Thematic Articles

Water condensed as ice beyond the water snowline, the location in the Sun’s natal gaseous disk where temperatures were below 170 K. As the disk evolved and cooled, the snowline moved inwards. A low temperature in the terrestrial planet-forming region is unlikely to be the origin of water on the planets, and the distinct isotopic compositions of planetary objects formed in the inner and outer disks suggest limited early mixing of inner and outer Solar System materials. Water in our terrestrial planets has rather been derived from H-bearing materials indigenous to the inner disk and delivered by water-rich planetesimals formed beyond the snowline and scattered inwards during the growth, migration, and dynamical evolution of the giant planets.

Water played a critical role in the evolution of rocky material and planetesimals in the early Solar System. Many primitive asteroids (the sources of chondrites) accreted a significant amount of water ice and were affected by aqueous alteration and/or fluid-assisted metamorphism. These secondary parent body processes have strongly modified the primary mineralogy of chondrites in favor of a wide diversity of secondary phases that formed by interaction with water. The mineralogical and isotopic character- ization of these secondary phases in chondrites and returned samples from hydrous asteroids Ryugu and Bennu can help us reconstruct the dynamical evolution of water in the early Solar System and understand the timing and mechanisms of aqueous alteration on hydrous asteroids.

The distribution of water in differentiated Solar System bodies depends on many factors including size, distance from the Sun, and how they incorporated water. Most of this water is likely locked as hydrogen in mantle minerals and could amount to several Earth oceans worth in mass for the largest planets. An essential compound for the development of life, water also has a tremendous influence on planetary evolution and volcanism. Only Earth has an active exchange of water between surface and mantle. Surface water on other differentiated bodies mostly results from degassing by volca- noes whose mantle sources are inherited from magma ocean processes early in their history. Airless bodies also acquire surface water by impacts, spallation, and from the solar wind.

Spacecraft-based missions have discovered an increasing number of ocean worlds in our Solar System, with even more candidates awaiting confirmation. The science of ocean worlds shares some commonalities with that of Earth’s oceans, making them exciting targets of future exploration. A major known difference, however, is that ice shells up to tens of kilometers thick may present barriers to the introduction of chemical gradients necessary for life’s development over the long term. Hence, ocean worlds differ substantially in terms of their energy budget and chemistry, with Europa and Enceladus being currently considered the most promising candidates for life-search missions.

Water is crucial for the emergence and evolution of life on Earth. Recent studies of the water content in early forming planetary systems similar to our own show that water is an abundant and ubiquitous molecule, initially synthesized on the surfaces of tiny interstellar dust grains by the hydrogenation of frozen oxygen. Water then enters a cycle of sublimation/freezing throughout the successive phases of planetary system formation, namely, hot corinos and protoplanetary disks, eventually to be incorporated into planets, asteroids, and comets. The amount of heavy water measured on Earth and in early forming planetary systems suggests that a substantial fraction of terrestrial water was inherited from the very first phases of the Solar System formation and is 4.5 billion years old.

Water played a key role in shaping the Solar System—from the formation of early solids to the processes of planetary and moon formation. The presence of water in molecular clouds influences the initial abundance and distribution of water in the circumsolar disk, which, in turn, affected the water budget of the terrestrial planets and, therefore, their geological activity and habitability. On Earth, surficial and deep-water cycles have largely governed the planet’s geodynamical and geochemical evolu- tion. This issue focuses on the past and present distribution of water within the Solar System and how this important molecule affects astrophysical and geological processes.