A New Dawn: Since 17 July 2011, NASA's spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684 ) use Dawn's observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687 ) report on the asteroid's overall geometry and topography, based on global surface mapping. Vesta's surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690 ) report on Vesta's complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694 ) describe two giant impact basins located at the asteroid's south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697 ) present the mineralogical characterization of Vesta, based on data obtained by Dawn's visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700 ) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet.

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 VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ ⁻¹ ), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

A New Dawn: Since 17 July 2011, NASA's spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684 ) use Dawn's observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687 ) report on the asteroid's overall geometry and topography, based on global surface mapping. Vesta's surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690 ) report on Vesta's complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694 ) describe two giant impact basins located at the asteroid's south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697 ) present the mineralogical characterization of Vesta, based on data obtained by Dawn's visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700 ) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet.