The second ExoMars mission will be launched in 2020 to target an ancient location interpreted to have strong potential for past habitability and for preserving physical and chemical biosignatures (as well as abiotic/prebiotic organics). The mission will deliver a lander with instruments for atmospheric and geophysical investigations and a rover tasked with searching for signs of extinct life. The ExoMars rover will be equipped with a drill to collect material from outcrops and at depth down to 2 m. This subsurface sampling capability will provide the best chance yet to gain access to chemical biosignatures. Using the powerful Pasteur payload instruments, the ExoMars science team will conduct a holistic search for traces of life and seek corroborating geological context information. Key Words: Biosignatures—ExoMars—Landing sites—Mars rover—Search for life. Astrobiology 17, 471–510. 1. Article Organization 2. Introduction 2.1. ExoMars origin 2.2. A difficult adolescence 2.3. Joint program 3. Early Mars as an Exobiology Target 3.1. A first window of opportunity for life 3.2. Separate ways 3.2.1. Young Earth 3.2.2. Young Mars 3.2.3. Young Venus 3.3. Lessons for ExoMars: when and where? 4. Biosignatures: Which and How Reliable? 4.1. Morphological biosignatures 4.2. Chemical biosignatures 4.2.1. Isomerism selectivity 4.2.2. Molecular weight fingerprints 4.2.3. Bulk isotopic fractionation 4.3. Importance of geological context for boosting biosignature confidence 4.4. Life's decision points 4.5. Examples using the ExoMars biosignature score 4.5.1. Kitty's Gap, N.W. Australia 4.5.2. Josefsdal Chert, Barberton, South Africa 4.5.3. Martian Meteorite ALH84001 4.5.4. Yellowknife Bay, Mars 5. The Martian Environment and the Need for Subsurface Exploration 5.1. Results from previous missions 5.2. Degradation of organic matter 5.3. Access to molecular biosignatures 6. The ExoMars Rover and Its Pasteur Payload 6.1. From panoramic to molecular scale through nested investigations 6.2. Pasteur payload instruments 6.2.1. Panoramic camera system 6.2.2. IR spectrometer 6.2.3. Shallow ground-penetrating radar 6.2.4. Subsurface neutron detector 6.2.5. Close-up imager 6.2.6. Drill IR spectrometer 6.2.7. Subsurface drill 6.2.8. Sample preparation and distribution system 6.2.9. MicrOmega 6.2.10. Raman laser spectrometer 6.2.11. Mars organic molecule analyzer 6.3. The reference surface mission 7. A Suitable Landing Site 7.1. Scientific constraints 7.2. Engineering constraints 7.3. Planetary protection constraints 7.4. Possible locations for landing 7.4.1. Oxia Planum (18.159°N, 335.666°E; −3 km MOLA) 7.4.2. Mawrth Vallis (22.160°N, 342.050°E; −2 km MOLA) 8. Conclusions Acknowledgments Author Disclosure Statement References Abbreviations Used

Von Kármán crater (diameter = ~186 km), lying in the northwestern South Pole‐Aitken (SPA) basin, was formed in the pre‐Nectarian. The Von Kármán crater floor was subsequently flooded with one or several generations of mare basalts during the Imbrian period. Numerous subsequent impact craters in the surrounding region delivered ejecta to the floor, together forming a rich sample of the SPA basin and farside geologic history. We studied in details the targeted landing region (45.0–46.0°S, 176.4–178.8°E) of the 2018 Chinese lunar mission Chang'E‐4, within the Von Kármán crater. The topography of the landing region is generally flat at a baseline of ~60 m. Secondary craters and ejecta materials have covered most of the mare unit and can be traced back to at least four source craters (Finsen, Von Kármán L, Von Kármán L', and Antoniadi) based on preferential spatial orientations and crosscutting relationships. Extensive sinuous ridges and troughs are identified spatially related to Ba Jie crater (diameter = ~4 km). Reflectance spectral variations due to difference in both composition and physical properties are observed among the ejecta from various‐sized craters on the mare unit. The composition trends were used together with crater scaling relationships and estimates of regolith thickness to reconstruct the subsurface stratigraphy. The results reveal a complex geological history of the landing region and set the framework for the in situ measurements of the CE‐4 mission, which will provide unique insights into the compositions of farside mare basalt, SPA compositional zone including SPA compositional anomaly and Mg‐pyroxene annulus, regolith evolution, and the lunar space environment.

The NASA Mars Science Laboratory (MSL) rover, Curiosity, has safely landed near a 35-km-long dark dune field in Gale Crater on Mars. This dune field crosses the landing site from the northeast to the southwest and lies along Curiosity’s traverse to Aeolis Mons. Here we present the first evidence of recent aeolian activity in the form of ripple and dune migration, and further estimate wind directions within the dune field through analysis of ripple and dune morphologies and the Mars Regional Atmospheric Modeling System (MRAMS). We measured a minimum ripple migration rate of 0.66 m per Earth year, and dune migration rate of 0.4 m per Earth year, in the southwest portion of the field. A strongly bidirectional ripple crestline orientation, nearly orthogonal dune slipfaces, and linear seif or oblique dunes indicate a bidirectional wind regime with winds mainly coming from the ENE and from the northwest; however, MRAMS results indicate primary winds from the ENE. Our constraints on the wind regime provide the unique opportunity to use ground measurements from MSL to test the accuracy of winds predicted from orbital data.

The Rümker region (41–45°N, 49–69°W) is located in northern Oceanus Procellarum of the Moon. Mons Rümker is the most distinctive geological feature in the area. The region is characterized by prolonged lunar volcanism (Late Imbrian Period to Eratosthenian Period), forming multiple geologic units in the area, including very low‐Ti to low‐Ti mare basalts, high‐Ti mare basalts, and volcanic complexes. Each geologic unit has distinct element composition and mineral assemblages. The Rümker region, overlying the Procellarum KREEP Terrain, was selected as the landing region for China's Chang'E‐5 lunar sample return mission. Prelanding analyses of the geologic context and scientific potential are reported in this contribution. We conducted detailed geological mapping using image, spectral, and altimetry data. Fourteen geological units were defined, a geologic map was constructed, and the geologic history was outlined. The western mare units (Im1, Im2, and Im3) are Imbrian‐aged (~3.4–3.5 Ga) representing the major stage of lunar mare eruptive volcanism. The eastern young mare units (Em3 and Em4; <2 Ga) are among the youngest mare basalts on the Moon. They have never been explored in situ or studied in the laboratory. We suggest that samples returned from the eastern mare unit (Em4) could answer many fundamental questions and that this unit should be listed as the top priority landing site for Chang'E‐5 sample return mission.

Thousands of phyllosilicate‐rich outcrops, mainly iron or magnesium‐rich are exposed on Noachian terrains in the Martian southern highlands. We analyzed 90 CRISM observations and more than a hundred HiRISE images located on the plateaus surrounding Valles Marineris. We mapped an extensive Al‐ and Fe/Mg‐phyllosilicate‐rich formation covering at least ∼197,000 km², for which we introduce the name “Plateau Phyllosilicates.” Tens of meters in thickness, this light‐toned formation crops out at various elevations on top of the Noachian units Npl₁ and Npl₂, as flat exposures on plateaus and along scarps such as valley walls, chasma walls, pit walls and impact crater rims. The Fe/Mg‐phyllosilicate‐rich lower member of the formation is composed of Fe/Mg‐smectites (nontronite, saponite) and vermiculite. The Al‐phyllosilicate‐rich upper member of the formation contains Al‐smectites (montmorillonite, beidellite) and locally kaolinite and/or halloysite. We suggest that the Plateau Phyllosilicates were mainly formed by pedogenesis related to the weathering of the Noachian bedrock by percolation of meteoric water or melted snow under a temperate and subarid climate during the Noachian Epoch in an alkaline to neutral environment. Kaolinite and/or halloysite may have formed in areas of more intense drainage at the surface under slightly acidic environments during the Noachian and Hesperian Epochs. Fluvial activity and deuteric alteration may have locally contributed to the genesis of phyllosilicates. This study suggests that the alteration of the Noachian basement of the plateaus surrounding Valles Marineris was widespread during the Noachian Epoch, and was still active during the Hesperian Epoch even though the water availability was limited.