Remaining Uncertainties:

1. Eberswalde Crater Site Specific Pros of Site: • Setting - • Eberswalde shows excellent preservation of what is highly likely to be a fluvial-deltaic deposit emplaced into a standing body of water that integrates sedimentary material from a broad source region. • The landing site provides the opportunity to quantitatively reconstruct the sedimentary and hydrologic conditions during deposition and specific formation models allow prediction of locations to target for exploration with MSL. Bottom set beds from each lobe of the delta can be defined and identified for exploration. Provide targets for organics if present • Evidence for episodic channel-switching is recorded in the delta and its deposition may have extended towards the long end model predictions (105+ years). • Smaller fluvial-deltaic systems coupled with other putative lacustrine and other exploration targets make the site a mix of land on and go to targets. • Diversity - • In addition to the delta-associated deposits, and sinuous ridges in Eberswalde, hills of older crater ejecta megabreccia and basement with possible hydrothermal veins occur in the landing ellipse and provide an exploration target. Collectively, the materials in the ellipse and the nearby delta include clay minerals whose distribution is associated with different outcrop characteristics. • Preservation - • Clay minerals have been found near the bottom of the delta front, maybe in bottom set deposits that form a well-defined target for exploration. There are also potential lake deposits within the landing ellipse that offer exploration targets. Such deposits may concentrate and preserve organics and evidence for habitability and life. • Well defined science targets and evaluation of mineralogical diversity within and outside of the ellipse defines short and long term exploration strategy. Lacustrine sediments likely exposed in and near the ellipse and distribution is becoming well-mapped. 23.9°S, 327°E Overarching Hypothesis: • Eberswalde Crater stratigraphy, geomorphology, and mineralogy record the evolution of a crater lake, associated fluvial-deltaic systems, and climate, and additionally represent a sedimentary, potentially habitable environment that is favorable to the preservation of organic materials and/or other kinds of biosignatures. • Possible megabreccia could be accessed and allow sampling of geologically diverse basement materials. Remaining Uncertainties: Little evidence of a shoreline/bench in Eberswalde crater corresponding to the elevation of the delta surface and the spillway to the eastern basin, some aspects of the Eberswalde system (poorly defined shorelines, etc) suggest it may have been ice-covered. No deformation of delta that might be expected. Predictions made that can help to evaluate in situ. Emplacement may be consistent with delivery of water and sediment by the Holden impact and cannot be ruled out, but tests are proposed that can resolve this in situ. Sediment contributions to the delta from Holden ejecta are uncertain. Careful mapping of Eberswalde tributaries and characteristics of incision will help answer these questions. Delta is no older than early Hesperian and mayhave been deposited as late as the early Amazonian or around the time of the Amazonian-Hesperian, but there is no consensus whether a post-Noachian age is of a concern for preserving organics or for preserving evidence for past habitability or life on Mars. Possible Cons of Site: • Relatively limited variety and modeled abundance of minerals known to preserve organics. • Science in landing ellipse is secondary to that outside of the ellipse.

2. Gale Crater Site Specific Pros of Site: Setting - Diverse stratigraphy in the 5 km mound and adjacent stratigraphy, including hydrated minerals, are well defined and the lower mound likely reflects deposition during changing environmental conditions, possibly global, with some contributions by fluvial processes. The landing ellipse contains alluvial materials and inverted channels whose form and distribution record hydrologic conditions when they were emplaced and provides the opportunity to sample materials weathered and eroded from the crater walls. Diversity - Multiple mineralogical and stratigraphic units within the 5 km thick mound sequence with alternating inter-bedded phyllosilicate and sulfate bearing beds in the lower mound. Stratigraphy comprising the mound is continuous over many km and well characterized in places. Preservation – The phyllosilicate-bearing units in the lower mound and moat include smectites and suggest they are well preserved and may contain and help preserve organics. Biosignatures may be best preserved in the sulfate bearing strata in the mound. The specific distribution of science targets within and outside of the ellipse is well defined. MSL would look for preserved organics in high thermal inertia unit in fan in ellipse and may be playa, in clay rich layer that may not have sulfates, and in sulfates. 4.5S, 137.4E Overarching Hypothesis: • Strata within the 5 km thick mound of layered sediments within Gale crater reflect a sequence of aqueous habitable environments over an extended time period • Strata contain diverse hydrous minerals (sulfates, phyllosilicates) that indicate varying aqueous environmental conditions. • Alluvial materials within the landing ellipse enable sampling crater rim materials that may record environmental conditions during their emplacement. Possible Cons of Site: • The original extent and timing of processes responsible for the current mound morphology need better definition and the regional and global stratigraphic context of the mound is not firmly established, but may be better resolved by measurement of crater statistics. • Science in landing ellipse on and near alluvial fan is secondary to that outside of the ellipse and observations there may be encumbered by a ubiquitous layer of dust. Remaining Uncertainties: Although the mineralogy of the mound is becoming better understood with several testable models for emplacement being established, uncertainty remains about the depositional setting for much of the stratigraphy. The source of the lower mound sediments is unknown but likely from outside of the crater. It is uncertain whether the mound is part of a larger deposit, though it is morphologically similar to deposits seen elsewhere on Mars. Crater statistics suggest Gale is late Noachian, whereas floor deposits onlapping the lower mound and including fan in ellipse are Early Hesperian. Hence, age of lower mound may be constrained to the intervening period. Age of upper mound is less constrained, so the total record of Martian history is recorded in the mound remains uncertain. The depositional provenance of the mound strata remains uncertain relative to dominant processes responsible for their emplacement, but bed continuity and morphology implies origin of lower section likely involved deposition into wet or standing water (from runoff to snow melt and with or without ponding) with some evidence for fluvial redistribution within the mound. The source of water remains uncertain, if sediments were deposited in a lake, the relative paucity of associated valleys supporting a lake suggests groundwater as opposed to meteoric sources, though valleys breaching at a higher stratigraphic level now eroded away may have contributed fill to the crater and/or in lake. Preservation potential in the sulfate units may be compromised by the known presence of iron oxides.

3. Specific Pros of Site: Setting - Fans in the ellipse and light-toned layered deposits comprise one of the largest and best preserved alluvial systems on Mars. The diverse and potentially weathered sediments record the conditions responsible for their formation during the Hesperian perhaps into the Early Amazonian. This sequence is underlain by the light-toned layered deposits and overlying Uzboi flood deposits and enable the age of the target deposits to be related to global stratigraphy. Diversity - Diversity is represented by fan sediments, phyllosilicate-bearing light toned layered deposits, Uzboi flood deposits, and mega-breccias in the crater walls/floor. The mineralogical diversity in the light toned layered deposits and crater walls/floor include both altered and primary compositions. Preservation - Strata comprising the light toned layered deposits may be the equivalent of bottom set beds emplaced in a lacustrine setting, which may preserve organics for interrogation by the MSL. Targets for interrogation by MSL have been made for within and outside the landing ellipse. Targets within the ellipse offer access to all major units for interrogation, though thicker sections of the light-toned layered materials and megabreccias occur farther to the south. Holden Crater Site 26°S, 325°E Overarching Hypothesis: • Holden crater preserves evidence of a closed fluvial-lacustrine system that provides the opportunity to apply a geomorphic systems approach to evaluating and preserving evidence for a sustained, habitable environment. • Diverse and widespread megabreccias enable sampling ancient crustal materials that may include hydrothermal environments. • Alluvial materials within the landing ellipse enable sampling crater rim materials that may record environmental conditions during their emplacement. Specific Cons of Site: • Origin of light toned layered deposits as lacustrine versus alternate depositional processes remains uncertain, but in situ evaluation of bedding character and chemistry is likely to distinguish origin. • Relatively limited variety of minerals known to preserve organics. Remaining Uncertainties: There are no shorelines or stratal geometries and limited evidence for other properties of the light toned layered deposits and fan deposits whose distribution can be mapped and used to more confidently define their origin and genetic relationships. Diverse mega-breccia occurrences within the ellipse, walls, and rims may support an impact-induced hydrothermal system. Light-toned layers high on the west wall of Holden may relate to older beds excavated from the pre-existing Holden basin. Age of light toned layered deposits and adjacent alluvial fan surfaces may be relatively young at no older than Early Hesperian for the light-toned layered deposits and perhaps as young as the the Late Hesperian to Early Amazonian, respectively. There is no consensus whether this is an issue for habitability and evaluating conditions for life. Collectively, exploration targets may reflect deposition in intervals from the Noachian to the Hesperian and perhaps the Late Hesperian to Early Amazonian.

4. Mawrth Vallis Site Specific Pros of Site: Setting and Age - Exposes the oldest preserved layered stratigraphic section of the four candidate sites and provides an opportunity to explore Noachian crust to capture the processes that were active on early Mars. There is consensus that the rocks may be among the oldest preserved on Mars and may be from a period not recorded in the rock record on Earth. Relative ages are well constrained. Numerous hydrated minerals are present in several 10s of % by volume (most of any of the sites) that formed in aqueous environments and the section within and near the landing ellipse appears to be mineralogically representative of other Noachian crustal sections in Arabia Terra, thus allowing an understanding of what may have been widespread processes on early Mars. Overlying (capping) materials appear unaltered and may record changing conditions during the Hesperian and younger times on Mars. Diversity – The ellipse contains a diverse, complex mineralogical and stratigraphic sequence that records changing processes and environmental conditions. The sequence includes many of the hydrated minerals found on Mars including multiple phyllosilicates and sulfates, and was likely emplaced by multiple geologic mechanisms that probably included diagenetic, impact, fluvial, and/or pedogenic processes, and it remains in situ. Preservation - Several locations in close proximity with one another and within the ellipse may allow interrogation of a variety of rocks that will help define the early period of time when water was present and determine whether the environment was habitable. The phyllosilicate-bearing units include smectites, suggesting they are well preserved and may contain and help preserve organics. Phyllosilicates represent minerals that may preserve organics. Science Targets – a good list of prioritized targets within and outside the ellipse has been identified including sulfates outside of the ellipse. There is consensus that both the Mg/Fe and Al- bearing phyllosilicate units need to be interrogated to assess the nature and distribution of any organics. 24°N, 341°E Overarching Hypothesis: • Mawrth Vallis records the geologic processes during early Martian history, when aqueous phyllosilicate-forming processes were pervasive and persistent, and provides the opportunity to understand early habitability on the planet. • Strata contain diverse hydrous minerals (sulfates, phyllosilicates) that indicate varying aqueous environmental conditions. • Mawrth Vallis’ compositional stratigraphy records a change in surface aqueous alteration environments in the Noachian that may be representative of Mars globally. Possible Cons of Site: • There is no consensus on the depositional setting and the mechanisms for concentrating or preserving organics. • It is unlikely that the depositional setting will be further refined prior to landing and evaluation on the surface of Mars. Although textural and chemical profiles of the units completed using the MSL payload may distinguish between models for emplacement history, there is not a consensus that such an approach will be successful. Remaining Uncertainties: The depositional setting(s) associated with emplacement of the rocks and mineralogic units at Mawrth remains uncertain, and may include depositional, diagenetic, and impact processes (e.g., for Al-phyllo unit). Further constraint is unlikely using existing orbital data sets but may be possible in situ. It remains uncertain whether Oyama ejecta persists or which unit it might correspond to (some patchy deposits present or could be dark capping unit). The amount, source and duration of interaction with water in development of the units remains uncertain.