteorites from Mars.

Early Mars hydrology: Meridiani playa deposits and the sedimentary record of Arabia Terra

The Meridiani Planum region of Mars has been identified as a region of past aqueous activity by a combination of orbital and in situ observations that revealed evidence for sulfate‐rich dirty evaporites formed in a playa setting. We investigate the hydrology and sedimentary record of this area using global and regional hydrological models in which groundwater flow is driven by a combination of precipitation, evaporation, and the surface topography. Groundwater evaporation results in evaporite precipitation and cementation of aeolian sediments, allowing the accumulation of deposits of substantial thickness, which then affect the subsequent patterns of groundwater flow. Hydrological activity is initially predicted to be isolated to the deepest craters and depressions, primarily within the Arabia Terra region surrounding Meridiani. As these depressions fill with sediments, the groundwater upwelling spreads laterally across broad regions of Arabia Terra, including Meridiani Planum, as well as regional topographic lows such as the northern lowlands and large impact basins. The model predictions are borne out by observations of large intracrater deposits, inverted valley networks, finely layered deposits, spectral evidence for hydrated sulfates, and pedestal craters that preserve the remnants of a much larger deposit that once covered much of Arabia Terra. The results suggest that the inferred playa at Meridiani was part of a regionally extensive zone of groundwater upwelling. This hydrological cycle requires that conditions in the late Noachian to early Hesperian must have been conducive to the existence of liquid water at the surface throughout much of the low latitudes of Mars.

Br/Cl partitioning in chloride minerals in the Burns formation on Mars

Within Gusev Crater and Meridiani Planum on Mars, the Mars exploration rovers have found Br concentrations in soils and rocks in the hundreds of ppm range. Relative to Earth compositions, these are high Br concentrations. Because of low Br concentrations on Earth, Br largely precipitates from seawater as a minor constituent in halite crystals rather than as a separate phase mineral. This is also likely to be the case for Mars. But given that the surface chemistries on Mars are significantly different than on Earth, minerals other than halite could serve as sinks for Br. The specific objectives of this paper were to (1) incorporate Br solution phase chemistries into the FREZCHEM model, (2) integrate the Siemann–Schramm Br/Cl mineral model into FREZCHEM, and (3) apply this mineral model to Br/Cl partitioning in Burns formation rocks as an indicator of past environments in the Meridiani Planum region of Mars. We showed that: (1) a molar-based model for Br substitution into halite and bischofite provided a better fit to experimental data than the standard mass-based model; (2) the concentrations of all of the soluble salts (mainly of Na, Mg, Ca, Cl, Br, and SO 4) in the Burns formation, except for Ca, were significantly related to stratigraphic depth; (3) the likely precipitation of Ca as gypsum on Mars precluded Ca precipitating as a CaCl 2 salt and thus impacts the possible minimum eutectic brine temperatures relevant to the Burns formation; (4) bischofite (MgCl 2⋅6H 2O) was a much more important sink for Br than halite; (5) Br/Cl patterns in the Burns formation, and within the three formation layers, argued in support of salt upwelling through groundwater evaporation; and (6) the high concentrations of Br in the surface layers of the Burns formation suggested that there was little water leaching and removal of soluble phases from the upper part of the stratigraphic succession.

Water cycling on Mars

The Meridiani Planum region on Mars is rich in minerals derived from evaporation, but lacks a topography consistent with standing water. Do the deposits stem from upwelling groundwater early in the planet's history?

Meridiani Planum and the global hydrology of Mars

The Opportunity Mars Exploration Rover found evidence for groundwater activity in the Meridiani Planum region of Mars in the form of aeolian and fluvial sediments composed of sulphate-rich grains. These sediments appear to have experienced diagenetic modification in the presence of a fluctuating water table. In addition to the extensive secondary aqueous alteration, the primary grains themselves probably derive from earlier playa evaporites. Little is known, however, about the hydrologic processes responsible for this environmental history-particularly how such extensive evaporite deposits formed in the absence of a topographic basin. Here we investigate the origin of these deposits, in the context of the global hydrology of early Mars, using numerical simulations, and demonstrate that Meridiani is one of the few regions of currently exposed ancient crust predicted to have experienced significant groundwater upwelling and evaporation. The global groundwater flow would have been driven primarily by precipitation-induced recharge and evaporative loss, with the formation of the Tharsis volcanic rise possibly playing a role through the burial of aquifers and induced global deformation. These results suggest that the deposits formed as a result of sustained groundwater upwelling and evaporation, rather than ponding within an enclosed basin. The evaporite formation coincided with a transition to more arid conditions that increased the relative impact of a deep-seated, global-scale hydrology on the surface evolution.

Ultrastructural Study of Iron Oxide Precipitates: Implications for the Search for Biosignatures in the Meridiani Hematite Concretions, Mars

Two terrestrial environments that have been proposed as analogs for the iron oxide precipitation in the Meridiani Planum region of Mars include the Rio Tinto precipitates and southern Utah marble concretions. Samples of two typical Utah iron oxide concretions and iron oxide precipitates in contact with biofilms from Rio Tinto have been studied to determine whether evidence could be found for biomediation in the precipitation process and to identify likely locations for fossil microorganisms. Scanning electron microscopy, energy dispersive X-ray, and gas chromatography-mass spectrometry (GC-MS) were used to search for biosignatures in the Utah marbles. The precipitation of iron oxides resembles known biosignatures, though organic compounds could not be confirmed with GC-MS analysis. In contrast, textural variations induced by biological activity are abundant in the modern Rio Tinto samples. Although no compelling evidence of direct or indirect biomediation was found in the Utah marbles, the ultrastructure of the iron oxide cement in the concretion suggests an inward growth during concretion precipitation from an initially spherical redox front. No indication for growth from a physical nucleus was found.

Mars exploration rover surface operations: driving opportunity at Meridiani Planum

Since landing on the Meridiani Planum region of Mars in January 2004, the Mars exploration rover (MER) vehicle named Opportunity has been sending back pictures taken from several different craters that would provide evidence that the region did indeed have a watery past. This paper details the experience of driving Opportunity through this alien landscape during its first 400 days on Mars, from the point of view of the other rover planners, the people who tell the rover where to drive and how to use its robotic arm

Comparative planetary mineralogy: Implications of martian and terrestrial jarosite. A crystal chemical perspective

The importance of the discovery of jarosite at the Meridiani Planum region of Mars is discussed. Terrestrial studies demonstrate that jarosite requires a unique environment for its formation, crystallizing from highly acidic (pH < 4) S-rich brines under highly oxidizing conditions. A likely scenario for jarosite formation on Mars is that degassing of shallow magmas likely released SO 2 that reacted with aqueous solutions in shallow aquifers or on the martian surface. This interaction forms both H 2SO 4 and H 2S. A martian oxidant must be identified to both oxidize H 2S to produce the required acidity of the fluid, and to oxidize Fe 2+ to Fe 3+. We suggest that reactions involving both sulfur and the reduction of CO 2 to CO may provide part of the answer. The jarosite crystal structure is truly remarkable in terms of its tolerance for the substitution of a large number of different cations with different ionic radii and charges. The structure accommodates hydrogen, oxygen, and sulfur, the stable isotope systematics of which are strong recorders of low-temperature fluid-rock-atmosphere interactions. Jarosite has been proven to be a robust chronometer for Ar–Ar and K–Ar dating techniques, and there is every reason to believe that U–Pb, Rb–Sr, and Nd–Sm techniques for older jarosite from Mars will also be robust. Although the discovery of jarosite on Mars alone, with no other analytical measurements on the phase, has given us insights to martian surficial processes, the true power of jarosite can not be exploited until jarosite is sampled and returned from Mars. Mars sample return is a long way off but, until then, we should be vigilant about examining martian meteorites for alteration assemblages that contain jarosite. A suite of jarosite samples representing a significant time span on Mars may hold the key to reading the record of martian atmospheric evolution.

An astrobiological perspective on Meridiani Planum

Sedimentary rocks exposed in the Meridiani Planum region of Mars record aqueous and eolian deposition in ancient dune and interdune playa-like environments that were arid, acidic, and oxidizing. On Earth, microbial populations have repeatedly adapted to low pH and both episodic and chronic water limitation, suggesting that, to a first approximation, the Meridiani plain may have been habitable during at least part of the interval when deposition and early diagenesis took place. On the other hand, the environmental conditions inferred for Meridiani deposition would have posed a challenge for prebiotic chemical reactions thought to have played a role in the origin of life on Earth. Orbital observations suggest that the combination of sulfate minerals and hematite found in Meridiani rocks may be unusual on the martian surface; however, there is reason to believe that acidity, aridity, and oxidizing conditions were broadly distributed on ancient Mars. When these conditions were established and how much environmental heterogeneity existed on early Mars remain to be determined. Because sulfates and iron oxides can preserve detailed geochemical records of environmental history as well as chemical, textural and microfossil signatures of biological activity, Meridiani Planum is an attractive candidate for Mars sample return.

Hydration state of the Martian coarse‐grained hematite exposures: Implications for their origin and evolution

The discovery of areas of bulk gray hematite on the surface of Mars is among the most interesting of the results from Mars Global Surveyor (MGS). The individual outcrops are concentrated in equatorial, low‐albedo regions from the Valles Marineris to the Meridiani Planum region to the east. While the Thermal Emission Spectrometer (TES) team favored an aqueous process for the formation of this deposit, recent geologic studies have interpreted deposits in the region to have a volcanic origin. Competing ideas for the method of formation of bulk hematite range from aqueous alteration, burial diagenesis, and metamorphism to igneous processes. Here we show that some of the spectra collected over Meridiani Planum and Aram Chaos from the Mariner 6 and 7 spacecraft have a high 3 μm band depth. This feature is associated with hydrated minerals in the surface layer, and the hydrated spectra coincide with the mapped hematite regions. This supports an aqueous mode of hematite formation and, as hematite itself is not hydrated, suggests the presence of other hydrous mineral phases in these areas. We discuss the implications of this association for origin and evolution of these deposits.

NASA Rover finds evidence of a once wet and habitable Mars

NASA's Opportunity rover has found strong evidence that a rock outcrop in Mars' Meridiani Planum region once was a wet and habitable environment, rover science team members announced at a 2 March news briefing at NASA headquarters in Washington, D.C.“We have concluded that the rocks here were once soaked in liquid water”said Steven Squyres, Mars Exploration Rover principal investigator and astronomy professor at Cornell University. He said it was not yet possible to indicate how long ago or for how long the rocks were wet.

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

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 (800–1600 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.