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

The Philae lander provides a unique opportunity to investigate the internal structure of a comet nucleus, providing information about its formation and evolution in the early solar system. We present Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) measurements of the interior of Comet 67P/Churyumov-Gerasimenko. From the propagation time and form of the signals, the upper part of the “head” of 67P is fairly homogeneous on a spatial scale of tens of meters. CONSERT also reduced the size of the uncertainty of Philae’s final landing site down to approximately 21 by 34 square meters. The average permittivity is about 1.27, suggesting that this region has a volumetric dust/ice ratio of 0.4 to 2.6 and a porosity of 75 to 85%. The dust component may be comparable to that of carbonaceous chondrites.

The physical properties of cometary nuclei observed today relate to their complex history and help to constrain their formation and evolution. In this article, we review some of the main physical properties of cometary nuclei and focus in particular on the thermal, mechanical, structural and dielectric properties, emphasising the progress made during the Rosetta mission. Comets have a low density of \(480 \pm 220~\mbox{kg}\,\mbox{m}^{-3}\) and a low permittivity of 1.9–2.0, consistent with a high porosity of 70–80%, are weak with a very low global tensile strength \(<100\) Pa, and have a low bulk thermal inertia of \(0\mbox{--}60~\mbox{J}\,\mbox{K}^{-1}\,\mbox{m}^{-2}\,\mbox{s}^{-1/2}\) that allowed them to preserve highly volatiles species (e.g. CO, CO₂, CH₄, N₂) into their interior since their formation. As revealed by 67P/Churyumov-Gerasimenko, the above physical properties vary across the nucleus, spatially at its surface but also with depth. The broad picture is that the bulk of the nucleus consists of a weakly bonded, rather homogeneous material that preserved primordial properties under a thin shell of processed material, and possibly covered by a granular material; this cover might in places reach a thickness of several meters. The properties of the top layer (the first meter) are not representative of that of the bulk nucleus. More globally, strong nucleus heterogeneities at a scale of a few meters are ruled out on 67P’s small lobe.

The search for evidence of past or present life on Mars is the principal objective of the 2020 ESA-Roscosmos ExoMars Rover mission. If such evidence is to be found anywhere, it will most likely be in the subsurface, where organic molecules are shielded from the destructive effects of ionizing radiation and atmospheric oxidants. For this reason, the ExoMars Rover mission has been optimized to investigate the subsurface to identify, understand, and sample those locations where conditions for the preservation of evidence of past life are most likely to be found. The Water Ice Subsurface Deposit Observation on Mars (WISDOM) ground-penetrating radar has been designed to provide information about the nature of the shallow subsurface over depth ranging from 3 to 10 m (with a vertical resolution of up to 3 cm), depending on the dielectric properties of the regolith. This depth range is critical to understanding the geologic evolution stratigraphy and distribution and state of subsurface H₂O, which provide important clues in the search for life and the identification of optimal drilling sites for investigation and sampling by the Rover's 2-m drill. WISDOM will help ensure the safety and success of drilling operations by identification of potential hazards that might interfere with retrieval of subsurface samples. Key Words: Ground penetrating radar—Martian shallow subsurface—ExoMars. Astrobiology 17, 565–584.