Editorial: Topical Collection on the Delivery of Water to Proto-Planets, Planets and Satellites

Space Sci Rev (2018) 214:110 https://doi.org/10.1007/s11214-018-0545-y Editorial: Topical Collection on the Delivery of Water to Proto-Planets, Planets and Satellites Alessandro Morbidelli1 · Shun-Ichiro Karato2 · Masahiro Ikoma3,4 · Yann Alibert5 · Michel Blanc6,7 · Lindy Elkins-Tanton8 · Paul Estrada9 · Keiko Hamano10 · Helmut Lammer11 · Sean Raymond12 · Maria Schönbächler13 Published online: 11 October 2018 © Springer Nature B.V. 2018 The review papers of this Topical Collection of Space Science Reviews devoted to the delivery of water to proto-planets, planets and satellites provide a coherent and comprehensive portrait of the knowledge in this fascinating field. We provide here a key of lecture of the volume, by summarizing the content of each review and proposing a logical order of reading, then attempting a broad summary of the state of the art as it emerges from the reviews altogether. The Delivery of Water to Protoplanets, Planets and Satellites Edited by Allessandro Morbidelli, Michel Blanc, Yann Alibert, Lindy Elkins-Tanton, Paul Estrada, Keiko Hamano, Helmut Lammer, Sean Raymond and Maria Schönbächler B A. Morbidelli 1 Observatoire de la Cote d’Azur, Nice, France 2 Department of Geology and Geophysics, Yale University, New Haven, CT, USA 3 Department of Earth and Planetary Science, University of Tokyo, Tokyo, Japan 4 Research Center for the Early Universe (RESCEU), University of Tokyo, Tokyo, Japan 5 Physics Institute, University of Bern, Bern, Switzerland 6 International Space Science Institute ISSI, Bern, Switzerland 7 Institut de Recherche en Astrophysique et Planétologie, IRAP, Toulouse, France 8 School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA 9 SETI and NASA/Ames Res. Center, Mountain View, CA, USA 10 Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Tokyo, Japan 11 Space Research Institute, Austrian Academy of Sciences, Graz, Austria 12 Lab. D’Astrophysique de Bordeaux, Floirac, France 13 Institut für Geochemie und Petrologie, ETH Zurich, Zurich, Switzerland 110 Page 2 of 8 A. Morbidelli et al. The review by F. Westall and A. Brack focuses on the importance of water for life. It may be possible to conceive theoretically life based on compound-solvent pairs other than carbon-based molecules and liquid water, but this paper makes clear that organic material and liquid water have physical and chemical properties that make them optimal among all known molecules. This is the reason why water is at the core of exobiology and the focus of considerable research, as described in this Collection. Westall and Brack argue that hydrothermal environments most likely played a key role in the appearance of life, because hot rock-water interfaces can provide the chemical disequilibria to fuel reactions, and rocks and minerals can provide the reactive surfaces needed to help the formation and stability of prebiotic molecules. Moreover, once life emerged, rocks and minerals provided the required nutriments for life to prosper. This intimate relationship between liquid water, organic material and hot rocks for the sustainability of life comes back later in the collection when discussing habitability in the sub-surface oceans of some giant planet’s satellites (see the review by Grasset et al.) or water-rich extrasolar planets (see the review by Noak et al.). Four review papers make the inventory of water throughout the Solar System. The review by Alexander et al. describes the water budgets (and, more generally, volatile budgets) in small bodies. The review by Peslier et al. focusses on the water budget on Earth and its main reservoirs (core, mantle, crust and hydrosphere). The review by Greenwood et al. extends the analysis to the other terrestrial planets and the Moon, while the review by Grasset et al. describes the giant planets, their satellites, trans-Neptunian objects and the dwarf planet Ceres in the asteroid belt. Altogether, these reviews establish a clear distinction between a water-poor inner Solar System and a water rich outer Solar System, with the exception of the asteroid belt, where water-rich and water-poor asteroids co-exist in the same region (but might have been separated at origin). The water budget on Earth is uncertain because water concentrations in the lower mantle and the core are poorly known, but it seems likely that our planet is intermediate in terms of bulk water content between water-poor and water-rich asteroids. Venus and Mars have, or started with, water budgets comparable to the Earth’s, whereas this is unknown for Mercury. The Moon, long-thought to be bone-dry, seems now to have a considerable water budget, perhaps 1/10 of that of the Earth. From the isotopic point of view (D/H and 14 N/15 N ratios), the Earth and Mars are very similar to water-rich asteroids, but distinct from comets: they are probably made of a mixture of dryer bodies and about 2% carbonaceous chondrites. A signature of cometary bombardment, however, has been found by comparing the Xenon isotope composition in the escape-corrected terrestrial atmosphere with those in asteroids and comets (see the review by Alexander et al.), concluding that 20% of atmospheric Xenon should be of cometary origin. The corresponding amount of cometary material would provide a negligible contribution to the terrestrial water budget. Three review papers discuss the formation of planetary systems and the fate of water in the various phases of this process. The review by Hartmann et al. focusses on protoplanetary disks and in particular it discusses the evolution of the so-called snowline that separates the part of the disk where water is in vapor form from that where water ice is stable. If the detection of the water snowline is still beyond observational capabilities (and therefore remains an issue investigated on the basis of theoretical models), interferometric observations are now providing constraints on the positions of CO snow lines, testing disk models at large scales. The review by Paardekooper and Johansen is a very complete compendium of all processes leading to giant planet formation: dust coagulation in the form of pebbles, aerodynamic radial drift of these particles, formation of self-gravitating clumps of pebbles leading to the birth of large planetesimals, the subsequent growth of planetesimals due to mutual collisions and continued pebble accretion until forming protoplanetary cores, gas accretion onto these cores and—last but not least—planet migration in the different planetary mass regimes. The Editorial: Topical Collection on the Delivery of Water. . . Page 3 of 8 110 review of O’Brien et al. instead focusses on the terrestrial planets of the Solar System, describing the leading models that attempt to reproduce their orbital and physical properties. As the giant planets formed before the terrestrial planets, the Paardekooper and Johansen review sets the stage for the O’Brien et (...truncated)